Download Structure of the Synapse

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 neuron: before the synapse
• postsynaptic neuron: behind the
synapse with receptor molecules
• synaptic cleft: a narrow gap about 20 nm
wide
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
Types of Synapse
• 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