Neuroprosthetics: Theory and Practice (Second Edition)
All you know about the external world, and all you can do about it, depends on your peripheral nervous system and cranial nerves, the latter a rostral extension of the former. In this chapter we provide an overview focused on the structure of peripheral nerves responsible for cutaneous and proprioceptive sensation and for voluntary muscle control.
The peripheral nervous system (PNS) consists of two major divisions, autonomic and somatic. The autonomic system consists of a preganglionic component (with nerve cells originating in the spinal column or skull) and a postganglionic component (with nerve cells lying wholly outside the axial skeleton). The somatic system consists of nerve cells with cell bodies located within or near the spinal column or skull. The autonomic system is largely concerned with functions not normally under voluntary control, while the somatic system is concerned with sensory inputs that can be perceived and with voluntary control of skeletal muscles. The voluntary nervous system is divided on anatomical and functional grounds into sensory and motor components. Except for the cranial nerves, the sensory system consists structurally of nerve cells with somata located outside the spinal cord in aggregates called dorsal root ganglia. These cells have no dendrites, but instead possess a single process that bifurcates upon leaving the ganglion, one process travelling centrally through dorsal roots to enter the dorsal part of the spinal cord and the other process travelling distally through peripheral nerves to innervate or form distal sense organs. These nerve processes are called afferent nerve fibers because they conduct action potentials and, hence, information from the periphery to the central nervous system.
The sensory elements can be divided, following the example of Gray , into the “special” senses of audition and vestibulation, smell, taste, and vision, and the “general” senses which include just about everything else that gives rise to conscious sensation. The “general” senses fall into two major domains: somatosensory and proprioceptive. One can think of the somatosensory or exteroceptive system as providing information about the state of the external environment as it interacts with the body, and of the proprioceptive system as providing information about the orientation of the body in space. This reduces to a consideration principally of cutaneous receptors and muscle/joint receptors. The motor system consists structurally of nerve cells with somata located in the ventral quadrant (horn) of the spinal cord. These cells have dendrites ramifying through the ventral portions of the spinal cord, and single axons that leave the spinal cord through the ventral roots and join with the sensory fibers to form the peripheral nerves and eventually innervate skeletal muscle fibers. These nerve process are called efferent nerve fibers because they conduct action potentials and, hence, information from the central nervous system to the periphery.
Active movement of joints is produced by contraction of extrafusal muscle fibers. A single motor neuron will typically innervate several muscle fibers in a given muscle. Since there is normally a one-to-one relationship between an action potential on a motor neuron and an action potential in the post synaptic muscle fiber, the axon and all its innervated muscle fibers is called a motor unit. Under normal conditions, only a single nerve fiber innervates a given muscle fiber, although multiple innervation can occur transiently in response to partial denervation of muscle. The electrical properties of muscle fiber membrane makes it difficult to active these fibers by externally applied currents. Thus, except in the case of totally denervated muscle, electrical stimulation of muscle by externally applied current normally consists of activation of the terminals of motor neurons that in turn activate the muscle.
Within the spinal cord, afferent process make local synapses and may terminate within a few segments of their entry point or they may travel directly, via the dorsal columns, to the medulla of the brainstem. On the efferent side, the dendrites of motor neurons receive input from local circuits, including feedback from sensors in the skin and muscle, and input from descending pathways originating in the brain.
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