Nervous Coordination

Question 1

Neurones

Answer 1

Neurones are SPECIALISED CELLS, they are adapted to their function which is to carry electrical impulses from one part of the body to another.

There are different types of neurone: sensory neurone, relay neurone, motor neurone

Question 2

The structure of a motor neurone

Answer 2

Question 3

Schwann cells

Answer 3

Surround peripheral nerves and form myelin sheath

Question 4

How a resting potential is established

Answer 4

resting potential = -70mV
* active transport/pumping of sodium (ions across membrane);
* out of neurone/higher concentration outside;
* differential permeability to K+ and Na+;
* Membrane more permeable to K+ ions;

Question 5

How a nerve impulse is transmitted

Answer 5

the membrane potential goes form –70mV to + 40mV in a short period of time

Question 6

Depolarisation

Answer 6

Depolarisation = -55mV
* The high concentration of positive ions inside the cell is the ACTION POTENTIAL (+40mV)

Question 7

Repolarisation

Answer 7

Repolarisation = -70mV

Question 8

Hyperpolarisation

Answer 8

• The K+ ion channel proteins remain open longer than needed to reach resting potential, making the inside of the cell even more negative to about -90mV. (hyperpolarisation)
• The sodium potassium pump restores the resting potential back to -70mV
• K+ channel goes from open to leaky to make more positive

Question 9

The potential across the membrane is reversed when an action potential is produced.

Describe how.

Answer 9

• Sodium ion gates / channel (proteins) open;
• Na+ (rapidly) diffuse in;

Question 10

The All or Nothing response

Answer 10

• An action potential will only occur when the membrane is stimulated so that all the local Na+ voltage-gated channel proteins open.
• The minimum intensity of stimulus is called the THRESHOLD
• Sub-threshold no action potential will occur
• Above the threshold a full-size action potential is given regardless of the increase in the size of the stimulus
• This is the ALL OR NOTHING LAW

Question 11

The Refractory period

Answer 11

the time taken to restore the resting potential
- avoid waste of ATP - cause neuronal stress

• (Refractory period) limits number of impulses per second/frequency of nerve impulses;
• Maximum frequency of impulse transmission
• Period of time between threshold and resting membrane potential.
• When maximum frequency reached/exceeded, no further increase in information

Question 12

Impulse transmission along the axon

Answer 12

Action potential’s act as a stimulus to adjacent polarised areas of the membrane and this causes the action potential to be passed along

Question 13

Factors affecting speed of conduction of impulses: Myelin sheath and saltatory conduction

Answer 13

• The impulse travels by jumping from one node of Ranvier to the next node of Ranvier
• This is known as SALTATORY conduction
• It occurs because myelin sheath provides electrical insulation along axon and depolarisation can only occur at the nodes of Ranvier
• The electrical impulse depolarise the next node and the impulse is passed along by jumping form node to node
• This INCREASES the rate of transmission as depolarisation only occurs at the nodes / less of the axon membrane needs to be depolarised.

Question 14

Factors affecting speed of conduction of impulses: Temperature

Answer 14

• Higher temperatures increase the Kinetic energy so increases rate of diffusion of ions therefore increasing the rate of conduction

Question 15

Factors affecting speed of conduction of impulses: Axon diameter

Answer 15

• The larger the axon diameter the greater the speed of conductance as larger membrane surface area means there is an increases the number of channel proteins
• So less resistance to flow of ions;

Question 16

How the cholinergic synapse works

Answer 16

9. In a cholinergic synapse the enzyme acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid using water to break the ester bond.
10. Choline and ethanoic acid diffuse back across the synaptic cleft and are transported across the presynaptic neurone membrane.
11. ATP released by mitochondria is used to resynthesise acetylcholine which is then stored in vesicles inside the presynaptic neurone.

Question 17

Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.

