The pupillary reflex consists in a change in the diameter of the pupils when the retina is exposed to light, during the convergence of the eyeballs and under some other conditions .. The diameter of the pupils can vary from 7.3 mm to 2 mm, and the plane of the opening - from 52.2 mm2 to 3.94 mm2.
The reflex arc consists of four neurons:
1) receptor cells mainly in the center of the retina, the axons of which, as part of the optic nerve and the optic tract, go to the anterior two-humped body
2) the axons of the neurons of this body are directed to the nuclei of Yakubovich and Westphal-Edinger;
3) the axons of the parasympathetic oculomotor nerves go from here to the ciliary node;
4) short fibers of neurons of the ciliary node go to the muscles, which narrows the pupil.
Narrowing begins 0.4-0.5 s after exposure to light. This reaction has a defensive meaning, it limits too much illumination of the retina. Pupil dilation occurs with the participation of the center located in the lateral horns of the C8-Thi segments of the spinal cord.
The axons of nerve cells go from here to the upper lumbar node, and postganglionic neurons as part of the plexuses of the internal carotid artery-in the eyes.
Some researchers believe that there is also a cortical center of the pupillary reflex in the anterior sections of the frontal lobe.
Distinguish between a direct reaction to light (narrowing on the side of illumination) and friendly (narrowing on the opposite side). Pupils narrow when looking at close (10-15 cm) objects (reaction to convergence), dilate when looking into the distance. The pupils also dilate under the action of painful stimuli (the center in this case is the subthalamic nucleus), with irritation of the vestibular apparatus, with translocation, stress, rage, increased attention. The pupils also dilate with asphyxia, this is a formidable sign of danger. Atropine sulfate eliminates the influence of the parasympathetic nerves, and the pupils dilate.
Each reflex has two paths: the first is sensitive, through which information about some kind of impact is transmitted to the nerve centers, and the second is motor, which transmits impulses from the nerve centers to the tissues, due to which a certain reaction arises in response to the impact.
Under illumination, the constriction of the pupil occurs in the studied eye, as well as in the paired eye, but to a lesser extent. The constriction of the pupil ensures that the blinding light entering the eye is limited, which means better vision.
The reaction of the pupils to light can be direct, if the studied eye is directly illuminated, or friendly, which is observed in the paired eye without its illumination. The friendly reaction of the pupils to light is explained by the partial crossing of the nerve fibers of the pupillary reflex in the region of the chiasm.
In addition to the reaction to light, it is also possible to change the size of the pupils during convergence, that is, the tension of the internal rectus muscles of the eye, or accommodation, that is, the tension of the ciliary muscle, which is observed when the fixation point changes from a far-away object to a close one. Both of these pupillary reflexes arise from the tension of the so-called proprioceptors of the corresponding muscles, and are ultimately provided by fibers that enter the eyeball from the oculomotor nerve.
Strong emotional excitement, fear, pain also cause a change in the size of the pupils - their dilation. Narrowing of the pupils is observed with irritation of the trigeminal nerve, decreased excitability. Constriction and dilation of the pupils is also found through the use of drugs that affect directly the receptors of the muscles of the pupil.
11. Question number 11
Receptor part of the visual system. Structure of the retina. photoreception mechanisms
Visual analyzer. Peripheral department visual analyzer- photoreceptors located on the retina of the eye. Nerve impulses through the optic nerve (conduction section) they enter the occipital region - the cerebral section of the analyzer. In the neurons of the occipital cortex large brain manifold and different visual sensations arise. The eye consists of an eyeball and an auxiliary apparatus. The wall of the eyeball is formed by three membranes: the cornea, the sclera, or white, and the vascular. The inner (vascular) membrane consists of the retina, on which the photoreceptors (rods and cones) are located, and its blood vessels. The eye consists of the receptor apparatus located in the retina and the optical system. Optical system the eye is represented by the anterior and posterior surface of the cornea, the lens and the vitreous body. For a clear vision of an object, it is necessary that rays from all its points fall on the retina. The adaptation of the eye to a clear vision of objects at different distances is called accommodation. Accommodation is carried out by changing the curvature of the lens. Refraction is the refraction of light in the optical media of the eye. There are two main anomalies in the refraction of rays in the eye: farsightedness and myopia. Field of view is the angular space visible to the eye with a fixed gaze and a motionless head. Photoreceptors are located on the retina: rods (with rhodopsin pigment) and cones (with iodopsin pigment). Cones provide daytime vision and color perception, rods provide twilight, night vision. A person has the ability to distinguish a large number of colors. The mechanism of color perception according to the generally accepted, but already outdated three-component theory is that there are three sensors in the visual system that are sensitive to three primary colors: red, yellow and blue. Therefore, normal color perception is called trichromasia. With a certain mixture of the three primary colors, there is a feeling white... In case of malfunction of one or two primary color sensors, correct color mixing is not observed and color perception disorders occur. Congenital and acquired forms of color anomaly are distinguished. With congenital color anomaly, a decrease in sensitivity to blue, and when purchased - to green. Dalton's color anomaly (color blindness) is a decrease in sensitivity to shades of red and green. About 10% of men and 0.5% of women suffer from this disease. The process of color perception is not limited to the reaction of the retina, but significantly depends on the processing of received signals by the brain.
