Pupillary reflex. The study of pupillary reflexes How is the test

VISUAL PATH

The anatomical structure of the visual pathway is quite complex and includes a number of neural links. Within the retina of each eye is a layer of rods and cones (photoreceptors - the first neuron), then a layer of bipolar (second neuron) and ganglion cells with their long axons (third neuron). Together they form the peripheral part of the visual analyzer. The pathways are represented by the optic nerves, chiasma, and optic tracts.

The latter terminate in the cells of the lateral geniculate body, which plays the role of the primary visual center. From them already originate the fibers of the central neuron of the visual pathway, which reach the region of the occipital lobe of the brain. The primary cortical center of the visual analyzer is localized here.

The optic nerve is formed by the axons of retinal ganglion cells and ends in the chiasm. A significant part of the nerve is the orbital segment, which in the horizontal plane has an 8-shaped bend, due to which it does not experience tension when the eyeball moves.

For a considerable distance (from the exit from the eyeball to the entrance to the optic canal), the nerve, like the brain, has three shells: hard, arachnoid, soft. Together with them, its thickness is 4-4.5 mm, without them - 3-3.5 mm. At the eyeball, the hard shell fuses with the sclera and the Telon capsule, and at the optic canal, with the periosteum. The intracranial segment of the nerve and chiasm, located in the subarachnoid chiasmatic cistern, are dressed only in a soft shell. The intrathecal spaces of the ophthalmic part of the nerve (subdural and subarachnoid) are connected to similar spaces in the brain, but isolated from each other. They are filled with a liquid of complex composition (intraocular, tissue, cerebrospinal).

Since the intraocular pressure is normally twice the intracranial pressure (10–12 mm Hg), the direction of its current coincides with the pressure gradient. The exception is cases when intracranial pressure is significantly increased (for example, with the development of a brain tumor, hemorrhages in the cranial cavity) or, conversely, the tone of the eye is significantly reduced.

All primary fibers that make up the optic nerve are grouped into three main bundles. Axons of ganglion cells extending from the central (macular) region of the retina make up the papillomacular bundle, which enters the temporal half of the optic nerve head. Fibers from the ganglion cells of the nasal half of the retina run along radial lines into the nasal half of the disc. Similar fibers, but from the temporal half of the retina, on the way to the optic nerve head “flow around” the papillomacular bundle from above and below.

In the orbital segment of the optic nerve near the eyeball, the ratios between the nerve fibers remain the same as in its disk. Next, the papillomacular bundle moves to the axial position, and the fibers from the temporal retinal squares move to the entire corresponding half of the optic nerve. Thus, the optic nerve is clearly divided into right and left halves. Its division into upper and lower halves is less pronounced. An important clinical feature is that the nerve is devoid of sensitive nerve endings.

In the region of the skull, the optic nerves join above the sella turcica to form the chiasma, which is covered by the pia mater and has the following dimensions: length 4-10 mm, width 9-11 mm, thickness 5 mm. Chiasma from below borders on the diaphragm of the sella turcica (a preserved section of the dura mater), from above (in the posterior region) - on the bottom of the third ventricle of the brain, on the sides - on the internal carotid arteries, on the back - on the funnel of the pituitary gland.

In the region of the chiasm, the fibers of the optic nerves partially cross due to portions associated with the nasal halves of the retinas.

Moving to the opposite side, they connect with fibers coming from the temporal halves of the retinas of the other eye, and form the visual tracts. Here, the papillomacular bundles also partially intersect.

The optic tracts begin at the posterior surface of the chiasm and, having rounded the brain stem from the outside, end in the lateral geniculate body, the posterior part of the optic tubercle, and the anterior quadrigemina of the corresponding side. However, only the external geniculate bodies are the unconditional subcortical visual center. The remaining two formations perform other functions.

In the visual tracts, whose length in an adult reaches 30-40 mm, the papillomacular bundle also occupies a central position, and crossed and non-crossed fibers still go in separate bundles. At the same time, the first of them are located vecto-medially, and the second - pre-reolaterally. Visual radiation (fibers of the central neuron) starts from the ganglion cells of the fifth and sixth layers of the lateral geniculate body.

First, the axons of these cells form the so-called Wernicke's field, and then, passing through the posterior thigh of the internal capsule, fan-shaped diverge in the white matter of the occipital lobe of the brain. The central neuron terminates in the sulcus of the bird's spur. This area personifies the sensory visual center - the seventeenth cortical field according to Brodman.

The path of the pupillary reflex - light and to set the eyes at close range - is rather complicated. The afferent part of the reflex arc of the first of them starts from the cones and rods of the retina in the form of autonomous fibers that go as part of the optic nerve. In the chiasm, they cross in exactly the same way as the optic fibers and pass into the optic tracts. In front of the external geniculate bodies, the pupillomotor fibers leave them and, after a partial decussation, terminate at the cells of the so-called pretectal region. Further, new, interstitial neurons, after partial decussation, are sent to the corresponding nuclei (Yakutovich - Edinger - Westphal) of the oculomotor nerve. Afferent fibers from the macula of the retina of each eye are present in both oculomotor nuclei.

The efferent path of innervation of the iris sphincter starts from the nuclei already mentioned and goes as a separate bundle as part of the oculomotor nerve. In the orbit, the sphincter fibers enter its lower branch. And then through the oculomotor root to the ciliary node. Here the first neuron of the considered path ends and the second one begins. Upon exiting the ciliary ganglion, the sphincter fibers in the composition of the short ciliary nerves, passing through the sclera, enter the perichoroidal space, where they form the nerve plexus. Its terminal branches penetrate the iris and enter the muscle in separate radial bundles, that is, they innervate it sectorally. In total, there are 70–80 such segments in the sphincter of the pupil.

The efferent path of the dilator (expander) of the pupil, which receives sympathetic innervation, starts from the ciliospinal center Budge. The latter is located in the anterior horns of the spinal cord. Connecting branches depart from here, which through the border trunk of the sympathetic nerve, and then the lower and middle sympathetic cervical ganglia reach the upper ganglion. Here the first neuron of the path ends and the second begins, which is part of the plexus of the internal carotid artery. In the cranial cavity, the fibers that innervate the pupillary dilator leave the aforementioned plexus, enter the trigeminal (Gasser) node, and then leave it as part of the optic nerve. Already at the top of the border, they pass into the nasociliary nerve and then, together with the long ciliary nerves, penetrate into the eyeball. In addition, the central sympathetic pathway departs from the Budge center, ending in the cortex of the occipital lobe of the brain. From here begins the corticonuclear pathway of inhibition of the pupillary sphincter.

The pupillary dilator function is regulated by the supranuclear hypothalamic center, located at the level of the third ventricle of the brain in front of the pituitary infundibulum. Through the reticular formation, it is connected with the ciliospinal center Budge.

The reaction of the pupils to convergence and accommodation has its own characteristics, and the reflex arcs in this case differ from those described above.

With convergence, the stimulus for pupillary constriction is proprioceptive impulses coming from the contracting internal rectus muscles of the eye. Accommodation is stimulated by the vagueness (defocusing) of images of external objects on the retina. The effective part of the pupillary reflex arc is the same in both cases.

