what is a synapse
the junction between two neurones across which impulses are transmitted using neurotransmitters
describe the structure of a synapse
- synaptic cleft = 20nm (gap separating axon of a neuron and dendrite of another neuron)
- presynaptic neuron (impulse arrives here)
- postsynaptic neuron (receives neurotransmitter)
- synaptic knob (swollen end of presynaptic neuron, has many mitochondria+endoplasmic reticulum to make neurotransmitters)
- synaptic vesicles (contain neurotransmitters and fuse with presynaptic membrane to release contents into synaptic cleft)
- neurotransmitter receptors (molecules which neurotransmitters bind to in postsynaptic membrane)
describe excitatory neurotransmitters
- result in depolarisation of postsynaptic neuron
- if threshold is reached in postsynaptic membrane, an action potential is triggered
- e.g. acetylcholine
describe inhibitory neurotransmitters
- result in hyperpolarisation of postsynaptic membrane
- prevents an action potential being triggered
- e.g. GABA
describe the process of synaptic transmission
- action potential reaches the end of the presynaptic neuron
- depolarisation of the presynaptic membrane causes calcium ion channels to open
- calcium ions diffuse into presynaptic knob
- synaptic vesicles fuse with presynaptic membrane and release neurotransmitters into synaptic cleft by exocytosis
- neurotransmitters diffuse across synaptic cleft and bind with receptors on postsynaptic membranes
- causes sodium ion channels to open
- sodium ions diffuse into postsynaptic neuron
- triggers action potential and impulse is propagated along postsynaptic neuron
- re-uptake occurs to remove neurotransmitters from the synaptic cleft and prevent the response from happening again
what are cholinergic synapses
- use acetylcholine
- common in the CNS of vertebrates and at neuromuscular junctions (motor neuron and muscle (effector) meet)
describe the process of transmission across cholinergic synapses
- acetylcholine is released from vesicles in the presynaptic knob
- diffuses across synaptic cleft and binds to specific receptors on postsynaptic membrane
- triggers action potential in postsynaptic neuron (or muscle cell)
- acetylcholine is hydrolysed by acetylcholinesterase (also on the postsynaptic membrane) to form choline and ethanoic acid
- breakdown products are taken back to presynaptic knob to be reformed into acetylcholine
explain the role of synapses
- ensure impulses are unidirectional
- allow an impulse from one neuron to be transmitted to a number of neurons at multiple synapses (single stimulus=many simultaneous responses)
- number of neurons go to the same synapse with one postsynaptic neuron (many stimuli=single result)
define summation
amount of neurotransmitter builds up sufficiently to reach the threshold and trigger am action potential
what is spatial summation
- many presynaptic neurons connect to one postsynaptic neuron
- enough neurotransmitters released to trigger an actionn potential in the postsynaptic neuron
what is temporal summation
- single presynaptic neuron releases neurotransmitter due to action potential several times over a short period
- builds up in synapse until enough to trigger action potential
how do drugs that stimulate the nervous system work
- mimicking shape of neurotransmitter
- stimulating release of more neurotransmitter
- inhibit enzyme responsible for breaking down neurotransmitter in the synapse
how do drugs that inhibit the nervous system work
- blocking receptors so the neurotransmitter can longer bind to it
- binding to specific receptors on the post-synaptic membrane and changing the shape of the receptor so binding of neurotransmitter increases and this increases activity
