Nervous Coordination

Question 1

Describe the general structure of a motor neurone

Answer 1

Cell Body: Contains organelles and a high amount of RER
Dendrons: Branch into dendrites which carry impulses towards the cell body
Axon: Long unbranched fibre that carries nerve impulses away from the cell body

Question 2

Describe the additional features of a myelinated motor neurone

Answer 2

Schwann cells: wrap around axon
Myelin sheath: Made of schwann cells
Nodes of Ranvier: gaps between schwann cells where there is no myelin sheath

Question 3

How does an action potential pass along an un-myelinated neurone?

Answer 3

1. stimulus leads to an influx of Na+ ions, the first section of the membrane depolarises
2. Local electrical currents cause sodium voltage gated channels further along the membrane to open
3. A wave of depolarisation occurs

Question 4

Explain why myelinated axons conduct impulses faster than un-myelinated axons

Answer 4

Saltatory conduction: the impulse ‘jumps’ between each node of Ranvir. Depolarisation cannot occur where myelin sheath act as electrical insulators.
Impulses don’t travel along the whole length of the axon

Question 5

What is resting potential?

Answer 5

The potential difference (voltage) across a neurone when it is not stimulated. Usually about -70mV.

Question 6

How is resting potential established?

Answer 6

1. Membrane is more permeable to K+ than Na+

2. Sodium potassium pump actively transports 3 Na+ out of the cell and 2K+ into the cell

Question 7

What stages are involved in generating an action potential?

Answer 7

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

Question 8

What happens during depolarisation?

Answer 8

1. Stimulus- facilitated diffusion of Na+ ions into cell down electrochemical gradient
2. P.d across membrane becomes positive
3. If membrane reaches threshold potential, voltage gated Na+ channels open
4. Significant influx of Na+ ions reversers the p.d to +40mV

Question 9

What happens during repolarisation?

Answer 9

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 10

What happens during hyperpolarisation?

Answer 10

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 membrane potential to threshold
3. voltage gated K+ channels close and sodium potassium pump re establishes resting potential

Question 11

Explain the importance of the refractory period.

Answer 11

No action potential can be generated in hyperpolarised sections of membrane.

Ensures a unidirectional impulse
Ensures discrete impulses
Limits frequeny of impulse transmission

Question 12

What is the all or nothing principle?

Answer 12

An action potential will only occur if the threshold is reached

Question 13

What 3 factors affect the speed of conductance?

Answer 13

Myelin sheath
Axon diameter
Temperature

Question 14

How can organisms detect the strength of a stimulus?

Answer 14

Larger stimulus raises membrane to threshold potential more quickly meaning a greater frequency of impulses

Question 15

What is the function of a synapse?

Answer 15

Electrical impulses cannot travel over the junction between neurones
Neurotransmitters send impulses between neurones
New impulses can be initiated in several neurones at the same time

Question 16

What happens in the presynaptic neurone when an action potential is transmitted from one neurone to another?

Answer 16

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

Question 17

How do neurotransmitters cross the synaptic cleft?

Answer 17

Via simple diffusion

Question 18

What happens in the postsynaptic neurone when an action potential is transmitted from one neurone to another?

Answer 18

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

Question 19

What is summation?

Answer 19

A neurotransmitter from several sub threshold impulses accumulates to generate an action potential

Question 20

What is the difference between temporal and spatial summation?

Answer 20

Temporal= One presynaptic neurone releases neurotransmitter several times in a short period
Spatial= Multiple neurones release a neurotransmitter

Question 21

What are cholinergic synapses?

Answer 21

Use acetylcholine as their primary neurotransmitter, excitatory of inhibitory

Question 22

What happens to acetylcholine from the synaptic cleft?

Answer 22

1. Hydrolysis into acetyl and choline by acetylcholinesterase
2. Acetyl and choline diffuse back into presynaptic membrane
3. ATP is used to reform acetylcholine

Question 23

What happens in an inhibitory synapse?

Answer 23

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

Question 24

What is a neuromuscular junction?

Answer 24

Synaptic cleft between a presynaptic neurone and a skeletal muscle cell

Question 25

Give 3 differences between cholinergic synapses and a neuromuscular junction

Answer 25

Cholinergic- To another neurone, excitatory or inhibitory, motor, sensory and relay neurones

Neuromuscular junction- Skeletal muscle, always excitatory, only motor neurones

Question 26

How might drugs increase synaptic transmission?

Answer 26

Mimic shape of neurotransmitter

Question 27

How might drugs decrease synaptic transmission?

Answer 27

Inhibit release of neurotransmitter

Decrease permeability of postsynaptic membrane to ions

Hyperpolarise postsynaptic membrane