Basic Characteristics of Muscle
Muscle is a soft contractile tissue, originated from the embryonic mesodermal layer. Muscle consists of muscle cells or muscle fibers. Contraction of muscle fibers generates force and that causes motion (i.e., locomotion or movement of visceral organs).
The word “muscle” derived from the Latin word “musculus,” which means “little mouse.” It may be due to the shape of muscles like mouse, or contracting muscles look like mouse moving under the skin. Muscle fibers contain contractile filaments myosin (also known as thick filament) and actin (also known as thin filament). These protein filaments slide over one another and produce contraction. According to structure, situation, and function, muscles are generally classified into skeletal muscle (attached to bones), smooth muscle (present in the visceral organs), and cardiac muscle (found in heart). On the basis of action, muscles are classified into voluntary and involuntary muscle. Muscles which can be controlled by animal’s own will are voluntary muscle like skeletal muscle, whereas muscles which are not under voluntary control are called involuntary muscle like smooth and cardiac muscle. Depending on the presence of striation, muscles can be classified into striated and nonstriated muscles. Skeletal muscle and cardiac muscle are striated muscles, whereas smooth muscle is a nonstriated muscle. The energy for muscle contraction is provided by ATP molecules, which are generated mainly through oxidation of fats and carbohydrates, but anaerobic reactions also occur.10.1.1 Functions of Muscle
1. Movement of body or locomotion: The major function of muscular tissue is locomotion or movement of body. The movement is of two types, i.e., gross movement and fine movement. Gross movement includes large, coordinated movements like walking, running, and swimming. Fine movement includes smaller movements, which occur in the limbs.
The movements are mainly under voluntary control, but some movements are reflexive.2. Maintenance of posture of the animal: Skeletal muscles maintain the body in the right position when an animal is in sitting or standing condition. Posture of an animal depends on strong and flexible muscles, whereas stiff, weak, or rigid muscles result in abnormality in posture. This abnormality in posture for a long time may cause pain in joint and muscles.
3. Joint stability: Tendons help in joint stability. Tendons of knee and shoulder joint are very important for stabilization, and muscles of the abdomen, back, and pelvic region are also involved in stabilization of the body and help in different activities.
4. Respiration: Breathing process in animals depends on the contraction of respiratory muscles, mainly diaphragm and intercostal muscles. During inspiration, contraction of diaphragm results in movement of air into the lungs down the pressure gradient, whereas during expiration, relaxation of the diaphragm leads to increase in pressure which pushes air out of the lungs. Forced breathing or deep breathing requires help from other muscles, like muscles of abdomen, back, and neck.
5. Circulation: Cardiac muscle helps in contraction and relaxation of heart, which results in circulation of blood throughout the body. Smooth muscles are present in blood vessels and regulate the constriction and relaxation of blood vessels, blood flow, as well as blood pressure.
6. Digestion: In the gastrointestinal tract (GI tract), contraction of the smooth muscle helps in the movement of the food particle by a wavelike motion called peristalsis. This process also helps in the mixing of food particles with stomach acid and enzymes. Smooth muscles also help to pass the undigested food out of the body as feces.
7. Urination: In the urinary system, smooth as well as skeletal muscles are there and the muscles along with nerves work together to hold and release urine from the urinary bladder.
Some abnormal cases of urinary system like poor bladder control or retention of urine are caused by damage of the nerves that carry signals to the muscles.8. Parturition: During parturition, contraction of uterine smooth muscles helps in expulsion of fetus as well as fetal membrane. Oxytocin hormone initiates the contraction process.
9. Vision: Muscles adjacent to the eye control the movements of eyeball. These muscles around eyes also help the eyes to maintain a stable image, scan the surrounding area, and track moving objects.
10. Protection of organ: Muscles provide protection to different parts of the body, bones, as well as visceral organs. Muscle absorbs shock and protects a visceral organ or bone.
11. Thermoregulation: Muscular system has a significant role in thermoregulation. During heat stress, relaxation of smooth muscles of blood vessels results in increased peripheral circulation and quick loss of heat from the surface of the body. However, during cold stress, contraction of smooth muscles of blood vessels reduces peripheral blood circulation and helps in the reduction of heat loss and maintaining body temperature. During cold stress, shivering thermogenesis also helps in body temperature maintenance.
12. Communication: Muscles help in communication between animals as well as between birds by making sounds and different types of activities.
Except these functions discussed above, muscles are also involved in different physiological functions. These functions are discussed in details later in this chapter (in the discussion portion of individual muscles).
10.1.2 Properties of Muscle Tissue
Muscle fibers have some special properties, which help them to carry out their functions and differentiate them from other types of cells in the body.
The properties are excitability, contractility, extensibility, and elasticity:
1. Excitability: The ability of muscle cells to respond to a stimulus is known as excitability. The stimuli are
Fig.
10.1 Histological structure of three types of muscles. (a) Skeletal muscle. Muscle fibers showing striations with multiple peripherally located nucleuses. (b) Smooth muscle. No striations are present. Eachcell contains single centrally located nucleus. (c) Cardiac muscle. Muscle fibers are branched having intercalated disc and one nucleus per cell
neurochemical, mechanical, and chemical in nature. When the muscle fibers are properly stimulated, then the muscle will respond to the stimulus.
2. Contractility: Contractility is the ability of a muscle to contract and generate pulling force when properly stimulated.
3. Extensibility: Muscle cells can lengthen in response to stretch, which is called extensibility. This property is more evident in smooth muscle compared to skeletal muscle.
4. Elasticity: It is the ability of muscle fiber to recoil to its original resting length once stretched. present in the muscle, and cells are uninucleated (Fig. 10.1).
3. Cardiac muscle: Cardiac muscle is located only in the heart, controls the cardiac contractions, and pumps blood all over the body. Like skeletal muscle, cardiac muscle is also striated. The contraction is slow and rhythmic and involuntary. Cells are branched, and intercalated discs are present. Lengths of cardiac myocytes are lesser than skeletal muscle fiber and contain 1-2 centrally located nuclei (Fig. 10.1 and Table 10.1).
10.1.3 TypesofMuscles
Three types of muscles are there in the body, i.e., skeletal muscle, smooth muscle, and cardiac muscle.
1. Skeletal muscle: Skeletal muscles are mainly attached with bones (via tendons), maintain posture of the animal, and control the movement or locomotion. Some skeletal muscles are directly attached with other muscles or skin like in the face where different muscles control facial expression. Skeletal muscle is under voluntary control and innervated by somatic motor neurons but can maintain posture or balance even in subconscious state. Muscle fibers are striated, elongated, and tubular in shape with multiple nuclei located peripherally (Fig. 10.1).
2. Smooth muscle: Smooth muscle is present in the walls of visceral organs, like organs of digestive system, respiratory system, blood vessels, glands, uterus, eye, and skin. Smooth muscles regulate the functions and movement of such systems, such as movement of food through GI tract via peristalsis or expulsion of fetus during parturition, propel urine, dilate/constrict pupils, regulate blood flow, etc. In some locations, they are autorhythmic like in GI tract. Smooth muscle is controlled involuntarily by endocrine and autonomic nervous systems. No striation is
10.2