17540 netter9

NEUROPHYSIOLOGY

Synaptic Transmission: Chemical Synaptic Transmission

s    Excitatorv-\ , /- Inhibitory

in synaptic

Presynaptic

Synaptic delt

Postsynaptic

Whon impulse roaches oxcitatory synaptic bouton. it causes release of a transmitter substance into synaptic cleft. This increases permeability ul postsynaptic membranę to Na* and K* Morę Na* moves into postsy-naptic celi than K¹ moves out. due to great-er electrochemical gradient

At inhibitory synapsę, transmitter substance re leased by an impulse increases permeability of the postsynaptic membranę to Cl^-. K* moves out of post-synaptic celi but no net (Iow of Cl* occurs at resting membranę potential

Resultant net ionic rurrent flow is in a direc-tion that tends to depolarize postsynaptic celi If depolarization reaches finng threshold, an impulse is generated in postsynaptic celi

Currenl flow and potential change

Resultant ionic cunrent flow is in rłirection that tends to hyperpolarize postsynaptic celi. This makes depolariza tion by excitatory synapses morę ditficult—morę depolarization is required to reach threshold

msec

Figurę 2.8 Chemical Synaptic Transmission _

Chemical synaptic transmission between neurons may be excitatory or inhibitory. During excitation (left column), a net inerease in the inward flow of Na¹ compared with the outward flow of K' results in a dcpolarizing potential change (cxcilatory postsynaptic potential |EP$P|> that dnves the postsynaptic celi doser to its threshold for an action potential. During inhibition iright column i, the opening of K' and Cl* channels drives the membranę potential away from thresh-old (hyperpolarizationj and decreases the probability that the neuron will reach threshold (inhibitory postsynaptic. potential llPSPlł for an action potential.

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