lemniscal pathway. Second-order neurons for the spinothalamic projection are also located in the dorsal horn of the spinal cord [17]. The synaptic connections for the different sensory fiber types are made in different layers of the dorsal horn. Thus, Aδ fibers synapse primarily in laminae I and V; C fibers connect mainly in laminae II and I; Aβ input goes to lamina V neurons.
Control of Pain Transmission
Pain transmission from the spinal cord can be modified by nonpainful sensory input as well as by activation of descending pathways from various brain nuclei. Painful sensations evoked by activity of nociceptors (Aδ and C fibers) can be decreased by simultaneous stimulation of low-threshold mechanoreceptors, which may underlie the pain-reducing effects of rubbing of the skin and transcutaneous and dorsal column electrical stimulation. This derives from the gate theory of pain[18], according to which pain results from the balance of activity in nociceptive and nonnociceptive afferent fibers. Thus, nonnociceptive Aβ fiber activity “closes” the central transmission gate whereas nociceptive activity “opens” it. The detailed spinal cord circuitry responsible for this effect is not known, but neurons in lamina V receive convergent input from Aβ. Aδ, and C fiber afferents. Furthermore, Aβ fiber activity can suppress firing of lamina V neurons via inhibitory interneurons in lamina II.
Spinal pain transmission is also modulated by descending inputs [19,20]. Neurons in the periaqueductal gray matter of the midbrain make connections with the cells in the rostroventral medulla, particularly in the nucleus raphe magnus. These in turn project to the spinal cord and make inhibitory connections with neurons in laminae I, II, and IV. Thus, electrical stimulation of periaqueductal gray or raphe nuclei inhibits dorsal horn neurons, including those giving rise to the spinothalamic tract, providing profound analgesia. There are several important neurotransmitter systems involved. The periaqueductal gray area has a very high density of opioid receptors. The raphe nucleus contains many serotonergic neurons. Another descending pathway, from the midbrain locus ceruleus, is noradrenergic. The serotonergic and noradrenergic fibers stimulate dorsal horn interneurons that release the endogenous opiate neurotransmitter enkephalin to pre- and postsynaptically inhibit spinothalamic tract projection neurons.
Motor Output and Sensorimotor Integration in the Spinal Cord
The final common motor pathway to the skeletal muscles is via the motoneurons, whose axons make up a substantial proportion of the myelinated fiber population of peripheral nerve. The major motor output of the spinal cord comes from the α-motoneurons, which directly stimulate skeletal muscle force production by synaptic activation at the motor end plate. The other cord output, from γ-motoneurons, exerts an indirect influence on muscle tension by controlling muscle spindle sensitivity and dynamic range, which consequently affects reflex activation of α-motoneurons.
The basic element of motor control is called the motor unit, which comprises an α-motoneuron together with all of the muscle fibers that it innervates [21]. The collection of α-motoneurons that innervates a single skeletal muscle is termed the motor unit pool of that muscle. The size of the motor units varies greatly. For muscles involved in high-precision movements such as those of the digits or face, the number of muscle fibers may be only 10. For large muscles of the trunk and limbs there may be 3000-4000 fibers per motor unit. These fibers may be distributed over the entire area of the muscle, so the territory of a single motor unit may be considerable. All muscle fibers in an individual motor unit are biochemically, histochemically, and physiologically identical, indicating the determination of muscle fiber properties by their innervation. In terms of muscle and contractile properties, there are three motor unit types. Those based on type I muscle fibers are characterized by relatively slow contraction speeds, reliance on aerobic energy metabolism, and a profuse capillary supply-features that confer extreme fatigue resistance. These units are active for much of the time, being involved in postural control, and are preferentially recruited by muscle spindle afferent input to the spinal cord. In contrast, type IIB motor units have fast contraction times, rely on anaerobic metabolism, have relatively poor vascular supply, and fatigue rapidly. They are preferentially recruited for large, fast movements such as limb withdrawal from a painful stimulus. Type IIA units are somewhere in between: the muscle fibers are fast contracting, well supplied with capillaries, have both aerobic and anaerobic energy production, and are moderately fatigue-resistant.
Fig. 2.8 Distribution of dermatomes and peripheral nerve patterns. Mapping of sensory innervation of the skin by the dorsal roots is shown on the left of the subject: cervical (C1-C7), thoracic (T1-T12), lumbar (L1-L5), and sacral (S1-S5). There is no dorsal root at C1, only a ventral (motor) root. The innervation patterns of peripheral nerves are shown for comparison on the left. Individual peripheral nerves have fibers that arise from several adjacent dorsal roots, leading to rather larger fields of innervation and overlap in the area innervated by each segment. (From [2], with permission)
Fig. 2.9 Schematic of the stretch or myotatic spinal reflex circuitry. The essential component is an excitatory (+) monosynaptic connection between muscle spindle la afferent fibers and the α-motoneuron pool for that muscle. The reflex may be evoked transiently by tapping the muscle tendon, which stretches the muscle and spindle endings, causing a short reflex contraction. Also shown is a connection via an inhibitory interneuron (−, black cell body), which suppresses activity in the antagonist muscle
There are two fundamental ways of varying muscle tension production: altering the frequency of action potentials transmitted by an individual a-motoneuron, and altering the range or number of motor units activated in that muscle. This is dependent on the synaptic input to the α-motoneurons, of which there are three sources: muscle spindle afferents, the corticospinal projection, and spinal cord interneurons. The latter category forms a complex control mechanism as it is in turn strongly influenced by both afferent input and all descending motor pathways.
The elegance and simplicity of the spinal cord circuitry involved in sensorimotor integration is apparent for the myotatic or stretch reflex. Thus, Sherrington [22] noted that when a muscle is stretched it tends to contract. This was traced to an excitatory monosynaptic reflex arc between muscle spindle afferents, which are stimulated by the stretch, and the α-motoneurons, which cause that muscle to contract (Fig. 2.9). The operation of this circuit is used as a clinical tool, observing the reflex jerk when the tendons are tapped to rapidly stretch muscle. Physiologically, however, this simple circuit acts tonically as a length servo feedback loop, which is crucial for postural stability and has important antigravity functions, for example in the major leg extensors. This circuit is elaborated by interneurons to ensure that antagonistic muscle groups controlling the same joint do not work against each other [23]. Thus, collateral branches of the spindle afferents also synapse on inhibitory interneurons that in turn innervate the motor unit pool of the antagonist muscle.
Another spinal circuit involving proprioceptor input is the reverse myotatic (or clasp knife) reflex arc (Fig. 2.10). Golgi tendon organs, which signal muscle tension, are the sensory arm and they innervate inhibitory interneurons that synapse with the α-motoneurons [1,2]. Thus, the reflex is polysynaptic, with increasing muscle
