Biology 218 – Human RIDDELL

Chapter 10 Adapted form Tortora 10th ed.

LECTURE OUTLINE

A. Introduction (p. 265) 1. Bones provide leverage and form the framework of the body, but motion results from alternating contraction (shortening) and relaxation of muscles. 2. also stabilizes the body’s position, regulates organ volume, generates heat, and propels fluids and food through various body systems. 3. The study of muscles is called myology. B. Overview of Muscle Tissue (p. 266) 1. There are three types of muscle tissue (see Table 10.2): i. tissue a. moves bones (and, in some cases, skin and other soft tissues) b. striated c. voluntary ii. tissue a. forms most of the wall of the heart b. striated c. involuntary d. some cells have autorhythmicity iii. tissue a. located in the walls of hollow internal structures (and arrector pili muscles) b. nonstriated, i.e., smooth c. involuntary d. some cells have autorhythmicity 2. Muscle tissue has four major functions: i. producing body movements ii. stabilizing body positions iii. storing and moving substances within the body iv. producing heat 3. Muscle tissue has four major characteristics that enable it to perform its functions: i. Electrical excitability is the ability to respond to certain stimuli by producing electrical signals called action potentials (impulses) ii. Contractility is the ability to shorten, thus developing tension (force of contraction) a. in an isometric contraction, a muscle develops tension but does not shorten b. in an isotonic contraction, tension remains relatively constant while the muscle shortens iii. Extensibility is the ability of the muscle to be stretched without being damaged iv. Elasticity is the ability to return to its original shape after contraction or extension

C. Skeletal Muscle Tissue (p. 267) 1. Connective Tissue Components: Page 1 of 5 Biology 218_Lecture Outline_10 Muscles 1 Biology 218 – Human Anatomy RIDDELL

i. is a sheet or broad band of fibrous connective tissue beneath the skin (superficial fascia) or around muscles or organs (deep fascia). ii. Three layers of connective tissue extend from the deep fascia: a. outer encircles the whole muscle b. surrounds fascicles (bundles of muscle fibers) c. surrounds individual muscle fibers iii. Extensions of these three layers are continuous with: a. , a cord of dense connective tissue that attaches a muscle to the periosteum of a bone; some are enclosed in tendon (synovial) sheaths which permit tendons to slide back and forth more easily b. , a broad, flat sheet of connective tissue that attaches a muscle to the periosteum of a bone, another muscle, or the skin 2. Microscopic Anatomy: (p. 267) i. Growth of a skeletal muscle is achieved primarily by hypertrophy because skeletal muscle fibers do not significantly increase in number after birth; skeletal muscle tissue does have limited powers of regeneration due to the presence of satellite cells which can fuse with or replace existing skeletal muscle fibers to a limited extent

- if significant skeletal muscle damage or degeneration occurs, skeletal muscle tissue undergoes fibrosis, the replacement of muscle fiber by fibrous scar tissue ii. Each muscle fiber has a sarcolemma (plasma membrane) that surrounds the sarcoplasm (cytoplasm); the latter contains an oxygen-binding protein called myoglobin iii. The sarcoplasm contains numerous myofibrils; a. The myofibrils contain numerous parallel (protein) myofilaments called thin filaments and thick filaments that are arranged in functional units called sarcomeres b. Adjacent sarcomeres are separated by Z discs (lines) and have regions called the A band, I band, H zone, and M line and a zone of overlap. c. Thick filaments consist of myosin molecules with heads or crossbridges. d. Thin filaments consist of actin molecules plus two regulatory proteins called troponin and tropomyosin. e. Sarcomeres also contain structural proteins (e.g., titin and dystrophin) which keep the thick and thin filaments properly aligned, give the myofibril elasticity and extensibility and link the myofibrils to the sarcolemma and . iv. The sarcoplasm contains the sarcoplasmic reticulum (SR) with dilated end sacs called terminal cisterns which store calcium ions; release of calcium ions into the cytosol triggers . v. The sarcolemma has tunnel-like infoldings called transverse tubules (T tubules) that penetrate into the muscle fiber at right angles to the myofilaments; these T tubules conduct muscle action potentials which cause the release of calcium ions from the SR.

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3. and Blood Supply: (p. 273) i. Motor neurons send impulses down their axons to stimulate muscle fibers to contract. ii. The junction or synapse between a motor neuron and a skeletal muscle fiber is called a neuromuscular junction (NMJ). iii. When a nerve impulse (action potential) arrives at this junction, the synaptic vesicles in synaptic end bulbs release acetylcholine (neurotransmitter molecules) which diffuse across the synaptic cleft and bind to acetylcholine receptor molecules on the motor end plate; this triggers a muscle action potential that travels along the sarcolemma and ultimately initiates contraction of the muscle fiber. iv. A motor unit consists of a motor neuron plus all the muscle fibers that it innervates; all fibers in a motor unit therefore contract and relax simultaneously a. the average motor unit contains 150 muscle fibers but there is a wide range that extends from 2–3 muscle fibers per motor unit to 2000-3000 muscle fibers per motor unit b. small motor units provide precise control of movement while large motor units provide greater strength of contraction v. Blood vessels called capillaries deliver nutrients and oxygen to muscle fibers and carry wastes and heat away from muscle fibers. 4. Contraction and Relaxation of Skeletal Muscle Fibers: (p. 274) i. Muscle contraction is triggered when a muscle action potential is propagated along the sarcolemma through the T tubule system and to the sarcoplasmic reticulum, where it causes the release of calcium ions from the SR into the cytosol ii. Muscle contraction occurs via the sliding filament mechanism: - in the presence of ATP and calcium ions, myosin cross bridges pull thin filaments toward the center of the sarcomere, resulting in a shortening of the sarcomere iii. The effect of acetylcholine is brief because it is rapidly broken down by acetylcholinesterase located in the synaptic cleft. 5. Muscle tone is a sustained, partial state of contraction that gives firmness to a relaxed skeletal muscle; in a constantly shifting pattern, a few motor units become active while others become inactive within a skeletal muscle. This is essential for maintaining posture.

