5.1.3 - Neuronal Communication

Question 1

What do sensory receptors do?

Answer 1

Detect a stimulus/changes in the internal and external environments of an organism

Question 2

How do receptors act as a transducer?

Answer 2

They convert different types of stimuli into electrical nerve impulses

Question 3

Which receptors detect light?

Answer 3

Photoreceptors (rods and cones)

Question 4

Which receptors detect heat?

Answer 4

Thermoreceptors (skin)

Question 5

Which receptors detect pressure?

Answer 5

Mechanoreceptor (Pacinian corpuscle in skin)

Question 6

Where are the Pacinian corpuscles found?

Answer 6

Deep in the skin mainly in fingers and feet or in joints

Question 7

What is the structure of a Pacinian corpuscle?

Answer 7

It has a single neurone, the ending of which is surrounded by layers of tissue separated by a gel

Question 8

How does the Pacinian corpuscle respond to the stimulus of change in pressure?

Answer 8

• The membranes of the pacinian corpuscle have stretch mediated sodium channels
• When pressure is applied it deforms the neurone plasma membrane and stretches and widens the sodium channels so sodium ions diffuse in which leads to the establishment of a generator potential

Question 9

What are the 3 types of neurones?

Answer 9

• Sensory
• Relay
• Motor

Question 10

What are the common features of all 3 neurones?

Answer 10

• Cell body: contains the organelles found in a typical animal cell including the nucleus and proteins and neurotransmitter chemicals are made here
• Dendrons: carry the action potentials to surrounding cells
• Axon: conductive, long fibre that carries the nervous impulse along the neurone

Question 11

What is the function of a sensory neurone?

Answer 11

Transmits electrical impulses from sensory receptors to other neurones (relay, motor or brain)

Question 12

What is the structure of a sensory neurone?

Answer 12

They have a long dendron which carries the impulse from the sensory receptor cell to the cell body of the neurone and then an axon to carry the impulse from the cell body to the next neurone

Question 13

What is the function of a relay neurone?

Answer 13

Transmits electrical impulses from sensory neurones to motor neurones

Question 14

What is the structure of a relay neurone?

Answer 14

They have multiple short axons and dendrons

Question 15

What is the function of a motor neurone?

Answer 15

Transmits electrical impulses from the CNS to the effector (muscles or glands)

Question 16

What is the structure of a motor neurone?

Answer 16

They have one long axon and multiple short dendrons

Question 17

What are myelinated neurones?

Answer 17

They have Schwann cells which wrap around the axon to form the myelin sheath which is a lipid and so does not allow charged ions to pass through it

Question 18

What is the function of a myelin sheath?

Answer 18

It acts as an electrical insulator speeding up the conduction of electrical impulses

Question 19

What are the gaps in the myelin sheath called?

Answer 19

nodes of Ranvier

Question 20

Why is the transmission of electrical impulses faster in myelinated neurones than in non-myelinated neurones?

Answer 20

The action potential jumps from node to node (saltatory conduction) which means the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length, just at the nodes of Ranvier

Question 21

What is the resting potential?

Answer 21

• When a neurone is not conducting an impulse there is a difference between the electrical charge inside and outside of the neurone
• There are more positive ions, Na+ and K+ outside compared to inside so the inside of the neurone is comparatively more negative

Question 22

What is the potential difference when the neurone is in resting state?

Answer 22

-70mV

Question 23

How is the resting potential established and maintained?

Answer 23

• The resting potential is maintained by a sodium potassium pump involving active transport and ATP
• The pump moves 2K+ ions in and 3Na+ ions out
• This creates an electrochemical gradient causing K+ to diffuse out and Na+ to diffuse in
• The membrane is more permeable to K+ so more are moved out resulting in the -70mV

Question 24

What is needed for an action potential to generate?

Answer 24

Stimuli need to reach a threshold potential

Question 25

What is the threshold potential?

Answer 25

-50mV

Question 26

What is the all or nothing principle?

Answer 26

• If the depolarisation does not exceed -50mV an action potential and impulse are not produced (nothing)
• Any stimulus that does trigger depolarisation to -50mV will always peak at the same maximum voltage (all)
• Bigger stimuli just increase the frequency of action potentials

Question 27

Why is the all or nothing principle important?

Answer 27

It ensures that animals only respond to large enough stimuli rather than responding to every slight change in the environment

Question 28

What is depolarisation?

Answer 28

• When the distribution of the charge reverses as more sodium ions flow into the axon due to the membrane becoming more permeable to Na+ ions
• The action potential is reached as more sodium ions flow across the membrane which continues until all voltage gated sodium ion channels are open which is an example of positive feedback

Question 29

What is repolarisation?

Answer 29

• When sodium ions stop flowing into the axon because membrane permeability for Na+ ions decreases
• The membrane permeability for K+ ions increases so they flow out of the axon returning the potential difference to -70mV

Question 30

What is hyperpolarisation?

Answer 30

When the cell’s membrane potential becomes more negative than the resting potential

Question 31

What is the refractory period?

Answer 31

Where there is no membrane permeability to sodium and potassium ions and the neurone is unresponsive

Question 32

Why is the refractory period important?

Answer 32

• It ensures that discrete impulses are produced. An action potential cannot be generated immediately after another which makes sure that each is separate
• It ensures that action potentials travel in one direction. This stops the action potential from spreading out in 2 directions which would prevent a response
• It limits the number of impulse transmission which is important to prevent over reaction to a stimulus

Question 33

What happens after the refractory period?

Answer 33

Neurone returns to its resting potential

Question 34

What is the gap between 2 neurones called?

Answer 34

Synapse/synaptic cleft

Question 35

What transmits an action potential across a synapse?

Answer 35

Neurotransmitter

Question 36

What are cholinergic synapses?

Answer 36

Synapses which use acetylcholine as the neurotransmitter

Question 37

How does an action potential travel across a synapse?

Answer 37

• An action potential arrives at the end of the presynaptic axon (synaptic knob). This causes an influx of calcium ions which cause vesicles containing ACh to fuse with the membrane and release ACh into the cleft by exocytosis
• ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane delaying the impulse by about 0.5ms
• An influx of sodium ions occurs as ion channels open, depolarising the cell. If the threshold potential is reached an action potential will be generated
• An enzyme (acetylcholinesterase) on the postsynaptic membrane hydrolyses ACh. The breakdown product choline is reabsorbed into the synaptic knob and recycled into ACh

Question 38

What is summation?

Answer 38

The rapid build-up of neurotransmitters in the synapse to help generate an action potential

Question 39

What is spatial summation?

Answer 39

When impulses from different presynaptic neurones collectively reach the threshold to cause an action potential

Question 40

What is temporal summation?

Answer 40

When several impulses arrive in quick succession from a single presynaptic neurone causing an action potential

Question 41

What do excitatory inputs do?

Answer 41

Cause depolarisation and action potentials

Question 42

What do inhibitory inputs do?

Answer 42

Cause the neurotransmitters to bind with the postsynaptic neurone allowing negative chloride ions to enter it which hyperpolarises the postsynaptic neurone making depolarisation less likely