13.5 synapses

chemicals that transmit impulses across the synapse

neurotransmitters

synaptic cleft

the gap which separates the axon of one neuron from the dendrite of the next neurons (20-30nm accross)

presynaptic neurons

neuron along which the impulse has arrived

postsynaptic neuron

neuron that removes the neurotransmitter

synaptic knob

the swollen end of the presynaptic neurone. It contains many mitochondria and large amounts of endoplasmic reticulum to enable it to manufacture neurotransmitters

synaptic vesicles

vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membranes and release their contents into the synaptic cleft

neurotransmitter receptors

receptor molecules which the neurotransmitter binds to in the postsynaptic membrane

excitatory neurotransmitters

these neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered. eg. ACh

inhibitory neurotransmitters

these neurotransmitters result in the hyper polarisation of the postsynaptic membrane. This prevents an action potential from being triggered. GABA is an example of this

synaptic transmission occurs are a result of the following

• the action potential reaches the end of the presynaptic neuron

• depolarisation of the presynaptic membrane causes calcium ion channels to open

• calcium ions diffuse into the presynaptic knob

• this causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis

• this causes sodium ion channels to open

• sodium ions diffuse into the postsynaptic neuron

• this triggers and action potential and the impulse is propagated along the postsynaptic neurone

what happens which a neurotransmitter has triggered an action potential in the postsynaptic neurone

• it is important that the stimulus is not maintained, and so another stimulus can arrive and affect the synapse

• any neurotransmitter left in the synaptic cleft is removed

• acetycholine is broken down by enzymes which also releases them from the recepors on the postsynaptic membrane

• the products are taken back in the presynaptic knob

• removing the neurotransmitter from the synaptic cleft prevent the response from happening again

transmission across cholinergic synapses

1. the arrival of an action potential at the end of the presynaptic neuron causes calcium ion channels to open and calcium ions to enter the synaptic knob

2. the influx of calcium ions into the presynaptic neurones causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into the synaptic cleft

3. acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neuron. This causes the sodium ion channels to open, allowing sodium ions to diffuse rapidly along a concentration gradient

4. the influx of sodium ions generates a new action potential in the postsynaptic neuron

5. acetylcholinesterase hydrolyses ACH into choline and ethanoic acid, which diffuses back across the synaptic cleft into the presynaptic neuron. In addition to recycling the choline and ethanoic acid, the breakdown of ACh prevents it from continuously generating a new action potential in the postsynaptic neuron

6. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetycholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylecholine in the receptor site

role of synapses

• ensure impulses are unidirectional. As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neuron to the postsynaptic neuron

• they can allow an impulse from one neuron to be transmitter to a number of neurons at multiple synapses. This results in a single stimulus creating number of simultaneous responses

• alternatively, a number of neurons may feed in to the same synapse with a single postsynaptic neuron. This results in a single stimuli from different receptors interacting to produce a single result

summation

is the amount of neurotransmitter builds up sufficiently to reach the threshold, then this will trigger an action potential

spacial summation

this occurs when a number of presynaptic neurons connect to one postsynaptic neurone. Each one releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neuron

temporal summation

this occurs when a single presynaptic neuron releases neurotransmitters as a reult of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient to trigger an action potential