Monday, September 26, 2016

Introduction to Neuroanatomy - Part 3 (Brain Structures - Telencephalon and Diencephalon)


Brain Structures and their Functions


To learn more about neuroanatomy, please check out: "Neuroanatomy Text and Atlas," by John H. Martin.


In the last post, we ended explaining the development of the spinal cord. We will continue this discussion by exploring the structures of the brain from a bottom-up perspective and describe their functions. 

Cranial and Non-cranial Nerves

Image result for alar plate migrating laterallyIf you read the last post (Here is the link: http://hbookreviews.blogspot.com/), you should remember that the spinal cord was divided by two plates: basal and alar. When we move up from the spinal cord into the brain we would observe a collection of structures known as the brainstem. In here, the plates become nerves. The alar plate becomes the cranial nerve sensory nuclei and the basal becomes the cranial nerve motor nuclei. Remember that a nerve is a bundle of axons in the central nervous system (CNS) and that nuclei are a collection of cell bodies that are also located in the CNS. Thus, the phrase "cranial nerve motor nuclei" tells us what the body part is (nerve=bundle of axons (1)), where it is (the nuclei is located inside the CNS, which is either the brain or the spinal cord (2)), and what it does (motor refers to sending information across the spinal cord that will eventually create movement. As you can see from the picture to the right, the dorsal or alar plates moved laterally in the brainstem.


Image result for brainstemImage result for brainstemThe brain stem is composed of three structures. They are the medulla, the pons, and the midbrain (3). Before we go into their functions, I want to keep talking about nerves. Each of these structures has non-cranial nerve nuclei. The inferior olivary nuclei are located in the medulla and they are responsible for hearing functions. The pontine nucleus is located in the pons and it is responsible for skilled movement control. The red nucleus and the substantia nigra are located in the midbrain. They are responsible for descending motor and ascending motor information, respectively. We will cover more of the brain stem later. 

There is a total of twelve cranial nerves. Next to them I'll write their functions. They are:

  • Olfactory - This nerve helps with the sense smell (6).
  • Optic - This nerve helps with vision by transmitting information from the retina to the brain (5).
  • Oculomotor - This nerve  helps with eye movement and accommodation, as well as pupil constriction (4).
  • Trochlear - This nerve helps the eye move up and down (7).
  • Trigeminal - This nerve sends somatosensory (this means sensory information from the skin) information from the face and movement of the jaw (8).
  • Abducens - This nerve helps with side to side movement of the eye (9).
  • Facial - This nerve helps send somatosensory information from the ear and sensory information from the tongue. As well as, movements of the face (facial expressions) (10).
  • Vestibulocochlear - This is a sensory nerve that deals with audition and balance (11).
  • Glossopharyngeal - This nerve sends somatosensory information from the tongue and the pharynx. In addition, it sends sensory (taste) information from the posterior one-third of the tongue (12).
  • Vagus - This nerve has some motor and sensory functions that range from swallowing and gland control to taste and involuntary muscle movement in the viscera (13). 
  • Accessory - This nerve helps with the movement of the head (14).
  • Hypoglossal - This nerve helps with the movement of the tongue (15). 

Prosencephalon

Image result for telencephalon
Now, that we covered the twelve cranial nerves, we are going to explore the structures of the brain using an embryological perspective. Do you remember the three main divisions? (It was covered in the second post) They were the prosencephalon, mesencephalon, and rhombencephalon. The former was two subdivision, they are the telencephalon and the diencephalon. The first one can be divided into three smaller parts: the cerebral cortex, the basal ganglia, and the limbic system, which is made up of the amygdala and the hippocampus. I know.. I know... It may seem as if it is too much information to process at once, but we will break it down apart in order to make it easier to understand. Think of the telencephalon as the superior part of the brain, as the cerebral hemispheres. The picture to the right also includes the corpus callosum in the telencephalon. This is the structure that connects both hemispheres, it helps by letting them communicate with each other (It is Latin for callous body). 

