The main function of the central nervous system (CNS) is to receive sensory signals and send out the appropriate motor responses. This integration is mainly done by interneurons. The CNS receives thousands of inputs every minute. Interneurons decide which signals are important and propagates them, and which signals are not important to respond to. These signals are inhibited, meaning that the signal does not continue on to generate a response to the stimulus. Inhibition can occur by two major mechanisms: lateral inhibition—where excited neurons can inhibit their neighbors to prevent cross-activation, and inhibitory postsynaptic potentials (IPSPs)—where the postsynaptic neuron (second neuron) is inhibited by the release of certain neurotransmitters that prevents the signal from continuing onward. It is important to note that most neurons are responding to an intricate balance of excitatory and inhibitory signals to respond in the proper way that allows individuals to function at “normal.” If not enough signals are inhibited, it can result in neurologic disease. Similarly, patients are adversely affected if too many signals are inhibited (possibly an individual would not be able to feel pain in certain body parts).
Lateral inhibition is the ability of an electrically excited neuron to inhibit the neurons around it. This is important because if lateral inhibition were not in place, it is possible that cross-excitation would occur. Cross-excitation is the excitation of neighboring neurons by the transduction of an action potential down one neuron. Lateral inhibition also is incredibly important in sensory perception such as touch, sound, and sight. This is because it allows a contrast to be made between strong, light, and nonexistent senses. The retina is a major region where lateral inhibition is used. This mechanism increases sharpness and contrast of images captured by the retina. This is because rods that pick up light dampen the rods surrounding them, causing their signal to indicate darkness. There is also some evidence of a link between color perception and lateral inhibition. For the most part, the cells that utilize lateral inhibition are located in the cerebral cortex and the thalamus.
Inhibitory Postsynaptic Potential
IPSPs are utilized to inhibit the propagation of neural signals by most neurons. IPSPs can happen at any synapse that utilizes neurotransmitters to pass the electrical signal to the next neuron. This is a type of synaptic potential, which makes it less likely that the postsynaptic neuron will be able to generate an action potential. The neurons that use IPSPs inhibit the postsynaptic neuron by releasing neurotransmitters that bind to postsynaptic receptors and change the electrical current that the postsynaptic cell experiences. The binding of these neurotransmitters to the postsynaptic receptor increases the negative postsynaptic potential of the second neuron. This means that when the generation of an action potential in the postsynaptic neuron occurs, it must depolarize more than it would normally do to overcome the inhibitory response. Depolarization from the resting threshold has to go further to get to the action potential threshold—the electrical potential has to become more positive than normal. This type of inhibition is very similar to what occurs in a hyperpolarization event after the generation of the action potential. Each postsynaptic neuron receives signals via neurotransmitters from both ISPSs and ESPSs. It is the intricate balance between the two signals that allows the nervous system to function as well as it does.
There are two main type of inhibitory receptors used in ISPSs: ionotropic receptors and metabotropic receptors. The two receptors are classified by the way that they function after binding of the neurotransmitter to the receptor. Ionotropic receptors take advantage of ion channels nearby in the membrane. When a neurotransmitter binds to an ionotropic receptor, it signals the nearby ion channels to open, allowing ions to flow into the postsynaptic neuron’s intracellular compartment. The influx of ions polarizes the cell’s potential, making it take longer for an action potential to be generated. This type of receptor is important in modulating the speed and size of action potentials generated in the postsynaptic neuron. Ionotropic receptors can act fairly quickly since they are in more direct contact with the ion channels.
Metabotropic receptors have two major regions or domains—the extracellular domain and the intracellular domain. Neurotransmitters bind to the extracellular domain of the metabotropic receptor. The intracellular domain is coupled to a G protein. When the neurotransmitter binds to the extracellular domain, it sends a signal down the receptor to activate the G protein within the cell. This activates a pathway within the cell that interacts with ion channels to close them. By closing ion channels, the cell prevents a depolarization event and thus an action potential event from occurring. Metabotropic receptors generate a long response because of the time it takes for the G protein pathway to be complete before any interactions with ion channels can occur.
Riannon C. Atwater
See also: Action Potential; Excitation; Membrane Potential: Depolarization and Hyperpolarization; Retina
Bear, Mark F., Barry W. Connors, & Michael A. Paradiso. (2007). Neuroscience exploring the brain (3rd ed.). Baltimore, MD: Lippincott Williams & Wilkins.
Grobstein, Paul. (2003). Tricks of the eye, wisdom of the brain. Retrieved from http://serendip.brynmawr.edu/bb/latinhib.html