Summary

Module 10.1: Overview of Muscle Tissue

The three types of muscle tissue are skeletal, cardiac, and smooth muscle tissue.
All muscle cells turn chemical energy into mechanical energy by contracting to generate muscle tension.
All types of muscle tissue consist of muscle cells and the surrounding extracellular matrix, called the endomysium.
All types of muscle tissue share common properties, including contractility, excitability, conductivity, distensibility, and elasticity.
The plasma membrane of a muscle cell is the sarcolemma, and the cytoplasm is the sarcoplasm. The sarcoplasm contains myofibrils and the sarcoplasmic reticulum (SR).

Module 10.2: Structure and Function of Skeletal Muscle Fibers

Skeletal muscle fibers are very long cells that consist of striated, multinucleated voluntary muscle cells.
The sarcolemma has inward extensions called T-tubules that surround myofibrils. The Ca²⁺-storing SR swells where it meets T-tubules to form terminal cisternae.
Myofibrils are composed of myofilaments. Myofilaments are composed of contractile proteins, regulatory proteins, and/or structural proteins.
There are three types of myofilaments:
- Thick filaments are composed of myosin, a contractile protein.
- Thin filaments are composed of contractile actin proteins and the smaller regulatory proteins troponin and tropomyosin.
- Elastic filaments are composed of the structural protein titin.
The striations of skeletal muscle tissue are due to the arrangement of myofilaments:
- The I bands are the light regions of striations where only thin filaments are found. In the middle of each I band is the Z-disc.
- The A bands are the dark regions of striations where the thick and thin filaments overlap. The central region of the A band is the H zone. It is bisected by the M line.
The sarcomere is the functional unit of contraction. It is defined as the area from one Z-disc to the next.
The sliding-filament mechanism is the currently accepted model of muscle contraction. In this process, the thick and thin filaments slide past one another.

Module 10.3: Skeletal Muscle Fibers as Electrically Excitable Cells

A separation of charges occurs across the sarcolemma, which is an electrical gradient or electrical potential. Membrane potentials are electrical potentials across plasma membranes.
An unstimulated muscle fiber shows a decrease in voltage across the membrane, called the resting membrane potential. Muscle fibers at rest have a resting membrane potential of about −90 mV.
Ions move across the sarcolemma via channels. Two types of channels are leak and gated channels.
The concentration of Na⁺ is higher in the extracellular fluid, and the concentration of K⁺ is higher in the cytosol. The Na⁺/K⁺ pump maintains this gradient.
The negative resting membrane potential is due to the loss of K⁺ through leak channels and the actions of the Na⁺/K⁺ pumps.
Ion movement is driven by the forces of the concentration gradient and the electrical gradient. The sum of these two forces is the electrochemical gradient.
An action potential is a temporary, quick reversal in the membrane potential. It consists of: (1) depolarization, in which Na⁺ enter the fiber, causing the membrane potential to become more positive; and (2) repolarization, in which K⁺ exit the fiber, returning it to its resting state.

Module 10.4: The Process of Skeletal Muscle Contraction and Relaxation

All skeletal muscle fibers are innervated by motor neurons. The neuron meets the fiber at the neuromuscular junction, or NMJ. The NMJ consists of the axon terminal, the synaptic cleft, and the motor end plate.
Skeletal muscle contraction can be divided into three parts: excitation, excitation-contraction coupling, and contraction.
The excitation phase begins as the axon terminal releases ACh into the synaptic cleft, which binds ACh receptors on the motor end plate. The result is an end-plate potential.
In excitation-contraction coupling, the end-plate potential triggers an action potential in the sarcolemma, which propagates down the T-tubules. This triggers the opening of Ca²⁺ channels in the SR, and Ca²⁺ flood the cytosol.
In preparation for contraction, the Ca²⁺ in the cytosol bind troponin, which allows tropomyosin to move away from the active sites of actin.
The contraction phase then begins, and the crossbridge cycle occurs as ATP hydrolysis “cocks” the myosin head and it binds to actin. When myosin undergoes a power stroke, it pulls actin toward the center of the sarcomere.
Muscle relaxation has two components: (1) The ACh in the synaptic cleft is broken down, and (2) the Ca²⁺ concentration in the cytosol returns to the resting level.

Module 10.5: Energy Sources for Skeletal Muscle

Immediate energy sources for muscle fibers include stored ATP within the cytosol and creatine phosphate.
Glycolytic catabolism is an anaerobic process during which glucose in the cytosol is split and ATP is produced.
Oxidative catabolism is an aerobic process that takes place in the mitochondria, during which the products of glycolysis, fatty acids, and amino acids are oxidized to generate ATP.