Retinal structure
The retina is the inner sensitive membrane of the eye (tunicainternasensoriabulbi, or retina), which lines the cavity of the eyeball from the inside and performs the functions of perception of light and color signals, their primary processing and transformation into nervous excitement.
In the retina, two functionally different parts are distinguished - the visual (optical) and the blind (ciliary). The visual retina is the large portion of the retina that is loosely attached to choroid and attaches to the underlying tissues only in the area of the optic nerve head and at jagged line... The free-lying part of the retina, which is in direct contact with the choroid, is retained by the pressure exerted by the vitreous body, as well as by the thin bonds of the pigment epithelium. The ciliary part of the retina covers the posterior surface of the ciliary body and iris, reaching the pupillary edge.
The outer part of the retina is called the pigment part, the inner part is the photosensitive (nerve) part. The retina is made up of 10 layers that contain different types of cells. The retina in the section is presented in the form of three radially located neurons (nerve cells): external - photoreceptor, middle - associative, and internal - ganglionic. Between these neurons are the so-called. plexiform (from Latin plexus - plexus) layers of the retina, represented by the processes of nerve cells (photoreceptors, bipolar and ganglion neurons), axons and dendrites. Axons conduct a nerve impulse from the body of a given nerve cell to other neurons or innervated organs and tissues, dendrites conduct nerve impulses in the opposite direction - to the body of the nerve cell. In addition, interneurons, represented by amacrine and horizontal cells, are located in the retina.
Pupil is called the hole in the center of the iris through which all the rays of light that enter the eye pass. Pupil improves the clarity of the image of objects on the retina, passing only the central rays and eliminating the so-called spherical aberration.
Spherical aberration consists in the fact that the rays hitting the peripheral parts of the lens are refracted stronger than the central rays ( rice. 209). Therefore, if the peripheral rays are not eliminated, light scattering circles should be obtained on the retina.
The muscles in the iris are able to change the size of the pupil and thereby regulate the flow of light into the eye. If you cover the eye from the light, and then open it, then the pupil, which has dilated with darkening, quickly narrows. This narrowing occurs reflexively.
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In the iris, there are two types of muscle fibers surrounding the pupil: some are circular (m. Sphincter iridis), innervated by parasympathetic fibers of the oculomotor nerve, others are radial (m.d ilatator iridis), innervated by sympathetic nerves ( rice. 210). The contraction of the former causes the constriction of the pupil, the contraction of the latter causes the expansion of the pupil. Rice. 210. Scheme of innervation of the iris and ciliary muscle. 1 - ciliary ganglion; 2 - short ciliary nerves; 3 - upper cervical sympathetic node; 4 - sympathetic nerves; 5 - ciliary muscle; 6 - fibers of the lens capsule; 7 - iridescent sheath; 8 - cornea; 9 - lens; 10 - circular muscles of the iris; 11 - radial muscles of the iris. Accordingly, adrenaline causes dilation of the pupil, and acetylcholine and eserine cause constriction. With emotions, accompanied by the excitement of the simatic system (fear, rage, pain), the pupils dilate. |
The pupils also dilate with asphyxia. Therefore, dilatation of the pupils with deep anesthesia indicates the onset of asphyxia and is a formidable sign indicating the need to reduce anesthesia.
The pupils of both eyes in healthy clothes are equally dilated or narrowed. When one eye is illuminated, the pupil of the other also narrows; such a reaction is called friendly.
