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Excitable Cells > Transmission within Neurons > Flashcards

Transmission within Neurons Flashcards

1
Q

What does it mean that action potentials have directionality?

A

The current can only spread forwards to neighbouring axons

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2
Q

What is transmission?

A

The ability to relay signals within or between neurons

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3
Q

What are the two types of transmission?

A
  • Electrical: within neurons

- Chemical: between neurons

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4
Q

Is the current attenuated in axons or dendrites?

A

dendrites

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5
Q

What does it mean that the current is attenuated?

A

Some of the current is lost

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6
Q

Why is current attenuated in dendrites?

A

Current leaks out through the cell membrane

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7
Q

What is the equation for voltage over a cable?

A
  • V = V0 e^x/lambda
  • Vo is starting voltage at source station
  • V is the voltage measured at point x
  • Lambda is called the length constant
  • Length constant is the distance over which the voltage drops to 37% of its original value
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8
Q

To get efficient transmission in cables what do we want?

A

Cables with big length constants

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9
Q

What does the length constant depend on?

A
  • Length constant depends on Rm, Ri and diameter
  • Rm = leakiness
  • Ri = conductivity
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10
Q

What are a cable engineer’s options to get good transmission?

A
  • Increase Rm: better insulation
  • Decrease Ri: better conducting cores
  • Increase d: fatter cables
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11
Q

Why is attenuation in dendrites not a big problem?

A

 Short distances involved

 Many inputs (big starting signal)

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12
Q

What does it mean that dendritic transmission is passive?

A

It does not involve a wave of action potentials

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13
Q

What did Oliver Heaviside do?

A
  • Developed the mathematic theory that became known as the ‘telegrapher’s equations’
  • Later predicted the existence of a layer in the atmosphere that reflects radio waves. This allows for radio signals to be sent to destinations beyond the horizon by bouncing them off the reflective layer and is of huge importance in international radio communication
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14
Q

Give some properties of axons regarding it’s action potential

A
  • Axons have a far higher density of sodium channels than dendrites
  • This is the key to non-attenuated transmission: an AP wave
  • Axon hillock has a high density of sodium channels – point at which action potential starts in axon
  • When current reaches axon hillock the sodium channel opens and an action potential is passed along the axon
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15
Q

In terms of a big length constant how does nature achieve it with axons?

A
  • increases Rm (better insulation - myelin sheath)

- Increases diameter - giant squid axon

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16
Q

Why don’t animals with complex nervous systems increase axonal diameter?

A

Because it would mean their head would be too large

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17
Q

Explain how the giant squid axon is useful is studying the nervous system

A
  • The squid giant axon can approach a diameter of 1mm compared to the mammalian axon of about 20 uM
  • This means that the giant axon can be visualised and manipulated very easily using low magnification and thus makes experiments on axonal action potentials much simpler
  • Because the giant axon is so large, it is relatively easy to insert microelectrodes into it and measure the electrical currents that underlie the action potential. If one electrode is inserted that can measure voltage and another than can be used to ‘inject’ current, it is possible to ‘clamp’ the voltage at a particular value and measure the current that must be injected to keep the axon at that voltage
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18
Q

What does myelination do?

A

Plugs up holes in the axonal membrane

19
Q

Describe the myelination of axons

A
  • Non continuous myelin sheath
  • Bare patches of axons are called nodes of Ranvier
  • Nodes of Ranvier have a high density of sodium channels
  • Sections of myelin sheath are called internodes: about 1mm long and spaced at regular intervals across axon
20
Q

Describe the density of sodium channels in the axon

A
  • High density of Na+ channels in nodes of Ranvier (1200/um^2)
  • In internodes under myelin Na+ channel density v. low (20/ um^2)
  • High density of channels in node decreases rise time of AP
21
Q

What is Saltatory Conduction?

A

a wave of action potentials interspersed by segments of rapid cable conduction

22
Q

What is the increase in speed between myelinated neurons and non-myelinated neurons due to?

A
  • Density of sodium channels (node 10x higher than unmyelinated neuron)
  • Decreased capacitance of membrane, so… cable transmission through internodes is much faster than AP wave
  • Speed of myelinated neuron is up to 150 m/s whereas in non-myelinated neuron it is up to 10 m/s
23
Q

What is the myelin sheath provided by?

A

Oligodendrocytes of Schwann cells

24
Q

What is the Myelin sheath an extension of?

A

the cytoplasm of a glial cell

25
Q

What is the basic pattern in conduction velocities?

A

the fatter the axon, the faster the conduction but for neurons of the same diameter, the myelinated neuron is faster

26
Q

What is Multiple Sclerosis?

A
  • It is the most common auto-immune disease in Northern Europe
  • It is a disease of younger adults that can lead to decades of disability
  • It is a perfect illustration of the importance of myelination in nerve conduction
27
Q

Who is MS common in?

A

Women under 50

28
Q

What is the cause of the pathology in MS?

A
  • demyelination of CNS neurons. This is caused by an immune attack on the oligodendrocytes
  • the damage to the protective covering and the nerves disrupts the smooth flow of nerve impulses. So messages from the brain and spinal cord going to other parts of the body may e delayed and have trouble reaching their destinations
29
Q

What are areas of inflammation of MS called?

A

Lesions

30
Q

What are ‘plaques’ in MS?

A

Areas of thick scar tissue that may eventually form along areas of permanently damaged myelin

31
Q

What is MS characterised by?

A

periods when it worsens (relapses) and periods where the patient may somewhat recover (remissions) (called relapsing-remitting)

32
Q

What area of the body is most commonly affected by MS?

A

The eye

33
Q

What is diagnosis and evaluating MS disease activity normally done by?

A

neurologists through a neurological history and examination

34
Q

What is often used to diagnose MS?

A
  • Tests that can indicate MS and rule out ‘MS mimickers’ are also performed
  • Lesions may be viewed on an MRI scan
  • Inflammation can be evaluated with gadolinium (or contrast) enhancement – a type of dye given to the patient via injection prior to the MRI
  • Lumbar puncture is used to diagnose MS. CSF is collected and laboratory testing may be performed to evaluate cellular and chemical abnormalities
  • Evoked potential (EP) tests can be used to help diagnose MS. These measure the speed of the brain’s response to visual, auditory or sensory stimuli using electrodes taped to the patient’s head. Delayed responses can indicate possible damage to the nerve pathways.
35
Q

What is the main treatment for MS?

A

Treatment with a long-term, disease modifying therapy (DMT)

36
Q

What is special about the C fibres that are involved in pain signalling and what does this mean?

A

C fibres that are involved in pain signalling are unmyelinated. They transmit signals slower than myelinated neurons of the same diameter, but the signals still arrive at their destination

37
Q

What is the difference between unmyelinated neurons and demyelinated neurons?

A

The difference between unmyelinated neurons and demyelinated neurons is that unmyelinated neurons will have sodium channels all across the axon, whereas demyelinated neurons will only have a high density of sodium channels where the Nodes of Ranvier were. The density of the sodium channels in the now bare internodes is insufficient to support a wave of action potentials.

38
Q

What is the visual evoked potential test and what do you see with patients with MS?

A
  • This is similar to an EEG except that it is stimulated brain activity rather than spontaneous activity that is measured.
  • In a VEP test, electrodes are placed on the scalp and the patient is shown a checkerboard pattern
  • The electrical activity that this stimulus provokes is picked up by the electrodes and recorded
  • In patients with MS there is a delay in the response. This is because demyelinated neurones conduct slower than normal
39
Q

What do MRI scans reveal with MS?

A

MRI scans can reveal the presence of sclerotic plaques in the brain and spinal cord – these are hallmarks of MS and can help distinguish it from disorders with similar clinical symptoms.

40
Q

There is a link between MS and low levels of what…during development?

A

low levels of vitamin D during development

41
Q

In what geographical location are you less likely to get MS?

A

If you live closer to the equator

42
Q

What is Guillan Barre syndrome and what are the symptoms?

A
  • Guillan Barre syndrome (GBS) is an autoimmune disorder in which Schwann cells are attacked and peripheral nervous system neurons become demyelinated.
  • This results in progressive weakness, numbness and pain as sensory motor neurons lose their ability to transmit signals efficiently
  • About 20% of people with GBS are unable to walk 6 months after onset of the disease and many will suffer from impaired respiratory function
43
Q

What can often be precursors to Guillan Barre syndrome?

A
  • Campylobater food poisoning
  • An infection with cytomegalovirus
  • after an infection with Cv-19 maybe

Excitable Cells (22 decks)

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