Neuroscience Basics: The Nervous System - Part 2
In a previous article, I described the various branches of nerves originating from the spinal cord and their corresponding functions throughout the body. Today, I will break down the different parts of the brain and illustrate a synergy between them which creates our conscious experience. This bottom-up view of the nervous system helps us develop a greater sense of how humans, or any vertebral organism for that matter, represent the world in relationship to the body. Everything we perceive overlaps onto the anatomical structure that we inhabit.
To clarify, the way in which it is possible to experience the universe is more or less constrained by the finite components that we are given. Of these components, the brain is one of the most remarkable in both capacity and secrecy. Like many other systems such as the immune system, endocrine system or cardiovascular system, the nervous system is contextually driven by everything inside and outside the body. However, the brain has a way of building upon itself by acting upon itself. It has the ability to represent the world abstractly and draw conclusions from simulated events that have yet to occur. Such behavior is simultaneously constructed and represented at the cellular level. This is perhaps the most advantageous aspect of the human brain; giving rise to self-awareness and steadily increasing intelligence.
The brain is like an ongoing construction project beginning with a foundation upward into beautiful columns, winding corridors, and a spectacular orchestration of communal activity. It is the temple of both social and individual experience.
Divisions of the Cortex
To properly deconstruct the brain, we have to subdivide it according to its primary functions. In figure 1 below, you'll notice the three main divisions of the cortex...
- Forebrain (Prosencephalon)
- Midbrain (Mesencephalon)
- Hindbrain (Rhombencephalon)
*Disclaimer - Most components of the brain are interdependent. Sometimes we have to go back and forth between sections to make sense of things as we go along.
Forebrain - Part 1 - Telencephalon
The forebrain is the largest division of the cortex and can be subdivided into two smaller divisions called telencephalon and diencephalon. The iconic folded outer structure of the forebrain is called "gyri". These indentations are formed to compensate for the lack of space available in the skull. Gyrification occurs mostly during fetal development.
This portion of the forebrain consists of the four major lobes as seen in figure 2 below.
Figure 2 - Lobes of the Forebrain
Figure 3 - Corpus Callosum
The forebrain can be further divided into two distinct hemisphere's (left/right). A thick band of fibers called the "corpus callosum" connects the two hemispheres allowing communication to take place between them (figure 3).
Each hemisphere specializes in certain tasks. The left brain is particularly adept at algorithmic and logical processing while the right brain is concerned with novelty and creativity. This dichotomy is not as cut and dry as it sounds. Any given task or mental activity requires both hemispheres working together. We do know, however, that the left brain controls motor function of the right side of the body and vice versa.
In humans, this part of the brain is larger and more developed than any other organism. The frontal lobe governs cognition including emotion, memory, problem solving, identity and appraisal. The frontal lobe is also responsible for primary motor function and conscious control of muscle activity.
In figure 3, you'll notice small regions in the left hemisphere called the "Broca's area" in the frontal lobe and "Wernicke's area" in the parts of the temporal and parietal region. Both vital to language processing and speech. This is a great example of how particular functions of the brain are not always locally isolated.
Figure 3 - Broca's & Wernicke's Area
The PFC in 60 Seconds
Divisions of the Frontal Lobe
You guessed it - more divisions. The prefrontal region of the brain is the most recent addition and is the last to fully develop during late adolescence. As mentioned before, the frontal lobe - as a whole - is involved in higher-level computation. However, the closer we move toward the medial (middle) region of the cortex, the more we find an overlap between cognition and motor function. In essence, it's the part of ourselves that asks what do we do with the information that we have?
The red shaded area in the figure above is what's called the prefrontal cortex or PFC. It is often considered the "seat of consciousness" because this is where our attention and executive control is governed. Most sensory-motor information converges here so we can plan and act accordingly. Underdevelopment of the PFC is often associated with risk taking and lack of inhibition. (As commonly demonstrated by teenagers/adolescents).
The lightly shaded blue portion is the premotor functional area of the frontal lobe. Learned behaviors and some motor skills are carried out here. While much of this area is not well understood, it is generally regarded as an intermediary point between the PFC and motor cortex.
In the dark blue region in the diagram is a small strip of tissue called the motor cortex. It is rich in neural connections that govern our bodily movement. In the figure below is a representation of body parts as they relate to the density of nerves devoted to their sensory input and function.
Figure 4 - Homunculus
This is how the body is represented in the brain. If you could tell a story about this peculiar creature what would it be? Hint: Communication, consumption, migration, and manipulation of objects (tools).
The parietal lobe consolidates various forms of sensory information to help us navigate the world. This includes the sense of touch as well as visuospatial processing to gauge distance and depth. Part of the aforementioned homunculus is interfaced between the frontal and parietal lobe (figure 4). The further toward the rear of the cortex, the more the parietal lobe is devoted to integrated visual information.
The left portion of the parietal lobe is also associated with symbolic interpretation such as mathematics and language while the right portion handles map reading and spatial relationships.
Overall, parietal function involves language comprehension, object concept, body position and movement, neglect/inattention, left-right differentiation and visual perception.
The occipital lobe makes up the majority of the visual cortex. It is primarily devoted to processing raw visual information. Light is captured by sensory receptors in the eyes (retina) and relayed by the optic nerve through a pathway called the optic tract to the posterior (back) region of the occipital lobe containing specialized neurons which make up the map of our visual field (figure 5). Note the use of the word "map" and not a direct experience of the world. Our perceptual experiences are merely a prediction; a triangulation or hypothesis of what's out there.
After we're first born and we begin to open our eyes, visual neurons begin to colonize this part of the brain. If a baby is born blind, neurons devoted to auditory processing will colonize this area instead. The reverse is true for babies born deaf.
