6B: Nervous Coordination

Question 1

What are neurones?

Answer 1

Neurones are nerve cells which are specially adapted to carry nerve impulses (electrochemical changes) to one part of the body to another.

Question 2

Function of sensory neurone?

Answer 2

transmit nerve impulses from a receptor to an intermediate or motor neurone. They have one
dendron that is often very long. It carries the impulse towards the cell body and one axon that carries it away from the cell body.

Question 3

Function of the motor neurone?

Answer 3

transmit nerve impulses from an intermediate or relay neurone to an effector, such as a gland or
a muscle. Motor neurones have a long axon and many short dendrites.

Question 4

Function of intermediate/relay neurone?

Answer 4

transmit impulses between neurones, for example, from sensory to motor neurones. They have numerous short processes

Question 5

Structure of myelinated motor neurone?

Answer 5

● Cell body – which contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum.
This is associated with the production of proteins and neurotransmitters
● Dendrons – extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve
impulses towards the cell body
● An axon – a single long fibre that carries nerve impulses away from the cell body
● Schwann cells – which surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis to
remove cell debris and play a part in nerve regeneration. Schwann cells wrap themselves around the axon many times, so that
layers of their membranes build up around it. This forms the myelin sheath.
● Myelin sheath – a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in
a lipid known as myelin. Neurones with a myelin sheath are called myelinated neurones. The myelin sheath is an insulator as
the lipid does not allow charged ions to pass through to the axon.
● Nodes of Ranvier – constrictions (gaps) between adjacent Schwann cells where there is no myelin sheath. The constrictions
are 2- 3 µm long and occur every 1- 3 mm in humans

Question 6

How is the resting potential maintained?

Answer 6

1. 3 Na+ binds to specific receptors on the intracellular side of the protein channel
2. Na+ actively transported out of the axon
3. 2 K+ bind to specific receptors on the protein channel
4. K+ actively transported into the axon
5. The outward active transport of Na+ ions is greater than the inward active transport of potassium ions meaning the inside is more negative than the outside (3 to 2)
6. Also more Na+ in the tissue fluid than in the cytoplasm and more K+ in the cytoplasm than in the tissue fluid →creating an electrochemical gradient.
7. The membrane is more permeable to K+ at rest so K+ ions begin to diffuse by facilitated diffusion down the concentration gradient back out of the axon in potassium ion channels
8. There are more open potassium voltage gated channels than sodium voltage gated channels in the phospholipid bilayer
   of the axon so only some Na+ diffuse back into the cell by facilitated diffusion
9. As even more positive charge is leaving the cell, the resting potential is maintained as more positive outside than
   inside the cell

Question 7

What happens in an action potential?

Answer 7

• When a stimulus (which is big enough) is detected by a receptor in the nervous system, sodium ion channels open and there is an influx of positive charge into the axon, increased permeability to Na+
• There is a temporary reversal of the charges either side of this part of the axon membrane, inside the membrane goes from -70 mV → +40 mV (axon membrane depolarised).
• This depolarisation occurs because the channels in the axon membrane change shape, and hence open or close depending on the voltage across the membrane.

Question 8

The sequence of events in an action potential?

Answer 8

1. RESTING POTENTIAL
   ○ At resting potential some K+ voltage-gated channels are open (permanently) but the Na+ voltage-gated channels are closed.
2. STIMULUS
   ○ The energy of the stimulus causes some Na+ voltage-gated channels in the axon membrane to open and therefore membrane permeability to Na+ ions increases
   ○ Na+ diffuse into the axon through these channels along their electrochemical gradient.
   ○ Being positively charged, they trigger a reversal in the potential difference across the membrane.
3. DEPOLARISATION
   ○ As the Na+ ions diffuse into the axon, if the potential difference reaches the threshold (-55 mV), it causes more sodium ion channels
   to open, causing an even greater influx of sodium ions by diffusion.
4. REPOLARISATION
   ○ Once the action potential of around +40 mv has been established:
   i. ii. the voltage gates on the Na+ ion channels close, preventing further influx of Na+ ions.
   The voltage gates on the K+ ion channels begin to open, membrane is more permeable to K+
   ○ K+ diffuse out of the axon through these channels along their electrochemical gradient.
   ○ The electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium
   ion channels to open. This means that yet more potassium ions diffuse out, starting repolarisation of the axon.
5. HYPERPOLARISATION
   ○ The potassium ion channels are slow to close and as there is the outward diffusion of these K+ ions it causes a temporary overshoot of the electrical gradient.
   ○ The inside of the axon is more negative (relative to the outside) than at the resting potential of -70 mV
6. RESTING POTENTIAL
   ○ The voltage-gated ion channels on the K+ ion channels now close and the activities of the Na+-K+ pumps cause sodium ions to be pumped out and potassium ions in to re-establish and maintain the resting potential (-70 mV) until next stimulation.

