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Module: BIOM - 1010

Lecturer: Dr Zhu

Date: 3/11/16

The Nervous System Part 4
o

Neurone to neuron transmission is connected by synapses
 They can also transfer information from neurone to effector cell

o

The neurones preceding and succeeding the synapse have special names:
 Presynaptic neurone
 Conducts the impulse to the synapse
 Postsynaptic neurone
 Transmits electrical signal away from synapse
 Can be neurone, muscle cell or gland cell in PNS

o

There are two types of synapse:
 Chemical synapse
 This is the most common type of the two
 These synapses are specialised for the release and reception of chemical
neurotransmitters
 They are composed of two parts:
 Axon terminal
o This is on the presynaptic neurones
membrane
These are
o These contain synaptic vesicles filled with
connected via
neurotransmitter
the synaptic cleft
 Receptor region
o This on the postsynaptic neurones membrane
o This area receives the neurotransmitter
 The synaptic cleft prevents impulses being spread directly
to
the next neurone
 It also ensures that the neurone path is unidirectional
 The process of chemical synaptic transfer:
 The action potential arrives at the terminal
 Voltage gated Ca2+ channels open and Ca2+ enters the terminal
 Ca2+ causes synaptic vesicles to release neurotransmitter and be exocytosed
 The neurotransmitter diffuses across the synaptic cleft and binds to receptors
on the postsynaptic neurone
 The binding of neurotransmitter causes ion channels to open and create
graded potentials
 The neurotransmitter is the retaken by the presynaptic neurone via membrane
proteins or enzyme degradation
 Electrical synapse
 These are less common than a chemical synapse
 The neurones are electrically coupled
 They are joined by gap junctions that connect the cytoplasm of the two neurones
 Communication is very fast and can be unilateral or bilateral
 These are found in some regions of the brain that are responsible for eye
movement and in the hippocampus in areas involving memory and emotion
 These are most abundant in embryonic nervous tissue

o

Depending on the amount of neurotransmitter released, and the amount of time it stays in the cleft,
the graded potentials can be of different strengths
 Depending on the effect of the postsynaptic neurone, postsynaptic potentials can be:
 EPSP: excitatory postsynaptic potentials
 IPSP: Inhibitory postsynaptic potential

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Excitatory postsynaptic potentials are characterised by the local depolarisation of the postsynaptic
membrane
 Neurotransmitter binding to chemically gated ion channels allows Na+ and K+ to pass
simultaneously
 The Na+ influx is greater than the K+ efflux and so depolarisation occurs bringing the
membrane closer to the threshold
 This causes an action potential

o

Inhibitory postsynaptic potentials are characterised by the local hyperpolarisation of the
postsynaptic membrane
 Neurotransmitter binding to the ion channels causes the membrane to be permeable to either K+
or Cl This causes the neurone to be further away from the threshold and so makes an action
potential less likely

o

A single excitatory signal cannot induce an action potential alone
 However, many can summate together to influence the neurone
 Inhibitory signals can also summate
 Many neurones receive both excitatory and inhibitory signals from thousands of neurones at
the same time
 There are two types of summation (see below):
 Temporal (right)
 One or more presynaptic neurones transmit impulses in rapid order
 Spatial (left)
 The postsynaptic neurone is stimulated by a large number of terminals
simultaneously



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