Muscular system

Chapter 4 Muscular system




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 limb. 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 limb. 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.

The muscular system includes all the skeletal or striated muscles within the body. Striated muscle is that tissue attached to the skeleton that is under voluntary or conscious control (for microscopic structure see Ch. 2).



Muscle structure and function



Contraction


Muscle is stimulated to contract when it receives a nerve impulse from the central nervous system. Each striated muscle fibre is composed of myofibrils made of thin actin filaments and thick myosin filaments. These fibres overlap in such a way that under the microscope muscle has the appearance of alternating light and dark bands or striations. These bands are separated into units called sarcomeres, which are the units of contraction.


During contraction, the actin and myosin filaments slide over one another and cross-bridges form between the heads of the myosin filaments and the heads of the actin filaments. The cross-bridges swing through an arc, pulling the thin filaments past the thick ones, and the sarcomere shortens. Once this movement is completed the cross-bridge detaches itself from the thin filament and reattaches itself further away – in other words, the cross-bridges between the myosin and actin filaments act as a ratchet mechanism, thus shortening the muscle (Fig. 4.1). This process requires energy input, which is provided by adenosine triphosphate (ATP) molecules; calcium ions are also essential to the process of muscle contraction.


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Fig. 4.1 The ‘ratchet mechanism’ involving actin and myosin within striated muscle fibres. A Muscle before contraction (shortening). B When the muscle shortens, the thick and thin filaments slide in between one another. The dark bands remain the same width in the shortened (contracted) muscle but the light bands and ‘H zones’ get narrower. C Close-up view of the ‘ratchet mechanism’, showing cross-bridges between the thin actin and thick myosin filaments.


The nerve that stimulates the muscle to contract enters the muscle and then splits up into many fibres to innervate the bundles of muscle fibres. The number of muscle fibres supplied by one nerve fibre will vary depending on the type of movement for which the muscle is responsible. If it is a delicate movement then a nerve fibre may only innervate a small number of muscle fibres. However, in larger movements, such as those made by the muscles of the limbs, one nerve fibre may supply 200 or more muscle fibres. A single nerve together with the muscle fibres that it supplies is called a motor unit.



Muscle tone


Many of the skeletal muscles in the body are always in a slight state of tension, known as muscle tone. 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 ‘twitchy’ 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.

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.



Muscle atrophy or wasting may be the result of many factors, e.g. lack of use as a result of lameness, fracture, denervation (injury to the nerve that supplies it) or a generalised disease condition, such as neoplasia. Muscle enlargement or hypertrophy may result from overexercise or overuse. This may be seen where a patient has fractured a limb and the muscles in the other limb must work harder to support it.



Anatomy of a muscle


A ‘classically’ shaped muscle (Fig. 4.2) has a thick, fleshy central part called the belly and tapers at each end – the head. Here, the connective tissue muscle sheath is continuous with the dense fibrous connective tissue of the tendon that attaches the muscle to a bone. A muscle is attached to a bone at two points: its starting point is called its origin; this moves least during contraction. The opposite end, where the muscle inserts on the bone, is called the insertion. However, a muscle can have more than one belly, all inserting at one point, in which case the muscle is said to have a number of heads, e.g. the biceps muscle has two heads. The length of the tendon attaching the muscle to a bone will vary, and in some cases tendons are far longer than the muscle itself, e.g. flexor and extensor tendons running over the digits.


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Fig. 4.2 Basic structure of a muscle. The connective tissue between the muscle fibres is continuous with that of the tendon.


Not all muscles take the ‘classical’ shape described above. Sometimes they are present in flat sheets, in 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 the 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 synovial or tendon sheath (Fig. 4.3).


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Fig. 4.3 A Structure of a bursa. B Structure of a synovial sheath.


The skeletal muscles of the body can be classed as 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 joints in that part only; for example, 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 skeletal muscles


To simplify the study of these muscles we can look at one area of the body at a time.



Muscles of the head



Muscles of facial expression


The muscles of facial expression are intrinsic muscles that move the lips, cheeks, nostrils, eyelids and external ears. A number of muscles are responsible for these movements, all of which are innervated by the facial nerve (cranial nerve VII).



Muscles of mastication


The main muscles responsible for the masticatory or ‘chewing’ action of the jaw (Fig. 4.4) are:


• Digastricus – this muscle opens the jaw, aided by gravity, and is located on the caudoventral surface of the mandible. Its origin is the jugular process of the occipital bone and it inserts on the angle and ventral surface of the mandible
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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Muscular system
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