5.1.3 Neuronal Communication

Question 1

what is the resting potential?

Answer 1

where the neurone is not conducting an impulse

Question 2

what are the common features of a neurone?

Answer 2

• Has a cell body, which contains the organelles found in a typical animal cell. Proteins and neurotransmitters are also made here.
• Dendrons, which carry the action potentials to surrounding cells.
• An axon, which is a long conductive fibre that carries the nervous impulse along the motor neurone.

Question 3

what is the name for a bundle of neurones?

Answer 3

A nerve

Question 4

what makes up the myelin sheath?

Answer 4

Schwann cells

Question 5

what are the 3 main types of neurone?

Answer 5

sensory, relay, and motor

Question 6

what do the sensory neurones do?

Answer 6

they carry electrical impulses from the receptor to the CNS

Question 7

what do the relay neurones do?

Answer 7

they carry electrical impulses between the sensory and motor neurones

Question 8

what do the motor neurones do?

Answer 8

they carry an electrical impulse from the CNS to the effectors

Question 9

describe the structure of a motor neurone

Answer 9

• a large cell body at one end that lies within the spinal cord or brain.
• a nucleus that’s always in its cell body
• one long axon
• many highly branched dendrites

Question 10

describe the structure of a relay neurone

Answer 10

• short
• highly branched

Question 11

describe the structure of a sensory neurone

Answer 11

• a cell body that branches off in the middle of the cell
• single long dendron
• short dendrites

Question 12

what is a cell that responds to a stimulus called?

Answer 12

a receptor cell

Question 13

why are receptor cells transducers?

Answer 13

because they convert energy from one form into an electrical impulse

Question 14

what are Pacinian corpuscles?

Answer 14

a type of mechanoreceptor found deep in the skin that responds to changes in pressure

Question 15

describe the action of Pacinian corpuscles

Answer 15

1) When there is no pressure, stretch-medium sodium ion channels are too narrow. Sodium consequently remains outside the membrane and the resting potential is maintained.
2) When pressure is applied, the layers become distorted, causing the sodium channels to open. Sodium ions then enter the axon of the secondary neurone.
3) A generator potential is then established. An influx of sodium ions changes the potential of the axon which causes depolarisation of the axon. If enough generator potentials are produced, an action potential will be established and a nerve impulse will move along the axon

Question 16

what is the voltage of a neuron at its resting potential?

Answer 16

• 70mV

Question 17

describe what happens during an action potential

Answer 17

• A stimulus triggers sodium ion channels in the axon to open and this allows sodium ions to diffuse into the neurone down an electrochemical gradient
• The membrane depolarises (becomes less negative) and reaches a threshold value of around -55mV. This triggers depolarisation.
• Voltage-gated sodium ion channels open and due to the electrochemical gradient built up by the sodium/potassium pump, sodium ions diffuse into the neurone.
• As the concentration of sodium ions increases, the membrane becomes more depolarised until it reaches +40mV. This is an example of positive feedback.
• Voltage-gated sodium ion channels then close and voltage gated potassium ion channels open.
• Potassium ions diffuse out of the neurone due to the electrochemical gradient built up by the sodium/potassium pump. The voltage inside the cell in now negative again (-70mV) and so this is called repolarisation.
• Potassium ion channels are slow to close and as a result, hyperpolarisation occurs and the voltage inside the neurone is now -75mV.
• The voltage-gated potassium channels then close and the potential difference of -70mV is restored.

Question 18

what happens after an action potential has been generated?

Answer 18

the membrane enters a refractory period where it can’t be stimulated because sodium ion channels are recovering and so can’t be opened.

Question 19

Why is the refractory period important?

Answer 19

• because it ensures that discrete impulses are produced. An action potential can’t be generated immediately after another and this makes sure that each impulse is separate.
• because is ensures that action potentials travel in one direction. This stops the action potential from spreading out in 2 directions (could prevent a response).
• because it limits the number of impulse transmissions. This is important to prevent overreaction to a stimulus.

