Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Sensory systems
Somatosensory system

The Human Body is covered by the Skin. The skin consists of a superficial epithelial layer and deeper layers (the dermis), formed by dense irregular Connective Tissue and subcutaneous adipose tissue. In addition, there are Skin Appendages—Hair, Nails, sebaceous, and Sweat Glands. The Structure OF THE Skin is described in detail in Chapter 5.

In addition to its protective (barrier) function, the skin performs a variety of other roles. It is involved in thermoregulation and excretion, and also houses A large number of receptor structures. These receptors perceive information about tactile, painful, thermal, and other stimuli applied to various areas of the skin. In other words, The surface of our body (soma) possesses a sensitivity known as somatic sensation. Several pathways exist to conduct these impulses, transmitting information to various PARTS OF THE CNS, including the Cerebral Cortex. Each type of sensation has its own projections, whose somatotopic Organization makes it possible to determine which part of our body is being stimulated, as well as the intensity and modality of the stimulus (Touch, pressure, vibration, Temperature, pain, etc.).

Several types of receptor structures exist to perceive these stimuli. All of them are primary sensory receptors, meaning they are the terminal branches of sensory nerve fibers. Depending on the presence or absence of surrounding accessory structures, such as connective tissue capsules, they are classified as encapsulated or unencapsulated (free) nerve endings, respectively.

Free nerve endings. These nerve fiber endings are terminal branches stripped of their myelin sheath. They are located in the dermis and the deep layers of the epidermis, extending up to the stratum granulosum (Fig. 3.76). These endings perceive mechanical stimuli and also respond to heat, cold, and painful (nociceptive) stimuli. The endings are formed by thin myelinated or unmyelinated fibers. For example, in the event of a burn, the former provide a rapid response (withdrawing the hand), while the latter produce a prolonged burning sensation. Thin myelinated fibers are sensitive to cooling, whereas unmyelinated ones are sensitive to heating. At the same time, extreme cold or heat can cause pain followed by itching.

In addition, in hairy skin, hair shafts and follicles are surrounded by the endings of 5—10 sensory fibers (Fig. 3.76). These fibers lose their myelin sheath and penetrate the basal lamina of the hair shaft. They respond to the slightest deflection of the hair.

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Fig. 3.76. Free nerve endings: A — in human skin (after Dogiel); B — receptor apparatus of a hair:

1 — hair ROOT; 2 — nerve trunk; 3 — nerve fibers; 4 — branching around the hair follicle; 5 — nerve endings

Fig. 3.77. Encapsulated nerve endings (after Dogiel):

A — Pacinian corpuscle; B — Merkel discs; C — Meissner's corpuscle; D — Ruffini endings

Encapsulated nerve endings are specialized structures designed to perceive specific types of stimuli. They are the terminals of thicker myelinated fibers compared to those that form free nerve endings. This allows for a faster rate of signal transmission to central structures.

Pacinian corpuscles (Vater-Pacini corpuscles) are among the largest receptor structures of this kind (Fig. 3.77, A). They are located in the deep layers of the dermis, as well as in the connective tissue sheaths of Muscles, the periosteum, mesenteries, etc. At one pole, a myelinated nerve fiber enters the corpuscle and immediately loses its myelin sheath. The fiber passes through the corpuscle within the inner bulb and expands at its terminal end, forming irregularly shaped processes. Surrounding the inner bulb is the outer bulb, composed of numerous concentrically arranged lamellae derived from Schwann Cells, with Collagen fibers and interstitial fluid in between. Externally, the corpuscle is covered by a connective tissue capsule that merges continuously with the endoneurium of the afferent fiber. The deeper the Pacinian corpuscle is located, the more layers its inner and outer bulbs contain. These endings are sensitive to touch, pressure, and high-frequency vibration, which is crucial for perceiving the texture of an object. When a stimulus, such as pressure, is applied, the capsule layers are displaced, generating excitation in the afferent fiber.

Merkel discs lie more superficially beneath the epithelium, near its lower boundary (Fig. 3.77, B). They are sensitive to static tactile stimuli (touch, pressure).

Meissner's corpuscles lie at the Base of the dermal papillae and are sensitive to light touch and low-frequency vibration (Fig. 3.77, C). They are particularly abundant in the skin of the palms and soles, Lips, eyelids, and nipples. Meissner's corpuscles are oval structures about 100 μm long, oriented perpendicularly to the epithelial surface. The corpuscle is formed by flattened, modified Schwann cells stacked on top of each other, mostly arranged transversely. A myelinated afferent fiber approaches the Meissner's corpuscle, loses its myelin, and branches repeatedly. Thus, up to 9 of its branches enter the corpuscle, winding spirally in the spaces between the cells. Externally, the corpuscle is covered by a connective tissue capsule that merges with the endoneurium outside. The capsule of the corpuscle is anchored to the lower boundary of the epithelium by bundles of collagen fibers.