Pupil constriction also occurs when examining close objects, when accommodation and convergence of the visual axes of both eyes occurs (convergence).
In some cases, the size of the pupils of both eyes is different (anisocoria). This can occur due to damage to the sympathetic nerve on one side, which entails constriction of the pupil (miosis) and, at the same time, narrowing of the palpebral fissure (Horner's symptom). Pupil dilation (mydriasis) of one eye can occur due to paralysis n. ociiloinatorius or due to irritation n. sympathicus.
Translated from German by N.A. Ignatenko
There is one advantage when examining the eyes: most structures are visible, therefore, already during clinical research can be diagnosed. In any case, very important during clinical examination the patient is to take anamnesis, since often eye changes are a sign of a systemic disease.
The sequence of the ophthalmological examination is focused on the anatomical structure of the eye and depends on it. Great importance has a strictly systematic approach. First, an examination is necessary, and only then further measures, such as palpation, eversion of the third eyelid, staining of the cornea, dilation of the pupil for ophthalmoscopy, etc.
A detailed study of both eyes is mandatory, even if changes are observed in one.
Anamnesis
In ophthalmology, as in all areas of veterinary medicine, a detailed history taking is very important. It is necessary to start with how long the animal has been with these owners, how long and under what circumstances changes in vision have been noticed. Owners' perception of eye problems in a pet can be an important point in determining the sequencing of the disease, for example, in the development of blindness.
With severe bilateral cataract, examination of the fundus becomes impossible. If the pet owner says that his pet could see "until the pupils turned white," then cataracts may be the only reason for the loss of vision. If the owner is sure that "the pupils were normal", and the pet is already blind, then, perhaps, in addition to cataracts, we can also talk about retinal degeneration. In general, questions to the owner are aimed at understanding the sequence of changes in the eyes of his pet. The following questions can be asked about blindness:
Can the patient see better under certain lighting conditions?
Does vision loss correlate with moving, rearranging furniture, or walking in unfamiliar areas (such as visiting a clinic)?
How did the owner understand that his pet no longer sees? Does the pet try to stay close to the owner's leg all the time?
Have there been changes general condition the patient's health (for example, symptoms of diabetes, etc.)?
Examination of the anterior chamber of the eye
During this exploration, you should try to avoid stress as much as possible. If the patient's eye is very painful, and there is a risk of further damage during the study, then it is necessary to put the animal in a short-term anesthesia. First, the patient is examined in a lighted room at some distance (observation). In this case, you need to pay attention to the following points:
Are we talking about unilateral or bilateral changes?
What is the ratio of the eye to the orbit, to the eyelids, to the second eye?
Estimate the size of the eyeball: large, small, normal?
What position does eyeball: are exophthalmos or endophthalmos observed?
Are the axes of both eyes the same?
Is there a loss of the third century?
Is there a discharge from the eyes? Are both pupils of the same size, or is there anisocoria (pupils different sizes)? Is there dilation of the pupils (mydriasis) (Fig. 1, 2)?
At the final stage, the auxiliary parts of the eye are examined using a focal (direct and lateral) light source. This can be done using an otoscope or a slit lamp. The slit lamp principle is based on focal illumination. It enables precise examination of the anterior and middle parts of the eye at fifteen times magnification. In this case, the assessment is carried out binocularly. Lateral illumination through the light slit makes it possible to examine the optical layers.
It is also necessary to pay attention to inflammation, neoplasms, anatomical abnormalities (congenital and acquired), the integrity of the cornea, the presence or absence of moisture, foreign bodies, signs of trauma, pain (probable self-injury, blinking). Any changes must be properly recorded, for example by means of a sketch (Fig. 3, 4).
For the study of the structures that are located behind the lens, medically achievable mydriasis is mandatory (see Section of ophthalmoscopy).
Neurological examination of the eye
Reflex test
Pupillary reflex
In order to assess the direct pupillary reflex, a light source is directed into the eye being examined.
It can be helpful to direct light onto the retina in the temporal region, as it is very sensitive. It is best to conduct the study in a room with normal lighting in order to immediately assess the symmetry of the pupils without complications that may arise in the dark due to changes in parasympathetic tone.
