3.6.2 Nervous Coordination

Question 1

Structure of a myelinated motor neurone

Answer 1

Cell body, contains organelles eg nucleus and lots of rough endoplasmic reticulum, produces proteins and neurotransmitters
Dendrons, extensions of cell body, divide into smaller branches called dendrites, carry nerve impulse towards cell body
Axon, a single long fibre carry nerve impulses away from cell body
Schwann cells, surround axon, provide electrical insulation, carry out phagocytosis and regenerate nerves, wrap around generating many layers
Myelin sheath, covers axon made up of the Schwann cell membranes (lipid called myelin)
Nodes of ranvier, areas between adjacent Schwann cells where there is no myelin sheath, transmission of impulse is faster

Question 2

Explain resting potential

Answer 2

3Na+ actively transported out of axon, 2K+ actively transported into axon both using carrier proteins, occurs due to sodium potassium pump
More Na+ in tissue fluid, more K+ in cytoplasm of axon, an electrochemical gradient is created
Facilitated diffusion moves Na+ into axon and K+ out of axon, channel protein has gates which are closed for Na+ movement (membrane is less permeable to Na+)
Inside of axon is -ve charged compared to tissue fluid making it polarised

Question 3

What’s a nerve impulse

Answer 3

Reversal of electrical potential difference across the axon membrane
Has resting potential and action potential

Question 4

What happens during depolarisation

Answer 4

Energy from a stimulus causes some sodium voltage gated channels in axon to open, Na+ diffuses (facilitated diffusion)into axon, reverse potential difference across membrane
40mV action potential is established

Question 5

What happens during repolarisation

Answer 5

40mV action potential is established, voltage gates on Na+ channels close, voltage gates of K+ open, electrical gradient allows K+ to diffuse out of axon

Question 6

What happens during hyperpolarisation

Answer 6

K+ ions diffusing out of axon cause an overshoot of electrical gradient, inside of axon is more negative than tissue fluid
Gates on K+ ion channels are closed
Sodium potassium pump continues, resting potential reestablished

Question 7

What’s action potential

Answer 7

Neurones voltage increase beyond a set point, continual wave of depolarisation, generates nervous impulse

Question 8

What’s the passage of an action potential across an unmyelinated axon

Answer 8

Resting potential, Na+ in tissue fluid, -ve charge inside axon, axon membrane is polarised
Depolarisation, Na+ moves into axon, inside is now +ve, membrane is depolarised
Localised electrical current by Na+ cause sodium voltage gates channels to close, potassium ones open, moves along axon membrane
Axon membrane behind action potential is repolarised, sodium potassium pump continues across the axon

Question 9

What’s the all or nothing principal

Answer 9

If depolarisation doesn’t exceed threshold voltage then an action potential isn’t generated

If a stimulus triggers depolarisation, there will be a peak to the same max voltage, larger stimuli increase frequency of action potentials

Ensures animals only respond to large stimuli

Question 10

What’s the passage of an action potential across a myelinated axon

Answer 10

Action potential can occur at nodes of ranvier, action potential jumps between adjacent nodes this is called saltatory conduction

Action potential passes along a myelinated neurone faster

Question 11

What’s the refractory period

Answer 11

After an action potential is created Na+ voltage gated channels close prevents Na+ moving into the cell

Question 12

What’s the importance of the refractory period

Answer 12

Action potentials propagated in one direction, action potential move from active to resting, cannot be propagated into a refractory region, move in one direction
Production of discreet impulses, new action potential can’t be formed immediately after another due to refractory period, ensures action potentials are separated
Limits number of action potentials, action potentials are separated from each other, only a certain amount can pass in a given time, strength of stimulus that can be detected is limited

Question 13

What shows the strength of a stimulus

Answer 13

Frequency of action potentials

Question 14

What factors effect the speed of conduction of an action potential

Answer 14

Myelination and saltatory conduction
Axon diameter
Tenperature

Question 15

How does myelination and saltatory conduction effect speed of an action potential

Answer 15

Myeline sheath acts as electrical insulator, causes action potential to jump between adjacent nodes of ranvier this is called saltatory conduction

