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Chapter 2: Somatosensory Systems

Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD Anderson Cancer Center
(content provided by Chieyeko Tsuchitani, Ph.D.)


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The somatosensory systems inform us about objects in our external environment through touch (i.e., physical contact with skin) and about the position and movement of our body parts (proprioception) through the stimulation of muscle and joints. The somatosensory systems also monitor the temperature of the body, external objects and environment, and provide information about painful, itchy and tickling stimuli. The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face. Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face. 

This first series of chapters on somatosensory systems concentrates on the somatosensory systems that provide accurate information about the location and temporal features of stimuli and about sharp pain, tactile stimuli and the position and movement of body parts. This chapter describes somatosensory stimuli, the sensations produced when they are applied, and the cutaneous, muscle, and joint receptors that are responsible for initiating the perceived somatic sensations. Subsequent chapters describe the pathways processing other pain, temperature, crude touch and visceral sensations.

2.1 Somatic Stimuli

Modality Specificity in the Somatosensory System. The somatosensory systems process information about, and represent, several modalities of somatic sensation (i.e., pain, temperature, touch, proprioception). Each of these modalities can be divided into sub-modalities, as shown in Table 1 (e.g., pain into sharp, pricking, cutting pain; dull, burning pain; and deep aching pain). Discriminative touch is also subdivided into touch, pressure, flutter and vibration. Each of these sensations (i.e., sub-modalities) is represented by neurons that exhibit modality specificity. That is, when a somatosensory neuron is stimulated naturally (e.g., by skin warming) or artificially (e.g., by electrical stimulation of the neuron), the sensation perceived is specific to the information normally processed by the neuron (i.e., warm skin). Consequently, a "warm" somatosensory neuron will not respond to cooling of the skin or to a touch stimulus that does not "warm" the skin. The somatosensory receptor and its central connections determine the modality specificity of the neurons forming a somatosensory pathway.

Table I
The Sensory Modalities Represented by the Somatosensory Systems

Modality Sub Modality Sub-Sub Modality Somatosensory Pathway (Body) Somatosensory Pathway (Face)
Pain sharp cutting pain   Neospinothalamic Spinal Trigeminal
dull burning pain   Paleospinothalamic
deep aching pain   Archispinothalamic
Temperature warm/hot   Paleospinothalamic
cool/cold   Neospinothalamic
Touch itch/tickle & crude touch   Paleospinothalamic
discriminative touch touch Medial Lemniscal  Main Sensory Trigeminal
pressure
flutter
vibration
Proprioception Position: Static Forces muscle length
muscle tension
joint pressure
Movement: Dynamic Forces muscle length
muscle tension
joint pressure
joint angle

Tactile Stimuli. Tactile stimuli are external forces in physical contact with the skin that give rise to the sensations of touch, pressure, flutter, or vibration. We normally think of touch as involving minimal force on-or-by an object that produces very little distortion of the skin. In contrast, pressure involves a greater force that displaces the skin and underlying tissue. Time varying tactile stimuli produce more complex sensations such as object movement or object flutter (20 to 50 Hz) or vibration (100 to 300 Hz). An initial clinical examination of discriminative touch often involves testing the vibratory sense by applying a 128 Hz tuning fork over a bony prominence.

Proprioceptive Stimuli.1 Proprioceptive stimuli are internal forces that are generated by the position or movement of a body part. Static forces on the joints, muscles and tendons, which maintain limb position against the force of gravity, indicate the position of a limb. The movement of a limb is indicated by dynamic changes in the forces applied to muscles, tendons and joints. An initial clinical examination of proprioception often involves testing the position sense by having the patient, with eyes closed, touch one finger with another after the target finger has been moved. 

Proprioception is critical for maintaining posture and balance. Somatosensory proprioceptive cues are combined with vestibular proprioceptive cues and visual cues to control motor responses to changes in body/head position. During a clinical examination, the Romberg test requires the patient to maintain balance while standing with feet together and eyes closed. It tests whether the proprioceptive components are working properly when the visual cues are missing and proprioceptive cues are the major sources of information. 

Sharp Cutting Pain Stimuli. Painful (nociceptive) stimuli are tissue-damaging sources of energy that may be external or internal to the body surface. Sharp, cutting pain is the sensation elicited on initial contact with the painful stimulus. The sensation of dull, burning pain may follow as a consequence of tissue inflammation. An initial clinical examination of the pain sense often involves testing sharp, cutting pain sensitivity by asking the patient, who has her/his eyes closed, what they feel when pricked with a pin. Pain mechanisms and pathways are described in detail in later chapters.