Question 9

The all or nothing principle for impulses

Answer 9

If the depolarisation as a result of the stimulus reaches the threshold level an action potential is triggered.
Depolarisation below the threshold value (-55mV) - NOTHING
● No action potential →no impulse generated.
● So any stimulus, of whatever strength, that is below the threshold value will fail to generate an action potential.
Depolarisation above the threshold level (-55mV) - ALL
● Action potential generated →nerve impulse will travel.
● All action potentials are more or less the same size so always peak at the same maximum voltage.

Question 10

How can an organism perceive the size of a stimulus if all action potentials are the same size?

Answer 10

1. By the number of impulses passing in a given time (frequency). The larger the stimulus, the more impulses that are generated in a given time
2. By having different neurons with different threshold values. The brain interprets the number and type of neurons that pass impulses as a result of a given stimulus and thereby determines its size.

Question 11

Why is the all or nothing principle important?

Answer 11

● It makes sure that animals only respond to large enough stimuli
● Rather than responding to every slight change in the environment which would overwhelm them

Question 12

Passage of an action potential- Myelinated axon

Answer 12

1. In myelinated axons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing action
   potentials from forming.
2. At intervals of 1- 3 mm there are breaks in this myelin insulation, called nodes of Ranvier.
3. Action potentials can occur at these points as depolarisation can happen here.
4. The localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials jump from node to
   node in a process known as saltatory conduction.
5. As a result, an action potential passes along a myelinated neurones faster than along the axon of an unmyelinated
   one of the same diameter.
6. This is because in an unmyelinated neuron, the events of depolarisation have to take place all the way along an axon and thus takes more time

Question 13

Nature of the refractory period

Answer 13

Once an action potential has been created in any region of an axon, there is a period afterwards when inward movement
of sodium ions is prevented because the sodium voltage-gated channels are closed (repolarisation)
● During this time it is impossible for a further action potential to be generated as Na+ channels inactivated
● During the refractory period ion channels are recovering and cannot be opened.
● This means there is a time delay between one action potential and the next.

Question 14

Importance of the refractory period

Answer 14

● No overlap of action potentials - discrete impulses
○ A new action potential cannot be formed immediately behind the first one
● There is a limit to the frequency at which the nerve impulses can be generated
○ As action potentials are separated from one another, it limits the number of them that can pass along an axon
in a given time
○ This limits the strength of the stimulus that can be detected.
● Action potentials are unidirectional (only in one direction)
○ Can only pass from an active region to a resting region
○ This is because action potentials cannot be created in a region in refractory

Question 15

How does myelination affect the speed of conductance?

Answer 15

● Myelin sheath is an electrical insulator preventing an action potential forming in the part of the axon covered in myelin.
● Sodium ion channels are concentrated at the Ranvier nodes between Schwann cells.
● In myelinated neuron, depolarisation only happens at nodes of Ranvier (where sodium ions can get through through the membrane).
● Neurons cytoplasm conducts enough electrical charge to depolarise the next node
● Action potentials impulse jumps from one node of Ranvier to another - saltatory conduction.
● This speeds up conductance.
● In non-myelinated neuron, impulse travels as a wave along the whale length of the axon so depolarisation happens along the whole length
of the membrane - slower than statutory conduction.

Question 16

How does axon diameter affect rate of conductance?

Answer 16

● Action potentials are conducted quicker along axons with bigger diameters:
● because there is less resistance to the flow of ions that are in the cytoplasm of a smaller axon.
● due to less leakage of ions from a large axon (leakage makes membrane potentials harder to maintain).
● With less resistance, depolarisation reaches other parts of the neuron cell membrane quicker.

Question 17

How does temperature affect the rate of conductance?

Answer 17

● Speed of conduction increases as temperature increases as ions can diffuse faster
● Increases up to around 40oC as the proteins begin to denature and impulses fail to be conducted at all
● Respiration provides ATP for active transport, this is controlled by enzymes

Question 18

Functions of synapses

Answer 18

They allow:
● A single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse.
○ This allows a single stimulus to create a number of simultaneous responses
● A number of impulses to be combined at a synapse.
○ This allows nerve impulses from receptors reacting to different stimuli to contribute to a single response.

