Cycles Shorter than a Day

I have made this letter longer than usual because I lacked the time to make it short. 

Activities repeated for periods shorter than a day are common in certain places and conditions. The beating of cilia (small hairlike fibers) in the respiratory airways to propel mucus and dust particles toward the mouth is in this category. So is the movement of flagella (a fine hairlike process of unicellular organism) in microbes (see Section 2.9.2). 

Nemal activity is often periodic with a high frequency. Thermoreceptors found in the skin produce a series of pulses, the frequency of which corresponds to a particular tanperature (see Section 6.20.1). Other neural sensors (for taste, smell, and touch, for instance) do the same. It is thought that there are neural circuits in the brain that form timing loops and enable the organism to judge elapsed time. 

Certainly, the periodic contraction of the myocardium is a cyclic operation of short period. There is a regular series of events beginning with the generation of the cardiac rhythm in the sinoatrial node, a wave of depolarization that spreads aaoss the myocardium, and then contraction of the muscle. Each cycle of this process is typical of the others, and can be studied to learn the details. The respiratory cycle may seem to be similar, but it is initiated in the brain rather than in the respiratory muscles themselves. 

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