After a preliminary chapter on the structural elements of the nervous system, the discussion of its major functional components and mechanisms now begins with the perceptual processes mediated by receptor organs: as depicted earlier in ▶Fig. 1.1, these organs are the site of origin of information flow in the nervous system, in accordance with the basic organizing principle, perception → processing → response. Somatosensory impulses from the periphery are conducted along an afferent nerve fiber to its neuronal cell body, which lies in a dorsal root ganglion (spinal ganglion). The impulses are then conducted onward into the central nervous system, without any intervening synapses, along the central process (axon) of the same neuron. This axon makes synaptic contact with a second neuron in the spinal cord or brainstem, whose axon, in turn, proceeds further centrally, and crosses the midline to the opposite side at some level along its path. The third neuron lies in the thalamus, the so-called gateway to consciousness; it projects to various cortical areas, most importantly the primary somatosensory cortex, which is located in the postcentral gyrus of the parietal lobe.
Receptors are specialized sensory organs that register physical and chemical changes in the external and internal environment of the organism and convert (transduce) them into the electrical impulses that are processed by the nervous system. They are found at the peripheral end of afferent nerve fibers. Some receptors inform the body about changes in the nearby external environment (exteroceptors) or in the distant external environment (teleceptors, such as the eye and ear). Proprioceptors, such as the labyrinth of the inner ear, convey information about the position and movement of the head in space, tension in muscles and tendons, the position of the joints, the force needed to carry out a particular movement, and so on. Finally, processes within the body are reported on by enteroceptors, also called visceroceptors (including osmoceptors, chemoceptors, and baroceptors, among others). Each type of receptor responds to a stimulus of the appropriate, specific kind, provided that the intensity of the stimulus is above threshold.
The encapsulated, differentiated end organs are probably mainly responsible for the mediation of epicritic sensory modalities such as fine touch, discrimination, vibration, pressure, and so forth, while the free nerve endings mediate protopathic modalities such as pain and temperature. The evidence for this functional distinction is incomplete, however (see below).
Various receptor organs of the skin and its appendages are depicted in ▶Fig. 2.1, including mechanoreceptors (for touch and pressure), thermoreceptors (for warm and cold), and nociceptors (for pain). These receptors are located mainly in the zone between the epidermis and the connective tissue. Many of these receptors are operated by TRP ion channels, which constitute a large family of molecular sensors of a large variety of stimuli including temperature and mechanical stress. The skin can thus be regarded as a sensory organ that covers the entire body.
Fig. 2.1 Somatosensory receptors in the skin. (a) Free nerve ending (pain, temperature). (b) Tactile disk of Merkel. (c) Peritrichial nerve endings around a hair follicle (touch). (d) Tactile corpuscle of Meissner. (e) Vater–Pacini lamellar corpuscle (pressure, vibration). (f) End bulb of Krause (cold?). (g) Ruffini corpuscle (warmth?).
Special receptor organs. The peritrichial nerve endings around the hair follicles are found in all areas of hair-bearing skin and are activated by the movement of hairs. In contrast, the tactile corpuscles of Meissner are found only on glabrous skin, particularly on the palms and soles but also on the lips, the tip of the tongue, and the genitals, and respond best to touch and light pressure. The laminated Vater–Pacini corpuscles (pacinian corpuscles) are found in deeper layers of the skin, especially in the area between the cutis and the subcutis, and mediate pressure sensations. The end bulbs of Krause were once thought to be cold receptors, while the corpuscles of Ruffini were thought to be warm receptors, but there is some doubt about this at present. Free nerve endings have been found to be able to transmit information about warmth and cold as well as about position. In the cornea, for example, only free nerve endings are present to transmit information about all of these sensory modalities. Aside from the receptor types specifically mentioned here, there are also many others in the skin and elsewhere whose function mostly remains unclear.
Free nerve endings. These are found in the clefts between epidermal cells, and sometimes also on more specialized cells of neural origin, such as the tactile disks of Merkel (▶Fig. 2.1). Free nerve endings are present, however, in not just the skin but also, practically, in all organs of the body, from which they convey nociceptive and thermal information relating to cellular injury. Merkel’s disks are mainly located in the pads of the fingers and respond to touch and light pressure.