2.2 Introduction to Peripheral Organization of Somatosensory Systems

Peripheral Somatosensory Neurons. The cell bodies of the first-order (1°) somatosensory afferent neurons2 are located in posterior root or cranial root ganglia (i.e., are part of the peripheral nervous system, Figure 2.1). The 1° afferents 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.

Somatosensory Receptor Organ. 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. These sensory receptor organs "house" the somatosensory receptors and deliver the somatosensory stimuli to the receptors.

Sensory Receptors. Specialized sensory receptor cells (e.g., the photoreceptors of the eye) are located in specialized receptor organs, produce receptor potentials, contain synaptic specializations, and release neural transmitters (Figure 2.2). Specialized sensory receptors may be modified neurons (e.g., the photoreceptors and olfactory receptors) or modified epithelial cells (e.g., taste receptors and the auditory and vestibular hair cells).

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Figure 2.1
The somatosensory first-order (1°) afferent is a pseudounipolar neuron, which has a single process that divides into a peripheral process and a central process. The peripheral process is part of the peripheral nervous system (PNS) and terminates to form or end on a somatosensory receptor in skin, muscle or joint. The central process travels tso the central nervous system (CNS) where it terminates on a spinal cord or brain stem neuron.

 

Figure 2.2
The specialized sensory receptors of the auditory and visual systems. These cells are specialized neurons (A. visual receptors) or specialized epithelial cells (B. auditory receptors) that generate receptor potentials and contain synaptic vesicles.

There is only one type of sensory receptor cell in the somatosensory system, the Merkel cells, and they are found only in skin. The vast majority of somatosensory receptors are not specialized receptor cells. That is, they are formed by the endings of the somatosensory 1° afferent peripheral axon and adjacent tissue (Figure 2.3). There is no synaptic specialization or neurotransmitter within the adjacent tissue. The adjacent tissue also does not generate receptor potentials.

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Figure 2.3
(A) When stimulated, the auditory receptor cell generates a receptor potential (1), which results in the release of neurotransmitter at its synapse with the auditory 1° afferent. The neurotransmitter depolarizes the 1° afferent, which generates action potentials (2 & 3) that travel to the 1° afferent synaptic terminals on 2° afferents in the central nervous system. The 2° afferent generates action potentials (4) in response to the transmitter release by the 1° afferent.

 

(B) Most somatosensory receptors are not specialized receptor cells and are formed by the terminal endings of the somatosensory 1° afferents. It is the 1° afferent terminal that produces a generator potential (1) which, in turn, initiates action potentials (2 & 3) in the 1° afferent axon. The 1° afferent releases neurotransmitter on 2° afferents in the central nervous system. The 2° afferent generates action potentials (4) in response to the transmitter by the 1° afferent.

Instead of ending on specialized receptors, most peripheral axons of somatosensory 1° afferents travel to skin, muscle or joint, branch near their terminal sites, and end in the skin (Figure 2.4), muscle, tendon or joint tissue.

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Figure 2.4
The primary (1°) somatosensory afferent neuron. The 1° afferent's cell body is located in the ganglion of a cranial or posterior (spinal) nerve root. The 1° afferent's peripheral process travels to skin, muscle or joint - where it branches into terminal fibers. Each terminal fiber forms, or ends on, a somatosensory receptor. The 1° afferent's central process joins a cranial or spinal nerve and enters the brain stem or spinal cord - where it synapses with a 2° somatosensory neuron.

All the peripheral terminal branches of a 1° somatosensory axon end in a specific type of tissue (e.g., skin) and not in multiple types of tissue (i.e., not in skin and muscle). All the peripheral terminal branches of a 1° axon form only one type of somatosensory receptor.

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Figure 2.5
The locations of somatosensory receptors in the body.

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. The complex of encapsulated tissue and afferent endings and the complex of hair follicle and afferent endings play a role in the receptor transduction process, and each complex is considered to form a "somatosensory receptor". Many other 1° somatosensory axons branch and terminate in skin, muscle, or joint as free nerve endings. These endings are bare of myelin, are not encapsulated and are not associated with a specific type of tissue.

The sensitivity of the receptors to specific stimuli (e.g., touch verses muscle stretch) is determined by the location of the receptor and by the non-neural tissue surrounding the 1° afferent terminal (Figure 2.6).

