P2.9.2^9.2. General Properties of Muscle^275^275^,,^14868^14954%
9.2
General Properties of Muscle
Muscle Characteristics
    LEARNING OUTCOMES

    After reading this section, you should be able to:

  1. Explain the four functional properties of muscle tissue.
  2. Summarize the major characteristics of skeletal, smooth, and cardiac muscle.

Muscle tissue is highly specialized. It has four major functional properties: contractility, excitability, extensibility, and elasticity.

  1. Contractility is the ability of muscle to shorten forcefully. For example, lifting this text book requires certain muscles to contract. When muscle contracts, it either causes the structures to which it is attached to move or increases pressure inside a hollow organ or vessel. Although muscle shortens forcefully during contraction, it lengthens passively; that is, other forces cause it to lengthen, such as gravity, contraction of an opposing muscle, or the pressure of fluid in a hollow organ or vessel.

  2. Excitability is the capacity of muscle to respond to a stimulus. Normally, the stimulus is from nerves that we consciously control. For instance, if you decide to wave to a friend, the conscious decision to lift your arm is sent via nerves. Smooth muscle and cardiac muscle can contract without outside stimuli, but they also respond to stimulation by nerves and hormones.

  3. Extensibility means a muscle can be stretched beyond its normal resting length and still be able to contract. If you stretch to reach a dropped pencil, your muscles are longer than they are normally, but you can still retrieve the pencil.

  4. Elasticity is the ability of muscle to recoil to its original resting length after it has been stretched. Taking a deep breath demonstrates elasticity because exhalation is simply the recoil of your respiratory muscles back to the resting position, similar to releasing a stretched rubberband.

Types of Muscle Tissue

Table 9.1 compares the major characteristics of skeletal, smooth, and cardiac muscle. Skeletal muscle, with its associated connective tissue, constitutes about 40% of the body's weight and is responsible for locomotion, facial expressions, posture, respiratory functions, and many other body movements. The nervous system voluntarily, or consciously, controls the functions of the skeletal muscles.

 
TABLE 9.1
Comparison of Muscle Types
  Skeletal Muscle Smooth Muscle Cardiac Muscle
Location Attached to bones Walls of hollow organs, blood vessels, eyes, glands, and skin Heart
Appearance
Cell Shape Very long and cylindrical (1 mm–4 cm); extends the length of muscle fasciculi, which in some cases is the length of the muscle Spindle-shaped (15–200 µm in length, 5–8 µm in diameter) Cylindrical and branched (100–500µm in length, 12–20 µm in diameter)
Nucleus Multiple nuclei: peripherally located Single, centrally located Single, centrally located
Special Cell-Cell Attachments None Gap junctions join some visceral smooth muscle cells together Intercalated disks join cells to one another
Striations Yes No Yes
Control Voluntary and involuntary (refl exes) Involuntary Involuntary
Capable of Spontaneous Contraction No Yes (some smooth muscle) Yes
Function Body movement Moving food through the digestive tract, emptying the urinary bladder, regulating blood vessel diameter, changing pupil size, contracting many gland ducts, moving hair, and having many other functions Pumping blood; contractions provide the major force for propelling blood through blood vessels

Smooth muscle is the most widely distributed type of muscle in the body. It is found in the walls of hollow organs and tubes, in the interior of the eye, and in the walls of blood vessels, among other areas. Smooth muscle performs a variety of functions, including propelling urine through the urinary tract, mixing food in the stomach and the small intestine, dilating and constricting the pupil of the eye, and regulating the flow of blood through blood vessels.

Cardiac muscle is found only in the heart, and its contractions provide the major force for moving blood through the circulatory system. Unlike skeletal muscle, cardiac muscle and many smooth muscles are autorhythmic; that is, they contract spontaneously at somewhat regular intervals, and nervous or hormonal stimulation is not always required for them to contract. Furthermore, unlike skeletal muscle, smooth muscle and cardiac muscle are not consciously controlled by the nervous system. Rather, they are controlled involuntarily, or unconsciously, by the endocrine and autonomic nervous systems (see chapters 16 and 18).

ASSESS YOUR PROGRESS
  1. Identify the four specialized functional properties of muscle tissue, and give an example of each.

  2. Distinguish among skeletal, smooth, and cardiac muscle as to their locations, appearance, cell shape, and cell to cell attachments.

  3. Outline the differences in control and function for skeletal, smooth, and cardiac muscle.