Somatosensory System Part 1

General Organization of Sensory Systems

Our knowledge of the environment around us depends on the information that we receive from peripheral receptors that are specialized nerve endings of sensory neurons. The major sensory systems include somatic, visual, auditory, vestibular, taste, and olfactory (smell) systems. In this topic, the somatosensory system is described. The sensory neurons in each system project centrally, where they make synaptic contact with the second-order neurons that, in turn, project to higher order neurons. The different components of sensory systems and the terminology associated with them are described in the following sections.

Sensory Receptors

Initial contact with our environment occurs at the sensory receptors, which are specialized neural structures. The sensations experienced by the peripheral receptors include touch, position of the body, pain, sight, sound, smell, and taste. Each stimulus has the following characteristics.

Modality

The term modality refers to the type of stimulus (e.g., mechanical, thermal, chemical, visual, or auditory) that activates sensory receptors.

Intensity

The strength of the stimulus determines the intensity of sensation. The smallest intensity at which a particular sensation is detected is called the sensory threshold.

Duration

Duration is the period of time the sensory stimulation continues. Usually, intensity of the sensation diminishes when the stimulus is continuous for an extended period of time. This is called adaptation.


Location

Awareness of the sensory experience includes the ability of the subject to identify the site of stimulation and the ability to distinguish between stimuli that are applied at close distances. The ability to distinguish between two stimuli applied at a close distance is determined by measuring the minimum distance between the two stimuli; this measure has been defined as two-point discrimination. There is a marked variation in two-point discrimination in different regions of the skin. For example, when a pair of calipers is applied to the skin on a fingertip, the sensation induced by each prong is felt even when the distance between the two prongs is very small (e.g., 5 mm). However, when the pair of calipers is applied to the forearm, the distance between the two prongs has to be at least 40 mm in order to feel sensations induced by each prong. This difference in two-point discrimination is explained by the observation that the mechanoreceptors that respond to two-point discrimination are much more numerous in the skin on the fingertips than on the forearm.

Stimulus Transduction

The sensory receptor converts a stimulus into neural activity; this conversion involves a process called stimulus transduction. A stimulus induces a generator (or receptor) potential in the receptor membrane.Usually, the stimulus depolarizes the membrane by opening channels, thus selectively permitting influx of Na+ (sodium) and efflux of K+ (potassium). Specific characteristics, such as intensity and duration, are converted into specific patterns of action potentials that are called neural codes. For example, an increase in the intensity of the stimulus elicits an increase in the magnitude of the receptor potential, which, in turn, produces an increased rate and number of action potentials. Similarly, an increase in the duration of the stimulus usually decreases the amplitude of the generator (receptor) potential that, in turn, results in the adaptation of the response. The adaptation of the response may be rapid (e.g., Pacinian corpuscle) or slow (e.g., Merkel’s receptor). These receptors are discussed later in this topic under "Tactile Sensations."

Receptive Field

The space in which the sensory receptor is located and where it produces the transduction of the stimuli is called the receptive field of the receptor.

Relay Nuclei

The thalamus contains a number of relay nuclei that serve to transmit the sensory information to different sensory receiving areas of the cerebral cortex. One exception to this general rule is that sensory information from the olfactory system can be transmitted to the prefrontal cortex through pathways that do not make a synapse in the thalamus.

Cortical Mechanisms

The sensory areas of the cerebral cortex play a critical role in the perception of the sensation.

Classification of Nerve Fibers

Nerve fibers have been divided into the following four groups: (1) myelinated fibers from annulospiral endings of the muscle spindle (type Ia or Aa fibers) or myelinated fibers from Golgi tendon organs (type Ib or Aa fibers); (2) myelinated fibers from flower-spray endings of the muscle spindles (type II or Ap fibers); (3) myelinated fibers that conduct crude touch, temperature, and pain sensations (type III or AS fibers); and (4) unmyelinated fibers that carry pain and temperature sensations (type IV or C fibers). The nomenclature, diameters, and conduction velocities of these fibers are listed in Table 15-1.

Somatosensory System

Sensory Modalities

Sensations mediated by this system include tactile sensations (touch, pressure, and vibration); perception of joint position, joint movements, and direction and velocity of joint movements (conscious proprioception or kinesthesia); nonconscious proprioception (sensations mediated by muscle spindles and Golgi tendon organs); pain; and temperature.

