Muscles

Here is some simple information for GCSE biology students about the different types of muscle tissues and why muscles work in antagonistic pairs.

Index

	Striped Muscle
	Smooth Muscle
	Cardiac Muscle
	Antagonistic Muscles
	Diagram of Biceps Contracting
	Diagram of Triceps Contracting

Striped Muscle

Striped muscle is also called voluntary muscle. It is sometimes called striped muscle because this is what it looks like through a microscope. It is also called voluntary muscle because this is the muscle which you have direct control over when you want to make a movement. You may also be told that it is called Skeletal muscle, this is because it is attached to the skeleton. You can use any one of these names.

Striped muscle is the muscle (or meat) on you legs arms etc., you have direct control over these muscles and can make almost any movements which you want to. These muscles are also found in your face and jaws, so they are used when you smile or frown and when you talk, eat or drink.

Striped muscles are always found in pairs of antagonistic muscles. A pair of muscles, the biceps and triceps, are used to bend and straighten the elbow. When you make your biceps contract, it flexes the elbow (bends it). When you contract your triceps, it extends the elbow (straightens it). The elbow, which is a hinge joint, only needs one pair of muscles to make it work, but your shoulder, which is a ball and socket joint, needs three pairs of muscles. This is because a ball and socket joint can make three kinds of movement. Your arm can be moved forwards and backwards at the shoulder; it can also be moved sideways up and away form your body and back down to your side; thirdly, you can twist your arm round. In this third movement you can make the palm of your hand point down or backwards and up and forwards. Some of this twisting happens at the shoulder, but some of the twisting happens in your forearm.

Smooth Muscle

This is called smooth or involuntary muscle. It is called smooth muscle because you cannot see any stripes when you look at it through the microscope; it is also called involuntary muscle because you cannot make it contract and relax through conscious control. Smooth muscle contracts and relaxes automatically. This muscle is found in your intestines and in the iris of your eye. When you take a mouthful of food, smooth muscles in your salivary gland squeeze the gland so that saliva squirts into your mouth. Sometimes the muscles squeeze the salivary gland so strongly that it hurts. If you bite into a sour apple, you feel a pain at the back of your jaw just underneath your ear.

The muscles in your intestines also work in pairs. When the circular muscles contract the make the intestines longer (and thinner) and when the longitudinal muscles contract they make the intestines shorter (and fatter). These muscles move food along the guts (peristalsis) and help to mix food with your digestive juices.

Cardiac Muscle

The muscle of your heart is also striped but it is involuntary. It is called cardiac muscle: cardiac means "of the heart". Cardiac muscle contracts and relaxes automatically without you having to think about it. When you take exercise your heart beats faster and with a bigger volume. This increase in cardiac output (how fast the blood is pumped around your body) is produced by a hormone called adrenaline. The vagus nerve can make the heart go slower; this happens when you are sitting or lying down.

Antagonistic Muscles

All muscles work in pairs. Whether they are striped muscle, smooth muscle or cardiac muscle makes no difference, all muscles must work in pairs. This is because they can contract and relax but cannot push or stretch themselves.

When your biceps contracts it flexes (bends) the elbow joint. At the same time it also pulls the triceps to make it longer. So the triceps is stretched by the biceps pulling it. When the triceps contracts is extends (straightens) the elbow joint, and at the same time it pulls the biceps and makes it longer. So these two muscles work together. Neither muscle can stretch itself, it must be stretched by its antagonist (partner).

Here is a diagram as seen on my "blackboard" at Kingsbury High School:

Hope you can read my handwriting! When the biceps contracts and the triceps relaxes, you can see that the triceps will be stretched. The elbow joint acts as a pivot or fulcrum. As you can see, if the biceps gets shorter, the triceps must get longer, the triceps is pulled by the end of the ulna: but if both muscles contract then nothing will happen. The diagram shows the two muscles in red, the tendons are in yellow, and the bone started off in blue but got turned into a multicolour mess when I converted from a BMP file to a GIF file.


Here the triceps is shown contracted and the biceps has been stretched: