Sensitivity – the ability of the body to perceive stimuli coming from the environment or from its own tissues and organs.
The mechanisms of sensitivity are explained on the basis of the theory of eb analyzers, the founder of which is I. P. Pavlov. The analyzer consists of three parts: the receptor, the conductor part and the cortical part. Receptors are terminal formations of sensitive nerve fibers that perceive changes in or outside the body and transmit it in the form of impulses. The receptors are divided into three groups: externo-, proprio– and interoreceptors. Extroreceptors are represented by tactile, pain and temperature, interoreceptors are located in the internal organs-chemo-and baroreceptors. Proprioreceptors are located in the muscles, ligaments, tendons, and joints.
Thanks to them, a person has an idea of the position of his gel in space. There are several types of sensitivity. Surface sensitivity combines pain, temperature, and tactile sensitivity.
Deep sensitivity includes vibration, muscle-joint feeling, pressure and mass feeling, and two-dimensional spatial feeling. Impulses from the receptors enter the cortical parts of the analyzer along a conducting pathway consisting of three neurons.
The first neurons of the pathways of any kind of sensitivity are located in the spinal nodes.
The second surface sensitivity neuron is located in the posterior horns of the spinal cord, where the axons of the first neurons enter through the posterior roots. There, the axons of the second neurons cross, forming part of the lateral cords of the spinal cord. They end in the visual hillock.
The third neuron is located in the ventrolateral nucleus of the visual tubercle. The axons of the third neuron terminate in the cortex of the posterior central gyrus, passing first through the pedicle of the inner posterior capsule. The section of the pathway to the third neuron is called the lateral spinothalamic pathway. From the third neuron, the thalamocortical pathway begins.
Impulses of a superficial type of sensitivity enter the cortex of the brain from the opposite side of the body. The first deep sensitivity neuron is located in the spinal ganglion. Its axons as part of the posterior roots fall into the posterior cords of the spinal cord of the same side. In the posterior ropes, there is a Gaulle bundle, more medial, and a Burdach bundle, more lateral.
The first contains fibers from the lower extremities, the second – from the upper.
The second neuron of the pathway is located in the nuclei of the posterior ropes in the medulla oblongata. There, the fibers cross and form a medial loop, in which the fibers of all types of sensitivity of the opposite half of the body are located.
Impulses of proprioceptive sensitivity also enter the cerebellar worm through the Flexig and Govers pathways. Thus, the pathways of the surface and deep types of sensitivity have both similarities and differences. The similarity is that the first neurons are located in the spinal ganglion, the axons of the second neuron cross, the third neurons are located in the nuclei of the thalamus, their axons pass through the posterior leg of the inner capsule and end in the cortex of the posterior central gyrus.
There are four types of sensitivity disorders: peripheral, segmental, conductive, and cortical.
The peripheral variant develops as a result of damage to the peripheral nerve and is located in the area of its innervation.
The segmental variant develops as a result of damage to the posterior root or spinal ganglion in the case of deep sensitivity, in the case of superficial sensitivity-also in the case of damage to the posterior horn or the anterior gray spike of the spinal cord.
The conductor variant of sensory impairment occurs when the posterior or lateral cords of the brain, the brainstem, the thalamus, the inner capsule, or the white subcortical substance are damaged. This disorder is characterized by a change in sensitivity below the level of the lesion of the pathway.
The cortical variant occurs when a certain area of the cerebral cortex is affected. At the same time, there is a local loss of sensitivity.
Sensitivity disorders, their symptoms Anesthesia – complete loss of sensitivity of all types. Anesthesia is divided into hemianesthesia – loss of sensitivity of half of the body and monoanesthesia-loss of sensitivity of one limb. If a separate type of sensitivity falls out, then the anesthesia is called partial.
Hypesthesia – decreased sensitivity.
Hyperesthesia – increased sensitivity.
Analgesia – loss of pain sensitivity, thermoanesthesia-loss of temperature sensitivity. The pathology of sensitivity includes a split sensation of pain. At the same time, as a result of the needle prick, the patient initially feels a touch, and then only pain.
A single irritation can be perceived as multiple-polyesthesia. The patient may incorrectly localize the irritation.
It usually points to a symmetrical area on the opposite side of the body – the allocheirium. There may be a perversion of perception (for example, heat in the form of cold, a prick in the form of a touch of hot, etc.) – dysesthesia. There may be spontaneous sensations of tingling, crawling goosebumps, tightening-paresthesia.
