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3 CHAPTER III SENSATION AND ITS DISORDERS. (Anatomy of Sensory Pathways. Syndromes of Sensory System Lesion on Different Levels)
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3 CHAPTER III SENSATION AND ITS DISORDERS. (Anatomy of Sensory Pathways. Syndromes of Sensory System Lesion on Different Levels)

somapath

 

Figure 3.1. Sensory pathways (Lecture notes. The University of Western Ontario Department of Anatomy and Cell Biology, 2007).

Sensation is the ability of an organism to perceive the irritations caused by the environment or by its own tissues and organs. Receptors are special sensitive structures, capable to perceive any changes inside or outside of an organism and to transform them to nervous impulses. All kinds of external effects are transformed into nervous impulses, and reach the brain as signals.

Depending on functional features the receptors are subdivided into exteroceptors, that respond to external stimuli (mechanical, thermal, optic, acoustic, olfactory, gustatory), proprioceptors (they give the information about tendon and muscle tension, movements and position of the body) and interoceptors (informing of the condition inside an organism). There are osmoreceptors, chemoreceptors, baroreceptors and others. Skin receptors are divided into mechanoreceptors (touch, tension), thermoreceptors (cold, heat) and nociceptors (pain). A great number of these receptors are situated between epidermis and connective tissue. Therefore the skin can be considered as a sensitive organ covering the whole surface of the body. There are free nerve endings and capsulated nervous bodies in the skin. Free nerve endings are located between epidermal cells and perceive painful irritations. Tactile Merkel’s corpuscles are located basically on tips of fingers and react to touch. Meissner’s corpuscles are on palms, soles, lips, tongue tip, mucous membrane of genitals and are very sensitive to touch. Pacinian Vater-Pacini corpuscles, located in deep layers of skin, perceive tension. Krause's end bulbs are considered as cold receptors and Ruffini’s corpuscles are thermal receptors.

Receptors are founded in deeper tissues such as muscles, tendons, fasciae and joints. Muscular receptors include some types, most important of which are neuromuscular spindles. They react to muscle strain and are responsible for the realization of the stretch reflex. Golgi-Mazzoni corpuscles are located between the tendon and the muscle.

From the receptor, information is transmitted to the afferent fibers of the pseudounipolar spinal ganglion cells, whose efferent fibers reach the spinal cord by way of the dorsal root. A synapse onto a second neuron in the sensory pathway is made either immediately, in the posterior horn of the spinal cord (protopathic system), or more rostrally, in the brain stem (lemniscal system). The highest level of the somatosensory pathway is the contralateral primary somatosensory cortex. The somatotopic organization of the somatosensory pathway is preserved at all levels.

The classification of sensation:

1) Superficial sensation (exteroceptive) such as pain, temperature and tactile;

2) Deep sensation (proprioceptive) such as joint position sense, vibratory sense, the sense of body weight and pressure sense, kinesthesia;

3) Compound sense such as sense of localization, posture sense (the recognition of signs and letters written on the skin), sense of discrimination (the recognition of pricks made at close distance), stereognosis;

4) Interoceptive sensation.

Protopathic sensation is phylogenetically a more ancient kind characterised by limited opportunities of differentiation of irritations by their modality, intensity and localization. Epicritic sensation is phylogenetically new kind of sensitivity.

The general scheme of the structure of sensory system is the following:

1. All the first neurons of sensation are in spinal ganglions (sensory ganglions of cranial nerves).

2. The axons of the second neurons always pass onto the opposite side (the neurons of superficial sensation in the spinal cord, neurons of deep sensation in the brain).

3. The third neurons of all kinds of sensation are in the thalamus.

Sensory irritations are conducted by two sensory systems, namely by the extralemniscus system (superficial sensation) and the lemniscus system (deep sensation).

The extralemniscus system includes conductors of pain, temperature and partly tactile sensation. The first neuron is in the spinal ganglion. The dendrites of these pseudounipolar neurons as a part of peripheral nerves arrive at skin where they end by specific receptors. The axons of these neurons form the dorsal root of the spinal nerve, enter into the posterior horn of the spinal cord and contact with second sensory neuron. Their axons come over to the opposite side and in lateral columns of the spinal cord go to the thalamus (lateral spinothalamic tract). Central fibers of the first sensory neuron ascend 1 or 2 segments before making a synapse in the substantia gelatinosa of the posterior horn. Fibers from the posterior horn decussate and enter the lateral spinothalamic tract, which, like the anterior spinothalamic tract, projects to ventral posterolateral nucleus; which projects in turn to the postcentral gyrus. Fibers from the posterior horn decussate and enter the lateral spinothalamic tract, which, like the anterior spinothalamic tract, projects to ventral posterolateral nucleus, which projects in turn to the postcentral gyrus.

The lemniscus system includes the conductors of deep sensation. The dendrites of pseudounipolar neurons of deep sensation of the spinal nerves as a part of peripheral nerves pass to receptors in muscles, tendons, periosteum and to tactile receptors of the skin. The axons of first sensory neurons don’t enter to the grey substance. They form posterior columns in which there is somatotopical localization. Fibers mediating sensation in the legs are in the fasciculus gracilis (medial), while those for the arms are in the fasciculus cuneatus (lateral). These fibers synapse onto the second sensory neuron in the corresponding somatosensory nuclei of the lower medulla (nucleus gracilis, nucleus cuneatus). Fibers from these nuclei decussate and ascend in the contralateral medial lemniscus and ascend to the postcentral gyrus through thalamus (ventral posterolateral nucleus) and internal capsule.



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