Hypothalamic Sampling Hormones Requires Blood Flow

Sampling of hormones (including the sex hormones) by the hypothalamus requires consistent blood flow. In the body, blood carries hormones released by endocrine glands and carries them to body parts that need them.

In parasympathetic withdrawal, diagnosis is usually considered adrenal fatigue. The volume of blood shifts from the muscles and brain to the central abdominal compartment. The blood flow to the brain is not stopped when this occurs. The flow is reduced and Poiseuille’s Laws come into play.

The circulatory system provides many examples of Poiseuille’s law in action—with blood flow regulated by changes in vessel size and blood pressure. Blood vessels are not rigid but elastic. Adjustments to blood flow are primarily made by varying the size of the vessels, since the resistance is so sensitive to the radius. This is done by the Abdominal Brain through the release of NeuroEndocrine transmitters, i.e. Serotonin – sero = “blood”, tonin = “pertaining to”.

A 19% decrease in flow is caused by a 5% decrease in radius of the blood vessels. The body may compensate by increasing blood pressure by 19%, but this presents hazards to the heart and any vessel that has weakened walls.

This decrease in radius is surprisingly small for this situation. To restore the blood flow in spite of this buildup would require an increase in the pressure difference of a factor of two, with subsequent strain on the heart.

ISCHEMIC PENUMBRA OF PARASYMPATHETIC DOMINANCE

In severe and/or chronic illness, profound changes occur in the hypothalamic-pituitary-thyroid axis. Ischemia and inflammation disrupt the porous Blood-Brain-Barrier surrounding the hypothalamus. The observed decrease in serum concentration of both hormones and neuroendocrine transmitter (neurotransmitters in the blood) are not compatible with a negative feedback loop.

Ischemia is a restriction in blood supply to tissues, causing a shortage of oxygen and glucose needed for cellular metabolism (to keep tissue alive, healthy and functioning properly). Ischemia is generally caused by problems with blood vessels, with resultant damage to or dysfunction of tissue or organs. It also means local anemia in a given part of a body sometimes resulting from congestion (such as vasoconstriction, red blood cell aggregation due to insulin resistance/diabetes). Ischemia comprises not only insufficiency of oxygen, but also reduced availability of nutrients and inadequate removal of metabolic wastes.

Hepatic Portal Hypertension

Parasympathetic Withdrawal (vasodilation) with blood pooling in the Abdominal Compartment makes the Movement Compartment and Brain/Spinal Cord Ischemic. At the periphery of the ischemic region, the so-called ischemic penumbra, neuronal damage throughout the body develops more slowly because blood flow arising from adjacent vascular territories (collateral flow) keeps blood perfusion above the threshold for immediate cell death. In the ischemic core, the major mechanism of cell death is energy failure caused by Oxygen/Glucose Deprivation (O2/GD). The hypothalamus and midbrain are most vulnerable to ischemia.

Neuron Vulnerability

Neurons in the most vulnerable areas cease to respond or show only faint responses and develop irreversible ischemic or post-ischemic damage. The hypothalamus responds to ischemic insults rigorously without having irreversible ischemic or post-ischemic damage.

The thalamus-hypothalamus interface represents a discrete boundary where neuronal vulnerability to ischemia is high in thalamus (like more rostral neocortex, striatum, hippocampus). In contrast hypothalamic neurons are comparatively resistant, generating weaker and recoverable anoxic depolarization similar to brainstem neurons, possibly the result of a Na/K pump that better functions during ischemia.

There is a well recognized but poorly understood caudal-to rostral increase in the brain`s vulnerability to neuronal injury caused by metabolic stress (insulin resistance).

Several brain regions, including the caudate, hippocampus, and hypothalamus, are vulnerable to hypoxic–ischemic brain injury. During O2/GD, hypothalamic neurons gradually depolarized during ischemic exposure. The O2/glucose deprivation (O2/GD) response induces failure of the Na+/K+ pump. The recovery is slow with chronic ischemic penumbrance

Without oxygen and glucose, neurons cannot generate the ATP needed to fuel the ionic pumps that maintain the ionic gradient across the neuronal membrane, mainly the Na+−K+ ATPase.

In the ischemic penumbra, the flow reduction is not sufficient to cause energy failure, and neurons remain viable for a prolonged period of time after the insult, but the neurons are stressed and critically vulnerable to pathogenic events that may tip their fragile metabolic balance. Excessive extracellular accumulation of glutamate is a major factor contributing to production of cytotoxic nitric oxide, free radicals and arachidonic acid metabolites. These events lead to necrosis or programmed cell death depending on the intensity of the insult and the metabolic state of the neurons. Injured and dying cells have a key role in post-ischemic inflammation because they release danger signals that activate the immune system.

Neurons that demonstrate particular vulnerability to ischemic challenges have been termed “selectively vulnerable neurons”. Of the entire forebrain, the neurons of the hippocampus are the most vulnerable.

Summary: Parasympathetic Dominance causes Ischemia to the Hippocampus, Hypothalamus, and Pituitary producing alterations in the HPA, HPT, HPD and HPG axis.

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