The center for setting the eye at close range is believed to be in Brodmann's eighteenth cortical area.

The eyes are quite an important organ for the normal functioning of the body and a full life. The main function is the perception of light stimuli, due to which the picture appears.

Structural features

This peripheral organ of vision is located in a special cavity of the skull, which is called the orbit. From the sides of the eye is surrounded by muscles, with the help of which it is held and moved. The eye consists of several parts:

1. Directly the eyeball, which has the shape of a ball about 24 mm in size. It consists of the vitreous body, the lens and aqueous humor. All this is surrounded by three shells: protein, vascular and mesh, arranged in reverse order. The elements that make up the picture are located on the retina. These elements are receptors that are sensitive to light;
2. The protective apparatus, which consists of the upper and lower eyelids, the orbit;
3. adnexal apparatus. The main components are the lacrimal gland and its ducts;
4. The oculomotor apparatus, which is responsible for the movements of the eyeball and consists of muscles;

Main functions

The main function that vision performs is to distinguish between various physical characteristics of objects, such as brightness, color, shape, size. In combination with the action of other analyzers (hearing, smell, and others), it allows you to adjust the position of the body in space, as well as determine the distance to the object. That is why the prevention of eye diseases should be carried out with enviable regularity.

Presence of a pupillary reflex

With the normal functioning of the organs of vision, with certain external reactions, the so-called pupillary reflexes occur, in which the pupil narrows or expands. The pupillary reflex, the reflex arc of which is the anatomical substrate of the pupil's reaction to light, indicates the health of the eyes and the whole organism as a whole. That is why, in some diseases, the doctor first checks for the presence of this reflex.

What is a reaction?

The pupil reaction or the so-called pupillary reflex (other names are the iris reflex, irritant reflex) is some change in the linear dimensions of the pupil of the eye. Constriction is usually caused by contraction of the muscles of the iris, and the reverse process - relaxation - leads to the expansion of the pupil.

Possible reasons

This reflex is caused by a combination of certain stimuli, the main of which is a change in the level of illumination of the surrounding space. In addition, a change in the size of the pupil can occur for the following reasons:

• action of a number of drugs. That is why they are used as a way to diagnose the state of drug overdose or excessive depth of anesthesia;
• changing the point of focus of a person's view;
• emotional outbursts, both negative and positive equally.

If there is no reaction

Lack of pupil reaction to light may indicate various human conditions that pose a threat to life and require immediate intervention by specialists.

Diagram of the pupillary reflex

The muscles that control the work of the pupil can easily influence its size if they receive a certain stimulus from the outside. This allows you to control the amount of light that enters the eye directly. If the eye is covered from the incoming sunlight, and then opened, then the pupil, which previously expanded in the dark, immediately decreases in size when the light appears. The pupillary reflex, the reflex arc of which begins on the retina, indicates the normal functioning of the organ.

The iris has two types of muscles. One group is circular muscle fibers. They are innervated by parasympathetic fibers of the optic nerve. If these muscles contract, this process causes pupil constriction. The other group is responsible for pupil dilation. It includes radial muscle fibers that are innervated by sympathetic nerves.

The pupillary reflex, the scheme of which is quite typical, occurs in the following order. Light that passes through the layers of the eye and is refracted in them hits the retina directly. The photoreceptors that are located here, in this case, are the beginning of the reflex. In other words, this is where the path of the pupillary reflex begins. The innervation of the parasympathetic nerves affects the work of the sphincter of the eye, and the arc of the pupillary reflex contains it in its composition. The process itself is called the efferent shoulder. The so-called center of the pupillary reflex is also located here, after which various nerves change their direction: some of them go through the legs of the brain and enter the orbit through the upper fissure, others - to the sphincter of the pupil. This is where the path ends. That is, the pupillary reflex closes. The absence of such a reaction may indicate any disturbances in the human body, which is why it is given such great importance.

Pupillary reflex and signs of its defeat

When examining this reflex, several characteristics of the reaction itself are taken into account:

• pupil constriction;
• the form;
• the uniformity of the reaction;
• pupillary mobility.

There are several of the most popular pathologies, indicating that the pupillary and accommodative reflexes are impaired, which indicates malfunctions in the body:

• Amaurotic immobility of the pupils. This phenomenon is a loss of a direct reaction when illuminating a blind eye and a friendly reaction if vision problems are not observed. The most common causes are various diseases of the retina itself and the visual pathway. If the immobility is unilateral, is a consequence of amaurosis (retinal damage) and is combined with pupil dilation, albeit slight, then there is a possibility of developing anisocoria (pupils become different sizes). With such a violation, other pupillary reactions are not affected in any way. If amaurosis develops on both sides (that is, both eyes are affected at the same time), then the pupils do not react in any way and even when exposed to sunlight remain dilated, that is, the pupillary reflex is completely absent.
• Another type of amaurotic immobility of the pupils is hemianopic immobility of the pupil. Perhaps there is a lesion of the visual tract itself, which is accompanied by hemianopsia, that is, blindness of half of the visual field, which is expressed by the absence of a pupillary reflex in both eyes.

• Reflex immobility or Robertson's syndrome. It consists in the complete absence of both direct and friendly reaction of the pupils. However, unlike the previous type of lesion, the reaction to convergence (narrowing of the pupils if the gaze is focused on a certain point) and accommodation (changes in the external conditions in which the person is located) is not impaired. This symptom is due to the fact that changes occur in the parasympathetic innervation of the eye in the case when there is damage to the parasympathetic nucleus, its fibers. This syndrome may indicate the presence of a severe stage of syphilis of the nervous system, less often the syndrome reports encephalitis, a brain tumor (namely in the legs), as well as a traumatic brain injury.

The causes may be inflammatory processes in the nucleus, root or trunk of the nerve responsible for eye movements, a focus in the ciliary body, tumors, abscesses of the posterior ciliary nerves.

The friendliness and simultaneity of the movements of the eyeballs is carried out by a synergistic contraction of several external mts. This is possible due to a special system that connects the nuclei of the oculomotor nerves of both sides and provides them with connections with other parts of the NS - the beginning from the nucleus of Darkshevich, which lies anterior to the nucleus of the third pair - the posterior longitudinal bundle (left and right). Pass through the brainstem close to the midline and give collaterals to the III, IV and VI pairs of cranial nerves. Also, the composition includes fibers from the cells of the vestibular nuclei of its and the opposite side. The posterior longitudinal bundle descends into the anterior cords of the spinal cord. It ends near the cells of the anterior horns of the cervical segments. With cortical gaze palsy - the eyes look towards the focus, with a bridge (stem) - in the contralateral side of the focus. pupillary reflexes : 1) into the light; 2) for convergence. Constriction of the pupil due to impaired sympathetic innervation is usually combined with endophthalmos and narrowing of the palpebral fissure (Bernard-Horner syndrome). Irritation of the sympathetic nerve gives, in addition to the expansion of the pupil, exophthalmos and expansion of the palpebral fissure (Pourfure du Petit syndrome). If the pupil is dilated due to damage to the oculomotor nerve, then at the same time its reaction to light and convergence with accommodation are weakened. With a weakening or absence of a direct and friendly reaction of the pupil to light, the oculomotor nerve is affected. If the direct reaction to light is impaired, and the friendly of the same eyeball is preserved, the afferent part of the reflex arc (n. opticus) is affected.

