Chapter 17
The Somatosensory System I: Tactile Discrimination and Position Sense
Posterior Column–Medial Lemniscal System
Primary Somatosensory (SI) Cortex
Additional Cortical Somatosensory Regions
Anterior and Posterior Trigeminothalamic Tracts
Receptive Field Properties of Cortical Neurons
Neuroimaging and Functional Localization
Plasticity and Reorganization in the Primary Somatosensory Cortex
Posterior Spinocerebellar Tract
When you reach into your pocket to determine the types of coins present, you are gathering information through the activation of specialized receptors of the somatosensory system. Specifically, the size of a coin is determined by noting the joint angles when the coin is held between the forefinger and thumb. “Heads and tails” may be identified with the use of slowly adapting receptors sensitive to stimuli that indent the skin. Dimes can be distinguished from pennies by stroking their edges with the fingertips and activating rapidly adapting receptors. This information is transmitted to the cerebral cortex by a multisynaptic pathway called the posterior column–medial lemniscal system. At the same time, much of this information, along with information concerning muscle tension and length, is also transmitted to the cerebellar cortex, where it is used to regulate muscle activity that allows manipulation of the coins. The spinocerebellar pathways are among those that subserve these nonconscious somatosensory functions.
OVERVIEW
In general, the somatosensory system transmits and analyzes touch or tactile information from external and internal locations on the body and head. The result of these processes leads to the appreciation of somatic sensations, which can be subdivided into the submodalities discriminative touch, flutter-vibration, proprioception (position sense), crude (nondiscriminative) touch, thermal (hot and cold) sensation, and nociception (pain). The following anatomically and functionally discrete pathways transmit these signals: (1) the posterior column–medial lemniscal pathway, (2) the trigeminothalamic pathways, (3) the spinocerebellar pathways, and (4) the anterolateral system.
This chapter describes pathways that transmit discriminative touch, flutter-vibration, and proprioceptive information. These pathways are the posterior column–medial lemniscal pathway, portions of the trigeminothalamic pathways originating in the principal trigeminal sensory nucleus, and the spinocerebellar pathways. The pathways subserving the submodalities of nociception (commonly referred to as pain), thermal sense, and crude touch, itch, and tickle comprise the anterolateral system. These and portions of the trigeminothalamic pathways are described in Chapter 18.
POSTERIOR COLUMN–MEDIAL LEMNISCAL SYSTEM
The posterior column–medial lemniscal system (PCMLS), shown later in Figures 17-7 and 17-8, is involved with the perception and appreciation of mechanical stimuli. It underlies the capacity for fine form (size and shape) and texture discrimination, form recognition of three-dimensional shape (stereognosis), and motion detection. This pathway is also involved in transmitting information related to conscious awareness of body position (proprioception) and limb movement (kinesthesia) in space.
Characteristic features of the PCMLS include transmission on somatic afferent (SA) fibers that have fast conduction velocities, a limited number of synaptic relays in which processing of the signal occurs, and a precise somatotopic organization. These features provide the basis for the accurate localization of the body region touched. There is only limited convergence along the pathway; consequently, the signal is transmitted with high fidelity and a high degree of spatial and temporal resolution. This pathway signals somatic sensations by use of frequency and population codes. In frequency coding, a cell’s firing rate signals stimulus intensity or temporal aspects of the tactile stimulus. In population coding, the distribution in time and space of activated cells in the central nervous system signals location of the stimulus as well as its motion or direction, if any.
The high degree of resolution in the PCMLS is the result of inhibitory mechanisms such as feed-forward, feedback, and lateral (surround) inhibition. This mechanism is a feature found initially within the posterior column nuclei and is present through all the relays of the PCML pathway. It sharpens and enhances the discrimination between separate points on the skin and is critical for two-point discrimination. The ability to discriminate between two points simultaneously applied varies widely over different parts of the body.
Peripheral Mechanoreceptors
The first step in evoking somatic sensations is the activation of peripheral mechanoreceptors. Mechanical pressure, such as skin deformation, is transduced into an electrical signal in the peripheral process of a primary afferent neuron (see Chapter 3). This leads to a depolarizing graded membrane potential across the membrane of the neuron. If this potential depolarizes the trigger zone, located at the first myelin segment of the axon, to threshold, an action potential is produced (see Chapter 3). In most receptors, transduction occurs between the mechanoreceptor and the subjacent primary afferent membrane. However, in some cases (i.e., Merkel cells), the nonneural cells of the receptor complex may influence their associated primary afferent axon by vesicular release of a transmitter substance.
Each morphologic type of mechanoreceptor responds to different tactile stimuli. Cutaneous tactile receptors (Table 17-1; Fig. 17-1) are located in the basal epidermis and dermis of glabrous (palms, soles, lips) and hairy skin. These low-threshold mechanoreceptors may be encapsulated, such as Meissner, Pacinian, and Ruffini corpuscles, or unencapsulated, such as Merkel cell–neurite complexes (commonly referred to as Merkel cells) and hair follicle receptors. Meissner corpuscles, some hair follicle receptors, and Pacinian corpuscles respond to transient, phasic, or vibratory stimuli. These receptors respond to each initial application or removal of a stimulus but fail to respond during maintained stimulation. Consequently, they are rapidly adapting (RA) receptors (Fig. 17-2A). Hair follicle receptors are also capable of signaling motion, its direction or orientation, and its velocity.
Table 17-1 Cutaneous Mechanoreceptors and Their Associated Fiber Types and Sensations
RA, rapidly adapting; SA, slowly adapting.
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.
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
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 17-6. 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 17-8.
