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Case 6 Summary - ILOs
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Case 6 summaryGeneral ILOs
- To demonstrate knowledge of the cellular composition of the CNS
- To demonstrate knowledge of neuronal structure and function including the
propagation and conduction of action potentials
Within the CNS, there are:
Microglia – scavenger cells, resemble macrophages and remove debris.
Macroglia – come in three types:
o Oligodendrocytes: myelin formation around axons (CNS)
o Schwann cells: myelin formation around axons (PNS)
o Astrocytes: come in fibrous (found in white matter) and protoplasmic (grey
matter) subtypes. Form the blood-brain barrier.
Neurons can be:
Unipolar
Bipolar
Multipolar
Pseudounipolar
The transport of molecules within the axon is carried out by axoplasmic flow. The transport
of molecules towards the synapse is known as orthograde transport. It occurs within
microtubules and requires molecular motors:
Dynein
Kinesin
1
Guillaume Antem – MBChB Y2
, The carrying of a signal is done by conduction. Within the neuron, the resting membrane
potential is typically -70mV. This potential is maintained by transport channels and
represents the equilibrium reached by ion movement. Relative concentrations are as follows:
K+ = high inside cell
Na+ = high outside cell
Ca2+ = high inside cell
A stimulus activates voltage-gated Na+ channels, which allows for an influx of Na+. If a
threshold is reached, a positive feedback loop is stimulated, moving the potential of the cell
to +60mV. Once this point is reached, the voltage-gated Na+ channels close and become
inactive for a few milliseconds.
To return to the normal resting potential (-70mV), voltage-gated K+ channels open which
brings the polarity of the cell down (as potassium flows out).
These channels are slower to open and close than voltage-gated Na+ channels so
hyperpolarisation occurs.
Ion concentrations are re-established by passive channels and Na-K+ pumps (3xNa+ out,
2xK+ in).
The carrying of the signal is done by repeating the above process in adjacent structures of the
axon. The hyperpolarisation which results from opening of voltage-gated K+ channels
establishes a direction to the signal.
Flow can be improved by:
Increasing axon diameter
Insulating axons: the myelin sheath is responsible for this. It eases conduction by
reducing loss and allowing the signal to travel by saltatory conduction.
Hyperpolarisation is also known as ‘refractory period’. These come in two types:
Absolute – no stimulus will excite the nerve (result of voltage-gated Na+ channels
inactivation)
Relative – stronger than normal stimuli cause excitation (result of delayed voltage-
gated K+ channels opening)
- To demonstrate knowledge of the brainstem (emphasising cranial nerves)
2
Guillaume Antem – MBChB Y2
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