Nerves

Question 1

Resting potential (3 points)

Answer 1

• membrane of axon is not transmitting a nerve impulse
• inside of axon is negatively charged relative to the outside (polarised axon)
• -65mV

Question 2

Resting potential sequence

Answer 2

1. Sodium ions actively transported out of the axon by sodium-potassium pumps in membrane
2. Potassium ions actively transported into the axon by sodium-potassium pumps in membrane
3. Outward movement > inward movement
   3 sodium ions move out, for ever 2 potassium ions move in
4. Creates electrochemical gradient- more sodium ions in tissue fluid around axon than cytoplasm. More potassium ions in cytoplasm than tissue fluid
5. Sodium ions diffuse naturally back into axon, potassium ions begin to diffuse out of axon
6. Gates of channels that allow sodium ions to move are closed
7. Gates of channels that allow potassium ions are open
8. Negatively charged inside relative to outside.
9. Axon is polarised

Question 3

Action potential (3 points)

Answer 3

• membrane or axon is transmitting a nerve impulse
• stimulus causes reversal of charge on the axon membrane (depolarised)
• +40mv

Question 4

Action potential (sequence of events)

Answer 4

1. Energy of stimulus causes some sodium channel voltage gates to open.
   Sodium ions diffuse into axon along electrochemical gradient
2. Greater influx of sodium ions
3. Once action potential of +40mv has been established, sodium channel voltage gates close
4. Voltage gates of potassium channels open, more potassium ions diffuse out
5. Outward diffusion of potassium ions causes inside of axon to be more negative relative to outside = hyperpolarisation
6. potassium channel gates close. Sodium- potassium pumps work again
7. Resting potential (-65mv) re-established
   Axon is repolarised

Question 5

Inhibition of action potential

Answer 5

1. Presynaptic neurone releases a neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
2. Causes chloride ion protein channels to open
3. Chloride ions move into postsynaptic neurone by facilitates diffusion
4. Binding of neurotransmitter causes potassium protein channel to open
5. Potassium ions move out of postsynaptic neurone
6. Combined effect = chloride ions in, potassium out, inside of postsynaptic neurone more negative and outside more positive
7. Membrane potential increases = -80mv
8. Hyperpolaristion - less likely a action potential will be created. Larger influx of sodium ions is required

Question 6

Cholinergic synapse (2 points)

Answer 6

Neurotransmitter called acetylcholine

Occur in central nervous system and at neuromuscular junctions

Question 7

Cholinergic synapse (sequence of events)

Answer 7

1. Action potential at presynaptic neurone, causes calcium ion protein channels to open
   Calcium ions enter synapse by facilitated diffusion
2. Influx of calcium ions in presynaptic neurone, cause synaptic vesicles to fuse with presynaptic membrane
3. Acetylcholine diffuses across and binds to receptor sites on sodium ion protein channels in postsynaptic neurone membrane, open
   Sodium ions diffuse rapidly across a concentration gradient
4. Influx of sodium ions, generates action potential, in postsynaptic n
5. Acetylcholinesterase hydrolyses acetylcholine, diffuses back to presynaptic n
   Prevents it from continuously generating action potentials
6. ATP used to recombine acetylcholine, stores in vesicles for future use, sodium ion protein channels close in absence to acetylcholine in receptor sites

Question 8

Neuromuscular junction (3 points)

Answer 8

• point between a motor neurone meets a skeletal muscle fibre
• many neuromuscular junctions spread throughout the muscle: contraction of a muscle is rapid & powerful when it is simultaneously stimulated by action potentials
• motor unit: all muscle fibres act together when supplied by a single motor neurone

Question 9

Neuromuscular junction (sequence)

Answer 9

1. Nerve impulse receive at neuromuscular junction
2. Synaptic vesicles fuse with presynaptic membrane and release acetylcholine
3. Acetylcholine diffuses into postsynaptic membrane (muscle fibre)
4. Attach to sodium protein channels, sodium released rapidly. Depolarises membrane
5. Contraction
6. Acetylcholine hydrolysed by acetlycholinesterase, absorbed back, stored in vesicles, mitochondria release ATP