Module 10.6: Muscle Tension at the Fiber Level

A single contraction-relaxation cycle of a muscle fiber is called a twitch. It consists of the latent period, the contraction period, and the relaxation period. Muscle fibers may be classified as fast-twitch or slow-twitch fibers.
The amount of tension generated by the contraction of a muscle fiber depends on the frequency of stimulation by the motor neuron and the resulting concentration of Ca²⁺.
- If a muscle fiber is stimulated before the relaxation period is completed, unfused tetanus results.
- If a muscle fiber is stimulated 80–100 times per second before the relaxation period begins, fused tetanus results.
Per the length-tension relationship, a contraction can produce a maximal amount of tension at the optimal length of a sarcomere.
Type I muscle fibers are slow-twitch fibers that use primarily oxidative catabolism. Type II muscle fibers are fast-twitch fibers that use primarily glycolytic catabolism.

Module 10.7: Muscle Tension at the Organ Level

A motor unit consists of a single motor neuron and the muscle fibers that it innervates.
- Recruitment occurs when more motor units are activated for more forceful contractions.
- Muscle tone is produced by small, involuntary contractions of alternating motor units.
There are three types of muscle contractions: an isotonic concentric (shortening) contraction, an isotonic eccentric (lengthening) contraction, and an isometric (unchanging length) contraction.

Module 10.8: Skeletal Muscle Performance

Endurance and resistance training result in changes in the structure and biochemistry of skeletal muscle fibers.
Disuse leads to atrophy of the muscle fibers.
Muscular fatigue is caused by depletion of key metabolites, inadequacy of oxygen delivery to muscle fibers, accumulation of certain metabolites, and environmental conditions.
Excess postexercise oxygen consumption (EPOC) occurs after exercise to correct the homeostatic imbalances that were caused by exercise.

Module 10.9: Smooth and Cardiac Muscle

The functions of smooth muscle tissue include peristalsis, forming sphincters, and regulating the flow of material through hollow organs.
Smooth muscle cells are uninucleate; lack T-tubules, striations, and sarcomeres; and have a less extensive SR than do skeletal muscle fibers.
Smooth muscle contraction is stimulated by the autonomic nervous system, stretch, hormones, and the activity of pacemaker cells.
Smooth muscle cell contraction is activated by Ca²⁺ binding to calmodulin. This activates myosin light-chain kinase (MLCK), which initiates crossbridge cycles.
Smooth muscle cell relaxation involves removal of Ca²⁺ from the cytosol.
There are two types of smooth muscle tissue:
- Single-unit smooth muscle cells contract together as a single unit.
- Multi-unit smooth muscle cells that can contract independently of one another.
Cells of cardiac muscle are joined physically and electrically by intercalated discs. Their electrical activity is coordinated by pacemaker cells.

Learning Objectives

Upon completion of this chapter, you should be able to:

Describe the major functions of muscle tissue.
Name and describe the structural elements and properties common to all types of muscle cells.
Compare and contrast the characteristics of skeletal, cardiac, and smooth muscle tissue.
Describe the structural properties and components of a skeletal muscle fiber.
Explain the organization of a myofibril.
Describe the structure and components of thick, thin, and elastic filaments.
Name and describe the function of each of the contractile, regulatory, and structural protein components of a sarcomere.
Explain the sliding-filament mechanism of muscle contraction.
Contrast the relative concentrations of sodium and potassium ions inside and outside a cell.
Differentiate between a concentration gradient and an electrochemical gradient.
Describe how the resting membrane potential is generated.
Describe the sequence of events of a skeletal muscle fiber action potential.
Describe the anatomy of the neuromuscular junction.
Describe the events at the neuromuscular junction that elicit an action potential in the muscle fiber.
Explain excitation-contraction coupling.
Describe the sequence of events involved in the contraction cycle of a skeletal muscle fiber.
Explain the process of skeletal muscle fiber relaxation.
Describe the immediate energy sources available to muscle fibers.
Describe the glycolytic and oxidative mechanisms that muscle fibers use to obtain ATP for muscle contraction.
Explain the duration of activity that each ATP source can fuel.
Describe the stages of a twitch contraction, and explain how a twitch is affected by the frequency of stimulation.
Relate tension production to the length of a sarcomere.
Compare and contrast the anatomical and metabolic characteristics of type I and type II muscle fibers.
Describe the structure and function of a motor unit.
Explain how muscle tone is produced.
Compare and contrast the three types of contractions.
Describe the effects of physical conditioning on skeletal muscle tissue, and compare endurance and resistance training.
Explain the factors that contribute to muscular fatigue.
Summarize the events that occur during the recovery period.
Describe the structure, location in the body, and functions of smooth and cardiac muscle tissue.
Describe the contraction process of smooth muscle fibers, and contrast it with skeletal muscle fiber contraction.

Tasks to Complete

Upon completion of this chapter, you will need to complete both an assignment and a quiz.