Neurones and nervous transmission

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  • Created by: Emily.T
  • Created on: 20-01-18 19:56
What is the nervous system made up of?
A complex network of cells called neurones
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What are neurones?
Specialised nerve cells that transmit electrical impulses around the body to respond to changes in the internal & external environment
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What type of neurones are there?
Sensory neurones, relay neurones and motor neurones
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What do sensory neurones do?
They transmit nerve impulses from receptors to the central nervous system (CNS)
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What do relay neurones do?
Transmit nerve impulses between sensory neurones and motor neurones
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What do motor neurones do?
Transmit nerve impulses from the CNS to effectors
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What is the central nervous system (CNS) made up of?
The brain and spinal cord
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What is a stimulus?
An internal or external environmental change which is detected by receptor cells which then sends a nerve impulse along a sensory neurone
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What is the structure of a neurone?
Cell body, dendrons, axons
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What does the cell body of a neurone contain?
Nucleus, vast amounts of ER to produce vesicles, mt for ATP production (to send impulse)
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What is a dendron?
An extensions from the cell body - multiple and shorter dendrons are called dendrites- transmits impulse TOWARDS cell body
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What are axons?
Elongated nerve fibres - transmit impules AWAY from cell body
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What type of axons and dendrites do each type of neurone have?
Sensory: long dendron & short axon Relay: short axons & short dendrites Motor: long axon & short dendrites
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What are myelinated neurones?
Neurones where the axon is covered in a myelin sheath, which is produced by Schwann cells and is a bilayer plasma membrane providing an insulating layer. Between the Schwann cells are nodes of Ranvier
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What does a myelinated sheath do?
It allows neurones to conduct the impulse at a much faster speed (up to 100m/sec) as depolarisation only occurs at the nodes of Ranvier
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What are nodes of Ranvier?
They are spaces between the Schwann cells, where sodium ion channels are concentrated there, where depolarisation only happens there so the impulse 'jumps' from node to node
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What do sensory receptors do?
They convert the stimulus detected into a nerve impulse. This info is passed through the NS to the CNS (brain)
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What are the two main features of a sensory receptor?
1) Specific to a single type of stimulus 2) Act as a transducer - convert a stimulus into an impulse
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What are the 4 different types of sensory receptors?
1) Mechanoreceptor 2) Chemoreceptor 3) Thermoreceptor 4) Photoreceptor
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What stimulus does a mechanoreceptor respond to?
Pressure/movement, e.g. pacinian corpuscle - skin
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What stimulus does a chemoreceptor respond to?
Chemicals, e.g. offactory (smells) - nose
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What stimulus does a thermoreceptor respond to?
Heat, e.g. end-bulbs of Krause - tongue
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What stimulus does a photoreceptor respond to?
Light, e.g. cone cell (detects light wavelengths) - eye
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What is a sensory neurone's role as a transducer?
To detect a range of different stimuli from sensory organse & converts it into a nervous impulse = generator potential
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What is the Pacinian corpuscle?
A specfic sensory receptor which detects mechanical pressure. Located deep in skin - most abundant in fingers, soles of feet & joints. End of sensory neurone is in the centre & surrounded by layers of connective tissue, each layer is separated by gel
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What are the sodium ion channels in the Pacinian corpuscle responsibel for?
Transporting Na+ across membrane. The neurone ending has stretch-mediated sodium (Na) channels - when they change shape their permeability to sodium changes (the channels open)
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How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse? (1)
Rest state: stretch-mediated Na channels too narrow for Na+ to pass through (has a resting potential), pressure applied to P.c. changes shape so mem around neurone stretches & Na+ channels widen & Na+ diffuse into neurone
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How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse? (2)
Influx of +ve Na+ changes mem potential = depolarised - creates a generator potential which creates an action potential (nerve impulse) that passes along sensory neurone
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What does it mean when a neurone is at 'resting potential'?
The neurone is not transmitting an impulse - the outside of the mem is more positively charged than inside as there are more +ve ions outside the axon - is polarised with a potential difference of -70mV
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How is a resting potential created and maintained in a neurone?
Sodium-potassium pumps actively pump out 3 Na+ and pump in 2 K+ = more Na+ outside than K+ inside as the mem is impermeable to Na+ so cannot diffuse back in = Na+ electrochemical gradient - K+ diffuse out via potassium ion channels
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What is an action potential?
The change in the potential difference across the neurone membrane of the axon when stimulated (approx +40mV)
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What is the 1st part of an action potential?
Stimulus excites the neurone mem = sodium voltage-gated ion channels open = mem more permeable to Na+ so they diffuse into neurone down Na+ electrochemical grad - makes inside of neurone LESS -ve
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What is the 2nd part of an action potential?
