topic 9.5 - nervous transmission

Question 1

what do nerve impulses depend on

Answer 1

• concentration of sodium ions (Na+) & potassium ions (K+) outside axon is different than the concentration inside axon

Question 2

how is a resting potential created

Answer 2

• due to the unequal distribution of Na+ and K+ ions across the membrane

Question 3

nerve cells polarised - explain

Answer 3

• when the inside of the cell is slightly negative that the outside of the cell

Question 4

how is resting potential created and maintained

Answer 4

by the sodium potassium ion pump

Question 5

sodium potassium ion pump - how it works

Answer 5

• 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

Question 6

what helps with the transport of ions across the membrane?

Answer 6

carrier proteins.
- 2 K+ ions into the axon
- 3 Na+ ions actively transported out the axon

Question 7

what two factors contribute to resting potential

Answer 7

1. transfer of 3 Na+ out of axon for every 2 K+ in
2. outwards movement of K+ ions down electrochemical gradient via faciltated diffusion

Question 8

what is the resting potential value

Answer 8

-70mV

Question 9

what is an action potential

Answer 9

• 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

Question 10

what triggers the action potential

Answer 10

• 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

Question 11

how does the inside of the cell become +40mV

Answer 11

due to Na+ ions; polarity changes from negative to positive

Question 12

what can be the causes of these changes

Answer 12

• light
• sound
• touch
• taste
• smell in sensory neuron
• arrival of neurotrasnmitter in motor neuron

Question 13

stages that generate action potential

Answer 13

• depolarisation
• repolarisation
• hyperpolarisation
• recovery

Question 14

depolarisation

Answer 14

• 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

Question 15

repolarisation

Answer 15

• K+ channels open
• K+ diffuses out of the cell taking positive charge with it (facilitated diffusion)
• inside becomes more negative
• K+ channels close

Question 16

hyperpolarisation

Answer 16

• large amount of K+ ions diffuse out the axon
• inside of the axon becomes more negative than the resting potential

Question 17

recovery

Answer 17

• K+ attracted back into cell by negative charge when membrane is hyperpolarised
• p.d. rises
• resting potential equilibrium stored

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Question 18

what is the refractory period

Answer 18

• 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

Question 19

how is a nerve impulse propogated

Answer 19

• 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

Question 20

saltatory induction

Answer 20

• nodes of Ranvier are the only regions of a myelinated nerve fibre that can depolarise
• impulse ‘jumps’ from one node to next

Question 21

Schwann cells

Answer 21

• depolarisation of one node causes depolarisation of the next node
• much faster conduction along nerve fibres

Question 22

why is speed of transmission greater along myelinated axons than non-myelinated axons?

Answer 22

• in myelinated neurons ions can only pass in/out of axon at the nodes of Ranvier (mo myelin sheath)
• myelin sheath is impermeable

Question 23

what is a synapse

Answer 23

• a junction between two neurons
• nerve impulses cross a synapse via neurotransmitters

Question 24

what is a synaptic cleft

Answer 24

• the gap between the pre and post synaptic membranes in synapse

Question 25

synaptic knobs

Answer 25

• bulges at the end of presynaptic neuron where neurotransmitters are made

Question 26

how do synapses work

Answer 26

1. action potential arrives at presynpatic knob
2. calcium ion channels open → calcium ions diffuse into neuron
3. vesicles fuse with the presynaptic membrane → neurotransmitter released
4. neurotransmitter diffuses across synaptic cleft
5. neurotransmitter binds to receptors on postsynaptic membrane
6. postsynaptic membrane may depolarise

this can have 2 effects; excitation or inhibition

Question 27

neurotransmitter in PNS

acetylcholine (ACh)

Answer 27

• 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

Question 28

what are nerves that use ACh called

Answer 28

cholinergic nerves

Question 29

what is the enzyme that breaks down ACh

Answer 29

acetylcholinesterase

Question 30

neurotransmitter in PNS

noroadrenaline

Answer 30

• 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

Question 31

what are nerves that use noroadrenaline called

Answer 31

adergenic nerves

Question 32

excitatory postsynaptic potentials (EPSP)

Answer 32

• Na+ ion channels open →lots of Na+ enter nerve fibre
• p.d. is lowered
• reduced p.d. raises membrane to threshold potential → an action potential is created

Question 33

how are the sodium channels opened?

Answer 33

• neurotransmitters bind to specific protein receptors on sodium channels of postsynaptic membrane
• this stimulates the opening of Na+ channels

Question 34

inhibitory postsynaptic potential

Answer 34

• different ion channels open in membrane → movement of negative ions
• inside is more negative than normal resting potential
• less likely that action potential will occur
• p.d. becomes more negative → hyperpolarisation

Question 35

how are the anion channels opened?

Answer 35

• neurotransmitters bind to specific protein receptors on postsynaptic membrane
• neurotransmitter has opposite effect
• stimulates opening of anion channels