11. V pair of FMN - trigeminal nerve, syndromes of sensitivity disorders (peripheral, nuclear, stem and hemispheric), chewing disorders.

V pair, n. trigeminus. Trigeminal nerve (mixed), has sensory and motor fibers. The sensitive pathway from superficial and deep receptors begins with peripheral and then central processes of sensitive bipolar cells (1st sense neuron) located in a powerful trigeminal (Gasser) node. The trigeminal node lies on the anterior surface of the pyramid of the temporal bone between the sheets of the dura mater. The peripheral processes of bipolar ganglion cells, distributed in 3 nerve trunks, make up 3 branches of the trigeminal nerve. Scheme of the sensory pathway of the trigeminal nerve: 1st neuron - bipolar cells of the trigeminal ganglion, 2nd neuron - sensitive nuclei of the trigeminal nerve - gives off a process that crosses and reaches the thalamus with fibers of the medial loop, the 3rd neuron is located in the thalamus; its process runs in the posterior third of the posterior pedicle of the internal capsule and ends in the projection zone of the central gyrus. The ophthalmic nerve (N. ophthalmicus) conducts impulses of superficial and deep sensitivity from the skin of the forehead and anterior scalp, upper eyelid, inner corner of the eye and back of the nose, eyeball, mucous membrane of the upper part of the nasal cavity, frontal and ethmoid sinuses of the meninges, and also from the periosteum and muscles of the upper third of the face. The maxillary nerve (N. maxillaris) conducts sensory impulses from the skin of the lower eyelid, the outer corner of the eye, the upper part of the cheeks, the upper lip, the upper jaw and its teeth, the mucous membrane of the lower part of the nasal cavity and the maxillary sinus. The mandibular nerve (N. mandibularis) conducts sensory impulses from the lower lip, lower cheek, from the lower jaw and its teeth, chin, back of the lateral surface of the face, from the mucous membrane of the cheeks, the lower part of the oral cavity of the tongue. The mandibular branch, unlike the upper and middle branches, is a mixed nerve that carries motor fibers to the masticatory muscles of M. masseter, M. temporalis, M. pterygoideus externus et medianus, M. digastricus (anterior belly). Qualitative and quantitative sensitivity disorders with the defeat of the trigeminal nerve, the same as with the defeat of the conductors of the sensitivity of the trunk and limbs: hyperesthesia, hypoesthesia or anesthesia, hyperpathia, dysesthesia, polyesthesia, pain, phantom sensations and other forms of sensitivity disturbance can be observed. The defeat of one of the three branches of the V nerve leads to a violation of all types of feelings according to the peripheral type - in the zone of innervation by this branch, to the appearance of pain, as well as to a decrease in the corresponding reflexes. The defeat of the trigeminal node or sensitive root (radix sensoris) is accompanied by a violation of all types of sensitivity in the innervation zones of all 3 branches. With a localized lesion in the region of the brain bridge, dissociated sensory disturbances may occur. With a complete lesion of the nucleus of the spinal tract of the Vth nerve, superficial sensitivity drops out on half of the face according to the segmental type. Segmental damage to this nucleus leads to a loss of sensitivity in certain segmental annular skin zones of Zelder. Foci in the middle part of the pons of the brain and in the medulla oblongata can simultaneously capture the fibers of the spinothalamic tract along with the nucleus of the Vth nerve, causing alternating hemianesthesia: a disorder of superficial sensitivity on the face on the side of the focus according to the segmental type, and on the trunk and limbs - according to the conductive type on the opposite side. Localization of the pathological process in the region of the pontine nucleus of the V nerve is accompanied by a loss of deep sensitivity of half of the face on the side of the focus. The defeat of the visual tuberosity and the posterior third of the posterior leg of the internal capsule causes a contralateral loss of all types of sensitivity on the face, trunk, limbs. Loss of feelings on half of the face can also occur when the lower third of the posterior central gyrus of the opposite side is destroyed. With trigeminal neuralgia associated with the defeat of one or another branch, the resulting pain can be radiating in nature, capturing the lower and upper jaws, eye, ear, etc. To determine the localization of the main lesion, it is of great importance to identify pain points at the exit points of the branches of the trigeminal nerve on the surface of the face: for the first branch, the supraorbital foramen (For. supraorbitalis), for the second, the infraorbital foramen (For. infraorbitalis), for the third, the mental foramen ( For mentalis).

12. VII pair of cranial nerves - facial nerve, central and peripheral paresis of mimic muscles.

VII pair, n. facialis - motor nerve. Innervates the mimic muscles, the muscles of the auricle and the subcutaneous muscle of the neck. The nucleus of the facial nerve is located deep in the lower part of the brain bridge on the border with the medulla oblongata. The fibers from the nucleus first rise up and go around the nucleus of the VI nerve, forming the inner knee of the facial nerve, then exit between the bridge and the medulla oblongata under the overhanging cerebellar hemisphere, in the so-called cerebellar pontine angle (the roots of the V, VI, VIII nerves also pass here). The facial nerve, together with the intermediate and VIII nerves, enters the internal auditory foramen of the temporal bone and soon penetrates through the opening at the base of the internal auditory meatus into the fallopian canal. Here, the facial nerve changes its horizontal direction to a vertical one, forming the external knee, and exits the skull through the styloid mastoid opening, penetrating the parotid gland, and divides into a number of terminal branches (crow's foot). In the canal of the temporal bone, three branches depart from the trunk of the facial nerve: the stony nerve, the stapedial nerve, and the tympanic string. Damage to a peripheral neuron (nucleus, trunk of the facial nerve) arises peripheral paralysis of facial muscles on the side of the focus. The face is asymmetrical. The tone of the muscles of the healthy half of the face "pulls" the mouth to the healthy side. The affected side is mask-like. There are no nasolabial and frontal folds. The eye is open (paralysis of the circular muscle of the eye) - lagophthalmos- hare eye. With lagophthalmos, it is usually observed lacrimation. The development of lacrimation is due to the fact that tears do not reach the lacrimal punctum, where they are usually pushed through by periodic closing of the eyelids, and pour out over the edge of the lower eyelid. Constantly open eye contributes to increased lacrimal reflex. On the affected side, the corner of the mouth is motionless, a smile is impossible. Due to the defeat of the circular muscle of the mouth, whistling is impossible, speech is somewhat difficult, liquid food on the affected side pours out of the mouth. Muscle atrophy occurs. There is a decrease in the superciliary, corneal and conjunctival reflexes . Damage to the nucleus of the facial nerve often accompanied by the involvement of the fibers of the pyramidal pathway in the process, as a result of which an alternating Miylard-Jublé syndrome: peripheral paralysis of the facial muscles on the side of the focus and contralateral spastic hemiplegia. Damage to the nucleus or inner knee of the facial nerve is sometimes accompanied by involvement in the pathological process, in addition to the pyramidal pathway, of the nucleus of the VI nerve. At the same time, an alternating Fauville syndrome: on the side of the focus - peripheral paralysis of the facial muscles and abductor muscles of the eye (converging strabismus), and on the opposite- spastic hemiplegia. With damage to the facial nerve root , which exits along with the V, VI and VIII nerves in the cerebellopontine angle, paralysis of the mimic muscles can be combined with symptoms of damage to these nerves. Symptoms of damage to the facial nerve in the fallopian canal depend on the level of localization. If the large stony nerve is damaged before the departure of the large stony nerve, all accompanying fibers are involved in the process and in the clinic there are, in addition to peripheral paralysis of the mimic muscles, dry eye, hyperakia, taste disturbance on the front 2/3 of the tongue. A lower localization of the lesion above the origin of the stapedial nerve is accompanied by hyperacusis and taste disorder. Dryness of the eye is replaced by increased lacrimation. With a lesion above the departure of the tympanic string, there is lacrimation and taste disturbance in the anterior 2 /z language. With a lesion below the departure of the drum string, paralysis of mimic muscles and lacrimation. Peripheral paralysis of mimic muscles is sometimes accompanied by pain in the face, ear, mastoid process. This is due to the involvement in the pathological process of the fibers of the V nerve (which can pass in the fallopian canal), the trigeminal ganglion or the root of the V nerve. With the defeat of the cortical-nuclear fibers on the one hand, developing central paralysis of the mimic muscles of the lower part of the face(upper - receives bilateral cortical innervation) on the opposite side of the hearth. At the same time, on the same side (contralateral to the focus) central paralysis of half of the tongue, and in case of involvement of the corticospinal tract - and hemiplegia.

