Neurophysiology Quiz: Action Potential Phases

Neurons and Action Potential

• Neurons are the basic functional unit of the central nervous system.
• Action potential is composed of four principal phases: polarization, depolarization, hyperpolarization, and repolarization.
• The refractory period ensures unidirectional propagation of the action potential and limits the frequency of action potential.

Refractory Period

• The refractory period consists of two phases: absolute refractory period and relative refractory period.
• During the absolute refractory period, the axonal membrane cannot initiate another action potential, regardless of stimulation.
• During the relative refractory period, a second action potential can be produced only by a stronger-than-usual stimulus.
• The length of the refractory period varies for different types of neurons.
• A longer refractory period results in a greater delay period before a new action potential can be initiated and a lower frequency of response to repeated stimulation.

Action Potential and Synapses

• Action potential occurs in an all-or-none fashion.
• A stronger stimulus triggers a greater number of action potentials per second.
• The junction between two neurons is called the synapse.
• A neurotransmitter carries the signal across a synapse.
• Some synapses excite the post-synaptic neurons, while others inhibit.
• The synaptic delay is approximately 0.5-1 millisecond.

Neurotransmitters and Synaptic Activity

• Neurotransmitters are quickly removed from the synaptic cleft to wipe the post-synaptic slate clean.
• The grand post-synaptic potential depends on the sum of activities in all pre-synaptic neurons.
• There are four possible outcomes of the grand post-synaptic potential:
  • Excitation and action potential
  • Facilitation and increased likelihood of action potential
  • Inhibition and prevention of action potential
  • No effect due to balance of excitatory and inhibitory inputs

Neuromodulators

• Neuromodulators are chemical messengers that bind to neuronal receptors at non-synaptic sites.
• They activate second messenger systems that produce long-term intracellular biochemical effects, altering the effectiveness of ongoing synaptic activity.
• Neuromodulators may act at pre-synaptic or post-synaptic sites.
• They can fine-tune synaptic responses by influencing neurotransmitter synthesis or post-synaptic receptor sensitivity.