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Figure 2.6
The locations of cutaneous (somatosensory) receptors in hairy and non-hairy (glabrous) skin.

2.3 Sensory Transduction

The Adequate Stimulus. The adequate somatosensory stimulus (i.e., the stimulus to which a somatosensory neuron is most sensitive) is either a mechanical force, a temperature change, tissue damage, or a chemical action. The discriminative touch and proprioceptive systems are most sensitive to mechanical force. Consequently, their sensory receptors are of the mechanoreceptor category.

Sensory Transduction. The non-neural tissue surrounding the peripheral ending of the somatosensory 1° afferent helps concentrate and deliver the stimulus (e.g., mechanical force) onto the 1° afferent terminal membrane. Somatosensory mechanoreceptors function to transduce the applied mechanical force into an electrical potential change in the 1° afferent neuron.

The mechanoreceptor 1° afferent terminal membrane contains ion channels that respond to mechanical distortion by increasing sodium and potassium conductance (i.e., the channels are stress gated). Generator potentials are produced as sodium and potassium flow down their electrochemical gradients to depolarize the terminal ending (see Figure 2.3B). In most cases, the magnitude and duration of the generator potentials are related to the applied mechanical force: the greater the mechanical force, the greater is the depolarization, and the longer the mechanical force is applied, the longer the terminal remains depolarized (Figure 2.7). Terminals that do not sustain the depolarization for the duration of the mechanical distortion are called rapidly adapting. Terminals that sustain the depolarization with minimal decrease in amplitude for the duration of a stimulus are called slowly adapting.

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Figure 2.7
At the TOP of this figure, two 1° somatosensory neurons are illustrated. A mechanical force (A) is applied and the responses are measured by a recording electrode in the somatosensory receptor (B), and a recording electrode in the axon (C). BELOW The responses of somatosensory 1° afferent neurons to stimulation of the receptor with a sustained stimulus are illustrated for rapidly adapting afferents (LEFT panel) and slowly adapting afferents (RIGHT panel). The time course of the applied force or skin displacement (A); generator potential recorded in the receptor (B); and the action potentials recorded from the 1° afferent axon (C) are illustrated. Notice that the Ruffini corpuscle and Merkel disk and their 1° afferent responses are best suited to transduce and transmit information about long-lasting (maintained or sustained) stimuli that do not vary over time.

The generator potential spreads passively along the 1° terminal fiber to the axon trigger zone - that part of the 1° afferent axon containing voltage-sensitive sodium and potassium channels (see Figure 2.3B). If the depolarization reaches threshold at these voltage-sensitive sites, action potentials are generated by the 1° afferent peripheral axon. When the action potentials reach the central terminals of the 1° afferent, they initiate the release neurotransmitters on 2° afferents within spinal cord or brain stem nuclei. If, as in the example in Figure 2.8, the generator potential is slowly adapting, the 1° afferent produces a sustained discharge of action potentials that continue for the duration of the stimulus.

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Figure 2.8
Stretching the Ruffini corpuscle produces a slowly adapting (sustained) generator potential in the 1° afferent terminal that degrades slowly for the duration of the stretch. If the force applied to the 1° afferent terminal produces a generator potential that is of sufficient amplitude at the axon trigger zone, a train of action potentials is generated that travel along the axon to the terminals of the its central process. The action potentials in the central terminals initiate the release of neurotransmitters on 2° somatosensory afferent neurons within the central nervous system, which results in a discharge of the 2° afferent.

If the generator potential is rapidly adapting (Figure 2.9), the 1° afferent produces a transient, short burst of action potentials and falls silent even in the continued presence of the stimulus.

Figure 2.9
Bending a hair produces a rapidly adapting discharge of action potentials in the 1° afferent axon that does not last the duration of the bending force. If the force applied to the 1° afferent terminal produces a generator potential that is of sufficient amplitude at the axon trigger zone, one or more action potentials are generated that travel to the terminals of the 1° afferent central process. The action potentials in the central terminals initiate the release of neurotransmitters on 2° somatosensory afferent neurons within the central nervous system. The 1° afferent axon response is rapidly adapting and action potentials are only generated when the hair is bent.

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The rapidly adapting receptors produce generator potentials and action potential discharges that follow the time-varying waveform of pressure changes produced by a vibrating stimulus (Figure 2.10, left panel). In contrast, the slowing adapting receptors produce generator potentials and action potential discharges that are sustained and unable to mimic the time-varying pattern of the stimulus (Figure 2.10, right panel). Consequently, the responses of rapidly adapting 1° afferents are best suited for representing time varying (e.g., vibrating or moving) stimuli, whereas slowly adapting 1° afferents better represent static stimuli (e.g., sustained pressure).