13. VIII pair of cranial nerves - vestibulocochlear nerve, auditory and vestibular system; the role of the vestibular apparatus in the regulation of coordination of movements, balance and posture; signs of damage at different levels; nystagmus, vestibular vertigo, vestibular atasia, Meniere's syndrome.

VIII pair, n. acusticus. The vestibulocochlear nerve consists of the cochlear part (pars cochlearis) and the vestibular part (pars vestibularis). The auditory pathways originate in the neurons of the spiral ganglion of the cochlea - first neuron, which is located in the snail labyrinth. The peripheral processes of these neurons are sent to the organ of Corti, where special receptors are located. The central processes through the internal auditory opening enter the cranial cavity and end in the two nuclei of the brain bridge - the anterior and posterior cochlear nuclei. fibers second neurons start from these nuclei, form a trapezoid body, pass to the other side and, as part of the lateral loop, end in the primary auditory subcortical centers - in the nuclei of the lower colliculus and in the internal geniculate bodies. Third neuron starts from the internal geniculate body, passes through the internal capsule and the radiant crown and ends in the cortical auditory region - the posterior section of the superior temporal gyrus (Geshl's gyrus). The vestibular part begins from the vestibular node, which lies at the bottom of the internal auditory meatus. The peripheral processes of the node cells (the first neuron) come from the ampullae of three semicircular canals and two membranous sacs of the vestibule - elliptical and spherical. The central processes of these cells make up the Pars vestibularis, which enters the cranial cavity through the internal auditory foramen and goes to the cerebellopontine angle. The fibers of the vestibular nerve terminate in the nuclei located in the area IV ventricle: outer nucleus (Deiters), superior nucleus (Bekhterev) and medial and inferior vestibular nuclei of the vestibule VIII nerve. The second neurons of the vestibular pathway originate from all nuclei, but mainly from the nuclei of Deiters and Bekhterev. From Bekhterev's nucleus, through the inferior cerebellar peduncle, the fibers are directed to the nucleus of the tent of the cerebellar vermis, mainly on their own side. The central vestibular pathway from the vestibular nuclei is connected through the optic tubercle with the cortical section of the vestibular analyzer, which is located in the parietotemporal region. Most often observed: 1) dizziness - can occur paroxysmal, sometimes only with certain positions of the head and torso. Sometimes it seems to the patient that all objects around him rotate in a certain direction counterclockwise or clockwise, the earth sways. Such dizziness is called systemic. It is very characteristic of vestibular lesions. In some cases, dizziness is aggravated by looking up or turning the head sharply. Against the background of this symptom, nausea, vomiting, blackout of consciousness may occur. 2) nystagmus - rhythmic twitching of the eyeballs. According to the direction of these movements, horizontal, vertical, rotatory nystagmus are distinguished. In some cases, nystagmus is observed constantly, in others it is detected only at a certain position of the head and body. Usually, two components can be distinguished in nystagmoid movements: a fast movement in one direction and a slow return back. The direction of nystagmus is determined from the fast component. With irritation of the vestibular apparatus, nystagmus occurs in the direction of irritation, with damage - in the opposite direction. 3) Impaired coordination of movements - consists in staggering, violation of the index test when it is carried out with closed eyes; similar symptoms can be observed with damage to the cerebellum.

14. IX and X pairs of cranial nerves - glossopharyngeal and vagus nerves, autonomic functions of the vagus nerve; signs of damage at different levels, bulbar and pseudobulbar syndromes.

