Muscles
Muscle Overview
There are three major types of muscle these are outlined in the table below.
| Type | Information |
|---|---|
| Striated Muscle | Also known as skeletal muscles, these are attachced to the bones by tendons and are responsible for controlling your physical movement (walking, talking, dancing). These are controlled by the brain and are voluntary (although maintaining posture is subconscious). |
| Smooth Muscle | Are found in the walls of organs such as the oesophagus, bronchi and bladder where they are responsible for peristalisis for example. These are under the control of the autonomic nervous system and as such are involuntary. |
| Cardiac Muscle | Specialized muscle that is only found in the heart. They are myogenic so can contract by themselves, the brain only controls the rate of contraction. Cardiac muscles also don't fatigue: the heart can't stop. |
Muscles very often have antagonistic pairs, which means two muscle work together. A good example is the iris when adjusting the pupil size. The most obvious example in striated muscle are the bicep and tricep.
When the arm is moved up the bicep contracts and tricep relaxes. When the arm is extended the reverse is true and the bicep relaxes and the tricep contracts.
Muscle Structure
Muscles have quite a complicated structure, with a lot of different words to describe all the features. The muscle is made up of muscle fibres, which is itself made up of myofibrils and this in turn consists of actin and myosin proteins.
The sarcomere is a short length of muscle fibre. It has characteristic banding patterns produced by the actin and myosin, depending on what state it is in.
The upermost pair of diagrams shows the banding that occurs in the sarcomere. The M line is where the thick filaments join and is in the centre of the sarcomere. The Z lines are where the thin filaments are joined and marks the ends of the sarcomere.
There are several bands, the dark band spans from the edges of either dark area, in the centre of it is a lighter area known as the H zone (from Helle, the German for light). Then at the extremities of the sarcomere are the light bands.
The black and white diagram below, shows how the bands occurs as a result of overlaping of filaments. The dark bands are the result of overlap of actin and myosin. When the sarcomere contracts there is a larger dark band because the actin is moved closer to the centre and more overlap occurs.
Muscle Contractions
A muscle contraction begins with a nervous impulse. The impulse arrives at a neuromusclular junction, and the impulse travels as if leaving a synapse. However, the acetylcholine binds to the sarcoplasmic reticulum and makes it more permeable to calcium ions.
The calcium binds to a molecule called troponin which is on the actin (step 1 on diagram). This changes its shape so that it will displace tropomyosin (step 2), which leaves the binding site free.
The myosin head can now bind with the binding site (step 3) and form a cross bridge. The head pulls backwards which moves the actin along a tiny bit. Now an ATP molecule is used to detach the myosin head and the myosin is said to be 'recocked': able to bind further along to continue t moving the filaments.