Nervous System - Part 2 Notes

K+ Equilibrium

• Equilibrium potential: $-90mV$
• Concentration gradient pulls $K^+$ outside.
• Electrical gradient pulls $K^+$ inside.

Na+ Equilibrium

• Equilibrium potential: $+66mV$
• Concentration gradient pulls $Na^+$ inside.
• Electrical gradient pulls $Na^+$ outside.

Resting Membrane Potential and Equilibrium Potentials

• Increased ion permeability: resting membrane potential moves towards that ion's equilibrium potential.
• Decreased ion permeability: resting membrane potential moves away from that ion's equilibrium potential.

Changes in Membrane Potential

• Depolarization: Membrane becomes less polarized.
  • Impulse changes $Na^+$ permeability, leading to action potential.
• Hyperpolarization: Membrane becomes more polarized.
  • Inhibitory impulse changes $K^+$ permeability, leading to hyperpolarization.

Membrane Channels

• Membrane potential changes with membrane permeability due to specific channel opening/closing.
  • Chemically-gated ion channels.
  • Voltage-gated ion channels.
  • Mechanically-gated ion channels.

Chemically-Gated Ion Channels

• Open due to specific chemical binding (e.g., acetylcholine).
• Typically on cell body and dendrites of a neuron.

Voltage-Gated Ion Channels

• Respond to changes in membrane potential.
• Typically in the axon of a neuron.
• Three states:
  • Closed (deactivated/resting).
  • Open (activated).
  • Closed (inactivated).

Graded Potentials

• Restricted changes in membrane potential.
• Produced by stimulus opening gated channels, allowing $Na^+$ to enter, causing depolarization.

Repolarization & Hyperpolarization

• Repolarization: Stimulus removed, membrane potential returns to normal.
• Hyperpolarization: Excess $K^+$ leaves the neuron.

Characteristics of Graded Potentials

• Stronger stimuli, greater changes.
• Can be depolarization or hyperpolarization.

Action Potentials

• Propagated changes in membrane potential.
• Begin at axon hillock and affect entire excitable membrane.
• Stimulated by graded potential to threshold ($-55 mV$).

Voltage-Gated Ion Channels

• $Na^+$ rushes inside the cell due to its gradient, depolarizing the membrane to $+30mV$.
• $K^+$ rushes out of the cell due to its gradient, repolarizing (then hyperpolarizing) the membrane to $-70 mV$.

Action Potential Generation

• Threshold ($-55 mV$): Voltage-gated $Na^+$ channels open.
• Membrane potential reaches $+30 mV$: $Na^+$ channels close, $K^+$ channels open.
• Repolarization overshoots to $-90 mV$ (hyperpolarization).

Steps of an Action Potential

1. Threshold: Graded potential reaches $+55 mV$ at axon hillock.
2. Depolarization: $Na^+$ rushes into cytosol.
3. Channel switch at $+30 mV$: $Na^+$ channels inactive, $K^+$ channels open.
4. Repolarization: $K^+$ moves out of cytosol.
5. Hyperpolarization: Membrane is hyperpolarized to $-90 mV$.

All-or-None Principle

• Any stimulus reaching threshold WILL cause an action potential.
• All action potentials are the same.

Refractory Period

• From the beginning of action potential to resting state.
• The membrane will not respond normally to additional stimuli.

Refractory Periods

• Absolute refractory period:
  • Neuron cannot generate another action potential, no matter how strong a stimulus.
  • Corresponds to the depolarization and early repolarization phases of an action potential
• Relative refractory period:
  • Neuron can generate an action potential only if a stimulus is relatively strong
  • Corresponds to the late repolarization phase and hyperpolarization of an action potential

Propagation

• Transmission of an action potential along an axon.
  • Saltatory (myelinated).
  • Continuous (unmyelinated).

Continuous Propagation

• Occurs in unmyelinated axons.
• Relatively slow.

Saltatory Propagation

• Occurs in myelinated axons
• Action potentials "leap" from node to node due to local current.
• Faster than continuous propagation.

Affects of Axon Size on Transmission Speed

• Larger axons propagate faster due to:
  • Reduced internal resistance to ion flow.
  • More surface area for increased ion channels

Synapse

• Point of communication between excitable cells.

Types of Synapses

• Chemical synapses: Signal transmitted by neurotransmitters.
• Electrical synapses: Direct connection allowing ion exchange.

Chemical Synapses

• Axon terminal releases neurotransmitters that bind to postsynaptic plasma membrane
• Produce graded potentials

Cholinergic Synapses

• Release acetylcholine (ACh) at neuromuscular junctions.
• Acetylcholinesterase breaks down ACh.

Events at a Cholinergic Synapse

1. Action potential arrives and depolarizes the membrane.
2. $Ca^{2+}$ enters, triggering exocytosis of ACh.
3. ACh binds to receptors, causing depolarization.
4. ACh is removed by acetylcholinesterase.