Ruffini endings lie in the deep layers of the dermis, being particularly abundant on the plantar surface of the FOOT, and are oval corpuscles measuring 1x0.1 mm (Fig. 3.77, D). A thick myelinated afferent fiber approaches the corpuscle, loses its sheath, and branches. Numerous terminal arborizations intertwine with collagen fibers, which also form the core of the corpuscle. Displacement of the collagen fibers excites the afferents. The thin capsule of the corpuscle merges into the endoneurium.

Krause end bulbs are located in the conjunctiva of the eye, the Tongue, and the external genitalia. The corpuscles are surrounded by a thin-walled capsule. Before entering the capsule, the afferent fiber loses its myelin and branches. These endings likely perform a mechanoreceptive function.

In addition to receiving information about stimuli acting on the skin, The Nervous system receives impulses from The Musculoskeletal System that signal the body's position in space. Previously, this sensory system was referred to as the motor analyzer, but today a different terminology has become generally accepted (Table 3.3).

As can be seen from the table, these three terms overlap to some extent. Proprioception integrates sensory signals from the Skeleton and Muscles and, therefore, includes Muscle sense. Kinesthesia is the sense of BODY POSITION AND limb movement, as well as the sensation of effort, force, and heaviness. All receptors of the musculoskeletal system and skin participate in providing it.

The receptor structures that mediate these types of sensation have a rather complex structure.

Muscle receptors—muscle spindles—serve to detect the degree of muscle stretch. They are particularly abundant in muscles that control precise movements. These receptors are spindle-shaped structures enclosed in a thin, stretchable connective tissue capsule. Spindles are arranged longitudinally within the muscles and stretch as the muscle stretches. Each spindle is formed by several fibers (from 2 to 12) called intrafusal fibers (from the Latin *fusus*—spindle) (Fig. 3.78). These fibers are bathed in interstitial fluid. There are two types of intrafusal fibers. The central region of most fibers contains a chain of a single row of Cell nuclei. The second type of fibers carries a cluster of nuclei in the center (nuclear bag fibers); these fibers are longer and thicker than the former. The peripheral ends of both types of fibers are capable of stretching.

Table 3.3 Types of sensory modalities associated with movement and their corresponding receptors (after Shepherd)

Intrafusal fibers are innervated by afferent Cytology/practical/65.html">Myelinated nerve fibers. A thick nerve fiber, which has a high impulse conduction velocity, approaches the central region of the intrafusal fiber and wraps spirally around the nuclear bag or the region containing the nuclear chain. This type of ending is called a primary ending. On either side of the primary endings, thinner afferent fibers form secondary endings, which may resemble a spray. The primary ending responds to both the degree and rate of muscle stretch, whereas the secondary ending responds only to the degree of stretch and changes in muscle position.

Fig. 3.78. Muscle spindles:

A — muscle spindles in the extraocular muscles: 1 — nerve fiber; 2 — muscle fiber; 3 — tendon; B and D — nuclear bag fibers; C and E — nuclear chain fibers; [A, B, C — histological preparations; D, E — diagrams]; 1 — capsule; 2 — intrafusal fibers; 3 — extrafusal fibers; 4 — primary endings; 5 — secondary endings; 6 — trail endings (efferent); 7 — primary afferent; 8 — secondary afferent; 9 — efferent nerve fibers

When a muscle is stretched, information from the nerve endings travels to the Spinal Cord, where a portion of it synapses onto motor Neurons in the anterior horns. Their resulting reflex impulses trigger Muscle contraction. Another portion of the impulses synapses onto interneurons and is transmitted to other Regions of the nervous system (see below).

Muscle spindles also have efferent innervation, which regulates their sensitivity to stretch. Efferent fibers reach the muscle spindles from spinal motor neurons, but not from those that innervate the main muscle itself, whose fibers are called extrafusal. However, in some cases, muscle spindles receive motor innervation via axon collaterals projecting to the muscle fibers. This is observed, for example, in the extraocular muscles.

In addition to the receptor endings located within the muscles themselves and responding to their stretch, there are receptors at the junctions of muscles and tendons. These are known as Golgi tendon Organs (receptors) (Fig. 3.79). They are encapsulated and innervated by thick myelinated fibers. The myelin sheath is lost as the fiber penetrates the capsule, and the fiber forms terminal branches among the collagen bundles of the tendon. These endings are stimulated when compressed by tendon fibers During muscle contraction, while muscle spindles remain inactive; conversely, when the muscle is stretched, spindle activity increases, whereas tendon organ activity decreases.

A large number of receptor endings are located in the joints (Fig. 3.79). Receptors similar to tendon organs lie within the articular ligaments, while the connective tissue joint capsules contain abundant free nerve endings, as well as structures analogous to Pacinian and Ruffini corpuscles. They are sensitive to tension and compression occurring during movement, thereby signaling body position in space and the movement of its individual parts (kinesthesia). Free nerve endings can, additionally, perceive pain.

