Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 21 2

Receptors in Deeper Regions of the Body

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 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 3­10 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. For further details, see p. 30 (monosynaptic intrinsic muscle reflex; polysynaptic reflexes).

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, p. 34).

Other receptor types. In addition to the muscle spindles and Golgi tendon organs, receptor types in the deep tissues include the laminated Vater­Pacini corpuscles and the Golgi­Mazzoni corpuscles as well as other terminal nerve endings that mediate pressure, pain, etc.

Peripheral Nerve, Dorsal Root Ganglion, Posterior Root

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 of one of the types described above are conducted centrally along an afferent fiber, which is the peripheral process of the first somatosensory neuron, whose cell body is located in a dorsal root ganglion (see below). The afferent fibers from a circum-

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222 · 2 Somatosensory System

Blood vessel	Unmyelinated fibers,	Myelinated,
	mostly autonomic

	Fat	segmented fibers,
		motor or sensory

Endoneurium

Perineurium

Epineurium

Fig. 2.3 Cross section of a mixed peripheral nerve

scribed 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

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 23		2
	Posterior root
C1–C8	Dorsal ramus

T1 T1 T2 T3 T4 T5 T6 T7 T8

T9

T10

T1–T12

Anterior root

Ventral ramus

T11

T12

L1–L5

S1–S5

Coccygeal nn. and filum terminale

Fig. 2.4 Nerve root segments and their relationship to the vertebral bodies. a Anatomy of the anterior and posterior spinal roots.

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 p. 70, Chapter 3 (Motor System).

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224 · 2 Somatosensory System

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 p. 100ff. in 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.

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 Figure 2.15 (p. 40), 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.

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 25				2
Nerve root
(posterior root)
Spinal
cord	Plexus	Peripheral n.	Dermatome

Nerve root	Plexus	Peripheral n.
Radicular (anterior root)
segments		Myotome

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).

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226 · 2 Somatosensory System

Dorsal root ganglion. The dorsal root ganglion is macroscopically visible as a swellingofthedorsalroot,immediatelyproximaltoitsjunctionwiththeventral 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 receptororgans.Theotherprocess(thecentralprocess)travelsbywayoftheposterior rootintothespinalcord,whereiteithermakessynapticcontactwiththesecond sensory neuron immediately, or else ascends toward the brainstem (see Fig. 2.17, p. 43). There are no synapses within the dorsal root ganglion itself.

Somatosensory Innervation by Nerve Roots and Peripheral Nerves

The fibers of individual nerve roots are redistributed into multiple peripheral nerves by way of the plexuses (cf. p. 24), and each nerve contains fibers from multiple adjacent radicular segments (see also Figs. 3.31, 3.32, and 3.33, p. 100­ 102). The fibers of each radicular segment regroup in the periphery, however (Fig. 2.5), to innervate a particular segmental area of the skin (dermatome). 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 Figure 2.6. The metameric organization of the dermatomes is easiest to see in the thoracic region.

As shown in Figure 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 Figure 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 adja-

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits				· 27 2
		Ophthalmic n.
C2		Maxillary n.
C3		Mandibular n.
C4
			T2
	T2		T3
			T4
C5	T3		T5
	T4		T6
	T5
	T6		T7
C6	T7	T1	T8
	T8		T9
T1	T9		T10
			T11
	T10		T12
			L1
	T11
			L 2
	T12			S5
	L1		L 3	S4
C7	L 2	S2
			S3
C8
			L 4
	L 3
			S2
			S1
	L 4		L5
L5
			S1
S1
	S1		L4
			L5

Fig. 2.6 Segmental innervation of the skin (after Hansen−Schliack). a Anterior view. b Posterior view.

cent roots, a dermatomal deficit of touch is generally hard to demonstrate, while one 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

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2 28 · 2 Somatosensory System

		Fig. 2.7 Segmental innervation
	C2	of the skin: simplified diagram
		of dermatomal topography
	T1	S1
	L1

prominent motor deficit in addition, we will defer further discussion of plexus lesions to the next chapter on the motor system (p. 100).

When a peripheral nerve is injured, the fibers within it, 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. The sensory deficit thus has a different distribution from that of 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.

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 29			2
Ophthalmic n.		Ophthalmic n.
Mandibular n.	Trigeminal n.	Greater occipital n.
Maxillary n.		Lesser occipital n.
		Great auricular n.
Great auricular n.		Dorsal rami
Transverse cervical n.		of cervical nn.
Supraclavicular nn.		Supraclavicular nn.
Ventral rami
of thoracic nn.
Lateral rami
	Dorsal rami of thoracic nn.
	Axillary n.
Lateral cutaneous rami of intercostal nn.
	Intercostobrachial n.
Posterior brachial cutaneous n.
Medial brachial cutaneous n.
Posterior antebrachial cutaneous n.
Lateral antebrachial cutaneous n.
	Medial antebrachial
	cutaneous n.
	Radial n.
	Ulnar n.	Ilio-
	Median n.	hypo-
		gastric n.
Iliohypogastric n.		Cluneal nn.
Genitofemoral n.		Dorsal rami
Ilioinguinal n.		of lumbar nn.
Obturator n.		Dorsal rami
		of sacral nn.

Lateral femoral cutaneous n.

Posterior femoral cutaneous n.

Femoral n.

Common peroneal (fibular) n.

Superficial peroneal (fibular) n.

		Saphenous n.
		Sural n.
		Saphenous n.
	Deep peroneal	Lateral plantar n.
		Medial plantar n.
	(fibular) n.

V1

Greater occipital n. C2–C3

V2

Lesser occipital n. C2

V3

Great auricular n. C2–C3 Transverse cervical n. C2–C3

Fig. 2.8 Innervation of the skin by peripheral nerves. a Anterior view. b Posterior view. c The areas innervated by the three divisions of the trigeminal nerve and by the cervical cutaneous nerves.

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