Essay 7

Do not memorise these answers! These are guidelines. Be creative, devise your own expression and organisation! You can elaborate on the points in your own language.
Remember that there is no 'perfect' answer - a little shortfall in content is easily compensated by presentation.
You should aim at writing an essay in this examination in approximately 20 minutes.

Structure and function in skeletal muscle.

Ultrastructural and histological organisation reflects the functional properties of all tissues. This is illustrated very well in muscle tissue. A named skeletal muscle is also much more than muscle cells - it has its own organisation of connective tissue, blood vessels and nerves. In short, a named skeletal muscle is an organ in its own right.

Ultrastructurally, all muscle tissue uses the properties of actin and myosin (both proteins) filaments. In contrast with the random arrangement in smooth muscle, skeletal muscle shows these filaments arranged in the form of sarcomeres. Each sarcomere has actin filaments attached to z membranes on two sides, overlapping with myosin filaments in the centre. A vast number of sarcomeres put end-to-end form a myofibril. In a skeletal muscle cell all myofibrils are arranged side-by-side so that their actin and myosin sets are at the same levels. This gives rise to the striated appearance in a light microscopic section. This arrangement allows concerted action of all sarcomeres together. The entire skeletal muscle cell contracts and relaxes in a matter of a few milliseconds.

Skeletal muscle cells have great lengths. In fact a single skeletal muscle cells is formed during embryonic period by the fusion of many undifferentiated cells (myoblasts). As a result, a skeletal muscle cell is multinucleated, with all nuclei close to the cell membrane.

Skeletal muscle is nerve dependent - it can contract only on stimulation by a nerve fibre. A single nerve fibre can supply many skeletal muscle cells. Such a group is called a motor unit. Members of a motor unit are usually scattered over a large number of bundles.

Connective tissue elements serve as passages for blood vessels and nerves. They also are the means of transmission of mechanical forces to bones. Many muscles have well-organised connective tissue bundles in the form of tendons and aponeuroses.

Bundles of muscle cells, connective tissue elements, blood vessels and nerves together reflect an organisation which gives a named skeletal muscle the status of an organ by itself.

Discussion

This may be a tricky topic - you may feel that there is too much factual material. But note that the emphasis is on correlation. As long as the key ideas are expressed, the length does not matter. In a question on skeletal muscle you may omit references to smooth or cardiac muscle. If a question asks for a comparison or refers to muscle tissue in general, you can add functional features of other types and cut down factual details - the fact that filaments are arranged in sarcomeres in skeletal and cardiac muscle and random in smooth muscle is adequate.

In a topic like this your presentation skills matter!

Compare this with cardiac muscle - striated for fast contraction, long refractory period so that it cannot undergo fatigue, intercalated discs so that it can contract as one mass! Smooth muscle on the other hand is slow to contract and relax.