The Somatosensory System I: Tactile Discrimination and Position Sense

RA, rapidly adapting; SA, slowly adapting.

Figure 17-1. Proprioceptive receptors and cutaneous mechanoreceptors and their afferent fibers. Cutaneous receptors are either rapidly adapting (RA) or slowly adapting (SA).

Figure 17-2. A, Diagrammatic action potentials (top trace) evoked by skin indentation and removal of a cutaneous stimulus or joint movement (bottom trace) in primary afferent fibers innervating slowly adapting (red) and rapidly adapting (green) cutaneous mechanoreceptors. B, Diagrammatic action potentials (blue) evoked in a Pacinian corpuscle afferent fiber by sinusoidal stimulation of the skin surface (bottom trace).

Merkel cells, Ruffini corpuscles, and some hair follicle receptors signal tonic events such as discrete small indentations in the skin. They provide input related to both the displacement and velocity of a stimulus. They are also capable of encoding stimulus intensity or duration because they are slowly adapting (SA) and are active so long as the stimulus is present (Fig. 17-2A). For example, Merkel cells are crucial to reading of Braille.

Deep tactile mechanoreceptors are found within the dermis of the skin, in the fascia surrounding muscles and bone, and in the periodontium. These receptors include Pacinian corpuscles, Ruffini corpuscles, and other encapsulated nerve endings located in the periosteum, the deep fascia, and the mesenteries. The receptors of this group respond to pressure, vibration (Fig. 17-2B and Table 17-1), skin stretch and distention, or tooth displacement.

Proprioceptive receptors (Table 17-2; Fig. 17-1) are located in muscles, tendons, and joint capsules. These receptors include muscle spindles and their associated nuclear bag and chain muscle fibers that are innervated by Ia and II afferent nerve fibers. The Golgi tendon organs and their group Ib fibers and the encapsulated Ruffini-type joint receptors also function in this capacity. They respond to static limb and joint position or to the dynamic movement of the limb (kinesthesia) and are important sources of information for balance, posture, and limb movement.

Table 17-2 Muscle and Joint Proprioceptors and Their Associated Fiber Types and Sensations

SA, slowly adapting.

The accuracy with which a tactile stimulus is localized depends on the density of receptors and the size of their receptive fields (Fig. 17-3). The greatest density of cutaneous tactile receptors is found on the tips of the glabrous digits and in the perioral region. Other regions, like the back, have much lower density, thus creating a receptor density gradient between various body parts. The receptive field is the area of skin innervated by branches of an SA fiber, the stimulation of which activates its receptors (Fig. 17-3). Small receptive fields are found in areas such as the fingertips, where receptor density is high and each receptor serves an extremely small area of skin. In such regions, the individual is able to discriminate small variations in a variety of sensory inputs. In other regions, receptor density is low and each receptor serves an expansive area of skin, creating large receptive fields with resultant reduction in discriminative ability.

Figure 17-3. A to C, Variation in the size of receptive fields as a function of peripheral innervation density. The greater the density of receptors, the smaller the receptive fields of individual afferent fibers.

At all levels of the tactile pathway, densely innervated body parts are represented by greater numbers of neurons and take up a disproportionately large part of the somatosensory system’s body representation. In this respect, there is an inverse relationship between the size of the receptive field and the representation of that body part in the somatosensory cortex. For example, the trunk, with its large receptive fields, has a small representation in the somatosensory cortex, whereas the fingers, with their small receptive fields, have a large representation in the somatosensory cortex (compare Fig. 17-3 with Fig. 17-10). As a result, the fingertips and lips provide the central nervous system with the most specific and detailed information about a tactile stimulus.

Primary Afferent Fibers

As initially described in Chapter 9, primary afferent SA fibers consist of (1) a peripheral process extending from the posterior root ganglion either to contact peripheral mechanoreceptors or to end as free nerve endings, (2) a central process extending from the posterior root ganglion into the central nervous system, and (3) a pseudounipolar cell body in the posterior root ganglion. The peripheral distribution of the afferent nerves arising from each spinal level delineates the segmental pattern of dermatomes. In clinical testing, these ribbon-like strips of skin are associated primarily with fibers and pathways that convey pain and thermal information; they are considered in Chapter 18.

