Muscle Tissue and Physiology of Muscle Contraction
All body functions involving movement require muscle activity. Muscle activity is required for skeletal movement, heart contraction, food moving through the gut, urination, and breathing, among many oth-ers. Certain muscles, even when not producing move-ment, remain contracted to maintain posture and op-pose the effects of gravity.
To perform these varied activities, three types of muscle are present in the body: skeletal, cardiac, and smooth. The skeletal muscles constitute about 40% to 50% of body weight. They are attached to the skeleton and are responsible for skeletal movement and stabilizing body position. Cardiac muscle is lo-cated in the heart; smooth muscle is present in the gut, around the bronchi in the lungs, urinary tract, reproductive organs, and blood vessels. Cardiac mus-cle helps move blood throughout the body, and smooth muscle helps move fluid and food matter. Smooth muscle also helps regulate flow out of certain organs (e.g., a ringlike arrangement of smooth mus-cle at the lower end of the urinary bladder regulates outflow of urine). Although the contractile mecha-nism is the same in the three muscle types, the types vary in microscopic appearance, strength of contrac-tion, duration of contraction, control by the nervous system, and in other ways—all adaptations according to the job performed.
Muscles produce heat during activity, and this heat is used to maintain the core body temperature.
This section addresses the structure of skeletal muscle and the process of muscle contraction.