The one about our Brain
Professor Jeff Lichtman starts off his course at Harvard by asking his students the question “If everything you need to know about the brain is a mile, how far have we walked in this mile?” He says students give answers like half a mile, a quarter of a mile, etc. — but that he believes the real answer is “about three inches.” Our brain remains the most complex object in the known universe. But still, I will attempt to lay out a beautiful and easy to understand picture of how our brain is organized and how it works.
A BRIEF HISTORY OF BRAIN
Imagine a sponge under the ocean about 700 million years ago.
This sponge has no brain nor any neurons. Essentially it has no way to process any information or stimuli. It can just exist and hope food floats by. Since it’s so long ago when there were barely any predators, our sponge doesn’t need to worry about getting eaten.
Then about 600 million years ago came Jelly fish.
The jelly fish had nerves and they were all interconnected like a neural net. If one part of its body was injured, it could pass the information to the next neuron, which could pass it to the next one and so on. Till all neurons are aware and as a collective they could move the body away from the danger zone.
Then about 550 million years ago came the flatworm.
The flatworm figured that we could get more work done, if there was someone in charge of all the neurons. Hence, came the first nervous system boss — Brain. So instead of arranging themselves in a net shape, the flatworm’s nervous system all revolved around a central highway of messenger nerves that would pass messages back and forth between the boss and everyone else.
As time passed and Earth’s animals, like reptiles, started inventing intricate new body systems like hearts and lungs and this idea of a nervous system boss caught on quickly. This reptilian brain controlled body’s vital functions such as heart rate, breathing, body temperature and balance. This is done by the brain stem and cerebellum. The reptilian brain is reliable but tends to be somewhat rigid and compulsive.
Eventually, about 225 million years ago, Earth had its first mammal.
Mammals were a bit more complicated. They had more going on than just survival instincts. So, a second boss developed on top of the old reptilian brain — Limbic system. This limbic brain could record memories of behaviours that produced agreeable and disagreeable experiences. It was responsible for what are called emotions in human beings. The main structures of the limbic brain are the hippocampus, the amygdala, and the hypothalamus. The limbic brain is the seat of the value judgments that we make, often unconsciously, that exert such a strong influence on our behaviour. It controls our emotional instincts like fear, love, anger.
As mammals grew more and more complex, a new brain was needed. This Neocortex was built on top of the limbic system. This is the wrinkly stuff we are used to seeing when we picture brain. Neocortex has been responsible for the development of human language, abstract thought, imagination, and consciousness. The neocortex is flexible and has almost infinite learning abilities. The neocortex is also what has enabled human cultures to develop.
Here is a summary picture of the brain history:
These three parts of the brain do not operate independently of one another. They have established numerous interconnections through which they influence one another. The neural pathways from limbic system to the cortex, for example, are especially well developed.
BRAIN ANATOMY
Starting from the outside, there are a bunch of layers before you hit the skull.
Then even more layers before you get to the brain:
There are three meningial layer. The outermost membrane is the dura mater, a tough, fibrous coat that surrounds the brain and spinal cord like a loose fitting bag. Beneath the dura lies the arachnoid mater which is a translucent, collagenous membrane that, like the dura, loosely envelops the brain and spinal cord. The innermost of the meninges is the pia mater, a delicate membrane of miscroscopic thickness that is firmly adherent to the surface of the brain and spinal cord, closely following the contours. Between the arachnoid and pia is the subarachnoid space though which cerebrospinal fluid circulates. The subarachnoid space also house the arteries and veins that nourish the brain. Meningitis causes an inflammation of these layers and pressure on dura mater results in headaches.
Let’s look at what all was included in the reptilian brain.
Cerebellum: This “Little Brain” helps with balance, posture, and coordinated movements.
Medulla Oblongata: Controls autonomic (involuntary) functions from heart rate and breathing to vomiting and sneezing
Pons: This is essentially a bridge between cerebellum and brain stem. Pons relays sensory information between various sensory areas of brain including the thalamus which we will learn about shortly.
Midbrain: There are nuclei (neuronal cell bodies) that process visual and auditory information and coordinate reflexive motor responses to these stimuli. This region also contains centers involved with the maintenance of consciousness.
