Baker: Notes on the biology of time – The News-Messenger

Ken Baker, Ph.D. | Columnist

Heres an entertaining little exercise: Define the word time without using any terms that refer to duration like amount of or length of.

If your definition, like mine, seems hollow, youre not alone. Ive never found anything approaching a good, common sense definition of the word.

Consider:

Time is the measured or measurable period during which an action, process, or condition exists or continues Merriam Webster.

Time is the indefinite continued progress of existence and events in the past, present, and future regarded as a whole Oxford Dictionaries

Sure, but how would the lexicographers who gave us these definitions define the terms period or continued progress not to mention past and future without using the word time? The religious, philosophical and scientific implications of time have been discussed and debated for as long as there have been theologians, philosophers and scientists.

Physicists have convinced us that space, speed, gravity and time are integrated in astonishing ways: Clocks tick ever so slightly more slowly near a massive object than a less massive one (gravitational time dilation), and also more slowly the faster a clock moves through space compared to a stationary clock (relative velocity time dilation).

Not much help, really. Such insights may describe how time-keepers behave under different circumstances, but not what it is they actually are measuring. So maybe time is like art. You know it when you see it, or in this case, when your brain senses it.

Except our brains cant sense time any more than they can define it.

Which is not to say we dont possess all manner of biological clocks enabling us to perceive that time however defined has passed, is passing, or is predicted to pass. Lets consider two bits of what biologists have learned; one concerning our conscious monitoring of time and the other that only the cells of our bodies know about.

A. Research within the past several years is providing strong evidence our awareness of time appears to be tied to the way our brains record memories of a series of events and the location in which they occurred, what neuroscientists refer to as episodic memory.

Lets suppose its Sept. 22, 2022, and youre observing a series of events taking place, say the bizarre last play of the Browns-Steelers game in which Denzel Ward recovers a fumble in the Steelers end zone to ice the game for the Browns, 29-17.

As the play unfolds, a network of specialized neurons called time cells within the hippocampus (a pair of curved tubes at the base of the brain) is activated when Steelers quarterback Mitch Trubisky throws the ball. A different network of time cells is activated when Diontae Johnson catches the ball and laterals it to Chase Claypool, at which point yet another group of time cells is triggered as Chase laterals to Najee Harris and so on.

To summarize, each separate event within the play receives a unique time stamp of activated nerve cells in the hippocampus.

Later on, when recounting the play to Grandma, those groups of time cells will be re-activated in your hippocampus in the same order in which they were originally activated. And each population of time cells appears to be linked to the memory of what was happening at the moment it was first activated.

So our sense of time seems to be related, at least in part, to different populations of time cells in the hippocampus being activated as each event real or imagined during the day unfolds.

B. Circadian (daily) rhythms have long been a topic of biological interest. Most of us get sleepy at night and awaken in the morning. We tend to be more coordinated early in the day, strongest in the late afternoon, and even our body temperature, blood pressure and immune system vary predictably throughout a 24-hour day.

When light strikes special cells in the back of the retina, signals are sent to a part of the hypothalamus called the suprachiasmatic nucleus, SCN. (This operates even in blind people.) The SCN is a sort of master clock that uses this information to synchronize circadian processes throughout the body, largely by triggering the secretion of hormones, adjusting their activity to changes in daylength as the seasons advance.

At a deeper level of our biology, each of the 37 trillion or so cells in our bodies possesses its own set of cellular clocks regulating how and when it performs its various tasks.

The biochemistry is complex, but often involves certain genes in the nucleus triggering production of proteins that build up in the cell to the point where they cause the genes to cease production. Later on, as those proteins decay, the genes reinitiate production. Cycling in the abundance of these proteins regulates a wide array of molecular activity within the cell.

In the final installment of this three-part series on time, Ill consider how animals perceive time.

Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.

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