It is often difficult to assess the response to light from the non-stimulated eye (indirect pupillary reflex), since room light can reflect on the cornea and complicate the assessment of the pupil. This can be avoided with the following techniques:
Use of a direct ophthalmoscope, during which the direct response in each eye can be assessed under room lighting. You can darken the room or turn off the light and move away from the patient so that the reflection of the fundus of the eyeball can be seen in both pupils using an ophthalmoscope with "0" diopter. The assistant shines first into one, then into the second eye, during which one can observe the reaction of the eye, which is not receiving a direct light source.
The so-called torch test can be performed without an assistant and without darkening the room. First, it is necessary to establish precisely that each eye is showing a direct response. Then the light source is directed to the right eye. If the pupil responds (or if the pupil does not respond after one to two seconds), the light source is quickly directed to the left eye. If the reaction was in the left eye, then the left pupil should remain constricted (if this is not done quickly enough, then the left pupil will dilate slightly again and show a normal direct reaction to light). You must act in the same way for the other side.
The evaluation of the reflex response is described below.
Corneal reflex
It is controlled trigeminal nerve(V sensitive branch) and facial nerve (VII motor branch). Therefore, each touch or painful stimulation of the cornea leads to reflex occlusion of the eye through the contraction of the orbicular muscle of the eye ( M. orbicularis oculi). A distinction is made between a direct corneal reflex (irritated eye reaction) and a contralateral eye reaction.
Threat reflex
It is also known as the blink reflex. Controlled by the optic nerve (II afferent branch) and the facial (VII motor branch). Consequently, the subcortical reflex, which is triggered by sudden stimulation of the visual system (for example, foreign body, which moves in the direction of the eye), leads to reflex closing of the eye and twitching of the head. The reflex may contain cortical components, since it requires intact (intact) photosensitive and motor areas of the cerebral cortex on the ipsilateral side. Opacity of the eyes and color deviations can lead to misdiagnosis. If the patient, for example, has a complete cataract, then the study of the threat reflex will have no practical value. The threat reflex may not directly correlate with the animal's ability to see. There are situations in which the patient sees, but the threat reflex is negative, or vice versa, the patient does not see, and the threat reflex is positive.
Reaction to light
This is an involuntary reaction of the eye to a light source. Especially if strong light shines directly into the eye, reactions include blinking, bulging of the third eyelid (if there is a third eyelid) and sometimes moving the head in the direction opposite to the light source. Despite the neuroanatomical support for this response, it is not entirely clear whether a positive response in common feature undisturbed conduction of the visual impulse to the brain and whether it can be taken as a sign of preserved vision. This reflex is a more reliable sign of vision preservation than the threat reflex, and is especially useful in those patients who have cloudy eyes. various reasons... Even complete cataracts or corneal lesions have no effect on this reflex.
Visual impairment
Visual aptitude testing
Since we cannot ask our patients about their visual abilities, it is worth observing their behavior for a few minutes. Rather, the integrity of neuroanatomical structures is checked by means of the pupillary reflex, the threat reflex and the reaction to light. All of these tests can be positive, and the patient is nevertheless unable to bypass obstacles or work their way.
Obstacle course
You should have a simple obstacle course at your disposal, but some animals, especially cats, will not make contact.
The obstacle course must be passed in daylight (to control photopic vision) and in the dark (to control scotopic vision) in order to test the visual ability of cones and rods. Red light is useful for stimulating scotopic (rod) vision.
It is very difficult to differentiate vision loss in cats. You can put the cat on the table and observe how confident it is when jumping and landing on its paws, how purposeful its jump was.
If there is a suspicion of one-sided blindness, then the animal must pass the obstacle course with a glued eye. In either case, both eyes should be assessed, as some patients refuse to pass an obstacle course with their eye glued on, whether they are blind or not.
Movement test
The undulating movement of the hand in front of the eye can cause the patient to blink only due to the vibrations of the air, even if he does not have the ability to see. In order to reduce the draft, you can hold a transparent plastic sheet between your hand and eye. Alternatively, a piece of cotton wool can be used, which is dropped in front of the patient and watched as he follows the fall. The cotton wool test can also check the volume of the visual field, which is greatly reduced in glaucoma. To check, the cotton ball should always fly from above, from the temporal edge, down to the nasal edge.
Signs of blindness
Sudden complete blindness, as a rule, is accompanied by slower, more careful movements, the animal begins to bump into objects. With gradually emerging or congenital blindness the patient very often seems to see, since he compensates for the missing vision with other senses (hearing and smell). Animals know their surroundings and move around without problems.
