Neurophysiology: Membrane Excitability and Synaptic Transmission
Membrane potentials and electrical signaling
Biophysical basis of excitability
Neuronal excitability arises from ionic gradients maintained by pumps and selectively permeable channels. The membrane potential reflects differential permeability to ions such as , , , and , while voltage-gated channels enable regenerative depolarization.
Useful equation
The Goldman-Hodgkin-Katz framework and the Nernst equation are central for reasoning about ionic equilibrium and membrane voltage.
What is the primary cause of the rising phase of the action potential in most neurons?
Rapid sodium influx depolarizes the membrane and produces the upstroke of the action potential.
Correct answer: Opening of voltage-gated sodium channels
What is an excitatory postsynaptic potential?
EPSPs are usually mediated by synaptic conductances that bring the membrane closer to threshold.
Correct answer: A graded depolarization that increases the probability of generating an action potential.
Synaptic transmission
Chemical synapses convert presynaptic action potentials into postsynaptic currents through calcium-dependent vesicle fusion. Synaptic efficacy depends on release probability, receptor composition, dendritic location, short-term plasticity, and postsynaptic integration.
Electrical vs chemical synapses
Electrical synapse
- Fast, often bidirectional coupling
- Useful for synchrony
Chemical synapse
- Slower but highly modulable
- Supports amplification and plasticity
Name one mechanism that increases synaptic strength on short time scales.
Short-term plasticity can transiently enhance transmission after recent activity.
Correct answer: Increased presynaptic release probability or facilitation.
Which ion is most directly associated with neurotransmitter vesicle release?
Presynaptic calcium entry triggers vesicle fusion and neurotransmitter release.
Correct answer: Calcium