Excitatory and Inhibitory Effects of Neurotransmitters

Excitatory neurotransmitters like acetylcholine and glutamate increase the likelihood of an action potential by causing sodium influx and potassium efflux, depolarizing the postsynaptic membrane. Inhibitory neurotransmitters like glycine and GABA decrease this likelihood by allowing potassium efflux or chloride influx, hyperpolarizing the membrane and making firing less probable.

Excitatory neurotransmitters bind to ion channel receptors on the postsynaptic cell, triggering rapid sodium influx and slow potassium efflux. These ion movements change the electrochemical gradient, causing membrane depolarization and generating an excitatory postsynaptic potential (EPSP). At the axon hillock, summation of all EPSPs triggers an action potential that travels down the axon.

Inhibitory neurotransmitters bind to postsynaptic ion channel receptors, allowing potassium efflux or chloride influx. Both processes make the cell cytoplasm more negative, creating an inhibitory postsynaptic potential (IPSP) through hyperpolarization. This hyperpolarization decreases the likelihood of the postsynaptic neuron firing an action potential.

Ligand-gated ion channels open when neurotransmitters bind to them at the postsynaptic membrane. This binding allows specific ions to flow across the membrane, either depolarizing the cell through sodium influx or hyperpolarizing it through potassium efflux or chloride influx, directly determining whether the postsynaptic neuron fires.

At the axon hillock, multiple excitatory postsynaptic potentials (EPSPs) sum together to determine whether the threshold for an action potential is reached. Inhibitory postsynaptic potentials (IPSPs) counteract this summation by hyperpolarizing the membrane. The balance between excitatory and inhibitory inputs determines whether the neuron fires, making the role of ion channels in neuronal computation essential.

A molecule is classified as a neurotransmitter if it is present in the presynaptic neuron, released in response to strong presynaptic depolarization, and has specific receptors on the postsynaptic cell membrane. These criteria ensure that the molecule can reliably transmit signals across chemical synapses and influence postsynaptic cell firing.

Alterations in neurotransmitter levels cause neurological dysfunction. Parkinson's disease results from dopamine loss in the substantia nigra, causing movement problems. Schizophrenia involves excess dopamine release, leading to hallucinations and altered thought processes. These examples demonstrate how disrupted neurotransmitter signaling directly impairs neural communication and behavior.