Skeletal Muscle Structure, Function, and Physiology Study Guide

Skeletal Muscle Structure and Function

Functions of Skeletal Muscle

Skeletal muscles are essential for movement and play several vital roles in the human body.

• Producing movement: Muscles contract to move bones and body parts.

• Maintaining posture and body position: Continuous muscle contractions stabilize joints and maintain posture.

• Supporting soft tissues: Muscles protect and support internal organs.

• Guarding body entrances and exits: Sphincter muscles control openings of the digestive and urinary tracts.

• Maintaining body temperature: Muscle contractions generate heat.

• Storing nutrients: Muscles store glycogen and proteins for energy.

Muscle Structure

Muscles are composed of bundles of fibers, connective tissues, and tendons.

• Tendon: A tendon is a tough, fibrous connective tissue that connects muscle to bone. It is formed by the merging of the muscle's connective tissue layers.

• Broad tendinous sheet: Known as an aponeurosis, it serves as a wide, flat tendon for muscle attachment.

• Layers of skeletal muscle: (Refer to figure 10-1)

  • Epimysium: Surrounds the entire muscle.

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

  • Endomysium: Surrounds individual muscle fibers.

• Organization from outside to inside: Epimysium → Perimysium → Endomysium → Muscle fiber

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a specialized endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction.

Sarcomere Structure and Regions

Key Components of the Sarcomere

The sarcomere is the basic contractile unit of muscle fiber, defined by the area between two Z lines.

• Z line: Bisects the I band; marks boundaries between adjacent sarcomeres.

• A band: Dark band; contains both thick (myosin) and thin (actin) filaments. Includes the zone of overlap.

• H band: On either side of the M line; contains thick filaments but no thin filaments.

• I band: Light band; contains thin filaments but no thick filaments.

• Zone of overlap: Region where thick and thin filaments overlap.

• M line: Center of the sarcomere; anchors thick filaments.

Region	Filament Type	Function/Location
Z line	Actin (thin)	Boundary between sarcomeres
A band	Myosin (thick) & Actin (thin)	Contains zone of overlap
H band	Myosin (thick)	No thin filaments
I band	Actin (thin)	No thick filaments
M line	Myosin (thick)	Center of sarcomere

Filament Proteins

• Thin filament: Composed mainly of actin; associated with the protein actin.

• Thick filament: Composed mainly of myosin; associated with the protein myosin.

• Titin: Anchors thick filaments to the Z line, providing elasticity and stability.

Sliding Filament Theory and Contraction Cycle

Sliding Filament Theory

This theory explains how muscles contract by the sliding of actin and myosin filaments past each other, shortening the sarcomere.

• All events of the sliding filament theory should be known, including cross-bridge formation, power stroke, and detachment.

Contraction Cycle Order

The contraction cycle involves several steps:

1. Calcium (Ca2+) ion gates open

2. Ca2+ influx into axon terminal

3. Once open, synaptic vesicles with acetylcholine (ACh) move toward axon terminal end plate

4. ACh released into synaptic cleft

5. ACh binds to sodium-gated channels (skeletal muscle motor end plate)

6. Sodium (Na+) gates open, Na+ influx into skeletal muscles

7. Muscle skeletal relaxation

8. ACh (acetylcholinesterase) binds to ACh

9. Breaks ACh down into acetic acid and choline

10. Acetic acid and choline reabsorbed back into the axon terminal

Neurotransmitters in Muscle Contraction

• Acetylcholine (ACh): The main neurotransmitter at the neuromuscular junction.

• Acetylcholinesterase: Enzyme that breaks down ACh, ending muscle contraction.

• If ACh is broken down, muscle contraction would stop.

Muscle Contraction Types and Tension Production

Types of Contractions

• Isotonic contraction: Muscle changes length (concentric: shortens; eccentric: lengthens) while tension remains constant.

• Isometric contraction: Muscle length remains the same while tension increases.

Tension Production

• Treppen: A stair-step increase in tension caused by repeated stimulation immediately after relaxation phase.

• Incomplete tetanus: Muscle produced near maximum tension, caused by rapid cycles of contraction and relaxation.

• Complete tetanus: Stimulus frequency is high and stays that way, producing smooth, steady tension.

• Recruitment: Increase in the number of active motor units.

Levers and Muscle Actions

Classes of Levers

Levers in the body help muscles move bones efficiently.

• 1st class lever: Load–fulcrum–applied force or applied force–fulcrum–load. The fulcrum is always in the middle.

• 2nd class lever: Load–applied force or applied force–load–fulcrum. The load is always in the middle.

• 3rd class lever: Applied force–load or load–applied force–fulcrum. The applied force is always in the middle.

Muscle Groups and Actions

Muscle Classification and Actions

• Agonist/Prime mover: Main muscle responsible for movement.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist.

• Fixator: Stabilizes the origin of the agonist.

Muscle Fiber Types

• Fast muscle fibers: Contract quickly, fatigue rapidly; used for short bursts of activity.

• Slow muscle fibers: Contract slowly, resist fatigue; used for endurance activities.

Fascicle Arrangement

• Muscles are grouped based on fascicle arrangement, affecting their range of motion and power.

Specific Muscles and Movements

Major Muscles and Their Actions

• Quadriceps femoris: Extends the knee.

• Hamstrings: Flex the knee.

• Sternocleidomastoid: Turns the head side to side.

• Pectoralis major: Flexes, adducts, and rotates the arm.

• Flexor carpi ulnaris: Flexes and adducts the wrist.

• Zygomaticus major: Causes one to smile.

• Rotator cuff muscles: Stabilize the shoulder joint.

• Gluteus maximus: Extends the hip.

• Muscle that abducts the shoulder: Deltoid.

• Muscle known as the kissing muscle: Zygomaticus minor.

Neural Control of Muscle

Action Potential Generation

Action potentials are electrical signals that trigger muscle contraction.

• Main steps include depolarization, repolarization, and return to resting potential.

Neuroglial Cells

• All neuroglial cells of the CNS and PNS have specific functions, such as support, insulation, and protection of neurons.

Peripheral Nervous System (PNS)

• Know the divisions, receptors, and effectors of the PNS.

• Afferent division: Sensory input to CNS.

• Efferent division: Motor output from CNS.

Neuron Structure and Classification

• Neurons are classified structurally and functionally; most abundant type in CNS is multipolar.

• Parts of a neuron: cell body, axon, dendrites.

• Axon hillock connects the cell body to the axon.

Synaptic Transmission

• Neurotransmitters are released at synapses to transmit signals between neurons.

• Adrenergic synapses use norepinephrine; cholinergic synapses use acetylcholine.

Ion Movement and Resting Membrane Potential

• Sodium and potassium move through exchange pumps to maintain resting membrane potential.

• Chemical, voltage, and leak channels regulate ion flow.

Additional Information

• A decrease in dopamine neurotransmitter can cause Parkinson-like symptoms.

Key Equations

• Resting Membrane Potential: $V_m = E_K \frac{[K^+]_{out}}{[K^+]_{in}}$ Additional info: This is a simplified version; the full Goldman-Hodgkin-Katz equation is used for precise calculation.