Intro | Presynaptic Neuron | Postsynaptic Neuron | Terminal Button | Axon | Neural Impulse | Synaptic Vesicles | Neurotransmitter Molecules | Cell Membrane | Transmitter does not fit at receptor | Transmitter fits receptor | Receptor Sites | Synaptic Cleft
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
10. Transmitter fits the receptor site, binds to the cell membrane, and produces a postsynaptic potential -
When the neurotransmitter released into the synaptic cleft fits into a receptor site on the postsynaptic neuron, a postsynaptic potential (PSP) will occur. PSP's are not all-or-none events like action potentials, but rather a graded response to the influence of all presynaptic neurons at a given point in time. PSP's affect the probability of a neuronal impulse based on their size and polarity. For example, if the PSP becomes more positive the neuron will move closer to the threshold for triggering an impulse. This type of PSP is called excitatory. If, however, the PSP becomes more negative, the neuron will move farther away from threshold. This type of PSP is called inhibitory.
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Neurotransmitter molecules are distinguished based on the effect they exert at the postsynaptic membrane. There are three mechanisms of action, ionotropic, metabotropic, and modulatory. Glutamate, GABA, and acetylcholine are all ionotropic; they induce the opening of specific ionic gates in the membrane. Glutamate opens Na+ gates, thereby allowing the movement of Na+ into the neuron and leading to depolarization. Glutamate is therefore excitatory. GABA opens Cl- gates, thereby allowing the movement of Cl- into the neuron and leading to hyperpolarization. GABA is therefore inhibitory. Acetylcholine is ionotropic at nicotinic synapses (synapses that respond to nicotine), inducing the opening of Na+ gates. These ionotropic effects are rapid (occurring typically within 10 milliseconds after neurotransmitter release) and last from 1 to 20 milliseconds. Metabotropic effects are much slower to occur (30 milliseconds after neurotransmitter release), are longer-lasting (up to several seconds in duration), and are more complicated involving a sequence of metabolic reactions. These effects typically involve a "second messenger", molecule that may open or close an ion gate, alter the production of proteins, or alter the structure of the postsynaptic cell. The biogenic amine, epinephrine, is a metabotropic transmitter. Finally, neuromodulators are peptide molecules (short proteins) whose function lie somewhere between that of a neurotransmitter and a hormone. Neuromodulators may influence a number of neurons in the region of release, including the releasing neuron. They act by prolonging or limiting the effects of a neurotransmitter. These effects are slow to occur and long lasting (from seconds to minutes, and possibly hours, in duration). The endorphins are neuromodulators. Most neurons synthesize and release both neurotransmitters and neuromodulators. This combination supports a broader range of effects.