6.2: Nervous coordination

Question 1

Where do synapses occur

Answer 1

Only occur in the grey matter of the brain (contains cell bodies)

Question 2

What does white matter in the brain contain

Answer 2

contains myelinated axons

Question 3

What does grey matter in the brain contain

Answer 3

Contains cell bodies

Question 4

What are neurones

Answer 4

specialised nerve cells

Question 5

What is a nerve

Answer 5

tissue - group of neurones

Question 6

Describe and explain the structure of a myelinated motor neurone

Answer 6

A cell body containing organelles (nucleus). Dendrites extend from the cell body and carry impulses toward the cell body which is carried away by the axon. The axon has a myelin sheath composed of Schwann cells that wrap around the axon providing electrical insulation. The gaps between the Schwann cells are called Nodes of Ranvier. The end of the axon is the synaptic knob (or axon terminal)

Question 7

What does the axon membrane contain

Answer 7

phospholipids, voltage-gated Na+ channel, Na+/K+ pump, K+ channel, voltage-gated K+ channel

Question 8

Describe the permeability of the axon membrane

Answer 8

Relatively permeable to K+
selectively permeable to Na+

Question 9

What is the resting potential

Answer 9

The potential difference across the axon membrane when no nerve impulse has been generated

Question 10

Explain the events in establishing a resting potential

Answer 10

Sodium/potassium pump actively transports 3 sodium ions out of the axon and 2 potassium ions into the axon. The axon membrane is more permeable to potassium ions than sodium ions, potassium ions move back out of the axon by facilitated diffusion. The membrane is impermeable to sodium ions so they remain outside of the axon. The uneven distribution of ions causes the membrane to be polarised, and a potential difference of -70mV is established across the membrane. This is called the resting potential. The inside of the axon is negatively charged relative to the outside.

Question 11

What is the action potential

Answer 11

a sudden, fast, transitory, and propagating change of the resting membrane potential. From -70mV to +40mV to -70mV in milliseconds

Question 12

Explain the process of depolarisation

Answer 12

When the neurone is stimulated, the voltage gated Na+ channels open, Na+ rapidly move into the axon by facilitated diffusion. The membrane is depolarised and the inside of the axon becomes positively charged (+40mV)

Question 13

Explain the process of repolarisation / hyperpolarisation

Answer 13

The Na+ gated channel proteins close and the Na+/K+ pump actively transports Na+ out of the axon. The K+ gated channel proteins open allowing K+ to diffuse faster out of the axon. The increased permeability to K+ causes the membrane to be hyperpolarised, with the inside of the axon negatively charged (-80mV). Once these K+ gated channels are all closed, the resting potential is reestablished by the Na+/K+ pump (-70mV)

Question 14

Explain the events in generating an action potential

Answer 14

A stimulus causes some voltage-gated Na+ channels to open. Na+ ions diffuse into the axon down their electrochemical gradient, the potential difference across the membrane becomes less negative as Na+ ions diffuse in. Once threshold has been reached (-55mV), more voltage gated Na+ channels open and Na+ ions flood into the axon rapidly by diffusion. This depolarises the membrane, once the action potential reaches +40mV the voltages gated Na+ channels close and voltage gated K+ channels open. K+ ions diffuse rapidly out of the axon repolarising the membrane, at resting potential these K+ channels begin to close but a little slowly causing a temporary overshoot of the usual resting potential, membrane is hyperpolarised (-80mV). Once these voltage gated K+ channels are all closed, resting potential is reestablished by the Na+/K+ pump (-70mV)

Question 15

What is the threshold for an action potential to occur

Answer 15

-55mV

Question 16

What causes voltage-gated ion channels in the cell-surface membrane of an axon to open

Answer 16

Neurotransmitter binding to a neurone

Question 17

Explain what is meant by the all or nothing principle

Answer 17

If a stimulus doesn’t cause threshold to be reached, there will be no action potential. If threshold is reached an action potential will be set up and it is independent to the size of the stimulus

Question 18

What is the effect on impulses when the intensity of the stimulus is greater

Answer 18

the greater the intensity of the stimulus above the threshold, the greater the frequency of impulses sent along the axon rather than a larger action potential

Question 19

Explain what is meant by the refractory period and why action potentials are prevented

Answer 19

The time following an action potential during which another action potential cannot take place. This is during the time the membrane is depolarised as the voltage gated Na+ channels are already open, continuing into the stage when the membrane is hyperpolarised

Question 20

Explain the importance of the refractory period

Answer 20

1- Prevents action potentials merging together - each action potential is discrete
2- limits the frequency of action potentials along a neurone
3- ensures the impulse travels along the axon in one direction only

Question 21

Explain the process of the propagation of the nerve impulse

Answer 21

The generation of an action potential in one section of the axon, sets up localised electrical circuits with the next section of the axon. This causes the Na+ gated channels to open in the next section, Na+ rapidly diffuse into the next section, the threshold is reached and an action potential is established in the next section. This process repeats along the entire length of the axon. When the section of the neurone is repolarising/hyperpolarised, it is in the refractory period -> preventing the impulse travelling back along the neurone

