Download Electrochemical Impulse

Electrochemical Impulse
Warm-Up

What is an electrochemical gradient? In what organelles do we find
these in a cell?
Learning Goals

to determine how an impulse travels down an axon of a neuron

to identify how release of neurotransmitters can stimulate an action potential
in a neighbouring neuron
The Sodium Potassium Pump

When a neuronal membrane is at rest
(that is, not sending an impulse), lots is
actually happening at its membrane.
The Na+/K+ pump is hydrolysing lots of
ATP to move 3 Na+ ions out, and 2 K+
ions in.

Sodium Potassium Pump
Membrane Potential

Due to the work of the sodium
potassium pump and large negatively
charged proteins in the cytoplasm, a
charge separation is produced.

Keep in mind that the sodiumpotassium pump is using lots of ATP
to maintain this separation.
Resting Membrane Potential

Researchers have measured the
potential difference across the
membrane of a neuron, and found it
to be -70 mV.
Channels at Membrane

Embedded in the membrane of axons
are channels known as voltage-gated
ion channels. These channels remain
locked at resting potential until a
change in membrane potential
occurs, which opens them up and
allows specific ions to flow through.
Changes in Membrane Potential

There are many things that can
cause a change in membrane
potential of a neuron. Specialized
receptors exist in your skin and
organs that can change
membrane potential due to
environmental changes. These
receptors will open channels
allowing cations like sodium into
the cell body.
Threshold Potential

If circumstances can cause
enough cations to enter the cell
body and they diffuse towards
the axon hillock, a neuron may
reach a threshold potential of 55 mv.
All or Nothing

Once the threshold
potential of -55 mV is
reached, an impulse will be
sent along the axon. This is
an all or nothing response
(if it doesn’t reach -55 mV,
no impulse is sent).
Action Potential

The action potential is a very
quick depolarization and
repolarization of the membrane,
and it occurs due to the opening
of the voltage gated channels in
specific order.
Action Potential

When threshold potential of -55 mV
is reached:

Na+ voltage gated channels
open, allowing Na+ to flood into
the cytoplasm down its gradient.

This causes depolarization

These ions diffuse, causing
threshold potential to be
reached further down the axon.
Action Potential

Once Na+ has flooded in to reverse
the charge to +40 mV:

the Na+ channels close

K+ voltage gated channels open,
allowing K+ to rapidly diffuse out
of the cell which begins to
repolarize the membrane.
Action Potential

Once enough K+ has diffused out to
hyperpolarize the membrane at -90 mV:

Na+ channels are locked so the impulse can’t
travel backwards. This is called the
refractory period.

leaky membrane channels begin to return the
membrane back to -70 mV
Propagation down the Axon

The impulse travels down the length of
the axon by the spreading of
depolarization down the membrane.

Once the depolarization reaches the end
of the axon, it causes release of
neurotransmitters, which can start an
impulse in the next neuron of the
circuit.
Saltatory Conduction
Schwann cells coat the axon in a myelin
sheath, which insulates the charge
movement in the axon, speeding up
propagation.
 Myelinated axons only allow action
potentials to occur at the unmyelinated
nodes of Ranvier that occur between the
myelination.
 It is by this restriction that saltatory
conduction propagates an action potential
along the axon of a neuron at rates
significantly higher than would be possible
without the myelination of the axon (200
m/s compared to 2 m/s)

Transmission Across a Synapse
Transmission Across a Synapse

Action Potential Animation
Review

Explain what the resting membrane potential is, and why it is
significant to the functioning of neurons.

Identify three factors that contribute to the resting membrane
potential of a neuron.

Summarize how the sodium-potassium pump contributes to the
separation of charge and the resulting electrical potential difference
across the membrane of a neuron.

Draw diagrams that summarize the changes that occur in an axon as a
nerve impulse is transmitted.

Explain the importance of repolarization in the transmission of a
nerve impulse.

Tetrodotoxin is a neurotoxin found in puffer fish. This large molecule
blocks the sodium channels in neurons. Infer the effect tetrodotoxin
would have on the propagation of an action potential in a neuron.