This section is about aspects of the hypothalamus. Scroll down here to Chapter 38 for its being a timekeeper, to Chapter 39 for its control of body rhythms and glands, and Chapter 40 for its affect on sexuality.
Updated 20 September 2021.
Introduction to the Hypothalamus:
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The hypothalamus may be considered your body's master neuro-gland. It contains neural centers that regulate sleep & wake, food, water intake and temperature, and that tell time; and it is the top tier of the autonomic nervous system's controlling of blood pressure and heart rate. It interacts in a controlling fashion with all the endocrine glands, and feeds the info to the cerebral cortex and to the other brain structures and the periphery.
Trauma (basal skull fractures), tumor (relatively frequent in old age) or infectious disease (meningitis, encephalitis) each have major affects. It is a part of the brain that allows substances in the blood easily to get into the brain because of lowering of the usual blood-brain barrier to passage of chemicals into the brain. In the next 3 chapters three of its more important functions will be presented.
38. Brain, the Body's Time Keeper
Every function of your body runs on time: sexual feelings, appetites, alertness & sleep patterns, energy, and your intellectual brilliance levels and moods; they all cycle. We may not notice the cycles because they are chemically based and only documented by blood testing, and as modern civilized persons, the external influences in our daily lives allow us to override cyclic patterns that would be inconvenient. But it is important to be aware of Brain's time-keeping because it explains many feelings and illnesses and helps us to be more efficient.
The time-keeper is based first on a biological clock in the hypothalamus. In the lowest part of hypothalamus is the suprachiasmatic nuclei (the SCN; right & left neurons located supra, or above the optic nerves crossing). These neurons tick our time away. Their setting, averaged out in humans, is 24 hours and 11 minutes plus or minus 16 minutes (95% of people fall within the plus or minus). The clock can reset daily to the 24-hour light-dark cycle.
How did it get set and how may it reset? In the retinas of your eyes are special retinal ganglion time-keeping neurons. These cells have only one function: to signal the external light and darkness from the daytime/night time cycle. The retinal neurons are directly connected each by a nerve fiber and the fibers run in the left and right optic nerve and form a tract, the retinal-hypothalamic tract, or RHT, and they have set the SCN cell clock to the c.24-hour rhythm, called circadian (circa- approximate and -dian day)
But what about the artificial lighting in our modern life? Don't it interfere with resetting the clock?
It sure do! The biological circadian clock is part of all animal and plant life on Earth. It evolved over billions of years since life on Earth. For 99.9% of human existence we ran around without the benefit (and distractions) of artificial light. Everything was day-sunlight or night-blackness (and in between a li'lle moonlight or firelight). Over the millennia, human circadian rhythm by its daily resetting developed because we are daytime animals who slept at night. (Mice and rats are night-time-active animals and as a result tend to have a less than 24-hr circadian rhythm) Today, with modern life, we override many rhythms and cues but the basic rhythm is so deeply inserted in our DNA that each person's SCN beats to a basically similar human clock - the cells remember. This explains phenomena like jet lag and the problem of 24-hour shift work and most insomnia.
The RHT-SCN connection is key to the body's biologic clock. All the vital organs function according to the clock with highs and lows in the 24-hour period. But the SCN is pure nervous system. So in order to signal other parts of the body, we have the pineal gland, a small mid line structure like a little plum that grows out of the brain substance and dangles into the 3rd ventricle of the brain and is bathed in the cerebrospinal fluid. Like the SCN, the pineal is connected to the eye retina by a branch of the RHT.
The pineal gland, during unlit night, produces the hormone melatonin and releases it into the cerebrospinal fluid and blood. So, the body knows when it is daytime or nighttime outside. It knows because light (sunlight or strong artificial light) excites the retina ganglion neurons to send a signal that stops the release of melatonin. (Even in usual type blindness, the RHT works because the retinal light-sensitive cells are not the rod and cone cells we see with) And when darkness descends around each of us, the stop-melatonin retinal signal ceases and melatonin gets released. This, in the natural state, results in a diurnal (day & night) absence and presence of melatonin from blood and CSF, and it signals cells all around the body what is the time of day and the melatonin helps you fall asleep. But when it clashes with the realities of one's daily living, insomnia develops.
39. Body Rhythms and the Glands
The hypothalamus controls our glandular systems - body energy by the thyroid gland, growth by the pituitary growth hormone and the liver's insulin-like growth factor, stress by the adrenal cortex gland, sex & reproduction by the ovaries and testes via the pituitary sex hormones, and lactation & maternal behavior by the pituitary hormone prolactin. All are under the timing from the SCN; our hormones being released in a cyclic fashion in hourly or more pulses and also over a daily and monthly cycle.