Question 20

Describe the transmission of an electrical impulse along an axon after an action potential has been generated

Answer 20

• An action potential triggered in the neurone causes depolarisation of that section of the axon.
• The current causes the opening of sodium ion channels a little further up the axon.
• This causes an influx of sodium ions in this section of the axon generating an action potential in this direction
• The previous section of the axon is in the repolarisation stage (the sodium channels are closed and potassium channels are open) and is unresponsive
• This makes the action potentials discrete events and means the impulse can only travel in one direction

Question 21

what is the all or nothing principle?

Answer 21

the idea that if an impulse isn’t strong enough, then a threshold is not reached and an action potential is not generated

Question 22

what affects the speed of conduction?

Answer 22

• myelination
• the diameter of the axon
• temperature

Question 23

what is the only place that action potentials can occur?

Answer 23

at the nodes of ranvier

Question 24

what is saltatory conduction?

Answer 24

where the action potentials jump from one node of Ranvier to the next. This can only happen in myelinated neurones.

Question 25

is it true that an impulse will be conducted at a higher speed along neurones with thicker axons compared to thinner ones?

Answer 25

yes

Question 26

why are impulses conducted at a higher speed along neurones with thicker axons compared to thinner ones?

Answer 26

because a thicker axon has an axon membrane with a greater surface areas over which the diffusion of ions can occur.

Question 27

describe transmission at a synapse

Answer 27

- An impulse arrives at the end of the synaptic knob of the pre-synaptic neurone. The synaptic knob becomes depolarised and this leads to the opening of calcium ion channels and so calcium ions diffuse into the synaptic knob. -Neurotransmitters are released from vesicles at the presynaptic membrane. The neurotransmitters then diffuse across the synaptic cleft and temporarily bind with the receptor molecules on the post-synaptic membrane. - Sodium ion channels on the post synaptic membrane open and sodium ions diffuse into the neurone. If there's enough neurotransmitter, then enough sodium ions diffuse in and post-synaptic neurone becomes depolarised. - The neurotransmitter is then degraded and released from the receptor. - The sodium ion channels then close and the post-synaptic neurone can re-establish resting potential. - The neurotransmitter is then transported back into the presynaptic neurone where it's recycled.

Question 28

give an example of a neurotransmitter

Answer 28

acetylcholine (ACh)

Question 29

what is the enzyme for acetlycholine?

Answer 29

acetylcholine esterase

Question 30

describe summation

Answer 30

where the rapid build-up of neurotransmitters in the synapse helps generate an action potential

Question 31

what is spatial summation?

Answer 31

where many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value

Question 32

what is temporal summation?

Answer 32

where one neurone releases a neurotransmitter repeatedly over a short period of time to add up enough to exceed the threshold value.

Question 33

what are synapses that use ACh known as?

Answer 33

cholinergic synapses

Question 34

what are the 4 types of receptor?

Answer 34

- mechanoreceptor - chemoreceptor - thermoreceptor - photoreceptor

Question 35

Describe the components and events involved in the reflex arc

Answer 35

- There's an action potential in the sensory neurone - Nerve impulse reaches relay neurone - Nerve impulse in motor neurone then passes to effector, either a muscle or gland

Question 36

Describe the difference between the 2 action potentials and suggest explanations for the differences you have decribed

Answer 36

- The dopamine neurone is less polarised. - No clear refractory period - Repolarisation is slower - Different number of sodium/potassium pumps in neurone membrane - Different number of potassium ion channels - More sodium and potassium ion channels in the purkeyne neurone

Question 37

what does a high frequency of action potentials show?

Answer 37

that a stimulus is intense

Question 38

Outline the role of synapses in the nervous system

Answer 38

- To transmit electrical impulses between neurones. - To ensure that electrical impulses are only transmitted in one direction. - Convergence/ divergence/ summation - To filter out low level stimuli. - Prevents overstimulation and fatigue. -