Question 16

How does the diameter of an axon effect the speed of an action potential

Answer 16

Greater axon diameter means faster speed of conductance, less leakage of ions, easier to maintain membrane potential

Question 17

How does temperature effect the speed of an action potential

Answer 17

Only in cold blooded animals
Higher temperature increases rate of diffusion for ions
Enzymes work faster at higher temps up to an optimal temp, respiration releases energy for active transport, faster movement of ions
Higher temps increase the speed and strength of muscle contractions

Question 18

What’s a synapse

Answer 18

Where one neurone communicates with another or with an effector

Question 19

Structure of a synapse

Answer 19

Synaptic cleft, small gap
Presynaptic neurones, releases neurotransmitter
Synaptic knob, swollen end of Presynaptic neurone, lord of mitochondria, manufactures neurotransmitters
Synaptic vesicles, stows neurotransmitters
Postsynaptic neurones, specific receptor proteins on membrane to receive neurotransmitter

Question 20

What’s a neuromuscular junction

Answer 20

Motor neurone meters skeletal muscle fibre, many of these junctions spread across muscle for rapid and coordinated contraction

Nerve impulse received at junction, synaptic vesicles fuse with Presynaptic membrane releasing acetylcholine, diffuses into Postsynaptic membrane, more permeable to Na+, Na+ enters, membrane depolarised, acetylcholine broken down by acetylcholinesterase, choline and acetyl diffuse back into neurone, recombine using energy from mitochondria

Question 21

Transmission in a cholinergic synapse

Answer 21

Action potential at synaptic knob (summation), depolarisation at knob, Ca2+ channels open, Ca2+ diffuses into synaptic knob
Vesicles containing acetylcholine move towards and fuse with Presynaptic membrane, acetylcholine released to synaptic cleft
Acetylcholine diffuses down a conc gradient across synaptic cleft to Postsynaptic membrane, acetylcholine binds to complimentary shaped receptors, Na+ protein channels open, Na+ diffuses in, generates new action potential in Postsynaptic neurone
Acetylcholinesterase hydrolyses acetylcholine into acetyl and choline, diffuse back across synaptic cleft to Presynaptic neurone, prevent continuous generation of action potentials, discreet transfer of info (unidirectionality)
ATP released by mitochondria recombine choline and acetyl to acetyl choline, stored in synaptic vesicles, Na+ protein channels close

Question 22

What’s unidirectionality

Answer 22

Synapses only pass info from Presynaptic neurone to Postsynaptic neurone

Question 23

What’s summation

Answer 23

Spatial summation, different Presynaptic neurones release enough neurotransmitter to exceed threshold of Postsynaptic neurone, trigger new action potential

Temporal summation, single Presynaptic neurone releases neurotransmitter lots of times in a short period, if the conc of neurotransmitter exceeds threshold of Postsynaptic neurone, new action potential is triggered

Question 24

What’s inhibition by inhibitory synapses

Answer 24

Presynaptic neurone releases neurotransmitter binds to Cl- protein channels on Postsynaptic neurone, Cl- protein channel opens, Cl- moves into Postsynaptic neurone by facilitated diffusion,
K+ channels also open, K+ moves out of Postsynaptic neurone into synapse, inside of Postsynaptic membrane is more -ve than outside, hyperpolarisation, more Na+ ions needed to produce action potential

Question 25

Cholinergic synapse vs neuromuscular junction

Answer 25

Both neurotransmitters acetylcholine transported by diffusion
Both have receptors on which bind to acetylcholine causing influx of Na+
Both use Na+ K+ pump to depolarise axon
Both use acetylcholinesterase to hydrolyse acetylcholine

NMJ
Links neurones to muscle
Only involves motor neurones
Action potential ends

CS
Links neurones to neurones, or neurones to effectors
Uses motor, sensory and intermediate neurones
A new action potential can be produced

Question 26

How movement of K+ and Na+ controlled

Answer 26

Channel proteins across the phospholipid bilayer, contain specific gates,if open all the time ions can move through by facilitated diffusion

Carrier proteins actively transport Na+ and K+ ions through the sodium potassium pump