Tactile Sensations (Touch, Pressure, and Vibration)

Receptors. Skin is important for tactile sensations. There are two types of skin: hairy (e.g., skin on the back of the hand) and hairless (glabrous) skin (e.g., skin on the palms of the hand). The cutaneous and deeper subcutaneous mechanoreceptors (Table 15-2) respond to external stimuli. Different receptors mediating tactile sensations are shown in Figure 15-1 and described in the following sections.

Hair Follicles. Each hair grows from a hair follicle, which is embedded in skin and innervated by nerve endings that surround it or run parallel to it (Fig. 15-1). When a hair is bent, a deformation of the follicle and the tissue around it activates adjacent nerve endings.

TABLE 15-1 Classification of Nerve Fibers

Nerve fiber

Numerical Nomenclature

Alphabetical Nomenclature

Fiber Diameter (mm)

Conduction Velocity (meters/sec)

Examples

Myelinated

Ia

tmp15F-46

17 (approx)

80-120

Fibers from annulospiral endings of the muscle spindle

Ib

tmp15F-47

16 (approx)

80-120

Fibers from Golgi tendon organs

II

tmp15F-48

8 (approx)

35-75

Fibers from flower-spray endings of the muscle spindles; cutaneous tactile receptors

III

tmp15F-49

1-5

5-30

Fibers conducting crude touch, temperature, and pain sensations

Unmyelinated

IV

tmp15F-50

0.2-1.5

0.5-2

Fibers carrying pain and temperature sensations

TABLE 15-2 Mechanoreceptors

Mechanoreceptor Type Receptor

Function

Examples

Cutaneous and subcutaneous: involved in touch, pressure, and vibration

Meissner’s corpuscle (low-threshold, rapidly adapting); found in glabrous skin

Touch, vibration below 100 Hz

Merkel’s receptor (low-threshold, slowly adapting); found in glabrous skin

Pressure

Pacinian corpuscle* (low-threshold, rapidly adapting); found in both hairy and glabrous skin

Rapid indentation of skin, e.g., the sensation caused by a high frequency vibration (100-400 Hz)

Ruffini’s corpuscle (low-threshold, slowly adapting); found in both hairy and glabrous skin

Magnitude and direction of stretch

Muscle mechanoreceptors

Muscle spindles

Limb proprioception

Golgi tendon organ

Limb proprioception

*Pacinian corpuscle is also present in the mesentery.

Meissner’s Corpuscles. This receptor consists of stacks of horizontally flattened epithelial cells enclosed in a connective tissue sheath. One to four myelinated axons enter the capsule, the myelin sheath (in case of myelinated axons) terminates, and the axon arborizes among the epithelioid cells. Meissner’s corpuscles are located beneath the epidermis (Fig. 15-1) of the fingers, palm of the hand, plantar surface of the foot, and the toes (glabrous skin). They are low-threshold, rapidly adapting mechanorecep-tors and are sensitive to touch and vibration.

Merkel’s Receptors (Merkel’s Disks). These receptors are located in the skin below the epidermis (Fig. 15-1) especially on the lips, distal parts of the extremities, and external genital organs (glabrous skin). Merkel’s receptors consist of a large epithelial cell in the basal layer of the epidermis that is in close contact with an axon. They are low-threshold, slowly adapting mechanoreceptors, and are sensitive to pressure stimuli.

Pacinian Corpuscles. These receptors are located deep in the dermis layer of both hairy and glabrous skin (Fig. 15-1). For example, these receptors are located in the skin of hands, feet, nipples, and mammary glands. They are also found in the walls of the mesenteries, vessel walls, periosteum, and joint capsules. Pacinian corpuscles consist of concentric lamellae of flattened cells that are supported by collagenous tissue. The spaces between the lamellae are filled with fluid. A myelinated nerve enters the corpuscle, the myelin sheath disappears, and a bare nerve terminal occupies the center of the corpuscle. These receptors are low-threshold and rapidly adapting and are sensitive to rapid indentation of the skin caused by vibration of high frequency.

