Part Of: [Neuroanatomy] sequence

Table Of Contents

• Circulatory System and BBB
• Ventricular System and BCSFB
• Endocrine System and CVOs
• Visceral vs Cognitive Processing
• The Folly Of Spock

Circulatory System and BBB

Cells need blood to survive. Neurons are cells like any other. Thus, we expect the circulatory system to extend into the brain.

Blood flow in the brain is organized around a structure poetically named the Circle Of Willis.

Unfortunately, capillaries sometimes leak, releasing noxious bacteria. Such infections are usually addressed by your immune system. However, certain areas of the body cannot tolerate such risks. If you had to choose, which areas of the body which you want special protection from infection?

Natural selection has produced specialized protection for three areas of the body: the brain, the eyes, the gonads.  Damage to these structures are particularly destructive to an organism’s fitness. Capillaries in these “protected zones” are given essentially several extra layers of “armor”, which prevent bacteria from escaping.

This protective mechanism is known as the blood-brain barrier (BBB). This armor is metabolically expensive, which explains the absence of “blood-foot barriers”.

Ventricular System and BCSFB

Your brain does not rest against the base of your skull. That would destroy brain tissue. Instead, it is immersed in a fluid bath, which protects and supports the brain.  Formally, the fluid is known as cerebrospinal fluid (CSF), and it is found as a support for the brain (ventricular system) and surrounding/cushioning the brain (in the subarachnoid space).

Cerebrospinal fluid (CSF) takes no direct connection to the circulatory system. In fact, since the CSF directly contacts nervous tissue, it also requires extra protection from infection. The blood-cerebrospinal fluid barrier (BCSFB) exists for precisely this reason.

Taken together, the BBB and the BCSFB insulate the brain from the circulatory system, while still allowing nutrients to reach neurons:

Endocrine System and CVOs

In Towards Body Architecture we explored how the nervous system interacts with other bodily systems (respiratory, digestive, etc).  In fact, the nervous system acts as a control system, influencing how other systems perform. No other anatomical system can claim that…

…except one. Your endocrine system also acts as a control system! Hormones play a role in digestion, cell growth, reproduction, etc – a whole host of bodily processes. Speaking generally, nervous signals act quickly (nerve signals do their work within seconds), whereas endocrine control (hormone signals enact change more slowly, usually in minutes or hours).

Two independent control systems is a poor way to design a body. We have every reason to benefit from them synchronizing their efforts, exchanging information. Let’s examine stress as an example, which requires the involvement of both systems.

• The nervous system is responsible for detecting stressful stimuli. It then transmits a signal, via nerves of the sympathetic nervous system to the adrenal gland, which immediately begins manufacturing cortisol (a hormone which mediates stress). This is fight-or-flight, a fast response.
• The endocrine system concurrently releases adrenocorticotropic hormone (ACTH) which also prompts the adrenal gland to manufacture cortisol. However, ACTH takes longer to take effect, and stays in your system longer. Thus, the endocrine system supports non-transient forms of stress.

These two phenomena are obviously correlated. But how? Your endocrine system is confined to your circulatory system – hormones travel via the bloodstream. But we just learned that the brain is insulated from the bloodstream…

The nervous system and the endocrine system coordinate with one another via circumventricular organs (CVOs). CVOs puncture the blood-brain barrier in a controlled way. They come in two flavors:

• Sensory CVOs translate hormone messengers into neural signals.
• Secretory CVOs translate neural signals into hormone messengers.

As you might expect by now, these two categories of CVOs provide neuroendocrine integration.

Visceral vs Cognitive Processing

In my last post, I presented the cognitive perception-action cycle. Let us consider these cybernetic systems side by side.

This image situates visceral and cognitive processes together. This reflects the medial viscera principle:

Visceral processes tend to reside in the center of the brain (medial regions).

The neuroendocrine system does not operate alone. Rather, it works alongside the autonomic nervous system, which regulates the body via two complementary systems:

1. Sympathetic Nervous System, which promotes “fight for flight” readiness.
2. Parasympathetic Nervous System, with a restorative “rest or digest” function.

In this way, the visceral perception-action cycle has two arms:

1. The autonomic nervous system, which quickly perceives and regulates the body.
2. The neuroendocrine system, which operates at a slower, more deliberate pace.

We can call the visceral perception-action cycle the “hot loop”, in contrast with the cognitive cycle, or “cold loop”. These loops represent a central organizing principle of the nervous system.

 

The spinal cord bears nerve fibers in service of both loops, both autonomic nerves bearing sympathetic & parasympathetic signals, and somatic nerves which regulate the skin and musculature. The cranial nerves include exteroceptive nerves as well (vision, hearing, smell, etc) which together encode information about the external world. 

The Folly Of Spock

As we will see later, the visceral perception-action cycle participates in the neural basis of emotion.

Next time you watch Star Trek, you may safely infer the Spock’s species has not achieved neuroendocrine integration.

Emotional processes reside at the center of the brain. Emotion is thus ideally situated to modify, modulate, and alter your decision-making capabilities.

This tension between visceral and cognitive, between hot and cold, is one of the hallmarks of being human.

Until next time.