Image result for cerebral cortex



To understand the function of the cerebral cortex, we have to explore first the lobes of the brain. As you can see there are four major lobes, they are the frontal, parietal, temporal, and occipital lobes. The first lobe is in charge of a variety of functions (16). The prefrontal cortex is located in the most rostral part of the frontal lobe. It is in charge of executive function, which is a set of cognitive skills that include planning, inhibition, and logical thinking. The frontal lobe also handles movement. There is a fissure that separates the frontal and parietal lobe is called the central fissure. The gyrus before the central fissure is called the precentral gyrus and it is where the majority of motor tasks are processed. Before the precentral gyrus, there is a structure known as Broca's area. This is the location where speech is produced. The parietal lobe, which is located after the central sulcus or fissure, handles somatosensory information (16). The postcentral gyrus is where the primary somatosensory cortex is located. This means that it handles sensory info from the skin such as pain, pressure, and temperature. The occipital lobe is where the primary visual cortex is located. Primary auditory cortex is located in the temporal lobe. In this lobe, there is a structure known as Wernicke's area. This is the opposite of Broca's area because it handles the understanding of speech. It is important to note that once sensory information arrives at its primary location in the cortex, it then goes to its respective association cortex where it undergoes further processing (17). In addition, the cortex has two divisions. They are neocortex and allocortex. The former is the most recent cortex in terms of evolution and it helps with higher cognitive functions (18). The latter is the older cortex and deals with more primitive functions. It has two subdivisions: archicortex and paleocortex. Both of them develop "in association with the olfactory system" and it doesn't have a layered structure (19).




Image result for basal gangliaThat was the first part of the telencephalon, the second structure would be the basal ganglia. Remember that ganglia are collections of cell bodies outside of the central nervous system (CNS), however, the basal ganglia is an exception because it is located in CNS. It is one of the structures that deal with movement. The structure that deals with memory looks like a seahorse that is why its name is hippocampus (latin for seahorse (20)). Close to the hippocampus is the amygdala (amygdala is latin for almond, it was given this name because of its shape), which is the structure that is responsible for emotions (21). In summary, the telencephalon is made up of the cerebral cortex, the basal ganglia and the limbic system, which includes the hippocampus and amygdala

Image result for diencephalon structuresThe second subdivision of the prosencephalon is the diencephalon. This subdivision is made up of several structures. One of them is the thalamus, which is a structure made up of nuclei that are responsible for sending sensory information to its respective cortex (22). Another important structure in the diencephalon would be the hypothalamus. This is responsible for the four F's: fighting, fleeing, feeding, and fornicating. As you can see this part of the brain is very primitive from an evolutionary perspective because of it functions. The epithalamus is also located in the diencephalon and it is made up of the habenula and the pineal gland (23). The former is involved in motivation and reward (24) and the latter is not where the mind and body interact like Rene Descartes used to believe, but rather it is responsible for segregating melatonin and it regulates the circadian rhythm (25).

C-Shaped Structures

Image result for brain during development
During development, the telencephalon undergoes an incredible expansion in terms of neurogenesis. On average, there are about 250, 000 neurons being developed per minute (22). As a result of this, structures start to develop a c-shape. One of the structures that has this shape is the cortex, which includes the four lobes that were covered before. Other structures include the ventricles, which are cavities that contain cerebrospinal fluid (22), the corpus callosum, which connects the two hemispheres, and the caudate nucleus.

References

1. http://www.dictionary.com/browse/nerve.

2. http://www.med.umich.edu/lrc/coursepages/m1/anatomy2010/html/modules/NS_overview_module/NS_Overview_06.html

3. https://www.britannica.com/science/brainstem

4. http://medical-dictionary.thefreedictionary.com/oculomotor+nerve

5. https://www.sciencedaily.com/terms/optic_nerve.htm

6. http://medical-dictionary.thefreedictionary.com/olfactory+nerve

7. http://www.medicinenet.com/script/main/art.asp?articlekey=7603

8. http://www.ncbi.nlm.nih.gov/books/NBK384/

9. http://www.meddean.luc.edu/lumen/meded/grossanatomy/h_n/cn/cn1/cn6.htm

10. http://www.meddean.luc.edu/lumen/meded/grossanatomy/h_n/cn/cn1/cn7.htm

11. http://www.meddean.luc.edu/lumen/meded/grossanatomy/h_n/cn/cn1/cn8.htm

12. http://www.meddean.luc.edu/lumen/meded/grossanatomy/h_n/cn/cn1/cn9.htm

13. http://emedicine.medscape.com/article/1875813-overview.

