THE SYNAPSE Where nerve impulses convert to neurotransmitters

THE SYNAPSE Where nerve impulses convert to neurotransmitters The Sanger Institute

The synapse is where the nerve impulse passes from one cell to the next ¢ The electrical signal (the action potential) stops and a chemical signal takes over to cross the gap between the cells ¢ The chemical messenger is called a neurotransmitter ¢ The neurotransmitter crosses the gap by diffusion, which creates a small delay ¢ © 2008 Paul Billiet ODWS

Designer signals The advantage of using neurotransmitter is that the nerve impulse can be given some more specificity ¢ Neurotransmitters can also control the operation of the nervous system by inhibition or excitation ¢ Many drugs that try to cure problems in the nervous system operate at synapses ¢ © 2008 Paul Billiet ODWS

Neurotransmitters and hormones In many ways neurotransmitters are hormones working over a very short distance (about 20 nm) ¢ Some of them work both at synapses and in the circulatory system ¢ © 2008 Paul Billiet ODWS

1. Action potential arrives at terminal button Vesicle storing neurotransmitter Ca 2+ channel Membrane receptor for neurotransmitter © 2008 Paul Billiet ODWS

Dept of Biology, Saint Louis University

3. Ca 2+ stimulates vesicles to fuse with membrane 2. Depolarisation opens Ca 2+ channels Ca 2+ enters terminal button Ca 2+ © 2008 Paul Billiet ODWS Ca 2+ 4. Exocytosis of neurotransmitter It diffuses 20 nm across the synaptic cleft

The passage across the synapse ¢ ¢ ¢ An action potential travels down an axon to the terminal buttons or synaptic knobs at the end The action potential depolarises the membrane of a terminal button causing voltage-gated Ca 2+ channels to open Ca 2+ ions flood into the terminal button This stimulates hundreds of synaptic vesicles, packed with neurotransmitter, to fuse with the membrane of the terminal button By exocytosis The Ca 2+ ions are then pumped out again © 2008 Paul Billiet ODWS

5. Neurotransmitter receptor sites on the postsynaptic membrane are ion channels. They open when the neurotransmitter binds 6. Localised depolarisation as ions leak in or out of membrane. © 2008 Paul Billiet ODWS

The passage across the synapse ¢ ¢ The neurotransmitter diffuses across cleft to postsynaptic membrane The neurotransmitter molecules bind with specific receptor sites on postsynaptic membrane The receptor sites are part of a ligand-gated ion channel These channels let Na+ ions in or K+ ions out causing localised depolarisation of the membrane © 2008 Paul Billiet ODWS

7. Action potential generated which travels down the postsynaptic cell. © 2008 Paul Billiet ODWS 8. Neurotransmitter destroyed by enzymes in the cleft. Stops signal being perpetuated.

A new action potential ¢ ¢ If the localised depolarisations build up to the nerve cell threshold, a full action potential will be produced This will travel away, down the postsynaptic neurone The action of the neurotransmitters stops: (i) as they dilute by diffusion in the synaptic cleft (ii) by hydrolysis through the action of enzymes there Important: The signal must not be perpetuated indefinitely © 2008 Paul Billiet ODWS

The neuromuscular junction is a synapse The motor end plate is the terminal button of a motor neurone that makes contact with a muscle cell ¢ The motor end plate releases the neurotransmitter acetylcholine that ultimately causes the muscle cell to contract ¢ © 2008 Paul Billiet ODWS

Motor end plates © David B. Fankhauser, Ph. D. , Professor of Biology and Chemistry, University of Cincinnati Clermont College