230 · 2 Somatosensory System

Peripheral Regulatory Circuits

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 and Polysynaptic Reflexes

Monosynaptic intrinsic reflex. As illustrated in Figure 2.11 (p. 34), the large-di- ameter 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 travel, along 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.

Such monosynaptic reflex arcs provide the neuroanatomical basis for the regulation of muscle length (see below).

Reflex relaxation of antagonist muscles. In a strict sense, 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, p. 37).

Polysynaptic flexor reflex. Another important reflex arc is that of the polysynaptic flexor reflex, a protective and flight reflex that is mediated by many interneurons and is thus polysynaptic.

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 31 2

Fig. 2.9 Intrinsic neurons and polysynaptic connections in the spinal cord. Note: interneurons are also called “intercalated” or “internuncial” neurons (from Latin nuntius, messenger).

	Funicular neuron
	Lissauer zone
	Commissural
	neuron
	Association
	neuron
	Interneuron
Fasciculus
proprius	Motor neuron

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.

A reflex of this type requires the coordinated contraction of multiple muscles, which must contract in the right sequence and with the right intensity, while others (the antagonist muscles) must relax at the appropriate times. The intrinsic neuronal apparatus of the spinal cord is the computerlike, interconnected network of cells that makes this process possible.

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232 · 2 Somatosensory System

Cerebrum	Fig. 2.10 Flexor reflex with polysynaptic

connections

Brainstem

Cerebellum

Painful stimulus

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 33	2

In another paradigmatic situation, stepping on a sharp rock generates nociceptive impulses that initiate a complex but unvarying sequence of events (Fig. 2.10): the painful foot is raised by flexion of the hip, knee, and ankle, while the opposite leg is extended so that the individual can stand on it alone (crossed extensor reflex). The sudden redistribution of weight does not cause the individual to fall over, because it is immediately compensated for by reflex contraction of muscles of the trunk, shoulders, arms, and neck, maintaining the body’s upright posture. This process requires synaptic communication among many different neurons in the spinal cord, with simultaneous participation of the brainstem and cerebellum. All of this happens in a fraction of a second; only afterward does the individual feel pain, look to see what caused it, and check whether the foot has been injured.

These monosynaptic and polysynaptic reflexes are unconscious processes occurring mainly in the spinal cord, yet the last example shows that higher components of the CNS must often be activated at the same time, e. g., to preserve balance (as in the example).

Regulation of Muscle Length and Tension

As discussed above, monosynaptic and polysynaptic reflex arcs serve different purposes: polysynaptic reflex arcs mediate protective and flight responses, while monosynaptic reflex arcs are incorporated in functional circuits that regulate the length and tension of skeletal muscle. Each muscle, in fact, contains two servo-control (feedback) systems:

A control system for length, in which the nuclear bag fibers of the muscle spindles serve as length receptors

A control system for tension, in which the Golgi tendon organs and the nuclear chain fibers of the muscle spindles serve as tension receptors

Stretch and tension receptors. Muscle spindles are receptors for both stretch (length) and tension. These two distinct modalities are subserved by two different kinds of intrafusal fibers, the so-called nuclear bag and nuclear chain fibers (Figs. 2.11 and 2.12). Fibers of both of these types are typically shorter and thinner than extrafusal muscle fibers. The two types of intrafusal fiber are depicted separately for didactic reasons in Figures 2.11 and 2.12, but, in reality, the shorter and thinner nuclear chain fibers are directly attached to the somewhat longer nuclear bag fibers. Muscle spindles generally consist of two nuclear bag fibers and four or five nuclear chain fibers. In the middle of a nuclear bag fiber, the intrafusal muscle fibers widen to a form a bag containing about 50 nuclei, which is covered by a network of sensory nerve fibers known as a primary or annulospiral ending (from Latin annulus, ring). This spiral ending

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234 · 2 Somatosensory System

Central Pyramidal input tract

Nuclear bag muscle spindle with annulospiral ending: receptor for changes in muscle length (stretch)

Ia fiber

γ1 motor neuron

α fiber

γ1 fibers

Renshaw cell α1 motor neuron

Fig. 2.11 Regulatory circuit for muscle length

Pyramidal				Reticulospinal	Tendon organ (Golgi organ):
tract				tract	receptor for muscle tension

Nuclear chain muscle spindle with a primary ending and a flower-spray ending

Tonic stretch reflex

II fiber

Ib fiber

α2 fiber

γ2 fiber

Ia fiber

γ2 motor neuron

α2 motor neuron

Fig. 2.12 Regulatory circuit for muscle tension

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 35	2

reacts very sensitively to muscle stretch, mainly registering changes in muscle length; the nuclear bag fibers are thus stretch receptors. The nuclear chain fibers, on the other hand, mainly register a persistently stretched state of the muscle, and are thus tension receptors.

Maintenance of constant muscle length. The extrafusal muscle fibers have a certain length at rest, which the organism always tries to maintain constant. Whenever the muscle is stretched beyond this length, the muscle spindle is stretched along with it. This generates action potentials in the annulospiral ending, which travel very rapidly in Ia afferent fibers and are then relayed across a synapse to motor neurons in the anterior horn of the spinal cord (Fig. 2.11). The excited motor neurons fire impulses that travel in equally rapidly conducting, large-diameter α1 efferent fibers back to the working extrafusal muscle fibers, causing them to contract to their former length. Any stretch of the muscle induces this response.

