Contractile animal tissue that produces locomotion and power and maintains the movement of body substances. Muscle contains very specialized animal cells - long cells - that can contract to between one-half and one-third of their relaxed length.
Muscle tissue is sometimes found in large amounts, forming muscles, that are organs. Muscle tissue enables movement. It may move the whole body, or part of it, or some material along a tube within it. Muscles can only do this work by contracting. This explains why muscles are usually found in pairs (antagonistic pairs) where the work done in the contraction of one causes the stretching of the other.
Muscle is categorized into three main groups: striped (or striated) muscles are activated by motor nerves under voluntary control - their ends are usually attached via tendons to bones; involuntary or smooth muscles are controlled by motor nerves of the autonomic nervous system, and are located in the gut, blood vessels, iris, and various ducts; cardiac muscle occurs only in the heart, and is also controlled by the autonomic nervous system.
Functions of muscle within the body
The circulation of blood around the body is dependent on muscle. Much of the wall of the heart is made from muscle. It is used to pump blood around the body. Arteries have thick walls containing muscle and elastic fibres.
The breathing system needs muscle to achieve ventilation. Muscles between the ribs contract, pulling the ribcage upwards. At the same time the diaphragm muscles contract causing the diaphragm to flatten. These two movements cause air to enter the lungs.
The eye adjusts to focus on near and distant objects. To do this the shape of the lens is changed. To focus on a near object the lens has to be fatter, or more curved. The ciliary muscle contracts to achieve this.
The biceps and triceps muscles form an antagonistic pair of muscles used to raise and lower the arm when the elbow joint is used. The bones of the arm are rigid to provide a support for the attachment of the muscles. The force of the muscles contracting is carried to the bone by tough, fibrous tendons. Tendons have tensile strength and a little elasticity.
Muscles are controlled by nerve impulses that travel to them via nerve cells. In a simple reflex arc nerve impulses pass from a receptor along a sensory nerve cell to the spinal cord or brain, then along a motor nerve cell to a muscle. The muscle contracts, bringing about a response.
Energy for muscles
The work expended in muscle contraction needs energy. This energy is released from glucose by respiration. This is usually aerobic respiration. During vigorous exercise, muscle cells may be short of oxygen. They can then obtain energy from glucose by anaerobic respiration. However, this involves only the incomplete breakdown of glucose and produces lactic acid. Because the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration. This anaerobic respiration results in an oxygen debt that has to be repaid when the exercise has stopped.
Muscles may shiver when the body is cold. This causes more respiration to occur and releases some energy as heat.
Regular exercise is essential in order to keep muscles toned, so the fibres are slightly tensed and ready to contract; increase muscle strength and prevent muscles from feeling stiff and sore after exercise; and maintain an efficient supply of blood to the muscles, the heart, and the lungs.
The elongated cells or fibres that constitute muscle are classified into three types: striated, cardiac, and smooth. Striated muscle cells are so called because of their striped appearance under the microscope. They comprise the bulk of the body musculature (about 40% of the total body weight). Striated muscle fibres contract as a result of nervous stimulation and they are mostly under voluntary control. The muscles of the arms and legs are examples. Striated muscle is also known as striped, skeletal, somatic, or voluntary muscle. Cardiac muscle also has cross-striations, but, unlike somatic muscle fibres, its fibres branch so that the mass of muscle tends to function as one unit. Cardiac muscle will continue to contract rhythmically when its nerve supply is cut. Smooth muscle lacks the visible cross-striations of somatic and cardiac muscle; it is found mostly in the walls of hollow viscera such as the intestines, and its continuous contractions provide the motive power for digestion, secretion, and excretion. Like cardiac muscle, smooth muscle is normally under the dual control of hormones and the autonomic nervous system, but it also has intrinsic contractility. It is not under voluntary control.
Striated muscles are composed of a large number of parallel cylindrical fibres supported by non-contractile connective tissue. The connective tissue extends at both ends of the muscle to be continuous with the tendons, aponeuroses, or other dense connective tissue that attaches the muscle to bone or cartilage. Usually a striated muscle is linked by connective tissue to bones at both its ends, and spans one or two joints in between. When the muscle fibres contract as a result of nervous stimulation, they tend to bring the bones closer together through a rotatory movement at the intervening joint(s). When such a movement takes place, as when the hand is brought up to the shoulder, the muscle shortens and becomes fatter, but the tension generated within it remains approximately constant. Such contractions are called isotonic.
If the muscle is prevented from moving the bones to which it is attached, either by an external force or antagonistic muscular action, as when the arm is used to carry a heavy suitcase, the tension generated inside the muscle increases, but the length remains constant. Such contractions are called isometric.
