Thermoregulation and Circadian Rhythms Part 2: Our Internal Clock

We humans are creatures of habit.  Most of our normal activities (eating, sleeping, etc) are done at roughly the same time every day.  This is not by coincidence.  Our brains contain an internal clock which regulates a plethora of bodily functions.  From hormone production and cell regeneration to brain wave activity and body temperature, all of them show a cyclical pattern coinciding with certain hours of the day.  It’s actually essential to our survival, without balancing energy demands of sleeping, repair, and other processes it would be difficult for us to function properly.  In fact, the idea of timing processes is so evolutionarily ancient, that it can even be seen in the simple fungus Neurospora.  The fungus only conducts DNA replication at night to protect from UV radiation.  Scientists call our 24 hour internal cycle our circadian rhythms.

By a classical definition, a body process that is associated with circadian rhythms must meet two requirements.  The rhythms must be maintained even without cues (i.e. light) and the rhythms must be able to be adjusted to local times.  As I mentioned, these rhythms control all sorts of processes from hormones to brain rhythms, but how?

The maintenance of circadian rhythms in humans is rather complex with “clock” cells located in several places throughout the body.  However, this study is intended to focus on the brain, so I will only talk about out “master clock.” Our central and most important brain region for circadian rhythms is the suprachiasmatic nucleus or SCN for short.  This is a tiny little piece of the brain located in the anterior hypothalamus, just behind the eyes.  In this area reside “pacemaker cells.”  These are neurons which, through some internal cellular mechanism which is not completely understood, keep time for the body dictating the circadian rhythm.  While each individual cell is capable of keeping time, the pacemakers must be connected in order to stay synchronized and have a meaningful effect on the rest of the body.  A very interesting finding of recent research is that these neurons do not need to communicate through sodium gated action potentials as most neurons do.  It has been shown that calcium dependent GABA release is a more important factor in their synchronization.  In fact, by directly influencing the levels of GABA in the SCN researchers were able to raise and lower the level of synchronization, with more GABA leading to better synchrony.

Beyond communication with each other, pacemaker cells need to be able to adjust to the local time.  They do this through light detection.  Another fascinating discovery is that traditional photoreceptors (our eye’s rods and cones) are not necessary to entrain (that is calibrate) our pacemaker cells.  They can utilize a small photosensitive protein called melanopsin which is located in a small number of retinal ganglion cells.  When struck by light this protein stimulates cells in the retinohypothalamic tract (RHT) which in turn releases the neurotransmitter glutamate stimulated the pacemaker cells and putting them in synch with the local light dark cycle.

Cells from the SCN reach all over the brain in order to have the far reaching effects on many systems.  It will be an interesting and difficult task to find the exact correlation with body temperature for my next entry.

For more information see:

Brain Research Reviews

Functional neuroanatomy of sleep and circadian rhythms

Alan M. Rosenwasser