Download SYNAPTIC TRANSMISSION

SYNAPTIC
TRANSMISSION
The Synapse:
 How does the impulse travel from one neuron to another?
 How does an impulse travel from a neuron to a muscle or gland (effector)?
 How can they do this when they are not physically connected (there’s a GAP)?
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When an impulse arrives at the end of an axon, it must make a connection to the next nerve cell, or to the
muscle or gland.
BUT neurons do NOT directly contact each other
There is a small space, termed the SYNAPTIC GAP / CLEFT, which the impulse must cross.
 How does this work?
PRESYNAPTIC MEMBRANE
POSTSYNAPTIC MEMBRANE
Encloses synaptic vesicles filled with neurotransmitters
= n.t. (a chemical message manufactured by the axon).
Contains protein receptor sites within the membrane to
recognize specific neurotransmitters
Synaptic transmission:
STEP 1:
 When an impulse arrives at the end of an axon, the sodium gates open and sodium floods into the axon bulb /
terminal.
STEP 2:
2+
 At the same time, the CALCIUM GATES OPEN and calcium (Ca ) also moves into the axon bulb / terminal of
the presynaptic neuron.
STEP 3:
 The calcium binds with CONTRACTILE PROTEINS (microfilaments) attached to the vesicles and this causes
them to contract, thus pulling the vesicles towards the pre-synaptic membrane
o This requires ATP
STEP 4:
 EXOCYTOSIS occurs as the vesicles release neurotransmitters into the synaptic gap. The neurotransmitter
diffuses across the gap.
STEP 5:
 Neurotransmitters bond with the receptor sites on the postsynaptic membrane.
STEP 6:
 When an excitatory neurotransmitters attaches to the receptors, the voltage of the post-synaptic membrane
changes to cause the sodium gates to open.
o This depolarizes the membrane.
 If an inhibitory neurotransmitter is released and attaches to the receptors, the post-synaptic membrane will be
hyperpolarized which makes it more difficult for the threshold to be reached (less likely an action potential
will occur).
STEP 7:
 If the synapse is between an axon and dendrite, then the Action Potential will continue down the next neuron
 If the synapse is between an axon and a muscle cell, then the muscle will contract.
 If the synapse is between an axon and a gland, then the gland will release a hormone.
STEP 8:
 The synaptic gap contains enzymes that will destroy the neurotransmitters, thus returning the synapse to its
original condition prior to the arrival of the impulse
STEP 9:
 The calcium ions are returned to the synaptic gap by active transport.
 Because only the axon bulb has the neurotransmitters and only the dendrite has
the receptors, synaptic transmission may only occur in one direction.
 The energy for this entire process comes from the mitochondria that can be found in abundance in the synaptic /
axon bulb.