Module 5.3 - Neuronal Communication

Question 1

What are (sensory) receptors?

Answer 1

Specialised cells that detect stimuli

Can be described as a transducer as they convert one form of energy to another

Question 2

What is a Pacinian corpuscle?

Answer 2

Touch/pressure receptors
Pressure on the skin causes the connective tissue to deform which cause sodium ion channels to distort and open so sodium ions can diffuse into the axon and produce an action potential

Question 3

Points about sensory neurones

Answer 3

Carry action potential from a sensory receptor to CNS

Long dendron, short axon

Question 4

Points about relay neurones

Answer 4

Connects sensory and motor neurones in CNS

Short dendrites, no dendron, short axon

Question 5

Points about motor neurones

Answer 5

Carries action potential from CNS to effector

Short/no dendron, long axon

Question 6

How is the resting potential of an axon maintained?

Answer 6

Sodium-potassium pumps in axon membrane transport 3 Na+ out for every 2 K+ moved in (via active transport)
Within the axon membrane are more open K+ channels than Na+ channels so the membrane is around 100 times more permeable to K+ and so K+ move out via diffusion (transports positive charge from inside to outside)
Inside axon has overall negative charge due to presence of organic anions (e.g. glucose, proteins, amino acids) that cannot cross the membrane

Question 7

Stages of an action potential

Answer 7

The membrane is polarised (-60mV) and at its resting potential
Stimulus is an energy change, causing voltage gated Na+ channels to open and Na+ to diffuse into the cell
Membrane depolarises, reaching threshold potential (-50mV)
More Na+ flood into the axon and potential difference across plasma membrane reaches +40mV
Na+ channels close and K+ channels open
K+ diffuse out of the axon, cell is undergoing repolarisation
The p.d. overshoots slightly, making the cell hyperpolarised
Original p.d. is restored so the cell returns to its resting potential

Question 8

What are local currents?

Answer 8

Ions diffusing along the neurone

Question 9

Roles of synapses

Answer 9

Transmit information between neurones
Ensure one way transmission of impulses
Acclimatisation
Divergence of nervous pathways
Avoid overstimulation
Temporal summation
Spatial summation

Question 10

Explain acclimatisation using synapses

Answer 10

Synapses fatigue and stop responding to stimuli
Helps avoid overstimulation of effectors which could cause damage (run out of neurotransmitter vesicles after repeated stimulation)

Question 11

Explain divergence of nervous pathways using synapses

Answer 11

One presynaptic neurone diverges into several postsynaptic neurones
Transmitted to several parts of the nervous system

Question 12

Explain temporal summation using synapses

Answer 12

Several action potentials come from the same presynaptic neurone to create an action potential in the post synaptic neurone

Question 13

Explain spatial summation using synapses

Answer 13

Action potentials arrive from converging presynaptic neurones and cause a few vesicles each to be released into the same synapse causing an action potential in the postsynaptic membrane

Question 14

How is an action potential transmitted through myelinated cells?

Answer 14

Myelin sheath is an electrical insulator and is impermeable to Na+ and K+ as there are no ion channels in myelinated regions
The movement of ions that causes action potentials can only occur at the nodes of Ranvier
This makes the local currents longer and Na+ ‘jump’ from one node of Ranvier to the next (saltatory conduction)

Question 15

What is the enzyme that hydrolyses ACh and what are the products?

Answer 15

Acetylcholiesterase

Acetic acid and choline

Question 16

Why is ACh hydrolysed?

Answer 16

Stops continuous production of action potentials in postsynaptic neurone
Enables repolarisation of postsynaptic membrane by unblocking receptors and stops Na+ channels from staying open
Recycles the ACh

Question 17

Describe the refractory period

Answer 17

Follows an action potential
Sodium voltage-gated channels close to stop another impulse being generated as the resting potential needs to be restored (redistribute Na+ and K+)
Also ensures impulses are separate and they pass in only one direction along the axon