Question 22

What 3 factors affect the speed of an impulse

Answer 22

Temperature, Axon diameter, Myelination

Question 23

How does an increase in temperature affect the speed of an impulse

Answer 23

Increase in temperature increases the speed, Na+/K+ have more kinetic energy, increased rate of diffusion, increased rate of respiration ->more ATP for Na+/K+ pump

Question 24

How does an increase in axon diameter affect the speed of an impulse

Answer 24

As diameter increases, speed of impulse increases, because the wider the axon, the more cytoplasm, so a greater number of ions present. Increased surface area of membrane, so more Na+ channels/K+channels due to decreased longitudinal resistance

Question 25

How does myelination affect the speed of an impulse

Answer 25

myelin sheath insulates the axon - prevents movement of Na+/K+ into/out of the axon action potentials can only be generated at nodes of Ranvier, action potential jumps from node to node - saltatory propagation

Question 26

What is a synapse

Answer 26

A gap/junction between two neurones

Question 27

How do two neurones communicate across a synapse and give 2 examples

Answer 27

Via chemical neurotransmitters: 1- Acetylcholine (ACH) - found at a cholinergic synapse 2- noradrenalin

Question 28

Why is it that myelination increases the speed of an impulse compared to no myelination

Answer 28

The whole membrane must become depolarised when there is no myelination, when myelinated, depolarisation only occurs at the nodes

Question 29

What are the functions of synapses

Answer 29

Act as junctions 1- Ensure nerve impulse travels in one direction only 2- Some are inhibitory synapses 3- Allow summation

Question 30

How do synapses ensure nerve impulses travel in one direction only

Answer 30

vesicles containing the neurotransmitter are only found in the pre-synaptic neurone, receptors for the neurotransmitter are only found in the post-synaptic membrane. Neurotransmitter diffuses down a concentration gradient from the pre to post synaptic membrane

Question 31

How do some synapses work as inhibitory synapses

Answer 31

prevent the generation of an action potential in the post-synaptic neurone, binding of a neurotransmitter may open K+ gated channels, more K+ diffuses out of the axon, less likely that the threshold is reached

Question 32

What is summation by impulses and what are the two types

Answer 32

when several impulses are added together in order to release enough neurotransmitter to reach the threshold in the next neurone spatial summation and temporal summation

Question 33

What is the difference between spatial and temporal summation

Answer 33

spatial summation is where several impulses arrive at the same time from different neurones, releasing enough neurotransmitter to trigger an action potential in the post-synaptic neurone temporal summation is where several impulses arrive at the synapse from the same pre-synaptic neurone, releasing enough neurotransmitter to trigger an action potential in the post-synaptic neurone

Question 34

Describe the detailed structure of a synapse

Answer 34

Impulse travels down the axon to the synaptic knob of the presynaptic neurone where there are many mitochondria and vesicles containing neurotransmitter (ACh if a cholinergic synapse). There are also calcium ion channels which are closed until the membrane is depolarised. The post synaptic neurone has receptors for the neurotransmitters which travel across the synaptic cleft

Question 35

Fully explain the sequence of events involved in transmission of an action potential from one neurone to another

Answer 35

1- Action potential arrives at the synaptic knob depolarising the presynaptic neurone 2- Causes Ca2+ gated channels to open, Ca2+ diffuse into the synaptic knob 3- Causes synaptic vesicles to move to and fuse with the pre-synaptic membrane, releasing the neurotransmitter into the synapse by exocytosis (Ach if a cholinergic synapse) 4- Neurotransmitter diffuses across synaptic cleft and binds to receptors on post-synaptic membrane 5- Causes Na+ channels on the membrane to open as the receptors are linked to a Na+ gated channel 6- Na+ rapidly diffuse into the post-synaptic neurone 7- If threshold is reached, an action potential is produced in the post-synaptic neurone 8- After the action potential, the Ach detaches from the receptor and binds to the active site of Achase (acetylcholinesterase). This closes Na+ channel, Na+/K+ pump actively transports Na+ out of the post-synaptic neurone, post-synaptic neurone is replenished 9- As the Achase hydrolysed Ach, acetate and choline diffuse back across the synapse and are taken up by endocytosis into synaptic vesicles in the pre-synaptic neurone, Ach is reformed which uses energy from ATP hydrolysis

Question 36

Explain what a neuromuscular junction is

Answer 36

The synapse between a motor neurone and a muscle fibre, these are excitatory, cholinergic synapses where the neurotransmitter is Ach, and the binding of Ach will lead to muscle contraction

Question 37

What are the two types of drugs that affect nerve impulses

Answer 37

Agonists (stimulants) and Antagonists (depressants)

Question 38

How do agonists affect nerve impulses and what are 3 ways they do this

Answer 38

causes the formation of multiple action potentials in the post-synaptic neurone 1- mimic neurotransmitter 2- prevent the re-uptake of neurotransmitter 3- inhibit Achase, so Ach not broken down

Question 39

How do antagonists affect nerve impulses and what are 3 ways they do this

Answer 39

prevent the formation of an action potential in the post-synaptic neurone 1- block Ca2+ channels 2- prevent the release of neurotransmitter 3- prevent the binding of neurotransmitter to the receptors on the post-synaptic membrane