Nervous coordination 3

Synapses

SynapsesNeurones communicate with each other through junctions called synapses.

The photograph shows a single neurone in the central nervous system of a snail, covered with nerve fibres from other neurones, each ending in a synaptic knob.

Neurotransmitters diffuse across the synaptic cleft, passing the information on from the presynaptic to the postsynaptic cell.

Action of a synapseSynaptic vesicles containing

neurotransmitter

Receptors in post-synaptic membrane

Sodium gates (closed)

Calcium gates (closed)

Mitochondria

Synaptic cleft

Sodium gates open on arrival of action potential

Depolarisation of presynaptic membrane

causes calcium gates to open: Ca2+ ions flood in.

Calcium ions trigger fusion of vesicles with presynaptic membrane …

Transmitter diffuses across cleft …

Action of a synapse

Binding of transmitter to receptors on postsynaptic membrane opens sodium gates, depolarising post-synaptic cell.

This depolarisation is an EXCITATORY POST-SYNAPTIC POTENTIAL. If it reaches threshold, an action potential is triggered.

The transmitter is immediately broken down by enzymes present in the cleft and the sodium gates in the postsynaptic membrane close.

Transmitter components are reabsorbed by the presynaptic cell to be re-assembled into transmitter molecules and incorporated into new vesicles.

Action of a synapse: recap

Why have synapses?

Synapses act as one-way valves: only one side of the synapse has vesicles, only the other side can respond to the transmitter: impulses can therefore travel only one way across a synapse.

Where several neurones meet at a synapse their effects can be ‘added up’ to decide whether or not impulses are triggered on the other side: this is called summation.

Summation at synapses

Here three presynaptic cells synapse with a single dendritic spine on a postsynaptic cell.

This oscilloscope registers membrane potential in one of the presynaptic cells. At present it shows resting potential.

This oscilloscope registers membrane potential in the postsynaptic cell. At present it shows resting potential.

Summation at synapses

An action potential arrives at one presynaptic terminal, causing release of neurotransmitter.

An EPSP is registered in the postsynaptic cell, but it does not reach threshold and no action potential is transmitted.

But if action potentials arrive at two of the presynaptic terminals ...

… the EPSPs sum, reach the threshold level, and an action potential is transmitted in the postsynaptic cell.

But if action potentials arrive at all three of the presynaptic terminals ...

… in this case the yellow terminal generates an inhibitory postsynaptic potential (IPSP), which subtracts from the EPSPs and prevents them from reaching threshold level.

These are examples of spatial summation.

Summation at synapses

Again, an action potential arrives at a presynaptic terminal, causing release of neurotransmitter.

An EPSP is registered in the postsynaptic cell, but it does not reach threshold and no action potential is transmitted.

But if two action potentials arrive in quick succession ...

… the second EPSP ‘builds’ on the first, reaches threshold. and triggers an action potential.

This is temporal summation.

Effects of summation

Synapses can act as ‘logic gates’, giving an output determined by the sum of the inputs: they can act as AND, OR, NAND, NOR gates etc., allowing simple decision-making to be buils in at this level.

Temporal summation allows synapses to act as filters, eliminating ‘noise’ and insignificant signals.

Drugs and synaptic transmission

A number of drugs act by affecting synaptic transmission

Drugs may mimic, enhance or block the effects of a neurotransmitter

Nicotine mimics the effect of acetylcholine at certain cholinergic synapses

Nicotine Acetylcholine

Drugs and synaptic transmission

Nicotine binds to receptors on the postsynaptic membrane of nicotinic cholinergic synapses, causing sodium channels to remain in the open position

Small doses of nicotine act as a stimulant (agonistic affect)

Prolonged exposure blocks the action of acetylcholine (antagonistic effect)

Nicotine also stimulates adrenaline and endorphin release