CAVE: An absent pupillary reflex does not indicate blindness, just as its presence does not always mean that the animal is seeing.
Differential diagnosis of vision loss
Loss of vision (blindness) can be unilateral and bilateral, and it can also be due to neurological and ophthalmological problems. Sometimes careful neurologic and ophthalmologic examinations are needed to find the cause. In some cases, specialized studies (electroretinography) are necessary.
1. Unilateral blindness
Loss of vision in one eye or one visual field can result from unilateral damage to the retina, optic nerve, optic tract, radiance, or cerebral cortex.
If the cause of the loss of vision lies in the optic nerve, then there is one-sided blindness and loss of pupil response to light in both eyes. If the light source is directed to the blind eye, then the pupils may be symmetrical, or the pupil in the blind eye may be slightly larger than the pupil of the healthy eye.
If the cause of blindness is in the optic tract, visual radiance or cerebral cortex, then in this case there is a loss of the visual field with a normal reaction of the pupil. The animal will also show other symptoms of cerebral disease associated with a lesion in this area. Loss of vision occurs on the side opposite from damage to the central nervous system. The size of both pupils is the same.
2. Bilateral blindness
If the lesions are located in the region of the retina, optic nerve or optic tract, then blindness is accompanied by maximally dilated pupils that do not respond to light. No other neurological symptoms not visible.
If the lesion is located in both radiant fields or the visual cortex, then a complete loss of vision occurs, but the pupils normal size... You can also see the normal response to light upon visual stimulation.
Nystagmus
Involuntary rhythmic movements of both eyes are called nystagmus. Distinguish between physiological and artificially induced nystagmus (provocative nystagmus), as well as pathological spontaneous nystagmus. The latter will be discussed in more detail.
Classification
Pathological nystagmus has two characteristics: in its direction and in what causes it. Both can provide information about the location of the violation.
1. In the direction of oscillatory movements are distinguished:
a) horizontal: fluctuations from one side to the other in most cases indicate peripheral disease, the rapid oscillation goes from the side of the lesion to the opposite;
b) rotational: the eye rotates clockwise or counterclockwise, which does not indicate the specific localization of the lesion;
c) vertical: the eye rotates ventrally in relation to the level of the head. This form of nystagmus is usually observed in diseases of the central nervous system;
d) direction changes: if the direction of nystagmus changes at different positions of the head, then this indicates a disease of the central nervous system.
2. By the type of occurrence in relation to movement:
a) persistent nystagmus: observed when the head of the animal is in the normal position. Typically, this type of nystagmus occurs in peripheral diseases;
b) position-related nystagmus: observed when the head is not parallel to the floor. It lasts more than one minute after the head has stopped moving. Positional nystagmus is observed in diseases of the central nervous system.
Causes
Pathological nystagmus is considered a symptom of peripheral or central diseases of the vestibular apparatus. It can also be associated with following symptoms: ataxia, curvature, circular motion and dizziness. Central vestibular disorders can be caused by damage to:
In the brain stem. Will be expressed in weakness and proprioceptive deficiency;
In the cerebellum. They will be characterized by tremor, hypermetria, absent threat reflex in normal vision... The cause of nystagmus is the asymmetry of the muscle tone of the eyeball. When the right vestibular apparatus falls out, only the left vestibular apparatus is stimulated, this leads to a slow tonic deviation of the eyeball to the right with a rapid return to the left. In this case, the fast phase acts towards the defeat. The reason for the rapid correction phase is probably in the cerebral cortex. A characteristic feature of vestibular nystagmus is that it does not correlate in any way with vision tests and can be observed in blind animals.
1. Nystagmus in peripheral vestibular disease:
a) It is very pronounced at the onset of the disease and decreases throughout the disease (rarely observed for more than a few weeks).
b) In most cases, involuntary and always independent of the position of the head.
c) It is basically unidirectional and maintains this direction, regardless of the position of the animal's head.
d) Its direction is in most cases horizontal.
e) If its appearance is due to a lesion in the area inner ear, then symptoms of damage to the VII pair of facial nerves and Horner's syndrome will also be detected. If the lesion is located in the area peripheral nerves, then in this case there will be no other symptoms.