These parts of the brain available for reptiles takes care of all basic, autonomic survival functions.
Moving on to the limbic brain (shown in orange) that our mammalian ancestors possessed:
Amygdala: Includes two almond shaped cluster of nuclei involved primarily in emotional responses (fear, anxiety, aggression) as well as emotional learning.
Thalamus: In its central position in the brain, the thalamus also serves as a sensory middleman that receives information from your sensory organs and sends them to your cortex for processing. When you’re sleeping, the thalamus goes to sleep with you, which means the sensory middleman is off duty. That’s why in a deep sleep, some sound or light or touch often will not wake you up. If you want to wake someone up who’s in a deep sleep, you have to be aggressive enough to wake their thalamus up. The exception is your sense of smell, which is the one sense that bypasses the thalamus. That’s why smelling salts are used to wake up a passed-out person.
Hypothalamus: The hypothalamus contains a variety of important control and integrative centers. It’s involved in subconscious control of skeletal muscle contractions, control of autonomic functions, coordination of nervous and endocrine system, secretion of hormones, production of emotional and behavioral drives, regulation of body temperature, and control of circadian rhythm.
Hippocampus: This is like a scratch board for memory. It plays an essential role in processing working memory and storing them in cortex as long term memories.
None of these parts give us conciousness and the power to think. That is the job of the cortex.
THE CORTEX
The cerebral cortex is necessary for conscious awareness and thought, memory and intellect. It is the region to which all sensory modalities ultimately ascend (mostly via the thalamus) and where they are consciously perceived and interpreted in the light of previous experience. The cerebral cortex is highest level at which the motor system is represented. It is here that actions are conceived and initiated.
We have over 100 billion neurons with trillions of individual connections. How does our brain fit all that in the tiny space? With the help of folds found in the cortex, our brain has increased the surface area for information storage and processing significantly. Look at this picture for reference:
The thickness of this unfolded cortex is 2 mm. Our cortex is divided into four main lobes:
Frontal lobe: Responsible for higher level thinking, reasoning, planning, execution and personality. Frontal lobe also has the primary motor cortex which has the mapping of entire body. This is an extremely important strip of brain and I will explain it shortly.
Parietal lobe: This area is involved with sensing and perception. Parietal lobe has the primary somatosensory cortex which complements the motor cortex in frontal lobe. I will explain this shortly as well.
Temporal lobe: This holds the auditory cortex which aids in processing of input from our ears. This area is also active when we hear things in our mind. Temporal lobe is also big in memory storage.
Occipital lobe: This is our visual cortex. Because processing of visual information is so complicated and detailed, almost entirety of this lobe is dedicated to that.
BROADMAN’S AREAS
Different areas of our cortex are dedicated to specific things such as motor activity, speech, understanding language, vision etc. The idea that different regions of the brain are involved with different psychological functions came to prominence in nineteenth century with the work of Franz Josef Gall. But the concept of ‘cerebral localization’ didn’t firmly take roots in neuroscience until Paul Broca in 1861. He proposed an area of brain in left frontal lobe dedicated to speech production. This is now known as Broca’s area. Similarly, Wernicke’s area also found in left hemisphere is involved in comprehension of written and spoken language. Later Korbinian Broadman organized even more section of the cortex based on their physiology and function. He noticed cortex was 6 layers deep with each layer containing different kinds of neurons each performing a specific function in the distribution of the information throughout the brain. And different areas of the cortex were organized differently giving the idea that each area of cortex had a specific function. He numbers the areas now known as Broadman’s area (1–52). For example, Broca’s and Wernicke’s areas that we just discussed are in areas 41 and 40 respectively. Area 17 is the visual cortex. Area 41 and 42 are the auditory cortex.
MAPS IN OUR BRAIN
Broadman area 4 is the primary motor cortex body and area 3 and 2 are sensory cortex. Both these areas have the entire map of our body as depicted below. The right side of our body is mapped out on left motor cortex and vice versa. The areaa dedicated to each body part id not proportional to the actual body size. For example, almost half the area is dedicated to face and hands.
