In popular culture, it is fairly common knowledge that there is a portion of the nervous system dedicated to the “Fight or Flight” response. We call it the “Sympathetic Nervous System” and it largely consists of a chain of ganglia (clusters of neuron cell bodies) that run parallel to the spinal cord and have origins in the brainstem and thalamus to prepare the body for activity by increasing heart rate, breathing, tighten muscles, dilate pupils and nostrils and flood the bloodstream with adrenaline.
A similar system, the “Parasympathetic Nervous System” serves the functions of “Feed and Breed” and is responsible for hunger, satiety, body temperature, and hormonal control of the body. The origins of the latter are in the hypothalamus, which acts as the brain’s major control interface with many separate systems of the body.
The diagram at the right shows a slightly expanded illustration of the different nuclei of the hypothalamus. You can see from the diagram that hypothalamus lies under the thalamus (hence the name). It also lies just forward of thalamus and brainstem (mesencephalon) and in the center of the brain - compared to the thalamus that forms the base of the two lateral hemispheres of the brain. This is not to say that the hypothalamus doesn't have right/left divisions. The picture below left shows the same structures from a 90 degree rotated angle (the thalamus is outlined in yellow). [By the way, the beautiful Frank Netter diagram above was first commissioned by CIBA pharmaceuticals in the 1930's. Netter is the acknowledged foremost medical illustrator, and his diagrams are considered *the* standard in medical textbooks.]
One of the characteristics of many neurons in the hypothalamus is that instead of releasing their neurotransmitters directly onto other neurons, they release into the blood. At this stage, the chemicals are termed "neuromodulators" because they act to alter neuron activity, instead of directly causing inhibition or excitation of a neuron. The more familiar name is "hormone. The blob hanging down from the middle of the hypothalamus above is the pituitary gland. It is most familiar for it's role in promoting adrenaline release from the adrenals glands sitting on top of the kidneys. However, it has other functions as well, as will be detailed below.
Regions of the hypothalamus include:
Anterior Medial (front, center) Region:
- Medial Preoptic nucleus - which controls urinary bladder pressure and decreases heart rate and blood pressure
- Supraoptic nucleus (located just over the optic nerve) - which releases oxytocin (uterine contraction) and vasopressin (blood pressure).
- Paraventricular ("between the ventricles") nucleus - which releases oxytocin, vasopressin and corticotropin releasing hormone (CRH, ultimately responsible for triggering the adrenal cortex to release cortisone/cortisol).
- Anterior hypothalamic nucleus - which controls panting, sweating, internal temperature regulation and release thyrotropin (regulates the thyroid).
- Suprachiasmatic (literally "Over the 'X'" referring to the crossover point of the optic nerve) nucleus - which releases vasopressin and regulates circadian rhythm (day/night, sleep/wake cycle)
- Lateral portions of preoptic and supraoptic nuclei
- Lateral nucleus - which regulates thirst and hunger (via secretion of a hormane called Orexin or Hypocretin, which is also shown to affect wakefulness)
- Dorsomedial Hypothalamic Nucleus - which regulates blood pressure and heart rate, as well as stimulating contractions in the intestines
- Ventromedial Nucleus - which regulates satiety (Orexin again, plus another hormone known as Gallanin), and control of neuroendocrine glands (such as thyroid, pancreas, ovaries, testes and adrenals)
- Arcuate Nucleus - which releases Gonadotropin-releasing hormone (GRH, aka Luteinizing hormone releasing hormone, LHRH) to regulate ovaries, releases dopamine (to alter prolactin release from pituitary and regulate lactation), and regulates feeding behaviors - largely in the form of altering taste to seek out essential nutrients.
- Mammillary nuclei - part of the Mammillary Bodies (named for their shape, not their function) - This is actually one end of the Limbic System, and is involved in memory.
- Posterior Nucleus - involved in the release of norepinephrine, a neurotransmitter similar to adrenaline, this area regulates blood pressure (causes arterial contraction), pupil dilation and shivering.
- Lateral Nucleus - actually a continuation of the Anterior Lateral Nucleus
There are two divisions - Anterior, also known as "Adenohypophysis" (named for its effect on glands) makes and secretes:
- Adrenocorticotropic Hormone (ACTH) which stimulates the adrenal gland. ACTH release is stimulated by CRH from the Paraventricular Nucleus)
- Thyroid-Stimulating Hormone
- Growth Hormone
- Prolactin (regulates mammary milk production and ovaries/uterus)
- Luteinizing Hormone (in women) / Interstitial Cell Stimulating Hormone (men) which regulates ovaries/testes
- Follicle Stimulating Hormone which stimulate ova release from ovaries
- melanocyte stimulating hormone which leads to melanin (skin pigment) formation
So, that's a lot of functions. If the Thalamus is the controller for neuron signals, hypothalamus is the controller for chemical signals. Together with the basal ganglia (next topic) they form the "diencephalon" which is essentially the *core* of the brain, and is the bare minimum structures necessary for most of what we think of as the essential functions of the brain apart from conscious thought.
Not bad for something about the size of a walnut. Kind of makes you rethink those prejudices against dinosaurs, right?
Until next time, "Take care of your Brain, it's the only one you've got!"