Structure of the Synapse

The Structure and function of the synapse

• WALT

• That synapses occur between neurones

• How the nervous impulse is transmitted across the synapse

• That synapses use different neurotransmitters

Definitions

• presynaptic neuronpresynaptic neuron: : before the synapsebefore the synapse

• postsynaptic neuronpostsynaptic neuron: behind the : behind the synapse with receptor moleculessynapse with receptor molecules

• synaptic cleftsynaptic cleft: a narrow gap about 20 nm : a narrow gap about 20 nm widewide

Synapse

• When a nerve impulse arrives at the synaptic knob it alters the permeability of the presynaptic membrane to Ca+, which therefore enters

• Synaptic vesicle fuse with membrane & discharge its transmitter substance

Synapse

• The transmitter diffuses across synaptic cleft & fuses with receptor molecules

• This alters permeability of postsynaptic membrane to Na+

• If excitatory postsynaptic potential (EPSP) EPSP by summation exceeds threshold

• Action potential results

Synapses

• Transmitter substance (acetylcholine) is hydrolysed by the enzyme acetylcholinesterase on the postsynaptic membrane.

• Its breakdown can be reused to synthesize acetylcholine again at the synaptic knob, with energy from mitochondria.

Synapse

Synapse

Neurotransmitters

• Acetylcholine - released by all motor neurones, activating skeletal muscles involved in the parasympathetic nervous system (relaxing responses)                      - cholinergic synapses

• Noradrenaline - involved in the sympathetic nervous system ('fight or flight' responses)                       - adrenergic synapses

Functions of the synapse

1. Transmit information between neurons

2. Pass impulses in one direction only

3. Act as junctions

4. Filter out low level stimuli

5. Allow adaptation to intense stimulation

Task• Draw and annotate the transmission of an

impulse across the synapse

• What is meant by adaptation?

• Where else do synapses occur?

• What name is given to those synapses that use acetylcholine as neurotransmitter?

• Questions 9 and 10 page 61

• Explain summation

Types of Synapse

• Excitatory ion channel synapses -neuroreceptors are Na+ channels.  When Na+ channels open, local depolarisaition occurs, if threshold is reached then action potential is initated

• Inhibitory ion channels - neuroreceptors are Cl- channels.  When Cl- channels open, hyperpolarisation occurs, making action potential less likely

• Non channel synapses - neuroreceptors are membrane-bound enzymes.  When activated, they catalyse the 'messenger chemical', which in turn can affect the sensitivity of the ion channel receptors in the cell

• Neuromuscular junctions - synapses formed between motor neurones and muscle cells.  Always use the neurotransmitter acetylcholine, and are always excitatory

• Electrical synapses - the membranes of the two cells actually touch and they chare proteins.  The action potential can pass directly from one membrane to the next

Types of Synapse