IX pair, n. glossopharyngeus- mixed nerve. X pair, n. vagus - mixed nerve. These two nerves are usually considered together, since they have common nuclei in the brain stem, jointly provide sensory and motor innervation of the pharynx, larynx of the soft palate; the study of their functions is carried out simultaneously. The IX nerve has four nuclei: gustatory - the nucleus of a single path, common with the intermediate and X nerve; salivary - lower salivary nucleus; sensitive - the core of the gray wing, common with the X nerve, providing sensitivity to the larynx, trachea, pharynx, soft palate, middle ear; motor - a double nucleus, common with the X nerve, innervating the muscles of the pharynx, larynx, epiglottis, soft palate. In addition to the three nuclei in common with the IX nerve, the X nerve has its own nucleus - parasympathetic - the posterior nucleus of the vagus nerve, which provides parasympathetic motor innervation of the internal organs and gives off secretory fibers going to the stomach, pancreas, and intestines. The system of IX and X nerves includes two sensitive nodes - the upper node, the lower node. In the nodes of the IX and X nerves, there is the first neuron of sensory pathways from the receptors of the mucous membrane of the pharynx, larynx, trachea, as well as from the taste buds of the tongue. Taste. Sensitive taste impulses from the tongue enter the primary taste center of the trunk - the gustatory nucleus through three main channels: from the anterior 2/3 of the tongue - along the intermediate nerve (first neuron) - the bipolar taste cell in the geniculate node, from the posterior 1/3 of the tongue - through IX and X nerves (bipolar taste cell in upper and lower nodes). Having collected all the taste information, the taste nucleus, in which the second taste neuron is located, sends it to the nucleus of the thalamus opticus of the opposite side. Here the third taste neurons begin, the axons of which pass through the posterior 1/3 of the posterior leg of the internal capsule and end in the cortical taste region (limbic region, lower parts of the posterior central gyrus, insula). Taste sensations are perceived differently by different parts of the tongue. Sweet is better felt by the tip of the tongue, sour - by the edges, bitter - by the back third, salty - equally by the entire surface of the tongue . Decreased taste is called hypogesia, the loss - ageusia, increase - hypergesia. Irritation of the cortical gustatory area causes gustatory hallucinations. Unilateral destruction of cortical taste centers does not cause noticeable taste disorders, since each hemisphere is associated with taste receptor fields on both sides. The salivary function is provided by the activity of the upper and lower salivary parasympathetic nuclei that innervate the lacrimal gland, submandibular, sublingual and parotid salivary glands. The neurons of the upper nucleus give off processes that go as part of the trunk of the intermediate nerve to the sublingual and submandibular salivary and lacrimal glands, and the neurons of the lower nucleus as part of the IX nerve to the parotid gland. The salivary fibers of the IX nerve, leaving its trunk, are sent as part of the tympanic nerve, and then as part of the small stony nerve to the ear node. Postganglionic fibers to the parotid gland go as part of the ear-temporal nerve. With damage to the salivary nucleus or glossopharyngeal nerve, dry mouth occurs due to inactivity of the powerful parotid salivary gland. Damage to the vrisberg nerve or string tympani does not lead to dry mouth if the parotid gland is functioning normally. The sensory nucleus and the motor nucleus, common to the glossopharyngeal and vagus nerve, provide sensitivity to the mucous membrane of the pharynx, larynx, trachea, soft palate and motor innervation of the muscles of the soft palate, epiglottis, pharynx, and larynx. With the defeat of any of these nuclei or trunks of the IX and X nerves there is a decrease or loss of the pharyngeal and palatine reflexes due to a break in the reflex arc, the afferent part of which is represented by processes of bipolar ganglion cells and neurons of the sensory nucleus, and the afferent part is represented by neurons of the double nucleus. With bilateral damage to the double nucleus swallowing is disturbed, patients choke. As a result of paralysis of the muscles of the epiglottis liquid food enters the larynx and trachea, and due to paralysis of the muscles of the soft palate, it flows into the cavity of the nasopharynx and nose. The patient's speech acquires nasal connotation, since the sound resonates in the nasopharynx, not closed by the palatine curtain. Unilateral lesion of the motor nucleus appears drooping of the soft palate on the side of the lesion, immobility or lagging behind on this side when pronouncing the sound "a". The tongue (uvula) deviates to the healthy side. Unilateral vocal cord paralysis is detected by laryngoscopy. The voice becomes hoarse. The pharyngeal and palatal reflexes are reduced or fall out on the affected side.Damage to the nucleus of the gray wing (Nucl. alae cinereae) or sensory fibers going to it along the trunk of the IX and X nerves, accompanied by anesthesia of the mucous membrane of the soft palate, pharynx. The posterior nucleus of the vagus nerve provides parasympathetic innervation to the smooth muscles of the vessels, stomach, intestines, trachea, bronchi, heart muscles, glands of the respiratory and gastrointestinal tract. Bilateral damage to these nuclei causes death due to the cessation of cardiac activity and respiratory arrest. With damage to the IX nerve: 1) violation of taste in the back third of the tongue; 2) denervation of the parotid gland, accompanied by dry mouth; 3) anesthesia of the pharynx on the affected side; 4) decrease in pharyngeal and palatine reflexes on the side of the lesion; 5) paralysis of the soft palate on the side of the lesion, deviation of the uvulae to the healthy side; choking when swallowing; nasal tone of voice. With damage to the X nerve: 1) violation of taste in the back third of the tongue; 2) anesthesia of the pharynx, larynx, trachea on the affected side; 3) decrease or loss of pharyngeal and palatine reflexes on the side of the lesion; 4) unilateral paralysis of the soft palate, choking when swallowing, sagging of the vocal cord; hoarse voice with a nasal tint; 5) parasympathetic denervation of internal organs on the affected side. bulbar syndrome. The combined defeat of the glossopharyngeal, vagus and hypoglossal nerves of the peripheral type leads to the development of the so-called bulbar palsy. It occurs when the nuclei of the IX, X and XII pairs of cranial nerves in the region of the medulla oblongata or their roots at the base of the brain, or the nerves themselves, are damaged. It can be either unilateral or bilateral. The latter is incompatible with life. It is observed in amyotrophic lateral sclerosis, circulatory disorders in the medulla oblongata, trunk tumors, stem encephalitis, syringobulbia, polioencephalomyelitis, polyneuritis, anomalies of the foramen magnum, fracture of the base of the skull, etc. There is paralysis of the soft palate, epiglottis, larynx. The voice becomes nasal, deaf and hoarse (aphonia), speech is slurred (dysarthria) or impossible (anartria), the act of swallowing is disturbed: liquid food enters the nose, larynx (dysphagia), there are no pharyngeal and palatine reflexes. On examination, immobility of the palatine arches and vocal cords, fibrillar twitching of the muscles of the tongue, their atrophy are revealed, the mobility of the tongue is limited up to glossoplegia. Violations of the vital functions of the body (respiration and cardiac activity) are observed. Similar disorders of swallowing, phonation and articulation of speech can occur when not the IX, X and XII pairs of cranial nerves themselves are affected, but the cortical-nuclear pathways connecting the cerebral cortex with the corresponding nuclei of the cranial nerves. Since in this case the medulla oblongata is not affected, this syndrome is called "false" bulbar paralysis (pseudobulbar syndrome). pseudobulbar syndrome. The main difference between the pseudobulbar syndrome is that, being a central paralysis, it does not lead to the loss of unconditioned stem reflexes associated with the medulla oblongata. With a unilateral lesion of the supranuclear pathways, no disorders from the glossopharyngeal and vagus nerves occur due to the bilateral cortical innervation of their nuclei. The dysfunction of the hypoglossal nerve that occurs in this case is manifested only by a deviation of the tongue when protruding in the direction opposite to the lesion (i.e., towards the weak muscle of the tongue). Speech disorders are usually absent. Thus, pseudobulbar syndrome occurs only with bilateral damage to the central motor neurons of the IX, X and XII pairs of cranial nerves. As with any central paralysis, there is no muscle atrophy and changes in electrical excitability. In addition to dysphagia, dysarthria, reflexes of oral automatism are expressed: nasolabial, labial, proboscis, palmo-chin Marinescu-Radovici, etc., as well as violent crying and laughter. Damage to the corticonuclear pathways can occur in various cerebral processes: vascular diseases, tumors, infections, intoxications, and brain injuries.

15. XI pair of cranial nerves - accessory nerve, symptoms of the lesion.

XI pair, n. accessorius- motor nerve. The nucleus of the nerve is located in the lower part of the medulla oblongata and gray in-ve s / m at the level of C 1 -C 5 . The roots of the s / m part go to the lateral surface of the cervical s / m, merge into a common nerve trunk, which rises up and enters the cranial cavity through the foramen magnum, then, after merging with the bulbar part of the nerve, exits through the jugular foramen (For. jugulare). The XI nerve innervates the sternocleidomastoid and trapezius muscles. Functions of the muscles: tilting the head to one side with turning the face in the opposite direction, raising the shoulder and the acromial part of the scapula up (shrugs), pulling the shoulder girdle backwards and bringing the scapula to the vertebrae. To study the function of the XI nerve, the patient is asked to turn his head to the sides, shrug his shoulders, raise his arms above the horizontal line. When defeated nucleus, root, nerve trunk having developed peripheral paralysis of the sternocleidomastoid and trapezius muscles and it is difficult to turn the head to the healthy side, the shoulder on the affected side is pubescent, the shoulder blades move away from the vertebra with a lower angle, it is difficult to shrug the shoulder, raising the arm above the horizontal line is limited . The nucleus of the accessory nerve has a bilateral cortical innervation, therefore, the central paralysis of the muscles innervated by it can occur only with bilateral damage to the cortical-nuclear pathways. A friendly turn of the head and gaze is carried out due to the connections of the nuclei of the accessory nerve with the system of the posterior longitudinal bundle.

