to inhibit further contraction. This may be demonstrated, for example, when a subject is asked to actively resist bending of their knee. At a high level of applied force, the strongly contracted extensors suddenly relax and the resistance disappears. This reflex arc, in extreme circumstances, may function to protect muscles from inappropriately high and potentially damaging tension production. However, the circuitry forms a physiological tension servo feedback mechanism to maintain a particular contracted state of the whole muscle, for example when some fibers weaken or drop out due to fatigue. It also is important for steady tension production required in fine motor control, for example in holding a fragile object. Thus, the tension servo mechanism acts in conjunction with the length servo mechanism provided by the muscle spindles to maintain precise positioning of limbs, postural maintenance, and manipulation of grip.
A third important spinal circuit involves nociceptive input and the complex polysynaptic reflex arc that mediates flexion or withdrawal of a limb from an aversive stimulus [1,2], for example standing on a nail (Fig. 2.11). Pain fibers enter the spinal cord and branch profusely to activate excitatory interneurons, which in turn excite α-motoneurons to flexor muscles. The magnitude of the noxious stimulus governs the size of the withdrawal response and the number of flexors responding: a highly painful stimulus will excite all the flexors of the affected limb. Thus, this reflex crosses dermatomal boundaries and involves integration between several spinal segments. As with the myotatic reflex, this circuit is elaborated to incorporate reciprocal inhibition of the antagonistic extensor muscles. Furthermore, a postural component is added, the crossed-extensor reflex, to support the weight of the body on the contralateral leg when the foot is with-drawn.
Fig. 2.10 Circuits for the inverse myotatic or clasp knife spinal reflex. This uses information from the lb afferents of Golgi tendon organs, which monitor tendon stretch and muscle tension. The reflex may be evoked by attempting to stretch a muscle during an isometric contraction. This causes a rapid increase in tension and a massive volley of action potentials from the tendon organs. Via inhibitory interneurons (–, black cell body), α-motoneuron activity is suppressed and the limb collapses, similar to the closing of a clasp knife blade
From this brief description, it is clear that the spinal cord carries out the basic steps of sensorimotor integration, which are further elaborated by the brain. However, the spinal cord circuits are not there simply to carry out reflex actions to sensory stimulation. The same circuits are recruited by descending inputs from the brain to simplify voluntary movement and postural adjustment. For example, the crossed-extensor reflex pathway (one leg flexed, the contralateral leg extended, and vice versa) is also an element in the sequencing of walking.
Blood flow to the nervous system is influenced or regulated by a number of factors including local tissue metabolism, oxygen and carbon dioxide tension, pH, circulating vasoactive agents, the intrinsic innervation of the blood vessels, and systemic perfusion pressure. This has been extensively documented for the cerebral circulation (see reviews [24–26]). However, the precise details of vascular regulation differ in the central and peripheral nervous system: moreover, the vascular supply has different characteristics in dorsal root ganglia and nerve trunks of the somatosensory nervous system. Given the importance of impaired blood flow in several peripheral nerve disease states, including diabetic neuropathy (see Chapter 4, page 115-123), a brief overview of the salient features of the normal vascular supply to the peripheral somatosensory system is appropriate.
Fig. 2.11 Circuits for the flexion (withdrawal) and crossed-extension reflexes. These reflexes are mediated by polysynaptic pathways in the spinal cord. Noxious stimulation of Aδ fibers causes excitation of α-motoneurons supplying the ipsilateral flexor muscles, which withdraw the limb from the threat of damage. Excitatory interneurons also connect to the α-motoneurons of the extensors on the contralateral limb, causing contraction to support the weight of the body during limb withdrawal. α-Motoneurons supplying the antagonistic muscles on both sides of the cord are inactivated via inhibitory interneurons during this reflex
Peripheral Nerve Trunk and Spinal Roots
The vasculature of peripheral nerve is relatively unique. Peripheral nerve and its spinal dorsal and ventral roots have a good vascular supply composed of two integrated but independent systems, termed the extrinsic and intrinsic circulations [27–29]. The extrinsic system comprises vessels that arise from local large arteries and veins as well as offshoots from the vessels supplying adjacent muscles and periosteum. These are arranged segmentally and follow the surface along the length of the nerve (Fig. 2.12). They form a highly anastomotic plexus within the epiperineurial layers of nerve sheath, vessels being mainly arterioles, venules, and arteriovenous shunts. This provides numerous connections with the intrinsic circulation. The latter, or vasa nervorum, comprises vessels on the perineurium and in the endoneurial vascular bed. Terminal arterioles from the perineurium penetrate the nerve fascicles and form the endoneurial capillary bed, which consists of a network of intrafascicular capillaries that run longitudinally, along with the nerve fibers, throughout the length of the nerve. Endoneurial capillaries are lined by a continuous layer of endothelial cells connected by tight junctions, which forms part of the blood-nerve barrier, analogous to, although somewhat less efficient than, the blood-brain barrier that restricts ingress of blood-borne substances to the central nervous system. The other component of the blood-nerve barrier is the inner layer of the perineurium, which is continuous with the arachnoid membrane of spinal cord [30].
Fig. 2.12 Peripheral nerve gross structure and blood supply. Peripheral nerves are surrounded by a loose connective tissue structure, the epineurium, which contains a plexus of blood vessels supplied by radial branches from multiple feed arteries. Nerves are divided into fascicles by the perineurium, which is a strong connective tissue that isolates the fascicles of nerve fibers physically to form part of the blood-nerve barrier. The other component of the barrier is the tight endothelial lining of the endoneurial capillaries
The endoneurial capillaries have a large diameter compared to those in other tissues such as brain and skeletal muscle, and the intercapillary distance is relatively great [31,32]. The latter would tend to render nerve susceptible to ischemic or hypovolemic stresses or nerve edema [33]. However, the extensive anastomotic connections between extrinsic and intrinsic circulatory systems minimize the effect of local disruptions to nerve blood supply, and, coupled with a low metabolic rate, make peripheral nerve relatively resistant to mild ischemia. The intrinsic circulation consists predominantly of capillaries, and there is a paucity of vascular smooth muscle in the endoneurium [32]. Thus, the main neural and humoral control of nerve perfusion is exerted at the level of the epiperineurial arterioles. These vessels are densely innervated by nerve fibers [34,
