Monday 1 May 2017

super-sensitive me

Today's post is about four named entities Meissner Merkel Pacini Ruffini which will intrigue and baffle all my readers except those who a) have recently taken a human physiology course or b) are doctors who can still remember 4 of the 10,000 new words they acquired in medical school: hello to all three of you.

I was started off on this MMPR jag because early early one morning I was cooking chapattis - The Beloved likes them and we had company. They are real simple and close to instant gratification. This is what you do:
  • Mix flour, a pinch of salt and enough water to make a stiff but pliable dough
  • Work it between your hands
  • Leave to allow the wheat to absorb the water
  • Pinch off a child's fistful and roll between palms
  • Heat a dry cast-iron skillet - not too hot
  • Roll out your doughball - not too thin ?3mm? keeping it roughly circular
  • Slap it onto the dry spit-hot skillet for a few seconds
  • Toss over before the top starts to blister
  • The outsides are now sealed and the insides can start to steam cook
  • After 'a while' flip the chapatti directly onto the gas flame
  • It will blow up like a football
  • Flip it over and over with your fingers until each side is just short of charring
The problem is that the bread puffs up like a ball because the water inside is boiling and forming live steam which soon finds its way out the least failure in the integrity of the crust. You can both see and hear this and you need to avoid the steam which a) can pop out unexpectedly from anywhere along the seam between top and bottom and b) can strip the skin off your fingers very efficiently.  I was deftly juggling these chapattis when I felt the least whisper of wind on one one finger and dropped the chapatti back down - a millisecond later the now out-of-reach finger felt a stab of heat. That told me that my light-touch receptors were faster than my pain receptors.

In my human physiology course (yours, if you are lucky enough to get one, will differ), I start off with a glance at the Central Dogma of molecular biology which was first put out by Francis "DNA-structure" Crick in 1958 and can be summarised as
  1. DNA makes RNA
  2. RNA makes proteins
  3. Proteins make everything else
Developments of our understanding of the living world over the last 60 years have shot many holes in this ideology but it still serves as a summary of the process whereby genes and their variants explain the differences between me and thee and between us and sheep. I go on to explain that tenet 3 of the Central Dogma works because all receptors and all enzymes are proteins.
  1. Enzymes are the biological catalysts of all the cellular activities on which we depend: alcohol dehydrogenase to de-toxify the booze; lipase to make other things out of the lard in which we fry the chips; phenylalanine hydroxylase which is banjaxed if you are born with PKU;  acetylcholinesterase which is inhibited by the nerve gas sarin. Notice, I say to my students, that all those enzymes end in -ase.
  2. Receptors otoh sit in the membrane of particular cells and recognise specific chemicals as they pass by. When those chemical 'ligands' dock with the receptor then a cascade of events happens inside to produce an appropriate response: allowing glucose across the membrane when insulin meets the insulin receptor; producing anti-microbial peptides when a Toll-Like Receptor TLR detects a pathogen; propagating nerve signals across a synapse when acetylcholine meets its receptor on the downstream neuron.
Later in the course I have to make a reality check and confess that not all receptors are proteins: while olfactory and taste receptors and rhodopsin in the retina are all proteins, there are a number of sensory receptors [R for diagrammatic cartoon] in the skin which are not.  They each have different somewhat overlapping functions and are named for their discoverers.
  • Meissner's or tactile corpuscles consist of a single raw, unmyelinated neuron embedded in layers of supportive cells and connective tissue that are exquisitely sensitive to light touch, they are concentrated on the pads of the fingers and on the lips. They allow you to feel and be annoyed by the least particle of sand in your beach-picnic ham sandwich. You have about 4x more of them at menarche than you do at menopause (blokes ditto for equivalent ages). So I guess snogging isn't so much fun as you get older.
  • Merkel's Disks are found, like Meissner's, where the dermis meets the epidermis especially the furry parts of the skin. They are sensitive to low frequency vibrations.
  • Pacinian or lamellar corpuscles are located deeper in the dermis and look like 1mm-sized onions with the central neuron surrounded by layer upon layer of connective tissue. They are sensitive to vibration and pressure and help us pick up eggs - where just the right amount of pressure is needed to prevent either crushing or dropping. With their help you achieve this on a daily basis without the least conscious thought.
  • Ruffini or bulbous corpuscles are located deeper than M&Ms but not as far buried as Pacinian, they are located particularly around the finger-nails and are sensitive to skin stretch and joint bending.Their structure is rather different from Pacinian corpuscles with more, subdivided, neuron and less surrounding connective tissue. They are thus also important in getting just enough muscle activity to hold things and move them from one place to another. They are also sensitive to heat.
Who dey?
Georg Meissner (1829-1905) anatomist from U Göttingen;
Friedrich Merkel (1845-1919) also U Göttingen;
Filippo Pacini (1812-1883) anatomist U Firenze;
Angelo Ruffini (1864-1929) embryologist from U Siena.
If they were women I'd write more about each one.

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