Muscle tissue overview

Introduction

Muscle tissue is specialized for contraction and is responsible for body movements and changes in size and shape of internal organs. Muscle cells are usually elongated and arranged in parallel arrays.

Muscle is classified based on the appearance of its cells. The two principal types of muscle include stri­ated and smooth muscle. Striated muscle appears to have cross striations when viewed under the light microscope, whereas smooth muscle lacks such stria­tions. Striated muscle can be further subdivided into two types: skeletal muscle that is attached to bone and responsible for the movements of the axial and appen­dicular skeleton, and cardiac muscle that makes up the majority of the heart. Skeletal muscle and cardiac muscle are sometimes referred to as voluntary striated and involuntary striated muscles, respectively.

The prefixes myo and sarco refer to muscle. There­fore, terms such as myofibril or myofilament reference structures within a muscle. For example, the plasma membrane of a muscle cell is called the sarcolemma, the cytoplasm the sarcoplasm, and the endoplasmic reticulum the sarcoplasmic reticulum (SR). In addition, a single skeletal muscle cell is also called a muscle fiber.

Properties of muscles

Four properties of muscles enable them to perform their functions. These properties include

• Excitability. Sometimes called irritability. Muscle cells maintain a membrane potential and are able to respond to a stimulus such as a neurotransmit­ter by developing an electrical impulse. The stim­ulus is usually neurochemical, but can also be mechanical or chemical. The electrical impulse can migrate across the sarcolemma.

• Contractility. When stimulated, the electrical impulse spreading across a muscle cell can cause the cell to contract.

• Extensibility. In addition to contraction, muscle cells can lengthen in response to stretch.

• Elasticity. Once stretched, muscle fibers can recoil to their original resting length due to the elastic elements within the muscle.

Functions of muscles

Muscles serve four major functions, including produc­tion of movement, maintenance of posture, stabiliza­tion of joints, and generation of heat:

• Production of movement. One feature unique to animals compared with plants is their ability to move. The action of skeletal muscle is responsible for moving joints and thus allowing locomotion. However, movement can be viewed more broadly than locomotion. An animal can change its posture or facial features as a result of muscle contraction. In addition, generally as a result of smooth or cardiac muscle contraction, materials can be relo­cated within the body. For example, contraction of the heart helps propel blood through the vessels; contraction of the bladder or gastrointestinal tract can also move materials.

• Maintaining posture. Maintaining a position is generally an active, rather than a passive, process. Through the actions of signals generated from sensors located in joints, tendons, and muscles, minute adjustments are automatically made to maintain the position of joints.

• Stabilizing joints. In addition to moving joints, muscles also stabilize the joints, thus minimizing dislocations.

• Generating heat. Endotherms maintain a rela­tively constant body temperature over a range of environmental temperature. Skeletal muscles are an important organ in heat production, such as through the process of shivering.