MUSCLE SYSTEM Human Anatomy and Physiology

Muscle Tissue  The essential function of muscle is contraction  Three forms: cardiac, smooth, and skeletal  In all its forms, muscle makes up nearly half the body’s mass

Muscle Functions 1. Producing Movement 2. Maintaining Posture 3. Stabilizing Joints 4. Generating Heat

Muscle Tissue In some ways, the three types of muscle tissue are the same  All muscle cells are elongated. So they are all called muscle fibers.  The ability to contract depends on two types of myofilaments (actin and myosin)  Terminology – myo and mys mean muscle; sarco means flesh

Muscle Tissue The three types of muscle tissue differ in:  cell structure  body location  how they are stimulated to contract

Three types of Muscle Tissue

Cardiac Muscle  Cardiac muscle is found in only one place in the body – the heart  striated  involuntary  arranged in spiral or figure 8 shaped bundles  cardiac muscle fibers are branching cells joined by special junctions called intercalated disks  contracts at a fairly steady rate, but the heart can also be stimulated by the nervous system to shift into high gear for short periods

Cardiac Muscle  The mechanism of contraction in cardiac muscle is essentially the same as that for skeletal and smooth muscle, but with some differences.  Cardiac muscle has transverse tubules that supply extra calcium, and can thus contract for longer periods.  Complex membrane junctions, called intercalated disks, join cells and transmit the force of contraction from one cell to the next, as well as aid in the rapid transmission of impulses throughout the heart.  Cardiac muscle is self-exciting and rhythmic, and the whole structure contracts as a unit. 8 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Smooth Muscle  spindle shaped  single nucleus  arranged in layers  One layer usually runs circularly, and the other layer longitudinally.  As the two layers alternately contract and relax, they change the size and shape of the organ  no striations  involuntary  contractions are steady and tireless  Found in walls of hollow organs such as the stomach, urinary bladder, and respiratory passages

Skeletal Muscle  cigar-shaped  multinucleate cells  striated (appears striped)  voluntary  the largest of the muscle fiber types  contract rapidly and with great force, but tires easily and must rest after short periods of activity.

Smooth Muscles  Smooth muscle cells are elongated with tapered ends, lack striations and have a relatively undeveloped sarcoplasmic reticulum.  Two types of smooth muscles 1. In multiunit smooth muscle, such as in the blood vessels and iris of the eye, fibers occur separately rather than as sheets. 2. Visceral smooth muscle occurs in sheets and is found in the walls of hollow organs; these fibers can stimulate one another and display rhythmicity, and are thus responsible for peristalsis in hollow organs and tubes 13 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Smooth Muscle Contraction  The myosin-binding-to-actin mechanism is mostly the same for smooth muscles and skeletal muscles.  Both acetylcholine and norepinephrine stimulate and inhibit smooth muscle contraction, depending on the target muscle.  Hormones can also stimulate or inhibit contraction.  Smooth muscle is slower to contract and relax than is skeletal muscle, but can contract longer using the same amount of ATP. 14 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.