Muscle tone
Many of the skeletal muscles in (he body are always in a slight state of tension, known as muscle tune. Even when an animal is at rest, muscles, such as those responsible for maintaining posture, will not be truly relaxed.
Muscle tone is achieved by a proportion of the motor units within the muscle being activated so that some of the muscle fibres are contracting, while others are relaxed. The nervous system can adjust this, and the number of motor units stimulated will increase when the animal is in an anxious state, i.e. the muscles become ,lwitchy' or ‘jumpy’. Thus, muscles undergo two types of contraction:Isometric contraction - when tension is generated in the muscle, i.e. muscle tone is increased, but the muscle does not shorten
Isotonic contraction - the muscle actually moves or shortens.
which case the tendon is also drawn out into a flat sheet of connective tissue - this type of arrangement is called an aponeurosis, e.g. the muscles of Ihe abdominal wall. Some muscles form a circular ring and serve to control the entrance or exit to a structure, e.g. the stomach and the bladder. These are called sphincter muscles.
A bursa is a connective tissue sac lined with synovial membrane and filled with synovial fluid. These typically develop between a bony prominence and a tendon, ligament or muscle and their function is to reduce friction between the associated structure and the bone. Sometimes a bursa completely wraps around a tendon forming a Sifiiovial or tendon sheath (Fig. 431.
The skeletal muscles of the body can be classed as W
either intrinsic or extrinsic:
Intrinsic muscles lie completely within one region of the body where they have their origin and insertion.They act on the bones in that part only, e.g. when a dog bends its elbow it is using the intrinsic muscles of the forelimb.
Extrinsic muscles run from one region of the body to another and alter the position of the whole part, e.g. a limb, in relation to the other. The muscles that attach the foreleg of the dog to the trunk are extrinsic muscles; they move the whole foreleg in relation to the trunk.
The more a muscle is used or exercised, the larger it will become - it is said to be hypertrophied. However, if a muscle is not used for some reason, e.g. due to injury or illness when the animal may be recumbent, or if a limb is in a cast, it will wither or shrink in size - it is said to be atrophied.