16. XII pair - hypoglossal nerve, symptoms of damage.

XII pair, n. hypoglossus- motor nerve. The nucleus of the nerve lies at the bottom of the rhomboid fossa, begins in its central section and stretches to the 3rd cervical segment of the s/m. The roots exit between the pyramids and olives of the medulla oblongata, merge into a common trunk emerging from the cranial cavity through the canal of the hypoglossal nerve (canalis hypoglossi). With peripheral nerve damage there is paresis or paralysis of the corresponding half of the tongue - atrophy of the muscles of the tongue. When protruding, the tongue deviates towards paralysis, because. The geniohyoid muscle of the healthy side directs the tongue forward and in the opposite direction. When the nucleus is damaged hypoglossal nerve in the muscles of the tongue - fibrillar twitching. Nerve damage leads to speech impairment. She becomes indistinct, weaving (dysarthria). Mild dysarthria can be detected when patients pronounce difficultly articulated words ("serum from yogurt"). With complete bilateral lesion tongue is immobile and speech becomes impossible (anartria), chewing and swallowing disorders . With damage to the nerve nucleus with pyramidal tracts passing through the trunk develop peripheral paralysis of the muscles of the tongue and central hemiplegia on the opposite side (alternating "Jackson's syndrome"). With damage to the medulla oblongata a combination of lesions of various nuclei of the bulbar group of the IX, X and XI nerves, as well as the pyramidal tract with the development alternating syndromes of Avellis, Schmidt. Avellis syndrome characterized by symptoms of damage to the double nucleus (IX and X n) and the pyramidal path . At Schmidt syndrome on the side of the pathological process, symptoms of damage to the motor nuclei of the caudal group (N. ambiguus and nuclei XI n) are noted, on the opposite side - central hemiplegia . The nucleus of the hypoglossal nerve (XII) is connected only with the opposite hemispheres, with damage to the cortical-nuclear pathway, central paralysis of the muscles of the tongue develops , in which there is no atrophy of the tongue, fibrillar twitching. By the presence or absence of atrophy and fibrillar twitching, peripheral paralysis can be distinguished from central. Simultaneously with the defeat of the cortico-nuclear pathways to the nucleus of the XII nerve, the pyramidal path and fibers to the lower part of the nucleus of the VII nerve may be involved in the process (for example, when the lesion is localized in the internal capsule). There is a characteristic symptom complex, contralateral to the lesion : hemiplegia, central paralysis of mimic muscles and half of the tongue.

The main property of the visual system, which determines all aspects of its activity and underlies such functions as distinguishing the brightness, color, shape and movement of objects, assessing their size and distance, is the ability to respond to light.

The minimum amount of light energy that causes the sensation of light characterizes the absolute light sensitivity of the eye. Due to its changes, the visual system adapts, adapts to different levels of brightness in a wide range - from 10 -6 to 10 4 nits. Light sensitivity increases significantly in the dark, which allows you to perceive very weak brightness, and decreases when moving from less to more light.

Under conditions of such adaptation, a certain background activity of all levels of the visual system is established. If there are areas with unequal brightness in the field of view, then their difference is evaluated by means of the contrast, or distinctive, sensitivity of the eye. This allows you to determine the spatial configuration of images. Consequently, contrast sensitivity is the physiological basis for the perception of the shape and size of objects. The central region of the retina has the highest contrast sensitivity.

The functional unit of the visual system is the receptive field - a cell or group of cells of a given level of the system that sends a nerve signal to the overlying neuron. Some receptive fields react only to turning on the light (on-response), others only to turning it off (off-response), others - both turning on and turning off the light (on / off-response). There are fields with on-center and off-periphery or off-center and on-periphery, as well as with an intermediate on/off zone. Due to the opponent's on/off-reactions and the excitatory-inhibitory processes connected with them, the spatio-temporal structures of the signal become sharper.

Receptive fields change, depending on the changing conditions and tasks of visual perception, their functional restructuring takes place. In the region of the central fossa, the receptive fields are smaller than in the periphery. Unlike the receptive fields of the retina and geniculate body, which are characterized by a round shape, the cortical fields have an elongated shape and a much more complex structure.

Several cells of the underlying layer of the visual system are associated with one overlying cell, i.e., there is an ascending floor-by-floor convergence of sensory neurons. At the same time, as we move from the retina to the visual cortex, on each successive floor, the number of nerve elements and connections between them increases, so that one retinal ganglion cell is associated with thousands of cortical neurons. This improves the reliability (of the system) and reduces the likelihood that an erroneous signal will be sent.

The main stages of visual information processing can be represented as follows. In cones and rods of the retina, photophysical and photochemical processes of transformation of light energy into nervous excitation take place, which is transmitted to bipolars, and from them to ganglion cells. The code for the intensity of the signal sent to the brain along the axons of ganglion cells - the fibers of the optic nerve, is the frequency of impulse discharges.

At the level of the retina, due to the spatio-temporal summation of the light stimulus, as well as the inhibitory interaction between the zones within the fields themselves, the contours of the image are emphasized. Information is transmitted to the overlying parts of the visual system mainly about those parts of it where there is a difference, gradation of brightness and contains the most recent information. In the lateral geniculate body, lateral inhibition increases and the image contrast effect is enhanced.

At the next stage of visual information processing, there is a transition to spatial (topological) coding. It has been established that in the visual system, mainly in its higher parts, there are neurons that selectively respond only to certain characteristics of the image: areas of various shapes and brightness, the boundaries of the dark and illuminated zones, straight lines oriented in one direction or another, sharp and obtuse corners, ends of segments, curved contours, different directions of movement of objects. Three types of feed receptive fields associated with the coding of form elements are described: simple, complex, and supercomplex. The specific responses of neurons to the action of a light stimulus make it possible to isolate the elementary features of an image and form the basis for a concise and economical description of a visible object.

Simple features of the image serve as ready-made blocks for building the image. The final process of its recognition is determined by the functional organization of sets of neurons, the integrative activity of the visual system as a whole. As we move to higher and higher parts of it, there is a decrease in the number of neural channels involved in the transmission of visual information, and a transition from the description of image elements to the construction of entire images, the formation of visual images and their identification. It has been suggested that the distinction between the simplest configurations is an innate property of the visual system, while the recognition of complex images is based on individual experience and requires training.

In the cortical association areas, visual information is combined with information from other sensory systems. As a result, conditions are created for a complex perception of the external environment.

Neural links of the visual pathway:

1. Within the retina of each eye is a layer of rods and cones (photoreceptors - 1 neuron),
2. Then a layer of bipolar (2 neurons) and
3. Ganglion cells with their long axons (3 neurons).

Together they form the peripheral part of the visual analyzer. The pathways are represented by the optic nerves, chiasma, and optic tracts. The latter terminate in the cells of the lateral geniculate body, which plays the role of the primary visual center. The fibers of the central neuron of the visual pathway originate from them ( radiatio optica) that reach the area area striata occipital lobe of the brain. The primary cortical center of the visual analyzer is localized here.

