Nervous System Part 2
Synapse
• The gap between different
neurons is called a synapse.
• Definition: The gap between
the axon of a previous
neuron and the dendrites of
the next neuron.
• We have established how an
impulse can travel down a
neuron (electricity). How
does the impulse travel
between different neurons?
How does an impulse travel
across a synapse?
Chemical, not electric
• The way an impulse was passed down a single
neuron was electrical (meaning the impulse
related to positively and negatively charged
atoms).
• The way an impulse is transmitted across a
synapse is chemical, not electric.
Transmission at the Synapse
• The axon terminal contains
many tiny sacs containing
chemicals known as
neurotransmitters.
• When the impulse reaches
the end of an axon (the
axon terminal), it causes
the axon to release these
neurotransmitters into the
synapse.
Neurotransmitters
• These neurotransmitters
cause some of the
dendrite’s sodium
channels to open.
• This causes sodium to rush
in, making that area of the
neuron more positive,
causing the nearby
voltage-gated-sodiumchannels to open, and the
cycle begins.
Neurotransmitter Breakdown
• After the
neurotransmitters
stimulate the next
dendrite, they must be
removed or they will
continue to send impulses
down the next neuron.
• To do this, there are
enzymes that destroy the
neurotransmitters in the
synapse.
Multiple Synapses
• The nervous system is
actually much more
complex than that.
• Each dendrite can have up
to 1000 synapses with
different axons.
• The nervous system is not
a direct path – it is much
more twisted and
interconnected than a
single straight line.
2 Types of Neurotransmitters
Excitatory Neurotransmitters• These chemical start
impulses in the next
neuron. These make the
next dendrite more
permeable to sodium.
These would be sent when
you want muscles to
contract.
Inhibitory Neurotransmitters• These chemical prevent
impulses in the next
neuron. These make the
next dendrite less
permeable to sodium.
These would be sent when
you want muscles to stop
contracting.
What happens if a dendrite receives
contradictory messages?
• The dendrite has synapses with multiple
axons.
• Sometimes, different axons tell a dendrite
different things.
• When this happens, the cell body
(headquarters) takes the average of all of the
incoming impulses. If the majority are
excitatory, then it will send the impulse down
its axon. If the majority are inhibitory then it
will not send an impulse down its axon.