Learning Outcomes
- Discuss muscle tension and contraction
Muscle Twitch
We can improve our understanding of muscle contraction by examining the contraction of one muscle fiber. A twitch occurs when one muscle fiber contracts in response to a command (stimulus) by the nervous system. The time between the activation of a motor neuron until the muscle contraction occurs is called the lag phase (sometimes called the latent phase). During the lag phase a signal called an action potential moves to the end of the motor neuron (axon terminal). This results in release of acetylcholine and depolarization of the motor end plate. The depolarization results in the release of calcium by the sarcoplasmic reticulum and subsequent binding of calcium to troponin which causes the myosin binding site to be exposes. This is followed by the actual muscle contraction that develops tension in the muscle. This next phase is called the contraction phase. During the contraction phase the cross-bridges between actin and myosin form. Myosin moves actin, releases and reforms cross-bridges many times as the sarcomere shortens and the muscle contracts. ATP is used during this phase and energy is released as heat. Myosin releases from actin when a second ATP attaches to myosin. Myosin is now available for another cross-bridge formation. When the muscle relaxes the tension decreases. This phase is called the relaxation phase. During this phase calcium is actively transported back into the sarcoplasmic reticulum using ATP. The troponin moves back into position blocking the myosin binding site on the actin and the muscle passively lengthens.
Muscle Stimulus and Contraction Strength
A skeletal muscle fiber will produce a given amount of force if the stimulus is strong enough to reach the threshold for muscle contraction. This is called the all or none law. Let’s say that we are electrically stimulating a muscle fiber. We begin with a low amount of stimulation that does not reach the threshold to produce a contraction. The muscle fiber will respond by remaining relaxed, it will not contract. Now if we increase the stimulation so that enough is produced to reach the threshold the muscle fiber will respond by contracting. Finally if we continue to increase the stimulus so that it well exceeds the threshold the fiber will respond by contracting with the same force as when we just reached the stimulus. The muscle will not contract with greater force if the stimulus is greater. The muscle responds to stronger stimuli by producing the same force. In skeletal muscles a motor neuron can innervate many muscle fibers. This is called a motor unit. There are numerous motor units throughout skeletal muscles. Motor units act in a coordinated fashion. One stimulus will affect all of the muscle fibers innervated by a given motor unit.
Muscle Length-Tension Relationship
The length of a muscle is related to the tension generated by the muscle. Muscles will generate more force when stretched beyond their resting length to a point. Muscles stretched beyond this point will produce less tension. If the muscle is at its resting length it will not produce maximal tension because the actin and myosin filaments excessively overlap. Myosin filaments can extend into the Z-discs and both filaments interfere with each other limiting the number of cross-bridges that can form. If the muscle is stretched to a point the tension will increase in the muscle. The actin and myosin filaments can now optimally overlap so that the greatest number of cross-bridges can form. If the muscle is overstretched the tension will decrease. The actin and myosin filaments do not overlap causing a decrease in the number of cross-bridges that can form. The ideal length of a sarcomere during production of maximal tension occurs when thick and thin filaments overlap to the greatest degree.
Control of Muscle Tension
Neural control initiates the formation of actin–myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to pull on bones, causing skeletal movement. The pull exerted by a muscle is called tension, and the amount of force created by this tension can vary. This enables the same muscles to move very light objects and very heavy objects. In individual muscle fibers, the amount of tension produced depends on the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin, and more tension will be produced.
The ideal length of a sarcomere during production of maximal tension occurs when thick and thin filaments overlap to the greatest degree. If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree, and fewer cross-bridges can form. This results in fewer myosin heads pulling on actin, and less tension is produced. As a sarcomere is shortened, the zone of overlap is reduced as the thin filaments reach the H zone, which is composed of myosin tails. Because it is myosin heads that form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by this myofiber. If the sarcomere is shortened even more, thin filaments begin to overlap with each other—reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.
The primary variable determining force production is the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, the motor cortex of the brain only signals a few neurons of the biceps, and only a few myofibers respond. In vertebrates, each myofiber responds fully if stimulated. When picking up a piano, the motor cortex signals all of the neurons in the biceps and every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more, because the tropomyosin is flooded with calcium.
Types of Muscle Fibers
There are three major types of skeletal muscle fibers. These are called fast twitch, slow twitch and intermediate. Generally, fast twitch fibers generate high force for brief periods of time. Slow twitch fibers generate lower amounts of force but can do so for longer periods of time. Intermediate fibers have some characteristics of both fast and slow twitch fibers. Fast twitch fibers are also called Type II fibers. Fast twitch fibers are the predominant fibers in the body. They respond quickly to stimuli and can generate a good deal of force. They have a large diameter due to the large amount of myofibrils. Their activity is fueled by ATP generated from anaerobic metabolism. Slow twitch fibers respond much more slowly to stimuli than fast twitch fibers. They are smaller in diameter and contain a large number of mitochondria. They are capable of sustaining long contractions and obtain their ATP from aerobic metabolism. Slow twitch fibers are surrounded by capillary networks that supply oxygenated blood for use in the aerobic energy systems. They also contain a red pigment called myoglobin. Myoglobin can bind oxygen (like hemoglobin) and provide a substantial oxygen reserve. Because of the reddish color of myoglobin these fibers are often called red muscle fibers. Slow twitch fibers are also called Type I fibers. Intermediate fibers resemble fast twitch fibers because they contain small amounts of myoglobin. They also have a capillary network around them and do not fatigue as readily as fast twitch fibers. They contain more mitochondria than fast twitch but not as many as slow twitch fibers. The speed of contraction and endurance also lie between fast and slow twitch fibers. Intermediate fibers are also called Type IIa fibers. Muscles that have a predominance of slow fibers are sometimes referred to as red muscles such as in the back and areas of the legs. Likewise muscles that have a predominance of fast fibers are referred to as white muscles. It is interesting to note that there are no slow twitch fibers in the eye muscles or muscles of the hands.