The functional element of striated muscle is the muscle fibre, which has many fine threads or myofibrils running throughout its length. Although the myofibrils themselves can only be observed under an electron microscope, their regular banded structure gives rise to the striped appearance of the muscle fibre. The light bands are called isotropic bands, and the dark ones anisotropic bands. A myofibril contains two different types of still finer myofilaments. The thicker myofilaments consist of the protein myosin and each is surrounded by a hexagonal arrangement of thinner actin myofilaments. The myosin molecules each have a projecting head that forms a minute cross bridge with an actin myofilament. After nervous stimulation, electrical changes in the membrane surrounding each myofibril cause the release of calcium ions, which are normally stored in sacs along the fibril. It is thought that the free calcium ions stimulate the heads of the myosin molecule to swivel, pulling the actin filaments past the myosin filaments. The cross bridges then break temporarily, as the myosin heads swing back to their original angle and attach themselves to the actin filaments again, before repeating the operation. Each cycle of attachment, swivel, detachment, decreases the distance between adjacent Z lines (and therefore shortens the muscle) by about 1%, so it must be repeated many times in order to produce a significant shortening.
The immediate source of energy for muscle contraction is the ubiquitous compound adenosine triphosphate (ATP). When ATP is hydrolysed, that is, one of its three phosphate groups is removed, a great deal of energy is released. ATP is found in the heads of the myosin molecules in muscle, and the myosin protein itself catalyses the energy-releasing hydrolysis to adenosine diphosphate (ADP). ADP must be reconverted to ATP for contraction to continue. During severe exercise this is achieved by the breakdown of creatinine phosphate, a substance that is built up in the muscle at rest. Normally much of the energy for creatinine phosphate and ATP resynthesis comes from the breakdown of glucose to carbon dioxide and water, a process that requires oxygen.
Oxygen is carried to muscles by the blood, which runs in a plexus of fine capillaries in between the fibres. During exercise oxygen demand often outstrips supply, and the breakdown of glucose (one of the basic fuels used by the body) is halted at an intermediate stage with the production of lactic acid. This anaerobic (without oxygen) pathway produces a lower yield of ATP and is a short-lived process because lactic acid accumulates in the muscles. Nevertheless it permits much greater muscular exertion over short periods (as in sprinting) than would be possible with a purely aerobic (oxygen-consuming) process. After a period of exertion is over, extra oxygen is consumed in order to remove the excess lactic acid and to replenish the stores of creatinine phosphate and ATP. The period of breathlessness represents the replacement of the oxygen debt which is proportional to the amount that the capacity for aerobic glycolysis (breakdown of glucose) was exceeded. By training, athletes can increase this aerobic capacity, and therefore contract smaller oxygen debts for a given amount of exercise.
Nerve supply to voluntary muscles
The nerve supply to a striated muscle usually enters along with the blood vessels. The nerve to a muscle is mixed: it contains both motor fibres which convey impulses from the spinal cord to the muscle and sensory fibres which relay information back to the spinal cord. The motor fibres branch within the muscle, and by these branches one nerve cell supplies several muscle fibres distributed throughout the muscle.
However, each muscle fibre receives only one terminal branch of a nerve fibre at the neuromuscular junction. As a nerve branch approaches this junction it further divides into a spray of twiglike endings, collectively termed the motor end plate. The terminal twigs of the motor end plate occupy minute furrows formed in the membrane of the muscle fibre at that site. The two cell membranes, that of the nerve fibre (called the pre-synaptic membrane) and that of the muscle cell (called the post-synaptic membrane) are always separated by a synaptic gap. A nervous impulse arriving at the motor end plate causes the release of a chemical neurotransmitter, acetylcholine, which is stored in vesicles in the terminal nerve twigs. The acetycholine molecules migrate across the synaptic gap to receptor sites on the postsynaptic (muscle) membrane. Here they seem to act by making the muscle fibre membrane more permeable to sodium ions, which in turn leads to a wave of depolarization (movements of sodium and potassium ions) along the fibre that releases calcium ions and initiates the process of contraction described above.
A single nerve cell and all the muscle fibres which it innervates are collectively known as a motor unit. Where fine gradations of muscular activity are required, as with the muscles of the eyeball, motor units are small, perhaps comprising only one or two muscle fibres. On the other hand, powerful limb muscles need only relatively coarse control and a motor unit may include over a hundred muscle fibres. A healthy muscle at rest is said to exhibit muscle tone, a firm feel and characteristic elastic resistance to pressure. If the nerve supply to the muscle is cut, the muscle becomes flaccid, unresisting, and paralysed.
An artificial muscle fibre was developed in the USA in 1990. Besides replacing muscle fibre, it can be used for substitute ligaments and blood vessels and to prevent tissues sticking together after surgery (adhesion). In 1996, two US dentists discovered a new muscle; it is about 3 cm/1 in long, runs from the jaw to just behind the eye socket, and helps to support and raise the jaw.
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