Nervous Communication

Question 1

What is summation

Answer 1

The addition of excitatory pre-synaptic potentials to combine enough to cause an action potential in the post-synaptic neurone

Question 2

Distinguish between spatial summation and temporal summation, and how they’d differ on a graph

Answer 2

Spatial summation is several pre synaptic neurones contributing to producing an action potential in 1 neurone - looks like a ladder on the graph

Temporal summation is continuous firing of APs from one pre-synaptic neurone until they combine to reach threshold potential- lots of small ones on a graph until action potential reached

Question 3

What is habituation

Answer 3

After repeated stimulation neurotransmitter vesicles run out - the synapse is fatigued to avoid overstimulation which could cause damage

Eg. Why we get used to a smell, background noise or don’t ‘feel’ that we are wearing clothes

Question 4

How do synapses ensure APs are transmitted in right direction?

Answer 4

Only pre synaptic bulb contains acetylcholine vesicles

Question 5

How do synapses filter out low level stimuli, eg low level background noise?

Answer 5

Several acetylcholine vesicles must be released to generate an AP in the post-synaptic neurone

Question 6

What is the resting potential and how is it maintained ?

Answer 6

-60mV; when the neurone is at rest (not stimulated)

NaK pumps:
3Na+ pumped out for every 2K+ in
K+ channels open so diffuses out

Gated Na+ channels closed but non-voltage-gated K+ open:
So inside is more negative compared to outside

Question 7

How is an action potential generated? 7 marks

Answer 7

A stimulus opens voltage gated Na+ channels so Na+ diffuses in

Depolarisation (Inside gets less negative)
; the membrane potential is now -40mV = the threshold

Threshold triggers positive feedback: more Na+ channels open; depolarisation occurs until action potential is reached +30mV

At action potential, gated Na+ channels close & gated K+ channels open so K+ diffuses out rapidly repolarising the membrane

Membrane potential overshoots slightly (more neg than resting potential) = HYPERPOLARISATION

VOLTAGE GATED K+ channels close & resting potential restores = repolarised

Question 8

What is the refractory period? Why is this useful?

Answer 8

Period after an action potential when the neurone cant be stimulated again straight away; the ion channels are recovering

This prevents reverse propagation of an action potential; it only goes forward & non-overlap of separate impulses

Question 9

What are nerve impulses

Answer 9

wave of depolarisation caused by action potentials that start at one end of the neurone and are propagated across the axon to the other end

Question 10

how are action potentials transmitted across a neurone (nerve impulses propagated)

Answer 10

initial AP causes influx of Na+ at start of neurone (depolarisation at start)

the Na+ is attracted to the more negative area of the axon ahead of it- moves along electrochemical gradient = LOCAL CURRENT

Local current causes opening of gated Na+ channels further down the axon, so Na+ influx & depolarisation

then gated Na+ channels close & K+ channels open so K+ diffuses out- depolarises behind the wave

this continues across the length of the neurone, as a wave of depolarisation

Question 11

What is saltatory conduction and why is it better?

Answer 11

nerve impulse transmission in myelinated neurones: where action potentials jump from one node of Ranvier to the next along the axon = much faster & uses less ATP so more energy efficient than continuous depolarisation (opening&closing of ion channels & NaK pump)

Question 12

Why does saltatory conduction result in APs jumping from node to node?

Answer 12

depolarisation of axon can only occur at Nodes of Ranvier (gaps between myelin)

so longer localised currents arise between adjacent nodes

Question 13

All-or-nothing principle

Answer 13

no matter the size of the stimulus, if the threshold potential is reached it will always trigger the same action potential of same value
if not reached: no response

Question 14

Nerve Impulse Transmission Across a Cholinergenic Synapse

Answer 14

1. AP arrives @ synaptic knob of presynaptic neurone
2. stimulates voltage-gated Ca2+ channels to open- Ca2+ diffuses into synaptic knob & binds to acetylcholine vesicles
3. vesicles move towards & fuse w/ presynaptic membrane- release acetylcholine into synaptic cleft by exocytosis.
4. acetylcholine diffuses across cleft & binds to receptors on post-synaptic membrane; causes Na+ channels in postsynaptic membrane to open
5. influx of Na+ into post-synaptic knob = depolarised; action potential generated if threshold potential reached
6. neurotransmitter broken down by acetylcholinesterase enzyme in synaptic cleft to stop response

Question 15

What is a cholinergenic synapse?

Answer 15

a synapse that uses acetylcholine as its neurotransmitter

Question 16

diff between synapse and synaptic cleft

Answer 16

synaptic cleft = gap between 2 neurones

synapse = post&pre-synaptic membranes + cleft

Question 17

describe the structure & function of a motor neurone, and where it is found in the nervous system

Answer 17

transmits impulses from CNS to effector
short dendrites but long axon
cell body & dendrites in CNS
axon ends at effector (axon outside CNS)

Question 18

describe the structure & function of a relay neurone, and where it is found in the nervous system

Answer 18

transmits impulses from sensory neurone to motor neurone
= CNS neurone
short dendrites & short axon

Question 19

describe the structure & function of a sensory neurone, and where it is found in the nervous system

Answer 19

transmits impulses from sensory receptors to CNS
short axon but long dendrites
axon inside CNS
dendrites & cell body inside CNS

Question 20

what is a myelinated neurone

Answer 20

neurone associated with many Schwann cells: make up a fatty sheath around the axon

Question 21

what are sensory receptors?

Answer 21

cells/sensory nerve endings that respond to an internal/external stimulus by generating an action potential (they convert the input energy into electrical energy)

Question 22

which sensory receptor responds to a change in light intensity ?

Answer 22

rods & cones in retinas of eyes

Question 23

which sensory receptor responds to a change in temp?

Answer 23

thermoreceptors in skin & hypothalamus

Question 24

which sensory receptor responds to touch (change in pressure on skin)? and how ?

Answer 24

pacinian corpuscles in skin
they are oval shapes composed of rings of connective tissue
pressure deforms these rings, pushing them against the nerve ending

they only respond to changes that deform the connective tissue; so if pressure is constant, eg. wearing clothes, they stop responding

Question 25

which sensory receptor responds to taste; chemicals in food?

Answer 25

chemical receptors in taste buds on tongue

Question 26

which sensory receptor responds to the presence of smell; chemicals in air?

Answer 26

olfactory cells in epithelium lining nose

Question 27

which sensory receptor responds to sound ?

Answer 27

vibration receptors in cochlea of ear

Question 28

Who do we detect stimuli of different intensities if the all-or-nothing principle is true?

Answer 28

frequency of action potentials: larger stimuli have higher frequency (on a graph would be many more Maps per unit time)
Brain determines the strength of stimulus based on frequency of action potentials arriving at CNS

Question 29

Define Local Current

Answer 29

flow from + to - as Na+ moves along neurone as wave of depolarisaton

Question 30

state the function of:

1. dendrites
2. cell body
3. axon

Answer 30

1. receives signals from other neurones
2. normal cell processes; eg protein synthesis for ion channels
3. transmits impulse down neurone to next synapse