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Myelin again
• Myelin speeds up the nerve impulse because
nerve fibers have Schwann cells around them
– Schwann cells restrict ion movement
– So impulse “jumps” between the nodes of Ranvier
– This jumping is called saltatory transmission
Synaptic Transmission
• Saltatory transmission impulse along a neuron
• Synaptic transmission impulse between
neurons
– Happens at the end of an axon
– No actual connection between the terminus and
the membrane of the next cell
– Space between is called a synapse or synaptic
gap/cleft
Synaptic membranes
• Axon membrane is called the pre-synaptic
membrane
– Has Ca2+ gate
– Has synaptic vesicles with neurotransmitters
inside
• Membrane on the other side of synaptic cleft
is the post-synaptic membrane
– Has protein receptor sites
Steps
1) Impulse arrives at end of axon
2) depolarization of pre-synaptic membrane
3) Ca2+ gates open - Ca2+ in the synaptic cleft
moves into the axon
4) Vesicles with neurotransmitters inside fuse
with the pre-synaptic membrane
5) Neurotransmitters are released (exocytosis)
into the synaptic cleft and diffuse across to
the post synaptic membrane
axon
terminal
Ca2+
1. After an
action
potential
arrives at an
axon terminal,
Ca2+ enters,
and synaptic
vesicles fuse
with the
presynaptic
membrane.
synaptic vesicles
enclose neurotransmitters
synaptic cleft
axon
terminal
2. Neurotransmitters
are released
and bind to
receptors
on the
postsynaptic
membrane.
NT
Ca2+
synaptic vesicles
enclose neurotransmitters
synaptic cleft
6) Neurotransmitters bond to receptor sites on
the post-synaptic membrane
7) Step (6) causes the Na+ gates to open on post
synaptic membrane which starts the nerve
impulse along that cell
8) Synaptic cleft is returned to normal
- enzymes that destroy specific neurotransmitters
- Ca2+ returned to the synaptic cleft (active
transport)
axon
terminal
Ca2+
3. When an excitatory
neurotransmitter
binds to a receptor, Na+
diffuses into the
postsynaptic neuron,
and an action potential
begins.
NT
receptor
Na+
synaptic cleft
postsynaptic
neuron
Synaptic Transmission in General
• Energy for synaptic transmission comes from
mitochondria in the axon
• Synaptic transmission only occurs in one direction
due to nature of the membranes on either side of
the synaptic cleft
• 2 types of neurotransmitters
– Inhibitory NT – makes it harder for depolarization of
the next membrane
– Excitatory NT – promotes depolarization of the next
membrane
ANIMATION
• Transmission across a synapse