BIOSCI 107- module 5 neurons

1) Describe and recognise the structure of a neuron 2) Explain and discuss how the resting membrane potential is generated. 3) Explain and discuss how action potentials are generated and transmitted. 4) Explain and discuss mechanisms and features of synaptic communication.

Nervous system is made up of?

CNS and PNS

cells of the nervous system

Neurons and glia

A?

Dendrites

B?

Cell body

C?

Axon

D?

Axon terminal

Synaptic potentials flow?

Into the cell body through the dendrites (passive electrical signals)

Action potentials flow?

Out of the cell body towards the axon terminal (non passive)

Are action potentials passive or not?

Not passive

Are Synaptic potentials passive or not?

Passive electrical signals

Function of dendrite?

Receives input

Function of Cell body?

Passively conduct electrical signals

Function of axon initial segment?

Initiate action potentials (AP)

Function of Axon?

Propagate AP

Function of Axon terminals?

Releases chemical signals

Examples of electrical signals

Dendrites, cell body, axon

Example of chemical signals

Synapses

The RMP experiment

2x electrodes, one inside cell (recording electrode), one outside, measure voltage difference (-50 to -70mV), when both outside mV=0

In neurons and their processes, the ———- has a potential that is 50 to 70mV lower (ie. more negative) than the potential of the extracellular fluid.

Cytosol

In neurons and their processes, how much lower is the potetial of the cytosol compared to the extracellular fluid?

50-70mV

What is it called when cytosol of a neuron has a lower mV (potential) than the extracellular fluid?

Seperation of charges

What is seperation of charges?

a difference in potential, causing electrical potential and therefore current, therefore voltage.

How many cells in the body have RMP?

Almost all except red blood cells or dead / dying cells

What can RMP affect in the body?

circadian rytheym and wound healing

What are excitable tissues?

Tissues that can suddenly respond with a transient change of this potential (ie. an action potential) in response to a stimulus (neurons and muscle fibres)

What are the two kinds of excitable tissues?

Muscle fibers and neurons

Two ways of recording intracellular potentials?

Microelectrodes and patch clamp technique

Microelectrode recording technique

Same as the RMP experiment but electrodes are much smaller and made of glass

Patch-clamp technique

Clamp and rupture membrane, intracellular fluid fills pippette

Two kinds of signals

Electric and chemical

Two kinds of electric signals

Synaptic potentials (passive) and action potentials

What are synaptic potentials?

Movement of charges

What is an action potential?

When the cell actively regenerates, allowing signal to propagate a longer distance

Equilibrium potentials tell you what?

The potential for the ion (not cell)

What equation do we use to calculate equilibrium potential?

Nernst Equation

What does the Nernst equation include?

intracellular and extracellular ion conc, R, F, T, Z constants (61.5)

Does the Nernst equation calculate for 1 ion type or 2?

1

What equation calculates RMP?

Goldmans equation

Does goldmans equation calculate for 1 ion type or 2?

2

What factors does Goldmans equation have?

intracellular and extracellular ion conc, R, F, T, Z constants (61.5), permeability

Ek+

-80mV

ENa+

60mV

Pk+ / PNa+

40/1

RMP

~ -65mV

Receiving neurotransmitters is an ____ signal

Chemical

Propagating neurotransmitters is an _______ signal

Electrical

Potential inside neurons is not constant. When does it change?

membrane permeability or ion concentrations change

Hyperpolarisation

When the cell membrane potential becomes more negative than the resting membrane potential because the cell moves closer to the equilibrium potential of potassium

Depolarisation

When cell membrane potential moves positively (away from RMP) because something has changed that moves it closer to the equilibrium potential of sodium

Action potential is

A brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels which spreads, like a wave, along an axon.

Action potential occurs

after the membrane potential reaches a certain voltage called the threshold (~ -55 mV).

When talking about action potentials, we are only talking about______________, because they can suddenly change their membrane potentials

Excitable tissues (muscles and neurons)

Action potentials are a key element of _________ along (often very long) axons.

signal transmission

The _________ of action potentials encodes information (a language by which neurons communicate)

frequency

Why are action potentials a key part in signalling along long axons?

