The Sciatic Nerve
The sciatic nerve is a large nerve that originates from the distal spinal cord and extends along nearly the entire length of the hind limb. In most vertebrates, it's the major branch of the sacral plexus, a complex mass comprised of neurons that exit the spinal column via spinal nerves L4 through S4. The sciatic nerve innervates most of the hind limb. As is the case with many of the large nerves of the vertebrate nervous system, the sciatic nerve is a mixed-function nerve, meaning it is made up of the axons of sensory and motor neurons.
The sciatic nerve gives rise to branches as it progresses distally along the hind limb. Some of these branches contain motor & sensory neurons involved in control of the muscle groups of the upper leg (particularly the 'hamstring' group, which flexes the upper leg), and the lower leg (both flexors and extensors). In addition, sensory receptors in the skin of the entire lower leg and the posteriolateral surface of the upper leg transmit information to the brain via sciatic nerve neurons.
Damage to, or irritation of, the sciatic nerve at any point can result in a number of symptoms, some of them potentially serious. The malady we call sciatica is the result of inflammation of the sciatic nerve, usually caused by chronic irritation of one or more of the spinal nerves L4 S4. The usual causes are trauma to the intervertebral discs associated with the roots of spinal nerves L4 _ S4, but a number of other causes, including improperly administered hypodermic injections into the gluteal muscle, have been documented. Whatever the cause, sciatica is characterized by pain along the course of the sciatic nerve through the hip and down the back of the leg (can you suggest why the pain is felt in these parts of the leg?).
Pressure, either chronic or acute, applied to the sciatic nerve's dorsal and/or ventral roots can result in a number of symptoms in addition to pain. Impaired function of the motor neurons can result in weakness in the lower leg muscles. In extreme cases, inability of the lower leg muscles to control the ankle and foot can result in impaired gait due to foot drop (inability to dorsiflex the foot upward when stepping forward). Similarly, interference with normal function of the afferent fibers results in sensory disturbances such as paresthesia (a tingling or "pins and needles" sensation) or hyperthesia (increased or extreme sensitivity of receptors, particularly touch, temperature, and pain receptors). Severe sciatica can even result in wasting of the muscles of the lower leg as a result of a loss of normal stimulatory input to the muscle fibers.
Categories of Sciatic Nerve Neurons
As with other nerves in the vertebrate body, the sciatic nerve is comprised of the axons of hundreds of neurons. These axons vary greatly in diameter, from < 1 to 20 mm. Because conduction velocity is proportional to axon diameter, the conduction velocity of the sciatic nerve neurons also varies widely, from 0.2 to 150 m sec-1.
Neurons are often categroized on the basis of their morphology and/or function (e. g., sensory or motor). However, neurophysiologists often employ an alternate approach that groups neurons (often referred to as "fibers" in this context) according to their axon diameter and degree of myelination. So-called Type A fibers, have large diameters, thick myelin sheaths and correspondingly high conduction velocities (30 _ 150 m sec-1). These neurons are mostly motor (efferent) neurons that control activity of skeletal muscles, or sensory (afferent) neurons that convey information from receptors in the muscles, joints, and epidermal tissues to the spinal cord. Type B fibers have less well developed myelin sheaths and conduction velocities in the range of about 3 _ 15 m sec-1. Most of these fibers are part of the autonomic nervous system's efferent pathways that innervate internal organs and blood vessels and provide for regulation of their activties. The smallest diameter fibers, termed Type C fibers, lack myelin sheaths and have correspondingly low conduction velocities (< 2 m sec-1). Many of the Type C fibers are efferent neurons of the sympathetic nervous system and afferent pain neurons.