1. Overview of Muscular Tissue
The scientific study of muscles is known as myology.
The three types of muscular tissue are skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle tissue is mostly attached to bones. It is striated and voluntary.
Cardiac muscle tissue forms most of the wall of the heart. It is striated and involuntary.
Smooth muscle tissue is located in viscera. It is nonstriated and involuntary.
Muscular tissue has five key functions: producing body movements, stabilizing body positions, regulating organ volume, moving substances within the body, and producing heat.
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3. Skeletal Muscle Tissue
Skeletal muscles are separate organs composed of hundreds to thousands of cells, which are called muscle fibers because of their elongated shapes.
Connective tissue coverings associated with skeletal muscle include the epimysium, covering an entire muscle; perimysium, covering fascicles; and endomysium, covering individual muscle fibers.
Tendons are extensions of connective tissue beyond muscle fibers that attach the muscle to bone.
Skeletal muscles are well supplied with nerves and blood vessels, which provide nutrients and oxygen for contraction.
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4. Skeletal Muscle Tissue: Histology
Skeletal muscle consists of muscle fibers (cells) covered by a plasma membrane called the sarcolemma.
Transverse tubules tunnel in from the surface toward the center of each muscle fiber. The fibers contain sarcoplasm, multiple nuclei, many mitochondria, myoglobin, and sarcoplasmic reticulum.
Each fiber also contains myofibrils that contain thin filaments and thick filaments. The filaments are arranged in functional units called sarcomeres.
Thick filaments consist of myosin; thin filaments are composed of actin, tropomyosin, and troponin.
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5. Skeletal Muscle Tissue: Histology
Filaments overlap in specific patterns and form compartments called sarcomeres, the basic functional units of striated muscle fibers.
Sarcomeres are separated from one another by zig-zagging zones of dense protein material called Z discs.
Within each sarcomere a darker area, called the A band, extends the entire length of the thick filaments.
At the center of each A band is a narrow H zone, which contains only the thick filaments. At both ends of the A band, thick and thin filaments overlap.
A lighter-colored area to either side of the A band, called the I band, contains the rest of the thin filaments but no thick filaments.
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6. Contraction and Relaxation of Skeletal Muscle
Before a skeletal muscle fiber can contract, it must be stimulated by an electrical signal called a muscle action potential delivered by its neuron called a motor neuron. A single motor neuron along with all the muscle fibers it stimulates is called a motor unit.
The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber. The NMJ includes the axon terminals and synaptic end bulbs of a motor neuron plus the adjacent motor end plate of the muscle fiber sarcolemma.
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7. Contraction and Relaxation of Skeletal Muscle
At the NMJ, a motor neuron excites a skeletal muscle fiber in the following way:
Release of acetylcholine (ACh) from synaptic vesicles. ACh diffuses across the synaptic cleft and binds to ACh receptors, initiating a muscle action potential.
Generation of muscle action potential. The inflow of Na+ (down its concentration gradient) generates a muscle action potential.
Breakdown of ACh by an enzyme called acetylcholinesterase.
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8. Contraction and Relaxation of Skeletal Muscle
The sliding-filament mechanism of muscle contraction is the sliding of filaments and shortening of sarcomeres that cause the shortening of muscle fibers.
Both Ca2+ and energy, in the form of ATP, are needed for muscle contraction. An increase in the level of Ca2+ in the sarcoplasm, caused by the muscle action potential, starts the contraction cycle; a decrease in the level of Ca2+ turns off the contraction cycle.
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9. Contraction and Relaxation of Skeletal Muscle
The contraction cycle is the repeating sequence of events that causes sliding of the filaments:
Splitting ATP – myosin ATPase splits ATP and becomes energized
Forming cross-bridges – the myosin head attaches to actin, forming a cross-bridge
Power stroke – the cross-bridge generates force as it swivels or rotates toward the center of the sarcomere
Binding ATP & detaching – myosin detaches from actin. The myosin head again splits ATP, returns to its original position, and binds to a new site on actin as the cycle continues
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10. Metabolism of Skeletal Muscle Tissue
Skeletal muscle fibers often switch between virtual inactivity when relaxed and great activity contracting.
Muscle fibers have three sources for ATP production: creatine phosphate, anaerobic glycolysis, and aerobic respiration.
The transfer of a high-energy phosphate group from creatine phosphate to ADP forms new ATP molecules. Creatine phosphate and ATP provide enough energy for muscles to contract maximally for about 15 seconds.
Glucose is converted to pyruvic acid in the reactions of glycolysis, which yield two ATPs without using oxygen. These reactions, referred to as anaerobic glycolysis, can provide enough ATP for about 2 minutes of maximal muscle activity.
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11. Metabolism of Skeletal Muscle Tissue
Muscular activity that lasts longer than half a minute depends on aerobic respiration, mitochondrial reactions that require oxygen to produce ATP. Aerobic cellular respiration yields about 36 molecules of ATP from each glucose molecule.
The inability of a muscle to contract forcefully after prolonged activity is muscle fatigue.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

12. Control of Muscle Tension
A twitch contraction is a brief contraction of all of the muscle fibers in a motor unit in response to a single action potential in its motor neuron.
A record of a contraction is called a myogram. It consists of a latent period, a contraction period, and a relaxation period.
Wave summation is the increased strength of a contraction that occurs when a second stimulus arrives before the muscle has completely relaxed after a previous stimulus.
Repeated stimuli can produce unfused (incomplete) tetanus; more rapidly repeating stimuli will produce fused (complete) tetanus.
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13. Control of Muscle Tension
Motor unit recruitment is the process of increasing the number of active motor units.
On the basis of their structure and function, skeletal muscle fibers are classified as slow oxidative (SO) fibers, fast oxidative-glycolytic (FOG) fibers, and fast glycolytic (FG) fibers.
Most skeletal muscles contain a mixture of all three fiber types; their proportions vary with the typical action of the muscle.
The motor units of a muscle are recruited in the following order: first SO fibers, then FOG fibers, and finally FG fibers.
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14. Control of Muscle Tension
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15. Exercise and Skeletal Muscle Tissue
The relative ratio of fast glycolytic (FG) and slow oxidative (SO) fibers in each muscle is genetically determined and helps account for individual differences in physical performance.
For example, people with a higher proportion of FG fibers often excel in activities that require periods of intense activity, such as weight lifting or sprinting. People with higher percentages of SO fibers are better at activities that require endurance, such as long-distance running.
Exercises that require great strength for short periods produce an increase in the size and strength of fast glycolytic (FG) fibers. The increase in size is due to increased synthesis of thick and thin filaments.
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16. How Skeletal Muscles Produce Movement
Skeletal muscles are not attached directly to bones; they produce movements by pulling on tendons, which, in turn, pull on bones.
The attachment to the stationary bone is the origin. The attachment to the movable bone is the insertion.
The prime mover (agonist) produces the desired action. The antagonist produces an opposite action. The synergist assists the prime mover by reducing unnecessary movement. The fixator stabilizes the origin of the prime mover so that it can act more efficiently.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.