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Neural Control of Muscles in Exercise
triglycerides (which store fat). On the reverse side is catecholamine activity, which activates glycogenolysis in the muscle as well as lipolysis.
NEURAL CONTROL OF MUSCLES IN EXERCISE
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In order to move a load, the sarcomeres need to shorten. This shortening force is called muscle tension. This leads to two types of skeletal muscle contractions, which are isotonic and isometric contractions. In isotonic contractions, the muscle shortens and stays constant. There are two types of isotonic contractions, which are concentric and eccentric. Concentric contractions involve the shortening of the muscle in response to the load. Eccentric contraction occurs when the muscle tension decreases and the muscle lengthens. These are used for movement and balance in the body.
Isometric contractions occur as the muscle produces tension without actually changing the angle of the skeletal joint. There is sarcomere shortening and muscle tension that cannot overcome the weight of the load. This happens when lifting a heavy load. Most actions of the body are a combination of isotonic and isometric contractions that together produce a wide range of possible outcomes.
As you remember, every skeletal muscle fiber needs to have innervation by the axon terminal of a motor neuron in order to engage in the process of contraction. The group of muscle fibers that are innervated by a single motor neuron is referred to as a motor unit. The actual size of a motor unit depends on what the muscle is used for. Small motor units permit fine motor control. This is seen particularly in facial muscles. Extraocular muscles have six fibers per motor unit. The large motor units are seen when gross or large movements are necessary.
The smaller motor units will have lower-threshold motor neurons that are more excitable and that fire to small numbers of muscle fibers. There will be the resultant small degree of tension on the muscle. When more strength is needed, the larger motor units, which have a higher threshold of activation, will become active, resulting in an increase in muscle contraction called recruitment. Recruitment increases the strength of the muscle. This is how the nervous system controls the strength of a given muscle.
There is a relationship between the length of a sarcomere and the tension that can be created by the muscle. When a muscle fiber contracts, it can only contract over the space where the thick and thin filaments already overlap. This means that the total length of the sarcomere has a direct influence on the force that can be generated. This relationship is called the length-tension relationship.
The ideal length of a sarcomere necessary to produce maximal tension occurs at 80 to 120 percent of its resting length or about 100 percent. This will maximize the overlap of the of actin-binding sites and myosin heads. If the sarcomere is stretched beyond 120 percent, there is poor overlap of the thick and thin filaments. If the sarcomere is shortened less than 80 percent, the zone of overlap is insufficient. If the muscle is stretched to where the thick and thin filaments don’t overlap, no cross-bridges can form. This generally does not occur because there is connective tissue that supports the muscle fiber.
A single action potential sent from a motor neuron will cause a twitch of the muscle, which can be short or long, depending on the type of muscle. The tension produced by a single twitch can be measured in an electromyogram, which measures tension over time. There are three phases to any twitch. The first is the latent phase, which involves calcium release by the sarcoplasmic reticulum and propagation of the action potential. There is no actual contraction. This is followed by the contraction phase, where the sarcomere is actively shortening. The relaxation phase is last, when the tension is decreased and calcium ions are pumped back into the sarcoplasmic reticulum.
It actually takes a series of action potentials to produce a muscle contraction that will produce work. Muscle contraction is normally more sustained and modifiable by input from the nervous system in what’s called a graded muscle response. It takes both a highenough frequency of action potentials and a higher number of motor neurons transmitting action potentials to affect the actual tension produced by the muscle. The second action potential will add to the first in a process called “wave summation.” The process of tetanus happens when there is sustained muscle contraction over time.
When a skeletal muscle has been dormant for a prolonged period of time and then activated, the initial contractions will generate half the force of later contractions. This
