Physiology

Receptor cells
All living organisms have receptor cells to monitor their immediate external environmental conditions. These include;

• Chemoreceptors (AIR, gases)
• Thermoreceptors (HEAT)
• Photoreceptors (LIGHT)
• Hydroreceptors (WATER)
• Baroreceptors (PRESSURE)

Hypothalamus – From a human perspective, all the above receptors innervate the nuclei and other connected areas of the hypothalamus using the autonomic nervous system (ANS) as part of the peripheral nervous system (PNS).

• AIR (gases) – olfactory, gustatory via amygdalae; skin via transient receptors potential channels
• HEAT – cornea/retina via retinohypothalamic tract, temperature regulation involves evaporation of lacrimal fluid. Skin, sweat glands, blood thermodynamics via somatosensory system
• LIGHT – Rods, cones and intrinsically photosensitive retinal ganglion cells, retina to suprachiasmatic nucleus (SCN) via retinohypothalamic tract
• WATER – Cornea/retina to SCN via retinohypothalamic tract, water vapour olfactory, gustatory via amygdalae
• PRESSURE – atmospheric and mechanical pressure exerts force on cell walls dilating/contracting arterial flow via somatosensory system

All of these stimuli also involve nocireceptors which activate when specific limits of homeostasis are exceeded.

Autonomic system – All our vital internal organs of; heart, lungs, gastrointestinal system, endocrine system, blood and muscles innervate the hypothalamic areas. Sympathetic, parasympathetic and enteric nervous systems, parts of the autonomic system function automatically in response to the external stimuli of weather and the homeostasis mechanism.

Positive/negative feedback – In response to the external stimuli of air, heat light and water and to maintain the balance of homeostasis the hypothalamus signals the vital organs by way of the autonomic system part of the peripheral nervous system. In return the vital organs send signals back to the hypothalamus their requirements for essential nutrients (energy), water and heat. These actions create an unconscious desire for food, drink and comfort.

Homeostasis
From a human perspective homeostasis can be explained as the balance between; oxygen/CO2, hot/cold, light/dark, wet/dry, high/low pressure, high/low carbohydrates, salty/bland, sweet/sour, acid/alkaline, positive/negative electrochemical and electromagnetic reactions.

Essential nutrients including water – Humans require 17 dietary minerals, 2 fatty acids, 10 essential amino acids, and 14 vitamins. The other essentials for sustainable life are; AIR (gases), HEAT, LIGHT and WATER.

Fig 1 the homeostatic mechanism, nature’s balance, between the external stimuli of air, heat, light and water and the need for essential nutrients including water and comfort.

Brain, Body and Barrier – There is a distinct dividing barrier between the body and the brain. We consider the brain as areas containing cerebrospinal fluid (CSF); cerebral cortex, cerebellum, 3rd, 4th, left and right lateral ventricles, brain stem and spinal cord. These areas enclose the central nervous system. All other areas can be considered body and communicate via the peripheral nervous system. Brain functions can be considered as; thought, learning, emotion and memory.

The limbic system, including the hypothalamus is on the body side of the blood-brain (CSF)-barrier. Body functions are autonomic responses to the stimuli of air, heat, light and water, and therefore create unconscious actions.

The dividing barrier can be considered as the meninges and circumventricular organs including the choroid plexus. Whilst the main parts of the circumventricular organs are on the body side of the barrier, tightly packed epithelial cells in the walls of the organs only allow molecules less than 500 Daltons from crossing into areas of CSF. We believe there is two way neural communications between body and brain at the circumventricular organs and one way neural communication at the brain stem and spinal cord between CNS and PNS.