A second group of receptor organs lies deep to the skin, in the muscles, tendons, fasciae, and joints (▶Fig. 2.2). In the muscles, for example, one finds muscle spindles, which respond to stretching of the musculature. Other types of receptors are found at the transition between muscles and tendons, in the fasciae, or in joint capsules.
Muscle spindles. Muscle spindles are very thin, spindle-shaped bodies that are enclosed in a connective-tissue capsule and lie between the striated fibers of the skeletal musculature. Each muscle spindle itself usually contains 3 to 10 fine striated muscle fibers, which are called intrafusal muscle fibers, in contrast to the extrafusal fibers of the muscular tissue proper. The two ends of each spindle, composed of connective tissue, are fixed within the connective tissue between muscle fascicles, so that they move in conjunction with the muscle. An afferent nerve fiber called an annulospiral ending or primary ending winds around the middle of the muscle spindle. This afferent fiber has a very thick myelin sheath and belongs to the most rapidly conducting group of nerve fibers in the body, the so-called Ia fibers. Further details can be found in the discussion of mono- and polysynaptic reflexes.
Golgi tendon organs. Golgi tendon organs contain fine nerve endings, derived from branches of thickly myelinated nerve fibers that surround a group of collagenous tendon fibers. They are enclosed in a connective-tissue capsule, are located at the junction between tendon and muscle, and are connected in series to the adjacent muscle fibers. Like muscle spindles, they respond to stretch (i.e., tension), but at a higher threshold (see ▶Fig. 2.12).
Other receptor types. In addition to the muscle spindles and Golgi tendon organs, receptor types in the deep tissues include the laminated Vater–Pacini corpuscles and the Golgi–Mazzoni corpuscles as well as other terminal nerve endings that mediate pressure, pain, etc.
The further “way stations” through which an afferent impulse must travel as it makes its way to the CNS are the peripheral nerve, the dorsal root ganglion, and the posterior nerve root, through which it enters the spinal cord.
Peripheral nerve. Action potentials arising in a receptor organ (see ▶Receptor Organs) are conducted centrally along afferent fibers. These are the peripheral processes of the first somatosensory neurons, whose cell bodies are located in the dorsal root ganglia. The afferent fibers from a circumscribed area of the body run together in a peripheral nerve; such nerves contain not only fibers for superficial and deep sensation (somatic afferent fibers) but also efferent fibers to striated muscle (somatic efferent fibers) and fibers innervating the internal organs, the sweat glands, and vascular smooth muscle (visceral afferent and visceral efferent fibers). Fibers (axons) of all of these types are bundled together inside a series of connective-tissue coverings (endoneurium, perineurium, and epineurium) to form a “nerve cable” (▶Fig. 2.3). The perineurium also contains the blood vessels that supply the nerve (vasa nervorum).
Nerve plexus and posterior root. Once the peripheral nerve enters the spinal canal through the intervertebral foramen, the afferent and efferent fibers go their separate ways: the peripheral nerve divides into its two “sources,” the anterior and posterior spinal roots (▶Fig. 2.4). The anterior root contains the efferent nerve fibers exiting the spinal cord, while the posterior root contains the afferent fibers entering it. A direct transition from the peripheral nerve to the spinal nerve roots is found, however, only in the thoracic region. At cervical and lumbosacral levels, nerve plexuses are interposed between the peripheral nerves and the spinal nerve roots (the cervical, brachial, lumbar, and sacral plexuses). In these plexuses, which are located outside the spinal canal, the afferent fibers of the peripheral nerves are redistributed so that fibers from each individual nerve ultimately join spinal nerves at multiple segmental levels (▶Fig. 2.5). (In analogous fashion, the motor fibers of a single segmental nerve root travel to multiple peripheral nerves; cf. ▶Fig. 2.5 and ▶Chapter 3.) The redistributed afferent fibers then enter the spinal cord at multiple levels and ascend a variable distance in the spinal cord before making synaptic contact with the second sensory neuron, which may be at or near the level of the entering afferent fibers or, in some cases, as high as the brainstem. Thus, in general, a peripheral nerve is composed of fibers from multiple radicular segments; this is true of both afferent and efferent fibers.