The physician tests the intactness of this regulatory circuit with a quick tap on a muscle tendon, e. g., the patellar tendon for elicitation of the quadriceps (knee-jerk) reflex. The resulting muscular stretch activates the monosynaptic reflex arc. Intrinsic muscle reflexes are of major value for localization in clinical neurology because the reflex arc for a particular muscle occupies only one or two radicular or spinal cord segments; thus, a finding of an abnormal reflex enables the physician to infer the level of the underlying radicular or spinal lesion. The more important intrinsic muscle reflexes in clinical practice, the manner in which they are elicited, and the segments that participate in their reflex arcs are shown in Figure 2.13. It should be realized that the clinical elicitation of intrinsic muscle reflexes is an artificial event: a brief muscular stretch such as that produced with a reflex hammer is a rarity in everyday life.

Reflex relaxation of antagonist muscles. The reflex contraction of a stretched muscle to maintain constant length is accompanied by reflex relaxation of its antagonist muscle(s). The regulatory circuit for this likewise begins in the muscle spindles. The nuclear chain fibers of many muscle spindles contain secondary endings called flower-spray endings in addition to the primary (annulospiral) endings discussed above. These secondary endings react to stretch as the primary endings do, but the afferent impulses generated in them travel centrally in II fibers, which are thinner than the Ia fibers associated with the primary endings. The impulses are relayed via spinal interneurons to produce a net inhibition—and thus relaxation—of the antagonist muscle(s) (reciprocal antagonist inhibition, Fig. 2.14).

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2 36 · 2 Somatosensory System

C5

C6

Biceps

Musculocutaneous n.

Radius

Ulna

Bicepsreflex

Tricepsreflex

		L2
		L3
		L4
		L5
	Femoral n.	S1
		S2
	Quadriceps
		Tibial n.
	femoris

Gastrocnemius

Quadricepsreflex	Tricepssuraereflex
(patellar reflex,	(Achilles reflex,
kneejerk reflex)	ankle-jerk reflex)

C6

C7

Radial n.

Triceps

Fig. 2.13 The most important intrinsic muscle reflexes

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 37	2

Annulospiral

receptor

+

	Motor neuron	Association
		neuron

Agonist	Antagonist
Contraction	Relaxation

+	–		–
	–	Interneuron	Fasciculus
			proprius

–

Fig. 2.14 Monosynaptic reflex with polysynaptic inhibition of antagonist muscles

Setting of target values for muscle length. There is a special motor system whose function is to set adjustable target values in the regulatory circuit for muscle length.

As shown in Figure 2.11, the anterior horn of the spinal cord contains not only the large α motor neurons but also the smaller γ motor neurons. These cells project their axons (γ fibers) to the small, striated intrafusal fibers of the muscle spindles. Excitation by γ fibers induces contraction of the intrafusal muscle fibers at either end of a muscle spindle. This stretches the midportion

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238 · 2 Somatosensory System

of the spindle, leading the annulospiral ending to fire action potentials, which, in turn, elevate tension in the working muscle.

The γ motor neurons are under the influence of several descending motor pathways, including the pyramidal, reticulospinal, and vestibulospinal tracts. They thus serve as intermediaries for the control of muscle tone by higher motor centers, which is clearly an important aspect of voluntary movement. The γ efferents enable precise control of voluntary movements and also regulate the sensitivity of the stretch receptors. When the intrafusal muscle fibers contract and stretch the midportion of a muscle spindle, the threshold of the stretch receptors is lowered, i.e., they require much less muscular stretch to be activated. In the normal situation, the target muscle length that is to be maintained is automatically set by the fusimotor (γ) innervation of the muscle.

If both the primary receptors (nuclear bag fibers with annulospiral endings) and the secondary receptors (nuclear chain fibers with flower-spray endings) are slowly stretched, the response of the spindle receptors is static, i.e., unchanging in time. On the other hand, if the primary receptors are very rapidly stretched, a dynamic (rapidly changing) response ensues. Both the static and the dynamic responses are controlled by efferent γ neurons.

Static and dynamic γ motor neurons. There are presumed to be two types of γ motor neurons, dynamic and static. The former innervate mainly the intrafusal nuclear bag fibers, the latter mainly the intrafusal nuclear chain fibers. Excitation of nuclear bag fibers by dynamic γ neurons induces a strong, dynamic response mediated by the annulospiral ending, while excitation of nuclear chain fibers by static γ neurons induces a static, tonic response.

Muscle tone. Every muscle possesses a certain degree of tone, even in its maximally relaxed (resting) state. In the clinical neurological examination, the physician assesses muscle tone by noting the resistance to passive movement of the limbs (e. g., flexion and extension).

Total loss of muscle tone can be produced experimentally either by transection of all of the anterior roots or, perhaps more surprisingly, by transection of all of the posterior roots. Resting tone, therefore, is not a property of the muscle itself, but rather is maintained by the reflex arcs described in this section.

Adaptation of muscle tone to gravity and movement. The human body is continually subject to the earth’s gravitational field. When an individual stands or walks, anti-gravity muscles must be activated (among them the quadriceps femoris, the long extensors of the trunk, and the cervical muscles) to keep the body erect.

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