They communicate with minimum decay to signals, (wont get degraded when sent far away)

Stimulus

can be physical (eg electric current or mechanical stretch) or chemical (eg a drug or synaptic excitation), pain

What do all action potentials start with?

RMP

1st step of action potentials

Membrane potential reaches threshold, followed by fast depolarisation to ~ +30 mV (‘overshoot’)

Fast depolarisation

voltage gated sodium channels open allowing large amounts of sodium to enter the cell, going towards the equilibrium potential of Na

Where does graded depolarisation occur?

After RMP but before threshold

What is graded depolarisation?

When the depolarisation it shows is relative to the amount of stimulus you have

What happens in repolarisation?

Na channels shut, K+ channels open

When does repolarisation occur?

After fast depolarisation

What is the last step of Action potentials?

After-hyperpolarisation

Example of graded depolarisation

The more you stretch a receptor, the more it gets depolarised

2nd step of action potentials

Repolarisation

Area 1 and 2 are the

absolute refractory period

Area 3 is the

relative refractory period

If you used a second stimulus to try and stimulate the same neuron during this period, it wont do anything.

Absolute refractory period

Why will a neuron not be stimulated in the absolute refractory period?

Because all voltage gated ion channels already open after threshold, so there is no more to open

During this period the same neuron can be stimulated if a larger stimulus is used

Relative refractory period

during Relative refractory period, what is key to stimulating the neuron again?

A larger stimulus for action potential

After-hyperpolarisation AHP. Membrane potential shifts towards EK+ since PK/Pna becomes

~100:1

When MP reaches the ————- there is a sudden activation (opening) of voltage-gated —— channels (PNa increase). At this moment PK/PNa is 1: 20 (before it was 40:1), therefore MP shifts towards the ENa+ towards +60 mV = overshoot.

threshold, Na+

Opening of voltage-gated —- channels is short-lasting, as these channels inactivate quickly. This is followed by transient opening of voltage-gated —- channels, leading to repolarisation

Na+, K+

What happens when membrane potential reaches the threshold?

Sudden opening of voltage-gated Na+ channels (PNa increase)

When the voltage-gated Na+ channels open, they inactivate quickly. What happens next?

transient opening of voltage-gated K+ channels, leading to repolarisation

What is happening during purple period?

Resting membrane potential. Na+ channels are in the resting state and K+ channels are closed

What is happening during the blue period?

Stimulus causes depolarisation until threshold is reached

What is happening during the green period?

Voltage gated Na+ channel activation gates opened

What is happening during the yellow period?

Voltage gated K+ channels are open, Na+ channels are inactivating

What is happening during the orange period?

Also known as the relative refractory period, Voltage gated K+ channels are still open, Na+ chanels in resting state

What two periods make up the absolute refractory period?

Green and yellow

Activation gate closed, inactivation gate open- RMP

Voltage-gated Na+ channel in resting state

Depolarisation to threshold

When the voltage threshold is reached, sodium channels open and Na+ ions move into the cell along both the concentration and electrical gradient.

Influx of Na+ slows down and stops when:

The inside potential becomes positive (moves towards ENa) and thus attracts Na+ less, Na+ channels inactivate

Peak of arc, after a fraction of a millisecond the inactivation gate closes

Back to initial resting state when membrane repolarises (when it goes under threshold)

RMP, first step

Depolarisation to threshold, step 2

Inactivation gate, peak of arc, step 3

back to initial state after repolarising, under threshold, last step

When the cell stimulus allows the cell to reach threshold, your activation gate will now open, Na rushes in contributing to depolarisation phase. After being open for a fraction of a millisecond, Na+ channels inactivate, ball and chain shut channel.

Name each step of the line

RMP, depolarisation to threshold, fast depolarisation, repolarisation, after-hyperpolarisation

Why is each action potential is an all- or-none event?

Once voltage gated ion channels are open, they are ALL open. Unlike graded depolarisation or hyperpolarisations.

Frequency ______ with more stimulus

Increases

If a large signal needs recognising, what gives information about it?

the change in frequency of action potential, not amplitude of ap. This is because a large stimulus may invoke several APs closer together, but all APs are same.

Normals amplitude of an Action potential?

~ 100 mV

amplitude of action potentials does not depend on the ‘stimulus’ intensity as long as the stimulus is ————

above the threshold