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When a peripheral nerve axon divides, its branches end on a number of muscles fibers. The axon and the muscle fibers with which it connects are collectively called a motor unit. Many motor units fire in a given muscle action. The receptor sites for motor nerve endings on each skeletal muscle cell lie near the cell center. Hence, the action potential generated by the nerve fibers spreads in both directions to the ends of the muscle cell.
Acetylcholinesterase, an enzyme in the synaptic space of the neuromuscular junction, inactivates acetylcholine, thus preventing the prolonged and excessive action of acetylcholine on the muscle cell during a single muscle cell firing. Multiple successive contractions of a muscle's fibers can, however, be summed into a single, strong, continuous contraction called tetany, through the rapidly repetitive discharge of motor fibers to the muscle. The strength of the contraction may be further increased by increasing the number of motor fibers that discharge simultaneously.
The striated appearance of striated muscle is due to the geometric alignment of groups of myofibrils. Smooth muscle cells do not have this geometric alignment, but do use a modified actin-myosin interaction.
In assessing the various kinds of motor weakness, it is important to determine whether the problem lies within the nervous system or within the muscles themselves. Sometimes weakness is based on a problem within the central nervous system motor pathways, e.g. injury to a cerebral hemisphere or spinal cord motor pathway (upper motor neuron deficits). At other times weakness is due to peripheral nerve injury (lower motor neuron deficits).
Weakness may also originate with a defect at the neuromuscular junction. Myasthenia gravis, for instance, is an autoimmune disease which attacks the acetylcholine transport proteins at the neuromuscular junction, resulting in profound weakness. It may be treated with acetylcholinesterase inhibitors, which allow the acetylcholine that has been released into the synaptic space to stay around longer and thus act more effectively.
A defect in the plasma membrane (sarcolemma) of the striated muscle cell may cause weakness, either through unresponsiveness to stimulation (as occurs in periodic paralysis) or hyperresponsiveness to stimulation (as occurs in myotonia, where there is increased muscle contractility in an activity such as gripping an object, with difficulty in releasing the grip). In Duchenne muscular dystrophy there is a hereditary defect in dystrophin, a membrane protein necessary for the integrity of the sarcolemma.
Weakness may also result from deficits inside the muscle cell, which interfere with the normal contractile mechanism of the muscle cell.
The proper diagnosis of such conditions lies in part in sorting through the different clinical manifestation of the patient. (See Fig. 9-12 for a summary of the differences in upper versus lower motor neuron weakness). One may also find nerve and muscle biopsies helpful.
The electromyogram (EMG) and the testing of nerve conduction velocities may provide important clues as to the cause of weakness. In testing nerve conduction velocities, one electrically stimulates a periph eral nerve, thereby simultaneously firing many nerve fibers. One then determines how long it takes for the grouped action potential to reach some distant point either along the nerve, or in its end point. If the peripheral nerve is intact, the conduction velocity and amplitude of the action potential should be normal. The amplitude of the potential will be decreased, however, in conditions where peripheral nerve fibers have been lost. Also, the nerve conduction velocity will be decreased in certain intrinsic peripheral nerve diseases (peripheral neuropathies).
The EMG is an important complimentary test to nerve conduction velocitiy. A needle is inserted into the muscle that is undergoing testing. There are a variety of ways to gather information in this setting; one is to have the patient contract the muscle. In myopathies, since muscle fibers have disappeared, there may be lower amplitude motor unit spike responses on attempting a contraction of a given strength. Since there are fewer muscle fibers the patient has to try harder to contract the muscle, and does so by firing more motor units, to compensate for the decrease in number of muscle fibers in each motor unit. Thus the EMG may show high frequency, low amplitude discharges in myopathies.
On the other hand, the motor unit spikes may increase in amplitude in peripheral nerve disease, where some nerve fibers disappear, but others may sprout and spread to take over the vacated areas on denervated muscle fibers. This expands the breadth of the persisting motor units and may cause an increased amplitude of motor unit discharge.
The hyperexcitable muscle membrane in myotonia shows rapid-firing action potentials. Muscle fibers that have been denervated exhibit slow, repetitious action potentials, called fibrillations (as opposed to the macroscopic, visible twitching of denervated muscles, called fasciculations).
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