componentsof somatosensory system

The somatosensory systems process information about, and represent, several modalities of somatic sensation (i.e., pain, temperature, touch, proprioception).

11.2.1 Sensory Receptors

Receptors are specialized neuronal structures to detect diverse stimuli from the internal and external environment.

Many of the 1° somatosensory afferent terminals are enveloped in a connective tissue capsule along with sur­rounding muscle, tendon, or cutaneous cells, or end on hair follicles. The hair follicles and the encapsulated tissue adja­cent to the 1° afferent terminals (i.e., skin, muscle, tendon, and joint tissues) contain no synaptic specializations and do not generate receptor potentials or release neural transmit­ters.

Receptors have the ability to convert other forms of energy into electrical energy i.e. action potential, causes transduction if adequate stimuli is applied. They have dif­ferential sensitivities to different stimuli. The permeability of the receptor is modified and ends up in graded receptor potential. They show different adaptability to a stimulus and speed of adaption varies with receptors. Each receptor has specific acuity i.e. discriminative ability.

Sensory receptors are classified based on function of the receptors, location of receptors, source of the stimulus, and type of stimulus or modality.

11.2.1.1 Classification of Sensory Receptors

Based on the function of the receptors

1. Phasic receptors: Produce action potential during part of stimulation hence they initiate response. It does not provide clue for duration of stimulus.

2. Tonic receptors: Generate action potential upon the application of stimulus and propagation of action potential goes on till the stimulus is removed. This gives information regarding the duration of stimulus. In this type, the possibility of weaken­ing of the impulse irrespective of the maintenance of constant strength of stimulus due to prolonged exposure leading on to a phenomenon known as receptor adaptation.

Based on the location of receptors

1. Exteroceptors: Detect stimuli from outside the body in animals’ micro or macro environment (change in temperature, air pressure).

la. Superficial cutaneous sensations: Touch, light superficial pressure, temperature, cold, warmth, and pain.

lb. Deep sensations: Receptors in the muscle ten­don and joints together known as kinesthetic sensation mediated through the muscle spin­dle receptor and Golgi tendon organ to sense position and movement e.g. Special Sensory receptors and Skin receptors.

2. Interoceptors: Detect stimuli within the body (Blood pressure, O2 uptake). They are Proprioceptors: Convey information from the mus­cles, tendons, and joints, Visceroceptors: Carrying information from the viscera, Specialized recep­tors: Baroceptors, osmoceptors, chemoceptors, and volume receptors, Baroceptors: Present in carotid sinus and aortic arch to detect BP change, Osmoreceptors: Present in hypothalamus to detect osmotic change, and Chemoreceptors: Sensitive to chemical change. Present in tongue to detect taste and in nose to detect smell. Hypothalamus central chemoreceptors detect pH change, blood glucose, and hormonal levels.

Based on the source of the stimulus

Telereceptors: Detect the stimuli at a distance (Photo and auditory receptors)

Based on the stimulus or modality

1. Photoreceptors: Detect change in intensity of light mediating sense of vision.

2. Thermoreceptors: To detect change in temperature.

3. Electroceptors and Magnetoceptors: (Mostly in insects and birds). They detect electrical and mag­netic field respectively.

4. Mechanoreceptors: Involved in sensing touch, pain, hearing, balance (equilibrium).

5. Chemoreceptors: Detect chemical signals, sense of gustation, olfaction, blood O2, tension, and change in pH (internal environment).

Based on the location of receptors sensations can also be classified as

1. Superficial cutaneous sensations: Touch, light superficial pressure, temperature, cold, warmth, and pain.

2. Deep sensations: Receptors in the muscle tendon and joints together known as kinesthetic sensa­tion mediated through the muscle spindle recep­tor and Golgi tendon organ to sense position and movement.

11.2.1.2 Types of Somatosensory Receptor

11.2.1.2.1 CutaneousReceptars

The somatosensory receptors in skin are known as the cutaneous receptors. These are classified as encapsulated receptors which include Meissner corpuscles, Pacinian cor­puscles and Ruffini corpuscles. Other cutaneous receptors are unencapsulated i.e. do not end on or near specialized tissue. They may be mechanoreceptors, nociceptors or ther­moreceptors. It includes hair follicle receptor (the 1° affer­ent ends on hair follicles) and the Merkel complex (the 1° afferent ends at the base of a specialized receptor cell called the Merkel cell) e.g. sensory receptors of the crude touch, pain and temperature.

a. Meissner Corpuscle- It is an elongated, encapsu­lated stack of flattened epithelial (laminar) cells found in glabrous (i.e., hairless) skin, within the dermal papillae.

b. Pacinian corpuscle - It is football-shaped, encap­sulated, and contains concentrically layered epithelial (laminar) cells and in cross section it looks like a slice of onion. These receptors can be cutaneous, proprioceptive or visceral receptors, depending on their location. They are located in subcutaneous tissue beneath the dermis and in the connective tissues of bone, the body wall and body cavity.

