In the nervous system, neurons communicate with one another by nerve impulses or action potentials. The result is an excitatory or inhibitory response in the receiving neuron. This process is generally called excitation (excitatory circuits) or inhibition (inhibitory circuits). The normal brain is constantly balancing between excitation and inhibition as too much of one state can cause abnormal neurological activity. For instance, too much excitation can cause seizure-like activity while too much inhibition can make a person feel “hung-over” or lethargic.
For most adult neurons, excitation “turns on” a cell and inhibition “turns off” a cell. Both are very important for normal brain function. Excitation is like gasoline for a car, which allows it to turn on and move. On the other hand, inhibition is like the brakes for a vehicle. For example, a person driving a car who needs to turn must apply the brakes to navigate safely through the turn. However, if the brakes fail prior to the turn, the driver is unable to slow down to the proper speed, which results in a car accident. The case within a brain is similar. If inhibition is compromised, it can result in neurons or circuits not being turned off, which may result in seizures or seizure-like activity.
Anatomy and Physiology
Neurons consist of a cell body, several dendrites, and a single axon. The axon terminates at its target and may have collateral branches to reach additional targets. The target is usually another neuron, muscle cell, or organ. In general, action potentials are discrete electrical signals that are transmitted along the length of axons and propagate information within the nervous system. Specifically, these signals are generated as a result of a transient change in membrane permeability: from a state where it is more permeable to potassium (K+) than sodium (Na+), to a reversal of these permeability properties. During the action potential, a flow of Na+ into the neuron is responsible for rapid depolarization and a flow of K+ out of the neuron causes repolarization or hyperpolarization. These changes in membrane permeability are due to the opening and closing of voltage-gated ion channels. The speed at which the action potentials are conducted is based on the radius of the axon, the presence of a myelin sheath, and the number of ion channels.
As the action potential propagates to the terminal end of the axon, it reaches the synaptic cleft, a small space between the two neurons. The presynaptic neuron releases a neurotransmitter into this space and the postsynaptic neuron receives the chemical. This action results in a voltage change across the postsynaptic neuron’s membrane. Specifically, postsynaptic potentials (PSPs) represent graded voltage changes in the electrical membrane potential of the postsynaptic neuron in a chemical synapse. If the neurotransmitter is an excitatory chemical, like glutamate, NMDA (N-methyl-D-aspartate), or AMPA (α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), it results in an excitatory postsynaptic potential (EPSP). This is the opposite response of an inhibitory postsynaptic potential (IPSP), which is induced by inhibitory chemicals such as GABA (gamma-amino butyric acid) and glycine. When multiple EPSPs arrive at a postsynaptic membrane relatively closely together, they will summate their amplitudes and produce a larger EPSP. If the newly combined EPSP is large enough, it may increase the membrane’s potential to produce an action potential.
In neurophysiology experiments where the circuitry of the brain is being tested, neuroscientists often refer to the PSP of the “field” or an fPSP. In general, this is an extracellular recording of all neurons within a local circuit that are firing nearly at the same time. Since a single neuron’s action potential is too small to be recorded by an extracellular electrode, the population of the neurons within a small distance of the electrode’s tip can be measured. Thus, an fPSP is the aggregate of all regional neurons firing action potentials near the extracellular electrode. This is recorded as excitation in the brain.
Jennifer L. Hellier
See also: Axon; Inhibition; Membrane Potential: Depolarization and Hyper-polarization
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