The main function of the muscular system is movement. Muscle is the only tissue in the body that has the ability to contract and, therefore, move other parts of the body. Without muscles, nothing would work in the body.

Muscles control posture and body position as well. They often contract to hold the body still or in a particular position rather than to cause movement.The muscles responsible for the body’s posture have the greatest endurance of all muscles in the body—they hold up the body throughout the day without becoming tired.

There are three types of muscle tissue: visceral, cardiac, and skeletal.

Visceral muscle is found inside organs like the stomach, intestines, and blood vessels. The weakest of all muscle tissues, visceral muscle enables organs to contract so that substances can pass through the organ. Because visceral muscle is controlled by the unconscious part of the brain, it is known as involuntary muscle—it cannot be directly controlled by the conscious mind.

Found only in the heart, cardiac muscle is responsible for pumping blood throughout the body and is self-stimulating. The cells of cardiac muscle are branched X or Y shaped cells which are tightly connected together by special junctions called intercalated discs. Intercalated discs are made up of finger-like projections from two neighbouring cells that interlock and provide a strong bond between the cells. The branched structure and intercalated discs allow the muscle cells to resist high blood pressures and the strain of pumping blood throughout a lifetime. These features also help to spread electrochemical signals quickly from cell to cell so that the heart can beat as a unit.

Skeletal muscle is the only voluntary muscle tissue in the human body—it is controlled consciously. Every physical action that a person consciously performs (for example, speaking, walking, or writing) requires skeletal muscle.

So how do muscles work?

Muscles contract when stimulated by signals from their motor neurons. Motor neurons contact muscle cells at a point called the Neuromuscular Junction (NMJ). Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma (specialised cell membrane which surrounds muscle fibre cells) known as the motor end plate. The motor end plate contains many ion channels that open up in response to neurotransmitters and allow positive ions to enter the muscle fibre.

When the positive ions arrive, calcium ions (Ca2+) are released and allowed to flow into the myofibrils (long filaments that make up muscle fibre). Ca2+ ions bind to troponin, which causes the troponin molecule to change shape and move nearby molecules of tropomyosin. Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together.

So it’s like a chain reaction. A signal opens up channels which then allows calcium ions to connect with specific molecules, trigger other molecules to change shape, bump along other molecules out of the way so that myosin proteins in the thick filaments of the muscle cells can bend and pull on actin molecules in the thin filaments. And all of this is done in a matter of micro-seconds across millions of cells!

Myosin proteins act like oars on a boat, pulling the thin filaments closer to the centre of the muscle tissue. As the thin filaments are pulled together, the muscle shortens and contracts.

Muscles continue to contract as long as they are stimulated by a neurotransmitter. When a motor neuron stops the release of the neurotransmitter, the process of contraction reverses itself. Calcium returns to the sarcoplasmic reticulum; troponin and tropomyosin return to their resting positions; and actin and myosin are prevented from binding. Muscle tissues return to their elongated resting state once the force of myosin pulling on actin has stopped.

For all this to work, we need the presence of precise chemicals and specific molecules all acting in concert and all behaving in defined ways to create contraction of the muscle. If any one of these components is not present the muscle won’t function.

Other factors come into play as well. Muscle cells need energy and use aerobic respiration when we call on them to produce a low to moderate level of force. Aerobic respiration requires oxygen to produce about 36-38 ATP molecules from a molecule of glucose. Aerobic respiration is very efficient and can continue as long as a muscle receives adequate amounts of oxygen and glucose to keep contracting.

Furthermore, to keep muscles working for a longer period of time, muscle fibres contain several important energy molecules. Myoglobin, a red pigment found in muscles, contains iron and stores oxygen in a manner similar to haemoglobin in the blood. The oxygen from myoglobin allows muscles to continue aerobic respiration in the absence of oxygen.

When muscles run out of energy, the muscle quickly tires and loses its ability to contract. This condition is known as muscle fatigue. A fatigued muscle contains very little or no oxygen, glucose or ATP, but instead has many waste products from respiration, like lactic acid.

The body must take in extra oxygen after exertion to replace the oxygen that was stored in myoglobin in the muscle fibre as well as to power the aerobic respiration that will rebuild the energy supplies inside of the cell. This explains why you feel out of breath for a few minutes after strenuous activity—your body is trying to restore itself to its normal state.

A muscle, too, doesn’t act in isolation. It needs to be connected to bones and tissues and this marvellous inter-connection will be discussed in a future article.

All of this harmonious interaction of so many diverse factors supports the concept of design and forethought as opposed to random, blind chance.