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Figure 2.10
At the TOP of this figure, two 1° somatosensory neurons are illustrated; each in contact with a mechanical force (A), a recording electrode in the somatosensory receptor (B), and a recording electrode in the axon (C). BELOW The responses of the somatosensory 1° afferents to stimulation of the receptor with a vibrating stimulus are illustrated for rapidly adapting afferents (LEFT panel) and slowly adapting afferents (RIGHT panel). The time course of the applied force or skin displacement (A); generator potential recorded in the receptor (B); and the action potentials recorded from the afferent axon are illustrated (C). Notice that the Pacinian and Meissner corpuscles and their 1° afferent responses are best suited to transduce and transmit information about time-varying (vibrating or moving) mechanical stimuli.

2.4 Somatosensory Receptor Types

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Figure 2.11
The locations of cutaneous receptors. Click on the somatosensory receptor name (in green shaded area) to view a detailed drawing of the receptor. The location of the receptor will be circled in the larger drawing of the skin.

Cutaneous Receptors

Some of the somatosensory receptors in skin (i.e., the cutaneous receptors) are classified as encapsulated receptors as the 1° afferent terminal and surrounding cutaneous tissue are encapsulated by a thin sheath (Table II). The encapsulated cutaneous receptors include Meissner corpuscles, Pacinian corpuscles and Ruffini corpuscles (See Figure 2.11). Other cutaneous receptors are unencapsulated and include the 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). The sensory receptors of the crude touch, pain and temperature senses are bare or free nerve endings. That is, they are unencapsulated, do not end on or near specialized tissue, and may be mechanoreceptors, nociceptors or thermoreceptors.

As was noted earlier, the sensitivity (modality specificity) of the somatosensory receptor is determined by its location and by the structure of the non-neural tissue surrounding the 1° afferent terminal. The following describes the most commonly observed cutaneous receptors.

Meissner Corpuscle. The Meissner corpuscle is found in glabrous (i.e., hairless) skin, within the dermal papillae (Figure 2.11). It consists of an elongated, encapsulated stack of flattened epithelial (laminar) cells with 1° afferent terminal fibers interdigitated between the cells (Figure 2.12).

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Figure 2.12
The Meissner corpuscle consists of an encapsulated stack of flattened epithelial (laminar) cells with 1° afferent terminals interdigitated between these cells. The Meissner corpuscle is located within the dermal papilla, near the surface of the skin, with its long axis perpendicular to the skin surface.

A force applied to non-hairy skin (Figure 2.13) causes the laminar cells in the Meissner corpuscle to slide past one another, which distorts the membranes of the axon terminals located between these cells. If the force is maintained, the laminar cells remain in a fixed, albeit, displaced position, and the shearing force on the axon terminals' membranes disappears. Consequently, the 1° afferent axons produce a transient, rapidly adapting response to a sustained mechanical stimulus.

Figure 2.13
When a force is applied to the dermal papilla containing the Meissner corpuscle, the laminar cells in the corpuscle slide past one another. This shearing force distorts the membranes of the axon terminals located between the laminar cells, which depolarizes the axon terminals. If the force is sustained on the dermal papilla, the laminar cells remain in their displaced positions and no longer produce a shearing force on the axon terminals. Consequently, a sustained force on the dermal papilla is transformed into a transient force on the axon terminals of the Meissner corpuscle. The 1° afferent axon response of a Meissner corpuscle is rapidly adapting and action potentials are only generated when the force is first applied.

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The Meissner 1° afferent discharges "follow" low frequency vibrating (30 -50 Hz) stimuli, which produces the sensation of "flutter" (Figure 2.10, left panel). Because a single 1° afferent axon forms many, dispersed (3-4 mm) Meissner corpuscles, the 1° afferent can detect and signal small movements across the skin. Stimulation of a sequence of Meissner corpuscles have been described to produce the perception of localized movement along the skin.

Consequently, Meissner corpuscles are considered to be the discriminative touch system's flutter and movement detecting receptors in non-hairy skin.

Pacinian Corpuscle. Pacinian corpuscles are found in subcutaneous tissue beneath the dermis (Figure 2.9) and in the connectivetissues of bone, the body wall and body cavity. Therefore, they can be cutaneous, proprioceptive or visceral receptors, depending on their location.

