Excitatory signals enhance the probability of a neuron firing, whereas inhibitory signals reduce this probability.

Neurons communicate via a process called synaptic transmission. During this process, chemicals known as neurotransmitters are released from the axon terminal of a neuron (the presynaptic neuron) into the synaptic cleft. These neurotransmitters then bind to receptors on the adjacent neuron (the postsynaptic neuron), initiating a response.

Excitatory signals are commonly linked to neurotransmitters such as glutamate. When these neurotransmitters attach to the postsynaptic receptors, they open ion channels that permit the influx of positive ions into the neuron. This influx results in a rise in the internal voltage of the neuron, a phenomenon known as depolarization. If this voltage reaches a specified threshold, it triggers an action potential, which is the electrical signal that propagates along the neuron and leads to the release of neurotransmitters at the subsequent synapse. Consequently, excitatory signals increase the likelihood of the neuron generating an action potential.

In contrast, inhibitory signals are typically associated with neurotransmitters like GABA (gamma-aminobutyric acid). When these neurotransmitters bind to the postsynaptic receptors, they either open ion channels that allow negative ions to enter the neuron or facilitate the exit of positive ions. This alteration in ion flow decreases the internal voltage of the neuron, a process referred to as hyperpolarization. Hyperpolarization moves the voltage further from the threshold necessary to trigger an action potential, thereby making it less probable that the neuron will fire.

The balance between excitatory and inhibitory signals is essential for the proper functioning of the nervous system. Excessive excitation can lead to conditions such as epilepsy, characterized by frequent and uncoordinated neuronal firing, resulting in seizures. Conversely, excessive inhibition can contribute to conditions like depression, where certain brain regions become less active than normal. Understanding the mechanisms by which these signals operate is therefore critical for the development of effective treatments for various neurological disorders.