Fig. 2.4 Nerve root segmentsand their relationship to the vertebral bodies. (a) Anatomy of the anterior and posterior spinal roots. For the locations of the motor and sensory fibers and the spinal ganglion cells, see ▶Fig. 2.17. (b) Enumeration of the nerve root segments and the levels of exit of the spinal nerves from the spinal canal. The spinal cord grows to a shorter final length than the vertebral column, so that the nerve roots (proceeding caudally) must travel increasingly long distances to reach their exit foramina. See also ▶Chapter 3. nn, nerves
Fig. 2.5 Redistribution of afferent and efferent nerve fibers in a nerve plexus. The sensory fibers contained in a single peripheral nerve are distributed to multiple dorsal spinal nerve roots, and, analogously, the motor fibers of a single nerve root are distributed to multiple peripheral nerves. (a) In the periphery, the sensory fibers of a single radicular segment are grouped together once again to supply a characteristic segmental region of the skin (dermatome). (b) Radicular and peripheral nerve innervation of muscle: each muscle is supplied by a single peripheral nerve, which, however, generally contains fibers from multiple nerve roots (so-called polyradicular or plurisegmental innervation).
Digression: Anatomy of the spinal roots and nerves. In total, there are 31 pairs of spinal nerves; each spinal nerve is formed by the junction of an anterior and a posterior nerve root within the spinal canal. The numbering of the spinal nerves is based on that of the vertebral bodies (▶Fig. 2.4). Even though there are only seven cervical vertebrae, there are eight pairs of cervical nerves, because the highest spinal nerve exits (or enters) the spinal canal just above the first cervical vertebra. Thus, this nerve, the first cervical nerve (C1), exits the spinal canal between the occipital bone and the first cervical vertebra (atlas); the remaining cervical nerves, down to C7, exit above the correspondingly numbered vertebra; and C8 exits between the seventh (lowest) cervical vertebra and the first thoracic vertebra. At thoracic, lumbar, and sacral levels, each spinal nerve exits (or enters) the spinal canal below the correspondingly numbered vertebra. There are, therefore, just as many pairs of nerves in each of these regions as there are vertebrae (12 thoracic, 5 lumbar, and 5 sacral) (▶Fig. 2.4). Lastly, there is a single pair of coccygeal nerves (or, occasionally, more than one pair).
Spatial organization of somatosensory fibers in the posterior root. Nerve impulses relating to different somatosensory modalities originate in different types of peripheral receptor and are conducted centrally in separate groups of afferent fibers, which are spatially arranged in the posterior root in a characteristic pattern. As shown in ▶Fig. 2.15, the most thickly myelinated nerve fibers, which originate in muscle spindles, run in the medial portion of the root; these fibers are responsible for proprioception. Fibers originating in receptor organs, which mediate the senses of touch, vibration, pressure, and discrimination, run in the central portion of the root, and the small and thinly myelinated fibers mediating pain and temperature sensation run in its lateral portion.
Dorsal root ganglion. The dorsal root ganglion is macroscopically visible as a swelling of the dorsal root, immediately proximal to its junction with the ventral root (▶Fig. 2.4). The neurons of the dorsal root ganglion are pseudounipolar, i.e., they possess a single process that divides into two processes a short distance from the cell, in a T-shaped configuration. One of these two processes travels to the receptor organs of the periphery, giving off numerous collateral branches along the way, so that a single ganglion cell receives input from multiple receptor organs. The other process (the central process) travels by way of the posterior root into the spinal cord, where it either makes synaptic contact with the second sensory neuron immediately or ascends toward the brainstem (see ▶Fig. 2.17). There are no synapses within the dorsal root ganglion itself.
The fibers of individual nerve roots are redistributed into multiple peripheral nerves by way of the plexuses, and each nerve contains fibers from multiple adjacent radicular segments (see also ▶Fig. 3.31, ▶Fig. 3.32, and ▶Fig. 3.33). The fibers of each radicular segment regroup in the periphery, however, to innervate a particular segmental area of the skin (dermatome) (▶Fig. 2.5). Each dermatome corresponds to a single radicular segment, which, in turn, corresponds to a single “spinal cord segment.” The latter term is used even though the mature spinal cord no longer displays its original metameric segmentation.
The dermatomes on the anterior and posterior body surfaces are shown in ▶Fig. 2.6. The metameric organization of the dermatomes is easiest to see in the thoracic region.
As shown in ▶Fig. 2.5, the dermatomes of neighboring roots overlap considerably, so that a lesion confined to a single root often causes a barely discernible sensory deficit, or none at all.
