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9

Muscles and Muscle Tissue

Part A

Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint^® Lecture Slides prepared by Vince Austin, University of Kentucky

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Muscle Overview

The three types of muscle tissue are skeletal, cardiac, and smooth
These types differ in structure, location, function, and means of activation

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Muscle Similarities

Skeletal and smooth muscle cells are elongated and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments – actin and myosin
Muscle terminology is similar

Sarcolemma – muscle plasma membrane
Sarcoplasm – cytoplasm of a muscle cell
Prefixes – myo, mys, and sarco all refer to muscle

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Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton
Has obvious stripes called striations
Is controlled voluntarily (i.e., by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

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Cardiac Muscle Tissue

Occurs only in the heart
Is striated like skeletal muscle but is not voluntary
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in bodily needs

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Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages as well as blood vessels
Forces food and other substances through internal body channels
It is not striated and is involuntary

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Functional Characteristics of Muscle Tissue

Excitability, or irritability – the ability to receive and respond to stimuli
Contractility – the ability to shorten forcibly
Extensibility – the ability to be stretched or extended
Elasticity – the ability to recoil and resume the original resting length

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Muscle Function

Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood through the body
Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs
Muscles also maintain posture, stabilize joints, and generate heat

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Skeletal Muscle

Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue
The three connective tissue sheaths are:

Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber
Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles
Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle

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Skeletal Muscle

Figure 9.2 (a)

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Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins
Each skeletal muscle fiber (muscle cell) is supplied with a nerve ending that controls contraction
Contracting fibers require continuous delivery of oxygen and nutrients via arteries
Wastes must be removed via veins

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Skeletal Muscle: Attachments

Most skeletal muscles span joints and are attached to bone in at least two places
When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

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Microscopic Anatomy of a Skeletal Muscle Fiber

Each fiber (muscle cell) is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
Sarcoplasm has a unique oxygen-binding protein called myoglobin
Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules

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Myofibrils

Myofibrils are densely packed, rodlike contractile elements
They make up most of the muscle volume
The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident

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Myofibrils

Figure 9.3 (b)

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Sarcomeres

The smallest contractile unit of a muscle
The region of a myofibril between two successive Z-lines
Composed of myofilaments made up of contractile proteins

Myofilaments are of two types – thick (myosin) and thin (actin)

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Sarcomeres

Figure 9.3 (c)

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Myofilaments: Banding Pattern

Thick filaments – extend the entire length of an A band
Thin filaments – extend across the I band and partway into the A band
Z-lines – a sheet of proteins that anchors the thin filaments and connects myofibrils to one another

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Myofilaments: Banding Pattern

Thin filaments do not overlap thick filaments in the lighter H zone (H for helle – “bright”)
M lines appear darker due to the presence of the protein desmin, which connects Z-lines to each other
Another protein titin anchors thick filaments to each Z-line

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M line

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Myofilaments: Banding Pattern

Figure 9.3 (c, d)

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Ultrastructure of Myofilaments: Thick Filaments

Thick filaments are composed of the protein myosin
Each myosin molecule has a rodlike tail and two globular heads

Tails – two interwoven, heavy polypeptide chains
Heads – two smaller, light polypeptide chains called cross bridges

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Ultrastructure of Myofilaments: Thick Filaments

Figure 9.4 (a)(b)

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Ultrastructure of Myofilaments: Thin Filaments

Thin filaments are chiefly composed of the protein actin
The subunits contain the active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory proteins bound to actin

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Ultrastructure of Myofilaments: Thin Filaments

Figure 9.4 (c)

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Arrangement of the Filaments in a Sarcomere

Longitudinal section within one sarcomere

Figure 9.4 (d)

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Sarcoplasmic Reticulum (SR)

SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril
Paired terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular calcium levels
Elongated tubes called T tubules penetrate into the cell’s interior

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Sarcoplasmic Reticulum (SR)

Figure 9.5

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T Tubules

T tubules are continuous with the sarcolemma
They conduct impulses to the deepest regions of the muscle
These impulses signal for the release of Ca²⁺ from adjacent terminal cisternae

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Sliding Filament Model of Contraction

In the relaxed state, thin and thick filaments overlap only slightly
When contracting, thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree
Upon stimulation, myosin heads bind to actin and sliding begins

