Physiology Lecture #5 [Channels, Action Potentials and Chemical Synapses]

A typical neuron at rest has a membrane potential of approximately

-70 mV

If the potential decreases (inside changes towards 0 mV), the membrane is said to

depolarize

If the potential increases (inside changes further away from 0, even more than resting membrane potential), the membrane is said to

hyperpolarize

Reversal potential of ion determine direction and magnitude of

Ion flux

A threshold potential for a typical neuron is ~___

-50mV

Action potentials propagate down the axon without changing ____

amplitude

What is responsible for the propagation of an action potential (General)

Voltage-gated ion channels

Rapid depolarization is caused by

- Opening of voltage-gated Na channels

- Inward flux of Na ions

- Threshold dictated by opening of voltage-gated Na channels

Repolarization is caused by

- Slower opening of voltage-gated K channels

- Outward flux of K ions

- Inactivation of voltage-gated Na channels

In a resting cell when the voltage is low (-70) Na channels are (open or closed)

Closed

When you increase the membrane voltage the Na channels (open or close)

Open

Once the membrane is staying at a high voltage Na channels?

inactivate

Na channels inactivating is extremely critical for what phase

Repolarization Phase

- If the Na channels do not close then you are always going to have an inward flux of Na ions

To bring the membrane voltage down you need both

You need the K efflux and the Na influx to shut down

Do Na or K channels faster to open

Na open faster

Membrane depolarization causes _____ and the Na channel opening

conformational change (protein changes shape)

Once open Na channels remain open for about _____

1 millisecond

Na channels cannot re-open during what phase

inactivation

During membrane repolarization and refractory period, the Na channels are able to

return to the normal closed state during recovery after inactivation

A refractory period follows an

Action potential

Once an action potential passes a region of the axon, that region becomes temporarily

refractory to subsequent activation

The refractory period result from

Na channel inactivation

The refractory phase puts a cap on action potential

Frequency

During an action potential we get Na and K ions moving down their electrochemical gradients across the Plasma Membrane which results in a change in ion gradient. How do we restore the original gradient?

Na/K ATPase (pump)

Local anesthetics (LA) work by reversibly blocking

Na channels in nerves

Water and lipid friendly

Amphiphilic

the neutral base form of LA penetrates lipid membanes and inhibits the channel with low ____ which changes bilayer fluidity

affinity

The charged form of LA potently ____ the channel

inhibits

Charged form of LA is _______ into the Plasma membrane

Not able to enter

The charged for of LA can only ____ the channel from the outside

weakly inhibit

The basic form of LA are _____ into the Plasma membrane

able to enter

Once the basic form of LA goes through the membrane into the cell it is able to

Accept a proton in the cytoplasm becoming the charged form of LA

Once positive form of LA is in the cell it waits for?

the Na channel to open and then inserts itself into the channel

The Positive form of LA is able to

Effectively block the Na channel from the inside.

Myelin is composed of sheets of membrane derived from what supportive cells

- Oligodendrocytes in the Central nervous system

- Schwann cells in the peripheral nervous system

Myelin acts as an electric

insulator

Myelination ____ the conduction velocity of action potentials 10-30 fold

increases

Without myelin the strength of the electrical signal gets ____ as it moves down the axon

weakend

Continuous conduction occurs along _______ axons

unmyelinated axons

Saltatory conduction occurs along __________ axons.

myelinated: action potential "jumps" from node to node

Demyelination leads to

neurological disorders

- Slower conduction (due to lack of saltatory conduction)

-Multiple sclerosis (CNS: auto-immune disorder)

Where does the initial stimulus to the neuron come from

Other cells