The skeleton is covered by layers of skeletal muscle. Each
muscle is attached to two or more bones so that when the muscle contracts
(shortens) it produces MOVEMENT. Skeletal muscle makes up about 40
per cent of body weight. As well as producing movement, some muscles remain
partially contracted for long periods to maintain the body’s
posture.
A skeletal muscle contains many long fibres arranged in bundles
called fascicles. Each fibre consists of smaller strands, called myofibrils.
These contain yet smaller parts called myofilaments. A muscle contracts when
sets of these myofilaments slide past each other in response to nerve
signals.
There are more than 600 muscles in the body. Their sizes vary from
tiny, such as the muscles that move the eyeballs in their sockets, to very
large, such as some muscles in the thighs. They are arranged in layers; shown
here are the superficial (outer) muscles at the front of the body and, on this
side, some of the deeper muscles.
Skeletal muscle is also called striated or striped muscle. The
stripes, which can be seen clearly when a piece of muscle is viewed under a
microscope, are caused by the arrangement of myofilaments in individual muscle
fibres. These lead to the appearance of alternating light and dark bands.
Skeletal muscles cross joints and are attached to the
bones on either side by tough cords called tendons. They contract, to produce
movement, as a result of nerve signals sent from the brain and spinal cord.
Although our movements are under our conscious control, the brain can learn
patterns of movements so that we can perform certain tasks, such as walking,
without thinking.
MUSCLE ACTION IN MOVEMENT
To straighten the knee, one group of muscles at the front of the
thigh contracts, while other muscles at the back of the leg relax. Two groups
of muscles such as this are called opposing groups. Contractions of opposing
groups have opposite effects, such as knee straightening and bending.
To bring about a movement, the brain sends a series of signals
instructing specific muscles to contract, via a network of nerve cell fibres.
Each individual fibre divides into several branches before it reaches the
muscle, and each branch connects to a single muscle fibre. The region where the
nerve and muscle fibres meet is called a neuromuscular junction.
Skeletal muscle is not the only type of muscle in the body. There
are two other types: smooth muscle and cardiac (heart) muscle. Unlike skeletal
muscle, these muscles are not under our conscious control.
Smooth muscle is found in the walls of many organs, such as the
bladder, the womb, and the intestines, where it contracts to propel food along.
It has short, spindle-shaped fibres.
Cardiac muscle contracts tirelessly throughout life to pump
blood from the heart to the lungs and around the body. It is made up of a
network of branching muscle fibres.
