September 16, 2007

Electronic versus Biological Clock: A Comparative Analysis

Electronic clockElectronic clocks rely on oscillators to derive its clock signals. Modern day clocks employ crystals made of quartz, instead of employing CR (Capacitor-Resistor) or LC (Inductor(L)-Capacitor) networks, transistors or 'flip flop' integrated circuits, for the production of clock signals. In a typical electronic device, such as a computer or mobile phones, there is the need for not one but many different clock signals, for its different sub-systems. This is achieved by using decade counters (such as the TTL IC 7490, TTL stands for transistor transistor logic), which divide the clock frequencies by 10, or by using dividing by 12 IC 7492. By using such devices and others, in cascading manners, many different frequencies can be synthesized which may then be fed to the corresponding sub-systems.

In both plant and animal kingdoms, including humans, there are many 'clock generating' cells that functions as biological clocks. For example, there is one such clock in the hypothalamus (a part of our brain), which goes by the name of 'circadian clock'. This clock, also called the master clock, is actually a conglomeration of neurones at suprachiasmatic nucleus (SCN) of the hypothalamus, which produces its clock cycles in more or less the same manner as an electronic oscillator does. Circadian means about (circa) days (dies), in Latin, and as the name suggests, this clock enables important tasks, those have some relevance with the time of the day, to be executed in time. Activities such as release of some hormones (ACTH or adrenocorticotrophic hormone, cortisol etc), regulation of body temperature, sleep-wake cycle, behavior, growth and many other activities are controlled by the circadian clock and hence these activities comprise what we call circadian or diurnal rhythms. Like electronic clocks, they are also fed by reset signals, so that the clock may be synchronized with external stimuli, called 'zeitgebers', German for time givers. During daylight, light falls on the retina, the ganglion cells there are stimulated via their melanopsin photoreceptors, which then send this information to the SCN ('master clock') through the retino-hypothalamic tract. This way the clock remains up to date. This story of relay doesn't end here. The hypothalamus again relays this cue to pass on to the pineal gland, which produces 'melatonin' that plays important role in sleep wake cycle.
There other clocks in the brain too. For example, in the arcuate nucleus of the hypothalamus, a pulsatile clock, controls GnRH (gonadotrophin releasing hormone) secretion. This hormone, in turn, regulates the secretion of gonadotrophins: Leutinizing and Follicle stimulating hormone (LH & FSH), which are instrumental in reproduction. Then there are 'peripheral clocks' in areas such as the heart (cardiac pacemaker), intestinal smooth muscles, liver and other organs. In addition, each individual nucleated cell has its own in-built telomere operated clock, that counts 50 times after which the cell self apoptoses (Hayflick limit). This clock is DNA operated. It is likewise speculated that the circadian clock may also be DNA operated, and DNA transcription/translation feedback mechanism, may be the one that does the counting trick. Barring some exceptions, the ultimate effector is change in ionic fluxes in the neurones.

Whatever may be the mechanism of genesis of the clock pulses, it is to be borne in mind that these numerous clocks have to be coordinated, to be of relevance in physiology. This coordination is done sometimes through neuroendocrine mechanisms and sometimes through other means. Since ionic fluxes are involved, it may be possible that in the brain, where electrical circuits abound, electromagnetic coupling might play some role. Also, if the distance between the transmitting and receiving units is negligible and the chances of 'decoherence' is small, coupling through quantum mechanism by entanglement is also not an untenable option. Therefore we need to be open minded and logical if we were to come to a meaningful and universally accepted theory, before the sands of time run out.
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