6.2 - Nervous coordination

Question 1

Describe the general structure of a motor neuron.

Answer 1

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

Question 2

Describe the additional features of a myelinated motor neuron.

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 Schwamm cells where there is now 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 pass along an unmyelinated neuron?

Answer 4

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

Question 6

What is resting potential?

Answer 6

Potential difference (voltage) across neuron membrance when not stimulated (-50 to -90 mV, usually about -70 mV in humans).

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 the cell and 2K+ into the 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 the 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 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 diffusionb 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+ ions diffuse out = p.d. becomes more negative than resting potential.
2. Refractory period- no stimulus is large enough to raise membrance potential to threshold.
3. Voltage-gated K+ channels close & 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 actipn 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 the speed of conductance?

Answer 15

greater diameter = faster
- Less resistance to flow of ions (depolarisation & repolarisation).
- Less ‘leakage’ of ions (easier to maintain membrane potential).

Question 16

How does temperature affect the speed of conductance?

Answer 16

Higher temperature = faster
- faster rate of diffusion
- faster rate of respiration = more ATP for active transport to re-establish resting potential
- Temp too high = membrane proteins denature

Question 17

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

Answer 17

Students t-test (comparing means of continuous data)

Question 18

Suggest appropriate units for the maximum frequency of impulse conduction.

Answer 18

Hz

Question 19

How can an organism detect the strength of a stimulus?

Answer 19

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

Question 20

What is the function of synapses?

Answer 20

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

Question 21

Describe the structure of a synapse.

Answer 21

Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
synaptic cleft: 20-30 nm gap between neurons.
Postsynaptic neuron: has complementary receptors to neurotransmitter.

Question 22

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

Answer 22

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

Question 23

How do neurotransmitters cross the synaptic cleft?

Answer 23

Via simple diffusion

Question 24

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

Answer 24

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

Question 25

Explain why synaptic transmission is unidirectional.

Answer 25

Only presynaptic neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.
So impulse always travels presynaptic –> postsynaptic.

Question 26

Define summation.

Answer 26

Neurotransmitters from several sub-threshold impulses accumalates to generate action potential.

Question 26

Name the 2 types of summation.

Answer 26

• Temporal summation
• Spatial summation

Question 27

What is the difference between temporal and spatial summation.

Answer 27

• Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession.
• Spatial multiple presynaptic neurons release neurotransmitter

Question 28

What are colinergic synapses?

Answer 28

They use acetylcholine (ACh) as a primary neurotransmitter. They can either be ecitory or inhibitory and are located at:
- motor end plate (muscle contraction)
- preganglionic neurons (exciitation)
- parasympathetic postganglionic neurons

Question 29

What happend to ACh from the synaptic cleft?

Answer 29

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

Question 30

Explain the importance of acetylcholinesterase.

Answer 30

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

Question 31

What happens in the inhibitory synapse?

Answer 31

1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane & triggers K+ channels to open.
2. Cl- moves in and K+ moves out via facilitated diffusion.
3. Potential difference becomes negative: hyperpolarisation.

Question 32

Describe the structure of a neuromuscular junction

Answer 32

Synaptic cleft between a presynaptic neuron and a skeletal muscle cell.

Question 33

How might drugs increase synaptic transmission?

Answer 33

• Inhibit AchE
• Mimic the shape of a neurotransmitter

Question 34

How might drugs decrease synaptic transmission?

Answer 34

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