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Figure 2.14
The Pacinian corpuscle consists of a single, centrally placed 1° afferent terminal that is surrounded by concentrically layered epithelial (laminar) cells that are all encapsulated within a sheath. In skin, the Pacinian corpuscle is located deep in the subcutaneous adipose tissue.

The Pacinian corpuscle is football-shaped, encapsulated, and contains concentrically layered epithelial (laminar) cells (Figure 2.14). In cross section, the Pacinian corpuscle looks like a slice of onion, with a single 1° afferent terminal fiber located in its center. The outer layers of laminar cells contain fluid that is displaced when a force is applied on the corpuscle.

When a force is first applied on the Pacinian corpuscle (Figure 2.15), it initially displaces the laminar cells and distorts the axon terminal membrane. If the external pressure is maintained on the corpuscle, the displacement of fluid in the outer laminar cells dissipates the applied force on the axon terminal. Consequently, a sustained force on the corpuscle is transformed into a transient force on the axon terminal, and the Pacinian corpuscle 1° afferent produces a fast adapting response.

Figure 2.15
When a force is applied to the tissue overlying the Pacinian corpuscle (press PLAY), its outer laminar cells, which contain fluid, are displaced and distort the axon terminal membrane. If the pressure is sustained on the corpuscle, the fluid is displaced, which dissipates the applied force on the axon terminal. Consequently, a sustained force on the Pacinian corpuscle is transformed into a transient force on its axon terminal. The Pacinian corpuscle 1° afferent axon response is rapidly adapting and action potentials are only generated when the force is first applied.

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Pacinian corpuscles 1° afferent axons are most sensitive to vibrating stimuli (e.g., a tuning fork vibrating at 100 to 300 Hz, Figure 2.10, left) and unresponsive to steady pressure. The sensation elicited when cutaneous Pacinian corpuscles are stimulated is of vibration or tickle.

Pacinian corpuscles in skin are considered to be the vibration sensitive receptors of the discriminative touch system.

Ruffini Corpuscle. The Ruffini corpuscles are found deep in the skin (Figure 2.11), as well as in joint ligaments and joint capsules and can function as cutaneous or proprioceptive receptors depending on their location. The Ruffini corpuscle (Figure 2.16) is cigar-shaped, encapsulated, and contains longitudinal strands of collagenous fibers that are continuous with the connective tissue of the skin or joint. Within the capsule, the 1° afferent fiber branches repeatedly and its branches are intertwined with the encapsulated collagenous fibers.

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Figure 2.16
The Ruffini corpuscle consists of 1° afferent terminal fibers that are intertwined with collagenous fibers and together with the collagenous fibers are encapsulated in a fibrous sheath. The Ruffini corpuscles are oriented parallel to the skin surface and situated deep within the dermis.

The Ruffini corpuscles are oriented with their long axes parallel to the surface of the skin and are most sensitive to skin stretch. Stretching the skin (Figure 2.17) stretches the collagen fibers within the Ruffini corpuscle, which compresses the axon terminals. As the collagen fibers remain stretched and the axon terminals remain compressed during the skin stretch, the Ruffini corpuscle's 1° afferent axon produces a sustained slowly adapting discharge to maintained stimuli.

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Figure 2.17
When the skin is stretched, the collagen fibers in the Ruffini corpuscles are also stretched and compress their 1° afferent terminals. As the collagen fibers remain stretched and the axon terminals remain compressed during the skin stretch, the Ruffini corpuscle 1° afferent axon produces a sustained generator potential and a slowly adapting discharge to maintained stimuli.

Ruffini corpuscles in skin are considered to be skin stretch sensitive receptors of the discriminative touch system. They also work with the proprioceptors in joints and muscles to indicate the position and movement of body parts.

Hair Follicle. The hair follicle receptor is an unencapsulated cutaneous receptor (Figure 2.10). The 1° afferent terminal axons spiral around the hair follicle base or run parallel to the hair shaft forming a lattice-like pattern (Figure 2.18).

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Figure 2.18
The hair follicle 1° afferent terminal fibers enter the follicle to encircle or to form a lattice pattern around the hair shaft.

Most hair follicle 1° afferents are the fast-adapting type; displacement of the hair produces a transient discharge of action potentials at the onset of the displacement and a maintained displacement of the hair often fails to produce a sustained discharge (Figure 2.19). The hair follicle afferents respond best to moving objects and signal the direction and velocity of the movement of a stimulus brushing against hairy skin.

