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.

Explore the concepts of membrane potential, including the differences between resting and active states, and the role of refractory periods. Understand how the absolute refractory period prevents action potentials and the mechanisms behind sodium inactivation.