At the end of this module – brain basics for stroke carers – you will be able to describe the brain’s basic anatomy and functions that could be affected by stroke.
In Module 1 – Stroke basics for stroke carers – we peeped into the definitions of the word, stroke, different stroke types, what happens in a stroke, and who are at risk.
2.1. Brain covers
2.2. Brain surface map
2.2.1. Frontal lobe and Broca’s area
2.2.2. Parietal lobe
2.2.3. The “Homunculus” (“Two Little Humans”)
2.2.4. Temporal lobe
2.2.5. Occipital lobe
2.3. Brain’s blood supply
2.1. Brain covers
Brain basics for stroke carers begin with an introduction to brain covers. Our brain is a very pliable organ. Even a small finger pressure makes a dent over its surface. The bony skull protects it and, as Figure 1 shows, three layers underneath the skull wrap the brain snugly. The outermost cover ( the folded one in Figure 1) is called, “Dura mater” (meaning “tough mother”). As its name implies it is firm and thick. The other two inner covers (“spider mother” and “soft mother”) is thinner, transparent, as well as glistening. Blood vessels traverse through the two layers bathing a colorless “cerebrospinal fluid”. You can also see that the surface is folded into grooves (long convoluted ditches) and bumps (long convoluted surfaces) underneath the inner two covers.
Visual credit: Videos drawn from the NeuroLogic Exam and PediNeuroLogic Exam websites are used by permission of Paul D. Larsen, M.D., University of Nebraska Medical Center and Suzanne S. Stensaas, Ph.D., University of Utah School of Medicine. Additional materials were drawn from resources provided by Alejandro Stern, Stern Foundation, Buenos Aires, Argentina; Kathleen Digre, M.D., University of Utah; and Daniel Jacobson, M.D., Marshfield Clinic, Wisconsin. The movies are licensed under a Creative Commons Attribution-non-commercial-ShareAlike License.
2.2. Surface map
Our brain surface owns a highly advanced map. Let us see how it is organized. An understanding of this map is an essential part of brain basics.
Figure 2 shows the brain sans its covers. It consists of two equal halves – identified as left and right hemispheres. Both communicate with each other via neurons cells who travel through a short thick stalk deep in the middle. We do not see the stalk in this image.
Scientists name this densely wrinkled (or densely folded) surface as the “cerebral cortex”; it is a layer with thickness varying from 1mm to 4.5mm averaging about 2.5mm (one inch)1. This layer, grey in color (also called “grey matter”) is made up of tightly packed millions of neurons. Using its grooves and bumps as landmarks, scientists divide it into four large regions, also called “lobes”: Frontal lobe, Parietal lobe, Temporal lobe, and Occipital lobe (Figure 3).
2.2.1. Frontal lobe (“region”)
Figure 3 depicts the Frontal lobe blue in color. It extends front to back until it meets a groove (or a fissure), scientists name the”Central Sulcus”. This groove separates the Frontal lobe from another two regions (lobes): the Parietal lobe and the Temporal lobe.
Pre-central gyrus (Primary Motor Cortex)
Now, look at Figure 4 and focus on the red-colored strip that lies in front of the Central Sulcus. Scientists name this bump the “Precentral Gyrus” because of its placement. As you can see, this strip ascends parallel to the Central groove from its left side over to the top. It ends after descending about 1cm on its own. We have exactly a similar strip on the right half of the brain too. The nerve cells that populate this strip has a very special job to do; they send commands, with our approval, to muscles of the opposite side of the body to move. Because of this job, scientists call it the “Primary Motor Cortex”.
The nerve cells in the large grey-colored area that spans towards the front of this “Primary Motor Cortex” become very busy when we think, make decisions, behave in ways whatever we want. Because we, humans, are the only ones who can do these high-end activities, we own the largest Frontal lobe among all animals.
Broca’s area (Figure 5)
This is a special small area in the Frontal lobe that merits our attention. We find it over the left Frontal lobe. The nerve cells in this area specialize in word production both in speech and writing. They process information that receives from another area (Wernicke’s area that situates in the Temporal lobe) and coordinates those with another set of large neural networks2.
Why do we need to know about this? It is because the death of these nerve cells results in a special kind of speech difficulty, called Broca’s aphasia.
2.2.2. Parietal lobe (Figures 3 and 4)
Figure 3 depicts this lobe (“region”) pale white in color. It begins just behind the Central Sulcus (groove), spreads over the top, and descends towards the left side to meet another groove, called the Lateral Sulcus. And the lobe extends behind to meet another groove that separates it from the Occipital lobe.
Figure 4 depicts this area of strip blue in color. Beginning just behind the Central Sulcus (groove), it runs parallel to its counterpart, the Precentral Gyrus that lies on the opposite bank of the groove. Very much similar to its counterpart, the nerve cells in this area look after the exact same locations of the body’s opposite side. However, there is a marked contrast; unlike its counterpart nerve cells who send commands, these nerve cells receive information about touch, temperature, and pain. Then, they relay the information to the Frontal lobe nerve cells for necessary action. Because of the nature of this job, scientists name this strip the “somatosensory cortex”. For example, when we feel pain in the left arm, the area assigned in this strip receives the information and then relays the information to the Frontal lobe. After processing this information, the nerve cells in the “primary motor cortex” send instructions to move the left arm from the source of the pain.