Peripheral nerves are classified by two schemes. One is based on their contribution to a compound action potential (A, B, and C waves) recorded from an entire mixed peripheral nerve (e.g., sciatic nerve) after electrical stimulation of that nerve. The other scheme specific to cutaneous fibers (e.g., lateral antebrachial cutaneous nerve, sural nerve) is based on fiber diameter, myelin thickness, and conduction velocity (classes I, II, III, and IV) (Table 17-3; Fig. 17-4). The two schemes are related because conduction velocity determines a fiber’s contribution to the compound action potential. Discriminative touch, vibratory sense, and position sense are transmitted by group Ia, Ib, and II fibers (Tables 17-1 and 17-2).

Table 17-3 Peripheral Sensory and Motor Fibers: Groups, Diameters, and Conduction Velocities

Figure 17-4. Compound action potential evoked in a mixed nerve (A) and a cutaneous nerve (B) in response to electrical stimulation. Note the increase in the number of small-diameter fibers and the absence of the Aα fibers in the cutaneous nerve (B).

Spinal Cord and Brainstem

On the basis of cell size and fiber diameter, primary sensory fibers are categorized as large and small. Large-diameter fibers subserve discriminative touch, flutter-vibration, and proprioception (groups Ia, Ib, II, and Aβ; Tables 17-1 and 17-2). They enter the spinal cord via the medial division of the posterior root (see Chapter 9) and then branch (Fig. 17-5). One set of branches terminates on second-order neurons in the spinal cord gray matter at, above, and below the level of entry. These branches contribute to a variety of spinal reflexes and to ascending projections such as postsynaptic posterior column fibers. The largest set of branches ascends cranially and contributes to the formation of the gracile and cuneate fasciculi. These fiber bundles are collectively termed the posterior columns owing to their position in the spinal cord (Figs. 17-5 to 17-7).

Figure 17-5. A representative section of the cervical spinal cord showing large-diameter Aα and Aβ fibers on the right and small-diameter Aδ and C fibers on the left.

Figure 17-6. The general somatotopic arrangement of fibers of the posterior columns; lower portions of the body are medial, and progressively more rostral portions are more lateral within the posterior funiculus. The septomarginal fasciculus is composed of descending collaterals of primary afferent fibers from sacral, lumbar, and low thoracic levels; the fasciculus interfascicularis is composed of descending collaterals from upper thoracic and cervical levels. These fibers are involved in reflexes mediated by posterior column afferents.

Within the posterior columns, fibers from different dermatomes are organized topographically. Sacral level fibers assume a medial position, and fibers from progressively more rostral levels (up to thoracic level T6) are added laterally to form the gracile fasciculus (Figs. 17-5 and 17-6). Thoracic fibers from above T6 and cervical fibers form the laterally placed cuneate fasciculus in the same manner. Thus the lower extremity is represented medially and the upper extremity is represented laterally within the posterior columns (Figs. 17-5 and 17-6). Compromise of blood flow in the posterior spinal artery, which supplies the posterior funiculus, or mechanical injury to the posterior columns (as in Brown-Séquard syndrome) results in an ipsilateral reduction or loss of discriminative, positional, and vibratory tactile sensations at and below the segmental level of the injury. Symptoms indicative of damage to fibers of the posterior columns are also seen in tabes dorsalis (progressive locomotor ataxia). This disease is caused by infection with Treponema pallidum and is associated with neurosyphilis. The fibers of the posterior columns degenerate, and the patient has ataxia (related to the lack of sensory input, clinically referred to as sensory ataxia), loss of muscle stretch (tendon) reflexes, and proprioceptive losses from the extremities. In sensory ataxia, the patient may also have a wide-based stance and may place the feet to the floor with force in an effort to create the missing proprioceptive input.

The posterior column nuclei, the gracile and cuneate nuclei, are found in the posterior medulla at the rostral end of their respective fasciculi. They are supplied by the posterior spinal artery (Fig. 17-7). The cell bodies of the gracile and cuneate nuclei are the second-order neurons in the PCMLS. They receive input from first-order neurons having cell bodies in the ipsilateral posterior root ganglia (Figs. 17-7 and 17-8). The gracile nucleus receives input from sacral, lumbar, and lower thoracic levels via the gracile fasciculus; the cuneate nucleus receives input from upper thoracic and cervical levels through the cuneate fasciculus.

Figure 17-7. The posterior column–medial lemniscal system. Note the somatotopic arrangement of body parts at each level of this pathway.

Figure 17-8. The location of posterior column–medial lemniscus (PCML) fibers in magnetic resonance images at representative levels of the medulla, pons, and midbrain. This illustrates the location of PCML fibers when they are viewed in images routinely used in the clinical setting.

