Module 6 - Nervous coordination

Question 1

What is meant by a cell membrane being polarised?

Answer 1

At resting state, there are more positively charged ions outside the membrane creating a potential difference

Question 2

What is used to maintain the potential difference?

Answer 2

Sodium-potassium pumps and potassium-ion channels

Question 3

How is a sodium ion electrochemical gradient created?

Answer 3

Sodium ions move out of the cell membrane via the sodium-potassium pump but because the membrane isn’t permeable to sodium ions, they can’t diffuse back in

Question 4

Explain of a stimulus affecting a neurones and depolarisation

Answer 4

A stimulus will excite the cell membrane of the neurone and cause sodium-ion channels to open. The membrane becomes more permeable to sodium which causes them to diffuse into the cell. A potential difference will occur in the neurone which leads to a generator potential. once it reaches a threshold of around -55mv then an action potential will be generated.

Question 5

Explain the stage of repolarisation

Answer 5

A potential difference of around +30mv means sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so the potassium ion channels open, down the conc gradient bringing it back to its resting potential

Question 6

Explain the stage of hyperpolarisation

Answer 6

Potassium ion channels are slow to close which causes too many potassium ions to diffuse out of the neurone which is known as an overshoot. The potential difference becomes -70mv which is too negative

Question 7

How do action potentials move along the neurone?

Answer 7

When an action potential is generated, some of the sodium ions generated enter sideways which causes sodium ion channels in the next region of the neurone to open and sodium ions to diffuse into that part. A wave of depolarisation will travel along the neurone

Question 8

What is the refractory period?

Answer 8

This period acts as a time delay from one action potential and the next

Question 9

What are the 3 conditions of an action potential?

Answer 9

They don’t overlap,
There is a limit to the frequency,
They are unidirectional

Question 10

What are the 3 factors that affect the speed of conduction of action potentials?

Answer 10

1. Myelination
2. Axon diameter
3. Temperature

Question 11

What is meant by neurones being myelinated?

Answer 11

They have a myelin sheath which is an electrical insulator

Question 12

What are schwann cells?

Answer 12

In the peripheral nervous system the sheath is made up of a type of cell

Question 13

What are the Nodes of Ranvier?

Answer 13

Here sodium ions cannot get through the membrane and so sodium ions are very concentrated here. In a myelinated neurone, depolarisation only happens in the Nodes of Ranvier

Question 14

What is saltatory conduction?
What is the difference in saltatory conduction between myelinated neurones and non-myelinated neurones

Answer 14

The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the electrical impulse jumps from node to node. Then in a non-myelinated neurone, depolarisation takes place along the whole length of the axon

Question 15

The second factor is Axon diameter.
How does this impact the speed of conduction?

Answer 15

Action potentials are quicker along longer axons because there is less resistance to the flow of ions compared to a smaller axon
With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker

Question 16

The third factor is temperature. How does this impact the speed of conduction?

Answer 16

Speed of conduction increases when the temperature increases because ions can diffuse faster
Speed only increases up to 40degrees otherwise the proteins being to denature and the speed decreases.

Question 17

What is a synapse?

Answer 17

The junction between one neurone and another and the junction between one neurone and an effector cell

Question 18

What is a synaptic cleft?

Answer 18

The space between the presynaptic membrane and the post synaptic membrane

Question 19

What makes transmission across a synapse unidirectional?

Answer 19

The complementary receptors are only on the post-synaptic membrane which means that transmission can only go in one direction

Question 20

Explain the steps of transmission across a cholinergic synapse

Answer 20

1. An action potential stimulates voltage gated calcium ions which diffuse into the pre-synaptic knob
2. Influx causes synaptic vesicles to move to the presynaptic membrane and fuse. This releases ACh into the synaptic cleft
3. ACh diffuses over to postsynaptic membrane and binds to receptors which causes sodium ion channels to open
4. Influx of Na+ causes depolarisation and so action potential is generated.

Question 21

What is done to the synaptic cleft to remove neurotransmitters in a cholinergic synapse?

Answer 21

It is broken down by the enzyme acetylcholinesterase and products are re-absorbed by the presynaptic neurone

Question 22

What is meant by neurotransmitters being excitatory and inhibitory?

Answer 22

Excitatory - depolarises the postsynaptic membrane making it fire an action potential
Inhibitory - hyperpolarise the postsynaptic membrane, preventing it from firing an action potential

Question 23

What is summation?

Answer 23

Neurotransmitters being released from many neurones added together

Question 24

What is the difference between spatial and temporal summation?

Answer 24

Spatial - many neurones connected to one neurone
Temporal - two or more nerve impulses arrive in a quick succession

Question 25

What is a neuromuscular junction?

Answer 25

A synapse between a motor neurone and a muscle cell

Question 26

Differences between a neuromuscular junction and cholinergic synapses.

Answer 26

• Postsynaptic membrane has many folds that form clefts to store the enzyme that breaks down ACh
• Postsynaptic membrane has more receptors that other synapses
• ACh is always excitatory at a neuromuscular junction

Question 27

5 drugs that affect the action of neurotransmitters at synapses

Answer 27

• Drugs that are the same shape as receptors - more activated
• Drugs block receptors so they can’t be activated
• Inhibit enzyme breaking down neurotransmitter so more to bind to receptors
• Stimulation of neurotransmitter from presynaptic neurone so more receptors activated
• Some drugs inhibit neurotransmitter release so fewer receptors activated

Question 28

Why is transmission across a cholinergic synapse unidirectional?

Answer 28

• There are only receptors in the post-synapse
• Neurotransmitter is only released at the pre-synapse