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Figure 2.19
Bending a hair produces a transient force on the hair follicle base as the entire follicle is displaced by the bending force. The 1° afferent terminal may produce a rapidly adapting generator potential and the 1° afferent axon a transient discharge of action potentials — even to sustained bending of the hair.

 

As Meissner corpuscles are absent from hairy skin, the hair follicle endings are considered to be the discriminative touch system's movement sensitive receptors in hairy skin.

Merkel Complex. The Merkel complex is found in both hairy and non-hairy skin and is located in the basal layer of the epidermis (Figure 2.11). The Merkel complex is unencapsulated and consists of a specialized receptor cell, the Merkel cell, and a 1° afferent terminal ending, the Merkel disk3 (Figure 2.20). Thick, short, finger-like protrusions of the Merkel cell couple it tightly to the surrounding tissue. The Merkel cell is a modified epithelial cell, which contains synaptic vesicles that appear to release neuropeptides that modulate the activity of the 1° afferent terminal. Each 1° afferent axon often innervates only a few Merkel cells in a discrete patch of skin (Figure 2.18).

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Figure 2.20
The Merkel complex consists of a specialized Merkel cell, which contains synaptic vesicles, and the Merkel disk ending of a 1° afferent terminal fiber. A single 1° afferent axon often innervates only a few Merkel cells within a discrete patch of skin.

A force applied to the skin overlying the Merkel cell distorts it (Figure 2.21), which stimulates its release of a neuropeptide at its synaptic junctions with the Merkel disk. As the Merkel cell is mechanically coupled to the surrounding skin, it remains distorted for the duration of the force applied on the overlying skin. Consequently, the Merkel complex 1° afferent axon responds to small forces applied to a discrete patch of skin with a slowly adapting, sustained discharge.

Figure 2.21
The Merkel cell is coupled to the surrounding tissue and cannot shift its position relative to the surrounding tissue. Consequently, a force applied to the overlying skin (press PLAY), distorts the Merkel cell for the duration of the applied force. The distortion of the Merkel cell results in the release of a stream of neuropeptides at its synaptic junctions with the 1° Merkel disk. As a result the action potential discharges produced by the Merkel complex 1° afferent is slowly adapting.

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Merkel cells are considered to be the fine tactile receptors of the discriminative touch system that provide cues used to localize tactile stimuli and to perceive the edges (shape or form) of objects.

Free Nerve Endings. Free nerve endings are found throughout the body, in skin (Figure 2.11), muscles, tendons, joints, mucous membranes, cornea, body mesentery, the dura, the viscera, etc. The free nerve endings in skin are stimulated by tissue-damaging (nociceptive) stimuli that produce the sensation of pain or by cooling of the skin or the warming of skin or by touch. Notice that although all cutaneous free nerve endings appear very similar morphologically, there are different functional types of free nerve endings, with each responding to specific types of cutaneous stimuli (e.g., nociceptive, cooling, warming or touch).

Free nerve endings are considered to be the somatosensory receptors for pain, temperature and crude touch.

Table II
Cutaneous Receptors
Receptor Type Sensation Signals Adaptation
Meissner
corpuscle 
Encapsulated
& layered 
Touch: Flutter & Movement Frequency/Velocity & Direction  Rapid
Pacinian
corpuscle 
Encapsulated
& layered 
Touch: Vibration  Frequency: 100-300 Hz  Rapid
Ruffini
corpuscle 
Encapsulated
collagen 
Touch: Skin Stretch  Direction & Force Slow
Hair follicle Unencapsulated  Touch: Movement  Direction &
Velocity
Rapid
Merkel
complex 
Specialized
epithelial cell 
Touch, Pressure, Form  Location & Magnitude Slow
Free Nerve
Ending 
Unencapsulated  Pain, Touch, or Temperature  Tissue damage, Contact, or Temperature change Depends on information carried

2.5 Proprioceptive Receptors 

Proprioceptors are located in muscles, tendons, joint ligaments and in joint capsules. There are no specialized sensory receptor cells for body proprioception4. In skeletal (striated) muscle, there are two types of encapsulated proprioceptors, muscle spindles and Golgi tendon organs (Figure 2.22), as well as numerous free nerve endings. Within the joints, there are encapsulated endings similar to those in skin, as well as numerous free nerve endings.

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Figure 2.22
A muscle spindle receptor and Golgi tendon organ in the bicep muscle.

