Neuroscience Membrane Potential

Summation and Action Potential Generation

• Summation: The process of adding multiple subthreshold potentials to reach the threshold and generate an action potential.
• Temporal Summation: Involves two or more presynaptic inputs arriving at the same postsynaptic cell in quick succession from a single source.
• Spatial Summation: Occurs when two or more presynaptic inputs arrive at a postsynaptic cell simultaneously from multiple sources.

Action Potential Sequence

• Polarized State: The resting membrane potential ranges from -70 to -90 mV; Na+ activation and inactivation gates are closed while K+ activation gates are also initially closed.
• Depolarization Stage: Threshold is reached, leading to:
  • Rapid opening of Na+ activation gates.
  • Full closure of Na+ inactivation gates at peak.
  • Slow closure of K+ activation gates and slow opening of K+ channels.
  • Na+ influx results in a more positive membrane potential.
• Repolarization Phase: After reaching the peak of the action potential:
  • Na+ inactivation gates are closed while K+ activation gates are open.
  • Na+ flow into the cell ceases, and K+ exits, making the membrane potential more negative.
• Hyperpolarization:
  • K+ activation gates are slow to close, causing the membrane potential to drop below the resting level.
  • Membrane returns to resting potential once gates close.

Refractory Periods

• Absolute Refractory Period: No action potential can be generated because Na+ inactivation gates are closed.
• Relative Refractory Period: An action potential can be generated with a greater than usual stimulus, corresponding with the hyperpolarization phase.

Accommodation

• A hypo-polarized state where the membrane cannot return to resting potential.
• Na+ inactivation gates remain closed, preventing further action potential generation.

Action Potential Propagation

• Initiated at the axon hillock when depolarization occurs.
  • Local currents spread to adjacent inactive regions, opening voltage-gated Na+ channels.
• Conduction Velocity: Determines the speed of action potentials along nerve or muscle fibers.
  • Saltatory Conduction: Increased conduction velocity due to myelination; action potentials jump between nodes of Ranvier.
  • Diameter: Larger diameter increases action potential speed.

Skeletal Muscle

• Structure: Voluntary, striated, long cylindrical, multinucleated cells.
• Myofibrils: Composed of myofilaments—actin (thin) and myosin (thick) arranged in sarcomeres.
  • Thin filaments include actin, troponin, tropomyosin, anchored by nebulin.
  • Thick filaments have myosin heads, ATPase enzymes, and are held by titin.

Sarcomere Structure

• Functional Unit: Bounded by adjacent Z discs; main components include:
  • A Band: Dark; includes thick myosin and overlapping thin actin.
  • M Line: Center of A band holding myosin in place.
  • H Zone: Lighter area containing only thick filaments.
  • I Band: Light band with only thin actin filaments.

Skeletal Muscle Contraction

• Action potential travels down the sarcolemma and into T-tubules to reach myofibrils.
• Triggers calcium release from the sarcoplasmic reticulum, exposing binding sites on actin by moving tropomyosin.
• Cross Bridge Cycle: Myosin binds with actin, sliding thin filaments past thick filaments toward the M line and shortening the sarcomere.

Muscle Fiber Types

• Fast Twitch Fibers (Type II):
  • Large for powerful contractions, extensive sarcoplasmic reticulum, and rich in glycolytic enzymes for rapid energy release.
  • Fewer mitochondria and less blood supply than slow fibers, leading to easier fatigue.

Types of Muscle Contraction

• Isometric Contraction: Muscle length remains unchanged while tension changes.
• Isotonic Contraction: Muscle length changes while tension remains constant.
  • Concentric: Muscle shortens.
  • Eccentric: Muscle lengthens.

Neuromuscular Transmission

• Synapse Types:
  • Electrical Synapse: Allows fast, bidirectional signal transfer (e.g., cardiac and some smooth muscles).
  • Chemical Synapse: Involves neurotransmitter release across the synaptic cleft; operates unidirectionally with a transmission delay.
• Motor Unit: Composed of a single motor neuron and the muscle fibers it innervates; crucial for muscle control and movement.