Ruffini’s Corpuscles (Endings). These receptors are located in the dermis layer of both hairy and glabrous skin (Fig. 15-1) and are widely distributed. They consist of encapsulated bundles of collagen fibrils that are connected with similar fibrils of the dermis. The endings of a sensory axon ramify within the collagen fibrils. Ruffini’s corpuscles are low-threshold, slowly adapting, and sensitive to stretching of the skin. They provide information about the magnitude and direction of stretch.

Proprioception

There are two types of proprioception: conscious and nonconscious.

Conscious Proprioception. Unlike cutaneous mechanoreceptors that provide information in response to external stimuli, proprioceptors respond to mechanical forces generated within the body itself. In conscious proprioception, the receptors located in the joint capsules (proprioceptors) provide sensory information to the cerebral cortex, which, in turn, uses this information to generate conscious awareness of kinesthesia (i.e., the joint position, direction, and velocity of joint movements).

Receptors. Conscious awareness of kinesthesia is believed to depend predominantly on joint receptors. Receptors located in ligaments and joint capsules consist of free nerve endings and encapsulated receptors. The encapsulated joint receptors are low-threshold mechanoreceptors. Some of them are slowly adapting and provide information about the static aspect of kinesthesia (i.e., the ability of an individual to judge the position of a joint without seeing it and without a movement). Other receptors are rapidly adapting and provide information about the dynamic aspect of kinesthesia (i.e., ability of an individual to perceive the movement of a joint and to judge the direction and velocity of its movement).

The receptors mediating tactile senses. Hair follicle: located in the epidermis and dermis. Meissner's corpuscle: sensitive to touch and vibration, located beneath the epidermis. Merkel's receptor (Merkel's disk): mechanoreceptor sensitive to pressure stimuli, located deep to the epidermis. Pacinian corpuscle: receptor sensitive to rapid indentation of the skin caused by vibration of high frequency, located deep in the dermis. Ruffini's corpuscle (ending): located in the dermis and provides information about the magnitude and direction of stretch.

FIGURE 15-1 The receptors mediating tactile senses. Hair follicle: located in the epidermis and dermis. Meissner’s corpuscle: sensitive to touch and vibration, located beneath the epidermis. Merkel’s receptor (Merkel’s disk): mechanoreceptor sensitive to pressure stimuli, located deep to the epidermis. Pacinian corpuscle: receptor sensitive to rapid indentation of the skin caused by vibration of high frequency, located deep in the dermis. Ruffini’s corpuscle (ending): located in the dermis and provides information about the magnitude and direction of stretch.

Anatomical Pathways Conveying Tactile Information From the Body. Tactile sensation and conscious proprioception are mediated by the dorsal column (dorsal or posterior funiculus)-medial lemniscus system (see Fig. 9-7) The cell bodies of sensory neurons that mediate touch and conscious proprioception are located in dorsal root ganglia. The receptors that mediate tactile sensations (Meissner’s, Merkel’s, Pacinian, and Ruffini) and conscious proprio-ception (receptors located in the joints and joint capsules) are specialized endings of the peripheral process of the sensory neurons located in dorsal root ganglia. The central axons of these sensory neurons travel in dorsal roots and enter the dorsal (posterior) funiculus of the spinal cord.Fibers in the gracile and cuneate tracts ascend ipsilaterally in the spinal cord and synapse upon the neurons located in the dorsal column nuclei (nuclei gracilis and cuneatus) of the medulla. Collectively, the axons of these second-order (dorsal column) nuclei pass ventrome-dially and decussate as internal arcuate fibers to form the medial lemniscus. The latter ascends through the medulla, pons, and midbrain and then projects to neurons located in the ventral posterolateral nucleus of the thalamus. These third-order neurons in the thalamus then project to the primary somatosensory cortex of the parietal lobe in a somatotopic manner such that the head region is situated laterally, the upper limb is situated more dorsally, and the leg region is situated medially.

Deficits After Lesions in the Dorsal Column-Medial Lemniscus System. Patients with such lesions have loss of kinesthetic sensation and, thus, are unable to identify the position of their limbs in space when their eyes are closed and do not know if one of their joints is in flexion or extension. In addition, they cannot identify the shape, size, or texture of objects in their hands by means of touch. This deficit is called astereognosis. The opposite is stereognosis, which is defined as the appreciation of shape, size, or texture of objects by means of touch. These patients are unable to maintain steady posture when their eyes are closed or to perceive vibration when it is applied to their body.

