Download The Synapse

Neural communication
How do neurons send messages to
each other?
The Synapse
• Two neurons meet at a synapse.
• Children have roughly 1016 synapses.
Adults somewhere from 1-5  1015
• A typical synapse is about 20nm wide (20
billionths of a meter). This is called the
synaptic cleft
Neurotransmitter
• Neurons communicate by sending chemical
messages called neurotransmitters to other
neurons.
• These neurotransmitters travel from axon to either
the dendrite or the cell body across the synapse.
• Where a synapse is depends on what the
connection type is
– Excitatory: Axon to dendrite
– Inhibitory: Axon to cell body
Neurotransmitter release
• Neurotransmitters are held in the terminal
button in synaptic vesicles.
• The action potential opens voltage-gated
Ca+ channels in the terminal.
• This causes the vesicles to bind with the cell
membrane, which allows neurotransmitter
to be released into the synaptic cleft.
Neurotransmitter absorbtion
• On the post-synaptic side of the synaptic cleft,
neurotransmitters bind to chemical receptors that
open chemically-gated ion channels.
• Some of these ion channels are K+ channels,
which allow K+ ions to leave the cell. This has the
effect of hyperpolarizing the area, which inhibits
the post-synaptic cell.
• Others are Na+ channels, which allow sodium into
the cell, depolarizing the area. This excites the
post-synaptic cell.
Re-uptake
• Once the neurotransmitter has done its job, it
binds to a protein on the pre-synaptic membrane.
• This process allows the pre-synaptic cell to
recycle the neurotransmitter, ensuring each action
potential will generate the same sized signal in the
synapse.
• It also lessens the effect of the neurotransmitter so
that the receiving cell does not become
desensitized to its effects, unless a rapid sequence
of action potentials causes neurotransmitter to be
continually present.