D. Types of Skeletal Muscle Fibers (p. 276) 1. Skeletal muscle fibers are not all identical in structure or function: i. Muscle fibers that have a high content of myoglobin, a red oxygen-storing protein (as well as more mitochondria and a rich blood supply), are called red muscle fibers. ii. Muscle fibers that have a low content of myoglobin are called white muscle fibers. iii. Based on structural and functional characteristics, skeletal muscle fibers are classified into three types (see Table 10.1): a. slow oxidative (SO) fibers b. fast oxidative-glycolytic (FOG) fibers c. fast glycolytic (FG) fibers iv. Most skeletal muscles contain a mixture of all three types of skeletal muscle fibers, but their proportion varies depending on the usual action of

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the muscle; however, the muscle fibers in any one motor unit are all of the same type. 2. The total number of skeletal muscle fibers and the relative percentages of fast and slow fibers in each muscle do not change significantly after birth, but the characteristics of the muscle fibers present may be somewhat altered in various ways by various types of exercise (see p. 277).

E. Cardiac Muscle Tissue (p. 278) 1. Cardiac muscle tissue is located in the wall of the heart. 2. The cells have the following important characteristics: i. are striated ii. are involuntary iii. some cells have autorhythmicity iv. are branching cylinders that usually contain a single nucleus v. compared to skeletal muscle fibers, the sarcoplasm: a. is more abundant b. has larger and more numerous mitochondria c. has a less elaborate sarcoplasmic reticulum d. is penetrated by fewer but wider transverse tubules vi. neighboring fibers are connected by intercalated discs that contain: a. desmosomes, which strongly hold the fibers together b. gap junctions, which allow muscle action potentials to spread from one fiber to another fiber 3. Under normal resting conditions, cardiac muscle tissue rhythmically contracts and relaxes about 75 times per minute; this rhythm may be increased or decreased by nerve or hormonal stimulation. 4. Cardiac muscle tissue has characteristics that permit significant increases in heart rate but prevent the heart from undergoing tetanus. 5. Damaged cardiac muscle fibers are not repaired or replaced, and healing occurs by fibrosis; but they can undergo hypertrophy in response to increased workload.

F. Smooth Muscle Tissue (p. 278) 1. Smooth muscle tissue is usually activated involuntarily. 2. There are two types of smooth muscle tissue: i. visceral (single-unit) smooth muscle tissue is the more common type a. located in the walls of small blood vessels and hollow viscera b. it is autorhythmic; due to presence of gap junctions, entire muscle contracts as a single unit when one fiber is stimulated by neurotransmitter, hormone or autorhythmic signal ii. multiunit smooth muscle tissue a. located in the walls of large arteries, bronchioles, arrector pili muscles, irises of the eyes, and ciliary muscles of the eyes b. each fiber contracts individually upon stimulation 3. The cells have the following important characteristics: i. are narrow fibers with two tapering ends ii. have a single nucleus iii. the sarcoplasm: a. contains both thick and thin filaments but they are not arranged into orderly sarcomeres, i.e., nonstriated and therefore called smooth b. contains intermediate filaments attached to dense bodies iv. when a smooth muscle fiber contracts, it turns like a corkscrew

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v. compared to skeletal muscle, contraction starts more slowly and lasts much longer vi. the prolonged presence of calcium ions in the cytosol provides for smooth muscle tone 4. Most smooth muscle fibers contract or relax in response to neurotransmitters, hormones, autorhythmic signals, stretching or local factors. 5. Smooth muscle tissue exhibits the stress-relaxation response which allows it to be stretched considerably while still retaining the ability to contract effectively. 6. Smooth muscle fibers can undergo hypertrophy; some smooth muscle fibers can divide and new fibers can be formed from pericytes and, therefore, regeneration can occur to a limited extent.

G. Development of Muscles (p. 282) 1. Almost all muscles develop from mesoderm. 2. Skeletal muscles of the head and limbs develop from the general mesoderm and the remaining skeletal muscles develop from the mesoderm of somites. 3. Cardiac muscle and smooth muscle develop from mesodermal cells.

H. Aging and Muscular Tissue (p. 283) 1. After 30 years of age, there is a progressive decrease in skeletal muscle mass that is replaced primarily by fibrous connective tissue and adipose tissue. 2. This is accompanied by a decrease in maximal strength and a slowing of muscle reflexes. 3. In some muscles, there is a change in the proportions of the specific types of skeletal muscle fibers.

I. Key Medical Terms Associated with Muscular Tissue (p. 284) 1. Students should familiarize themselves with the glossary of key medical terms.

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