8.1 Overview of Muscle Tissues

Muscle is one of the four primary tissue types of the body (along with epithelial, nervous, and connective tissues), and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (Figure 8.1.1). All three muscle tissue types have some properties in common. They all exhibit a quality called excitability, meaning their plasma membranes can change electrical states (from polarized to depolarized) in response to stimuli and send electrical waves called action potentials along the entire length of their membranes. Other key characteristics common to all muscle tissue types are contractility, which is the ability of the cells to shorten and generate force in response to an action potential; extensibility, which is the ability to stretch and extend beyond the resting length of the cells without being damaged; and elasticity, which allows a muscle to recoil back to its original length and shape after being stretched.

Key differences also exist between the three muscle types. Skeletal muscle cells (also called muscle fibers) are unique in that they are multinucleated with the nuclei located on the periphery of the cell under the cell plasma membrane (also called the sarcolemma in muscle). During early development, embryonic myoblasts, each with its own nucleus, fuse with hundreds of other myoblasts to form long multinucleated skeletal muscle fibers. Cardiac muscle cells generally have one to two nuclei centrally located in each cell and are physically and electrically connected to each other so that the entire heart can contract as one unit. Smooth muscle cells contain a single nucleus and can exist in electrically linked units contracting together as a single-unit or multiunit smooth muscle where cells are not electrically linked. More broadly, skeletal muscle depends entirely on signaling from the nervous system to initiate and modulate muscle contraction, whereas both cardiac muscle and smooth muscle can be influenced by the nervous system as well as various hormones and local stimuli.

All muscles begin the mechanical process of contracting (shortening) when a protein called actin is pulled by a protein called myosin. This occurs in striated muscle (skeletal and cardiac) after specific binding sites on actin have been exposed in response to the interaction between calcium ions (Ca⁺⁺) and proteins (troponin and tropomyosin) that “shield” the actin-binding sites. Ca⁺⁺ also is required for the contraction of smooth muscle, although its role is different: here Ca⁺⁺ activates enzymes, which in turn activate myosin heads. All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca⁺⁺ is removed and the actin-binding sites are re-shielded.

Differences among the three muscle types also include the microscopic organization of their contractile proteins: actin and myosin. The actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells (referred to as fibers) in both skeletal muscle and cardiac muscle, which creates a pattern, or stripes, called striations. The striations are visible with a light microscope under high magnification (Figure 8.1.1) and are the reason why both skeletal and cardiac muscles are classified as striated muscle. Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber has a uniform, nonstriated appearance (resulting in the name smooth muscle).

Muscle Functions

The best-known feature of skeletal muscle is its ability to contract and cause whole body movements due to its direct connection with bone. Yet skeletal muscles are also responsible for stopping movement, such as resisting gravity to maintain posture. Small, constant adjustments of skeletal muscles are needed to hold a body upright or balanced in any position. Skeletal muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. In addition to attaching to bone, skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions such as swallowing, urination, and defecation to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs. Finally, skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat in an attempt to offset heat loss to the environment.

Cardiac muscle is only found in the heart and functions to generate force and build pressure gradients within the heart to drive blood flow throughout the body. Smooth muscle in the walls of blood vessels is a critical component that regulates blood pressure and blood flow through the circulatory system, smooth muscle in the skin is responsible for controlling the orientation of hair on our bodies, while smooth muscle in visceral organs and internal passageways is essential for moving materials throughout the body. Neither cardiac nor smooth muscle connects to bone and therefore cannot produce the gross movements we associate with skeletal muscle. Further, while skeletal muscle is responsible for both voluntary and involuntary movements, cardiac and smooth muscle are not subject to voluntary control.

This work, Human Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images from Anatomy & Physiology by OpenStax are licensed under CC BY except where otherwise noted.

Access the original for free at OpenStax.

Report an Error

Did you find an error, typo, broken link, or other problem in the text? Please follow this link to the error reporting form to submit an error report to the authors.