Visual tracts (traclus opticus) begin at the posterior surface of the chiasm and, rounding the brain stem from the outside, end in the lateral geniculate body ( corpus geniculatum laterale), the back of the thalamus ( thalamus opticus) and anterior quadrigemina ( corpus quadrigeminum anterius) of the respective party. However, only the external geniculate bodies are the unconditional subcortical visual center. The remaining two formations perform other functions.

In the visual tracts, the length of which in an adult reaches 30-40 mm, the papillomacular bundle also occupies a central position, and the crossed and non-crossed fibers still go in separate bundles. At the same time, the first of them are located ventromedially, and the second - dorsolaterally.

Visual radiation (fibers of the central neuron) starts from the ganglion cells of the fifth and sixth layers of the lateral geniculate body. First, the axons of these cells form the so-called Wernicke's field, and then, passing through the posterior thigh of the internal capsule, fan-shaped diverge in the white matter of the occipital lobe of the brain. The central neuron terminates in the sulcus of the bird's spur ( sulcus calcarinus). This area personifies the sensory visual center - the 17th cortical field according to Brodman.

Arc pupillary reflex

The arc of the pupillary reflex to light has afferent and efferent links.

Afferent part of the reflex arc the first of them starts from the cones and rods of the retina in the form of autonomous fibers that go as part of the optic nerve. In the chiasm, they cross in exactly the same way as the optic fibers and pass into the optic tracts. In front of the external geniculate bodies, the pupillomotor fibers leave them and, after a partial decussation, continue into the brachium quadrigeminum, where they end at the cells of the so-called pretectal region (area pretectalis). Further, new, interstitial neurons, after partial decussation, are sent to the corresponding nuclei (Yakubovich - Edinger - Westphal) of the oculomotor nerve. Afferent fibers from the macula of the retina of each eye are present in both oculomotor nuclei.

The afferent link begins with the ganglion cells of the retina, which transmit light (visual) and pupillary impulses through the fibers of the optic nerve, chiasm and optic tract. In the distal optic tract, packets of light and pupillary impulses are separated to reach different synaptic sites: light (visual) impulses are sent to the lateral geniculate nuclei, and pupillary impulses are directed to the pretectal nuclei. Each pretectal nucleus in the dorsal midbrain continues the transmission of pupillary impulses to the ipsilateral and contralateral Edinger-Westphal nuclei of the oculomotor complex.

In the nuclei of Edinger-Westphal begins efferent link pupillary reflex to the light and goes in a separate bundle as part of the oculomotor nerve ( n. oculomotorius). The size and reactivity of the pupils are the same as long as the signals emanating from the Edinger-Westphal nuclei are the same. That's why unequal pupil sizes- evidence of a unilateral efferent defect.

In the orbit, the sphincter fibers enter its lower branch, and then through the oculomotor root ( radix oculomotoria) - in the ciliary knot. Here the first neuron of the considered path ends and the second one begins. Upon exiting the ciliary ganglion, the sphincter fibers in the short ciliary nerves ( nn. ciliares breves), passing through the sclera, enter the perichoroidal space, where they form the nerve plexus. Its terminal branches penetrate the iris and enter the muscle in separate radial bundles, that is, they innervate it sectorally. In total, there are 70-80 such segments in the sphincter of the pupil.

Pupil dilator efferent pathway ( m. dilatator pupillae), which receives sympathetic innervation, starts from the ciliospinal center Budge. The latter is located in the anterior horns of the spinal cord (h) between Cvii and ThM. Connecting branches depart from here, which through the border trunk of the sympathetic nerve (l), and then the lower and middle sympathetic cervical ganglia (t, and t2) reach the upper ganglion (t3) (level C II -C IV). Here the first neuron of the path ends and the second begins, which is part of the plexus of the internal carotid artery (m). In the cranial cavity, the fibers innervating the pupillary dilator leave the mentioned plexus and enter the trigeminal (Gasser) node ( gangl. trigeminale), and then leave it as part of the optic nerve ( n. ophthalmicus). Already at the top of the orbit, they pass into the nasociliary nerve ( n. nasociliaris) and further along with long ciliary nerves ( nn. ciliares longi) penetrate the eyeball.

The pupillary dilator function is regulated by the supranuclear hypothalamic center, located at the level of the bottom of the third ventricle of the brain in front of the pituitary infundibulum. Through the reticular formation, it is connected with the ciliospinal center Budge.

The reaction of the pupils to convergence and accommodation has its own characteristics, and the reflex arcs in this case differ from those described above.

With convergence, the stimulus for pupillary constriction is proprioceptive impulses coming from the contracting internal rectus muscles of the eye. Accommodation is stimulated by the vagueness (defocusing) of images of external objects on the retina. The efferent part of the pupillary reflex arc is the same in both cases.

The center of setting the eye at a close distance is believed to be in the 18th cortical field according to Brodmann.

Reflexes are the most important function of the body. Scientists who studied the reflex function, for the most part, agreed that all conscious and unconscious acts of life are inherently reflexes.

What is a reflex

Reflex - the response of the central nervous system to irritation of recipes, which provides the body's response to a change in the internal or external environment. The implementation of reflexes occurs due to irritation of the nerve fibers, which are collected in reflex arcs. The manifestations of the reflex are the emergence or cessation of activity on the part of the body: contraction and relaxation of muscles, secretion of glands or its stop, constriction and expansion of blood vessels, changes in the pupil, and so on.

Reflex activity allows a person to quickly respond and properly adapt to changes around him and inside. It should not be underestimated: vertebrates are so dependent on the reflex function that even a partial violation of it leads to disability.

Types of reflexes

All reflex acts are usually divided into unconditional and conditional. Unconditional are inherited, they are characteristic of every biological species. Reflex arcs for unconditioned reflexes are formed even before the birth of the organism and remain in this form until the end of its life (if there is no influence of negative factors and diseases).

Conditioned reflexes arise in the process of development and accumulation of certain skills. New temporary connections are developed depending on the conditions. They are formed from the unconditional, with the participation of higher brain departments.

All reflexes are classified according to different criteria. According to their biological significance, they are divided into food, sexual, defensive, orienting, locomotor (movement), postural-tonic (position). Thanks to these reflexes, a living organism is able to provide the main conditions for life.

In each reflex act, all parts of the central nervous system are involved to one degree or another, so any classification will be conditional.

Depending on the location of the stimulus receptors, reflexes are:

• exteroceptive (external surface of the body);
• viscero- or interoreceptive (internal organs and vessels);
• proprioceptive (skeletal muscles, joints, tendons).

According to the location of neurons, reflexes are:

• spinal (spinal cord);
• bulbar (medulla oblongata);
• mesencephalic (midbrain);
• diencephalic (midbrain);
• cortical (cerebral cortex).

In the reflex acts carried out by the neurons of the higher parts of the CNS, the fibers of the lower parts (intermediate, middle, medulla oblongata and spinal cord) also participate. At the same time, the reflexes that are produced by the lower parts of the central nervous system necessarily reach the higher ones. For this reason, the presented classification should be considered conditional.

