5.3: Nervous System Flashcards
Function of dendrites
Receive action potential from preceding/ relay/ intermediate/ connector neurones
Function of myelin sheath
Insulates axon causing saltatory conduction
Function of Schwann cells
Secrete myelin
Explain how depolarisation occurs at a synapse
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
Explain how is a resting potential generated:
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
What is meant by a threshold stimulus?
The smallest stimulus that is capable of setting up an action potential
How are action potentials propagated along the axon during nerve impulse transmission:
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
Why do synaptic bulbs contain many mitochondria?
Provide ATP; to provide energy for combining of acetate/ choline to synthesise neurotransmitter; move vesicles (exocytosis)
How does the post-synaptic membrane become repolarised?
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
Distinguish between a motor neurone and a motor nerve:
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
Refractory period of a neurone:
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]
Distinguish between spatial and temporal summation at synapses:
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
All or nothing law:
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
Outline stages at resting state:
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
Outline stages of depolarisation:
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
Outline stages of repolarisation:
Na channels close; (voltage-gated) K channels open/ K+ move out of neurone; membrane potential becomes negative; positive feedback/ more K channels open
Outline stages of hyperpolarisation:
K ions continue to leave/ K channels slow to close; inside of cell becomes more negative than resting stage;
Outline how the first neurone communicates with the second neurone across the gap:
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
The relationship between strength of a stimulus and the resulting action potential:
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)
Outline roles of synapses in the nervous system:
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
Outline the importance of the junctions between neurones in the functioning of the nervous system:
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
Why is the Pacinian corpuscle described as a transducer?
Converts mechanical energy to electrical energy
Deformation of plasma membrane of the tip of the neurone causes the membrane to become more permeable to Na+ suggest why?
Increased pressure causes Na channels to open; (temporary) gaps appear between the phospholipids/ in the bilayer
Suggest an explanation for the fact that action potentials are not generated constantly whilst wearing clothes:
Na channels remain open/ resting potential not reestabilished; ions in the wrong place for movement across membrane
Explain why the pre-synaptic bulb contains many SER:
Synthesis and packaging of acetylcholinesterase.
Outline the ways in which structures of a sensory and motor neurones are similar:
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
Another name for cell body:
Centron
Describe and explain the effect of myelination on the rate of conduction of an action potential:
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
Explain how action potentials are transmitted along a nonmyelinated neurone and describe which parts of this process are different in myelinated neurones:
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)
Define reflex action:
Rapid/ fast acting; short-lived; automatic/ involuntary/ brain not involved; innate/ not learned; response same each time
Outline what happens in the membrane of the sensory receptor in response to pressure:
Distortion; Na gates/ channels open; Na enter; depolarisation; receptor/ generator potential; threshold value; action potential
Explain why impulses can only travel one direction:
Neurotransmitter only in presynaptic knob/ released from presynaptic membrane; receptors only on post-synaptic membrane; ref to refractory period/ hyperpolarisation
Define refractory period:
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;
Importance of refractory period:
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;
How are impulses transmitted from receptor to effector:
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
Describe structure of myelin sheath:
Myelin/ lipid/ fatty sheath; Schwann cell wrapped around axon; except at nodes of Ranvier
Effect of myelination:
Myelination: greater speed; unmyelinated needs larger diameter to produce same speed
Effect of axon diameter on speed of conduction:
Larger axon: higher speed;
Suggest why an increase in temperature results in increase in speed of conduction:
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
As the temperature continues to increase, impulse ceases why?
Ion channels/ pumps denatured; fluidity of membrane disrupted; synaptic enzymes denatured
Outline the events following the arrival of an action potential at the synaptic knob until ACh has been released into the synapse:
Ca channels open; Ca2+ diffuse in; ACh in vesicles; (synaptic) vesicles move towards presynaptic membrane; fuse with; release ACh by exocytosis into synaptic cleft
Describe how the structure of a motor neurone differs from sensory neurone:
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
Function of motor neurone differs from that of a sensory neurone:
MN carries impulses from CNS/ brain/ sc/ RN to effector/ muscle/ gland
SN from receptor to brain/ sc/ CNS/ RN
Outline the events that occur when an area of cell surface membrane undergoes an action potential:
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
Suggest why MS is described as an auto-immune condition:
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 ;
Explain why this damage leads to a loss of sensation.
(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 ;
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:
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
Hormones that increase heart rate:
(Nor)adrenaline/ (nor)epinephrine/ thyroxine/ corticosteroids
Action potential across myelinated and non-myelinated neurones:
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
