Lecture Notes on Neuron Function & Action Potentials

Sodium and Potassium Distribution

• Higher sodium concentration outside of the cell (resting state).
• Higher potassium concentration inside the cell.

Chloride Ions

• Higher concentration of chloride ions outside the cell.
• Chloride ions carry a negative charge (from gaining an electron to complete their valence shell).

Cell Membrane Structure

• Membrane consists of a hydrophobic interior due to fatty acid tails and a hydrophilic exterior from phosphate heads.
• Channels necessary for movement of ions due to the membrane's hydrophobic nature.

Types of Ion Channels

• Gated Channels: Open/close at specific times.
• Ligand-gated channels: Open when a chemical (ligand) binds, e.g., acetylcholine at the neuromuscular junction.
• Mechanically-gated channels: Open in response to physical changes, like touch or pressure.
• Voltage-gated channels: Open in response to changes in electric potential across the membrane.

Resting Membrane Potential

• The inside of a cell is more negative compared to the outside due to several factors:
• Higher concentration of potassium inside, leading to leakage out (positive charge leaving).
• Sodium-potassium pump: pumps 2 potassium ions in and 3 sodium ions out.
• Presence of large negatively charged proteins that cannot leave the cell.
• Typical resting membrane potential is around -70 mV.

Changes in Membrane Potential

• Depolarization: Membrane becomes less negative (e.g., moving from -70 mV to -55 mV).
• Hyperpolarization: Membrane becomes more negative (e.g., moving from -70 mV to -90 mV).

Action Potential Mechanism

• If the membrane reaches the threshold of around -55 mV, action potential is initiated.
• Sequence of events:

1. Depolarization: Sodium channels open, sodium ions enter, making the inside of the cell more positive.
2. Repolarization: Potassium channels open, potassium exits the cell, and membrane potential returns toward -70 mV.
3. Hyperpolarization: Membrane potential temporarily becomes more negative than -70 mV before returning to resting state.

• The process is all-or-none: once threshold is reached, the action potential is fully triggered.

Propagation of Action Potential

• Self-propagating along the neuron; initiated at one point and continues down the axon without additional stimuli.
• In myelinated neurons, the action potential jumps from node to node (Nodes of Ranvier), increasing conduction speed compared to unmyelinated neurons where the potential must propagate continuously.

Synapses

• Chemical Synapse: Neurotransmitters are released from one neuron and bind to receptors on another, can be excitatory (depolarization) or inhibitory (hyperpolarization).
• Electrical Synapse: Direct electrical communication between cells; faster synaptic transmission.

Neurotransmitters

• Examples include:
• Acetylcholine: Important at neuromuscular junctions.
• Epinephrine, Serotonin, Dopamine: Various roles in the nervous system.

Summary of Key Ion Movements

• Sodium (Na+): Enters during depolarization, moves extracellular to intracellular.
• Potassium (K+): Exits during repolarization, moves intracellular to extracellular.
• Calcium (Ca2+): Enters during synaptic transmission, important for neurotransmitter release.
• Chloride (Cl-): Typically enters, can lead to hyperpolarization.

Key Takeaways

• Understand the ion distributions (Na+, K+, Cl-) and their charges.
• Be familiar with the types of channels and mechanisms of action potentials, graded potentials, and synaptic transmission.