Motor Units, Skeletal Muscle Fibers and Effects of Exercise
Objectives
- Students will be able to describe the structure and function of a motor unit, motor unit recruitment and the phases of twitch contraction.
- Students will be able to apply this information to explain how the frequency of stimulation affects muscle tension and leads to muscle tone, and isotonic and isometric contractions.
- Students will be able to compare the structure and function of the three types of skeletal muscle fibers (cells).
- Students will be able to describe the effects of exercise on different types of skeletal muscle fibers.
Motor Units
As you have learned, every skeletal muscle cell must be innervated by a somatic motor neuron in order to contract. This somatic motor neuron will branch and innervate several skeletal muscle cells; additionally, each skeletal muscle cell is innervated by only one somatic motor neuron. A single somatic motor neuron and all the skeletal muscle cells that it innervates are known as a motor unit. The size of a motor unit is variable depending on the nature of the muscle.
A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle cells in a whole skeletal muscle. Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyes. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual neuromuscular junctions. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc.
A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor units are concerned with simple, or "gross," movements, such as powerfully extending the leg at the knee joint. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle, as its axon splits into thousands of branches.
There is a wide range of motor units within many skeletal muscles, which gives the nervous system a wide range of control over the muscle. The small motor units in the muscle will have smaller, more excitable motor neurons, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units, results in a relatively small degree of contractile strength (tension) generated in the muscle. As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smallest motor units in the muscle. This allows a coffee mug to be picked up using the biceps brachii arm muscle with minimal force, and a couch to be lifted by the same muscle by recruiting the largest motor units. Another way to think about recruitment is to remember back to a time when you have moved residences. When you move you pack everything into boxes and recruit help to aid you in moving which objects ... which needs more help: the box full of toiletries or the sectional couch? You recruit help for the couch because it requires more force to move that you alone can produce. If each person helping in the move, including yourself, are considered to be individual single motor units, it becomes clear only one motor unit is required to move smaller, lighter objects. But when moving very large, heavy objects more motor units are required, producing a great deal of force and successfully moving the object.
