6.2. Nervous Coordination

Question 1

Describe the general structure of a motor neurone

Answer 1

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

Question 2

Describe the additional features of a myelinated motor neurone

Answer 2

• Schwann cells: wrap around axon many times
• Myelin sheath: made from myelin-rich membranes of Schwann cells
• Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath

Question 3

Name 3 processes Schwann cells are involved in

Answer 3

• Electrical insulation
• Phagocytosis
• Nerve regulation

Question 4

How does an action potential pass along an unmyelinated neurone?

Answer 4

• Stimulus leads to influx of Na+ ions. First section of membrane depolarises
• Local electrical currents cause sodium voltage gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise
• 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 does not travel along whole axon length

Question 6

What is resting potential?

Answer 6

• Potential difference (voltage) across neurone 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
3) Establishes electrochemical gradient: cell contents more negative than extracellular environment

Question 8

Name the stages in generating an action potential

Answer 8

• Depolarisation
• Repolarisation
• Hyperpolarisation
• Return to resting potential

Question 9

What happens during depolarisation?

Answer 9

1) Stimulus -> facilitated diffusion of Na+ ions into cell down electrochemical gradient
2) Potential difference across membrane becomes more positive
3) If membrane reaches threshold potential (-50mV), voltage gated Na+ channels open
4) Significant influx of Na+ ions reverses potential difference 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) Potential difference across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

1) ‘Overshoot’ when K+ ions diffuse out = potential difference becomes more negative than resting potential
2) Refractory period: no stimulus is large enough to raise membrane potential threshold
3) Voltage gated K+ channels close and sodium potassium pump re-establishes resting potential

Question 12

What is the refractory period?

Answer 12

No action potentials can be generated in the hyperpolarised sections of the membrane

Question 13

Explain the importance of the refractory period

Answer 13

• Ensures unidirectional impulse
• Ensures discrete impulse
• Limits frequency of impulse transmission

Question 14

What is the ‘all or nothing’ principle?

Answer 14

Any stimulus that causes the membrane to reach threshold potential will generate an action potential

Question 15

Name the factors that affect the speed of conductance

Answer 15

• Myeline sheath
• Axon diameter
• Temperature

Question 16

How does axon diameter affect the speed of conductance?

Answer 16

• Greater diameter = faster
• Less resistance to flow of ions (depolarisation and repolarisation)
• Less leakage of ions - easier to maintain membrane potential

Question 17

How does temperature affect speed of conductance?

Answer 17

• Higher temperature - faster
• Faster rate of diffusion - depolarisation and repolarisation
• Faster rate of respiration = more ATP for active transport to re-establish resting potential

Question 18

What happens to the speed of conductance if the temperature is too high?

Answer 18

Membrane proteins denature so slower speed of conduction

Question 19

Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance

Answer 19

Student’s t-test (comparing means of continuous data)

Question 20

Suggest appropriate units for the maximum frequency of impulse conduction

Answer 20

Hz

Question 21

How can an organism detect the strength of a stimulus

Answer 21

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

Question 22

Function of synapses

Answer 22

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

Question 23

Describe the structure of a synapse

Answer 23

• Pre-synaptic neurone ends in a synaptic knob: contains lots of mitochondria, endoplasmic reticulum and vesicles of a neurotransmitter
• Synaptic cleft: 20-30 nm gap between neurones
• Post synaptic neurone: has complementary receptors to neurotransmitter

Question 24

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

Answer 24

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

Question 25

How do neurotransmitters cross the synaptic cleft?

Answer 25

Simple diffusion

Question 26

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

Answer 26

1) Neurotransmitter binds to specific receptor on post synaptic membrane
2) Ligand-gated Na+ channels open
3) If influx of Na+ ions raises membrane to threshold potential, action potential is generated

Question 27

Explain why synaptic transmission is unidirectional

Answer 27

• Only presynaptic neurone contains vesicles of neurotransmitter
• Only post synaptic membrane has complementary receptors

Question 28

Define summation

Answer 28

• Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
• No summation at neuromuscular junctions

Question 29

Name the two types

Answer 29

• Temporal summation
• Spatial summation

Question 30

What is the difference between temporal and spatial summation?

Answer 30

• Temporal: one presynaptic neurone releases neurotransmitter several times in quick succession
• Spatial: multiple presynaptic neurones release neurotransmitter

Question 31

What are cholinergic synapses?

Answer 31

• Use acetylcholine as primary neurotransmitter
• Excitatory or inhibitory

Question 32

Where are cholinergic synapses located?

Answer 32

• Motor end plate (muscle contraction)
• Preganglionic neurones (excitation)
• Parasympathetic postgangionic neurones (inhibition e.g. of heart or breathing rate)

Question 33

What happens to acetylcholine from the synaptic cleft?

Answer 33

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

Question 34

Explain the importance of AChE

Answer 34

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

Question 35

What happens in an inhibitory synapse?

Answer 35

1) Neurotransmitter binds to and opens Cl- channels on post synaptic membrane and triggers K+ channels to open
2) Cl- move in and K+ move out via facilitated diffusion
3) Potential difference becomes more negative: hyperpolarisation

Question 36

Describe the structure of a neuromuscular junction

Answer 36

Synaptic cleft between a presynaptic neurone and a skeletal muscle cell

Question 37

Contrast a cholinergic synapse and a neuromuscular junction

Answer 37

• Cholinergic has AChE on synaptic cleft, neuromuscular has AChE on postsynaptic membrane
• Cholinergic response is excitatory or inhibitory, neuromuscular is always excitatory
• Cholinergic has motor, sensory, relay neurones involved, neuromuscular only has motor

Question 38

How might drugs increase synaptic transmission?

Answer 38

• Inhibit AChE
• Mimic shape of neurotransmitter

Question 39

How might drugs decrease synaptic transmission?

Answer 39

• Inhibit release of neurotransmitter
• Decrease permeability of post synaptic membrane to ions
• Hyperpolarise postsynaptic membrane