Science SectionPeripheral Nerve: From the Microscopic Functional Unit of the Axon to the Biomechanically Loaded Macroscopic Structure
Section snippets
Nerve Fibers—The Functional Units
The microscopic functional unit of the peripheral nervous system (PNS) is the nerve fiber, made up of the axon and its associated Schwann cells. Myelinated nerve fibers have serially arranged Schwann cells, each of which has individually wrapped a segment of axon with multiple layers of plasma membrane, termed a myelin sheath. During development, only axons that are 0.7 mm or larger in diameter will be myelinated.1 The narrow region of the axon between myelinated segments is called a node of
The Neuron Action Potential
An action potential refers to the brief reversal of the electrical potential across a cell membrane, and is the method by which neurons transmit information (Figure 1). In response to mechanical, chemical, or thermal stimulation in sensory neurons, or neurotransmitter stimulation in motoneurons, sodium (Na+) channels in the plasma membrane open, elevating the electrical potential from its resting −70 mV. Although potassium (K+) channels are also opened, if enough Na+ ions enter the cell to raise
Axonal Transport
Neurons are polarized cells with distinctive dendritic and axonal processes. In some PNS neurons, the axon terminal may be up to 3 ft distant from the cell body and thus depend on regulated axonal transport for growth and maintenance. Neurons have developed directional fast axonal transport to deliver cargo in anterograde and retrograde directions, away from or toward the cell body, respectively, and slow anterograde axonal transport to deliver cytoskeletal proteins. Neuronal microtubules form
Motor or Efferent Fibers
The motor or efferent system includes somatic motor axons to skeletal muscle of somatic origin, branchial motor axons that innervate skeletal muscle of branchial arch origin, and visceral motor, autonomic axons to smooth muscle, cardiac muscle, and glands. Somatic motor and branchial motor axons are large diameter myelinated axons that release the neurotransmitter, acetylcholine, directly on skeletal muscle fibers at neuromuscular junctions. A single motor neuron and all of the skeletal muscle
Peripheral Nerves
Peripheral nerves extend from the spinal cord and brainstem, and may contain both motor and sensory nerve fibers. The cell bodies of somatic motoneurons are located in the ventral horn of the spinal cord. The cell bodies of autonomic, visceral motoneurons of the sympathetic nervous system are located in the intermediolateral cell column of the T1–L2 or L3 segments of the spinal cord. The cell bodies of visceral motoneurons of the parasympathetic nervous system are located in the brainstem
CNS–PNS Transition Zones
Transition zones, which mark the change from central nervous system (CNS) to PNS and provide some structural support to the nerve rootlets, are visible in histological sections from dorsal rootlets near their entry into the spinal cord and ventral rootlets near their exit from the cord surface. Transition zones vary in morphology; some are flush with the CNS surface, whereas others have extensive central tissue projections into the PNS tissue.17 In general, projections of central tissue are
Peripheral Nerve Connective Tissues
Peripheral nerves are supported by three layers of connective tissues whose structures have been investigated using electron microscopy and immunohistochemical and biochemical methods. Endoneurium is the innermost connective tissue between individual nerve fibers and consists of endoneurial fluid, longitudinally oriented types I and II collagen fibers and a network of fine collagen fibers closely associated with the continuous basal laminae that envelop myelinated and unmyelinated nerve fibers.
Vascular Supply
The vascular supply of peripheral nerve is derived from segmental vessels that enter the epifascicular epineurium and pass as arterioles and venules along the length of the nerve. These relatively permeable vessels possess fenestrations and clefts between endothelial cells35 and anastomose within the interfascicular epineurium. Small arterioles pass obliquely through the perineurial sheath, carrying a layer of perineurial cells. Within the endoneurial space, the vessels form continuous
Blood–Nerve Interface
The blood–nerve interface that controls the endoneurial environment is actively maintained by perineurial cells and by endothelial cells of endoneurial capillaries, and has been described recently as being under homeostatic regulation.39, 40 Endoneurial fluid is thought to be in communication with cerebrospinal fluid of the subarachnoid space,41 and flows at approximately 4–8 mm per hour in a proximo-distal direction.42 Despite this fluid flow rate, the endoneurial ionic environment must be
Fascicle Organization
The numbers and organization of fascicles within a nerve differs between nerves, between species, between individuals, and along the length of the nerve. Thus, it is not surprising that the debate over fascicle organization continues in the literature. A recent review provides support for both a “cable” organization in which fibers of particular nerve branches remain sequestered in discreet fascicles throughout the length of the nerve, and a “plexiform” arrangement in which fascicles branch and
Mechanosensitivity of the Nervous System
Mechanosensitivity refers to how sensitive or tolerant nerves are to mechanical stresses. It is the mechanism with which the neural tissue can respond to stress levels that are potentially harmful and is thought to be a normal protective mechanism. Little is known about how this mechanism works but there is some evidence that the nerve fibers innervating the connective tissue of the nerves themselves, the nervi nervorum, have nociceptive properties.54, 55 In addition, the development of an
Nerve Biomechanics
Peripheral nerves in the upper extremity need to accommodate a vast array of mechanical stresses associated with the diverse repertoire of human movement related to function and participation in daily activities. The structural elements of the PNS must support the ultimate function of conduction of electrical impulses through myriad positions, postures, and movements that often stress the nerves over multiple limb segments and joints simultaneously. It is well known that compressive and tensile
Quiz: Article #220
Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to JHTReadforCredit.com. There is only one best answer for each question.
- #1.
The following element of a peripheral nerve accommodates the length changes associated with joint motion
- a.
perineurial tissue
- b.
epineurial tissue
- c.
connective tissue
- d.
axonal tissue
- a.
- #2.
This dimension determines if a nerve is myelinated or not
- a.
diameter
- b.
length
- c.
distance between nodes
- d.
depth from skin
- a.
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