Physiology of the Brain: Focusing on Neurons
The brain: a unique, fascinating creature. And yes, I said the brain is a creature as it can be described as nothing less. It is the central part of our human nervous system. The brain makes us who we are; every human being on this earth their own living, thinking individual. Before we can talk about anything else (artificial intelligence, neural nets, Autism), it is crucial we dive deeply into the physiology of the brain, as this is what connects all these topics. The brain is a central organ made of nervous tissue and functions to control and regulate our body in every aspect possible.
Let’s start off the major landmarks of the brain, shall we? The three major portions would be the cerebrum, the cerebellum, and the brainstem. The cerebrum makes up 83% of brain volume, while the cerebellum contains 50% of the neurons (very important to xNeurals). The cerebrum is the seat of sensory perception, memory, thought, judgment, and voluntary motor actions. The next very important part of the brain is the cerebellum. This is highly important in motor coordination, aids in learning motor skills, maintains muscle tone and posture, smoothes muscle contraction, coordinates eye and body movements, coordinates the motions of different joints withe each other, and helps in ataxia (defined as a clumsy, awkward gait). Lastly, the brainstem is the portion of the brain that remains if the cerebrum and cerebellum are removed; the diencephalon, the midbrain, the pons, and the medulla oblongata.
While these are the important parts of the brain, an important sub-part, specifically part of the cerebrum, is crucial to neurons: the cerebral cortex. Neural integration is carried out in the gray matter of the cerebrum. The cerebral cortex is a layer covering the surface of the hemispheres. It is only about two to three millimeters thick and constitutes for about 40% of brain mass. The amazing thing about the cerebral cortex is that it contains 14 to 16 billion neurons! A six-layer tissue, called the neocortex, constitutes about 90% of the cerebral cortex. The neocortex is a relatively recent in evolutionary origin.
Next part is the meninges, consisting of three connective tissue membranes that envelop the brain. They work to protect the brain and provide structural framework for its arteries and veins. We can move from the meninges to the ventricles and cerebrospinal fluid. CSF, or cerebrospinal fluid is clear, colorless liquid that fills the ventricles and canals of the central nervous system. Every day, the brain produces and absorbs 500 milliliters of CSF. Production of CSF begins with the filtration of blood plasma through the capillaries of the brain. Ependymal cells modify the filtrate, so then cerebrospinal fluid has more sodium and chloride than plasma, but less potassium, calcium, glucose, and very little protein. The first function of CSF is buoyancy. This allows brain to attain considerable size without being impaired by its own weight and if it rested heavily on floor of cranium, the pressure would kill the nervous tissue. Second, CSF protects the brain from striking the cranium when the head is jolted. Shaken child syndrome and concussions do occur form severe jolting. Lastly, CSF works to provide chemical stability. The flow of CSF rinses away metabolic wastes from nervous tissue and use homeostasis to regulate its chemical environment. Neurons are located in ventricles as well, and send dendrites to signal CSF.
Another important part to discuss is blood supply and the brain barrier system. The brain is only 2% of the adult body weight, but receives 15% of the blood (about 750 millimeters per minute). Neurons have a high demand for ATP, and therefore, oxygen and glucose, so a constant supply of blood is critical to the nervous system. This means a ten second interruption of blood flow may cause loss of consciousness, a one to two minute interruption can cause significant impairment of neural function, and going four minutes without blood causes irreversible brain damage. The brain barrier system regulates what substances can get from the bloodstream into the tissue fluid of the brain. This guards two main points of entry: blood capillaries throughout the brain tissue and capillaries of the choroid plexus.
The blood-brain barrier protects blood capillaries throughout brain tissue. This consists of tight junctions between endothelial cells that form the capillary walls. From there, astrocytes reach out and contact capillaries with their perivascular feet. The blood-brain barrier induces the endothelial cells to form tight junctions that completely seals off gaps between them. Anything leaving the blood must pass through the cells, and not between them. The blood-CSF barrier protects the brain at the choroid plexus.
Another topic is the reticular formation. This is a loosely organized web of gray matter that runs vertically through all levels of the brainstem. It is clusters of gray matter scattered throughout the pons, midbrain, and medulla. The reticular formation also occupies space between white fiber tracts and brainstem nuclei. The most important thing to absorb for us it that this has connections with many areas of the cerebrum, especially more than 100 small neural networks without distinct boundaries. To broadly graze over it, through the reticular formation’s neural networks it performs somatic motor control, cardiovascular control, pain modulation, sleep and consciousness, and habituation.
Going back to the cerebrum, let’s focus on the cerebral lobes. There are four main lobes and an insula. The frontal lobe controls voluntary motor functions and aids in motivation, foresight, planning, memory, mood, emotion, social judgement, and aggression. Next, the parietal lobe receives and integrates general sensory information, taste, and some visual processing. The third lobe, the occipital lobe, is the primary visual center of the brain. Lastly, the. Temporal lobe has areas for hearing, smell, learning, memory, and some aspects of vision and emotion. The insula, an area not talked about often, works on understanding spoken language, and taste and sensory information from visceral receptors. The insula is hidden by other regions.
While this was all very important information to learn, I want to press on how without this information we would not be able to move forward to talk about future topics, such as neural networks and such. The brain has so many functions: sleep, memory, cognition, emotion, sensation, motor control, and language. However, it still somehow manages to make each human being a unique being.
References
Cerebellum (section 3, Chapter 5) Neuroscience online: An electronic textbook for The
NEUROSCIENCES: Department of neurobiology and anatomy — the University of Texas medical school at Houston. Cerebellum (Section 3, Chapter 5) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy — The University of Texas Medical School at Houston. (n.d.). https://nba.uth.tmc.edu/neuroscience/ m/s3/chapter05.html.
Interactive brain model. BrainFacts.org. (n.d.). https://www.brainfacts.org/3d-brain. Maldonado, K. A. (2021, May 9). Physiology, brain. StatPearls [Internet]. https://
www.ncbi.nlm.nih.gov/books/NBK551718/.
Peate, I. (2017). Anatomy and PHYSIOLOGY, 4. the brain. British Journal of Healthcare
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- Jayvanti Vanmanthai
