Topic 6 Electrotonic potentials (comparison with the action potential, examples)

Electrotonic conduction refers to the passive conduction of current. In order for a neuron to fire, there are two types of electrical potentials produced:

1. non-propagated local (passive current) potential called an electrotonic potential

2. Propagated impulse called an action potential.

Electrotonic potentials represent changes to the neuron's membrane potential that do not lead to the opening of gated ion channels. Electrotonic potentials can sum spatially or temporally. Because the ionic charge enters in one location and dissipates to others, losing intensity as it spreads, electrotonic spread is a graded response. Electrotonic spread is generally responsible for increasing the voltage of the soma (neuronal cell body) sufficiently to exceed threshold and trigger the action potential; Electrotonic potentials are conducted faster than action potentials, but attenuate rapidly so are unsuitable for long-distance signaling.

Comparison with action potential:

Action potential Electrotonic potential Stimulus intensity Above threshold Any intensity Direction Depolarization Depolarization or hyperpolarization Amplitude All or none Proportional to stimulus Amplitude during propagation

Without decrement With decrement

Refractory period Absolute and relative refractory periods None Summation None Temporal and spatial Function Propagation of excitation over long

distances Propagation of excitation over short distances receptor potential postsynaptic potential

Mechanism Voltage gated channels Ligand gated channels Mechanical stimulus gated channels Second messenger gated channels

Examples: EPP -EPSP, IPSP

EPSP/IPSP example: Chemical synapse secretes a neurotransmitter which alters the membrane potential of the post-synaptic neuron. A chemical synapse can be excitatory (generating an excitatory post-synaptic potential, EPSP) or inhibitory (inhibitory post-synaptic potential, IPSP). Integration of EPSP's and IPSP's determines the probability that the post-synaptic cell will fire.