Muscle and Nervous System: Structure, Function, and Physiology Study Guide

Muscle Microanatomy and Physiology

Functions of Muscle and Their Daily Relevance

• Movement: Muscles enable movement, maintain posture, and generate heat.

• Posture and Joint Stability: Muscles stabilize joints and maintain body position.

• Heat Generation: Muscle contractions produce heat, important for maintaining body temperature.

Antagonist Muscles

• Definition: Antagonist muscles produce movement in the opposite direction of another muscle (e.g., triceps extension vs. biceps flexion).

• Importance: Maintains position, controls rapid movement, and prevents injury.

• Example: Biceps brachii (flexion) and triceps brachii (extension) at the elbow.

Muscle Structure: From Organ to Myofilament

• Hierarchy: Muscle → Fascicle → Fiber (cell) → Myofibril → Myofilament

• Myofibrils: Composed of repeating units called sarcomeres.

Special Organelles in Muscle and Their Roles

• Myofibrils: Contain sarcomeres, the contractile units.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasmic Reticulum: Stores and releases calcium ions around each myofibril.

• T-tubules: Invaginations of the sarcolemma that conduct impulses deep into the muscle cell.

Connective Tissue Organization in Skeletal Muscle

• Epimysium: Surrounds entire muscle.

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

• Endomysium: Surrounds individual muscle fibers.

Smooth vs. Skeletal Muscle (Cellular Level)

• Skeletal Muscle: Long, cylindrical, multinucleated, striated.

• Smooth Muscle: Spindle-shaped, single nucleus, lacks striations.

Mechanism of Muscle Contraction

Sarcomere Structure and Function

• Components: Overlapping thick (myosin) and thin (actin) filaments; Z discs mark sarcomere boundaries.

• Sliding Filament Theory: Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

Thick and Thin Filament Specializations

• Thin Filaments:

  • Actin (2 strands), myosin binding sites, tropomyosin (blocks binding sites), troponin (binds calcium).

• Thick Filaments:

  • Composed of myosin molecules, each with a head for ATP binding and actin attachment.

Crossbridge Cycling: Chemical and Functional Steps

• 1) Myosin is at rest until ATP binds.

• 2) ATP binding causes myosin head to detach and "cock" into position.

• 3) Myosin binds actin, forming a crossbridge.

• 4) Power stroke occurs as ADP is released, pulling actin filament.

• 5) New ATP binds, causing myosin to detach and repeat the cycle.

Chemical Equation:

• $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (energy released powers contraction)

Muscle Activation

Neuromuscular Junction and Signal Transmission

• Process: Action potential arrives at axon terminal → acetylcholine (ACh) released → binds to receptors on muscle fiber → triggers muscle action potential.

• Result: Initiates muscle contraction via calcium release from sarcoplasmic reticulum.

Motor Units and Recruitment

• Motor Unit: A single motor neuron and all the muscle fibers it innervates.

• Small Motor Units: Fine control (e.g., fingers, eyes).

• Large Motor Units: Gross movement (e.g., thigh muscles).

• Recruitment: Increasing the number of active motor units increases force.

Temporal Summation vs. Recruitment

• Temporal Summation: Increased frequency of stimulation increases force (single motor unit).

• Recruitment: More motor units activated for greater force.

Muscular Bioenergetics

ATP Regeneration Mechanisms

• Direct Phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP. Fast, short duration.

• Anaerobic Pathway: Glycolysis produces ATP without oxygen, yielding lactic acid. Moderate speed, short duration.

• Aerobic Pathway: Uses oxygen, produces most ATP, supports long-duration activity.

Equation for Direct Phosphorylation:

• $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

Pros and Cons:

• Anaerobic: 2.5x faster, less efficient, produces lactic acid.

• Aerobic: Slower, more efficient, supports endurance.

Cells of the Nervous System

Basic Parts of an Axon

• Axon Hillock: Region where axon joins cell body; initiates action potentials.

• Initial Segment: First part of axon after hillock.

• Axon Terminals: Endings where signals are transmitted to other cells.

Afferent vs. Efferent Pathways

• Afferent: Sensory signals toward CNS.

• Efferent: Motor signals away from CNS.

• Somatic: Voluntary control (skeletal muscle).

• Visceral: Involuntary control (organs, glands).

Organization of the Nervous System

• CNS: Brain and spinal cord.

• PNS: Nerves and ganglia outside CNS.

Nuclei, Ganglia, Nerves, and Tracts

• Nuclei: Clusters of neuron cell bodies in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

• Nerves: Bundles of axons in PNS.

• Tracts: Bundles of axons in CNS.

Dendrites and Axon Specializations

• Dendrites: Receive inputs, transmit signals to soma.

• Axon: Transmits signal away from soma.

Myelination

• Function: Increases speed of signal conduction.

• Cells: Schwann cells (PNS), oligodendrocytes (CNS).

• Nodes of Ranvier: Gaps in myelin sheath that allow for saltatory conduction.

Neuroglia (Glial Cells)

• Support Cells: Astrocytes, microglia, ependymal cells, oligodendrocytes (CNS); Schwann cells, satellite cells (PNS).

• Functions: Support, protect, and nourish neurons.

Action Potentials

Resting Membrane Potential and Ion Channels

• Leakage Channels: Allow passive diffusion of ions, maintaining resting potential.

• Na-K Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining negative interior.

• Types of Ion Channels:

  • Leakage (always open)

  • Mechanically gated (open by physical deformation)

  • Ligand gated (open by chemical binding)

  • Voltage gated (open by changes in membrane potential)

Action Potential Generation and Propagation

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Threshold: Typically around -70 mV; must be reached to trigger action potential.

• All-or-None Principle: Action potentials either occur fully or not at all.

• Propagation: Action potential travels along axon, each segment depolarizing the next.

Key Steps:

• 1) Resting state: Na+ and K+ channels closed.

• 2) Depolarization: Na+ channels open, Na+ enters cell.

• 3) Repolarization: K+ channels open, K+ exits cell.

• 4) Hyperpolarization: K+ channels remain open briefly.

Graph Interpretation: Be able to read and interpret a graph of membrane potential over time, identifying phases of action potential.

Summary Table: Muscle Energy Pathways

Pathway	Speed	Oxygen Required?	ATP Yield	Byproducts
Direct Phosphorylation	Very Fast	No	1 ATP per CP	Creatine
Anaerobic Glycolysis	Fast	No	2 ATP per glucose	Lactic acid
Aerobic Respiration	Slow	Yes	32 ATP per glucose	CO2, H2O

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.