Where nerve impulses convert to neurotransmitters THE SYNAPSE www.sanger.ac.uk/.../gfx/070305_synapse_300.jpg Where nerve impulses convert to neurotransmitters © 2016 Paul Billiet ODWS

The synapse A gap (about 20nm) Where the nerve impulse passes from one cell to the next The electrical signal (the action potential)  a chemical signal to cross the gap between the cells Neurotransmitter The neurotransmitter crosses by diffusion This creates a small delay. © 2016 Paul Billiet ODWS

Designer signals Neurotransmitters give the nerve signal more specificity Neurotransmitters 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. © 2016 Paul Billiet ODWS

Neurotransmitters and hormones Neurotransmitters are hormones working over a very short distance Some of them work both at synapses and in the circulatory system E.g. Adrenalin (epinephrin). © 2016 Paul Billiet ODWS

1. Action potential arrives at terminal button Vesicle storing neurotransmitter Ca2+ channel Membrane receptor for neurotransmitter © 2016 Paul Billiet ODWS

starklab.slu.edu/neuro/synapse.jpg © 2016 Paul Billiet ODWS

3. Ca2+ stimulates vesicles to fuse with membrane 2. Depolarisation opens Ca2+ channels Ca2+ enters terminal button Ca2+ 4. Exocytosis of neurotransmitter It diffuses 20nm across the synaptic cleft © 2016 Paul Billiet ODWS

The passage across the synapse An action potential travels down an axon to the terminal buttons The membrane of a terminal button depolarises 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 Exocytosis The Ca2+ ions are then pumped out again. © 2016 Paul Billiet ODWS

They open when the neurotransmitter binds 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. © 2016 Paul Billiet ODWS

The passage across the synapse Neurotransmitter diffuses across cleft to postsynaptic membrane Neurotransmitter molecules bind with specific receptor sites on postsynaptic membrane The receptor sites are ligand-gated ion channels These channels let Na+ ions in or K+ ions out causing localised depolarisation of the membrane. © 2016 Paul Billiet ODWS

8. Neurotransmitter destroyed by enzymes in the cleft 8. Neurotransmitter destroyed by enzymes in the cleft. Stops signal being perpetuated. 7. Action potential generated which travels down the postsynaptic cell. © 2016 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 neuron 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 © 2016 Paul Billiet ODWS

The neuromuscular junction is a synapse The motor end plate is the terminal button of a motor neuron that makes contact with a muscle cell The motor end plate releases the neurotransmitter acetylcholine that ultimately causes the muscle cell to contract © 2016 Paul Billiet ODWS

biology.clc.uc.edu/fankhauser/Labs/Anatomy_... © 2016 Paul Billiet ODWS

education.vetmed.vt.edu/Curriculum/VM8054/Lab... © 2016 Paul Billiet ODWS

Blocking synapses Poisons Psychotic drugs Medication Botox Neonicotinoid pesticides © 2016 Paul Billiet ODWS

Nip & tuck Botox = a neurotoxin produced by a bacterium Chlostridium botuninum It causes food poisoning (botulism) The toxin blocks the release of acetyl choline neurotransmitter to muscles Causes muscle paralysis Cosmetically used to smooth lines and creases Medically used to relax muscles that causing illnesses such as spasms and strabismus (cross-eyed). © 2016 Paul Billiet ODWS

Neuro-active pesticides Neonicotinoids mimic the neurotransmitter acetylcholine Nicotinoids cannot be broken by the enzyme in the synapse So the signal continues Causing paralysis These pesticides have been associated with honey bee deaths in many countries. © 2016 Paul Billiet ODWS