3.6.2 Nervous coordination

Question 1

Describe the general structure of a motor neurone

Answer 1

Cell body: contains organelles and lots of RER
Dendrons: branch into dendrites which carry impulse to cell body
Axon: long, unbranched fibre carries nerve impulses away from cell body

Question 2

Describe the additional features of a myelinated motor neurone

Answer 2

Schwann cells: wrap around axon
Myelin sheath: made from myelin-rich membranes of schwann cells
Nodes of ranvier: very short gaps between neighbouring schwann cells where no myelin sheath

Question 3

Name 3 processes schwann cells are involved in

Answer 3

electrical insulation
phagocytosis
nerve regeneration

Question 4

How does an action potential pass along an unmyelinated neurone

Answer 4

1. Stimulus leads to influx of Na+. First section of membrane depolarises
2. Local electrical currents cause sodium voltage gated channels further along membrane to open. The section behind begins to repolarise
3. Sequential wave of depolarisation

Question 5

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 5

Saltatory conduction: Impulse ‘jumps’ from one node of ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse doesnt travel along whole axon length

Question 6

What is resting potential

Answer 6

Potential difference across neuron membrane when not stimulated ~70mV

Question 7

How is resting potential established

Answer 7

1. Membrane is more permeable to K+ than Na+
2. Sodium potassium pump actively transports 3Na+ out of cell and 2K+ into cell

Establishes electrochemical gradient: cell contents more negative than extracellular environment

Question 8

Name the stages in generating an action potential

Answer 8

1. Depolarisation
2. Repolarisation
3. Hyperpolarisation
4. Return to resting potential

Question 9

What happens during depolarisation

Answer 9

1. Stimulus –> facilitated diffusion of Na+ into cell down electrochemical gradient
2. p.d across membrane becomes more positive
3. If membrane reaches threshold (-50mV) voltage gated Na+ channels open
4. Significant influx of Na+ reverse p.d to +40mV

Question 10

What happens during repolarisation

Answer 10

1. Voltage gated Na+ channels close and voltage gated K+ channels open
2. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3. p.d across membrane becomes more negative

Question 11

What happens during hyperpolarisation

Answer 11

1. Overshoot when K+ diffuse out, p.d becomes more negative than resting potential
2. Refractory period: no stimulus is large enough to raise membrane potential to threshold
3. Voltage gated K+ channels close and sodium potassium pump re establishes resting potential

Question 12

Explain the importance of the refractory period

Answer 12

No action potential can be generated in hyperpolarised sections of membrane
Ensures unidirectional impulse
Ensures discrete impulses
Limits frequency of impulse transmission

Question 13

What is the all or nothing principle

Answer 13

Any stimulus that causes the membrane to reach threshold potential will generate an action potential
All action potentials have same magnitude

Question 14

Name the factors that affect the speed of conductance

Answer 14

Myelin sheath
Axon diameter
Temperature

Question 15

How does axon diameter affect speed of conductance

Answer 15

greater diameter = faster
Less resistance to flow of ions(depolarisation and repolarisation)

Less leakage of ions (easier to maintain membrane potential)

Question 16

How does temperature affect speed of conductance

Answer 16

Higher temp = faster
Faster rate of diffusion (depolarisation and repolarisation)
Faster rate of respiration = more ATP for active transport to re establish resting potential

Temp too high - membrane proteins denature

Question 17

How can organisms detect the strength of a stimulus

Answer 17

Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses

Question 18

What is the function of synapses

Answer 18

Electrical impulse cannot travel over junction between neurones
Neurotransmitters send impulses between neurons / from neurons to effectors
New impulses can be initiated in several different neurons for multiple simultaneous responses

Question 19

Describe the structure of a synapse

Answer 19

Presynaptic neuron ends in synaptic knob; contains many mitochondria, endoplasmic reticulum, vesicles of neurotransmitter

synaptic cleft; 20-30nm gap between neurons

postsynaptic neuron; complimentary receptors to neurotransmitter

Question 20

Outline what happens in the presynaptic neurone when an action potential is transmitted from one neuron to another

Answer 20

1. Wave of depolarisation travels down presynaptic neuron causing voltage gated Ca2+ channels to open
2. Vesicles move towards and fuse with presynaptic membrane
3. Exocytosis of neurotransmitter into synaptic cleft

Question 21

How do neurotransmitters cross the synaptic cleft

Answer 21

Simple diffusion

Question 22

Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another

Answer 22

1. Neurotransmitter binds to specific receptor on postsynaptic membrane
2. Ligand gated Na+ channels open
3. If influx of Na+ raises membrane to threshold, action potential is generated

Question 23

Explain why synaptic transmission is unidirectional

Answer 23

Only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors

Question 24

Define summation and name the 2 types

Answer 24

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
-temporal summation
-spatial summation

Question 25

What is the difference between temporal and spatial summation

Answer 25

Temporal; one presynaptic neuron releases neurotransmitter several times in quick succession
Spatial; multiple presynaptic neurons release neurotransmitter

Question 26

What are cholinergic synapses

Answer 26

Use acetylcholine as primary neurotransmitter. Located at;
Motor end plate (muscle contraction)
Preganglionic neurons (excitation)
Parasympathetic postganglionic neurons (inhibition eg heart or breathing rate)

Question 27

What happens to acetylcholine from the synaptic cleft

Answer 27

1. Hydrolysis into acetyl and choline by AChE
2. Acetyl and choline diffuse back into presynaptic membrane
3. ATP is used to reform acetylcholine for storage in vesicles

Question 28

Explain the importance of AChE

Answer 28

Prevents overstimulation of skeletal muscle cells
Enables acetyl and choline to be recycled

Question 29

What happens in an inhibitory synapse

Answer 29

1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open
2. Cl- moves in and K+ moves out by facilitated diffusion
3. p.d becomes more negative; hyperpolarisation

Question 30

Describe the structure of a neuromuscular junction

Answer 30

Synaptic cleft between a presynaptic neuron and a skeletal muscle cell

Question 31

Contrast a cholinergic synapse and a neuromuscular junction

Answer 31

Response - excitatory in neuro, excitatory or inhibitory in cho
Neurons involved - only motor in neuro, motor, sensor or relay in cho
AChE location - postsynaptic membrane in neuro, synaptic clef in cho

Question 32

How might drugs increase synaptic transmission

Answer 32

Inhibit AChE
Mimic shape of neurotransmitter

Question 33

How might drugs decrease synaptic transmission

Answer 33

Inhibit release of neurotransmitter
Decrease permeability of postsynaptic membrane to ions
Hyperpolarise postsynaptic membrane