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 20nm)

Some of them work both at synapses and in the circulatory system

© 2008 Paul Billiet ODWS

1. Action potential arrives at terminal button

Membrane receptor for neurotransmitter

Ca2+ channel

Vesicle storing neurotransmitter

© 2008 Paul Billiet ODWS

Dept of Biology, Saint Louis University

Ca2+ Ca2+ Ca2+ Ca2+

2. Depolarisation opens Ca2+ channelsCa2+ enters terminal button

3. Ca2+ stimulates vesicles to fuse with membrane

4. Exocytosis of neurotransmitterIt diffuses 20nm across the synaptic cleft

© 2008 Paul Billiet ODWS

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 Ca2+ channels to open

Ca2+ 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 Ca2+ ions are then pumped out again

© 2008 Paul Billiet ODWS

6. Localised depolarisation as ions leak in or out of membrane.

5. Neurotransmitter receptor sites on the postsynaptic membrane are ion channels. They open when the neurotransmitter binds

© 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

8. Neurotransmitter destroyed by enzymes in the cleft. Stops signal being perpetuated.

7. Action potential generated which travels down the postsynaptic cell.

© 2008 Paul Billiet ODWS

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