Muscles and Action
Muscle, the raw material of movement, is a contractile tissue that makes up from 35 to 45 per cent of the body’s weight and powers all its actions – from the merest flicker of an eyelid to the sustained effort needed to run a marathon race. Even during sleep some muscles remain active in order to power vital support systems.
There are 3 types of muscles, all different in structure and function. Skeletal muscle – the meat on our bones – is Best and most abundant. Smooth muscle, found in the arteries, intestines and other internal organs, perform slow, sustained contractions.
Cardiac muscle is specific to the heart, generating the powerful contractions that pump blood throughout the body.
Voluntary Muscles
Only skeletal muscle is under conscious control – governed directly by the central nervous system – and is therefore described as voluntary muscle. It is networked with blood vessels and nerve endings and consists of fibres up to a maximum of about 30 cm long. These fibres have the ability to contract along their whole length in response to nerve stimuli, sometimes shortening by as much as a third. Under a microscopic the fibres appear striped, which accounts for the name “striped” or “striated” muscle. Each voluntary muscle is bound within its own tough, elastic sheath and is further protected by layers of connective tissue.
Skeletal muscles, mostly attached to bone either directly or through tendons, include flexors to bend joints and extensors to straighten them. Abductor muscles are needed to pull the arm, for example, away from its natural position at the side of the body, and adductors return it to the side. Limbs are twisted by rotators. Each fibre in a muscle has a separate nerve branch; these branches meet at what is called the motor end plate. A nerve impulse from the brain or spinal cord releases a chemical transmitter substance that makes the fibre contract.
No single muscle, however, is of much use in isolation. Because contracted fibres requires an opposing force to expand, skeletal muscle is mostly arranged in antagonistic blocks with flexors working against extensors and abductors against adductors.
When muscles contract, using energy, heat builds up and carbon-dioxide. Water and lactic acid are formed. After strenuous exertion muscles are affected by an accumulation of lactic acid because there is not enough oxygen available for metabolism. This results in an aching sensation and in “heaviness” of the limbs. A build-up of lactic acid, combined with deprivation of salt and water or oxygen, may be the cause of cramp.
Involuntary Muscles
Involuntary muscle, the basis of internal support systems, has an equally vital role to play. Both types of involuntary muscle – smooth and cardiac – are in continuous use, maintaining such functions as respiration, digestion and circulation and heart contraction.
Smooth muscle fibres are arranged in two different patterns, depending on the delicacy of their task. There are multi-unit smooth muscles, for examples, in the arteries and in the iris of the eye. There are tiny individual fibres are separated, each contracting only when stimulated by a nerve impulse.
In the kind of smooth muscle found in the viscera – the intestines, bile ducts, ureters and other internal organs – smooth muscle fibres are packed together so densely that they are almost indistinguishable from each other. This is the kind of muscle that powers the wave-like movement known as peristalsis. The antagonistic mechanism vital to muscle function is achieved by the arrangement of two sets of muscle fibres – longitudinal and circular – that compose the muscular wall, and by two sets of autonomic nerves, the sympathetic and parasympathetic.
Early on in the digestive process there is an ingenious mixture of muscle types working in collaboration. This is in the oesophagus or gullet, the canal running from the mouth to the stomach. Voluntary muscle rings the upper third of this tube, but sensation of movement ceases as food reaches the middle section, which consists of mixed voluntary and involuntary muscle.
Voluntary and involuntary features are combined in the bladder. The smooth layers must first relax, allowing the bladder to stretch as it fills with urine, and then contract to expel the contents. The sphincter, a ring of striated muscles that is under conscious control, is the governing mechanism. Kept contracted while the bladder is filling, the sphincter seals off the outlet until, under voluntary direction, it is relaxed and urine is passed.
The muscle of the heart
Cardiac muscle – the fabric of the heart – is striated like voluntary fibres but it is not under conscious control. Highly adaptable, cardiac muscle is stretched as blood enters the heart chambers. The degree of expansion is signaled by pacemaker tissue to ordinary cardiac muscle cells, ensuring adequate and coordinated constriction to empty the heart completely. Once again the antagonistic principle comes into play and changes in the rate and strength of contraction are governed by the sympathetic and parasympathetic nerves. In the heart, as in the intestine, stimulation of one set of nerves increases contraction while the other set decreases it.
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