Fig. 3.79. Receptor endings in tendons (after Dogiel):

1 — myelinated nerve fiber; 2 — muscle fibers; 3 — terminal branching; 4 — collagen fiber bundle; 5 — axon (axis cylinder); 6 — Nucleus of a tendon cell (fibrocyte); 7 — Ruffini corpuscles; 8 — Pacinian corpuscles; 9 — Golgi receptors; 10 — free nerve endings

Conductive and central Divisions of the somatosensory system. Nerve impulses from receptors in the skin and musculoskeletal system (excluding the HEAD) travel via Spinal Nerves to the spinal ganglia, and then enter the spinal cord through the dorsal roots. Afferent fibers of each dorsal root conduct impulses from a specific area of the body — a dermatome (Atl. Fig. 159). The information entering the spinal cord serves two purposes: it participates in local Reflexes, whose arcs close at the spinal cord level, and is transmitted to higher divisions of the CNS via ascending pathways (Atl. Fig. 160). At the same time, a somatotopic organization is observed in the ascending tracts: axons joining at higher levels are situated closer to the Gray matter. Accordingly, axons coming from the lower part of the body lie more superficially.

As mentioned above, the gray matter of the spinal cord can be represented as laminae. Thin unmyelinated fibers reaching the spinal cord from pain and mechanoreceptors terminate in the superficial laminae, primarily in the substantia gelatinosa. Thin myelinated fibers mostly reach only the marginal zone (Fig. 3.80). Thick myelinated fibers bypass the dorsal horn, send collaterals to neurons of laminae III–IV, and enter the dorsal Column of the White matter. It has been established that most dorsal horn neurons receive afferent input of only one type; however, there are neurons where impulses from different receptors converge. This may form the basis for the interaction of various receptor systems. Axons of dorsal horn neurons can project into the white matter — into ascending tracts — or reach motor neurons of the anterior horns and participate in several spinal reflexes. For instance, impulses from cutaneous receptors trigger the flexor reflex, which occurs when a limb is withdrawn from a painful stimulus (such as a burn, etc.).

Fig. 3.80. Somatosensory pathways in the spinal cord (after Melzack, 1965):

1 and 2 — myelinated and 2 — unmyelinated fibers from mechanoreceptors (1 and 2) and pain receptors (2); 3 — interneuron; 4 — projection neuron; 5 — to the anterior horn (flexor reflex); 6 — to the spinothalamic tracts; I—V — gray matter laminae

Impulses from receptors of the somatosensory system are conducted along the gracile and cuneate fasciculi, as well as the spinothalamic and spinocerebellar tracts, and the trigeminal lemniscus.

The gracile fasciculus carries impulses from the body below the T5 thoracic segment, while the cuneate fasciculus carries them from the upper trunk and arms. These pathways are formed by the axons of sensory neurons whose cell bodies reside in the spinal ganglia, and whose dendrites form receptor endings in the skin, muscles, and tendons. Having traversed the entire spinal cord and the posterior part of the Medulla Oblongata, the fibers of the gracile and cuneate fasciculi terminate on neurons of the gracile and cuneate nuclei. The axons of these nuclear neurons project in two directions. Some — known as external arcuate fibers — cross to the contralateral side and, within the inferior cerebellar peduncles, terminate on Cells of the cerebellar vermis cortex (Atl. Fig. 160). The axons of the latter connect the vermis cortex with the cerebellar nuclei. The axons of these nuclear neurons, within the inferior cerebellar peduncles, project to the vestibular nuclei of the Pons. The other, larger portion of fibers from the gracile and cuneate nuclei decussates anterior to the central canal of the medulla oblongata, forming the medial lemniscus. Therefore, both of these pathways are referred to as the lemniscal system. The medial lemniscus ascends through the medulla oblongata, the tegmentum of the pons, and the Midbrain, terminating in the lateral and ventral nuclei of the thalamus. Along its path through the Brainstem, fibers of the medial lemniscus give off collaterals to the reticular formation. Axons of thalamic neurons project via the thalamic radiation to the cortex of the central regions of the cerebral hemispheres. Both the medullary nuclei and the thalamic and cortical projections of the gracile and cuneate pathways exhibit somatotopic organization. These pathways (especially the cuneate fasciculus) transmit fine sensation from the upper limbs, enabling delicate and precise Movements of the fingers. This is also facilitated by the small number of synaptic relays from neuron to neuron, which prevents the "spreading" of excitation across Brain AND SPINAL cord structures.

