excitatory
Summary
Excitatory describes input that makes a postsynaptic neuron more likely to fire, usually by depolarizing it toward threshold (an EPSP). Whether it fires depends on summation of excitatory and inhibitory inputs at the axon hillock, and the receptor, not the transmitter alone, sets the effect.
Detail
Meaning: An excitatory input makes a postsynaptic cell more likely to fire an action potential. The neurotransmitter binds a receptor, often a ligand-gated channel, that lets cations such as Na⁺ enter, so the membrane depolarizes toward threshold. This small graded change is an excitatory postsynaptic potential (EPSP). Glutamate is the major excitatory neurotransmitter in the central nervous system, and acetylcholine is excitatory at the skeletal neuromuscular junction.
On the MCAT: Passages show a neuron receiving several inputs and ask whether it fires. EPSPs and inhibitory postsynaptic potentials (IPSPs) are graded and add by temporal and spatial summation at the axon hillock. If the summed membrane potential reaches threshold, an action potential fires. Expect questions on how a channel blocker or an altered ion gradient shifts the membrane potential. You may also have to read a voltage trace, where a depolarizing deflection is an EPSP and a hyperpolarizing one is an IPSP.
Common confusion: A neurotransmitter is not excitatory or inhibitory by name alone. The postsynaptic receptor decides: acetylcholine excites skeletal muscle through nicotinic receptors but can slow the heart through muscarinic receptors. Also, an EPSP is not an action potential. One EPSP by itself often falls short of threshold.
Sources
- Campbell Biology
- Neuroscience (Purves et al.)
- Principles of Neural Science
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.