Question

Is a muscle twitch different from the type of muscle contraction we use everyday? With regards to skeletal muscle, define the term recruitment?

Answer 1

Muscle twitch is different than the everyday muscle contraction that we use.

Muscle twitch or muscle fasciculation is a small involuntary muscle contraction and relaxation of skeletal muscle that is seen under the skin. It is caused by direct stimulation or by a single action potential. In skeletal muscle, it is a single contraction and relaxation produced by the action potential in the muscle fiber. Mostly, they go unnoticed, such as after physical activity or stress. However, if severe, they indicate a neurological condition such as Loh Gehrig’s disease or muscular dystrophies. Muscle twitches are due to involuntary muscle contraction and are not spontaneous.

Everyday muscle contraction is spontaneous and voluntary. It can be isometric, isotonic or isokinetic. In isotonic contraction, the tension in muscle remains constant despite change in length of muscle. Motor units develop the tension in the muscle. For example, when a person hold a heavy object, tension develops in the muscle before the weight is lifted. When this tension exceeds the load, the muscle contracts to shorten the muscle and lift the load. These contraction can be concentric (when the muscle shortens) or eccentric (when the muscle lengthens). Isokinetic contractions are similar to isotonic contraction. However, in this contraction, the speed of the movements remains constant. For example, when riding a stationary bike or swimming. During swimming, the water provides the constant resistance to the movements. Isometric contraction generates force but do not change the length of the muscle. For example, when kicking a ball, the muscles of the feet do not change but enough force is generated to lift the ball and kicked.

Recruitment of skeletal muscle fibers is the ability to recruit additional motor units during muscle contraction to overcome the increased resistance in the muscle. This action increases the response to the stimulus. When skeletal muscles are stimulated by nerve impulses, individual skeletal muscle fibers are stimulated. A single motor unit in a skeletal muscle consists of a number of muscle fiber that is stimulated by a single neuron. When this neuron is activated, all fibers in the motor unit are stimulated. Lesser the number of muscle fibers in a motor unit, finer is the control.