Voluntary muscle contracts only when it is stimulated by nerve impulses from nerve fibres which originate from the central nervous system. These nerves, called motor nerves, tell the muscle when to contract and by how much. The instructions usually come from the brain via the spinal cord. The motor nerve branches at a point just before it reaches the muscle and individual branches then feed individual muscle fibres. There may be anything from five to 150 branches from one nerve. The nerve, together with all the muscle fibres it controls, is called a motor unit, and all the muscle fibres in one motor unit contract simultaneously when they are stimulated by their motor nerve. The point where the branch of the motor nerve connects with the muscle fibre is called the motor end plate. In the end plate is a small gap between the nerve ending and the surface membrane of the muscle fibre. An electrical impulse in the nerve triggers the release of a chemical messenger, a neurotransmitter, from tiny pits in the nerve ending; the neurotransmitter molecules then cross the junction to trigger a response in the muscle fibre.
In voluntary muscle the neurotransmitter is acetylcholine. Once the acetylcholine has initiated the muscle action potential, it is broken down and destroyed in the motor end plate to prevent further stimulation.
Patterns of control
The command for muscle contraction originates in the motor cortex, but co-ordination of muscles and muscle groups comes from the cerebellum. In order to do this it must receive information from muscles all over the body, concerning their state of contraction. There are two types of sensory receptors in muscles: one set, the Golgi tendon organs, detect muscle tension; another set, the muscle spindles, pick up information about the length of the muscle fibre; and both sets relay information to the brain along sensory nerve fibres. The brain can then send messages down to the motor nerves to make adjustments to the tension or length of the muscle. Such a feedback system is vital to maintain co-ordinated movement. The information supplied by the receptors therefore helps us to control our position and posture, and move in a smooth, well-regulated way. If the cerebellum is damaged or destroyed in a head injury the muscles are no longer well co-ordinated and movements become jerky and irregular.
When muscles contract without exerting a movement, usually when they are involved in maintaining posture, the contraction is termed ‘isometric’. Fibres
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Become tense, but they do not shorten. ‘Isotonic’ contraction occurs when muscles contract and physically shorten, to move a part of the body, but the muscles do not tense.
We have seen that the response of a muscle fibre to a single nerve impulse is ‘all or nothing’ – there is no halfway state. The electrical impulse from the motor nerve has to be strong enough to trigger an action potential in the muscle fibre, but beyond this a stronger impulse will not cause a stronger reaction. When a complete muscle, containing many hundreds or thousands of muscle fibres, is considered, the situation is more complicated. Various degrees of contraction occur when a muscle is stimulated, depending on how many individual fibres receive an impulse strong enough to bring about the changes that cause a muscle contraction. In general, a strong impulse along a motor nerve causes contraction of all the muscle fibres supplied by that one nerve. If a series of strong nerve impulses are sent in rapid succession the tension in the muscle rises progressively until a plateau is reached. The muscle is then in a state of constant contraction – a condition known as tetanus.