5.3: Nervous System

Question 1

Function of dendrites

Answer 1

Receive action potential from preceding/ relay/ intermediate/ connector neurones

Question 2

Function of myelin sheath

Answer 2

Insulates axon causing saltatory conduction

Question 3

Function of Schwann cells

Answer 3

Secrete myelin

Question 4

Explain how depolarisation occurs at a synapse

Answer 4

Action potentials arrive at the synapse
Ca2+ ions enter synaptic knobs
Causes release of acetylcholine from synaptic bulbs
Attaches to receptors on the sarcolemma
Making it permeable to Na+ ions
Which diffuse in fibre causing depolarisation

Question 5

Explain how is a resting potential generated:

Answer 5

Axon is impermeable to Na+ but permeable to K+; Na+ pumped out of axon to tissue fluid; more negative inside; inflow of K+ less than outflow of Na+ (3 Na out for 2 K in); ATP required for pumps

Question 6

What is meant by a threshold stimulus?

Answer 6

The smallest stimulus that is capable of setting up an action potential

Question 7

How are action potentials propagated along the axon during nerve impulse transmission:

Answer 7

Propagated by local currents; e- flow occurs at margins of depolarised (+) and resting/ repolarised (-) regions; these currents make the next bit of axon membrane permeable to Na+ and so the region of depolarisation spreads

Question 8

Why do synaptic bulbs contain many mitochondria?

Answer 8

Provide ATP; to provide energy for combining of acetate/ choline to synthesise neurotransmitter; move vesicles (exocytosis)

Question 9

How does the post-synaptic membrane become repolarised?

Answer 9

Acetylcholinesterase enzyme releases as soon as muscle is depolarised; this removes the acetylcholine from the receptors; by hydrolysing into acetate + choline; membrane of muscle reverts to being impermeable to sodium ions: resting potential restores

Question 10

Distinguish between a motor neurone and a motor nerve:

Answer 10

Motor neurone: single cell running from CNS to effector organ;
Motor nerve is made of many motor neurones running side by side to effector(s); contain bundles of neurones grouped together in connective tissue/ collagen sheaths

Question 11

Refractory period of a neurone:

Answer 11

The period which must elapse after a (first) stimulus before a second stimulus can produce a second action potential; axon must have repolarised almost completely before another potential can be set up
[book: Na voltage gated channels remain closed; so action potentials are unidirectional; no overlap]

Question 12

Distinguish between spatial and temporal summation at synapses:

Answer 12

The depolarisation effect of discharging synaptic knobs is cumulative (= summation); in spatial summation several synaptic knobs discharge simultaneously (onto the post-synaptic membrane); whereas in temporal summation they discharge in rapid succession

Question 13

All or nothing law:

Answer 13

If a stimulus is above its threshold value; it sets up a complete full sized action potential; larger stimuli don’t increase the size of action potential

Question 14

Outline stages at resting state:

Answer 14

Membrane is impermeable to Na; Na pumped out by active transport/ higher conc maintained outside; K enter along electrical gradient; influx of K can’t catch up with out flux of Na so membrane is charged

Question 15

Outline stages of depolarisation:

Answer 15

Stimulus causes membrane to become permeable to Na/ (voltage-gated) Na channels open; Na+ diffuse in the neurone (down electrochemical gradient); membrane potential becomes positive; positive feedback

Question 16

Outline stages of repolarisation:

Answer 16

Na channels close; (voltage-gated) K channels open/ K+ move out of neurone; membrane potential becomes negative; positive feedback/ more K channels open

Question 17

Outline stages of hyperpolarisation:

Answer 17

K ions continue to leave/ K channels slow to close; inside of cell becomes more negative than resting stage;

Question 18

Outline how the first neurone communicates with the second neurone across the gap:

Answer 18

Neurotransmitter released from pre-synaptic membrane; diffuses across synaptic cleft; attaches to receptors of Na channels on post-synaptic membrane; neurotransmitter broken down in cleft

Question 19

The relationship between strength of a stimulus and the resulting action potential:

Answer 19

Only stimuli that reach threshold value produce an action potential; all-or-nothing law: ap either occurs or not; so is same magnitude no matter the strength; strong stimulus produces many ap (in rapid succession)

Question 20

Outline roles of synapses in the nervous system:

Answer 20

Allows neurones to communicate/ cell signalling;
ensure transmission in one direction; allows divergence/ convergence;
filter out low level stimuli;
prevent over stimulation;
allow many low level stimuli to be amplified;
presence of inhibitory/ excitatory synapses allow impulses to follow specific paths;
permits memory/ learning

Question 21

Outline the importance of the junctions between neurones in the functioning of the nervous system:

Answer 21

Ensures movement of impulse/ action potential in one direction only; integration- one neurone can, connect to/ receive impulses from/ transmit impulses, many neurones;
allows summation;
filters out background/ low level stimuli- only stimulation strong enough will be passed on;
permits memory/ learning; acclimatisation;
prevents continuous stimulation of neurones;
2 types of synapses: excitatory inhibitory

Question 22

Why is the Pacinian corpuscle described as a transducer?

