Muscle Tissue and Skeletal Muscle Structure: Study Notes for Anatomy & Physiology

Muscle Tissue Types

Structure, Function, Location, and Control

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Each type has distinct structural features, functions, locations, and mechanisms of control.

• Skeletal Muscle: - Structure: Long, cylindrical, multinucleated fibers with striations. - Function: Voluntary movement of the skeleton. - Location: Attached to bones via tendons. - Control: Voluntary (somatic nervous system).

• Cardiac Muscle: - Structure: Branched, striated fibers with one or two nuclei; intercalated discs present. - Function: Pumps blood through the heart. - Location: Walls of the heart (myocardium). - Control: Involuntary (autonomic nervous system).

• Smooth Muscle: - Structure: Spindle-shaped, non-striated cells with a single nucleus. - Function: Moves substances through hollow organs (e.g., digestive tract, blood vessels). - Location: Walls of hollow organs (intestines, blood vessels, bladder). - Control: Involuntary (autonomic nervous system).

Muscle Fascicle and Muscle Fiber

Definitions

• Muscle Fascicle: A bundle of muscle fibers (cells) surrounded by perimysium.

• Muscle Fiber: A single muscle cell, also called a myocyte, which is multinucleated and contains myofibrils.

Tendons

Structure and Role

• Structure: Tendons are dense, regular connective tissue composed mainly of collagen fibers.

• Role: Tendons connect muscle to bone, transmitting the force generated by muscle contraction to produce movement.

Connective Tissue Coverings of Skeletal Muscle

Layers from Superficial to Deep

• Epimysium: Outermost layer; surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

Skeletal Muscle Fiber Structure

Key Components

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions.

• T-tubule (Transverse Tubule): Invaginations of the sarcolemma that conduct action potentials into the fiber.

• Terminal Cisternae: Enlarged areas of the SR adjacent to T-tubules; store and release calcium.

• Myofibrils: Rod-like units within muscle fibers, composed of repeating sarcomeres.

• Myofilaments: Protein filaments (actin and myosin) within myofibrils.

• Sarcomere: The functional contractile unit of muscle, defined by Z lines.

Myofilament Structure

Thick and Thin Filaments

• Thick Filaments: Composed mainly of myosin protein.

• Thin Filaments: Composed mainly of actin, along with troponin and tropomyosin.

• Major Proteins: Myosin (thick), Actin (thin).

Sarcomere

Structure and Function

• Definition: The sarcomere is the basic contractile unit of skeletal muscle, extending from one Z line to the next.

• Components: Contains thick (myosin) and thin (actin) filaments, with the Z line marking the boundaries.

• Function: Sarcomeres shorten during muscle contraction, producing force.

Neuromuscular Junction (NMJ)

Structure

• Axon Terminal: The end of a motor neuron that releases neurotransmitters.

• Neurotransmitter: Acetylcholine (ACh) is released into the synaptic cleft.

• Synaptic Cleft: The gap between the axon terminal and the muscle fiber's motor end plate.

• Synaptic Vesicles: Membrane-bound sacs in the axon terminal containing ACh.

• Motor End Plate: Specialized region of the muscle fiber's sarcolemma with ACh receptors.

• Acetylcholine Receptors: Proteins on the motor end plate that bind ACh, initiating muscle contraction.

• Acetylcholinesterase: Enzyme that breaks down ACh in the synaptic cleft.

Events at the Neuromuscular Junction

Summary

1. An action potential arrives at the axon terminal.

2. Synaptic vesicles release ACh into the synaptic cleft.

3. ACh binds to receptors on the motor end plate, triggering an action potential in the muscle fiber.

4. Acetylcholinesterase breaks down ACh, ending the signal.

Contraction Cycle Events

Role of Filaments, Cross Bridges, and ATP

1. Calcium ions are released from the sarcoplasmic reticulum.

2. Calcium binds to troponin, moving tropomyosin and exposing actin binding sites.

3. Myosin heads bind to actin, forming cross bridges.

4. ATP is hydrolyzed, causing myosin heads to pivot and pull actin filaments (power stroke).

5. New ATP binds to myosin, causing it to release actin and reset for another cycle.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

Origin and Insertion of Skeletal Muscles

Definitions

• Origin: The fixed attachment point of a muscle, usually proximal.

• Insertion: The movable attachment point, usually distal.

Muscle Roles: Prime Movers, Synergists, and Antagonists

Relationships and Examples

• Prime Mover (Agonist): The main muscle responsible for a specific movement.

• Synergist: Muscles that assist the prime mover.

• Antagonist: Muscles that oppose the action of the prime mover.

Examples:

• Biceps brachii (prime mover of elbow flexion), Brachialis (synergist), Triceps brachii (antagonist).

• Quadriceps femoris (prime mover of knee extension), Sartorius (synergist), Hamstrings (antagonist).

Summary Table: Muscle Tissue Comparison

Type	Structure	Location	Control	Function
Skeletal	Striated, multinucleated, long fibers	Attached to bones	Voluntary	Movement of skeleton
Cardiac	Striated, branched, intercalated discs	Heart walls	Involuntary	Pumping blood
Smooth	Non-striated, spindle-shaped	Walls of hollow organs	Involuntary	Movement of substances