Bones and Joints
The adult human skeleton consists of 206 bones. The bones along with approximately 700 skeletal muscles account for 50% of our body weight.
Bones provide protection and support. When two or more bones join together, a joint or articulation is formed. Several types of joints help in movements.
Muscles are elastic in nature. They are capable of contraction in response to stimuli from the central nervous system. The muscle cells function like biological machines that convert chemical energy into mechanical work.
The mechanical work involves various movements including vital processes like contraction of the heart and blood vessels. Approximately 40% of the body is skeletal muscle and almost 10% is smooth and cardiac muscles.
Structure of a Skeletal Muscle
A striated muscle is composed of many fibers arranged in bundles. The diameter of each fiber varies from 10 to 100 microns. The length of fibers ranges from 1 to 20mm.
Each fiber is surrounded by a membrane called the Sarcolemma. Each muscle fiber is made up of 4 to 20 thread-like structures called myofibrils.
They are parallel to each other. The myofibrils are 1 to 3 microns in diameter. In between the myofibrils, the sarcoplasm is present. A small segment of the myofibril is called the sarcomere.
Structure of Sarcomere
When a sarcomere is observed under a microscope we could see alternative dense (A band) and light bands (I band). The central region of the A band is often less dense and is known as the “H Zone“.
The ‘I band’ is bisected by a dense narrow line, the Z line. Thus each sarcomere includes repeating units between two Z lines in linear order as Z line, I band, A band, I band, and next Z line. There is the regular arrangement of 2 types of protein filaments.
A band contains a set of thick filaments formed of the contractile protein myosin. It may range up to 110 A in diameter and 1.5 microns in length. The second set of thin filaments (50 A diameter) overlaps the long filaments in the A band.
The second set of filaments extend partly in ‘I band’ and partly in ‘A band’. These filaments are formed of a substance called Actin.