Answer 17

• Depolarisation of presynaptic membrane;
• Ca2+ channels open and calcium ions enter (synaptic knob);
• synaptic vesicles fuse with presynaptic membrane and release acetylcholine
• Acetylcholine diffuses across synaptic cleft
• binds to receptors on the postsynaptic membrane;
• Sodium ions enter postsynaptic neurone leading to depolarisation;

Question 18

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.

Describe how.

Answer 18

• Nerve impulse causes Ca2+ channel (proteins) to open;
• Ca2+ enter by facilitated diffusion;
• Causes synaptic vesicles to fuse with presynaptic membrane;

Question 19

Describe how the inhibition of acetylcholinesterase affects the action of synapses.

Answer 19

• Acetylcholine not broken down
• Na+ ions (continue to) enter / (continued) depolarisation

Question 20

Synapses are unidirectional

Answer 20

• Vesicles containing neurotransmitter are only found in presynaptic neurone
• Receptors for neurotransmitter are only found on post synaptic neurone

The synapse will delay the impulse slightly.
Synapses prevent the impulse from going in the wrong direction.
Synaptic transmission is the same regardless of the neurotransmitter.

Question 21

The Neuromuscular junction

Answer 21

is a synapse between a motor neurone and a muscle cell. Neuromuscular junctions use acetylcholine which binds to nicotinic cholinergic (protein) receptors.

Neuromuscular junctions work in the same way as the cholinergic synapse but there are a few differences:
1. The postsynaptic membrane has lots of folds that form clefts. These clefts increase surface area so MORE acetylcholinesterase enzymes that hydrolyse acetylcholine at a faster rate.
2. The post synaptic membrane has more receptors than other synapses.
3. When a motor neurone fires an action potential, it always triggers a response in a muscle cell (this is not always the case for a synapse between two neurones).

Question 22

1. Spatial summation (Eye)

Answer 22

Different neurones converge at a single synapse. Action potentials arrive from several different neurones at the synapse.
This causes the release of enough neurotransmitter to reach threshold and cause an action potential in the post-synaptic neurone

Question 23

2. Temporal summation

Answer 23

In this case there is only one presynaptic neurone but the impulses arrive in rapid succession giving a cumulative effect which is sufficient to depolarise the post synaptic neurone

Question 24

Fatigue

Answer 24

If the rate of transmitter release is higher than the rate at which it is reformed, then it said to be fatigued. The presynaptic neurone cannot release enough neurotransmitter to generate an action potential in the post synaptic neurone until the transmitter is regenerated.

Question 25

Different types of neurotransmitters

Answer 25

Excitatory neurotransmitters cause action potential in post synaptic neurone by making the resting potential LESS negative, so less sodium ions are required to reach THRESHOLD. Inhibitory neurotransmitters (e.g. GABA: opens Cl- channel proteins) affect different receptors on post synaptic neurone. The resting potential becomes hyperpolarised and the postsynaptic membrane is therefore less likely to reach the threshold value and generate an action potential.

Question 26

The binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic neurones. Explain how this will inhibit transmission of nerve impulses by postsynaptic neurones.

Answer 26

* (Inside of postsynaptic) neurone becomes more negative/hyperpolarised; * More sodium ions required (to reach threshold) OR Not enough sodium ions enter (to reach threshold); * For depolarisation/action potential;

Question 27

The effects of drugs on synapses:

Answer 27

1. They can stimulate the nervous system by creating more action potentials in the post synaptic neurone- Excitatory effects. Drug may have similar shape to neurotransmitter and so bind to receptors on post synaptic membrane mimicking the effect of the neurotransmitter. Drug may cause an increase in the release of the neurotransmitter. Drug may inhibit enzyme that hydrolyses the neurotransmitter. 2. They can create fewer action potentials in the post synaptic neurone and have an inhibitory effect. Drug may inhibit release of neurotransmitter or bind and block the receptors on the post synaptic membrane.

Question 28

As a neurone transmits an impulse, its rate of oxygen consumption increases. Explain why.

Answer 28

* ATP required for active transport; * Na+ (actively) moved out only at nodes in myelinated