The major subcortical input to area 6 of motor cortex arises from the ventral lateral nucleus of the thalamus, which in turn comes from the basal ganglia (Fig. 2.4). These comprise the caudate nucleus and the putamen (collectively termed the striatum), the globus pallidus, the subthalamus, and the midbrain substantia nigra. These areas form a complex circuit that funnels or focuses activity from widespread areas of association cortex on to area 6, perhaps supplying basic motor programs for the desired action. This cortex → striatum → globus pallidus → thalamus → motor cortex loop is an integral part of the movement initiation process, and the other structures form side loops that modulate this pathway [6].Diseases of basal ganglia result in problems with voluntary movement initiation, involving hypokinesia, as in Parkinson's disease, or hyperkinesias, as in Huntington's disease. In addition to their role in initiation, basal ganglia also modulate posture and muscle tone, abnormalities of which are found in basal ganglia disease. The basal ganglia also have a role in nonmotor behaviors, including cognition and mood [7].
The cerebellum is involved with coordination of the sequence of muscle contractions during a movement. In man, there are three important functional subdivisions of the cerebellum. The anterior lobe (paleocerebellum, spinal cerebellum) and its associated deep cerebellar nuclei (fastigial, interposed, and lateral vestibular nuclei) is concerned primarily with processing of information from muscle, joint, and cutaneous mechanoreceptors. It also receives input pertaining to activity in the motor cortex via pontine nuclei and collaterals of corticospinal fibers. The anterior lobe output provides information to modulate the brainstem nuclei from which the reticulospinal tracts originate, and there is a projection to the red nucleus. These circuits are involved in the control of posture and gait.
The flocculonodular lobe (archicerebellum, vestibular cerebellum) and fastigial nucleus are involved with the coordination of the paraxial muscles associated with balance and equilibrium. The major input comes from the vestibular apparatus, and output goes to the vestibular nuclei and then to the vestibulospinal tracts. There is also an important vestibulo-ocular projection to the external ocular muscles.
The posterior lobe (neocerebellum, cerebral cerebellum) and dentate nucleus have massive reciprocal connections with cerebral cortex, including motor, premotor, sensory, and posterior parietal areas. This cerebellar subdivision is involved with coordination of voluntary movement sequences, particularly ballistic movements that are normally too fast to be under feed-back proprioceptive control. It is responsible for smooth and accurate execution of movements, and shows evidence of the synaptic plasticity necessary for the learning and refinement of complex motor skills [8].
Peripheral Nerve Fiber Types
Peripheral nerve consists of bundles of nerve fibers, generally mixed sensory and motor. Fiber types have been classified in two ways [2]. The first depends on axon diameter, with categorization into groups A, B, and C. The largest axons, which are myelinated, belong to group A. The smallest fibers, which are unmyelinated, belong to group C. The B group contains myelinated axons from autonomic preganglionic neurons, although this classification is rarely used today. The A group is further classified into the subgroups α, β, δ, and γ.
There is also a second classification system for some of the sensory axons, based primarily on conduction velocity, but also on origin and function. This categorization has numerical classes I–IV in descending order of conduction velocity. Because conduction velocity is directly related to axon diameter for myelinated fibers, the two classification systems can be related as shown in Table 2.1. Both terminologies are in common usage, although they were developed independently and do not overlap exactly in terms of fiber categories.
Somatosensory Receptors and Sensation
Somatosensory receptors can be divided into three groups: mechanoreceptors, thermoreceptors, and nociceptors (Table 2.2). Mechanoreceptors respond to deformation of their nerve endings, which contain specialized mechanosensitive ion channels whose gating depends on stretching or changes in tension of the surrounding membrane. The nerve endings of mechanoreceptors are usually associated with specialized nonneural structures that govern their detailed response characteristics, so determining the adequate stimulus. Mechanoreceptors mediate the sensations of light touch, pressure, vibration and flutter, and limb position and movement (kinesthesia). Examples of mechanoreceptors in hairy and hairless (glabrous) skin are shown in Figure 2.5.
Cutaneous mechanoreceptors [9] have punctate receptive fields whose size is determined by the area of nerve terminal branching and associated nonneural tissue. The information and sensation gleaned from these receptors is governed by their degree of adaptation to a constant stimulus. They may be classified into slowly adapting, moderately rapidly adapting, and very rapidly adapting categories. Slowly adapting receptors respond with an increased frequency of action potentials for the duration of a stimulus. Thus, they are able to accurately signal skin indentation and pressure. Merkel's disk receptors and Ruffini's endings fall into this category. Moderately rapidly adapting receptors respond with a burst of action potentials at stimulus onset and comprise the Meissner's corpuscle of glabrous skin and the hair receptor. They are best at signaling the velocity of movement of a stimulus, and are most sensitive to low-frequency (<50 Hz) repetitive stimulation. In this frequency range, a sinusoidal mechanical stimulus will give rise to the sensation of “flutter” where individual waves of the vibration are felt. This contrasts with higher frequencies, which are felt as a true unitary vibration, and this information is transmitted by the most rapidly adapting receptor type, the pacinian corpuscle.
Fig. 2.5 Schematic of hairy and glabrous (hairless) skin, showing the location of various mechanoreceptors. The receptors in glabrous skin are Meissner's corpuscles and Merkel's disks, located in the dermal papillae, and bare nerve endings. In hairy skin, there are hair receptors around the hair shafts. Merkel's disks, and bare nerve endings. Beneath both types of skin, in the subcutaneous region pacinian corpuscles and Ruffini's endings are found. (From [1], with permission)
The different receptor types work together along with hand movements and skin patterns such as fingerprints for shape and texture discrimination [10]. Thus, while the discharge of slowly adapting Merkel's disks is best at encoding the spatial characteristics of a stimulus, the more rapidly adapting receptors provide texture information from vibrations set up by the mechanical interaction between surface and fingerprints as the finger tip is moved across a surface.
The sensitivity of the skin to mechanical stimulation varies widely over the body [1], as can be seen from the results of two-point spatial discrimination tests (Fig. 2.6). Thus, highest sensitivity is noted for the fingers, lips, nose, and toes, whereas the trunk and upper limbs are relatively insensitive. This is a direct reflection of peripheral innervation density, the number of receptors per unit area of skin, and the average size of individual neural receptive fields, which are correspondingly larger in regions