anatomy chap 10

BI 231 - Anatomy & Physiology - Muscle Tissue

LISA EDENS, MBS, PHD

UMPQUA COMMUNITY COLLEGE

The Muscle System

•Nearly half of body's mass

•Transforms chemical energy (ATP) to directed mechanical energy  exerts force

•Three types

•Skeletal (a)

•Cardiac (c)

•Smooth (b)

•Four important functions

•Movement of bones or fluids (e.g., blood)

•Maintaining posture and body position

•Stabilizing joints

•Heat generation (especially skeletal muscle)

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Muscle Contraction Stimulation

•Skeletal muscles stimulated by somatic motor neurons

•Axons of motor neurons travel from central nervous system via nerves to skeletal muscle

•Each axon forms several branches as it enters muscle

•Each axon ending forms neuromuscular junction with single muscle fiber

•Usually only one per muscle fiber

Skeletal Muscle Cells – Muscle Fiber

•Long, cylindrical cells

•10 to 100 µm in diameter; up to 30 cm long

•Multiple peripheral nuclei

•Comprised of long parallel organelles called myofibrils

•Repeating chains of sarcomeres

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Sarcomere

•Smallest contractile unit (functional unit) of muscle fiber

•Made of contractile proteins

•Actin myofilaments = thin filaments

•Extend across I band and partway in A band

•Anchored to Z discs

•Myosin myofilaments = thick filaments

•Extend length of A band

•Connected at M line

•Globular protein heads

Pearson 2023; Mohan  Gupta 2022

Figure 10.6

•Motor End-Plate and Innervation

•At the NMJ, the axon terminal releases ACh.

•The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma.

•When Ach molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors.

Figure 10.7

•The T-tubule

•Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad—a “threesome” of membranes, with those of SR on two sides and the T-tubule sandwiched between them.

Figure 10.8

•Contraction of a Muscle Fiber

•A cross-bridge forms between actin and the myosin heads triggering contraction.

•As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.

Figure 10.9

•Relaxation of a Muscle Fiber

•Ca++ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands.

•A muscle may also stop contracting when it runs out of ATP and becomes fatigued.

Actin Activation

•Twisted double strand of fibrous protein consisting of globular actin proteins

•Troponin - regulatory protein

•Changes conformation when bound to calcium

•Tropomyosin – regulatory protein

•Rotates exposing myosin head attachment sites

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Sliding Filament Model

•In relaxed state, thin and thick filaments overlap only at ends of A band

•During contraction, thin filaments slide past thick filaments  actin and myosin overlap more

•Occurs when myosin heads bind to actin  cross bridges

•Myosin heads bind to actin; sliding begins

•Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere

•Causes shortening of muscle fiber

•Pulls Z discs toward M line    

•I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same)

Myosin Activation – Power Stroke

•Myosin heads contain 2 smaller, light polypeptide chains

•Myosin head binds actin forming cross bridges during contraction

•Myosin head bends and ADP + Pi are released

•New ATP binds myosin head causes dissociation from actin

•As ATP is hydrolyzed by myosin head cocks forward

•Repeat

Figure 10.12

•Muscle Metabolism

•Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds.

•Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle.

•Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.

Figure 10.13

•Types of Muscle Contractions

•During isotonic contractions, muscle length changes to move a load.

•During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate.

Figure 10.15

•A Myogram of a Muscle Twitch

•A single muscle twitch has a latent period, a contraction phase when tension increases, and a relaxation phase when tension decreases. During the latent period, the action potential is being propagated along the sarcolemma. During the contraction phase, Ca++ ions in the sarcoplasm bind to troponin, tropomyosin moves from actin-binding sites, cross-bridges form, and sarcomeres shorten. During the relaxation phase, tension decreases as Ca++ ions are pumped out of the sarcoplasm and cross-bridge cycling stops.

Figure 10.16

•Wave Summation and Tetanus

•(a) The excitation-contraction coupling effects of successive motor neuron signaling is added together which is referred to as wave summation. The bottom of each wave, the end of the relaxation phase, represents the point of stimulus. (b) When the stimulus frequency is so high that the relaxation phase disappears completely, the contractions become continuous; this is called tetanus.

Figure 10.17

•Treppe

•When muscle tension increases in a graded manner that looks like a set of stairs, it is called treppe. The bottom of each wave represents the point of stimulus.

Types of Muscle Fibers

Figure 10.18

•Marathoners

•Long-distance runners have a large number of SO fibers and relatively few FO and FG fibers. (credit: “Tseo2”/Wikimedia Commons)

Figure 10.19

•Hypertrophy

•Body builders have a large number of FG fibers and relatively few FO and SO fibers. (credit: Lin Mei/flickr)

Figure 10.20

•Atrophy

•Muscle mass is reduced as muscles atrophy with disuse.

Figure 10.21

•Cardiac Muscle Tissue

•Cardiac muscle tissue is only found in the heart. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Figure 10.22

•Cardiac Muscle

•Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.

Figure 10.23

•Smooth Muscle Tissue

•Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Figure 10.24

•Muscle Contraction

•The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.

Figure 10.25

•Motor Units

•A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor units through the smooth muscle.

Figure 11.2

•Prime Movers and Synergists

•The biceps brachii flex the lower arm. The brachoradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.

Figure 11.3

•Muscle Shapes and Fiber Alignment

•The skeletal muscles of the body typically come in seven different general shapes.

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Figure 11.6

•Understanding a Muscle Name from the Latin

Figure 11.8

•Muscles in Facial Expression

Figure 11.12

•Muscles for Tongue Movement, Swallowing, and Speech

Figure 11.21

•Muscles of the Perineum Common to Men and Women

Figure 11.24

•Muscles That Move the Humerus

Figure 11.26

•Muscles That Move the Forearm

Figure 11.27

•Muscles That Move the Wrist, Hands, and Forearm

Figure 11.30

•Gluteal Region Muscles That Move the Femur

Figure 11.31

•Thigh Muscles That Move the Femur, Tibia, and Fibula

Figure 11.33

•Muscles That Move the Feet and Toes

Figure 11.35

•Intrinsic Muscles in the Foot

Questions???