In humans, muscles constitute about 40 to 50 percent of the total body weight. In humans muscles are broadly classified into three categories.
1. Skeletal or striped or striated or voluntary muscles.
2. Smooth or unstriped or no striated or visceral or involuntary muscles.
3. Cardiac muscles Structure, location and functions of these muscles have been described further.
1. Flexor. This muscle bends one part of a limb on another at a joint, e.g., biceps. It brings the fore arm towards the upper arm.
2. Extensor. This muscle extends or straightens a limb, e.g., triceps. It extends the fore arm.
3. Adductor. This muscle brings a limb towards the mid-line of the body, e.g., altissimo dorsi. It presses the entire arm against the side.
4. Abductor. This muscle pulls a limb away from the mid-line of the body, e.g., deltoid us. It draws the entire arm to the side.
5. Pronator, This muscle turns the palm downward or to the posterior, e.g. promoter.
6. Supinator, This muscle turns the palm upward or to the anterior, e.g., separator.
7. Elevator. This muscle raises a part of the body, e.g., master. It lifts up the lower jaw to close the mouth.
8. Depressor. This muscle lowers a part of the body, e.g., depressor mandibles. It lowers down the lower jaw to open the mouth.
9. Rotator. This muscle rotates a part of the body, e.g., pyriformis. It raises and rotates the thigh.
10. Sphincter. This muscle decreases the size of an opening, e.g., pyloric sphincter between stomach and duodenum.
11. Dilator. This muscle enlarges the size of an opening.
(i) Antagonistic Muscles: Muscles which act in opposition to other muscles are called the antagonistic muscles. The biceps for example bends the arm and is called a flexor. Its antagonist, the triceps extends the arm and is termed as extensor.
(ii) Molecular Components of Muscles: The molecular components of muscles are- (i) contractile proteins (e.g., myosin, acting tropomysin and proponing); (ii) enzymes; (iii) fats and carbohydrates; (iv) organic phosphates (e.g., adenosine triphosphate (ATP) and phosphocreatine); and (v) ions of sodium, potassium, magnesium, calcium and chloride.
(iii) Motor Unit: A motor unit consists of a single motor neuron (nerve cell) and the muscle fibers it innervates.
(iv) Single Muscle Twitch: Single isolated contraction of muscle fibre is called single muscle twitch.
(v) Threshold Stimulus: For contraction, muscles fiber always requires a specific minimum strength or intensity of the stimulus or nerve impulse. This is called threshold stimulus.
(vi) All or none Law: According to this law when a fiber contracts, it contracts maximally.
(vii) Summation: Addition of one contraction to a previous one to produce greater shortening is called summation.
(viii) Rigor Mortis: The rigidity of muscles that occurs after death is called rigor mortis.