Answer 22

Converts mechanical energy to electrical energy

Question 23

Deformation of plasma membrane of the tip of the neurone causes the membrane to become more permeable to Na+ suggest why?

Answer 23

Increased pressure causes Na channels to open; (temporary) gaps appear between the phospholipids/ in the bilayer

Question 24

Suggest an explanation for the fact that action potentials are not generated constantly whilst wearing clothes:

Answer 24

Na channels remain open/ resting potential not reestabilished; ions in the wrong place for movement across membrane

Question 25

Explain why the pre-synaptic bulb contains many SER:

Answer 25

Synthesis and packaging of acetylcholinesterase.

Question 26

Outline the ways in which structures of a sensory and motor neurones are similar:

Answer 26

Both have: dendrites; axon; cell body with nucleus/ mitochondria/ Golgi/ SER/ RER; myelin sheath/ covered by Schwann cell/ nodes of Ranvier; voltage-gated channels/ Na+K+ pumps

Question 27

Another name for cell body:

Answer 27

Centron

Question 28

Describe and explain the effect of myelination on the rate of conduction of an action potential:

Answer 28

Myelinated fibres conduct more quickly than unmyelinated; myelin sheath acts as (electrical) insulator; lack of Na/ K gates in myelinated region; depolarisation occurs at nodes of Ranvier only; (so) longer local circuits; (action potential) jumps from one node to another/ saltatory conduction

Question 29

Explain how action potentials are transmitted along a nonmyelinated neurone and describe which parts of this process are different in myelinated neurones:

Answer 29

1) Na ions (inside axon), diffuse
2) towards resting/negative region
3) causes depolarisation of this region/ change in p.d to reach threshold value
4) (more) Na channels open
5) Na+ move in
!!! 3) to 5) linked to movement within axon!!!
6) ref to local circuits
7) one way transmission
8) ref to refractory period/ region of axon recovering behind action potential
9) ref to insulating role of, myelin sheath/ Schwann cells
10) depolarisation can’t occur through myelin
11) ref to nodes of Ranvier
12) longer local circuits
13) saltatory conduction
14) fewer (Na+ and K+) ion channels in myelinated region
15) ref to absolute and relative refractory period, ref to actual distance between nodes (1-3mm)

Question 30

Define reflex action:

Answer 30

Rapid/ fast acting; short-lived; automatic/ involuntary/ brain not involved; innate/ not learned; response same each time

Question 31

Outline what happens in the membrane of the sensory receptor in response to pressure:

Answer 31

Distortion; Na gates/ channels open; Na enter; depolarisation; receptor/ generator potential; threshold value; action potential

Question 32

Explain why impulses can only travel one direction:

Answer 32

Neurotransmitter only in presynaptic knob/ released from presynaptic membrane; receptors only on post-synaptic membrane; ref to refractory period/ hyperpolarisation

Question 33

Define refractory period:

Answer 33

Flowing an action potential; need to, redistribute Na+/ K+/ restoring resting potential; sodium voltage gated channels are closed; another impulse can’t be generated/ conducted;

Question 34

Importance of refractory period:

Answer 34

Another impulse can’t be generated/ conducted; ensures impulses are separated; determines maximum frequency of impulse transmission; impulse passes in one direction only along axon; ref to absolute and relative refractory period;

Question 35

How are impulses transmitted from receptor to effector:

Answer 35

1) ref to change in receptor
2) creates receptor/ generator potential
3) if greater than threshold value
4) depolarisation (of axon/ sensory/ afferent, neurone)
5) ref to action potential
6) ref to myelin sheath
7) saltatory conduction
8) ref to nodes of Ranvier
9) synapse with, motor/ effector/ efferent neurone
10) ref to, Ca2+/ channels
11) vesicles of neurotransmitters fuse with presynaptic membrane
12) acetylcholine
13) secretion/ exocytosis
14) diffusion across synaptic cleft
15) receptors on postsynaptic m
16) depolarisation (of postsynaptic m)
17) neuromuscular junction/ motor end plate
18) AVP: ion movement/ refractory period/ voltage-gated channels

Question 36

Describe structure of myelin sheath:

