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3.6 Organisms respond to changes in their environments
3.6.2 Nervous coordination
3.6.2.2 Synaptic transmission
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Dorcas ᶻ 𝗓 𐰁
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Biology > 3.6 Organisms respond to changes in their environments > 3.6.2 Nervous coordination > 3.6.2.2 Synaptic transmission
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Describe the role of neurotransmitters at a synapse.
Neurotransmitters are released from the
presynaptic
neuron into the
synaptic cleft.
They bind to
receptors
on the
postsynaptic
neuron, opening
ion channels
.
This triggers an
action potential
in the
postsynaptic
neuron if the
threshold
is reached.
Neurotransmitters are
broken
down
or
reabsorbed
to stop the signal
Explain how the structure of a synapse enables one-way transmission of nerve impulses.
The synapse consists of a
presynaptic
terminal,
synaptic cleft
, and
postsynaptic membrane
.
Neurotransmitters
are released by the
presynaptic neuron
and bind to
receptors
on the
postsynaptic neuron
.
This ensures signals only move in
one
direction
, as receptors are only on the
postsynaptic
membrane
How do enzymes at synapses control neurotransmitters?
Enzymes break down
neurotransmitters
in the
synaptic cleft
This prevents
continuous
stimulation of the
postsynaptic neuron
.
Spatial summation:
Multiple
presynaptic
neurons release
neurotransmitters
simultaneously.
Temporal summation: One
presynaptic
neuron releases
neurotransmitters
repeatedly in
quick
succession.
A neuromuscular junction is a
synapse
between a
motor
neuron
and a
muscle fibre
.
Summation is the
combined
effect of multiple
excitatory
or
inhibitory
signals to generate a response in the
postsynaptic
neuron.
Give one similarity and one difference between a synapse and a neuromuscular junction.
Similarity: Both involve
neurotransmitters
crossing a
synaptic cleft
to transmit signals.
Difference: Synapses connect
two
neurons
, while neuromuscular junctions connect a
neuron
and a
muscle cell
.
Acetylcholine
is released from neuromuscular junctions.
Neurotransmitter removal:
To prevent
continuous
stimulation,
neurotransmitters
must be removed from the
synaptic cleft
.
Enzymatic breakdown: E.g.,
acetylcholinesterase
breaks down acetylcholine.
Reuptake: Neurotransmitters are
reabsorbed
by the
presynaptic neuron
.
Diffusion: Some neurotransmitters
diffuse
away
from the cleft.
Excitatory synapses:
Cause
depolarisation
of the
postsynaptic membrane
.
Increase
the likelihood of an
action potential
.
Inhibitory synapses:
Open channels for
negative
ions (e.g.,
Cl⁻
) or cause
K⁺
to leave the
postsynaptic
neuron.
Hyperpolarise
the membrane, making action potentials
less
likely.
A synapse is the
junction
between
two
neurons
or a
neuron
and an
effector
cell.
What are the three main components of a synapse?
Presynaptic neuron
(with
synaptic knob
and
vesicles
).
Synaptic cleft
(the small
gap
between
neurons
).
Postsynaptic membrane
(with
receptor
proteins).
Sequence of events that occurs at a typical cholinergic synapse (uses
acetylcholine
as neurotransmitter):
Action potential causes
voltage-gated
calcium ion channels
in
presynaptic membrane
to open.
Calcium ions
(Ca²⁺) enter the presynaptic neuron by
diffusion.
Calcium ion influx causes
vesicles
containing
acetylcholine
(ACh) to fuse with the
presynaptic membrane.
ACh is released into the
synaptic cleft
by
exocytosis
.
ACh
diffuses
across the
synaptic cleft
and binds to
receptor
proteins on the
postsynaptic membrane
.
This opens
sodium ion
(Na⁺) channels on the
postsynaptic membrane.
Na⁺ enters the
postsynaptic
neuron, causing
depolarisation
.
If threshold is reached, a new action potential is triggered.
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