Overview
This lecture covers the physiological basis of membrane potentials, including resting membrane potential (RMP), graded potentials (GP), and action potentials (AP), outlining their generation, characteristics, and propagation in neurons.
Membrane Potential Basics
- Membrane potential is the electrical difference across a cell membrane due to unequal ion distribution.
- At rest, the inside of a neuron is slightly negative and the outside is slightly positive.
Resting Membrane Potential (RMP)
- RMP in neurons is about -70 mV, and -90 mV in muscles.
- Maintained by unequal ion distribution, non-diffusable anions (proteins, ATP), and the Na+-K+ ATPase pump.
- Na+-K+ ATPase pumps 3 Na+ out and 2 K+ in, making the inside more negative.
Ion Channels
- Leak channels are always open and maintain RMP; K+ leak channels are most numerous.
- Ligand-gated channels open with chemical signals; voltage-gated channels open with membrane potential changes.
- Mechanically gated channels respond to touch, pressure, or stretch.
Graded Potentials (GP)
- GPs are small deviations from RMP, causing depolarization (less negative) or hyperpolarization (more negative).
- Occur mainly in dendrites and cell bodies, diminishing as they spread (decremental conduction).
- GPs can combine through spatial and temporal summation.
Summation and Integration
- Spatial summation: input from multiple locations at the same time.
- Temporal summation: repeated input from one location over time.
- The net summation of excitatory (EPSP) and inhibitory (IPSP) potentials at the trigger zone determines AP generation.
Action Potentials (AP)
- APs are rapid, all-or-none electrical signals triggered when membrane potential reaches threshold (~-55 mV).
- Two main phases: depolarization (Na+ influx) and repolarization (K+ outflow), followed by after-hyperpolarization.
- Absolute refractory period: no second AP possible; relative refractory period: second AP possible only with strong stimulus.
Propagation of Action Potentials
- APs propagate without decreasing in strength via sequential opening of voltage-gated channels.
- Continuous conduction in unmyelinated axons; saltatory conduction (faster and energy-efficient) in myelinated axons, jumping between Nodes of Ranvier.
- Speed increases with more myelination, larger axon diameter, and higher temperature.
Types of Nerve Fibers
- A fibers: large, myelinated, fastest conduction.
- B fibers: small, myelinated.
- C fibers: small, unmyelinated, slowest conduction.
Synapses and Neurotransmitters
- Synapse: junction where neurons communicate or signal effector cells.
- Electrical synapses allow direct AP passage via gap junctions (fast, synchronous).
- Chemical synapses involve neurotransmitter release and have a synaptic delay (~0.5 ms).
- Key neurotransmitters: Acetylcholine, norepinephrine (PNS); GABA (CNS inhibitor); glutamate (CNS excitatory); dopamine (CNS).
Key Terms & Definitions
- Resting Membrane Potential (RMP) — The steady voltage across the membrane when the cell is inactive.
- Graded Potential (GP) — Small, variable changes in membrane potential, usually localized.
- Action Potential (AP) — Rapid, all-or-none electrical signal propagating along the axon.
- Depolarization — Membrane potential becomes less negative.
- Repolarization — Return of membrane potential to resting state.
- Hyperpolarization — Membrane potential becomes more negative than resting.
- Absolute/Relative Refractory Period — Periods after an AP when a neuron can't/can fire again with difficulty.
- Saltatory Conduction — Fast AP propagation in myelinated axons, "jumping" from node to node.
Action Items / Next Steps
- Review lecture notes and diagrams on membrane potentials and AP phases.
- Study the differences between graded and action potentials.
- Complete reading on neurotransmitter functions and synaptic transmission.