Thursday, 18 April 2024
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Hello. All is well. Continuing with chapter three, we examine the nervous system and the brain's structure.

The nervous system comprises two major systems. The central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS is made up of the brain and the spinal cord. I'll start with the spinal cord, then jump to the PNS, and finally back to the CNS, covering the brain's structure.

The purpose of the spinal cord is to connect the brain with the body, relaying sensory input from the body, and motor output from the brain. Functionally, it has 30 segments. Each segment, corresponding with a vertebrae, connects to a specific part of the body. These functional segments of the spine end at the bottom of the ribs, not at the base of the spine. If a segment is damaged, resulting in a loss of function, then all segments below lose function, too. The body parts associated with the lost segments become paralyzed. Interestingly, at the top of the spinal cord, automatic reflexes are processed. The brain is skipped, processing sensory input with automatic reflexive motor output, like a knee-jerk, removing a hand from the hot stove.

Now for the PNS, a bundle of nerves (axons) to connect all body parts with the spinal cord. The PNS can be subdivided into two systems. The autonomic nervous system and the somatic nervous system.

The autonomic system, as the name suggests, is responsible for automatic functions of internal organs and glands, like heart rate, or homeostatic processes. Homeostasis is a state of equilibrium for the body's biological condition, such as an ideal body temperature in a given context. We have two subsystems in the autonomic system that work together in a balancing act to reach homeostasis: the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic system is the stressor, responsible for the fight and flight response, while the parasympathetic system is the calmer, responsible for routine functions like digestion. The sympathetic system helps us respond to short-term, immediate danger: dilating pupils, increasing blood flow, increasing breathing, and releasing glucose. Meanwhile, the parasympathetic system halts its functions, conserving energy for external action, and returning to routine processes like digestion when the threat is over or the sympathetic system winds down. As you see, a balancing act, to regulate the body for optimal survival conditions, short-term and long-term.

I wonder how the autonomic nervous system relates to disease and sickness, like catching a cold. I imagine it includes this system, but the distinction is whether it's an external or internal threat, each having different responses, the internal response involving more functions of the immune system. Raising body temperature is one response to fighting off a virus, at least I believe so from personal experience, hehe.

Now for the second subsystem of the autonomic system, the somatic nervous system. It's responsible for sensory and motor function. We've got efferent fibers for motor output from the CNS and afferent fibers for sensory input to the CNS. Here, we take the perspective of the CNS, where efferent means to move away, sending signals of motor neurons away from the CNS to the PNS. Afferent means to move toward, sending signals from sensory neurons of the PNS towards the CNS. Uh, I think. I looked it up. I gathered it's commonly used to describe signals away (efferent) and towards (afferent) an organ, in this case, the spinal cord and the brain. I guess efferent is output, and afferent is input for an organ, oh, or exit and arrive, as the book calls it, matching first letters. Kewl.

Okay, we've discussed the spinal cord and the peripheral nervous system. Let's tackle the final part of the central nervous system: the brain.

The brain is a network of neurons. Billions of interconnections comprise the brain's subsystems, each with a specialized function, and together, compose the brain's all-encompassing system and function.

When you look at a brain from the outside, it looks a bit like a walnut. You notice its folds and groves. I like to call them hills and valleys. You can also compare the brain to an onion, having layers. The outer layer of the brain, the layer with fold and groves, we call the cerebral cortex.

The formation of folds and groves in this layer helps us to divide the brain's subsystems into physical subparts. At the top-front to the top-back of the brain, you'll notice a deep grove, known as the longitudinal fissure. It separates the brain into two parts, two hemispheres: the right and the left. Therefore, we call the brain bilateral. This is known as lateralization, we split the brain and see each part responsible for different functions. Studies to support this is mixed. Studies have shown that each hemisphere is counterintuitively responsible for the opposite body part, and they each have different responsibilities for language function. However, in between, in the middle of the brain, below the cerebral cortex, a chunky fiber band, comprising ~200 million axons, called the corpus callosum, connects the hemispheres. While they separately differ in function, together, compositional function emerge.

Okay, so we've looked at the brain as having two hemispheres and an outer layer, the cerebral cortex. To continue our structural dissection of the brain, we'll split the brain up in three parts: The forebrain, the midbrain, and the hindbrain.

The forebrain is the largest part, making up most of the brain. We divide it into two major parts. The cerebral cortex and the subcortical structures, the parts beneath the cerebral cortex.

I'll go back to the forebrain later to quickly cover the midbrain and the hindbrain. The midbrain is like a little nugget. A deep, central layer of the brain. The reticular formation takes center here, but it extends up the forebrain and down to the hindbrain, too. Underdetailed, its function involves the sleep-wake cycle, arousal, alertness, and motor function. The midbrain also has some other structures, confusingly named, but ultimately responsible for dopamine production and movement (not sure what they mean with movement, perhaps movement in external environment).

All right, that was the midbrain, our little nugget fella. Now, the hindbrain, you could think of as the behind brain. It's loosely at the bottom-back of the brain. It consists of the brain stem in the front, and the cerebellum at the back. The brainstem connects with the spinal cord, and consists of the pons and the medulla. The pons, at the top, simply acts as a bridge, connecting the hindbrain with the rest of the brain. The medulla, at the bottom, closer to the spinal cord, is responsible for functions in the autonomic nervous system. Do you remember? The system seeking to reach homeostasis, stuff like heart rate.

Side note, hypertension is a silent killer. Make sure to check your blood pressure regularly. On that side note, related, an annoying thing, well in the past mostly, when my autonomic system increased my breathing, I'd eat the air, yum yum, causing bloat in me stomach. At least that's what I think happens, maybe I'm so helpless I can't even figure out how to breathe properly, hah.

Ok, we're done with the midbrain and the hindbrain, back to the forebrain.

Quick note, the hemispheres only make up the cerebrum (containing subcortical structures) and the cerebral cortex, the outer layer above the cerebrum. Both are part of the forebrain.

The cerebral cortex is responsible for high-level functions. We dissect it into four physical lobes:
- Frontal lobe (located at the front-end, front-top of the brain, contains prefrontal cortex, premotor cortex, primary motor cortex)
- Parietal lobe (located at middle-top of the brain, contains the somatosensory cortex)
- Temporal lobe (located at the sides of the brain, contains the auditory cortex)
- Occipital lobe (located at the back-end, contains the primary visual cortex)

Finally, for the subcortical structures of the brain, we make two divisions; the thalamus and the limbic system.

The thalamus plays a central role in relaying sensory and motor signals in the brain. I think we can consider it an important hub for synchronizing the brain.

The limbic system comprises the following three parts, with its function in parentheses: the hippocampus (learning, memory), the amygdala (emotion), and the hypothalamus (homeostasis).

Right, that's it for now. I probably got some things wrong, but it'll be interesting sometime later to look more into all the stuff we covered today.

Lastly, I forgot, neuroplasticity. Basically, neurons are plastic or malleable, with some support from their buddies, glial cells. New synapses (connections between neurons) are formed, but even new neurons can form, and unused neurons are pruned. Hence, the brain is a self-organizing system, able to adapt its function, in turn, making it a resilient system. Very kewl.