Anatomical Pathways Conveying Tactile Information From the Head and Face.

Nonconscious Proprioception. The impulses arising from the proprioceptors mediating this type of sensation (muscle spindles and Golgi tendon organs) are relayed to the cerebellum rather than to the cerebral cortex. Propriocep-tion mediated by muscle spindles is predominantly non-conscious proprioception. These sensations are mediated by the following muscle receptors: muscle spindles and Golgi tendon organs (Table 15-2).

Muscle Spindles. Muscle spindles are present in skeletal (flexor as well as extensor) muscles (Fig. 15-2A). They are more numerous in muscles that control fine movements (e.g., muscles of the hands and speech organs and extraocu-lar muscles). Different components of muscle spindles are shown in Figure 15-2B. Each spindle consists of a connective tissue capsule in which there are 8 to 10 specialized muscle fibers called intrafusal fibers. The intrafusal fibers and the connective tissue capsule in which they are located are oriented parallel to the surrounding skeletal muscle fibers called extrafusal fibers. The intrafusal fibers are innervated by spinal gamma motor neurons, whereas the extrafusal fibers receive motor innervation from alpha motor neurons located in the spinal cord. There are two types of intrafusal fibers. The nuclear chain fiber contains a single row of central nuclei and is smaller and shorter than the nuclear bag fiber. The nuclear bag fiber has a bag-like dilation at the center where a cluster of nuclei is located. Efferent innervation is provided to the polar ends of both types of intrafusal fibers (i.e., nuclear bag and nuclear chain fibers) by efferent axons of gamma motor neurons that are located in the ventral horn of the spinal cord.

Two types of afferents arise from the intrafusal fibers: (1) annulospiral endings (primary afferents), which are located on the central part of the nuclear bag and nuclear chain fibers; and (2) flower-spray endings (secondary afferents), which are located on both types of intrafusal fibers on each end of the annulospiral endings. Annulospi-ral endings are activated by brief stretch or vibration of the muscle, whereas both types of afferent endings (annulospi-ral and flower-spray) are activated when there is a sustained stretch of the muscle. Thus, muscle spindles detect changes in the length of the muscle.

The Stretch Reflex (Myotatic Reflex). When a muscle is stretched during the myotatic reflex, the receptor endings located on the intrafusal fibers of the muscle spindle (flower-spray and annulospiral) are stimulated, impulses are initiated in the afferent nerve fibers (Ia type; see Table 15-1), which project to alpha motor neurons located in the ventral horn of the spinal cord. The alpha motor neurons supply efferents to the extrafusal muscle fibers. Activation of these efferents results in a reflex-induced contraction of the extrafusal fibers. Thus, as a result of activation of muscle spindles, there is a rapid increase in muscle tension that opposes the stretch.

Gamma Motor Neurons. These neurons are interspersed between alpha motor neurons in the ventral horn of the spinal cord. Gamma motor neurons do not receive afferents from muscle spindles. As noted in the section on stretch reflex, when a muscle is stretched, the afferents arising from the muscle spindles are activated, which, in turn, induce a reflex contraction of the parent muscle. Contraction of the parent muscle results in a decrease in the tension of the intrafusal fibers and cessation of activity in the afferents arising from the muscle spindles. However, during voluntary movements, alpha and gamma motor neurons are activated simultaneously by higher centers so that the sensitivity of muscle spindles to stretch is maintained. The mechanism by which gamma motor neurons regulate the sensitivity of muscle spindles is as follows. When gamma motor neurons are activated, the contractile parts of the muscle spindle (intrafusal fibers) attempt to shorten, but the length of the muscle spindle does not change because it is anchored at both ends (i.e., isometric shortening of the muscle spindle occurs). A pull is generated at the equatorial region of the muscle spindle that results in a distortion of the annulospiral and flower-spray endings, and they fire. As stated earlier, muscle spindle afferents synapse on alpha motor neurons, which are activated and elicit a contraction of extrafusal fibers in the homonymous muscle.

(A) Muscle spindles are located deep in the skeletal muscles parallel to the extrafusal muscle fibers, which are innervated by axons of alpha motor neurons. (B) Each spindle consists of a connective tissue capsule containing 8 to 10 intrafusal fibers (nuclear chain and nuclear bag fibers). Spinal gamma motor neurons provide efferent innerva-tion on the ends of the intrafusal fi bers. Note also the primary affer-ents (arising from annulospiral endings) and secondary afferents (arising from the flower-spray endings) located on the intrafusal fibers.

