the nervous system j. gilbert march 2004 biologymad.com
TRANSCRIPT
The Nervous System
J. Gilbert
March 2004
BiologyMad.com
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?
SynapsesBiologyMad.com
Nerve Impulses
Nerve 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.
BiologyMad.com
Menu
Resting Membrane Potential
Resting 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.
BiologyMad.com
Menu
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.
BiologyMad.com
Menu
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 +
cellm embrane
outside
inside
Na KNa K ATPase+ +
ATP ADP+P i
closed(leak)
closed(leak)
+
-BiologyMad.com
Menu
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 surroundingsThe resting membrane potential is –70mV
Menu
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
Menu
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.
BiologyMad.com
Menu
How do Nerve Impulses Start?
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.
Menu
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.
Menu
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.
Menu
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
BiologyMad.com
Menu
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
BiologyMad.com
Menu
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
BiologyMad.com
Menu
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
BiologyMad.com
Menu
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
Menu
AP - Overview
BiologyMad.com
Menu
(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
Menu
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.
BiologyMad.com
Menu
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.
BiologyMad.com
Menu
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.
BiologyMad.com
Menu
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.
BiologyMad.com
Menu
Action Potential
The sodium/potassium pump restores the resting concentrations of sodium and potassium ions
BiologyMad.com
Menu
Action Potential
BiologyMad.comBiologyMad.com
Menu
BiologyMad.com
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
Menu
BiologyMad.com
BiologyMad.com
AP - Refractory Period
Absolute refractory period = During the action potential, a second stimulus will not cause 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
BiologyMad.com
How Fast are Nerve Impulses?
BiologyMad.com
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
Menu
BiologyMad.com
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
Menu
BiologyMad.com
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.
Menu
Na+
Sodium channel
Na+ Na+
Nodes of Ranvier BiologyMad.com
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
BiologyMad.com
Synapses
Junction between two neurones is called a synapse
An AP cannot cross the synaptic cleft Impulse is carried by chemicals called
neurotransmitters
Menu
BiologyMad.com
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
Menu
BiologyMad.com
Synapses
Menu
Click here for animation BiologyMad.com
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
Menu
BiologyMad.com
Synapses
Calcium ions cause synaptic vesicles to fuse with the cell membrane
Neurotransmitters diffuse across the synaptic cleft
Menu
BiologyMad.com
Synapses
Neurotransmitter binds to neuroreceptors in the post-synaptic membrane
Channels open, Na+ flow in
Causes depolarisation AP initiated in post-
synaptic neurone
Menu
BiologyMad.com
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.
Menu
BiologyMad.com
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
Menu
BiologyMad.com
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
Menu
BiologyMad.com
Neurotransmitters
Noradrenaline– Released by neurones in the sympathetic
nervous system– Adrenergic synapses