2. Nystagmus with central vestibular lesion:
a) Prone to persistence. As long as the animal has the disease, nystagmus will be observed.
b) It is often progressive and gets worse over time.
c) The direction of the nystagmus may change when the head is tilted.
d) It often has vertical components as well.
Continued in the next issue.
The main signs and causes of violations. In amaurosis, when the perception of light is absent due to damage to the retina or optic nerve, both pupils are the same in size, when the blind eye is illuminated, neither of the pupils reacts, when a healthy (paired) pupil is illuminated, both pupils react, and the reaction to a near stimulus is also preserved.
With the Marcus-Hun symptom, when the retina or optic nerve is only partially damaged (for example, with neuritis), and visual acuity can be intact, the response of both pupils when the diseased eye is illuminated is slowed down. In order to more easily grasp the difference, the eyes are alternately illuminated with a mirror ophthalmoscope, trying to catch the presence of a paradoxical reaction: when light is quickly transferred from a healthy eye to a sick eye, its pupil not only does not narrow, but expands. It is believed that this is the result of a stronger than direct, friendly reaction (the pupil of a healthy eye begins to expand when light is transferred to the diseased eye).
Argyll Robertson's symptom is pathognomonic for neurosyphilis. It is characterized by a dissociation of reactions to light and a near stimulus: there is no reaction to light, but to "near" it is alive. Usually both pupils react, although asymmetry of reactions is not excluded, the pupils are narrow and dilate poorly under the influence of mydriatics. Dissociation is associated with a violation of the non-ironic connection between the pretectal nucleus and the Yakubovich - Edinger - Westphal nucleus (with loss of vision, the dissociation of reactions cannot be detected).
In the case of a tonic reaction of the pupil (Edie - Holmes), disorders are associated with the pathology of the postganglionic part of the III pair cranial nerves... This is the most common reason anisocoria in women aged 30-40 years who have undergone viral infection(tendon reflexes tend to weaken as well).
Wider, i.e. damaged, the pupil reacts weaker to light, and to "near" its reaction is very slow. Just as slowed down back reaction relaxation of the sphincter (it, as well as the ciliary muscle, is, as it were, in a state increased tone). To confirm the diagnosis, a 0.1% solution of pilocarpine is installed in the conjunctival sac of both eyes, while the pupil of the healthy eye almost does not react, and the patient's pupil is sharply narrowed.
This symptom is not a sign of a severe course of the process, although there is no effective treatment, but accommodation disturbances may decrease over time.
With a midbrain (tectal) nature of violations of pupillary reactions, the cause of their occurrence is considered to be compression of the third ventricle (for example, pinealoma). Along with the dilation of the pupil and the weakening of its reaction to light, long-term preservation of the reaction to "near" is characteristic, since the fibers providing it lie more ventrally than those on which the reaction to light depends. This symptom must be differentiated from Argyll Robertson's symptom.
Neurosyphilis is not the only reason for the dissociation of the pupil's reactions to light and a nearby stimulus (light-to-near dissociation). The latter are found in juvenile diabetes, myotonic dystrophy, inadequate regeneration of fibers of the third pair of cranial nerves, Parino syndrome, in which, in addition to pupillary disorders, there are limited upward gaze and convergent retraction nystagmus.
When the third pair of cranial nerves is damaged, for example, due to compression by a cerebral aneurysm, the efferent path of the pupillary reflex is disrupted and, naturally, reactions to light and "near" disappear on the damaged side. In the process of restoring the function of the damaged nerve, regeneration proceeds in an aberrant way, and then the adductor fibers are woven into the branch of the pupillary fibers, i.e. internal, rectus muscle of the eye. In this case, there may be a false dissociation "light - near" (the pupil of the pseudo-Argyle Robertson). Due to damage to the branch of the third pair of cranial nerves, which innervates the sphincter of the pupil, the reaction to light is impossible (or very weakened), but the reaction to "near" is observed. It is associated with the tension of the internal rectus muscle, in particular, during convergence (synkinetic movements of the pupil of this muscle were the result of pathological regeneration). Additional signs of an intracranial aneurysm are a pseudo-Graefe symptom, expressed in retraction of the upper eyelid in response to eye adduction, and segmental twitching of the sphincter of the pupil during eye movements.
