| It is essential to make a diagnosis of a disease which relapses and remits since the patient may show no abnormalities at the time of physical examination by the clinician. Lumbar puncture therefore plays a major role with the demonstration of oligoclonal bands in CSF which are absent from the parallel serum specimen. This means that there is an intrathecal rather than a systemic immune response. The converse is seen in, e.g. neurosarcoidosis, in which the systemically synthesized immunoglobulins are transferred passively into the spinal fluid, giving rise to a so-called 'mirror' pattern, where the oligoclonal bands are the same in both CSF and serum. The test involves isoelectric focusing of CSF with a parallel serum sample. The separated immunoglobulins are exposed to anti-IgG to identify bands which are present in CSF but absent from the corresponding serum. Such patterns indicate local synthesis of IgG within the brain. |
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| Because of the multitude of different synaptic inputs to a given neuron, the final algebraic summation results in a
'decision' at the level of the axon hillock (the site of origin of the axon from the cell body) as to whether or not to transmit an action potential down the axon as an all-or-none phenomenon. However, even before this decision is made, the input of a particular neurotransmitter can essentially be classified as excitatory or inhibitory.
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| In addition to the relatively short-term decisions concerning action potentials (Chapter 40), there is a longer-term modulation of the resting membrane potential, moving it either closer to (excitation) or further from (inhibition) the critical membrane potential, which is the level at which the resting membrane potential will finally trigger an action potential at the axon hillock. Many drugs have a longer-term effect on modulation, in addition to the short-term effect, which partially explains their addictive effect; this can be seen with alcohol or the opioid drugs. There are also long-term effects during treatment with various drugs - for example those used to treat endogenous depression - such that it may be weeks before any beneficial effects are seen.
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