The system depends on each gland having a releasing hormone produced by neurons in the hypothalamus, and the releasing hormone connecting to its receptor in the nearby, underlying pituitary gland. (The pituitary is on a stalk from the hypothalamus into which the releasing hormones are secreted by neurons of the hypothalamus) Once the releasing hormone contacts the pituitary receptor on the surface of the particular pituitary cell, the receptor causes the cell to make a pituitary-tropic hormone for the particular gland, the tropic hormone is secreted into the blood and goes to the gland (hence -tropic, or follows to or growth toward in contrast to -tropic, or feeding), and it produces the action hormone of the gland.
Example is thyroid gland. Hypothalamic neurons under the influence of the SCN are constantly releasing a pulse of thyroid releasing hormone (TRH) at approx 90-minute intervals (varies during a 24-hour) and the TRH is released from the hypothalamic neuron axon end-fiber that synapses on the cell of the anterior pituitary, and the cell gets stimulated through its TRH receptor to produce thyroid stimulating hormone (TSH) and release it into the blood (also at the same time intervals). The TSH circulates in the blood and reaches the thyroid gland in the front of the neck where it connects with receptors to produce the two iodine-containing thyroid hormones T3 & T4, (3 & 4 Iodine atoms) which then do their work of making the body metabolism more efficient by getting more energy out of oxygen and they also speed the heart and make you nervous when they are too high and make you dull when too low. The timing of the hormone release dictated at T3 & T4 level by the SCN is important to know. First we have 1- to-2-hour high-energy cycles. But over that, in a 24-hour period, the average thyroid hormone level in the blood will average high from the evening through 4 AM and then will drop and be low during the day. This is important for the thyroid-caused heart arrhythmia atrial fibrillation (AF) because AF starts most frequently at night due to the high thyroid. The timing should inform a patient when to take his daily hormone pill.
Finally, in this example, this is a negative feedback system, i.e., the T3 & T4 pulses caused by the hypothalamic SCN pulsatile releasing hormone will feed back to the receptors in the hypothalamus every 90 minutes or so and temporarily shut off the production and release of new hormone. Then the hormone level quickly drops and the lack of hormone in the hypothalamus stimulates a new pulse of TRH and the cycle renews.
A similar system occurs for the stress hormones (CRH, ACTH, corticosterone), for the growth hormones (GHRH and its opposite somatostatin, and for the insulin-like growth factor from liver), and for the reproductive sex hormones (GnRH, FSH & LH and testosterone & estradiole), the lactation hormone (Dopamine inhibition, prolactin and milk production) and for as yet undetermined glandular effects on eating, sleeping and other behavior.
So here, in the glands of our bodies we see an ultradian rhythm (A much-less-than-24-hour time rhythm that repeats throughout the 24-hour day) dictated by the hypothalamus and that also must affect our behaviors in many ways. The timing of reproductive cycles from external light extends to the monthly menstrual cycle that, obviously, became conditioned to the 28-day moon cycle in the sky. And in some animals the reproductive cycle may be as long as a year. The purpose is to ensure birth at an ideal time for maximal survival (i.e., births favored in late spring and summer when food is plentiful). Of course this is the natural state for an animal in the wild; humans and their pets have adapted to modern technology and reproduce when they please. Still, it is best to have a baby in spring-summer in terms of fetal abnormalities.
40. Brain & Sexuality
More than the other glands the sex/reproductive hormones are acted on by time cycles controlled by the SupraChiasmatic Nuclei (SCN). Puberty is programmed to occur in males and females based on the time clock in the SCN affected by how much light and dark at the age when puberty normally begins. And monthly cycles of sex hormones circulate under the time-keeping control of SCN. There are highs and lows of sexual desire cycling at 1 to 2 hour intervals (often not obvious to the average person), and diurnally high at night (Man is a night time sexual creature) and in women, peaking just before and at (minutes to hour) ovulation.
Recent data has also shown that a man's typical sexuality, and a woman's, too, are imprinted on the brain as a fetus in the womb and in childhood by the presence or absence of levels of testosterone in blood. Males, who normally have relatively high testosterone, become masculine sexually (attracted to women and not other men) while women, normally not having high testosterone, become "ladylike". Most interesting has been data that show that the hypothalamic brain structures in homosexual men are intermediate in size between non-homosexual men and normal women. Furthermore, experiments with rats have shown that stopping testosterone production in young male rats and/or exposing them to exogenous estrogen causes these male rats to engage in homosexual behavior as adults. This suggests that one of the factors that make a man homosexual may be exposure to excess female or lower than usual male hormone in the womb or in childhood. Also it favors the idea that homosexuals will have difficulty changing their behavior because it is fixed by their brains. However, it also suggests that the homosexual pattern could be reduced or eliminated by strict attention to pregnant women's hormonal internal milieu. (Strictly opposed by the homosexual community!) Still much more research needs to be done but this is a cutting edge area.
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