Nerve impulses

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  • Created by: Hindleyc
  • Created on: 09-04-19 14:51
What do all neurones have
The same basic structure but different types of nerve cells are specialised for their particular functions
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What does a neurone consist of
A cell body containing the nucleus with one or more long thin structures called processes.
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What are processes
extensions of the cytoplasm surrounded by a plasma membrane
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What are the processes that conduct nerve impulses towards the cell body called
Dendrons
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What do dendrons have
Very fine processes called dendrites
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What are the processes that carry nerve impulses away from the cell body called
Axons
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What are the axons of many neurones surrounded by
Schwaan cells which form a myelin sheath
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Why do they do this
Because as they grow the Schwann cells twist around the axon several times
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What Is the function of the myelin sheath
Increase the speed of conduction of the nerve impulse, electrically insulate the axons from each other
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What are the junctions between adjacent Schwann cells
Nodes of ranvier
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What do neurones link together at
Gaps called synapses
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Where do these mainly occur
in the CNS
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What can one single neurone have
many synapses with other neurones
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What are synapses
the only way In which impulses can pass from one neurone to the next
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At this junction what is there a gap of
about 20nm
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What is transmission across the gap via
chemical substances
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What does a nerve consist of
A bundle of neurones
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What does the cell body contain
dendrites that connect with other neurones to dendrons with a granular cytoplasm containing nissl granules (protein)
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What is the nerve fibre
axon with insulator of myelin sheath
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direction of impulse
--->
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What are axon terminals
nerve endings connecting with muscle or gland
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What does the cell membrane separate
2 electrical environments -1=extracelluar solution containing Na and Cl ions 2=intracellular solution contains more potassium
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What are all of these ions
Small and could find a way to cross the membrane using facilitated diffusion
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When a neurone isn't transmitting an impulse what is the outside of the axon
more positively charged compared with the inside
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What is the potential difference (voltage) called
the resting potential
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About
-70mv
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What does this negative indicate
The inside of the cell is less +ve (relatively) than the outside hence the -ve charge
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What are both the intra and extra cellular environments
Positively charged however 1 area may be more/less positive than the other
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What do different chemicals have
Different charges
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What is membrane said to be
polarised at this point
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What does stimulus do
cause PD to temporarily reverse Na floods in so inside +ve relative to outside
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How does RP exist
When the cell isn't electrically active (doesn't mean cell isn't doing anything)
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What is K+ conc
Higher inside the neurone than outside
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What is Na+ conc
Higher outside the neurone than inside
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What is a membrane with a potential difference across it said to be
Polarised
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When a membrane loses its potential difference what doesn't it have
a charge and is said to be depolarised
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What happens if cyanide is added to axon
Resting potential dissapears bc no ATP being produced- adding ATP to the axon restores the resting potential so ATP must therefore be needed to maintain the resting potential
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What are the 2 mechanisms involved in producing an AP
There is a Na+/K+ pump in the cell membrane of the axons which consists of the enzyme ATPase
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What is the pump
carrier protein and so it is called the Na+/K+ ATPase pump
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What is it used for
To pump Na+ and K+ in opposite directions across the axon membrane
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What is the Na+/K+ ATPase pump (Carrier proteins located on the CSM)
An AT mechanism maintaining a surplus of K+ inside the axon and a surplus of Na+ outside the axon.
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What happens each time the pump works
3 Na+ out and 2K+ moved in
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2nd way The resting potential is also established in the axon membrane
Permanently open diffusion channels in the membrane allowing limited passive diffusion of Na+ ions into the axon but more rapid diffusion of K+ out of the axon
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But what is the diffusion of K+
More rapid because K+ have a membrane permeability 20x greater than Na+ (far more K+ channels than Na+ channels so far more K+ moves out)
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What is the loss of K+
Greater than Na+ gain because the Na+/K+ ATPase pump moves more ions than are moved by FD and so a -ve charge build up inside the cell and a more +ve charge outside axon
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What is the Na channel
Closed
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What is K+ cahnnel
Closed but leaky
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What does resting potential mean
Gives resting neurone potential to transmit a nerve impulse
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Model answer, how the RP is established in an axon by the movement of ions across the membrane
AT pump Na+ out of AXON, Diff of K+ out of the axon and little diffusion of Na+ into axon
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What is happening during the resting potential
-70mv membrane potential, Na+ channel close and K+ channel closed
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Starting to depolarise
-50mv, Na+ channel open and K+ channel closed
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Repolarising
-20mv, Na+ closed and K+ open
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What is the action potential
When a N is stimulated a message passes down the length of its axon called a wave of depolarisation
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Why does it happen
Because of channels in the membrane of the axon that can change shape
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In the open position what do they do
Allow ions to pass through but in the closed position they block ion movement
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Because they can be open or closed they are called
Gated ion channels
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Whats a nerve impulse to
Changes in permeability of nerve cell membrane to K+ and Na+ ions
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What does this lead to
Changes in the potential difference and action potential
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What is resting potential
Store of potential energy in the axon
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What does the neurone use
This energy to produce an action potential
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What is the nerve impulse
Passage of an Action potential along an axon
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As well as the permanently open diffusion channels what are there also
Voltage gated channels for Na+ and K+ in the axon membrane that open when the voltage across the membrane falls
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What is regulation mainly achieved by
The Na+ gate which is closed in a resting cell
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What is this why
Membrane isn't very permeable to Na+ when resting
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When part of the membrane depolarises what happens
Some of the gated channels open Na+ diffuses in along their conc gradient and some +ve ions are pushed along the axon to the next part of the membrane so it becomes slightly depolarised
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Then what happens
Further sodium diffusion channels then start to open because they are sensitive to depolarisation of the membrane and that part of the membrane fully depolarises
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so
influx of Na+ reverses polarity of the membrane which pushed Na+ ahead to the next section
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What does PD peak at, why
Around +35mv because max conc of Na+ inside the axon
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What is this change of potential from -ve to +ve which travels down the axon membrane is called the
action potential
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What does depolarisation occur in
Short stretch of membrane and then recovers its resting potential and the AP moves on down the axon
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How does the membrane recover its -ve resting potential
Because of the gated K+ channels- shortly after Na+ gated channels let Na+ in (depolarising the membrane) K+ channels open
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Because of the high conc of K+ inside than out what happens
K+ moves out resulting in a net +ve charge outside and forming the resting potential once more
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What is this called
Repolarisation
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What does the membrane do
Return towards resting potential but slightly overshoots (hyper polarisation) because the membrane is slightly more permeable to K+ than usual due to the delay in closing all the K+ gates compared to the Na+ gates
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Card 5

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