Nurs 1002-Obj #9

Muscle Tissue

Overview

Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and heat production. Nearly half of the body's mass is muscle, which transforms chemical energy (ATP) into directed mechanical energy, enabling forceful contractions.

Major Connective Tissues Associated with Skeletal Muscles

Connective Tissue Sheaths

• Epimysium: The outermost layer, surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Endomysium: Surrounds each individual muscle fiber.

These sheaths support and reinforce the muscle, providing pathways for nerves and blood vessels.

• Direct (fleshy) attachments: Epimysium fused to periosteum of bone or perichondrium of cartilage.

• Indirect attachments: Connective tissue wrappings extend beyond muscle as a tendon (rope-like) or aponeurosis (sheet-like).

Types of Muscle Tissue

Classification and Characteristics

There are three types of muscle tissue, each with distinct structure and function:

Type	Location	Striated/Non-striated	Voluntary/Involuntary	Control
Skeletal	Attached to bones and skin	Striated	Voluntary	Consciously controlled
Cardiac	Walls of the heart	Striated	Involuntary	Cannot be consciously controlled
Smooth	Walls of hollow organs (e.g., stomach, urinary bladder, airways)	Non-striated	Involuntary	Cannot be consciously controlled

Key Terms: Myo, mys, and sarco are prefixes related to muscle. Sarcoplasm refers to the muscle cell cytoplasm.

Main Characteristics of Muscle Tissue

• Excitability (responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after stretching.

Functions of Skeletal Muscle Tissue

• Movement: Responsible for all locomotion and manipulation (e.g., walking, digesting, pumping blood).

• Maintaining posture and body position.

• Stabilizing joints.

• Heat generation: Muscles produce heat as they contract, helping maintain body temperature.

Skeletal muscle is an organ composed of muscle fibers, connective tissue sheaths, nerves, and blood vessels. Each muscle fiber requires a rich supply of oxygen and nutrients and efficient removal of waste products.

Structural Parts of Skeletal Muscle Tissue

Muscle Fiber Structure

• Muscle fibers: Long, cylindrical cells containing multiple nuclei.

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm, rich in glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Myofibrils: Densely packed, rodlike elements that make up ~80% of muscle cell volume.

Myofibril and Sarcomere Organization

• Striations: Alternating dark (A bands) and light (I bands) along the length of each myofibril.

• Sarcomere: The smallest contractile (functional) unit of muscle fiber; aligned end to end along myofibril like boxcars on a train.

• Myofilaments: Thick filaments (myosin) and thin filaments (actin), whose arrangement creates striations.

• Regulatory proteins: Tropomyosin and troponin are bound to actin and regulate contraction.

During contraction, myosin heads bind to actin, forming cross-bridges that generate force.

Levels of Organization

• Muscle (organ) → Fascicle (bundle of fibers) → Muscle fiber (cell) → Myofibril → Sarcomere → Myofilaments (actin & myosin)

Major Events of Contraction and Relaxation

Sliding Filament Model

Muscle contraction occurs when myosin heads form cross-bridges with actin filaments, pulling them toward the center of the sarcomere. This process shortens the muscle fiber without changing the length of the filaments themselves.

Neuromuscular Junction and Excitation-Contraction Coupling

• Action potential (AP) from a motor neuron reaches the neuromuscular junction.

• Acetylcholine (ACh) is released into the synaptic cleft, binding to receptors on the sarcolemma.

• Ligand-gated ion channels open, allowing Na+ influx and depolarization of the muscle fiber membrane.

• Voltage-gated ion channels propagate the AP along the sarcolemma and into the T-tubules.

• Calcium ions are released from the sarcoplasmic reticulum, enabling cross-bridge cycling and contraction.

• Relaxation occurs when ACh is degraded by acetylcholinesterase, Ca2+ is pumped back into the SR, and the muscle fiber repolarizes.

Refractory Period

• After an AP, the muscle fiber cannot be stimulated again until repolarization is complete. Resting conditions are restored by the Na+-K+ pump.

Energy Sources for Muscle Contraction

• ATP is the immediate source of energy for muscle contraction, required for cross-bridge cycling, Ca2+ reuptake, and ion balance.

• ATP stores are depleted within 4–6 seconds of activity and must be regenerated quickly.

ATP Regeneration Mechanisms

• Direct phosphorylation of ADP by creatine phosphate (CP): Fastest method, provides energy for ~15 seconds.

• Anaerobic pathway (glycolysis): Glucose is broken down to pyruvate and lactic acid, providing energy for short bursts.

• Aerobic respiration: Occurs in mitochondria, uses oxygen to generate ATP for prolonged activity.

"All or None Principle"

An action potential in a muscle fiber is always a full response; there are no partial action potentials. This ensures reliable transmission of signals and minimizes the chance of miscommunication.

Types of Skeletal Muscle Contraction

• Isometric contraction: Muscle tension increases but the muscle does not shorten; occurs when the load is greater than the force the muscle can generate.

• Isotonic contraction: Muscle changes length to move a load. Two types:

  • Concentric: Muscle shortens as it does work (e.g., lifting a book).

  • Eccentric: Muscle lengthens while generating force (e.g., lowering a book).

Clinical Implications

• Muscular dystrophy: Genetic disorder caused by defective dystrophin, leading to muscle fiber damage and progressive muscle weakness.

• Atrophy: Loss of muscle mass due to inactivity or loss of neural stimulation; can become irreversible if muscle tissue is replaced by connective tissue.

Summary Table: Comparison of Muscle Tissue Types

Feature	Skeletal	Cardiac	Smooth
Location	Bones, skin	Heart	Walls of hollow organs
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Cell Shape	Long, cylindrical, multinucleate	Branched, usually uninucleate	Spindle-shaped, uninucleate

Example: Biceps brachii contracts concentrically to lift a book and eccentrically to lower it.

Additional info: The notes above expand on the original slides by providing definitions, context, and examples for each key concept, ensuring a comprehensive and self-contained study guide for exam preparation.