With the development of a pathological process of different localization, pain symptoms may occur, they can be local, projected, radiating and reflected. Local pain is characterized by the appearance of irritation at the site. Projection pain is localized in the area of innervation of the affected nerve. Radiating pain occurs when a branch of the nerve is affected and is localized in the innervation zone of another branch of the same nerve. Reflected pain is localized in certain areas of the skin and occurs in the pathology of internal organs.
Pain includes causalgia. It is characterized by the appearance of burning paroxysmal pain, which increases when touched and other irritations. These pains are localized in the area of the affected nerve. Often there are phantom pains, which consist in the sensation of pain in the missing limb.
The occurrence of such pains is associated with the development of scarring processes in the nerve stump, which creates conditions for its constant irritation. Damage to the posterior roots of the spinal cord, nerve plexuses and trunks causes the appearance of tension symptoms. These include the symptoms of Laseg, Neri, Sikar, Mackiewicz, and Wasserman.
Laseg’s symptom is the occurrence of pain along the sciatic nerve when the leg is bent in the hip joint.
The symptom of Neri is the occurrence of pain in the lower back when bending the head forward.
Sikar’s symptom is pain along the sciatic nerve during the back flexion of the foot.
Mackiewicz’s symptom is pain on the anterior surface of the thigh when the leg is bent at the knee joint in the supine position. This symptom indicates a pathology of the femoral nerve.
Wasserman’s symptom is pain on the front surface of the thigh when lifting the extended leg in the supine position.
When the nerve trunks and plexuses are affected, pain points may appear. Erb points are located above the middle of the clavicle by 2 cm, and soreness in them occurs when the brachial plexus is affected. Gar points are located above the spinous processes of the IV and V lumbar and I sacral vertebrae.
Soreness occurs when the lumbosacral plexus is affected. The Valais points are located at the exit of the sciatic nerve from the pelvic cavity, in the area of the gluteal fold, in the popliteal fossa, posteriorly from the head of the fibula and posteriorly from the inner ankle. Soreness occurs in the same pathology.
The violation of sensitivity depends on the localization of the pathological process and the level of the lesion.
Damage to the nerve trunk leads to a violation of all types of sensitivity, which is localized in the place of its innervation.
The lesion of the nerve plexuses causes local soreness and sensitivity disorders of all kinds, which are localized in the innervation zone of all the nerves of this plexus.
Damage to the posterior roots of the spinal cord causes a violation of all types of sensitivity in the areas corresponding to the affected segment. If there is irritation of these formations, then there are pain of a shingling nature and paresthesia. If a lesion of the spinal ganglion is attached, then herpetic rashes appear in the corresponding segment.
Damage to the posterior horn of the spinal cord leads to a loss of the surface appearance of sensitivity on the same side. The deep sensitivity is preserved.
Bilateral damage to the posterior horns and the anterior gray spinal cord spike leads to a violation of the surface appearance of segmental-type sensitivity on both sides.
Damage to the posterior cord of the spinal cord leads to a violation of the deep and tactile sensitivity of the conductor type. There is also a violation of the coordination of movements, which increases when the eyes are closed – sensitive ataxia.
When the lateral cord is affected, the surface sensitivity below the lesion site on the opposite side of the conductor type is disturbed.
A partial lesion of the spinal cord causes the development of Brown-Sikar ssindrome. This syndrome consists in a loss of deep sensitivity on the same side, a violation of surface sensitivity on the opposite side. At the level of the affected segment of the spinal cord, segmental sensitivity disorders are noted. In the case of a complete transverse lesion of the spinal cord, all types of sensitivity along the conductor type on both sides are violated.
The lesion of the medial loop causes a complete loss of all types of sensitivity on the opposite side. Damage to the thalamus leads to the loss of all types of sensitivity on the opposite side.
In addition, trophic disorders, visual disturbances and hyperpathies are noted. Damage to the posterior pedicle of the inner capsule leads to a violation of all types of sensitivity on the opposite side, as well as to sensitive hemiataxia and hemianopsia. Damage to the cortex of the posterior central gyrus causes a complete loss of sensitivity of all types on the opposite side.
Proprioceptive regulation of movements is carried out without the intervention of consciousness, i.e. impulses from proprioceptors do not reach the cerebral cortex. Usually, such impulses form a closed feedback loop, which is essentially a reflex that ensures the maintenance of a certain pose or position of the body in space.