Change in charge = more Na+ channels open to allow more Na+ to diffuse into axon = positive feedback
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What is the 3rd part of an action potential?
When the potential difference is at +40mV (depolarisation), V-G Na+ channels close & V-G K+ channels open so Na+ no longer able to enter axon, but mem is more permeable to K+
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What is the 4th part of an action potential?
K+ diffuse out of axon down electrochemical grad, reducing the charge = inside of axon is more -ve than the outside. Is negative feedback during repolarisation
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What is the 5th part of an aciton potential?
K+ channels are too slow to close so too many diffuse out of neurone - insdie becomes MORE -ve (relative to outside) than normal resting state = hyperpolarisation. V-G K+ channels close & Na-K pump kicks in, returns to resting potential - repolarised
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What is depolarisation?
A change in potential difference from negative (-ve) to postive (+ve) across the mem of a neurone
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What is hyperpolarisation?
A change in an axon mem's potential that makes it more negative than the resting potential
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What is repolarisation?
A change in potential difference from postive back to negative across the mem of a neurone
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What is the refractory period?
This is a period of time after an action potential when the axon cannot be excited again. During this the V-G Na+ channels remain closed, preventing movement of Na+ into the axon
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What do refactory periods do?
They prevent the propogation of an action potential going backwards along the axon as well as forwards & makes sure action potentials are unidirectional and that they don't overlap and occur as discrete impulses
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What is the propagation of action potentials?
A nerve impulse is an action potential that starts at one end of the neurone and is propagated (transmitted) along the axon to the other end of the neurone
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How is an aciton potential propagated along an axon?
The initial stimulus causes change in sensory rec, triggering action potential so 1st region of axon mem is depolarised. Acts as stimulus for depolarisaiton of next region. Process continues along length of axon = wave of depolarisation
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How is the depolarisation of the next region triggered in the propagation of action potentials?
When Na+ are inside the axon, they are attracted by the negative charge ahead & the conc. grad. to diffuse further along inside the neurone.
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What is the 1st part in the propagation of action potentials?
Resting pot = Na+ conc outside axon is high relative to inside. K+ conc inside is high relative to outside. Overall conc of +ve ions is greater outside makingn it positive compared to inside. Mem is polarised.
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What is the 2nd part in the propagaiton of action potentials?
The stimulus causes an influx of Na+, reversing the charge of the mem = action potential & mem is depolarised
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What is the 3rd part in the propagaiton of action potentials?
Na+ influx causes V-G Na+ channels to open further along ax. Influx of Na+ in this region = depolarisation. Behind this new region of depolarisation, V-G Na+ channels close & K+ ones open. K+ diffuse out of axon
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What is the 4th part in the propagaiton of action potentials?
The action potential (depolarisation) continues along axon. K+ leave = mem behind action potential returns to original state (+ve out, -ve in) = repolarised
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What is the 5th part in the propagaiton of action potentials?
Following repolarisation, the axon mem returns to resting potential in preparation for a new stimulus
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What is saltatory conduction?
Where the nerve impulse 'jumps' from node of Ranvier to another to speed up the transmission
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What type of neurone does saltatory conduction occur in and why?
Neurones which are covered in a myelin sheath (usually sensory) as depolarisation of the axon mem only occurs at the nodes of Ranvier
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How does saltatory conduction work?
During depolarisation at the nodes, Na+ pass through the protien channels in the mem, so longer localised circuits arise adjacent nodes, so aciton potential 'jumps' = much faster than a wave of depolarisation along entire axon mem
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How is saltatory conduction a more energy efficient way of transmission?
Repolarisation uses ATP in the Na pump, so reducing the amount of repolarisation means the conduction of impulses is more energy efficient (saves energy)
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What is a localised circuit in saltatory conduction?
Repolarisation uses ATP in the Na-K pump, so reducing the amount of repolarisation decreases the amount of Na+ & K+ needed to be pumped to restore resting potential, saving energy
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What two factors affect the speed of transmisison?
1) Axon diameter 2) Temperature
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How does axon diameter affect the speed of transmisison?
The bigger the diameter, the faster the transmission due to less resistance to ion flow in the cytoplasm
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How does temperature affect the speed of transmission?
The higher the temperature, the faster the impulse due to faster ion diffusion. This usually only occurs up to 40'C as above that causes proteins to denature (such as Na-K pump)
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What is the all or nothing principle?
Where a certain level of stimulus, the threshold value, always triggers a response. Once the threshold value is reached an action potential is always created. No matter the stimulus' size, the same sized aciton potential will always be triggered
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What happens if the threshold value is not reached?
No action potential will be triggered
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What does the size of the stimulus affect?
The numer of aciton potentials that are generated in a given time. The larger the stimulus, the more frequently the action potentials are generated.
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