In addition to the somatotopic organization of projections to the posterior column nuclei, there is a submodality segregation of tactile inputs within these nuclei. The second-order relay neurons are arranged into a core “clusters” region surrounded by a covering “shell” region that allows submodality segregation of the excitatory primary afferent input. Rapidly adapting and slowly adapting inputs terminate centrally within the core. Muscle spindle and joint inputs project preferentially to the rostral shell region. Pacinian corpuscle input is restricted to the caudal shell region.

The posterior column nuclei have an inner core region containing large projection neurons surrounded by a diffuse shell of small fusiform and radiating cells. The shell area contains interneurons responsible for feedback inhibition in the posterior column nuclei. This feedback alters activity of projection neurons of the inner core. The posterior column nuclei also receive descending axons from the contralateral primary somatosensory cortex and from the medullary reticular formation (nucleus reticularis gigantocellularis). The presence of non–posterior column inputs to these projection cells suggests that information received by the posterior column nuclei is not simply relayed but undergoes signal processing.

The second-order cells in the core region of the posterior column nuclei send their axons to the contralateral thalamus (Figs. 17-7 and 17-8). In the medulla, the internal arcuate fibers, axons of cells in the posterior column nuclei, arc anteromedially toward the midline, decussate, and ascend as the medial lemniscus on the opposite side. Fibers in the medial lemniscus that arise in the cuneate nucleus are located in superior portions of the medial lemniscus (and convey information from the upper extremity), and those from the gracile nucleus are located in its inferior parts (and relay data from the lower extremity) (Figs. 17-6 and 17-8). The anterior spinal artery supplies the medial lemniscus in the medulla, and penetrating branches of the basilar artery (paramedian and short circumferential) supply it in the pons. Vascular damage at these brainstem levels leads to deficits in discriminative touch, vibratory, and positional sensibilities over the contralateral side of the body. As the medial lemniscus moves rostrally through the brainstem, it rotates laterally so that the upper extremity representation comes to lie medially and the lower extremity laterally in the pons (Figs. 17-7 and 17-8). As the medial lemniscus traverses the midbrain, it is shifted laterally and posteriorly by the appearance of medial structures such as the red nucleus (Figs. 17-7 and 17-8). The midbrain lesion in Figure 17-9 compromised only the medial lemniscus on the right side and resulted in a loss of discriminative touch and proprioception on the patient’s left side. This patient did not experience the loss of any other modality. This somatotopic organization is generally maintained as the medial lemniscus terminates on cells in the ventral posterolateral nucleus (VPL) of the thalamus.

Figure 17-9. The position and somatotopy of the medial lemniscus in the midbrain (A) and a small midbrain lesion that involved only the medial lemniscus (B), producing a left-sided loss of proprioception and discriminative touch. LE, lower extremity; T, trunk; UE, upper extremity.

The postsynaptic posterior column pathway, a small supplemental pathway in humans that relays nondiscriminative tactile signals to supraspinal levels, consists of non–primary afferent axons carrying tactile signals in the posterior columns (see Fig. 17-14). The cells of origin of this pathway are located in laminae III and IV of the posterior horn. Axons of the second-order postsynaptic posterior column pathway travel in the posterior columns and together with other tactile primary afferent fibers terminate in the posterior column nuclei. Cells of these nuclei relay this postsynaptic posterior column input to the contralateral thalamus via the medial lemniscus. Although this pathway is small, it may provide the morphologic basis for the return of some tactile sensation after vascular lesions involving the PCMLS.

Ventral Posterior Nucleus

The ventral posterior nucleus, sometimes called the ventrobasal complex, is a wedge-shaped cell group located caudally in the thalamus. Its lateral border abuts the internal capsule, and ventrally it borders on the external medullary lamina. The ventral posterior nucleus is composed of the laterally located ventral posterolateral nucleus (VPL) and the medially located ventral posteromedial nucleus (VPM). Although these nuclei have also been termed the ventralis caudalis externus and ventralis caudalis internus in humans, the more widely used and recognized terms VPL and VPM are used in this book. The VPL is separated from the VPM by fibers of the arcuate lamina. The ventral posterior nucleus (VPM and VPL) is supplied by thalamogeniculate branches of the posterior cerebral artery, and compromise of these vessels can result in loss of all tactile sensation over the contralateral body and head (Fig. 17-10).