How signals are sent through the nervous system
Synapse

Synapse = Junction between two
connecting neurons
 Synaptic cleft-between the neurons, signal
has to go across this space

Signals need to be sent from neuron to
neuron, not just from neuron to muscle
Synapse
Presynaptic Neuron: sender of the signal;
axon end (Before synapse)
 Postsynaptic Neuron: receiver of the signal
(After synapse); dendrite end

Synaptic Transmission

Transmission occurs when the message
crosses the synapse

Neurotransmitters are biochemicals that
complete this function
 Which neurotransmitter did we learn about
during the muscular unit?
Neurotransmitters

Distal end of axons have synaptic knobs
with synaptic vesicles (store
neurotransmitter)
Neurotransmitters

Can be:
 Excitatory: increase signal transmittance
○ More of this type = sending of signal
 Inhibitory: decrease signal transmittance
○ More of this type = no signal
Chemistry Review

Ions play an important role in the nervous
system signals

What is an ion?
 An atom that has lost or gained 1/more
electrons
 Ions are positive if electrons are lost and
negative if electrons are gained
 Examples:
Na+
K+
Mg+2
Cl- O-2
Neurons at rest
Neurons at rest

Resting potential: inside is more
negative than outside of the cell

AKA POLARIZED (think polar opposites)
Why?
Resting Neuron

To keep the cell in resting potential, a SodiumPotassium pump restores ions to where they
belong
Action Potential

Conditions must change in order for a
signal to be sent by the neuron

This electrochemical signal = ACTION
POTENTIAL

Which part of the neuron is the sender?
Depolarization
Environmental Stimuli (odor, touch,
sound,etc.)
2. Receptor cell releases neurotransmitter
3. ONLY Na+ channels open, Na+ ions go
into cell
1.
= DEPOLARIZATION
Depolarization

Inside of cell becomes more POSITIVE

This triggers an ACTION POTENTIAL

Will continue down rest of membrane
Repolarization

Quickly after the previous step, K+ is
able to move across membrane through
its channels; sodium can no longer
move
Repolarization

Inside is negative again (repolarized)
Refractory period

Sodium – Potassium pump uses active
transport to move Na+ & K+ back to where
they started

During this time, the neuron cannot
transmit an impulse

Known as REFRACTORY PERIOD

Membrane returns to true resting potential
All or None Response

Just like muscles if a
nerve responds, it
responds completely

Greater intensity of
stimulation triggers
more impulses per
second

Not a greater intensity of
impulse
Nerve Impulse

This “wave” of action potentials from one
neuron to the next is known as a
NERVE IMPULSE

Moves from dendrites down through
axon
Nerve Impulses

Unmyelinated neurons conduct impulses over their entire
membrane surface- SLOW

Myelinated neurons conduct impulses from node of Ranvier
to node of Ranvier - FAST
Synaptic Transmission
1. When an impulse
reaches the end of an
axon, synaptic vesicles
release neurotransmitters
2. The neurotransmitters
react with receptors on the
postsynaptic membrane to
open ion channels.
3. Ions flow into the
postsynaptic cell, eliciting a
response.
Neurotransmitters

Excitatory NTs: cause depolarization

Inhibitory NTs: lessen depolarization
Last Step

Neurotransmitters
are broken down
by enzymes, or

Reabsorbed by
presynaptic cell
 Called re-uptake