6.5

Question 1

what is the CNS

Answer 1

central nervous system:
the brain and the spinal cord

Question 2

what is the PNS

Answer 2

Peripheral Nervous System:
all the nerves which connect the body to the CNS

Question 3

what is a neuron

Answer 3

an individual cell which transmits electrical impulses

Question 4

what is a nerve

Answer 4

many neurons grouped together in a bundle

Question 5

how are motor neurons adapted for their function

Answer 5

dendrites-highly branched to increase surface area over which impulses are received
Mitochondria – provide ATP for sodium potassium pumps to maintain resting potential.
Long, thin axon – conducts impulses quickly over long distance.
Myelin sheath – electrical insulator for action potential, which speeds up conduction.
Nodes of Ranvier – gaps in myelin sheath
Motor end plate - specialised terminal of neurons and can synapse with effectors causing them to contract

Question 6

Resting potential

Answer 6

• when an neuron is not firing
• cell membrane is said to be polarised
• has a potential difference of -70mV because there are more Na+ ions outside than there are K+ ions inside this with respect to the outside the inside is more negative
• is maintained by Na/K pump as it pumps out 3 Na+ ions but only let’s in 2 K+ ions —> maintains concentration gradient of more Na+ outside than K+ inside
• has a slightly leaky membrane which allows Na+ ions to diffuse in and K+ ions to diffuse out, however more K+ ions diffuse out thus helps build positive charge outside
• inside of axon has some negatively charged ions (Cl-) which contributes to more negative charge inside

Question 7

Action potential: depolarisation

Answer 7

• when a nerve impulse is generated voltage gated Na+ channels open
• Na+ ions rapidly diffuse down their concentration gradient into the axon
• this reverses the potential difference which is now at around +40mV
• the inside of the axon is now positive with respect to the outside
• this is called depolarisation

Question 8

Action potential: Repolarisation

Answer 8

• voltage gated Na+ channels close so that no more Na+ ions can diffuse into the axon
• voltage gated K+ channels open & K+ ions rapidly diffuse down their concentration gradient outside the axon
• this decreases the potential difference & the inside of the axon is now negative with respect to the outside
• this is called repolarisation

Question 9

analysis of oscilloscope

Answer 9

Question 10

what is hyper-polarisation

Answer 10

the slight overshoot of repolarisation (slight dip in graph) which causes the membrane potential to become lower than it’s normal value

it’s restored by the Na/K pump

Question 11

refractory period

Answer 11

• the period of time in which an action potential cannot be generated and a neuron does not fire
• ensures that action potentials can only travel in one direction. they cannot go backwards to an area in the refractory period
• imposes a limit to the frequency of nerve impulses

Question 12

propagation of action potentials

Answer 12

• when Na+ ions from a depolarised part of an axon diffuse into resting neighbouring part (lower Na+ conc.), known as local currents, the potential difference rises to -50mV
• this is known as the threshold potential
• when the threshold potential is reached, voltage gated Na+ channels open causing depolarisation to occur which propagates the action potential along the axon

Question 13

myelination & saltatory conduction

Answer 13

• myelin sheath acts as an electrical insulator around the axon membrane
• action potential cannot form where myelin sheath is present as Na+ cannot
  enter the axon there
• however, action potentials can form at nodes of ranvier where myelin sheath is absent
• this means that nerve impulses jump from node to node = SALTATORY CONDUCTION

—> benefit is that it greatly speeds up the conduction of nerve impulses

Question 14

what is a synapse and how does an impulse cross it

Answer 14

• synapse is a junction between neurons
• an impulse crosses a synapse by chemical diffusion of a neurotransmitter across the synaptic cleft

Question 15

synaptic transmission

Answer 15

1. An action potential reaches the pre-synaptic knob on the pre-synaptic neuron. Depolarisation causes calcium channels to open and calcium ions diffuse into the knob.
2. The difference in calcium ions stimulates the movement of vesicles, containing neurotransmitters, towards the presynaptic membrane.
3. The vesicles fuse with the membrane and release the neurotransmitters into the synaptic cleft via exocytosis.
4. The neurotransmitters rapidly diffuse across the synaptic cleft and bind to receptors on the post synaptic membrane, which opens specific sodium channels in the membrane
5. Sodium ions diffuse into the postsynaptic neuron which generates an action potential.
6. Neurotransmitters are removed from receptors by enzymes which causes ion channels to close.

Question 16

cholinnergic synapses: acetylcholine

Answer 16

• A neurotransmitter which is found in the synapses between motor neurons and muscle fibres. It is responsible for muscle contraction.
• it is made of choline and an acetyl group.
• Acetylcholinase breaks down ACh from receptors on postsynaptic membrane to prevent over stimulation

Question 17

neonicotinoids

Answer 17

• They are pesticides, which irreversibly bind to ACh receptors in insect synapses, this prevents ACh binding to receptors and blocks signals to the muscles leading to paralysis
• neonicotinoids reduces damage to crops and is non-toxic to humans as it only binds to insect ACh receptors
• However, they are used widely, and they may be causing collapse of the honeybee population

Question 18

threshold potential

Answer 18

• an action potential will either occur, or it will not, and all or nothing principle
• Nerve impulse is only initiated if the threshold potential (-50mV) is reached as this is needed to open voltage gated sodium channels

Question 19

how does intensity of stimulus affect nerve impulses

Answer 19

an increased intensity results in an increase in frequency of action, potentials, not size of them

Question 20

What is summation and why is it useful?

Answer 20

spatial summation:
When a postsynaptic neuron receives neurotransmitters from multiple presynaptic neurons to reach the threshold potential.

Temporal summation:
When a postsynaptic neuron receives a neurotransmitter repeatedly from one pre-synaptic neuron to reach the threshold potential.

This is done to get to the action potential.