MOTOR AND SENSORY CORTEX
In the diagram above, the orange strip is the motor cortex where the wiring of all movement pathways start. The blue strip is the sensory cortex where the wiring for all sensory pathways terminate. So, each part of your skin is lined with nerves that originate in the sensory cortex. This is a homunculus man — 3d representation of how much area if dedicated to each body part.
There’s also a tonotopical map of cochlea in the auditory cortex and a map of retina in visual cortex.
CORTICAL LAYERS
The top most layer of the cortex is divided in 6 different layers. This is the grey matter area of the cortex.
LAYER I: The most superficial layer, consists few nerve cell bodies but many dendritic and axonal processes in synaptic interaction.
LAYER II: Contains many small neurons, which establish intracortical (within the cortex) connections
LAYER III: Contains medium-sized neurons giving rise to the specific thalamic nuclei
LAYER IV: Site of termination of afferent fibers from the specific thalamic nuclei
LAYER V: The origin of projection fibers of the extracortilal (outside of cortex) targets, such as the basal ganglia, thalamus, brain stem, and spinal cord. In the primary motor cortex of the frontal love, this layer contains the giant Beta cells, which project fibers into the pyramidal tract.
LAYER VI: Also contains association and projection fibers.
So, all Broadman areas including the motor, sensory, auditory, visual, Broca, Wernicke are organized in these six layers of neurons.
BRAIN LATERALIZATION
There’s a common misconception that the right side of our brain controls the left side of the body. For some factors, yes this is true. As we just saw that our motor and sensory cortex are mapped that way. But our visual field does a combination of both. The information received form the visual field on closer to our nose decussate and get process by opposite sides of brain. But the information coming from peripheral side is processed by same side of brain. Also, our language areas (Broca and wernicke) are mostly located in left hemisphere even in help landed people. So, language seems to be a primarily left brain task.
HOW DO NEURONS COMMUNICATE
Now that we have a general understanding of the layout of the brain, I think it is important to learn how neurons communicate before we can see how it all fits together.
A neuron is a brain cell. The cell body which houses the nucleus and dna has spiny projections called dendrites. The dendrites receive information from other neurons. The other end of the cell body has a long projection called Axon. Axons terminate at a dendrite of another neuron and send signals. Axons are covered in insulating myelin sheath which allow for more efficient and faster signal transfer. This information transfer between an axon of one neuron and dendrite of another neuron happens at the synapse. Synapse is a small gap between the axon and dendrites. Axons release neurotransmitters (certain chemical like dopamine, serotonin, norepinephrine, acetylcholine, GABA etc) in this gap which get taken up by the dendrites. The effect of the neurotransmitters can either be excitatory or inhibitory. Signal propagation is made possible because of an electrical potential difference. The inside of a neuron at rest has a slight negative charge compared to outside. All neurons have a bunch of axons ending at their dendrites and a bunch of dendrites starting at their axon terminals. And there is always some electrical activity going on. But a signal that propagates completely from a dendrite to another dendrite aka action potential is an all or none situation. A certain threshold of electrical potential must be reached before an informtaion can be transmitted to the next neuron in form of action potential. This is just like the binary language of computers. An action potential firing = 1 and not firing = 0. All of this is mediated by opening and closing of sodium and potassium channels to depolarize and hyperpolarize cells.
To learn a new skill, we practice. More practice means more action potentials. If enough action potentials are fired for an extended period of time, the connection between neurons will strengthen. And that skill is more likely to go into long term memory. Our brain is not entirely hardwired with fixed neural circuits. It is able to rewire itself in response to training or injury. This is known as neuroplasticity.
GREY and WHITE MATTER
The cell bodies of neurons are located in the 2 mm thickness of the cortex and some in brain stem and our limbic brain. These cell bodies make up the gray matter. The entire top layer of cortex is made of neuron cell bodies and is therefore gray. The cell bodies in gray matter receive signals and the output is sent via axons that form the white matter.
THE WIRING
This is the white matter of our brain composed of axons and made white due the mylin sheath which provide insulation.
Beneath the cortex’s surface lies and enormous mass of nerve fibers, all of which have their origin, or termination, or sometimes both, wihin the cortex. The fibers are classified into three types depending upon their origin and destination:
ASSOCIATION FIBERS: interconnect cortical sites lying within on hemisphere
COMMISURAL FIBERS: run from one cerebral hemisphere to another, connection two functionally related structures.
