Comprehensive Study Notes: Skeletal Muscle Structure, Function, and Physiology

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 and Organization

Muscles are composed of bundles of fibers, connective tissues, and associated structures.

• 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.

• 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: Specialized endoplasmic reticulum in muscle cells that stores and releases calcium ions, 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 specific regions and lines.

• Z line: Defines the boundaries of a sarcomere; anchors thin filaments.

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

• H band: Region within the A band containing only thick filaments.

• I band: Light region containing only thin filaments and titin protein.

• Zone of overlap: Area where thick and thin filaments overlap, crucial for contraction.

Region	Contents
Z line	Anchors thin filaments
A band	Thick and thin filaments (zone of overlap)
H band	Thick filaments only
I band	Thin filaments and titin

Proteins of the Sarcomere

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

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

• Titin: Provides elasticity and stabilizes thick filaments.

Sliding Filament Theory and Contraction Cycle

Sliding Filament Theory

This theory explains how muscles contract by the sliding of actin (thin) and myosin (thick) filaments past each other.

• Events: Myosin heads bind to actin, pulling thin filaments toward the center of the sarcomere, shortening the muscle.

• Order of contraction cycle:

  1. Calcium ions released from sarcoplasmic reticulum

  2. Calcium binds to troponin, exposing binding sites on actin

  3. Myosin heads attach to actin, forming cross-bridges

  4. Power stroke: Myosin heads pivot, pulling actin filaments

  5. ATP binds to myosin, causing detachment from actin

  6. ATP hydrolysis re-cocks myosin head

Muscle Excitation and Relaxation

• Excitation:

  1. Calcium (Ca2+) ion gates open

  2. Ca2+ influx into axon terminal

  3. Ca2+ opens vesicles with acetylcholine (ACh), which moves 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

• Relaxation:

  1. ACh (acetylcholinesterase) binds to ACh

  2. ACh broken down into acetic acid and choline

  3. Acetic acid and choline reabsorbed into the axon terminal

Neurotransmitters in Muscle Contraction

• Acetylcholine (ACh): Main neurotransmitter at the neuromuscular junction; initiates muscle contraction.

• Acetylcholinesterase: Enzyme that breaks down ACh, ending the signal for contraction.

• Effect of ACh breakdown: Muscle contraction would stop if ACh is broken down.

• Parkinson-like symptoms: A decrease in dopamine neurotransmitter can cause symptoms similar to Parkinson's disease.

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 tension increases, but length does not change.

Tension Production

• Treppen (staircase effect): A stepwise increase in tension due to repeated stimulation after relaxation.

• Incomplete tetanus: Muscle produces near maximum tension with rapid cycles of contraction and relaxation.

• Complete tetanus: Stimulus frequency is high; muscle remains in sustained contraction.

• Recruitment: Increase in the number of active motor units produces smooth, steady increases in tension.

Levers and Muscle Actions

Classes of Levers

Muscles and bones act as levers to produce movement. There are three classes of levers:

• 1st class lever: Fulcrum is between load and applied force (e.g., neck muscles).

• 2nd class lever: Load is between fulcrum and applied force (e.g., standing on tiptoe).

• 3rd class lever: Applied force is between fulcrum and load (e.g., biceps curl).

Lever Class	Arrangement	Example
1st Class	Fulcrum between load and force	Neck muscles
2nd Class	Load between fulcrum and force	Standing on tiptoe
3rd Class	Force between fulcrum and load	Biceps curl

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 in performing its action.

• Fixator: Stabilizes the origin of the agonist.

Muscle Fiber Types

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

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

Muscle Groups by Fascicle Arrangement

• Parallel: Fibers run parallel to the long axis (e.g., biceps brachii).

• Pennate: Fibers arranged at an angle to the tendon (e.g., rectus femoris).

• Convergent: Fibers converge on a single tendon (e.g., pectoralis major).

• Circular: Fibers arranged in concentric rings (e.g., orbicularis oris).

Specific Muscle Actions

• Muscles of facial expression: Zygomaticus major causes one to smile.

• Muscles passing through the zygomatic arch: Masseter.

• Muscles that abduct the shoulder: Deltoid.

• Muscles that extend the arm: Triceps brachii.

• Muscles of the rotator cuff: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Muscles that flex the foot: Tibialis anterior.

• Muscles that help with turning the head side to side: Sternocleidomastoid.

• Muscles that make up the quadriceps femoris: Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius.

• Muscles that make up the hamstrings: Biceps femoris, semitendinosus, semimembranosus.

Neural Control of Muscle Contraction

Action Potential and Synaptic Transmission

• Action potential: Electrical signal that travels along the neuron and muscle fiber, triggering contraction.

• Main steps in generation:

  1. Resting membrane potential

  2. Depolarization (Na+ influx)

  3. Repolarization (K+ efflux)

  4. Return to resting potential

• Neurotransmitter release: Adrenergic (norepinephrine) and cholinergic (acetylcholine) synapses.

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

Neuroglial Cells of CNS and PNS

• CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells.

• PNS: Schwann cells, satellite cells.

Structural Classification of Neurons

• Multipolar: Most abundant in CNS; many dendrites, one axon.

• Bipolar: One dendrite, one axon; found in sensory organs.

• Unipolar: Single process; found in sensory neurons of PNS.

Sensory Receptors

• Interoceptors: Monitor internal environment.

• Proprioceptors: Monitor position and movement of muscles and joints.

• Exteroceptors: Monitor external environment.

Divisions of the PNS

• Afferent division: Carries sensory information to CNS.

• Efferent division: Carries motor commands from CNS to effectors.

Other Key Concepts

• Structure of a neuron: Cell body, dendrites, axon.

• Axon hillock: Connects cell body to axon; site of action potential initiation.

• Sodium-potassium exchange pump: Maintains resting membrane potential by moving 3 Na+ out and 2 K+ in.

Equations and Formulas

• Resting membrane potential: $V_{rest} = -70\ \text{mV}$

• Sodium-potassium pump: $3\ \text{Na}^+ \text{out} : 2\ \text{K}^+ \text{in}$

Additional info:

• Some muscle names and actions were inferred based on standard anatomy knowledge.

• Tables and diagrams were recreated for clarity and completeness.