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The Nervous

System

OverviewNerve Impulses

(completed12/03/04)

Resting Membrane Potential(completed12/03/04)

How do nerve impulses start?(completed 19/03/04)

Action Potential(completed 19/03/04)

How Fast are Nerve Impulses?

Synapses

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Nerve ImpulsesNerve Impulses

� Neurones send messages

electrochemically – this means that

chemicals cause an electrical impulse.

� Chemicals in the body are ‘electrically

charged’ when they have an electrical

charge, they are called ions.

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Resting

Membrane

PotentialResting Membrane Potential

� When a neurone is not sending a signal, it is

at ‘rest’.

– The inside of the neurone is negative relative to

the outside.

– K+ can cross through the membrane easily

– Cl- and Na+ have a more difficult time crossing

– Negatively charged protein molecules inside the

neurone cannot cross the membrane.

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Resting Membrane Potential

� The membranes

contain sodium-

potassium pumps

(Na+K+ATPase).

– Uses ATP to

simultaneously pump

3 sodium ions out of

the cell and 2

potassium ions in.

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Resting Membrane Potential

� There are also sodium and potassium ion

channels in the membrane.

– These channels are normally closed, but even

when closed, they ‘leak’, allowing sodium ions to

leak in and potassium ions leak out – down their

concentration gradients.3Na

+

2K+

cellmembrane

outside

inside

Na KNa K ATPase+ +

ATP ADP+Pi

closed(leak)

closed(leak)

+

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Resting Membrane Potential

Ion Concentration inside

cell/mmol dm-3

Concentration outside

cell/mmol dm-3

K+ 150.0 2.5

Na+ 15.0 145.0

Cl- 9.0 101.0

�The imbalance of ions causes a potential

difference (or voltage) between the inside of

the neurone and its surroundings

�The resting membrane potential is –70mV

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Resting Membrane Potential

� Overall:– K+ pass easily into the cell

– Cl- and Na+ have a more difficult time crossing

– Negatively charged protein molecules (A-) inside the neurone cannot pass the membrane.

– The Na+K+ATPase pump uses energy to move 3 Na+ out for every 2K+ in to neurone

�This imbalance in voltage causes a potential difference across the cell membrane – called the resting membrane potential. BiologyMad.com

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Resting Membrane Potential

� Membrane potential is always negative inside the cell.

� The Na+K+ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them.

– Plant cells don’t need this as they have strong cells walls to prevent bursting.

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How do Nerve

Impulses Start?

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How do Nerve Impulses Start?

� Neurones are stimulated by receptor cells

– These contain special sodium channels that are

not voltage-gated, but are gated by the

appropriate stimulus.

� stimulus causes the sodium channel to open

– Causes sodium ions to flow into the cell

– Causes a depolarisation of the membrane

potential � affects the voltage-gated sodium

channels nearby and starts an action potential.

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How do Nerve Impulses Start?

� Some examples:

– chemical-gated sodium channels in

tongue taste receptor cells open when a

certain chemical in food binds to them

– mechanically-gated ion channels in the hair

cells of the inner ear open when they are

distorted by sound vibrations; and so on.

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How do Nerve Impulses Start?

In each case the correct stimulus causes the sodium channel to open (reaches the

threshold value)

↓

causes sodium ions to flow into the cell

↓

causes a depolarisation of the membrane potential

↓

affects the voltage-gated sodium channels nearby and starts an action potential.

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Action Potential

Action Potential (AP)

� The resting potential tells about what

happens when a neurone is at rest.

� An action potential occurs when a

neurone sends information down an

axon.

– Is an explosion of electrical activity

– The resting membrane potential changes

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AP - Depolarisation

� Resting potential is –70mv (inside the axon). When stimulated, the membrane potential is briefly depolarised– Stimulus causes the membrane at one part of the neurone to increase in permeability to Na+ ions

– Na+ channels open. This causes resting potential to move towards 0mV

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AP - Depolarisation

� When depolarisation reaches –30mV

more Na+ channels open for 0.5ms

– Causes Na+ to rush in � cell becomes

more positive

Na+

out

in

K

closed(leak)

open

+

-

Na

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AP - Repolarisation

� At a certain point, the depolarisation of

the membrane causes the Na+

channels to close

� This causes K+ channels open

out

in

Na

closed(leak)

open

+

-

K+

K

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AP - Repolarisation

� K+ rush out � making inside the cell more negative. – Since this restores the original polarity, it is called repolarisation

– There is a slight ‘overshoot’ in the movement of K+ (called hyperpolarisation).

– Resting membrane potential is restored by the Na+K+ATPase pump

out

in

Na

closed(leak)

open

+

-

K+

K

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AP - Overview

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(Click here for animation)

AP – All or nothing

� AP only happens if the stimulus reaches a

threshold value

– Stimulus is strong enough to cause an AP

– It is an ‘all or nothing event’ because once it

starts, it travels to the synapse.

� AP is always the same size

� Frequency of the impulse carries information

� strong stimulus = high frequency

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Action Potential

� At rest, the inside of the neuron is slightly negative

due to a higher concentration of positively charged

sodium ions outside the neuron.

