KEY POINTS
■ The muscular system comprises striated or skeletal muscle, i.e. muscle which is attached to the skeleton and brings about movement of a region.
■ Each striated muscle fibre is filled with myofibrils made of two contractile proteins, actin and myosin.
At the cellular level, muscle contraction results from the formation of cross-bridges between the actin and myosin molecules.■ Muscle fibres are stimulated to contract by nerve impulses carried by nerve fibres.The number of muscle fibres supplied by a single nerve fibre is called a motor unit. In muscles which perform accurate and delicate movements, a nerve fibre will supply a few muscle fibres, but in muscles which perform less accurate movements a nerve fibre will supply many muscle fibres.
Muscle tissue is always under a degree of tension, known as muscle tone.The tone increases when an animal is alert or frightened and decreases when it is relaxed or asleep.
All muscles consist of a central belly and. at the point of attachment to a bone, an origin (often called the head), and an insertion.
Skeletal muscles may be either:
- Extrinsic, i.e. attached from one major structure, such as the trunk, to another structure, such as a Iimb-These muscles bring about movement of the whole limb in relation to other body parts
- Intrinsic, i.e. attached at both ends within the one structure, such as a Iimb-These muscles bring about movement within the individual limb, e.g. bending an elbow.
Each area has a range of specialised muscles designed to bring about the specific types of movement necessary for the animal's normal function.