8.1 Case Study: The Control Centre of Your Body
Case Study: Fading Memory
Each of these brightly-coloured sticky notes (Figure 8.1.1) represents a piece of information that someone doesn’t want to forget. Although we are all forgetful sometimes, most people do not have trouble remembering things that are important or routine to them, such as a friend’s name or how to get to class. Our — the control centre of the and the rest of the body — normally allows us to retain and recall information. If, however, the brain is damaged, a person may need to rely excessively on external reminders — like this wall of sticky notes — rather than their own memory… if they can remember to write things down in the first place.
One person having trouble with her memory is 68-year-old Rosa. Rosa has been struggling to remember where she has set down objects in her house, and she forgot about a few doctor’s appointments and lunches she planned with friends. Her family began to notice that she would sometimes fail to recall recent conversations, requiring them to repeat things to her. Rosa would also sometimes struggle to find the right word in a conversation, and would put objects in unusual places, such as the milk in a cabinet instead of the refrigerator. While most people do things like this occasionally, it seemed to Rosa and her family that it was happening to her more regularly.
Rosa also had other symptoms that were impacting her life, such as having trouble paying her bills on time and managing her budget, which she had previously done well. She ascribed these lapses in memory and mental functioning to the normal effects of aging, but her family was concerned. They noticed that she was also more irritable than usual and would sometimes verbally lash out at them, which was not like her. When she became disoriented on a walk around her neighborhood and a neighbor had to escort her home, her family convinced her to see a doctor.
Besides a complete physical exam and lab tests, Rosa’s doctor interviewed Rosa and her family about her memory, ability to carry out daily tasks, and mood changes. He also administered a variety of tests to assess her memory and cognitive functioning, such as her ability to solve problems and use numbers and language correctly. Finally, he ordered a scan of her brain to investigate whether a tumor or some other observable cause was leading to changes in the functioning of her brain.
Based on the results of these tests, Rosa’s doctor concluded that she most likely has mild Alzheimer’s disease (AD). AD results from abnormal changes in the and of the brain, characterized by clumps of (called amyloid plaques) between brain cells and tangled bundles of protein fibres (called neurofibrillary tangles) within certain brain cells. The affected brain cells stop functioning properly, lose their connections to other brain cells, and will eventually die. Figure 8.1.3 shows part of a cross-section of a brain from a patient who had severe AD, compared to a similar cross-section of a healthy brain. You can see how severely shrunken the brain with AD is, due to the death of many brain cells.
AD is a progressive disease, which means the damage and associated symptoms get worse over time. Clinicians have categorized the progression into three main stages — mild, moderate, and severe AD. Typically, AD cannot be definitively diagnosed until after death, when the brain tissue can be directly examined for plaques and tangles. Based on Rosa’s symptoms and the results of her tests, though, her doctor thinks she most likely has mild AD. At this stage, the brain has started changing, but resulting symptoms are not yet severe.
Although there is currently no cure for AD and Rosa will eventually get worse, her doctor says that medications and behavioral therapies may improve and prolong her functioning and quality of life over the next few years. He prescribes a medication that improves communication between brain cells, which has been shown to help some people with AD.
As you read this chapter, you will learn more about how the brain and the rest of the nervous system work, along with the multitude of functions they control in the body. By the end of the chapter, you will have enough knowledge about the nervous system to learn more about why AD causes the symptoms that it does, how Rosa’s medication works, and some promising new approaches that may help physicians diagnose and treat AD patients at earlier stages.
Chapter Overview: Nervous System
In this chapter you will learn about the human nervous system, which includes the brain, spinal cord, and nerves. Specifically you will learn about:
- The organization of the nervous system — including the central and peripheral nervous systems — and their organs and subdivisions.
- The cells of the nervous system — neurons and neuroglia — their parts, and their functions.
- How messages are sent by neurons through the nervous system, and to and from the rest of the body.
- How these messages (or nerve impulses) are transmitted by electrical changes within neurons, and through chemical molecules to other cells.
- The structure and functions of different parts of the central nervous system, which includes the brain and spinal cord, and some of the things that can go wrong when they are damaged.
- The structure and functions of the peripheral nervous system, which includes the nerves that carry motor and sensory information to and from the body to control voluntary and involuntary activities.
- The human senses, as well as how visual information, sounds, smells, tastes, touch, and balance are detected by sensory receptor cells and then sent to the brain for interpretation.
- How legal and illegal drugs can have psychoactive effects on the brain — altering mood, perceptions, thinking, and behavior — which can sometimes lead to addiction.
As you read the chapter, think about the following questions:
- Based on Rosa’s symptoms, which parts of her brain may have been affected by Alzheimer’s disease?
- How are messages sent between cells in the nervous system? What molecules are involved in this process? In what ways can drugs alter this process?