FIGURE 15-2 (A) Muscle spindles are located deep in the skeletal muscles parallel to the extrafusal muscle fibers, which are innervated by axons of alpha motor neurons. (B) Each spindle consists of a connective tissue capsule containing 8 to 10 intrafusal fibers (nuclear chain and nuclear bag fibers). Spinal gamma motor neurons provide efferent innerva-tion on the ends of the intrafusal fi bers. Note also the primary affer-ents (arising from annulospiral endings) and secondary afferents (arising from the flower-spray endings) located on the intrafusal fibers.

Golgi Tendon Organ. These high-threshold receptors are located at the junction of the muscle and tendon. Golgi tendon organs are arranged in series with the muscle fibers, in contrast to muscle spindles, which are arranged parallel to the extrafusal muscle fibers. A tendon is composed of fascicles of collagenous tissue that are enclosed in a connective tissue capsule. A Golgi tendon organ consists of a large myelinated fiber that enters the connective tissue capsule of a tendon and subdivides into many unmyeli-nated receptor endings that intermingle and encircle the collagenous fascicles. Active contraction of the muscle or stretching of the muscle activates the Golgi tendon organs. Thus, Golgi tendon organs are sensitive to increases in muscle tension caused by muscle contraction. Unlike muscle spindles, they do not respond to passive stretch.This afferent fiber makes an excitatory synapse with an interneuron that then inhibits the alpha motor neuron, which innervates the homonymous muscle group. The net effect is that the period of contraction of the muscle in response to a stretch is reduced. This type of response (i.e., reduction of contraction of homonymous muscle) elicited by stimulation of Golgi tendon organs is referred to as the inverse myotatic reflex.

Anatomical Pathways. The sensation of limb proprioception is mediated by two pathways: the posterior (dorsal) spinocerebellar tract (see Fig. 9-8) and anterior (ventral) spinocerebellar tract (see Fig. 9-9). In both of these pathways, the cell bodies of the first-order sensory neurons, which mediate these senses, are located in dorsal root ganglia. As noted earlier, muscle spindles and Golgi tendon organs are specialized endings of the peripheral process of the sensory neurons located in dorsal root ganglia.

Dorsal (Posterior) Spinocerebellar Tract. The anatomical course of this tract is shown in Fig. 9-8, and its function is described here. The peripheral processes of the first-order neurons (located in dorsal root ganglia) innervate mainly muscle spindles and, to a lesser extent, Golgi tendon organs. The central axons of the sensory neurons of muscle spindles travel in dorsal roots and project to the ipsilateral nucleus dorsalis of Clarke in the spinal cord (i.e., the tract is uncrossed). Axons arising from these second-order neurons in the nucleus dorsalis of Clarke form the dorsal (posterior) spinocerebellar tract. This tract ascends ipsilaterally in the lateral funiculus of the spinal cord and then the lateral aspect of the medulla. It reaches the cerebellum via the inferior cerebellar peduncle (resti-form body), conveying impulses concerning the actions of individual muscles of the trunk and lower limb.

Central axons entering the spinal cord via dorsal roots above T6 (conveying signals from the individual muscles of the upper limb) ascend to the medulla ipsilaterally and synapse on neurons located in the accessory cuneate nucleus. Axons of the second-order neurons located in the accessory cuneate nucleus form the cuneocerebellar tract, which reaches the cerebellum via the inferior cerebellar peduncle.

Ventral (Anterior) Spinocerebellar Tract. The anatomical course of this tract is shown in Figure 9-9. Impulses from the Golgi tendon organs travel in the dorsal root, enter the posterior horn of the spinal cord, and synapse on neurons located in the lateral part of the base and neck of the dorsal horn. The axons of these second-order neurons cross to the contralateral lateral funiculus of the spinal cord. The axons then ascend through the medulla to the pons, join the superior cerebellar peduncle (brachium con-junctivum), cross to the contralateral side, and terminate in the vermal region of the anterior lobe of the cerebellum. The cerebellum receives information regarding movement of muscle groups through the ventral (anterior) spinocer-ebellar tract.

Next post:

Previous post: