S7) The Action Potential & Neuromuscular Junction Flashcards

1
Q

What is an action potential?

A

An action potential is a change in voltage across the cell membrane

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2
Q

What are the properties of action potentials?

A

Action potentials:

  • Depend on ionic gradients and relative permeability
  • Only occur if a threshold level is reached
  • Are propagated without loss of amplitude
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3
Q

Describe the sodium hypothesis of the action potential

A

1 - Threshold potential reached

2 - Na+ channels open

3 - Na+ channels inactivate & K+ channels open

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4
Q

Explain the term threshold potential

A
  • Threshold potential is the critical level to which a membrane potential must be depolarised to initiate an action potential
  • Once this membrane potential is reached, a positive feedback occurs as Na+ channels begin to open
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5
Q

Depolarisation is followed by repolarisation.

What happens during this process?

A
  • Potassium channels open causing an efflux of K+
  • Sodium channels inactivate stopping the influx of Na+
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6
Q

What is the absolute refractory period?

A

The absolute refractory period is the time between the initial opening and subsequent closing of the Na+ channels (approx. ~1 millisecond in nerves)

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7
Q

What is the relative refractory period?

A

The relative refractory period is the time needed for Na+ channels to recover back to their resting membrane potential so they can open again (approx. 4 milliseconds)

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8
Q

Local anaesthetics act by binding to and block Na+ channels, thereby stopping action potential generation.

How is this achieved?

A
  • Most local anaesthetics are weak bases and cross the membrane in their unionised form
  • They block Na+ channels easily when the channel is open
  • They also have a higher affinity for the inactivated state of the Na+ channel
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9
Q

In what order do local anaesthetics block the conduction in nerve fibres?

A
  • Small myelinated axons
  • Non-myelinated axons
  • Large myelinated axons
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10
Q

What is electrical stimulation and how is it done?

A

- Electrical stimulation is used to stimulate an axon / group of axons to threshold potential, thus intiating an action potential

  • Stimulation occurs under a cathode (negatively charged)
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11
Q

How does one calculate conduction velocity?

A

Conduction velocity = distance / time

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12
Q

How is an action potential conducted along an axon?

A
  • A change in membrane potential in one part can spread to adjacent areas of the axon by local currents
  • When local current spread causes depolarisation of part of the axon to threshold potential then an action potential is initiated in that location
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13
Q

What are the properties of the axon that lead to a high conduction velocity?

A
  • A high membrane resistance
  • A low membrane capacitance
  • A large axon diameter
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14
Q

What is capacitance?

A

Capacitance is the ability of a lipid bilayer to store charge

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15
Q

Why does a low capacitance lead to a faster conduction velocity?

A
  • A high capacitance takes more current to charge
  • It can cause a decrease in local current spread
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16
Q

What is membrane resistance?

A

The membrane resistance is a function of the number of open ion channels

17
Q

Why does a low membrane resistance lead to a slower conduction velocity?

A

The lower the resistance, the more ion channels are open and the more loss of local current occurs across the membrane

18
Q

Local currents cause an action potential to propagate down an axon.

Why do action potentials not move backwards?

A

The action potential will not move backwards because the area of axon that has just fired an action potential is refractory and cannot fire another action potential until it has recovered from being refractory

19
Q

Explain how the myelin sheath affects conduct velocity

A

Myelin reduces the capacitance and increases the resistance of the axonal membrane, hence increasing the overall conduction velocity

20
Q

Identify the cells which form myelin and describe their actions

A
  • Schwann cells myelinate peripheral axons
  • Oligodendrocytes myelinate axons in the CNS
21
Q

Describe the process of saltatory conduction in myelinated nerve fibres

A
  • Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node
  • This increases the conduction velocity of action potentials
22
Q

What is the relationship with fibre diameter and conduction velocity?

A
  • Myelinated fibres: velocity proportional to diameter
  • Unmyelinated fibres: velocity proportional diameter1/2
23
Q

In terms of myelination & conduction velocity, describe what occurs in multiple sclerosis?

A

Multiple sclerosis is an autoimmune disease wherein the myelin is destroyed in certain areas of the CNS, leading to decreased conduction velocity and/or the complete block of action potentials