Learning Objectives
1. Describe how neurotransmitter is released at chemical synapses, including the role of calcium.
The neurotransmitter at chemical synapses is packaged in intracellular vesicles. A presynaptic action potential causes presynaptic calcium to rise. Calcium is the trigger to release the contents of synaptic vesicles by the process of exocytosis.
2. Compare excitatory and inhibitory neurotransmitters and identify the major examples of each neurotransmitter type in the CNS.
The ‘workhorse’ neurotransmitters in the CNS are glutamate, which is excitatory and GABA and glycine, which are inhibitory. Outside the CNS, acetylcholine is excitatory.
3. Compare ionotropic and metabotropic receptors.
Ionotropic receptors are ion channels that open rapidly to change membrane potential. Metabotropic receptors are GPCRs that initiate second messenger signaling and hence produce slower onset and longer lasting changes in membrane potential.
4. Describe the mechanism by which neurotransmitter is cleared at chemical synapses. Membrane transporters clear glutamate, GABA, and glycine from the extracellular space into glia or back into neurons. Acetylcholine is destroyed by the extracellular enzyme acetylcholinesterase.
5. Describe how temporal and spatial summation of synaptic potentials affect postsynaptic responses.
Postsynaptic electrical responses are augmented by summation when one presynaptic nerve terminal fires rapidly in succession or when several nearby nerve terminals fire nearly synchronously.
6. Outline the key differences between chemical and electrical synapses.
Overall Summary
Chemical synapses use neurotransmittters to activate postsynaptic receptors and deliver excitatory or inhibitory messages. The receptors amplify the message. In contrast, electrical synapses transmit by electric currents flowing through gap junctions between two cells without amplification and without the possibility of sign inversion.
Synapses are the communication junctions formed by neurons to each other and to the cells of their target tissues. The word synapse comes from Greek, meaning joining together, and the process of communication is called synaptic transmission. This is a fundamentally important property; a nervous system without synaptic transmission would be like a society without language or any other form of communication. In addition to transmitting signals from one cell to another, synapses are the likely sites for many key brain functions such as decision making, learning, and memory.
Chemical synapses are the predominant form of synaptic transmission in the brain. Transmission at these synapses is mediated by a diffusible chemical transmitter released by the presynaptic cell in response to a change in voltage. Electrical synapses, in contrast, work through the direct flow of ions and other small molecules from one cell to another. They do not involve a chemical transmitter. Electrical synaptic transmission is relatively uncommon in the brain, but such electrical communication is important in sensory systems (e.g. retina), the liver, and cardiac muscle among other places. Electrical transmission is particularly effective at synchronizing the electrical signals in collections of cells – e.g. causing muscle cells to contract at the same time.
(Unless otherwise noted, all figures are from: Kandel ER, Schwartz JH, Jessell TM 2012, Siegelbaum SA, Hudspeth AJ. ‘Principles of Neural Science, 5th ed. McGraw-Hill, New York.)