Anatomic and Physiologic Considerations
The epineurium, a loose connective tissue layer, surrounds fascicles and protects (cushions) against external trauma. The outer condensation of this layer comprises the external epineurium. The portion of this tissue that extends into the substance of the nerve and defines fascicular groups is the internal epineurium. The external epineurium is very loosely connected to the surrounding tissues, permitting longitudinal nerve excursion and absorbing longitudinal stress (Fig 1). The thickness of the epineurium is variable along the course of the nerve; it is most abundant around joints, to protect the nerve from compression and friction.
Cross-sectional anatomy of peripheral nerve.
A strong layer of connective tissue, the perineurium, surrounds each fascicle and maintains neural mechanical integrity; it is an important mechanical barrier against trauma to the endoneural space and functions as a diffusion barrier. Individual blood vessels penetrate the perineurium, carrying with them a sleeve of perineural tissue.
Surrounding the individual axons is the collagenous tissue layer known as the endoneurium. This layer resists longitudinal stress and contributes to the formation of the endoneurial tube.
The fascicle is the smallest unit of the peripheral nerve that can be visualized and surgically dissected. It is composed of individual axons in endoneurial connective tissue and is surrounded by a sleeve of perineurium. Groups of fascicles are common and are defined by a thickening of the internal epineurium.
Internal nerve topography is important, but difficult to verify by surgical dissection. Recent investigations have found that fascicular groups travel close to one another over great distances, though interconnections between fascicular groups are common.[16,17]
The blood supply of peripheral nerves is composed of an extrinsic and intrinsic system. Fortunately, the orientation and interconnection of the intrinsic vascular supply permit nerve mobilization over great distances without significant intraneural ischemia.
Joint motion produces significant longitudinal excursion of peripheral nerve. After injury or surgery, restoration of nerve excursion is important in obtaining optimal results. Based on fresh cadaveric specimens, the longitudinal excursion of nerves is 15.5 mm in the median nerve and 14.8 mm in the ulnar nerve proximal to the carpal canal. With elbow motion, excursion of the median nerve is 7.3 mm proximal and 4.8 mm distal to the elbow. Ulnar nerve excursion with elbow motion is 9.8 mm proximally and 3.0 mm distally.
The basic cellular components of the peripheral nervous system are the neuron and the Schwann cell. The neuron is composed of a cell body and the axonal processes, which extend outward to the end organs. The cell body of the motor neuron is located in the anterior horn of the spinal cord. The sensory neurons are found in the dorsal root ganglia. The axonal processes connect the cell body to the periphery (Fig 2). Schwann cells encircle the axonal projections and may be myelinated or nonmyelinated. "Fast antegrade transport" occurs at speeds of 400 mm/day; retrograde transport occurs at approximately half this speed. Axon transection produces loss of appropriate neurotrophic factors, such as nerve growth factor (NGF), and death of the cell body.
Schematic illustration of cellular anatomy of peripheral nerves.
The transfer of information along the axon to a muscle fiber or from an end organ is accomplished by action potentials utilizing sodium (Na+) and potassium (K+) channels. The electrical resistance of the axoplasm and capacitance of the cell membrane inhibit the speed of the process. Myelin encircling the axon dramatically decreases capacitance, but inhibits ion flux across the membrane. The decrease in ion flux would negate the advantage of decreased capacitance, if the myelin sheath were continuous. However, at the junction between Schwann cells, the axon membrane is exposed. This interval is known as the node of Ranvier. This configuration allows a very rapid transmission of the action potential over myelinated segments with a momentary slowing of conduction at the nodes of Ranvier. The result is a dramatic increase in the speed of conduction through the process of saltatory conduction.
The motor unit describes a single motoneuron, its axonal projection, and the muscle fibers that it innervates. The motor unit includes multiple muscle fibers. The number of fibers innervated can vary greatly, from 100 to around 2,000. The smaller units are capable of higher-precision movement but less strength. The number of fibers included in the motor unit defines the innervation ratio. Individual axons join the innervated muscle fiber at a specialized site called the neuromuscular junction. Acetylcholine released by the axon diffuses across the synaptic cleft, binds to receptors on the muscle surface, and initiates muscle contraction. After ischemia, the axon remains capable of transmission for a period of time, indicating that the synaptic cleft is the first affected (Fig 3).
Motor unit and neuromuscular junction.
Exteroceptors can be grouped according to the way in which they adapt to stimuli. The rapidly adapting receptors (eg, Meissner and Pacinian corpuscles) produce an initial impulse that progressively dampens to zero with continuous and constant stimulus. The slowly adapting receptors (eg, free nerve endings, Merkel cell-neurite complex, and Ruffini end organ) continue their impulse at a dramatically lower rate from that of the initial impulse as long as the stimulus is maintained. A thorough review of sensory receptors has been presented by Dykes.
J South Orthop Assoc. 2001;10(2) © 2001 Southern Medical Association
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