Horner's syndrome (ocular palsy) stands apart, since with it, in addition to pupillary disorders, manifested only in the form of miosis, which is more noticeable with reduced illumination (all pupillary reactions are preserved), moderate ptosis of the upper eyelid is observed (due to paresis of Mueller's muscle), and also elevation of the lower eyelid (due to paresis of smooth muscles, normally leaning the "cartilaginous" plate of the lower eyelid to the eye). For these reasons, the palpebral fissure narrows, and therefore enophthalmos is mistakenly diagnosed, in which an increase in the accommodative ability of the eye to close distances is detected.
It is necessary to differentiate pre- and postganglionic (relative to the cervical ganglion) lesions. The first (bronchial carcinoma, aneurysm thoracic aorta etc.) are less favorable, the latter, in which sweating is also impaired, more often have vascular genesis, are the cause of headaches, but the course is more favorable.
Horner's syndrome is confirmed when a 4% cocaine solution is installed in the conjunctival sac, when the pupil of the diseased eye does not respond to drops, and the healthy eye expands.
Normal pupillary responses are a good predictor of improvement visual functions in various ambient light conditions, and persistent mydriasis at different distances is the cause of increased glare in bright light. Thanks to mydriasis, the depth of the focal zone and, consequently, the field, within which it is possible to simultaneously clearly see objects remote at different distances, decreases. Persistent miosis complicates orientation in conditions of reduced illumination, and with extreme degrees of pupil constriction due to diffraction, it becomes the cause of a decrease in visual acuity.
General guidelines for the study of the pupils. The study is carried out in a poorly illuminated room when the patient looks into the distance (for example, at the Sivtsev table), while his face is illuminated so that both eyes are evenly illuminated by oblique rays. Pupil diameter is measured directly with a millimeter ruler or with a pupillometer attached from the side of the temple on the investigated side, on which black circles with a diameter of 1.5 to 8 mm are presented next to the ruler with an interval of 0.5 mm. Since every fifth subject normally has mild anisocoria, lighting should be changed in search of pathology. So, in patients with Horner's syndrome, the difference is much more pronounced with reduced lighting.
It should be borne in mind that a one-sided decrease in vision in itself does not affect the size of the pupil, but in the pathology of the optic-nerve pathways, the pupillary reaction can be impaired, for example, according to the type of Hun's symptom.
To check the light reflex of the pupil, you can use an ophthalmoscope mirror or slit lamp illuminator. In case of suspicion of unilateral weakness of the pupillary reaction under direct illumination, the friendly reaction of the pupil of the other eye is checked. With the same severity of direct and friendly reactions, the afferent arc of the reflex is recognized as normal. In order to identify hemianopsic disorders in the reaction of the pupil, it is most convenient to use a point source of light from a slit lamp, transferring it alternately to the right and then to the left position and at the same time observing the severity of the pupil reaction through the binocular.
To assess the "near" reflex of the pupil, the patient is asked to first look into the distance, and then move his gaze to the tip of his own finger, attached to the nose. It is advisable to keep the chin slightly raised, since many people converge more easily when looking down. Sometimes you have to hold on upper eyelid to make it easier to follow the reaction of the pupil. The degree of narrowing can be assessed using a three- or four-point system.
Installations for registering the movements of the pupil were proposed in the last century (L. G. Bellyarminov and others); In our country, the Samoilov-Shakhnovich device is known, but in wide clinical practice, recording devices are dispensed with.
VISUAL WAY
The anatomical structure of the visual pathway is quite complex and includes a number of neural links. Within the retina of each eye, this 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. Pathways are represented by optic nerves, chiasm, and optic tracts.
The latter end in the cells of the lateral geniculate body, which plays the role of the primary visual center. From them originate already 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 located here.
The optic nerve is formed by the axons of the retinal ganglion cells and ends in the chiasm. A significant part of the nerve is the orbital segment, which has an 8-shaped bend in the horizontal plane, due to which it does not experience tension when the eyeball moves.
Over a considerable length (from the exit from the eyeball to the entrance to the optic canal), the nerve, like the brain, has three sheaths: hard, arachnoid, soft. Together with them, its thickness is 4–4.5 mm, without them - 3–3.5 mm. Near the eyeball hard shell grows together with the sclera and the telonic capsule, and in 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 orbital nerve (subdural and subarachnoid) are connected to similar spaces of the brain, but isolated from each other. They are filled with a complex liquid (intraocular, tissue, cerebrospinal).