Muscle Spindles. Muscle spindles are found in nearly all striated muscles. A muscle spindle is encapsulated and consists of small muscle fibers, called intrafusal muscle fibers, and afferent and efferent nerve terminals (Figure 2.23).

Figure 2.23
A muscle spindle with its sensory and motor innervation. The primary muscle spindle afferent terminates as annulospiral endings in the central area of the intrafusal muscles whereas the secondary muscle spindle afferent terminates as flower spray endings in more polar regions of intrafusal muscles. The motor endplates of gamma motor neurons are located in the polar regions. The muscle spindle is attached to the surrounding extrastriate muscles and lays with its long axis in parallel with the long axes of the surrounding muscle. 

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Intrafusal muscles are found exclusively in muscle spindle receptors and are distributed throughout the body among the ordinary extrafusal muscle fibers of skeletal muscles. The intrafusal fibers are attached to the larger, surrounding extrafusal muscle fibers. They are oriented in parallel with the extrafusal fibers but do not contribute directly to muscle strength when they contract because of their small size. 

There are two types of afferent terminals in the muscle spindle (Figure 2.23). The annulospiral endings wrap around the central region of the intrafusal fibers, whereas the flower-spray endings terminate predominantly in more polar regions (away from the central area) of the intrafusal fibers. The 1° afferents forming the annulospiral endings are called the primary muscle spindle afferents, whereas those forming the flower-spray endings are called the secondary muscle spindle afferents.

In addition to afferent terminals, the terminals (motor endplates) of gamma motor neurons end on intrafusal muscle fibers. They will be described in detail in the chapters covering motor systems.

In summary, 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. Muscle spindles are most numerous in muscles that carry out fine movements, such as the extraocular muscles and the intrinsic muscles of the hand. There are fewer spindles in large muscles that control gross movements of the body (e.g., the muscles of the back). 

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Figure 2.24
The Golgi tendon organ is located at the junction of muscle and tendon. The Golgi tendon organs resemble the Ruffini corpuscles. That is, the 1° afferent terminal fibers are intertwined with collagenous fibers of the tendon and the entire organ is encapsulated in a fibrous sheath.

Golgi Tendon Organs. Golgi tendon organs are found in the tendons of striated extrafusal muscles near the muscle-tendon junction (Figure 2.22). Golgi tendon organs resemble Ruffini corpuscles. For example, 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 (Figure 2.24). They are functionally "in series" with striated muscle.

The Golgi tendon organ collagen fibers are continuous with the extrafusal muscle at one end and with the muscle tendon at its opposite end. Consequently, the mechanical force on the organ is maximal when the extrafusal muscles contract, shorten, and increase the tension on the tendon. When the muscles contract, the 1° afferent terminals are compressed and remain compressed as long as the muscle remains contracted. The Golgi tendon organ 1° afferent response to sustained isometric muscle contraction is slowly adapting, and the 1° afferent generates action potentials as long as the tension is maintained. The responses of the Golgi tendon organ 1° afferent axon is maximal when the contracted muscle bears a load, e.g., when lifting a heavy object.

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). 

Joint Receptors. Joint receptors are found within the connective tissue, capsule and ligaments of joints (Figure 2.25). The encapsulated endings resemble the Ruffini and Pacinian corpuscles and the Golgi tendon organs.

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Figure 2.25
The joint receptors are free nerve endings and encapsulated endings in the joint capsule and joint ligaments. The encapsulated receptors in the joint capsule resemble Pacinian and Ruffini endings whereas those in the ligaments resemble Golgi tendon organs.

The joint 1° afferents respond to changes in the angle, direction, and velocity of movement in a joint. The responses are predominantly rapidly adapting with few joint 1° afferents signaling the resting (static) position of the joint. It has been suggested that information from muscles, tendons, skin and joints are combined to provide estimates of joint position and movement. For example, when the hip joint is replaced — removing all joint receptors — the ability to detect the position of the thigh relative to the pelvis is not lost. 

Free Nerve Endings. As mentioned above, free nerve endings of 1° afferents 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.

