Twitch Muscle Fibers

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muscle contraction


For a skeletal muscle fiber to contract, a stimulus must be applied to it. The stimulus is delivered by a nerve cell, or neuron. A neuron has a threadlike process called and axon that my run 91 cm or more to a muscle. A bundle of such fibers from man different neurons composes a nerve. A neuron that stimulates muscle tissue is called a motor neuron.

The motor neuron branches into terminal structures called telodendria that come into close approximation with a portion of the sarcolemma o a muscle fiber (muscle cell). The site where the neuron and muscle fiber meet is called a neuromuscular junction. At this junction the muscle fiber membrane is specialized to form a motor end plate.

The sensitivity of a muscle fiber to electrical stimulation through the skin surface is largely a function of the proximity of a neuromuscular junction, the thickness of the skin, and variations in skin conductivity. An electrical stimulus that causes a muscle twitch at one spot may have no effect on muscle fibers that are only a centimeter away. Specific spots on the skin where minimal electrical stimuli cause muscle twitching are called motor points. The purpose of this exercise is to identify specific spots on the skin where these motor points are located.

If the electrical stimulus is close enough to pain receptors (naked nerve endings) in the skin, one may experience a slight pricking sensation as the stimulus is applied. Since the skin has many of these pain receptors, this sensation is often experienced; however, a muscle twitch may or may not be produced simultaneously with the pricking sensation. Our intent will be to concentrate on the identification of motor points.

Muscles are made up of functional units called motor units. Each motor unit consists of a neuron which supplies one or more muscle fibers. When a motor unit is stimulated, its muscular component will contract with all of the force they (it) can generate, or they (it) are not contracting at all / in a given muscle there are many motor units, these units vary in their level of irritability. Tat is some will respond to a weak stimulus, while others may require a much stronger stimulus. Often the smaller units are less irritable than the larger units, therefore we often have better control when the task requires only slight strength, and we have much poorer control when greater strength is needed. The overall strength of a contraction of a muscle is the summation of the pull of the motor units operating at any one time.

There are two types of summation, spatial and temporal. An increase in the strength of the overall muscle contraction due to spatial summation is brought about by increasing the strength of a stimulus which increases the number of motor units that are operating. Remember that the muscle fibers that are contracting are doing so with all of their strength.

Temporal summation is brought about by increasing the rate of stimulus so that the same muscle fibers are contracting again before they have had a chance to completely relax. Each individual fiber can contract more powerfully when the stimuli arrive close together in this fashion, then it can contract in a single twitch. This may at first seem to violate the all or nothing principle. The explanation for temporal summation involves the elastic properties of muscles. Tension developed is translated through many structures. It is transmitted from the cross bridges through thick and thin filaments, across Z lines, extracellular connective tissue, muscle and bone. Each of these components has a certain amount of elasticity. Only when all of these elastic structures are taut can increasing contraction by the muscle occur. When a second stimulus occurs very close to the first the elastic structures are not yet slack and the result is the contraction is stronger than any simple muscle twitch. Temporal summation will not be studied at this time.

Human muscle can...
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