The muscle system is responsible for movement of the human body, posture, movement of substances inside the body andfor the generation of body heat. There are approximately 700 known and named muscles and on top of that, muscle tissue is also found inside of the heart, digestive organs, and blood vessels.
There are 3 main types of muscle in the human body:
Skeletal muscle is a voluntary muscle, which means that we can actively control its function. It’s attached to the bone and forms a distinct organ of muscle tissue, blood vessels, tendons, and nerves that covers our bones and allows movement.
Skeletal muscles often exist in pairs, whereby one muscle is the primary mover and the other acts as an antagonist. For example when you bend your arm, your biceps contracts whilst your triceps is relaxed. When your arm returns to the extended position it is the triceps that contracts and the biceps relaxes.
Skeletal muscle is a fascinating tissue with a complex structure. It consists of elongated multinuclear cells called the myocytes (or myofibers). The muscle cells can be anything from 1 mm to 30 cm in length. The longest muscle cell in our bodies can be found in the sartorius muscle and is 30 cm (nearly 12 inches!) long.
The individual muscle cells appear striated under the microscope (see image below). This is due to the highly organsied structure of the muscle fibers where actin and myosin myofilaments are stacked and overlapped in regular repeating arrays to form sarcomeres. Actin and myosin filaments slide against each other and are responsible for the muscle contraction.
To see how the muscle contracts and works, have a look at the video here.
The energy for muscle function comes from intracellular organelles called the mitochondria. Mitochondria are the powerhouses of every cell in our bodies and responsible for delivering energy that the cells need to function.
Muscles are ennervated by motor neurons. A motor neuron and the muscle fibers ennervated by it form a motor unit. Size of motor units varies in the body, depending on the function of the muscle. Fine movements (eyes) have fewer muscle fibers per neuron to allow for fine movement. Muscles that require a lot of strength have many muscle fibers per unit. The body can control strength by deciding how many motor units it activates for a given function.
There are two types of skeletal muscles in our bodies, which vary in function. Slow twitch muscle fibres are better for endurance activities and can work for a long time without getting tired. Fast twitch muscles are good for rapid movements as they contract quickly, but get tired fast, and consume lots of energy.
Most of our muscles are made up of a mixture of both slow and fast twitch muscle fibres. However, muscles involved in maintaining posture contain mainly slow twitch muscle fibres, and muscles responsible for eye movements are made up of fast twitch muscle fibres.
And for a bit of fun, here’s a lovely song describing all the muscles of the leg: