Topic 3: Synaptic Transmission

Question 1

What are the components of a monosynaptic knee-jerk reflex arc (in order)?

Answer 1

1) Muscle spindle (sensory receptor)
2) Sensory neuron (afferent)
4) Motor neuron (efferent)
5) Skeletal muscle (effector)

Question 2

The muscle spindle is a sensory receptor that monitors stretch. They are located parallel to contractile fibres attached to the tendons at either end of the muscle fibre. The spindle signals to the CNS when muscle length changes via changes to action potential frequency. Match the following diagrams of action potential frequency to the states of muscle.

a) —|—|—|——|———–|—–|—|—|—|

b) —|–|-||||-|–|—|—|—|—|—|—|

c) —|—|—|—|—|—|—|—|—|—|—|—|

• Tonic
• Stretched
• Contracted

Answer 2

a) Contracted
b) Stretched
c) Tonic

Question 3

What are the components of a polysynaptic reflex arc (Golgi tendon organ (GTO) reflex)?

Answer 3

1) Golgi tendon organ (sensory receptor, is in series with muscle fibres)
2) Sensory neuron (afferent)
3) Inhibitory interneuron (in spinal chord)
4) Motor neuron (efferent)
5) Skeletal muscle (effector)

Question 4

Where is the GTO, what is its function and match the action potential frequencies to the states of muscle:

a) —|—|—|——|———–|—–|—|—|—|

b) —|–|-||||-|–|—|—|—|—|—|—|

c) ————————————

• Stretched
• Tonic
• Contracted

Answer 4

The GTO is located in the tendon. Therefore the tendon is most stretched, and the action potential frequency is highest when the muscle is contracted. Its function is to prevent damage to the muscle by relaxing it if load is too great

a) Stretched
b) Contracted
c) Tonic

Question 5

All cells have a resting membrane potential. Is the inside of the cell positively charged or negatively charged and what is the cause of this?

Answer 5

At rest, the INSIDE is at negative potential compared to the OUTSIDE.
This is caused by unequal distribution of ions

Question 6

Differences in potential across a membrane of a neuron is partially due to unequal distribution of ions. What are the key ions that cause this (+ protein) and what are their intracellular and extracellular concentrations?

Answer 6

Inside Outside
Potassium 140nM 5nM
Sodium 15nM 150nM
Calcium 0.0001nM 2nM
Chloride 10nM 120nM
Protein 145nM 37nM

Question 7

Besides unequal distribution of ions, what are the other 2 main factors underlying the ionic basis of cell excitability?

Answer 7

• Selective ion permeability
• Electrochemical gradient

Question 8

What does the Nernst equation define and what is its important determinant?

Answer 8

The equilibrium potential (Ek, also known as reversal potential or the Nernst potential) for any permeant ion.

The important determinant is the concentration ratio of the ion in question

Question 9

What is the equilibrium potential / the Nernst potential?

Answer 9

It refers to the membrane potential at which there is NO net movement of a specific type of ion across the membrane.

Question 10

The Nernst equation is:

E = (RT/zF) * ln ([X]out/[X]in)

What do these symbols each stand for?

Answer 10

R = Gas constant
T = Temp
z = Valency (number of sign / charge)
F = Faraday’s constant

Question 11

Which ion determines resting membrane potential and why?

Answer 11

K+ ions. The plasma membrane of neurons is 100x more permeable to K+ than other ions, thus the resting membrane potential lies very near the the equilibrium potential for potassium (~70-90mV)

Question 12

Describe the features of a typical neuronal action potential

Answer 12

• All-or-nothing response
• Regenerative
• Stereotyped response (fixed amplitude and duration)

Question 13

Outline the process of sequential permeability with Na+ and K+

Question 14

Discuss the function of voltage gated ion channels during the action potential

Answer 14

1) At rest, Na and K gates closed
2) At threshold, Na gate opens and Na+ enters, depolarising the cell
3) At peak, Na gate closes (h-gate) preventing ENTRY of Na+
4) K gate slowly opened during depolarisation, K+ leaves
5) Cell is repolarised, Na gate fully shut
6) K gate slow to close allowing more K+ out causing hyperpolarisation

Question 15

What are the 2 refractory periods define the mechanisms and role of the refractory period of action potentials

Answer 15

Absolute:
- Na channels inactivated
- Cannot generate another action potential
- Sets max AP frequency

Relative:
- K channels slow to close
- Higher K permeability than at rest (hyperpolarised)
-Need stronger stimulus to reach threshold
- Controls AP frequency

The role of the refractory period is to control the unidirectional AP propagation

Question 16

Outline the role of the sodium pump (Na/KATPase) in restoring ion gradients

Answer 16

The sodium pump helps maintain the long-term ionic gradient by using ATP to pump 2K+ ions IN and 3Na+ ions OUT the cell.

Question 17

List the basic properties of voltage-gated Na+ and K+ ion channels

Answer 17

Na channels:
- m gate: opens rapidly with depolarisation
- h gate: blocks open channel

K channels:
n gate: opens slowly with depolarisation and closes slowly with repolarisation

Question 18

In case some specific question about channelopathies comes up,

Defects in ion channels can lead to channelopathies, What can a defect in the Na+ channel cause?

Answer 18

Na+ channels:

Epilepsy

K+ channels:

Every other channelopathy

Question 19

Describe the properties of the compound action potential

Question 20

Outline the classification of major fibre types in relation to function and conduction velocity

Answer 20

Fibre Type Function Conduction Velocity

A (alpha) Motor neurone, 70-120m/s
muscle spindle
afferent
A (beta) Touch/Pressure 30-70m/s
A (gamma) Motor to muscle 15-30m/s
spindle
A (delta) Pain, Touch 12-30m/s
B Preganglion 3-15m/s
autonomic
C Pain, Temp 0.5-2m/s

Question 21

Define the major determinants of AP conduction velocity; length, fibre diameter and membrane resistance

Answer 21

Length:
- Depolarisation declines with distance from the stimulus due to spreading of local ion flow in the axon.
- The length constant (lambda) is the distance from the site of stimulus at which an action potential can still be generated.
- The bigger the length constant, the HIGHER the conduction velocity

Fibre Diameter:
- Small diameter = slow conduction: Harder for ions to flow along the axon. Depolarisation decays more rapidly
- Large diameter = fast conduction: Easier for ions to flow

Membrane Resistance:
- Low membrane resistance = Slow conduction: easier for ions to flow out of axon.
- High membrane resistance = Fast conduction

Question 22

Outline the function of axon myelination (saltatory conduction) and the effect of demyelination disease

Answer 22

• Myelination increases conduction velocity by increasing membrane resistance which increases length constant
• Action potentials are regenerated at each node in between myelin sheaths leading to high conduction speed
• Demyelination results in slowing nerve conduction

Question 23

How do we monitor neuronal function non-invasively and what do a) the peaks and b) delay in peaks represent?

Answer 23

• Clinically, we cannot measure APs in a single neuron
• We record neuronal activity non-invasively from populations of axons in peripheral nerves
• This records COMPOUND action potential

a) Peaks represent sum of action potentials from individual axons (graded)
b) Delay in peaks reflects conduction velocity of different axons

Question 24

What does conduction velocity increase with?

Answer 24

• Axon diameter
• Axon myelination