Question 19

Cholinergic synapse impulse transmission

Answer 19

1. An action potential reaches the synaptic knob of the presynaptic neurone - DEPOLARISATION OF IT
2. This stimulates voltage-gated calcium ion channels in the presynaptic neurone to open
3. Calcium ions diffuse by facilitated diffusion into the synaptic knob
4. This influx of Ca ions causes the synaptic vesicles to move to the presynaptic membrane
5. They fuse with the membrane and releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft by
   exocytosis.
6. The ACh diffuses down its concentration gradient across the synaptic cleft to the postsynaptic membrane
7. It binds to complementary receptors on sodium ion protein channels on the postsynaptic cell surface membrane
8. This causes sodium ion channels to open and sodium to diffuse rapidly along a concentration gradient into the
   post-synaptic neurone
9. The influx of sodium ions causes the membrane potential to increase and if threshold is reached, it becomes
   depolarised so a new action potential is generated in the postsynaptic neurone.
10. Degradation occurs to the ACh which is released from the receptors as acetylcholinesterase AChE hydrolyses ACh into choline and ethanoic acid.
11. They diffuse back across the synaptic cleft to the presynaptic neurone and the products are reabsorbed and recycled to make more ACh. This prevents continuous generation of a new action potential so leads to discrete transfer of information across the synapse
12. In the pre-synaptic neurone, ATP released by mitochondria is used to recombine the chlorine and ethanoic acid → ACh which is stored in synaptic vesicles for further use.
13. The sodium ions channels in the receptor sites close and the resting potential is re-established in the post-synaptic neurone

Question 20

Features of synapse- Unidirectionality

Answer 20

Synapses only allow information to travel in one direction - pre → post synaptic neurone
● Neurotransmitters (e.g ACh) can only be generated in the presyanptic neurone and stored in synaptic vesicles
● Receptors complementary to the neurotransmitter (e.g sodium ion channels) are only on the postsynaptic
membranes on dendrites
● So neurotransmitters can only diffuse across the synaptic cleft from the presynaptic to the postsynaptic neurone

Question 21

Spatial summation

Answer 21

○ Many presynaptic neurones connect to one post synaptic neurone
○ They all release a small amount of neurotransmitter
○ Together this is enough to reach the threshold level and trigger action potential

Question 22

Temporal summation

Answer 22

○ A single presynaptic neurone releases neurotransmitter many times over a very short period
○ Nerve impulses arrive from the same presynaptic neurone in quick succession
○ If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered.

Question 23

Inhibition of synapses

Answer 23

1. The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the
   postsynaptic neurone
2. The neurotransmitter causes the chloride ion protein channels to open
3. Chloride ions move into the postsynaptic neurone by facilitated diffusion.
4. The binding of the neurotransmitter causes the opening of nearby potassium protein channels.
5. Potassium ions move out of the postsynaptic neurone into the synapse.
6. The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out
   is to make the inside of the postsynaptic membrane more negative and the outside more positive.
7. The membrane potential increases to as much as -80 mV compared with the usual - 70 mV at resting potential.
8. This makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one.

Question 24

Excitatory neurotransmitters

Answer 24

● Depolarise the postsynaptic membrane
● Making it fire an action potential is the threshold is reached
● Acetylcholine at cholinergic synapses in the CNS and neuromuscular junctions

Question 25

Inhibitory neurotransmitters

Answer 25

● Hyperpolarise the postsynaptic membrane
● Preventing firing of an action potential
● Acetylcholine in cholinergic synapses at the heart where potassium ion channels open so hyperpolarisation occurs

Question 26

Neuromuscular junction impulse transmission

Answer 26

1. When a nerve impulse is received at the neuromuscular junction, calcium ion channels open and influx of calcium
   causes the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine.
2. The acetylcholine diffuses to the postsynaptic membrane (which is the membrane of the muscle fibre)
3. It binds to nicotinic cholinergic receptors
4. This alters its permeability to sodium ions which enter rapidly, depolarising the membrane.
5. The postsynaptic membrane has lots of folds in it which form clefts
6. The clefts store AChE
   The acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated.
7. The resulting choline and ethanoic acid (acetyl) diffuse back into the neurone, where theyare recombined to form
   acetylcholine using energy provided by the mitochondria found there