Chapter 9: Muscles and Muscle Tissue – Structure, Function, and Disorders

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Muscle Tissue: Structure and Function

Characteristics of Muscle Tissue

Muscle tissue possesses several unique properties that enable it to perform its essential roles in the body, including movement, posture, and heat production.

Excitability (Responsiveness): The ability of muscle cells to respond to chemical signals, stretch, or other stimuli by generating electrical changes across their plasma membrane, similar to nerve cells.
Conductivity: The capacity of muscle cells to propagate an electrical excitation (action potential) along their length, leading to coordinated contraction.
Contractility: Muscle cells can shorten forcefully when stimulated, pulling on bones or other structures to produce movement.
Extensibility: Muscle cells can be stretched beyond their resting length without rupturing, up to three times their contracted length.
Elasticity: After being stretched, muscle cells can recoil to their original length, which is essential for maintaining muscle function and preventing injury.

Types of Muscle Tissue

Classification and Features

There are three main types of muscle tissue in the human body, each with distinct structure and function.

Skeletal Muscle: Striated and voluntary; attached to bones and responsible for body movement. Under a microscope, skeletal muscle cells appear striped due to the arrangement of actin and myosin filaments.
Cardiac Muscle: Striated but involuntary; found only in the heart. Cardiac muscle cells are branched and interconnected, allowing for coordinated contractions.
Smooth Muscle: Non-striated and involuntary; found in the walls of hollow organs (e.g., intestines, blood vessels). Smooth muscle cells are spindle-shaped and contract more slowly than skeletal muscle.

Example: Skeletal muscles control limb movement, cardiac muscle pumps blood, and smooth muscle regulates blood vessel diameter.

Muscle Structure and Organization

Muscle Fiber and Connective Tissue Layers

Muscles are organized into bundles of fibers, each surrounded by connective tissue layers that provide support and transmit force.

Muscle Fiber: Also called a muscle cell; elongated and multinucleated.
Endomysium: Thin connective tissue surrounding each muscle fiber.
Perimysium: Connective tissue that groups muscle fibers into bundles called fascicles.
Epimysium: Dense connective tissue that surrounds the entire muscle.

Muscle Fiber Anatomy

Sarcolemma: The plasma membrane of a muscle fiber.
Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and stored nutrients.
Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases calcium ions during muscle contraction.
Myofibrils: Long protein bundles within the sarcoplasm, composed of repeating units called sarcomeres.
Glycogen: Stored carbohydrate for energy during exercise.
Myoglobin: Oxygen-binding protein that supplies oxygen for muscle activity.

Myofibrils and Sarcomeres

Myofibrils are made up of repeating units called sarcomeres, which are the functional contractile units of muscle.

Thick Filaments: Composed of myosin proteins.
Thin Filaments: Composed of actin proteins.
A Band: Dark band; contains thick (myosin) filaments.
I Band: Light band; contains thin (actin) filaments.
Z Line: Defines the boundaries of each sarcomere.

Example: The sliding of actin over myosin filaments during contraction shortens the sarcomere, leading to muscle contraction.

Muscle Contraction Mechanisms

Sliding Filament Theory

The sliding filament theory explains how muscles contract at the molecular level.

During contraction, thin filaments (actin) slide past thick filaments (myosin), shortening the sarcomere without changing the length of the filaments themselves.
ATP and calcium ions are required for the interaction between actin and myosin.
Key steps include cross-bridge formation, power stroke, detachment, and re-cocking of the myosin head.

Equation:

Neuromuscular Junction

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber to initiate contraction.

Motor End Plate: Folded region of the muscle fiber's sarcolemma with receptors for neurotransmitters.
Synapse (Synaptic Cleft): The gap between the neuron and muscle fiber.
Synaptic Vesicles: Store the neurotransmitter acetylcholine (ACh), which is released into the synaptic cleft to trigger muscle contraction.
Cholinesterase: Enzyme that breaks down ACh, ending the signal for contraction.

Motor Unit

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Recruitment of more motor units increases the strength of muscle contraction.

Energy for Muscle Contraction

ATP Production and Usage

ATP is produced primarily by cellular respiration in mitochondria.
Creatine phosphate helps regenerate ATP during intense activity.
Only about 25% of the energy from cellular respiration is used for muscle work; the rest is released as heat to maintain body temperature.

Muscle Response and Adaptation

Key Terms and Concepts

Threshold Stimulus: Minimum stimulus strength required to trigger a muscle contraction.
All-or-None Response: Muscle fibers contract fully or not at all when stimulated above threshold.
Action Potential: Electrical signal that travels along the sarcolemma, initiating contraction.
Recruitment: Increasing the number of active motor units to produce stronger contractions.
Muscle Tone: Continuous, partial contraction of muscles, even at rest.
Hypertrophy: Increase in muscle size due to exercise or certain conditions.
Atrophy: Decrease in muscle size and strength due to disuse or disease.
Muscle Fatigue: Reduced ability to contract after prolonged activity.
Muscle Cramp: Sustained, involuntary contraction of a muscle.
Oxygen Debt: Extra oxygen required after exercise to restore metabolic conditions and remove lactic acid.

Muscle Twitch

A muscle twitch is a single, brief contraction and relaxation cycle in response to a single stimulus. Twitch strength can vary with temperature, hydration, and frequency of stimulation.

Muscle Fiber Types

Classification by Contraction Speed and Metabolism

Fiber Type	Color	Contraction Speed	Metabolism	Example Activity
Slow Oxidative (Type I)	Red	Slow	High mitochondria, aerobic	Distance running, posture
Fast Oxidative (Type IIa)	Red to pink	Fast	Many mitochondria, aerobic	Walking, sprinting
Fast Glycolytic (Type IIb)	White (pale)	Fast	Few mitochondria, anaerobic	Short, intense movements (e.g., baseball)

Muscle Disorders and Clinical Conditions

Common Disorders

Tetanus: Caused by a bacterial toxin that prevents breakdown of acetylcholine, leading to sustained muscle contraction. Requires immediate medical attention.
Myotonia: Delayed relaxation of muscles after voluntary contraction or stimulation; seen in certain genetic conditions (e.g., "fainting goats").
Myasthenia Gravis: Autoimmune disease where antibodies damage acetylcholine receptors, causing muscle weakness and fatigue.
Muscular Dystrophy: Group of genetic disorders causing progressive muscle weakness and degeneration; Duchenne MD is most common in boys.
Amyotrophic Lateral Sclerosis (ALS): Progressive degeneration of motor neurons, leading to muscle atrophy and loss of voluntary control.
Botulism: Caused by a toxin that blocks acetylcholine release, resulting in flaccid paralysis.
Strychnine Poisoning: Blocks inhibitory neurotransmitters, causing uncontrolled muscle contractions.
Rigor Mortis: Stiffening of muscles after death due to lack of ATP, which prevents relaxation; lasts about 72 hours.

Muscle Attachments and Movements

Origin and Insertion

Origin: The fixed, immovable attachment point of a muscle.
Insertion: The movable attachment point where the muscle exerts its action.
Example: The biceps brachii has two origins ("heads") and inserts on the radius to flex the forearm.

Practice Questions (Sample)

Which is NOT a characteristic of muscles? (Answer: Contains vesicles)
In myofibril, thick filaments are made of myosin and thin filaments are made of actin.
Which disorder causes degeneration of motor neurons and affects voluntary muscle control? (Answer: ALS)
Which is NOT found in the sarcoplasm of a muscle fiber? (Answer: Acetylcholine)