The other areas of the parietal lobe carry out more complex jobs than the above, however. It includes maintaining balance, recognizing objects, etc.
2.2.3. The two Homunculi (“two little humans”) (Figure 6 and 7)
This is a very useful excellent metaphor with practical value to understand the brain basics for stroke carers. In Figure 4, we found red-colored and blue-colored strips on either side of the Central Sulcus (groove). While the nerve cells of the former are responsible for sending commands to move muscles, the nerve cells of the latter strip process sensory information from the body.
Researchers have shown that both strips own a unique layout to execute their jobs. If we climb the strip (precentral gyrus) starting from the area closest to the left Lateral Sulcus (groove) (Look at Figure 4) until it ends at the other side, we will find a homunculus; a small replica of the human body. Compare Figures 6 and 7.
The layout is exactly similar to a homunculus: a small replica of a human body. The “Homunculus” refers to a fictional fully formed human being but extremely small in size; it is a miniature replica. Figures 6 and 7 re-create it beautifully. We own two homunculi on each side of the brain: The “motor homunculus – the strip that lies in front of the Central groove, and the “sensory” homunculus that lies just behind the Central groove. Another interesting characteristic is that the assigned area’s sizes are proportional to the complexity of the job to do. For example, the assigned areas for the fingers, tongue, and face are much larger than the rest of the areas.
Dr. Wilder Penfield and Edwin Boldrey in 1937 “discovered” and wrote a detailed 50-page article with drawings.
We already know what these “little humans” are doing there.
2.2.4. Temporal lobe (Figure 3 and 4)
Figure 3 depicts the left Temporal lobe (region) green in color. It extends from the “Lateral groove” (also called “Lateral Fissure”) back until it reaches the Occipital lobe’s territory. The nerve cells in this region process information about language understanding, hearing, and memory.
The neurons in this special area process information of language understanding and interpretation. Then it relays the information to the Broca’s area’s neurons for word production.
When we hear something, the hearing area receives information from the ear and then send this information to the Wernicke’s area. Similarly, when we see or read something, the visual area receives it and then sends that information to the Wernicke’s area. Neurons in this area work hard to retrieve suitable nouns appropriate to the context from the storeroom, set the language structure, and shoot the processed information to the Broca’s area.
The death of brain cells of the Wernicke’s area will result in a specific language disability called “Wernicke’s aphasia”. We will learn about it in another module.
2.2.5. Occipital lobe (Figure 3)
This lobe appears colored brown in Figure 3 and situates the brain’s back surface. Its neurons interpret images that receive from our eyes. The damage to neurons in this area causes loss of our sight even though the eyes and its nerves are intact.
The above layout of brain basics for stroke carers provides a solid foundation to improve care for those living with a stroke.
2.3. Brain’s blood supply
Another useful area of the brain basics for stroke carers are the brain’s blood supply architecture. The brain receives oxygen and food for survival via two main supply routes: carotid arteries and vertebral arteries. The carotid arteries climb from the sides of the neck while the vertebral arteries climb through our vertebral column which situates the back of the neck. Inside the brain, they form an inter-connected-circle, called the circle of Willis.
Figure 8 still-image taken from a video clip from a demonstration-explanation of the brain’s blood supply by a professor of the University of Utah shows the Left side carotid artery and its divide. This is the main supply route. We own a similar one on our right side too.
At the level of the neck, the carotid arteries, both left and right, branch out into two: External and internal. The latter again divides into three smaller ones: anterior (front), middle, and posterior (back).
Figure 9 illustrates their feeding areas in the brain; the first one carries blood to the neurons who manage information related to the lower part of the body including the legs; the second one (the middle branch) nourishes the neurons who manage information related to the upper part of the body including the face, tongue, arm. This also includes areas assigned to language and speech: Broca’s area and Wernicke’s area.
Why should we know about this information? This knowledge of brain basics for stroke carers is critical to treat and providing care to the affected. For example, if the right middle branch’s blood supply interrupts, the affected person will show difficulties in raising or feeling of the left arm and drooping of the right side of the face, etc. If the left middle branch’s blood supply interrupts, in addition to the above, we can observe speech and understanding difficulties also.
- Bruce Fischl, Anders M. Dale (2000): Measuring the thickness of the human cerebral cortex from magnetic resonance images bruce Fischl, Anders M. DaleProceedings of the National Academy of Sciences Sep 2000, 97 (20) 11050-11055; DOI: 10.1073/pnas.200033797.
- Redefining the role of Broca’s area in speech Adeen Flinker, Anna Korzeniewska, Avgusta Y. Shestyuk, Piotr J. Franaszczuk, Nina F. Dronkers, Robert T. Knight, Nathan E. Crone Proceedings of the National Academy of Sciences Mar 2015, 112 (9) 2871-2875.