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Sliding Filament Model of Contraction

Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere
As this event occurs throughout the sarcomeres, the muscle shortens

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Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:

Be stimulated by a nerve ending
Propagate an electrical current, or action potential, along its sarcolemma
Have a rise in intracellular Ca²⁺ levels, the final trigger for contraction

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Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons
Axons of these neurons travel in nerves to muscle cells and branch profusely as they enter muscles
Each branch forms a neuromuscular junction with a single muscle fiber

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Neuromuscular Junction

The neuromuscular junction is formed from:

Axonal endings, which have small membranous sacs (vesicles) that contain the neurotransmitter acetylcholine (ACh)
The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors

Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

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Neuromuscular Junction

Figure 9.7 (a-c)

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Neuromuscular Junction

When a nerve impulse reaches the end of an axon at the neuromuscular junction:

Calcium channels open and allow Ca²⁺ to enter the axon
Ca²⁺ inside the axon terminal causes vesicles to fuse with the distal membrane

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Neuromuscular Junction

This fusion releases ACh into the synaptic cleft via exocytosis
ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma
Binding of ACh to its receptors initiates an action potential in the muscle

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Destruction of Acetylcholine

ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
This destruction prevents sustained muscle fiber contraction in the absence of additional stimuli

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Excitation-Contraction Coupling

Figure 9.9

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Sequential Events of Contraction

Cross bridge formation – myosin cross bridge attaches to actin filament
Working (power) stroke – myosin head pivots and pulls actin filament toward M line
Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches
“Resetting” of the myosin head – energy from hydrolysis of ATP resets the myosin head into the high-energy state

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Sequential Events of Contraction

Myosin cross bridge attaches to the actin myofilament

1

2

3

4

Working stroke—the myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line

As new ATP attaches to the myosin head, the cross bridge detaches

As ATP is split into ADP and P_i, cocking of the myosin head occurs

Myosin head (high-energy configuration)

Thick filament

Myosin head (low-energy configuration)

ADP and P_i (inorganic phosphate) released

Figure 9.11

Thin filament

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Muscle Twitch

A muscle twitch is the response of a muscle to a single, brief threshold stimulus
The three phases of a muscle twitch are:

Latent period – �first few milli-�seconds after �stimulation �when excitation-�contraction �coupling is �taking place

Figure 9.13 (a)

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Muscle Twitch

Period of contraction – cross bridges actively form and the muscle shortens
Period of relaxation – Ca²⁺ is reabsorbed into the SR, and muscle tension goes to zero

Figure 9.13 (a)

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Muscle Tone

Muscle tone:

Is the constant, slightly contracted state of all muscles, which does not produce active movements
Keeps the muscles firm, healthy, and ready to respond to stimulus

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Muscle Metabolism: Energy for Contraction

ATP is the only source used directly for contractile activity
As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:

The interaction of ADP with creatine phosphate (CP)
Anaerobic glycolysis
Aerobic respiration

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Muscle Metabolism: Energy for Contraction

Figure 9.18

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Heat Production During Muscle Activity

Only 40% of the energy released in muscle activity is useful as work
The remaining 60% is given off as heat
Dangerous heat levels are prevented by radiation of heat from the skin and sweating

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Muscle Fiber Type: Functional Characteristics

Speed of contraction – determined by speed in which ATPases split ATP

The two types of fibers are slow-twitch and fast-twitch

ATP-forming pathways

Oxidative fibers – use aerobic pathways
Glycolytic fibers – use anaerobic glycolysis

These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

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Muscle Fiber Type: Speed of Contraction

Slow oxidative fibers contract slowly, have slow acting myosin ATPases, and are fatigue resistant
Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue
Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued

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Muscular Dystrophy

Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy

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Muscular Dystrophy

Duchenne muscular dystrophy (DMD)

Inherited, sex-linked disease carried by females and expressed in males (1/3500)
Diagnosed between the ages of 2-10

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Muscular Dystrophy

Caused by a lack of the anchoring protein dystrophin
As a result, poorly anchored muscle fibers tear themselves apart under stress of contraction
Free calcium enters the muscle cells causing cell death and fiber necrosis
There is currently no cure
25% survive to age 21