c. Ruffini Corpuscle - It is cigar-shaped, encapsu­lated, and contains longitudinal strands of collage­nous fibers that are continuous with the connective tissue of the skin or joint. These receptors can be cutaneous or proprioceptive receptors depending on their location. The receptors run parallel to the surface of the skin and are most sensitive to skin stretch.

d. Hair Follicle - It is an unencapsulated cutaneous receptor which run parallel to the hair shaft form­ing a lattice-like pattern. These receptors respond best to moving objects and signal the direction and velocity of the movement of a stimulus brushing against hairy skin.

e. Merkel Complex - It is unencapsulated cutaneous receptor consists of a specialized receptor cell, the Merkel cell, and a nerve ending, the Merkel disk.

f. Free Nerve Endings - Free nerve endings are found throughout the body, in skin, muscles, ten­dons, joints, mucous membranes, cornea, body mesentery, the dura, the viscera, etc. The free nerve endings in skin are stimulated by tissue­damaging (nociceptive) stimuli that produce the sensation of pain or by cooling of the skin or the warming of skin or by touch. Free nerve endings are considered to be the somatosensory receptors for pain, temperature and crude touch.

11.2.1.2.1.1 Mechanism of Action of Cutaneous Receptors

1. Thermoreceptors- The hypothalamus of the ani­mals has central thermoceptors to monitor their internal temperature. Two different receptors to mediate warmth and cold.

The warmth receptors are situated near deep blood vessels. They are Ruffini end organ which can also serve to detect tactile sensation (Poly modal receptor). Ruffini’s corpuscles are located in the connective tissue of the skin in associa­tion with collagen fibers. Hence detect skin stretch and are involved in proprioception. Warmth receptors detect change in temperature of 20° C to 47° C. The frequency of action potential increases with increase in the temperature.

The cold sensation is mediated by the end organ of Krause. The Cold receptors detect and are stimulated at the temperature between 10° C to 35° C. These are extremely sensitive and can detect change in even 0.5° C. There are also another thermoceptor which detect painful hot stim­uli. The temperature ranges above 45 ° C. The frequency of action potential generated is in accordance with the amount of pain sensation.

Free nerve ending of the thermoreceptor neurons have specific thermoreceptor proteins. They are known as Thermo TRPs. The thermo TRPs are many, variable to different temperature range, and some are to detect dis­tinct temperature. They get stimulated and act through ion channels and gating mechanism by modifying the protein configuration.

2. Nocioceptors- These detect pain sensation by naked nerve endings. This sensation does not have any special structure to mediate. These nociceptors are available in the superficial layers of dermis, which are represented by non-medul- lated fibers (Figure 11.1). They act as protective response to an injurious stimulus. These receptors have vibratory sense which is mediated by tactile and pressure receptors namely Merkel’s discs and Pacinian corpuscles and Itching and tickling sense which is mediated by stimulation of both tactile and pain receptors (Table 11.1).

TABLE 11.1

Cutaneous Receptors

11.2.1.2.2 Proprioceptive Receptors

The somatosensory receptors in muscles, tendons, joint ligaments, and joint capsules are known as propriocep­tors. There are no specialized sensory receptor cells for body proprioception. In skeletal muscle, these are muscle spindles and Golgi tendon organs, as well as numerous free nerve endings. In joints, these are encapsulated endings similar to those in the skin, as well as numerous free nerve endings.

a. Muscle Spindles - They are encapsulated and con­sist of small muscle fibers, called intrafusal muscle fibers, and afferent and efferent nerve terminals. Muscle spindles are found in nearly all striated muscles. Muscle spindles are most numerous in muscles that carry out fine movements, such as the extraocular muscles and the intrinsic muscles of the hand. The muscle spindles are propriocep­tors specialized to monitor muscle length (stretch) and signal the rate of change in muscle length by changing the discharge rate of afferent action potentials.

There are two types of intrafusal fibers. One of them is known as the nuclear bag, where their central portion is interrupted and dilated, which houses many nuclei. The nuclear bag region tapers towards the pole. Another intra­fusal fiber is known as the nuclear chain. They have a uni­form diameter throughout, but near the middle, a small but appreciable increase in diameter due to the increased number of cell nuclei. It is innervated by the gamma motor neurons or intrafusal fibers. The cell bodies of the gamma motor neurons are located in the ventral horn of the gray matter of the corresponding spinal segment. The skeletal muscles are innervated by α-motor neurons. These are referred to as extrafusal fibers. The gamma motor neurons are of two types: gamma plate neurons and gamma trail motor neurons.

The Gamma plate neurons innervate nuclear bag intra­fusal fibers and are tonic neurons generating sustained action potential. Gamma trail motor neurons supply nuclear chain intrafusal fibers. These are phasic neurons and pro­duce action potentials of short duration with high frequency.

b. Golgi Tendon Organ - They are encapsulated and contain intertwining collagen bundles, which are continuous with the muscle tendon, and fine branches of afferent fibers that weave between the collagen bundles. They resemble Ruffini corpus­cles and are found in the tendons of striated extra- fusal muscles near the muscle-tendon junction.

