Neuronal communication

Question 1

Explain the term refractory period and outline its importance

Answer 1

Following action potential

Need to redistribute Na and K ions to restore resting potential

Na voltage gated channels are closed

Another impulse cannot be generated

Ensures impulses are separated

Determines maximum frequency of impulse transmission

Can only pass in one direction along axon

Question 2

what is the role of sensory receptors

Answer 2

as transducers to convert stimulus into a nerve impulse

Question 3

what are sensory receptors?

Answer 3

detect change in environment and are each specific to a single type of stimulus

Question 4

give an example of a sensory receptor and explain how it works

Answer 4

the pacinian corpuscle

it detects mechanical pressure changes and is located deep within the skin

the nuerone ending has special sodium ion channels called stretch mediated sodium channels

at resting potential theses sodium channels are too narrow for sodium but when pressure is applied they change shape as membrane stretches and sodium channels widen so becomes more permeable to sodium

Question 5

describe the structure and function of a sensory neurone?

Answer 5

one long dendron carrying impulses from receptor cells to the cell body and one short axon carry impulses away from cell body

carries impulses from receptors to the CNS

Question 6

describe the structure and function of a motor neurone

Answer 6

many short dendrites to carry impulses to the cell body and one long axon that carries impulses away from cell body

carries impulses from CNS to effector cells

Question 7

describe the structure and function of a relay neurone

Answer 7

many short dendrites and many short axons

carry nerve impulses from sensory neurones to motor neurones

Question 8

what are myelinated nuerones?

Answer 8

nuerones covered in myelinated sheath- made of schwann cells wrapped around the axon many times forming many layers of plasma membrane. gaps inbetween schwann cells are called nodes of ranvier

Question 9

how is resting potential established?

Answer 9

sodium-potassium pumps actively transport sodium ions out of the axon and potassium into the axon in a ratio of 3:2 respectively.

potassium ion channels are open and due to electrochemical gradient of potassium ions made due to sodium-potassium pumps, potassium ions diffuse out of the axon.

meanwhile gated sodium channels are closed so axon has has negative potential difference as it is more positive on the outside.

Question 10

what is the resting potential?

Answer 10

-70mV

Question 11

How is an action potential generated?

Answer 11

energy of the stimulus triggers sodium voltage gated ion channels to open so sodium ions diffuse into axon down electrochemical gradient so the potential difference becomes more positive

the change in charge causes more sodium ion channels to open so more sodium diffuses in (positive feedback)

when the potential difference reaches +40mV the voltage gated sodium ion channels close and potassium ion channels open

potassium ion diffuse out of the axon initally at a fast rate, so becomes more negative than than resting potential - it is hyperpolarised.

the sodium-potassium pumps cause sodium to move out and potassium to move back in to repolarise the axon to resting potential

Question 12

what is the refractory period?

Answer 12

the short time after action potential when the axon cannot be excited

Question 13

how is action potential propogated along a neurone?

Answer 13

in non myelinated nuerone
- the depolaristation acts as a stimulus for the depolarisation of the next region of the membrane and when sodium ions diffuse into the axon, they are attracted by the negative charge ahead and diffuse further along inside the axon.

myelinated neurone
- by saltatory conduction
depolarisation can only occur at the nodes of ranvier where no myelin is present. this creates loner localised currents and action potentials jump between nodes

Question 14

what is the significance of the frequency of impulse transmission?

Answer 14

the size of a stimulus does not change the amount of depolarisation but it does change the frequency of impulse. when brain detects high frequecy of impulse, it interprets this as a large stimulus

Question 15

describe how an impulse is transmitted across the synapse

Answer 15

an action potential arrives at the presynaptic knob

the stimulates voltage gated calcium channels to open and calcium ions diffuse into presynaptic knob

the influx of calcium ions causes synaptic vesicles to move and fuse with the presynaptic membrane

this releases neurotrasmitter into the synaptic cleft by exocytosis.

the neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane

this causes sodium ion channels in the postsynaptic neurone to open

the influx of sodium ions causes depolarisation, if threshold potential is reached then an action potential is generated.

nuerotransmitter is removed from the synaptic cleft

Question 16

what is a cholinergic synapse?

Answer 16

a synapse with acetylcholine as the neurotransmitter. this is broken down in the synaptic cleft by acetylcholinesterase and the products are taken back to the presynaptic knob in which acetylcholine is remade.

Question 17

what is spatial summation?

Answer 17

when many neurones converge to provide nuerotramitter for one postsynaptic neurone. so that threshold potential is reached in the postsynaptic neurone and an action potential is triggered

Question 18

what is temporal summation?

Answer 18

when multiple nerve impulses arrive after each other from the same presynaptic neurone. this increases chance of action potential because more neurotransmitter is released into the synaptic cleft.

Question 19

what is the difference between inhibitory and excitatory synapses?

Answer 19

excitatory synapses result in the depolarisation of the postsynaptic neurone, if threshold potential is reached then an action potential is triggered

inhibitory synapses result in hyperpolarisation of the postsynaptic membrane, preventing an action potential being triggered

Question 20

what is the importance of synapses?

Answer 20

ensures impulses are unidirectional

gives ability to transmit an impulse to multiple neurones

gives ability for multiple nuerones to transmit impulse to one

Question 21

why are sensory receptors described as transducers?

Answer 21

they convert from one type of energy to electrical energy

Question 22

Deformation of the plasma membrane of the tip of the neurone causes the membrane to
become more permeable to Na+.
Suggest why

Answer 22

idea that deformation of membrane will allow more Na+
through because
1 (the increased pressure) causes
sodium (ion) channels to open ;
2 (temporary) gaps / holes / spaces , appear ,
between the phospholipids / in the bilayer ;

Question 23

The generation of an action potential follows the ‘All-or-Nothing’ law.
Explain what this means.

Answer 23

if the , stimulus is not strong enough /
threshold (value) is not reached /
depolarisation (of membrane) is insufficient ,
then , it / an action potential , is not ,
generated / AW ;

Question 24

why do you not feel your clothing on skin after a while?

Answer 24

action potentials not generated because
1 sodium (ion) channels (remain) open /
resting potential not re-established
;
2 idea of ions being in the wrong place for correct ion
movement (across membrane) ;