topic 9.5 - nervous transmission Flashcards
what do nerve impulses depend on
- concentration of sodium ions (Na+) & potassium ions (K+) outside axon is different than the concentration inside axon
how is a resting potential created
- due to the unequal distribution of Na+ and K+ ions across the membrane
nerve cells polarised - explain
- when the inside of the cell is slightly negative that the outside of the cell
how is resting potential created and maintained
by the sodium potassium ion pump
sodium potassium ion pump - how it works
- sodium-potassium pump creates a concentration gradient across the membrane
- ATP broken down into ADP - need energy
- two K+ ions diffuse out the axon
- electrochemical gradient will pull K+ ions back into cell
- three Na+ ions are actively transported out of the axon
- conc of sodium is lowered inside the axon - cannot diffuse back in
what helps with the transport of ions across the membrane?
carrier proteins.
- 2 K+ ions into the axon
- 3 Na+ ions actively transported out the axon
what two factors contribute to resting potential
- transfer of 3 Na+ out of axon for every 2 K+ in
- outwards movement of K+ ions down electrochemical gradient via faciltated diffusion
what is the resting potential value
-70mV
what is an action potential
- a change to the potential difference at a point on the membrane of the nerve cell in response to the transmission of a nerve impulse
what triggers the action potential
- triggered by the depolarisation of nearby membrane causing a change in p.d. to threshold potential
- inside of the cell changes from -70mV to +40mV
how does the inside of the cell become +40mV
due to Na+ ions; polarity changes from negative to positive
what can be the causes of these changes
- light
- sound
- touch
- taste
- smell in sensory neuron
- arrival of neurotrasnmitter in motor neuron
stages that generate action potential
- depolarisation
- repolarisation
- hyperpolarisation
- recovery
depolarisation
- neuron stimulated - excitation of neuron cell triggered
- voltage gated sodium channels open
- Na+ diffuses into cell carrying positive charge
- inside is more positive
- p.d. is now +40mV
- voltage gated sodium channels close
lasts around 1ms
repolarisation
- K+ channels open
- K+ diffuses out of the cell taking positive charge with it (facilitated diffusion)
- inside becomes more negative
- K+ channels close
hyperpolarisation
- large amount of K+ ions diffuse out the axon
- inside of the axon becomes more negative than the resting potential
recovery
- K+ attracted back into cell by negative charge when membrane is hyperpolarised
- p.d. rises
- resting potential equilibrium stored
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what is the refractory period
- time it takes for an area of the axon membrane to recover after action potential (go back to resting potential)
- depends on sodium/pottasium pump and on membrane permeability to pottasium ions
how is a nerve impulse propogated
- sodium ions entering during action potential flow along inside of nerve fibre
- addition positive charge reduces the p.d. to threshold
- voltage-gated sodium channels open
- Na+ flows both ways along the membrane - refractory period prevents impulse from returning
saltatory induction
- nodes of Ranvier are the only regions of a myelinated nerve fibre that can depolarise
- impulse ‘jumps’ from one node to next
- the mechanism by which the speed is increased
Schwann cells
- depolarisation of one node causes depolarisation of the next node
- much faster conduction along nerve fibres
why is speed of transmission greater along myelinated axons than non-myelinated axons?
- in myelinated neurons ions can only pass in/out of axon at the nodes of Ranvier (mo myelin sheath)
- myelin sheath is impermeable
what is a synapse
- a junction between two neurons
- nerve impulses cross a synapse via neurotransmitters
what is a synaptic cleft
- the gap between the pre and post synaptic membranes in synapse
synaptic knobs
- bulges at the end of presynaptic neuron where neurotransmitters are made
how do synapses work
- action potential arrives at presynpatic knob
- calcium ion channels open → calcium ions diffuse into neuron
- vesicles fuse with the presynaptic membrane → neurotransmitter released
- neurotransmitter diffuses across synaptic cleft
- neurotransmitter binds to receptors on postsynaptic membrane
- postsynaptic membrane may depolarise
this can have 2 effects; excitation or inhibition
neurotransmitter in PNS
acetylcholine (ACh)
- synthesised in synaptic knob using ATP
- found in all nerves in voluntary & parasympathetic autonomic system
- acetylcholinesterase breaks down ACh into acetate and choline (hydrolysis)
- acetycholine usually results in excitation at the post synaptic membrane
what are nerves that use ACh called
cholinergic nerves
what is the enzyme that breaks down ACh
acetylcholinesterase
neurotransmitter in PNS
noroadrenaline
- found in nerves of sympathetic autonomic nervous system
- binds to receptors in postsynaptic membrane
- when released from postsynaptic receptors, noroadrenaline is reabsorbed into the presynaptic knob
- most is repacked and reused
what are nerves that use noroadrenaline called
adergenic nerves
excitatory postsynaptic potentials (EPSP)
- Na+ ion channels open neurotransmitters bind to specific protein receptors
- lots of Na+ enter nerve fibre
- p.d. is lowered
- reduced p.d. raises membrane to threshold potential → an action potential is created
- action potential travels along the postsynaptic membrane
how are the sodium channels opened?
- neurotransmitters bind to specific protein receptors on sodium channels of postsynaptic membrane
- this stimulates the opening of Na+ channels
inhibitory postsynaptic potential
- different ion channels open in membrane → movement of negative ions inwards
- inside is more negative than normal resting potential
- less likely that action potential will occur
- p.d. becomes more negative → hyperpolarisation
how are the anion channels opened?
- neurotransmitters bind to specific protein receptors on postsynaptic membrane
- neurotransmitter has opposite effect
- stimulates opening of anion channels