The spinothalamic tract conducts excitation from receptors whose stimulation evokes pain and temperature sensations (Atl. Fig. 160). It also contains fibers from articular and tactile receptors. The Cell bodies of the sensory neurons of this pathway also reside in the spinal ganglia. The central processes of these neurons enter the spinal cord within the dorsal roots, where they terminate on the cell bodies of interneurons in the dorsal horns at the level of laminae IV–VI. The axons of the dorsal horn neurons partially cross to the contralateral side, while the rest remain ipsilateral, forming the spinothalamic tract deep within the lateral column. The latter ascends through the spinal cord, the tegmentum of the medulla oblongata, pons, and cerebral peduncles, and terminates on cells of the ventral Nucleus of the thalamus. Along its course through the brainstem, collaterals branch off from this tract to the reticular formation. From the thalamus, fibers project via the thalamic radiation to the cortex, terminating primarily in the postcentral gyrus.

The posterior and anterior spinocerebellar tracts conduct excitation from proprioceptors of the motor apparatus (Atl. Fig. 160). The sensory neurons of these pathways are located in the spinal ganglia, while the interneurons are in the dorsal horns of the spinal cord. The axons of the interneurons forming the posterior spinocerebellar tract remain ipsilateral in the lateral column of the spinal cord, whereas those forming the anterior tract cross to the contralateral side, also running in the lateral column. Both tracts enter the Cerebellum: the posterior via the inferior cerebellar peduncles, and the anterior via the superior cerebellar peduncles. They terminate on cells of the cerebellar vermis cortex. From there, impulses travel along the same pathways as those passing through the external arcuate fibers from the medulla oblongata. The spinocerebellar tracts enable the integration of information from muscle and joint receptors of the limbs with cerebellar mechanisms required for motor coordination, maintenance of muscle tone, and posture. This is particularly crucial for lower limb function during standing and locomotion.

The trigeminal lemniscus transmits impulses from mechanoreceptors, thermoreceptors, and pain receptors of the head (see Atl.). The sensory neurons are the cells of the trigeminal ganglion. The peripheral fibers of these cells run within the three Branches of the Trigeminal nerve, which innervate the skin of the face (Fig. 3.28). The central fibers of the sensory neurons emerge from the ganglion within the sensory root of the trigeminal nerve and enter the pons where it transitions into the middle cerebellar peduncles. Within the pons, these fibers bifurcate in a T-shape into ascending and long descending branches (spinal tract), which terminate on neurons forming the principal sensory nucleus of the trigeminal nerve in the pontine tegmentum, and its spinal nucleus in the medulla oblongata and spinal cord (Atl. Figs. 110, 111). The central fibers of the neurons of these nuclei decussate in the upper pons and, as the trigeminal lemniscus, ascend through the midbrain tegmentum to the thalamus, where they terminate either independently or alongside fibers of the medial lemniscus on cells of its ventral nucleus. The processes of neurons from this nucleus project via the thalamic radiation to the cortex of the lower postcentral gyrus, where sensitivity originating from head structures is primarily localized.

Fig. 3.81. Somatosensory cortex:

1 — leg; 2 — hip; 3 — trunk; 4 — neck; 5 — head; 6 — arm; 7 — elbow; 8 — forearm; 9 — hand; 10 — fingers; 11 — thumb; 12 — eye; 13 — Nose; 14 — face; 15 — lips; 16 — Teeth; 17 — Gums; 18 — jaw; 19 — tongue; 20 — Pharynx; 21 — Internal Organs

Somatosensory projections in the cerebral cortex are located in the postcentral gyrus. Fibers from the thalamus project here, carrying impulses from all receptors of the skin and musculoskeletal system. Here, as in the thalamus, the somatotopic organization of projections is highly pronounced (Fig. 3.81). In addition to the primary projection area, which receives afferent input solely from the thalamus, There is a secondary area where fibers from the primary area terminate alongside thalamic projections. In this area, sensory signals are processed, and from here they are directed to other regions, including motor areas of the cortex and subcortical structures.

Review Questions

1. What are the general structural patterns of Sensory Systems?

2. Structure of the Eyeball and its accessory apparatus.

3. How does the eye develop during ontogeny?

4. Peripheral, pathway, and cortical divisions of the visual system.

5. What are the characteristics of visual projections in the CNS?

6. Structure of the auditory sensory system. Which CNS structures are involved in conducting impulses from auditory receptors?

7. Structure and Development of the external, middle, and Inner ear.

8. Describe The structure of the Organ of Balance. Which CNS structures are involved in transmitting information about changes in body position in space?

9. Structure and development of the Olfactory sensory system. Which brain structures are involved in transmitting information from olfactory receptors?

10. Structure and development of the Gustatory sensory system. Its pathway and central divisions.

11. What types of receptors are found in the somatosensory system? Where are they located and what stimuli do they detect?

12. Pathway and central divisions of the somatosensory system. What is The Significance of the somatotopic organization of its projections within CNS structures? Provide Examples.