By the end of this section, you will be able to:
Describe structural and functional differences of skeletal, cardiac, and smooth muscle tissue.
- Describe the different types of muscle
- Contrast structural and functional differences of muscle tissue
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 1). All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane. While the nervous system can influence the excitability of cardiac and smooth muscle to some degree, skeletal muscle completely depends on signaling from the nervous system to work properly. On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.
A unique property common to all three types of muscle is contractility, which is the ability of the cells to shorten and generate force. While muscle tissue can shorten with contractions, it also displays extensibility or the ability to stretch and extend beyond the resting length of the cells. After being stretched, the elasticity of muscle allows it to recoil back to its original length.
The muscles all begin the mechanical process of contracting (shortening) when a protein called actin is pulled by a protein called myosin, and differences in the microscopic organization of these contractile proteins exist among the three muscle types. In both skeletal and cardiac muscle, the actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells, which creates an alternating light and dark striped pattern called striations. The striations are visible with a light microscope under high magnification (see Figure 1). Smooth muscle (named for it’s lack of striations), does not produces this striped pattern because the contractile proteins are not arranged in such regular fashion.
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 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 each generally have one nucleus centrally located in the cell, but the cells are physically and electrically connected to each other so that the contraction signals spread through cells and the entire heart contracts as one unit. Smooth muscle cells contain a single nucleus and can exist in electrically linked units contracting together as a single-unit or as multi-unit smooth muscle where cells are not electrically linked.
The best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. 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.
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.
Cardiac muscle is only found in the heart and functions to generate force and build pressure gradients to drive blood flow throughout the body. Smooth muscle in the walls of arteries is a critical component that regulates blood pressure and blood flow through the circulatory system. Smooth muscle in the skin, visceral organs, and internal passageways is also essential for moving materials through the body.
Muscle is the tissue in animals that allows for active movement of the body or materials within the body. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Most of the body’s skeletal muscle produces movement by acting on the skeleton. Cardiac muscle is found in the wall of the heart and pumps blood through the circulatory system. Smooth muscle is found in the skin, where it is associated with hair follicles; it also is found in the walls of internal organs, blood vessels, and internal passageways, where it assists in moving materials.
Critical Thinking Questions
1. Why is elasticity an important quality of muscle tissue?
2. What are the primary functions of skeletal muscle?
- cardiac muscle
- striated muscle found in the heart; joined to one another at intercalated discs and under the regulation of pacemaker cells, which contract as one unit to pump blood through the circulatory system. Cardiac muscle is under involuntary control.
- ability to shorten (contract) forcibly
- ability to stretch and rebound
- ability to undergo neural stimulation
- ability to lengthen (extend)
- skeletal muscle
- striated, multinucleated muscle that requires signaling from the nervous system to trigger contraction; most skeletal muscles are referred to as voluntary muscles that move bones and produce movement
- smooth muscle
- nonstriated, mononucleated muscle in the skin that is associated with hair follicles; assists in moving materials in the walls of internal organs, blood vessels, and internal passageways
Answers for Critical Thinking Questions
- It allows muscle to return to its original length during relaxation after contraction.
- Produce movement of the skeleton, maintain posture and body position, support soft tissues, encircle openings of the digestive, urinary, and other tracts, and maintain body temperature.