Depending on the response and the organs involved, reflexes are:

• motor, motor (muscles);
• secretory (glands);
• vasomotor (blood vessels).

However, this classification is applicable only to simple reflexes that combine some functions within the body. When complex reflexes occur that irritate the neurons of the higher parts of the central nervous system, various organs are involved in the process. This changes the behavior of the organism and its relationship with the external environment.

The simplest spinal reflexes include flexion, which allows you to eliminate the stimulus. This also includes the scratching or rubbing reflex, knee and plantar reflexes. The simplest bulbar reflexes: sucking and corneal (closing of the eyelids when the cornea is irritated). The mesencephalic simple ones include the pupillary reflex (pupil constriction in bright light).

Features of the structure of reflex arcs

A reflex arc is the path that nerve impulses travel through, carrying out unconditioned and conditioned reflexes. Accordingly, the autonomic reflex arc is the path from irritating nerve fibers to transmitting information to the brain, where it is converted into a guide to the action of a specific organ. The unique structure of the reflex arc includes a chain of receptor, intercalary and effector neurons. Thanks to this composition, all reflex processes in the body are carried out.

Reflex arcs as part of the peripheral nervous system (part of the NS outside the brain and spinal cord):

• arcs of the somatic nervous system, which provide skeletal muscles with nerve cells;
• arcs of the autonomic system that regulate the functionality of organs, glands and blood vessels.

The structure of the autonomic reflex arc:

1. Receptors. They serve to receive stimulus factors and respond with excitation. Some receptors are presented in the form of processes, others are microscopic, but they always include nerve endings and epithelial cells. Receptors are part not only of the skin, but also of all other organs (eyes, ears, heart, etc.).
2. Sensitive nerve fiber. This part of the arc ensures the transmission of excitation to the nerve center. Since the bodies of nerve fibers are located directly near the spinal cord and brain, they are not included in the CNS.
3. Nerve center. Here, switching between sensory and motor neurons is provided (due to instantaneous excitation).
4. motor nerve fibers. This part of the arc transmits a signal from the central nervous system to the organs. The processes of nerve fibers are located near the internal and external organs.
5. Effector. In this part of the arc, the signals are processed, and a response to receptor irritation is formed. The effectors are mostly muscles that contract when the center receives stimulation.

The signals of receptor and effector neurons are identical, since they interact following the same arc. The simplest reflex arc in the human body is formed by two neurons (sensory, motor). Others include three or more neurons (sensory, intercalary, motor).

Simple reflex arcs help a person involuntarily adapt to changes in the environment. Thanks to them, we withdraw our hand if we feel pain, and the pupils react to changes in lighting. Reflexes help to regulate internal processes, contribute to maintaining the constancy of the internal environment. Without reflexes, homeostasis would be impossible.

How does the reflex work?

The nervous process can provoke the activity of the organ or increase it. When the nerve tissue accepts irritation, it goes into a special state. Excitation depends on differentiated indicators of the concentration of anions and cations (negatively and positively charged particles). They are located on both sides of the membrane of the process of the nerve cell. When excited, the potential of electricity on the cell membrane changes.

When the reflex arc has two motor neurons at once in the spinal ganglion (nerve ganglion), then the cell's dendrite will be longer (a branched process that receives information through synapses). It is directed to the periphery, but remains part of the nervous tissue and processes.

The excitation speed of each fiber is 0.5-100 m/s. The activity of individual fibers is carried out in isolation, that is, the speed does not change from one to another.

Inhibition of excitation stops the functioning of the site of irritation, slowing down and limiting movements and responses. Moreover, excitation and inhibition occur in parallel: while some centers are dying out, others are being excited. Thus, individual reflexes are delayed.

Inhibition and excitation are interconnected. Thanks to this mechanism, the coordinated work of systems and organs is ensured. For example, the movements of the eyeball are carried out due to the alternation of muscle work, because when looking in different directions, different muscle groups contract. When the center responsible for the tension of the muscles of one side is excited, the center of the other slows down and relaxes.

In most cases, sensory neurons relay information directly to the brain using a reflex arc and a few interneurons. The brain not only processes sensory information, but also stores it for future use. In parallel with this, the brain sends impulses along the descending pathway, initiating the response of effectors (the target organ that performs the tasks of the central nervous system).

visual path

The anatomical structure of the visual pathway is represented by a number of neural links. In the retina, these are rods and cones, then bipolar and ganglion cells, and then axons (neurites, which serve as a path for an impulse emanating from the cell body to the organs).

This circuit represents the peripheral portion of the optic pathway, which includes the optic nerve, chiasm, and optic tract. The latter ends in the primary visual center, from where the central neuron of the visual pathway begins, which reaches the occipital lobe of the brain. The cortical center of the visual analyzer is also located here.

Components of the visual pathway:

1. The optic nerve starts at the retina and ends at the chiasm. Its length is 35-55 mm, and its thickness is 4-4.5 mm. The nerve has three sheaths, it is clearly divided into halves. The nerve fibers of the optic nerve are divided into three bundles: axons of nerve cells (from the center of the retina), two fibers of ganglion cells (from the nasal half of the retina, as well as from the temporal half of the retina).
2. Chiasma begins above the region of the Turkish saddle. It is covered with a soft shell, 4-10 mm long, 9-11 mm wide, 5 mm thick. This is where fibers from both eyes join to form the optic tracts.
3. The optic tracts originate from the posterior surface of the chiasm, go around the legs of the brain and enter the lateral geniculate body (the unconditioned visual center), the optic tubercle and the quadrigeminae. The length of the visual tracts is 30-40 mm. From the geniculate body, the fibers of the central neuron begin, and end in the furrow of the bird's spur - in the sensory visual analyzer.

Pupillary reflex

Consider the reflex arc on the example of the pupillary reflex. The path of the pupillary reflex passes through a complex reflex arc. It starts from the fibers of the rods and cones, which are part of the optic nerve. The fibers cross in the chiasm, passing into the optic tracts, stop in front of the geniculate bodies, partially twist and reach the pretectal region. From here, new neurons go to the oculomotor nerve. This is the third pair of cranial nerves, which is responsible for the movement of the eyeball, the light reaction of the pupils, and the elevation of the eyelid.

The return journey starts from the oculomotor nerve to the orbit and ciliary ganglion. The second neuron of the link emerges from the ciliary node, through the sclera into the perichoroidal space. Here a nerve plexus is formed, the branches of which penetrate the iris. The sphincter of the pupil has 70-80 radial neuron bundles that enter it sectorally.

The signal for the muscle that dilates the pupil comes from the ciliospinal center Budge, which is located in the spinal cord between the seventh cervical and second thoracic vertebrae. The first neuron goes through the sympathetic nerve and the sympathetic cervical ganglia, the second starts from the superior ganglion, which enters the plexus of the internal carotid artery. The fiber that provides nerves to the pupillary dilator leaves the plexus in the cranial cavity and enters the optic nerve through the trigeminal ganglion. Through it, the fibers penetrate into the eyeball.

The closed nature of the circular work of the nerve centers makes it perfect. Thanks to the reflex function, the correction and regulation of human activity can occur arbitrarily and involuntarily, protecting the body from changes and danger.