Neuronal Physiology: Resting Potential, Synapses, and Action Potentials

What is the resting membrane potential (RMP)?

The difference in electrical charge across a neuron's plasma membrane when the cell is at rest, typically around −70 mV.

What primarily causes the negative resting membrane potential?

The efflux of potassium (K+) through leak channels, leaving behind negatively charged molecules.

What is the function of the sodium-potassium (Na+/K+) ATPase pump?

It maintains concentration gradients by pumping 3 Na+ out of the cell for every 2 K+ it brings in, requiring ATP.

What is the concentration of K+ inside the cell compared to outside?

High concentration of K+ inside the cell; the net electrochemical gradient drives K+ out.

What is the concentration of Na+ inside the cell compared to outside?

High concentration of Na+ outside the cell; the net electrochemical gradient drives Na+ in.

What occurs during depolarization?

The membrane potential becomes less negative due to an influx of positive ions (e.g., Na+ or Ca2+).

What occurs during repolarization?

The membrane potential returns to the resting value, typically caused by an efflux of K+.

What is hyperpolarization?

The membrane potential becomes more negative than the resting membrane potential, often due to K+ efflux or Cl- influx.

What is a synapse?

A specialized junction where two electrically active cells connect and a nervous signal is passed.

What are the main structures of a typical chemical synapse?

Axon terminal (presynaptic cell), synaptic cleft, and neurotransmitter receptors (postsynaptic cell).

What triggers the release of neurotransmitters at the synapse?

The influx of Ca2+ ions into the presynaptic axon terminal after an action potential arrives.

What is the role of neurotransmitter receptors?

They bind neurotransmitters and open ligand-gated ion channels, causing changes in the postsynaptic membrane potential.

What is an excitatory postsynaptic potential (EPSP)?

A local depolarization that makes the neuron more likely to fire an action potential.

What is an inhibitory postsynaptic potential (IPSP)?

A local hyperpolarization that makes the neuron less likely to fire an action potential.

What are the characteristics of local (graded) potentials?

They are graded, decremental, reversible, dependent on ligand-gated channels, and can be excitatory (EPSP) or inhibitory (IPSP).

What is temporal summation?

Multiple postsynaptic potentials from one presynaptic neuron occurring close together in time, adding their effects.

What is spatial summation?

Multiple postsynaptic potentials arriving at different locations from multiple presynaptic neurons, adding their effects simultaneously.

How is neurotransmitter signaling terminated?

Through enzymatic breakdown, reuptake by the presynaptic neuron, or diffusion away from the synaptic cleft.

What is an action potential (AP)?

A sudden, fast, self-propagating electrical signal resulting in a pronounced change in membrane potential, typically about 100 mV.

What is the threshold voltage for triggering an action potential?

Approximately −55 mV.

What ions are involved in the phases of an action potential?

Na+ for depolarization and K+ for repolarization.

What happens during the depolarization phase of an action potential?

Vm rises to +35 mV due to Na+ rushing into the axon.

What happens during the repolarization phase of an action potential?

Vm falls back toward resting membrane potential as K+ rushes out of the axon.

What occurs during the hyperpolarization phase of an action potential?

Vm briefly dips below resting membrane potential due to continued K+ efflux.

What is the function of the Sodium-Potassium ATPase Pump?

It maintains concentration gradients by pumping 3 Na+ out for every 2 K+ in, requiring ATP.

What are the relative ion concentrations inside and outside the cell?

K+: High concentration inside the cell; Na+: High concentration outside the cell.

What is depolarization in terms of membrane potential?

It is when the membrane potential becomes less negative, moving toward 0 mV or positive due to an influx of positive ions (e.g., Na+ or Ca2+).

What is repolarization?

It is the return of the membrane potential to its resting value, typically caused by an efflux of K+.

What are the structures of a typical chemical synapse?

1. Axon Terminal (Synaptic Knob): Contains synaptic vesicles with neurotransmitters.

2. Synaptic Cleft: The gap between two cells.

3. Neurotransmitter Receptors: Proteins on the postsynaptic membrane that bind neurotransmitters.

What are the chronological events of synaptic transmission?

1. Action potential arrives at the presynaptic axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ rushes in.

3. Ca2+ triggers fusion of synaptic vesicles, releasing neurotransmitter.

4. Neurotransmitter diffuses across the cleft and binds to receptors.

5. Receptor binding opens ligand-gated ion channels, changing postsynaptic membrane potential.

What are the groups of neurotransmitters?

1. Acetylcholine (ACh)

2. Amino Acids: GABA (Inhibitory), Glutamate (Excitatory)

3. Monoamines: Epinephrine, Norepinephrine, Dopamine, Serotonin

4. Neuropeptides: Substance P (Pain), Endorphins

What is the difference between EPSP and IPSP?

1. EPSP (Excitatory Postsynaptic Potential): Local depolarization making the neuron more likely to fire an AP.

2. IPSP (Inhibitory Postsynaptic Potential): Local hyperpolarization making the neuron less likely to fire an AP.

What is summation of postsynaptic potentials?

PSPs are summed at the axon hillock. If the net sum of EPSPs and IPSPs reaches threshold voltage (e.g., -55 mV), an action potential is triggered.

What are the methods for synaptic transmission cessation?

1. Enzymatic Breakdown: Enzymes destroy the neurotransmitter (e.g., ACh by AChE).

2. Reuptake: Presynaptic neuron takes neurotransmitter back.

3. Diffusion: Neurotransmitter diffuses away from the synaptic cleft.

What are the phases of an action potential?

1. Threshold: Critical membrane voltage (≈-55 mV) to trigger AP.

2. Depolarization: Vm rises to +35 mV due to Na+ influx.

3. Repolarization: Vm falls back toward RMP due to K+ efflux.

4. Hyperpolarization: Vm briefly dips below RMP due to continued K+ efflux.

What is the difference between continuous conduction and saltatory conduction?

1. Continuous Conduction: Occurs in unmyelinated axons; AP is regenerated at every point on the membrane, slower.

2. Saltatory Conduction: Occurs in myelinated axons; AP 'jumps' from node to node, faster.

How does axon diameter affect conduction velocity?

Larger diameter axons have lower internal resistance, allowing current to spread faster, thus increasing conduction velocity.

What is the refractory period?

It ensures that the AP propagates only in one direction toward the axon terminal.

What is the absolute refractory period?

Voltage-gated Na+ channels are inactivated; a new AP cannot be initiated regardless of stimulus strength.

What is the relative refractory period?

Voltage-gated K+ channels are still open; a new AP can be initiated only by a suprathreshold stimulus strong enough to overcome K+ efflux.