I have just started reading The Rainbow and the Worm, by Mae-Wan Ho, one of the books recommended by my sister. I would like all of you to read the following two page selection taken from the introductory first chapter, "What is it to be alive?" The passage describes what happens when a muscle contracts. I find this passage simply astonishing. Think, as you read it, of how many thousands of research scientists had to carry through carefully conceived experimental programs in order for her to write this brief summary for the non-specialist. This is the sort of thing I wish Thomas Nagel had spent his time writing about in his book, Mind and Cosmos. By the way, Ho indicates that later in the book [I have only read chapter one], she will talk about quantum entanglement and a host of other topics from theoretical Physics in their relation to Biology.
"Another instructive example is muscle contraction. About 40% of our body is made up of skeletal muscle, i.e., muscle attached to bones, like those in our arms and legs and trunk. Another 5 or 10% is smooth muscle such as those of our gut and body wall, and cardiac muscle in the heart. Skeletal muscle consists of long thin muscle fibres, which may be discerned under a magnifying glass. these fibres are several centimetres in length, each of which is actually a giant cell formed by the fusion of many separate cells. A single muscle fibre, magnified a hundred times or more under the light microscope, can be seen to be made up of a bundle of 20 to 50 much smaller fibres, or myofibrils, each 1 to 2 μm (micrometre, one-millionth of a metre] in diameter. A myofibril has regular, 2.5μm repeating units called sarcomeres, along its length. Adjacent myofibrils are aligned so that their sarcomeres are in register. Under the much higher magnifications from the electronmicroscope -- thousands to tens of thousands of times -- one will see extremely regular arrays of the periodic structures. One will also see that each sarcomere consists of alternating thin and thick filaments, made up respectively of the two main muscle proteins, actin and myosin. In three dimensions, there are actually six thin actin filaments surrounding each thick myosin filament, and the six actin-filaments are attached to an end-plate, the Z-disc. Contraction occurs as the actin filaments surrounding the myosin filaments slide past each other by cyclical molecular tread milling between myosin 'head' groups and serial binding sites on the actin filament, forming and breaking cross-bridges between the filaments, in all three dimensions in the entire array.
The actin and myosin molecules are packed and arranged very precisely, approaching the regularity of crystals, and the study of the detailed structure of resting as well as contracting muscle is done by means of x-ray crystallography. There are 624 myosin head groups on each myosin filament, and when the entire muscle contracts, each sarcomere in it shortens proportionately. Thus, when a myofibrile containing a chain of 20,000 sarcomeres contracts from 5 to 4 cm., the length of each sarcomere shortens correspondingly from 2.5 to 2 μm. The energy for contraction comes from the hydrolysis of a special molecule that acts as the universal energy transacting intermediate in the body. In its 'charged up' form, it is ATP, or adenosine triphosphate, with three phosphate groups joined one to another in series and then to the chemical group adenosine. ATP 'discharges' its energy by splitting off a phosphate group at the end, to give the partially 'discharged' form, ADP or adenosine diphosphate.
Muscle contraction is triggered by an action potential at the site where a nerve impinges on the muscle-cell membrane. An action potential is a quick electrical discharge followed by recovery of the pre-existing baseline electrical potential. This releases calcium ions, Ca2+, from intracellular calcium ion stores to initiate contraction simultaneously in the entire cell within a millisecond. Contraction involves numerous autonomously occurring cycles of attachment and detachment of all the individual myosin heads to and from the binding sites on the actin filaments at the rate of 50 cycles or more per second -- each of which molecular event requiring the transfer of energy contained in one molecule of ATP -- precisely coordinated over the whole cell.
In a typical muscle contraction, all the cells in the muscle -- billions of them at the very least -- are executing the same molecular treadmilling in concert. Simply waving our arms about is a veritable feat requiring a series of actions coordinated instantaneously over a scale of distances spanning nine orders of magnitude from 10-9 metre [nanometre, one billionth of a metre] for intermolecular spacing between the actin and myosin heads, to about one metre for the length of our arm; each action, furthermore, involving the coordinated splitting of 1019 individual molecules of ATP. Now, think, imagine what has to happen when a top athlete runs a mile in under four minutes; the same instantaneous coordination over macroscopic distances involving astronomical numbers of molecules, only more so, and sustained for a long period without break."