Since the intraocular pressure is normally twice as high as the intracranial pressure (10–12 mm Hg), the direction of its current coincides with the pressure gradient. The exceptions are cases when the intracranial pressure(for example, with the development of a brain tumor, hemorrhage 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 from above and below “flow around” the papillomacular bundle.
In the orbital segment of the optic nerve near the eyeball, the relationships between nerve fibers remain the same as in its disc. Further, the papillomacular bundle moves to the axial position, and the fibers from the temporal squares of the retina - 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 sensory nerve endings.
In the skull region, the optic nerves are connected above the sella turcica, forming a chiasm, which is covered by the pia mater and has the following dimensions: length 4-10 mm, width 9-11 mm, thickness 5 mm. Chiasmus from below is bordered by the diaphragm of the sella turcica (a preserved area of solid meninges), from above (in the posterior part) - with the bottom of the third ventricle of the brain, on the sides - with the internal carotid arteries, behind - with the pituitary funnel.
In the area of the chiasm, the fibers of the optic nerves partially intersect due to portions associated with the nasal halves of the retinas.
Passing to the opposite side, they connect with the fibers coming from the temporal halves of the retinas of the other eye, and form the visual tracts. Here, papillomacular bundles also partially intersect.
The optic tracts begin at the posterior surface of the chiasm and, rounding the outer side of the brain stem, end in the external geniculate body, the posterior part of the optic tubercle and the anterior quadruple of the corresponding side. However, only the lateral geniculate bodies are the unconditioned subcortical visual center. The other two entities perform other functions.
In the optic tracts, the length of which in an adult reaches 30–40 mm, the papillomacular bundle also occupies a central position, and the crossed and uncrossed fibers still go in separate bundles. In this case, the first of them are located vectromedially, and the second - pre-rheolaterally. Visual radiance (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 field, and then, passing through hind thigh internal capsule, fan-shaped in the white matter of the occipital lobe of the brain. The central neuron ends in the avian spur furrow. This area also personifies the sensory visual center - the seventeenth cortical field according to Brodmann.
The path of the pupillary reflex - light and to set the eyes at a close distance - 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 run as part of the optic nerve. In chiasm, they intersect in the same way as the optic fibers, and pass into the optic tracts. Before outdoor geniculate bodies the pupillomotor fibers leave them and, after partial crossover, end in the cells of the so-called pretectal region. Further, new, interstitial neurons after partial crossing are directed to the corresponding nuclei (Yakutovich - Edinger - Westphal) of the oculomotor nerve. Afferent fibers from the macula of the retina of each eye are presented in both oculomotor nuclei.
The efferent pathway of innervation of the iris sphincter starts from the already mentioned nuclei 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 into the ciliary node. This is where the first neuron of the path under consideration ends and the second begins. At the exit from the ciliary node, the sphincter fibers as part of the short ciliary nerves, passing through the sclera, enter the perichoroidal space, where they form nerve plexus... Its terminal ramifications 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.
Efferent pathway of the dilator (dilator) of the pupil receiving sympathetic innervation, starts from the ci-liospinal center of Budge. The latter is located in the anterior horns of the spinal cord. From here, the connecting branches depart, 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 pathway ends and the second begins, which is part of the plexus of the internal carotid artery. In the cranial cavity, the fibers innervating the pupil dilator leave the mentioned plexus, enter the trigeminal (gasser) node, and then leave it as part of the optic nerve. Already at the apex of the border, they pass into the nasal nerve and then, together with the long ciliary nerves, penetrate into the eyeball. In addition, the central sympathetic pathway departs from the center of Budge, ending in the cortex of the occipital lobe of the brain. From here, the corticonuclear pathway of inhibition of the sphincter of the pupil begins.
The regulation of the function of the pupil dilator takes place with the help of the supranuclear hypothalamic center, located at the level of the third ventricle of the brain in front of the pituitary funnel. Through the reticular formation, it is connected with the Budge ciliospinal center.
The reaction of the pupils to convergence and accommodation has its own characteristics, and reflex arcs in this case are different from those described above.
During convergence, the stimulus to constriction of the pupil is proprioceptive impulses coming from the contracting internal rectus muscles of the eye. Accommodation is stimulated by blurring (defocusing) images of external objects on the retina. The effective part of the pupillary reflex arch is the same in both cases.
The center of setting the eye at close range is believed to be in the eighteenth cortical field according to Brodmann.
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