Other functions of the occipital lobe include color discrimination, motion detection, spatial processing, facial recognition, and analyzing body language.
Figure 5 - Visual Pathway
Damage to the visual cortex is likely to result in what is called "cortical blindness" even if the eyes remain functionally intact.
Figure 6 - Hippocampus
The location of the temporal lobe indicates convenience of function as it rests just behind the ear. It processes auditory stimuli which allow us to perceive the world in terms of sonic input including encoded patterns of speech (language). As seen earlier in figure 3, the Wernicke's Area is specifically involved in speech recognition whereas the Broca's Area is responsible for the formulation of speech.
Behind the temporal lobe is a component of the limbic system called the hippocampus (figure 6) which accounts for general knowledge and autobiographical memory. For our purposes, we will investigate that later.
Something to think about:
Before the printing press or computers, most of what was to be learned over the course of our evolution was learned through word of mouth. Our brains are highly adapted for recognizing and memorizing language. This is largely still the case. In addition to reading comprehension, we need to augment the flow of information with speech for an optimal learning experience. (Hence the video augmentation for several segments.)
Forebrain - Part 2
Diencephalon & Limbic System
Just below the cortical cap that we've just covered is what's known as the"interbrain" or "diencephalon" located between the cerebrum and the brain stem. It is made up of 4 main components:
- Pituitary Gland
In the image above you'll notice that these structures are smaller in size but are perhaps the most ancient and powerful motivators of the brain. They help regulate alertness, homeostasis, and reproductive activity.
Thalamus - Relay mechanism that traffics sensory information to various parts of the nervous system. It plays a large role in arousal and feedback between the brain and spinal cord.
Hypothalamus - The prefix hypo (below) refers to the location of this structure in relationship to the thalamus. It is the primary "seeking/expectancy" system that identifies novelty and motivates exploration. What's more, the hypothalamus regulates body temperature, appetite, and hormone regulation.
Epithalamus - (Above thalamus) Includes the pineal gland which helps regulate sleep and emotions.
Pituitary Gland - Known as the "master gland", the pituitary gland heavily regulates the secretion of adrenal and growth hormones throughout the body. However, it does not act alone but is constantly communicating with other surrounding mechanisms, namely the hypothalamus.
Hippocampus - Central to the organization of information and formation of new memories. The role of the hippocampus is often controversial among fields that study the brain but most agree that it is essential in declarative memory (information that can be explicitly verbalized.)
Amygdala - This is often regarded as the "fear center" of the brain as it plays a large role in emotional memory but this blanket description has since been refuted by researchers because many other contributions are made by other parts of the brain during emotional states. This organ is merely part of a larger relay system involved with fear conditioning in addition to other functions such as pheromone processing, appetite, and memory consolidation.
More importantly, the amygdala plays a large role in remembering threats from previous experiences. Over repeated exposure to sensory cues associated with stress, cues will become more salient to the amygdala, therefore, more susceptible to triggering the stress response in the future
Limbic System & Hypothalamus
Tectum & Tegmentum
Situated atop the brainstem just below the thalamic structures is the midbrain which is comprised of small nodules (see image above) that serve some basic but vital purposes.
Tectum - Rear portion of the midbrain comprised of two smaller swellings called superior (above) and inferior (below) colliculi.
- Superior Colliculus - Receives sensory input from the retina and visual cortex. Engaged in visual reflexes such as the tracking of moving objects.
- Inferior Colliculus - Receives sensory input from auditory nerves and auditory cortex.
Tegmentum - Located in front of the tectum. The tegmentum can be subdivided into three distinct parts according to function.
- Red nucleus - Sensorimotor input and coordination
- Periaqueductal gray - Pain transmission, analgesia, anxiety, and cardiovascular control.
- Substantia nigra - Learning, addiction & emotion.
Hindbrain - Rhombencephalon
The hindbrain makes up the lower portion of the brainstem. It is highly involved in autonomous functions such as breathing, balance and blood flow. To get a better idea of this ancient remnant of the nervous system, let's review its basic components.
- Cerebellum - Sometimes called the "little brain", the cerebellum has two hemispheres but contains 4 times the number of neurons than the larger outer cortex. It is most often associated with coordination, muscle activity, and cardiac function.
- Pons - Acts as an intermediary between the cerebrum and the cerebellum. Connects with major nerves responsible for facial motor activity such as chewing and swallowing in addition to the nerves which give rise to our ability to turn our heads left to right.
- Medulla - Put simply, the medulla is the junction between the brain and spinal cord. Much of its function is devoted to communicating sensory information between the two but is partially involved in autonomic activity such as digestion and respiration.
- Reticular Formation - A dense branch of neurons with numerous connections throughout the hindbrain and midbrain. It is most often known for its role in promoting arousal and consciousness. Damage to this region of the hindbrain is likely to result in permanent coma.
And finally, we reach the major nerve connections that bridge the gap between the brain and where we left off in part 1 of this article. Please enjoy the video below for the final segment of this crash course. Be sure to leave a comment below. Thank you for your support and take care of your brain!
Anil Seth on Consciousness
Ackerman, S. (1992) Discovering the Brain. Major Structures and Function of the Brain. NCBI. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK23415
Hines, T. (2016) Anatomy of the Brain. Mayfield Brain & Spine. Retrieved from https://www.mayfieldclinic.com/PE-AnatBrain.htm7/
NBIA (2018) Brain Structure & Function. Northern Brain Injury Association. Retrieved from http://nbia.ca/brain-structure-function/
© 2018 Jessie Watson