Table III
Receptor Type Sensation Signals Adaptation
Muscle Spindle  Encapsulated annulospiral and flower spray
endings 
Muscle 
stretch 
Muscle
length & velocity 
Rapid initial transient and slow sustained
Muscle: Golgi
Tendon Organ 
Encapsulated
collagen 
Muscle tension  Muscle
contraction
Slow
Joint:
Pacinian 
Encapsulated
& layered 
Joint Movement  Direction & velocity Rapid
Joint:
Ruffini 
Encapsulated
collagen 
Joint pressure  Pressure & Angle Slow
Joint: Golgi
Organ 
Encapsulated
collagen 
Joint torque  Twisting force Slow

2.6 Summary

In this chapter, you have learned about somatosensory stimuli and the receptors of three components of the somatosensory systems. These three components provide accurate information about the location, shape, texture, and movement of tactile stimuli, (discriminative touch), the position and movement of body parts (proprioception) and the application and location of painful stimuli (nociception). Tactile and proprioceptive stimuli are the mechanical forces produced when skin contacts external objects (discriminative touch), limbs oppose the force of gravity (body position) and muscles contract and body parts move. Painful stimuli are tissue-damaging forces. The sensations produced are those of touch, pressure, flutter, and vibration/movement (discriminative touch), body position and movement (proprioception), and sharp cutting pain. The discriminative touch receptors are encapsulated 1° afferent terminals (Meissner, Pacinian and Ruffini corpuscles), hair follicle endings and Merkel complexes in skin. The proprioceptive receptors in muscle are also encapsulated and include the muscle spindle and Golgi tendon organ. The joint receptors are similar to the encapsulated endings in skin and tendon and are found in the joint capsule and ligaments. The sharp cutting nociceptors are free nerve endings. 

Although it is convenient to subdivide somatosensory receptors and pathways for didactic, clinical and research purposes, it is important to keep in mind that most somatosensory stimuli act simultaneously and in varying degrees on all somatosensory receptors in the body part stimulated. For example, placing a heavy, cold object in an outstretched hand produces tactile, thermal, and proprioceptive sensations that allow us to appreciate the presence (touch, pressure), temperature, and weight of the object and provide proprioceptive information for finger, wrist and arm adjustments so we do not drop the object.

Test Your Knowledge

Make the best match between the receptor type and the sensation elicited when the receptor is stimulated.

  • Golgi tendon organ
  • A
  • B
  • C
  • D
  • E

A. Fine touch

B. Vibration

C. Flutter

D. Muscle tension

E. Muscle length

A. Fine touch This is an INCORRECT match.

B. Vibration

C. Flutter

D. Muscle tension

E. Muscle length

A. Fine touch

B. Vibration This is an INCORRECT match.

C. Flutter

D. Muscle tension

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter This is an INCORRECT match.

D. Muscle tension

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle tension This is the CORRECT match!

Golgi tendon organs are stimulated during muscle tension (contraction against a force), whereas the muscle spindles are stimulated during muscle stretch.

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle tension

E. Muscle length This is an INCORRECT match.

 

 

 

 

 

 

 

  • Meissner corpuscle
  • A
  • B
  • C
  • D
  • E

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch This is an INCORRECT match.

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration This is an INCORRECT match.

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter This is the CORRECT match!

Meissner corpuscle responds to time varying stimuli with frequency much below 100 cps.

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions This is an INCORRECT match.

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length This is an INCORRECT match.

 

 

 

 

 

 

 

  • Merkel complex
  • A
  • B
  • C
  • D
  • E

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch This is the CORRECT match!

Merkel complex responds to localized, static tactile stimuli.

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration This is an INCORRECT match.

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter This is an INCORRECT match.

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions This is an INCORRECT match.

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length This is an INCORRECT match.

 

 

 

 

 

 

 

  • Free nerve endings
  • A
  • B
  • C
  • D
  • E

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Pain

A. Fine touch This is an INCORRECT match.

B. Vibration

C. Flutter

D. Muscle contractions

E. Pain

A. Fine touch

B. Vibration This is an INCORRECT match.

C. Flutter

D. Muscle contractions

E. Pain

A. Fine touch

B. Vibration

C. Flutter This is an INCORRECT match.

D. Muscle contractions

E. Pain

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions This is an INCORRECT match.

E. Pain

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Pain This is the CORRECT match!

 

 

 

 

 

 

 

  • Pacinian corpuscle
  • A
  • B
  • C
  • D
  • E

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch This is an INCORRECT match.

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration This is the CORRECT match!

Pacinian complex are most responsive to time varying stimuli with frequency between 100 to 300 cps.

C. Flutter

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter This is an INCORRECT match.

D. Muscle contractions

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions This is an INCORRECT match.

E. Muscle length

A. Fine touch

B. Vibration

C. Flutter

D. Muscle contractions

E. Muscle length This is an INCORRECT match.

 

 

 

 

 

 

 

 

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