Nervous coordination and synaptic tranmission

Question 1

What is resting potential?

Answer 1

The neurone isn’t being stimulated
The outside of the membrane is positively charged compared to the inside
This makes the membrane polarised - there’s a difference in charge (called potential difference or voltage) across it
This is about -70 millivolts
Resting potential is created and maintained by the sodium-potassium pump and potassium ion channels in the neurones membrane
This is achieved by an electrochemical gradient of sodium and potassium ions

Question 2

How is an electrochemical gradient achieved across a neurone membrane?

Answer 2

The sodium-potassium pumps move sodium ions out of the neurone
The membrane is not permeable to sodium ions so they can’t diffuse back in
This creates a sodium ion electrochemical gradient because there are more positive sodium ions outside of the cell than the inside
The sodium-potassium pumps also move potassium ions into the neurone but the membrane is permeable to they can diffuse back out through potassium ion channels
This also makes the outside of the cell positively charged compared to the inside

Question 3

Action potential - stimulus

Answer 3

A stimulus excites the neurone cell membrane causing sodium ion channels to open
The membrane becomes more permeable to sodium ions and they diffuse into the neurone down the electrochemical gradient
The inside of the neurone is now less negative

Question 4

Action potential of a neurone - depolarisation (2)

Answer 4

If the potential difference reaches the threshold (around -55mv) more sodium ion channels will open and sodium will rapidly diffuse into the neurone
A wave of depolarisation is created
The wave moves away from the parts of the membrane in the refractory period as these parts can’t fire an action potential

Question 5

Action potential of a neurone - repolarisation (3)

Answer 5

At a potential difference of around -30mv the sodium ion channels close and the potassium ion channels open
The membrane is more permeable to potassium and so K+ ions diffuse out of the neurone down the potassium ion concentration gradient

Question 6

Action potential of a neurone - hyperpolarisation (4)

Answer 6

Potassium ion channels are slow to close so there’s a period where too many K+ ions have diffused out of the neurone
The potential difference becomes more negative than the resting potential

Question 7

Action potential of a neurone - resting potential is restored (5)

Answer 7

The ion channels are rest
The sodium potassium pump returns the membrane to its resting potential and maintains it until it is excited by another stimulus

Question 8

What is the refractory period?

Answer 8

During the refractory period, ion channels are recovering and can’t be opened - Na+ channels are closed during repolarisation and K+ channels are closed during hyperpolarisation
It is the time delay between one action potential and the next
It means that action potentials don’t overlap but pass along as discrete (separate) impulses
Action potentials are undirectional
There’s a limit to the frequency at which nerve impulses can be transmitted

Question 9

What is the all or nothing nature?

Answer 9

If the threshold isn’t reached, an action potential won’t fire
A bigger stimulus won’t cause a bigger action potential, it’ll cause them to fire more frequently

Question 10

What 2 factors affect the speed of conduction of action potentials?

Answer 10

2) Action potentials are conducted quicker along axons with bigger diameters because there’s less resistance to the flow of ions in the cytoplasm. With less resistance depolarisation can reach other parts of the cell membrane quicker
3) As temperature increases so does the rate of conduction as ions diffuse faster. This is only true up until around 40c where proteins will begin to denature

Question 11

How does myelination affect the speed of conduction of action potentials?

Answer 11

Some neurones are myelinated. The myelin sheath is an electrical insulator. In the peripheral nervous system the sheath is made out of Schwann cells. Between the Schwann cells are the nodes of Ranvier which have a high concentration of sodium ion channels
In a myelinated neurone depolarisation only happens at the nodes of Ranvier. The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node. This is saltatory conduction and is quicker than the conduction in a non-myelinated neurone as the impulse has to travel the whole length of the axon membrane

Question 12

What is a synapse?

Answer 12

The junction between a neurone and another neurone
Or between a neurone and an effector cell

Question 13

What is the synaptic cleft?

Answer 13

The gap between the cells at a synapse

Question 14

Transmission of a neurotransmitter across the synaptic cleft

Answer 14

The presynaptic neurone has a swelling called the synaptic knob
This contains synaptic vesicles filled with neurotransmitters
When an action potential reaches the end of a neurone, neurotransmitters are released into the synaptic cleft and diffuse across to the postsynaptic membrane and bind to specific receptors which may cause an action potential, muscle contraction or cause a hormone to be secreted from a gland cell
Neurotransmitters are removed from the cleft to ensure the response doesn’t keep happening

Question 15

How are synapses unidirectional?

Answer 15

Receptors are only present on the postsynaptic neurone

Question 16

What is a cholinergic synapse?

Answer 16

Synapses that use acetylcholine as their neurotransmitter
Their structure is the same

Question 17

How is a nerve impulse transmitted across a cholinergic synapse?

Answer 17

Depolarisation of presynaptic membrane
This stimulates voltage-gated calcium ion channels in the presynaptic neurone to open
Calcium ions diffuse into the synaptic knob which causes the synaptic vesicles to move and fuse with the presynaptic membrane
The vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic clefts.
Ach diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane . This causes sodium ion channels to open which cause depolarisation
An action potential is generated if the threshold is reached

Question 18

What does an inhibitory neurotransmitter do?

Answer 18

They hyperpolarise the postsynaptic membrane and so prevent it from firing an action potential

Question 19

What does an excitatory neurotransmitter do?

Answer 19

They depolarise the postsynaptic membrane and so make it fire an action potential if the threshold is reached

Question 20

What is spatial summation?

Answer 20

Sometimes many pre synaptic neurones connect to one post synaptic neurone
Each neurone may only release a small amount of neurotransmitter but altogether it is enough to reach the threshold in the postsynaptic neurone and trigger an action potential
If some neurones release an inhibitory neurotransmitter the total effect might result in no action potential

Question 21

What is temporal summation?

Answer 21

Where two or more nerve impulses arrive in quick succession from the same presynaptic neurone
This makes an action potential more likely because more neurotransmitter is released into the synaptic cleft

Question 22

What is a neuromuscular junction?

Answer 22

A synapse between a motor neurone and a muscle cell
They use the neurotransmitter acetyl choline (ACh) which binds to nicotinic cholinergic receptors

Question 23

Adaptations of a neuromuscular junction?

Answer 23

They work in basically the same way as a cholinergic synapse but
The postsynaptic membrane has lots of folds that from clefts. The clefts store the enzyme acetylcholinesterase (AChE)
The postsynaptic membrane has more receptors than other synapses
ACh is always excitatory

Question 24

Name 5 different ways drugs can affect the action of neurotransmitters at synapses

Answer 24

• Some are the same shape as neurotransmitters so mimic their action at receptors (agonists)
• Some block receptors so they can’t be activated by neurotransmitters (antagonists)
• Some inhibit the enzyme that breaks down a neurotransmitter
• Some stimulate the release of neurotransmitter from the presynaptic neurones so more receptors are activated
• Some inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated

Question 25

What enzyme breaks down acetylcholine

Answer 25

An enzyme called acetylcholinesterase (AChE) breaks down ACh and the products are re-absorbed into the presynaptic neurone