Sensory deficits due to radicular lesions. A demonstrable sensory deficit in a segmental distribution is usually found only when multiple adjacent nerve roots are involved by a lesion. As each dermatome corresponds to a particular spinal cord or radicular level, the dermatome(s) in which a sensory deficit is located is a highly valuable indicator of the level of a lesion involving the spinal cord or one or more nerve roots. The schematic representation of ▶Fig. 2.7 is intended for didactic purposes, to help the student remember where the boundaries between the cervical, thoracic, lumbar, and sacral dermatomal areas are located.
The dermatomes for the sense of touch overlap to a greater extent than those for pain and temperature. It follows that, in a lesion of one or two adjacent roots, a dermatomal deficit of touch is generally hard to demonstrate, while that of pain and temperature sensation is more readily apparent. Thus, nerve root lesions can be more sensitively detected by testing for hypalgesia or analgesia, rather than hypesthesia or anesthesia.
Sensory deficits due to peripheral nerve lesions. It is easy to see why a lesion affecting a nerve plexus or a peripheral nerve produces a sensory deficit of an entirely different type than a radicular lesion. As plexus lesions usually cause a prominent motor deficit in addition, we will defer further discussion of plexus lesions to Chapter 3 (▶Motor System).
When a peripheral nerve is injured, the fibers within it, which are derived from multiple nerve roots, can no longer rejoin in the periphery with fibers derived from the same nerve roots but belonging to other peripheral nerves—in other words, the fibers in the injured nerve can no longer reach their assigned dermatomes. Thus, the sensory deficit caused by a peripheral nerve injury has a different cutaneous distribution from the dermatomal deficit seen after a radicular injury (▶Fig. 2.8). Furthermore, the cutaneous areas innervated by individual peripheral nerves overlap much less that those innervated by adjacent nerve roots. Sensory deficits due to peripheral nerve lesions are, therefore, more readily apparent than those due to radicular lesions.
In the next section after this one, we will trace the ascending fiber pathways responsible for pain and temperature sensation, and for sensory modalities such as touch and pressure, as they travel up the spinal cord and into the brain. Before doing so, however, we will explain the function of a number of important peripheral regulatory circuits. Even though the current chapter is devoted to the sensory system, it will be useful, in this limited context, to describe not only the afferent (sensory) arm of these regulatory circuits, but their efferent (motor) arm as well.
Monosynaptic intrinsic reflex. As illustrated in ▶Fig. 2.11, the large-diameter afferent fiber arising in a muscle spindle gives off many terminal branches shortly after entering the spinal cord; some of these branches make direct synaptic contact onto neurons in the gray matter of the anterior horn. These neurons, in turn, are the origin of efferent motor fibers, and are therefore called motor anterior horn cells. The efferent neurites exit the spinal cord by way of the anterior root and then extend, by way of the peripheral nerves, to the skeletal muscles.
A neural loop is thus created from a skeletal muscle to the spinal cord and back again, composed of two neurons—an afferent sensory neuron and an efferent motor neuron. This loop constitutes a simple, monosynaptic reflex arc. Because the arc begins and ends in the same muscle, the associated reflex is called an intrinsic (or proprioceptive) muscle reflex.
Reflex relaxation of antagonist muscles. Strictly speaking, the monosynaptic reflex is not truly monosynaptic, because it also has a polysynaptic component: the reflex is manifested not only in contraction of the muscle in question, but also in relaxation of its antagonist muscle(s). The inhibition of muscle cells that leads these muscles to relax is a polysynaptic process occurring by way of interneurons in the spinal gray matter. Were this not the case, tension in the antagonist muscles would counteract agonist contraction (see ▶Fig. 2.14).
When a finger touches a hot stove, the hand is pulled back with lightning speed, before any pain is felt. The action potentials that arise in the cutaneous receptor (nociceptor) for this reflex travel by way of afferent fibers to the substantia gelatinosa of the spinal cord, where they are then relayed, across synapses, into cells of various types belonging to the cord’s intrinsic neuronal apparatus (interneurons, association neurons, and commissural neurons). Some of these cells—particularly the association neurons—project their processes multiple spinal levels upward and downward, in the so-called fasciculus proprius (▶Fig. 2.9). After crossing multiple synapses, excitatory impulses finally reach the motor neurons and travel along their efferent axons into the spinal nerve roots, peripheral nerves, and muscle, producing the muscular contraction that pulls the hand back from the stove.