EPSPs, IPSPs, Excitatory and Inhibitory neurotransmitters

In an EPSP, neurotransmitter binding opens chemically gated channels allowing

Na+ to enter the cell in large amounts

Na+ influx causes

local net graded potential depolarization = excitatory postsynaptic potential (EPSP)

If EPSP is of threshold strength

Actional potential is triggered

can spread to axon hillock and trigger opening of voltage-gated channels, generating an action potential

In an IPSP, neurotransmitter binding to receptor opens chemically gated channels that allow

entrance/exit of ions to cause hyperpolarization

makes the postsynaptic membrane more permeable to K+ or Cl-

if K+ channels open

K+ moves out of cell

if Cl- channels open

Cl- moves into cell

IPSP

reduces postsynaptic neuron’s ability to produce an AP

moves neuron farther away from the threshold (makes it more negative)

Summation

one EPSP/IPSP cannot induce an AP, but if several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation

Most neurons receive both

excitatory and inhibitory inputs from thousands of other neurons

an AP will be generated ONLY if

EPSPs predominate and bring to threshold

Excitatory neurotransmitter

depolarizing the postsynaptic membrane → bring neuron closer to threshold

inhibitory neurotransmitter

hyperpolarizes the postsynaptic membrane → move the neuron farther from threshold (more negative inside)

whether a neurotransmitter is inhibitory or excitatory is determined by

the receptor to which the neurotransmitter binds

examples of excitatory neurotransmitters

glutamate

Acetylcholine - at neuromuscular junctions in skeletal muscle

examples of inhibitory neurotransmitters

GABA

Glycine

ACh - in cardiac muscle

How excitatory neurotransmitters work

• They open chemically (ligand)-gated Na⁺ channels

• Na⁺ rushes in → inside becomes less negative

• This creates an EPSP (excitatory postsynaptic potential)

If the EPSP is strong enough to reach –55 mV, it triggers an action potential.

How do inhibitory neurotransmitters work

They open K⁺ or Cl⁻ channels:

• K⁺ leaves the cell → inside becomes more negative

• Cl⁻ enters the cell → inside becomes more negative

This creates an IPSP (inhibitory postsynaptic potential), making it harder for the neuron to fire.