Nervous Coordination

Question 1

Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.

Answer 1

1. Myelination provides insulation
2. Myelination allows saltatory conduction at nodes of Ranvier
3. Non-myelinated needs conduction across entire length of axon whereas myelination only needs it at nodes of ranvier

Question 2

what is meant by saltory conduction?

Answer 2

action potentials been generated at the nodes of ranvier

Question 3

what would happen if the neuron was unmyelinated?

Answer 3

• every part of the axon would need to generate an action potential
• would take much longer for action potential to reach axon terminal

Question 4

what are the nodes of ranvier

Answer 4

• where action potential is generated (only place on axon where ions move in and out)
• gaps between the myelin sheath

Question 5

what is the schwann cell

Answer 5

• wrap around the axon to form the myelin sheath
• myelin sheath: lipid so doesnt allow ions to pass through

Question 6

How is a resting potential maintained?

Answer 6

1. Sodium potassium pump actively transports 2K+ in, 3NA+ out
2. This establishes a electrochemical gradient
3. The cell membrane is more permeable to K+ ions so K+ moves back out the membrane by facilitated diffusion
4. More K+ channels than Na+ = more K+ diffuse out
5. K+ channels are mainly open and Na+ channels are closed ( until there is a high enough voltage to trigger them to open)

Question 7

How are action potentials generated?

Answer 7

1. Sodium ion channels in the axon membrane open
2. Sodium ions pass into the axon down the electrochemical gradient
3. This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – depolarisation
4. Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
5. If the potential difference reaches -55mV (threshold potential), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of +40mV
6. An action potential is generated

Question 8

What is repolarisation?

Answer 8

1. About 1 ms after an action potential, all the sodium ion channels in this section close.
2. Potassium ion channels in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon
3. This returns the potential difference to normal - repolarisation
4. There is actually a short period of hyperpolarisation where the potential difference briefly becomes more negative than the normal resting potential- refractory period
5. The potassium ion channels then close and the sodium ion channels in this section of membrane become responsive to depolarisation again

Question 9

What is the refractory period?

Answer 9

Until Na+ channels become responsive again this section of the axon membrane is in a period of recovery and is unresponsive - refractory period

Question 10

What is the importance of the refractory period?

Answer 10

1. ensues that discrete impulses ae produced
2. ensures action potential travels in one direction
3. limits the number of impulse transmission

Question 11

what is meant by the all or nothing principle?

Answer 11

• if depolarisation doesn’t exceed -55mV, then the action potential and the impulse are not produced - failed initiations
• ensures animals only respond to large enough stimuli, rather than responding to every slight change in environment as this would overwhelm the senses and hinder survival

Question 12

How does axon diameter affect speed of conduction?

Answer 12

• thicker axons have a higher speed of conduction
• greater SA → increased rate of diffusion
• fewer leakage of ions SO action potentials are generated faster

Question 13

How does temperature affect speed of conduction?

Answer 13

• speed of conductance increases
  1. ions diffuse faster
  2. enzymes involved in respiration work faster SO more ATP for active transport of Na+/K+ pump
• more kinetic energy for the facilitated diffusion of K+ and Na+

Question 14

how do you find the maximum frequency of impulses within a period of time?

Answer 14

time divided by refractory period

Question 15

function of a synapse

Answer 15

1. action potential arrives at synaptic knob, voltage gated Ca2+ channels open and calcium ions diffuse through into the synaptic knob
2. presynaptic vesicles (contain neurotransmitter) fuse with membrane, neurotransmitter is released into the synaptic cleft and diffuses down its conc gradient
   3.neurotransmitter binds to receptors complementary in shape found on post- synaptic membrane
3. receptors are attached to Na+ channels on post- synaptic membrane causing them to open: if enough neurotransmitter, then enough Na+ diffuses in, post synaptic neurone becomes depolarised and triggers an action potential
4. neurotransmitter is broken down and released from receptor to prevent triggering another action potential.
   - Na+ channels close to re-establish resting potential

Question 16

Describe transmission across a cholinergic synapse.

Answer 16

1. Action potential arrives at pre-synaptic neuron Ca2+ channels open so calcium ions diffuse into pre-synaptic knob
2. Presynaptic vesicles fuse with membrane to release ACh
3. Acetyl choline diffuses across synaptic cleft
4. Binds to receptors on post-synaptic membrane
5. Receptor proteins ocause Na+ channels open and Na+ diffuses into post synaptic knob
6. Depolarisation of post-synaptic membrane
7. If threshold reached action potential generated
8. The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline
9. The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives

Question 17

What is temporal summation?

Answer 17

Multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential and exceed the threshold

Question 18

What is spatial summation?

Answer 18

many different neurones collectively release enough neurotransmitter to trigger an action potential and exceed the threshold (-55mV)

Question 19

why is summation needed?

Answer 19

• some action potentials do not result in sufficient concentrations of neurotransmitter being released to generate a new action potential

Question 20

Describe inhibition by inhibitory synapses.

Answer 20

• opening the gated potassium ion channels in the membrane so that potassium ions are able to diffuse out
• opening chloride ion channels more chloride to move in
• this causes ** hyperpolarisation** therefore action potential is unlikely to be triggered
  ADV: don’t respond to ever stimulus which could hinder survival

Question 21

where is the neuromuscular junction found?

Answer 21

• occurs between motor neurone and muscle (the effector)
• found at the end of reflex arc

Question 22

Describe transmission across a neuromuscular junction.

Answer 22

1. Depolarisation of pre-synaptic neuron
2. Ca2+ channels open so Ca2+ diffuses into pre-synaptic knob
3. Cause the exocytosis of vesicles
4. Ach containing vesicles fuse with pre-synaptic membrane
5. Acetyl choline diffuses across the NMJ
6. ACh binds to sarcolemma receptors
7. Na+ channels open and Na+ diffuses into sarcolemma
8. Action potential passes along sarcolemma and down T-tubules
9. Ca2+ ions diffuse out of sarcoplasmic reticulum into sarcoplasm
10. Ca2+ binds with troponin, initiating muscle contraction

Question 23

Explain how a resting potential is maintained across the axon membrane in a neurone. (3)

Answer 23

1. Higher concentration of potassium ions inside and higher concentration of sodium ions outside (the neurone)
2. (Membrane) more permeable to potassium ions (leaving than sodium ions entering)
3. Sodium ions (actively) transported out and potassium ions in

Question 24

Describe the sequence of events involved in transmission across a cholinergic synapse. Do not include details on the breakdown of acetylcholine in your answer. (5)

Answer 24

1. Depolarisation of presynaptic membrane 2. Calcium channels open and calcium ions enter (synaptic knob); 3. Calcium ions cause synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter; 4.Acetylcholine diffuses across synaptic cleft 5. Acetylcholine attaches to receptors on the postsynaptic membrane; 6. Sodium ions enter postsynaptic neurone leading to depolarisation;

Question 25

Neuromuscular junction Vs cholingeric synapse

Answer 25

both: unidirectional due to neurotransmitter receptors only being on the post synaptic membrane NJ: - only excitary (trigger action potential) - connects motor neurone to muscle - end point of the action potential -Acetylcholine binds to receptors on muscle fibres CS: - excitary or inhibitory - connects to neurones - a NEW action potential is generated in the next neurone - Acetylcholine binds to receptors on post- synaptic membrane of a neurone