- Why can’t Rosa’s brain simply grow new cells to replace the ones that have died?
Attributions
Figure 8.1.1
Sticky notes by Richard Maguluko from Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
Figure 8.1.2
Washing hands [photo] by National Cancer Institute on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 8.1.3
Alzheimers_brain by National Institute on Aging/ NIH’s Medline magazine on Wikimedia commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain) .
Reference
Mayo Clinic Staff. (n.d.). Alzheimer’s disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/alzheimers-disease/symptoms-causes/syc-20350447
The central nervous system organ inside the skull that is the control center of the nervous system.
Created by: CK-12/Adapted by Christine Miller
Ribosome Review
The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body. This artwork brings to life an important structure in living cells: the , the cell structure where are synthesized. The slender silver strand is the messenger (mRNA) bringing the code for a out into the cytoplasm. The purple and green structures are ribosomal subunits (which together form a single ), which can "read" the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein. All living — whether they are or — contain , but only eukaryotic cells also contain a and several other types of .
What Are Organelles?
An is a structure within the of a that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the , , , , , and . are not enclosed within a membrane, but they are still commonly referred to as organelles in cells.
The Nucleus
The is the largest organelle in a cell, and it's considered the cell’s control center. It contains most of the cell’s (which makes up chromosomes), and it is encoded with the genetic instructions for making . The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called , which is similar in composition to the found in the cytoplasm outside the nucleus. Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red blood cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.
As you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the . This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model below is mainly involved in the assembly of . After being produced in the , ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.
Mitochondria
The mitochondrion (plural, ) is an organelle that makes available to the cell. This is why mitochondria are sometimes referred to as the "power plants of the cell." They use energy from organic compounds (such as ) to make molecules of (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy.
Mitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane. In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix. Does this sound similar to the requirements to be considered a cell? That's because they are!
Scientists think that mitochondria were once free-living organisms because they contain their own . They theorize that ancient prokaryotes infected (or were engulfed by) larger cells, and the two organisms evolved a relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as inside them. This theory is called , and it is widely accepted by biologists today. (See the video in section 4.3 to learn all about endosymbiotic theory.)
Endoplasmic Reticulum
The (ER) is an organelle that helps make and transport and . There are two types of endoplasmic reticulum: (rER) and (sER). Both types are shown in Figure 4.6.4.
- rER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER.
- sER looks smooth because it does not have ribosomes. sER makes lipids, stores substances, and plays other roles.
The Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.
Golgi Apparatus
The (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.
Vesicles and Vacuoles
Both and are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport and molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.
There are some vesicles which are specialized to carry out specific functions. Lysosomes, which use enzymes to break down foreign matter and dead cells, have a double membrane to make sure their contents don't leak into the rest of the cell. Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins.
Centrioles
are organelles involved in . The function of centrioles is to help organize the before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a protein named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured in Figure 4.6.5.
Ribosomes
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.
4.6 Summary
- An is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job. Although are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.
- The is the largest organelle in a eukaryotic cell, and it is considered to be the cell's control center. It controls gene expression, including controlling which proteins the cell makes.
- The mitochondrion (plural, ) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted , mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
- The (ER) is an organelle that helps make and transport proteins and lipids. (rER) is studded with ribosomes. (sER) has no ribosomes.
- The is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
- Both and are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.
- are organelles located near the that help organize the before so each daughter cell receives the correct number of chromosomes.
- are small structures where proteins are made. They are found in both and cells. They may be found alone or in groups within the or on the rER.
4.6 Review Questions
- What is an organelle?
- Describe the structure and function of the nucleus.
- Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.
- Why are mitochondria referred to as the "power plants of the cell"?
- What roles are played by vesicles and vacuoles?
- Why do all cells need ribosomes — even prokaryotic cells that lack a nucleus and other cell organelles?
- Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?
4.6 Explore More
https://www.youtube.com/watch?v=URUJD5NEXC8&t=121s
Biology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.
https://www.youtube.com/watch?v=Id2rZS59xSE&feature=youtu.be
David Bolinsky: Visualizing the wonder of a living cell, TED, 2007.
Attributes
Figure 4.6.1
Ribosomes at Work by Pedrik on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
Figure 4.6.2
Nucleus by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 4.6.3
Mitochondrion_structure.svg by Kelvinsong; modified by Sowlos on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 4.6.4
Endomembrane_system_diagram_en.svg by Mariana Ruiz [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.6.5
Centrioles by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 4.6.6
Ribosome_shape by Vossman on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
References
Blausen.com staff. (2014). Nucleus - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014
Blausen.com staff (2014). Centrioles - Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014
Nucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https://www.youtube.com/watch?v=URUJD5NEXC8&feature=youtu.be
TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https://www.youtube.com/watch?v=Id2rZS59xSE&feature=youtu.be
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.
The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.
A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.