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Action Potential

� When stimulated past the threshold, sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon.

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Action Potential

� The region of positive charge causes nearby

sodium channels to open. Just after the sodium

channels close, the potassium channels open wide,

and potassium exits the axon.

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Action Potential

� This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon.

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Action Potential

� The sodium/potassium pump restores the resting

concentrations of sodium and potassium ions

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Action Potential

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AP – Refractory Period

� There is a time after depolarisation where no new AP can start – called the refractory period.– Time is needed to restore the proteins of voltage sensitive ion channels to their original resting conditions

– NA+ channels cannot be opened, as it can’t be depolarised again

– Therefore impulses travel in one direction

– Can last up to 10 milliseconds – this limits the frequency of impulses

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AP - Refractory Period

� Absolute refractory period = During the action potential, a second stimulus will notcause a new AP

� Exception: There is an interval in which a second AP can be produced but only if the stimulus is considerably greater than the threshold = relative refractory period

� Refractory period can limit the number of AP in a given time.

� Average = about 100 action potentials/s

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How Fast are

Nerve Impulses?

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How fast are impulses?

� AP can travel 0.1-100m/s along axons

� Allows for fast responses to stimuli

� Speed is affected by:

– Temperature

– Axon diameter

– Myelin sheath

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Myelinated Neurones

� The axons of many neurones are encased in a fatty myelin sheath (schwann cells).

� Where the sheath of one Schwann cell meets the next, the axon is unprotected.

� The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier).

Na+Na+ Na+

Sodium channel Nodes of Ranvier

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Myelinated Neurones

� The in rush of sodium ions at one node creates just enough depolarisation to reach the threshold of the next.

� In this way, the action potential jumps from one node to the next (1mm) – called saltatory propagation (click here for animation)

– Results in much faster propagation of the nerve impulse than is possible in nonmyelinated neurons.

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Na+

Sodium channel

Na+ Na+

Nodes of Ranvier BiologyMad.com

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Facts about Propagation

� Nerve impulse conduction is really the bumping of positive charge down the axon

� AP initiated at one end of the axon is only propagate in one direction.

– The AP doesn’t turn back because the membrane just behind is in its refractory period i.e. voltage gated Na+ channels are inactivated

Facts about propagation

� To increase conduction velocity:– Increase the axonal diameter

– Myelin of the axon facilitates current flow down the inside of the axon. • Breaks in the myelin wrapping occur at the Nodes of Ranvier, which have increased concentrations of voltage gated Na+ channels. Regeneration of the AP occurs at the nodes

� Saltatory conduction – propagation and regeneration of an AP down myelinated axon

� E.g. Local anaesthesia temporarily blocks AP generation by binding the interior of voltage gated Na+ channels

Synapses

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Synapses

� Junction between two neurones is

called a synapse

� An AP cannot cross the synaptic cleft

� Impulse is carried by chemicals called

neurotransmitters

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Synapses - Neurotransmitters

� Neurotransmitters are made by the cell

sending the impulse (the pre-synaptic

neurone) and stored in synaptic

vesicles at the end of the axon

� The cell receiving the impulse (post-

synaptic neurone) has chemical gated

ion channels called neuroreceptors

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Synapses

�

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Synapses

� At the end of the pre-

synaptic neurone there

are voltage gated

calcium channels.

� When AP reaches the

synapse, the channels

open

� Calcium ions flow into

the cell

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Synapses

� Calcium ions cause

synaptic vesicles to

fuse with the cell

membrane

� Neurotransmitters

diffuse across the

synaptic cleft

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Synapses

� Neurotransmitter binds to neuroreceptors in the post-synaptic membrane

� Channels open, Na+

flow in

� Causes depolarisation

� AP initiated in post-synaptic neurone

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Synapses

� Function:– Prevents impulses travelling in the wrong direction. • An impulse can pass along an axon in either direction, but can only cross a synapse in one direction because the synaptic vesicles are only found in the synaptic knobs and end plates

– A vast number of synaptic connections allow for great flexibility. They are equivalent to the switchboard in an elaborate telephone exchange enabling messages to be diverted from one line to another and so on

Integrating Signals

� If the diffusion

of ions reaches

a threshold

value, it will

cause the AP

in the

postsynaptic

membrane.

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Neurotransmitters

� Neurotransmitters are broken down by a

specific enzyme in the synaptic cleft.

� Breakdown products are absorbed by the

pre-synaptic neurone

� Used to re-synthesise more neurotransmitter

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Neurotransmitters� Acetylcholine (ACh)

– Released by motor neurones onto skeletal muscle cells

– Released by neurones in the parasympathetic nervous system

– Cholinergic synapses

– Ach is removed from the synapse by acetylcholinesterase• Nerve gasses used in warfare (e.g. sarin) and the organophosphate insecticides (e.g. parathion) achieve their effects by inhibiting acetylcholinesterase this allowing Ach to remain active.

• Atropine is used as an antidote because it blocks ACh receptors

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Neurotransmitters

� Noradrenaline

– Released by neurones in the sympathetic

nervous system

– Adrenergic synapses