The Golgi tendon organ is a proprioceptor that monitors and signals muscle contraction against a force (muscle ten­sion), whereas the muscle spindle is a proprioceptor that monitors and signals muscle stretch (muscle length).

c. Joint Receptors - These are encapsulated endings that resemble the Ruffini and Pacinian corpuscles and the Golgi tendon organs and are located within the connective tissue, capsule, and ligaments of joints.

d. Free Nerve Endings - These are abundant in mus­cles, tendons, joints, and ligaments. These free nerve endings are considered to be the somato­sensory receptors for pain resulting from muscle, tendon, joint, or ligament damage and are not con­sidered to be part of the proprioceptive system.

11.2.1.2.2.1 Mechanism of Action of Proprioceptors These receptors are located in the skeletal muscle, tendons, and joints are known as the kinesthetic receptors. These inform the CNS about the movement and position of the limbs to maintain posture and equilibrium. The joint sense includes the sense of movement and the sense of position. The sense of movement is the ability to feel the movement with closed eyes. The sense of position is to feel the position of the body in relation to space. These two are mediated by receptors in and around the joint. Receptors involved are the spray nerve endings, Pacinian corpuscles, touch receptors, and free nerve endings located in the synovial membrane, ligaments, and tis­sues near the joint.

FIGURE 11.2 Propioceptive receptors of somatosensory system (Blumer et al., 2023)

The impulses generated from these reach the cerebral cortex through dorsal tracts, medial lemniscus, and thala­mus to end in conscious perception of the position of the body in relation to space. The impulses from the muscle spindle organ, Golgi tendon organ, and vestibular apparatus of the internal ear do not reach the cerebral cortex. They are concerned with the reflex adjustment of muscle tone with the involvement of the spinal cord to maintain posture and equilibrium. These are known as non-sensory propriocep­tive impulses as they never bring out conscious sensations.

Some of these proprioceptors send signals to the respi­ratory neurons about changes in the force exerted by the respiratory muscles and the movement of the ribcage. The function of the diaphragm with respiratory control is also mediated by one of the tendon organs in the external inter- costals, which inhibit inspiration upon stimulation by a small change in the force of contraction of these muscles. The stretching of the diaphragm results in the inhibition of its contraction and inspiration acting through the tendon organs. The intercostal reflexes are significant due to the presence of numerous muscle spindle organs in the external intercostals. The muscle spindles help to coordinate breath­ing during changes in the posture and stabilize ribcage dur­ing reduction in lung compliance.

11.2.1.2.2.2 Mechanism of Action of Muscle Spindle Gamma Motor Neuron Excitation of these neurons ends up in contraction of intrafusal muscles. This in turn stretches the nuclear bag fibers and nuclear chain fibers. Sensory receptor of the muscle spindle in limited to these areas and can detect the tension upon these receptors. Stretch initiates depolarization in the sensory receptor membrane. Intrafusal fibers have no control on the tension developed in the extrafusal fibers. The nuclear bag fibers are tonic and sarcolemma of these are not excitable. No appreciable action potential is generated, hence no muscle twitch occur. On contrary nuclear chain fibers respond to neural excitation and are phasic in nature. It is excitable when stimulated and generates action potential in response to gamma trail motor neurons. This creates a twitch type of contraction.

Afferent fibers from nuclear bag spiral around the intra­fusal muscle and form Annulospiral or Nuclear bag endings of the muscle spindle. These fibers are large and myelinated, responsible for initiation of myotatic reflex. Sensory fibers associated with nuclear chain intrafusal fibers terminate in plaque like ending namely the Floewer Spray ending. These fibers are small and are myelinated and are responsible for flexor reflex (Figure 11.2). These system functions to main­tain posture of an animal and muscle tone of the skeletal muscles. The effect exerted by Gamma plate motor neurons is referred to as Gamma loop mechanism (Table 11.2).

11.2.2 Somatosensory Neurons

The cell bodies of the first-order somatosensory afferent neurons are located in posterior root or cranial root ganglia and are pseudounipolar cells. The cell body gives rise to a single process that divides to form a peripheral axon and a central axon. The peripheral axon travels to and ends in the skin, muscle, tendon or joint, and the central axon travels to and ends in the central nervous system (Table 11.3).

11.2.3 Somatosensory Receptor Organ

The sensory receptor organs contain the somatosen­sory receptors and deliver the somatosensory stimuli to the receptors. The receptors of most sensory systems are located in specialized sensory receptor organs (e.g., the photoreceptors in the eye and the auditory and vestibular hair cells in the inner ear) or within a restricted part of the body (e.g., the taste buds in the mouth and the olfactory receptors in the olfactory mucosa of the nose). For the tac­tile component of the somatosensory system, the skin cov­ering the entire body, head, and face functions as the touch receptor organ, whereas joint tissues, muscles, and tendons act as the proprioception receptor organs.

TABLE 11.2

Proprioceptive Receptors

TABLE 11.3

Somatosensory Receptors and their Peripheral Axons