PROJECTION FIBERS: pass between cerebral cortex and subcortical structures such as thalamus, striatum, brain stem, and spinal cord
A bunch of axons wrapped together is called a nerve fiber bundle. A few bundles wrapped together is called nerve. Spinal nerves project out of the holes in the vertebral columns as an extension of the spinal cord. Most Cranial nerves project out of the skull.
There are two main kinds of wiring: ones going to the brain with sensory information are called afferent fibers and one leaving the brain are called efferent fibers. These efferent fibers are generally motor nerves. Some nerves terminating on skin and muscle are in our voluntary control. Whereas others ending on internal organs are autonomous. Some are excitatory like the sympathetic nervous system also known as the fight or flight response. Some like the parasympathetic system are inhibitory and are also known as our body’s rest and digest response. These same pathways are responsible for pupil constriction in excess light and dilation in darkness. Here’s a cross section of the spinal cord:
All these labeled tracts and fasciculus in the spinal cord cross section above are various pathways of nerves. And the name of the tract usually implies its route. So, for example, the spinocerebellar tract connects cerebellum to the spinal cord and corticospinal tract connects spinal cord to the cortex. There are many manyyy pathways in the brain from reward pathway involving dopamine to repetitive thoughts pathway of OCD to serotonin pathways originating in Raphe nuclei of brain stem.
Here is an example of a neural pathway or circuit:
Reward Pathway
When we experience something pleasurable, VTA (Ventral Tegmental Area) sends this information to several different brain regions: the Nucleus Accumbens, the amygdala, the septum, and the prefrontal cortex. This information is transmitted via the neurotransmitter, Dopamine. The nucleus accumbens then activates the individual’s motor functions, while the prefrontal cortex focuses his or her attention. Increased levels of dopamine in nucleus accumbens reinforces the behaviours by which we satisfy our fundamental needs. This information that we have just experienced something pleasurable is sent to VTA from the cortex itself. This way, like all other neural circuits, the loop closes back in itself.
Let’s see an example of a sensorimotor pathway of us sensing something and then responding to the stimuli.
Let’s say there’s a fly on my arm. Fly’s legs activate the touch receptors on my skin. This signal leaves my arm in a sensory nerve. The sensory nerve relays in the spinal cord. Here a big chunk of the information will go up the spinocortical tract to the primary sensory cortex. Some of the information may travel to the sensory relay station in thalamus via spinothalamic tract. Yet some information may go to our visual cortex and auditory cortex. The sensory information from sensory cortex gets relayed to the motor cortex. Here, whatever output the brain desires gets sent back to spinal cord and then to my arm. In this case, let’s say the brain wants me to move my arm in hopes the fly will fly away. This information will get relayed to my arm via a motor neuron. All of this feels like an instant. But not as fast as a reflex.
If this was a reflex pathway instead of a conscious one then the information to move my muscle will come directly from the spinal cord without any input from brain. This is a much faster process.
INTERESTING DISORDERS OF THE BRAIN
AGNOSIA (visual, sensory, motion): There are different kinds of agnosia where there is a disconnect between brain and one of the senses. For example, in visual agnosia, a person is unable to recognize an apple just by looking at it but will know it’s an apple if they touched it. The opposite of this would be sensory agnosia. People with motion agnosia see life in snapshots are unable to perceive motion.
ALICE IN WONDERLAND SYNDROME: People with this condition either feel like giants in a miniature world or an ant in a giant’s world. It’s possible that Lewis Carol may have suffered from this and that’s why it is called Alice in wonderland syndrome.
SYNESTHESIA: Condition where senses are mixed up and numbers can have tastes, alphabets can have smells.
COTARD’S SYNDROME: The affected person believes to themselves to be dead and putrefying.
One of my favorite neuroscience books is called Man who mistook his wife for a hat by Olive Sacks. Olive Sacks is a neuroscientist and this book is a collection of some of his cases dealing with rare brain disorders.
Hope you found this post informative as well as interesting.
REFERENCES: Neuralink and Brain’s magical future by waitbutwhy; Human Anatomy 6Th edition by Martini and Timmons; Neuroanatomy 3rd edition by Crossman and Neary
