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Calcium links electrical excitation to contraction,
although the source of the signal
and the proteins that interpret it differ between muscle types.
In skeletal muscle the dihydropyridine receptor on the T-tubule is a voltage sensor,
not a channel.
When the membrane depolarises, it mechanically opens the ryanodine receptor RyR1,
and calcium pours from the sarcoplasmic reticulum onto troponin C.
Smooth muscle reads the same signal differently: calcium binds calmodulin,
which activates myosin light chain kinase, and the phosphatase relaxes.
At the neuromuscular junction the receptor is nicotinic,
and it is this receptor that myasthenic antibodies attack.
Visceral and somatic afferents share second-order neurons in the spinal cord,
not the medulla, so the brain assigns visceral pain to the skin.
Gallbladder and diaphragmatic irritation travel with the phrenic nerve, C3 to C5,
and are felt in the right shoulder.
Cardiac ischaemia shares T1 to T4 and radiates to the left arm
and jaw.
Taste travels with the seventh, ninth and tenth nerves, never the trigeminal.
Only nasal retinal fibres cross at the chiasm,
which is why a pituitary tumour produces bitemporal hemianopia.
Calcium links electrical excitation to contraction,
although the source of the signal
and the proteins that interpret it differ between muscle types.
Acute loss of vision in one eye is sorted by three questions:
is it painful, how old is the patient,
and what does the disc show?
A young adult with pain on eye movement,
red desaturation and a relative afferent pupillary defect has optic neuritis.
In about two-thirds the lesion is retrobulbar,
so a normal disc excludes nothing.
Painless sudden loss in an older patient points to vascular occlusion:
a cherry-red spot in arterial occlusion, a blood-and-thunder fundus in venous occlusion.
Temporal headache,
jaw claudication and a high ESR after fifty mean giant cell arteritis,
and corticosteroids are started before the biopsy,
because the other eye will not wait.
The Optic Neuritis Treatment Trial fixed two rules.
Intravenous methylprednisolone hastens recovery and delays a second demyelinating event,
whereas standard-dose oral prednisone alone increases the recurrence rate
and is therefore forbidden.
Final acuity is similar whatever is given.
The MRI, not the steroid, determines the future.
With no white matter lesions the fifteen-year risk of multiple sclerosis is
about 25 per cent;
with one or more lesions it is about 72 per cent.
Pure abduction belongs to the lateral rectus and the sixth nerve;
depression in adduction belongs to the superior oblique and the fourth nerve.
A hypermetropic child accommodates at every distance, over-converges and develops accommodative esotropia,
for which full hypermetropic spectacles, not surgery, are the first treatment.
The sixth nerve runs the longest intracranial course, hugging the clivus,
so raised intracranial pressure or a posterior fossa tumour compresses it first.
An older diabetic with a sixth nerve palsy may be observed for about three
months,
but a child needs imaging at once.
Bilateral palsies suggest idiopathic intracranial hypertension,
and myasthenia may present with one drooping eyelid alone, worse by evening.
Calcium links electrical excitation to contraction,
although the source of the signal
and the proteins that interpret it differ between muscle types.
Calcium links electrical excitation to contraction,
although the source of the signal
and the proteins that interpret it differ between muscle types.