Answer 36

Myelin/ lipid/ fatty sheath; Schwann cell wrapped around axon; except at nodes of Ranvier

Question 37

Effect of myelination:

Answer 37

Myelination: greater speed; unmyelinated needs larger diameter to produce same speed

Question 38

Effect of axon diameter on speed of conduction:

Answer 38

Larger axon: higher speed;

Question 39

Suggest why an increase in temperature results in increase in speed of conduction:

Answer 39

Increase KE so ions diffuse into cell more quickly; faster movement of neurotransmitters;
Faster diffusion of ions leads to faster depolarisation; shorter duration of action potential/ refractory period; faster depolarisation

Question 40

As the temperature continues to increase, impulse ceases why?

Answer 40

Ion channels/ pumps denatured; fluidity of membrane disrupted; synaptic enzymes denatured

Question 41

Outline the events following the arrival of an action potential at the synaptic knob until ACh has been released into the synapse:

Answer 41

Ca channels open; Ca2+ diffuse in; ACh in vesicles; (synaptic) vesicles move towards presynaptic membrane; fuse with; release ACh by exocytosis into synaptic cleft

Question 42

Describe how the structure of a motor neurone differs from sensory neurone:

Answer 42

SN: CB at centre of cell; CB at PNS; dendrites at end of axon/dendron; shorter axon; dendron present; connects to/ starts at receptor

MN: CB at end of neurone/ cell; CB on CNS/ spinal cord/ brain; dendrites connected to CB; longer axon; no dendron; axon connects to effector/ motor end plate

Question 43

Function of motor neurone differs from that of a sensory neurone:

Answer 43

MN carries impulses from CNS/ brain/ sc/ RN to effector/ muscle/ gland
SN from receptor to brain/ sc/ CNS/ RN

Question 44

Outline the events that occur when an area of cell surface membrane undergoes an action potential:

Answer 44

Resting potential reduced;
threshold value;
ion channels/ gates/ gated proteins open;
Na+ enter; by diffusion/ down an electrochemical gradient;
interior of cell becomes positive;
depolarisation;
(ion gate) proteins close;
K+ move out of cell;
resting potential restored/ repolarisation;
ref to action potential spreading/ lowering resting potential of neighbouring regions of membrane;
-70mV to +40mV;
Refractory period prevents action potential propagating in reverse direction

Question 45

Suggest why MS is described as an auto-immune condition:

Answer 45

attacked by the body’s (own) immune system ;
(immune system) mistakes / treats / recognises ,
body cells / neurones / myelin ,
as , ‘foreign’ / non self ;
correct ref. to , antibodies / (named) phagocytes /
(named) B lymphocytes / (named) T lymphocytes ;

Question 46

Explain why this damage leads to a loss of sensation.

Answer 46

(damage to) myelin / sheath / Schwann cell(s) ;
removes / has less , insulation ;
interferes with / slows / stops ,
conduction of , (nerve) impulse / action potential
or
slows / stops / prevents , saltatory conduction / described ;
occurs , in sensory neurones /
towards brain / towards CNS /
from sensory organ / from receptor ;

Question 47

Tetradotoxin is poisonous to humans because it blocks gated sodium channels in cell membranes, preventing action potentials. This does not happen in the fish themselves. suggest why fish not affected:

Answer 47

channel/ receptor/ ion , is different; idea that toxin confined to, organelle/ organ/ part of the body; toxin not, in general circulation/ (circulated) in blood; toxin stored in inactive form; contains a compound that neutralises toxin

Question 48

Hormones that increase heart rate:

Answer 48

(Nor)adrenaline/ (nor)epinephrine/ thyroxine/ corticosteroids

Question 49

Action potential across myelinated and non-myelinated neurones:

Answer 49

6 7 8
9 10
11 12 13
14 15
ref to local circuits;
one way transmission;
ref refractory period/region of axon behind AP recovering;
ref to insulating role of, myelin sheath/Schwann cells;
depolarisation cannot occur through myelin/
++ impermeable to (Na and K ) ions/ora;
ref to nodes of Ranvier; longer local circuits; saltatory conduction/AW;
++
AVP; e.g. fewer (Na and K ) ion channels in myelinated region/ora.
AVP; ref. to absolute and relative refractory period, ref. to actual distance between nodes (1 – 3mm);
4. 1
2 towards, resting/negative region;
3 causes, depolarisation of this region/change of PD to reach threshold value;
4 (more) sodium channels open;
5 sodium (ions) move in;
marking points 3-5 only available if linked to sodium ions moving within axon
sodium ions (inside axon), move/diffuse