14. http://www.ncbi.nlm.nih.gov/books/NBK387/

15. http://www.ncbi.nlm.nih.gov/books/NBK388/

16. https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_11.html

17. http://www.ncbi.nlm.nih.gov/books/NBK11109/

18. http://www.dictionary.com/browse/neocortex

19. http://medical-dictionary.thefreedictionary.com/archicortex

20. https://www.britannica.com/science/hippocampus

21. http://www.dictionary.com/browse/amygdala.

22. "Neuroanatomy text and atlas," by John H. Martin.

23. http://medical-dictionary.thefreedictionary.com/epithalamus

24. https://www.hindawi.com/journals/aneu/2014/862048/

25. http://www.ncbi.nlm.nih.gov/pubmed/15589268

Monday, September 5, 2016

Introduction to Neuroanatomy - Part 2 (The Brain and the Spinal Cord)

The Brain and the Spinal Cord


To learn more about neuroanatomy, please check out: "Neuroanatomy Text and Atlas," by John H. Martin.

Glial Cells

Image result for glial cellsIn the last cover, we covered primarily neurons, but the glial cells are also very important in the nervous system. Something important to note is that the same parts in the central nervous system (CNS) and the peripheral nervous system (PNS) have different names. We will see an example of this with a specialized type of glia cell. They outnumber neurons, however, the exact number is still being debated. One textbook affirms that the ratio is ten to one(1), while another one asserts that it is three to one (2). Whether is one or the other, the fact that there is more glial cells remains true. This might suggest that their function is vital for the nervous system. There are two types of glial cells: Microglia and macroglia. The former acts as immune cells because if there is an infection or damage detected in the central nervous system, this subdivision is in charge of phagocytizing (this means destroying a cell (3), remember that phago- means consuming (4), and cyte- means cell(5)). The latter is a group of cells that provide support or aid neurons with nutrition (1). For example, neurons have something known as the myelin sheath. This helps action potentials reach the end of the neuron in a faster manner and there are some long neurons that without the myelin sheath the action potential would never reach the terminal buttons. The cells that provide the myelin sheath to neurons are macroglia. Remember how we mentioned that the same parts have different names depending whether they are in the central nervous system (CNS) or the peripheral (PNS) one? Well, the cells that myelinate axons in the PNS are called Schwann cells and in the CNS they are called oligodendrocytes. Both of these cells 


Image result for nervous system classification
In this post, we will cover the central nervous system (CNS), which includes the brain and the spinal cord. The former is composed of the cerebral hemispheres and the brainstem. The other parts of the nervous system will be covered in detail in the following posts. However, I want to cover them in a superficial manner. The other component of the nervous system is the peripheral nervous system (PNS). Like stated before the PNS is everything outside the CNS (remember peripheral means outside of (6)). The PNS has two subdivision the somatic and the autonomic nervous systems (ANS). The former is in charge of the skin and the muscles (7) (remember that soma means the body (8)). The latter is in charge of automatic (duh!) actions such as secreting hormones (9). The ANS also has two divisions, they are the sympathetic and the parasympathetic nervous systems. The former is in charge of creating involuntary actions like sweating, however, this is best seen active when stress is experienced. What happens is that epinephrine, also known as adrenaline (10), helps the "flight or fight system" by increasing heart rate, etc (11). The parasympathetic tries to do the opposite by conserving energy (12). A part that is sometimes included in the nervous system is the enteric system (as you can see it is not included in the graph to the left. I'm going to leave a link if you are interested in to explore more about this topic: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/gi_nervous.html). Another type of macroglia would be an astrocyte, which has a complex range of functions, one of them would be aiding neurons in synaptic transmission (20). They are also an integral part of the blood-brain barrier, as well as helping neurons migrate (1). The last type of macroglia is the ependymal cells. They are in charge of regulating the direction of where the cerebrospinal fluid is going (21).

The Development of the Central Nervous System

If you saw carefully the graph above, you may have noticed that there are certain subdivisions in the brain: the telencephalon, etc. These will be covered in detail in this section, where we will cover the development of the CNS. 
Image result for human embryo and its 3 layers


Image result for neural tube and neural crestThe embryo is composed of three layers: the endoderm, the mesoderm, and the ectoderm. They are the inner, middle, and external layers, respectively, in the embryo. The neural plate is a part of the ectoderm that later develops into the neural tube and the neural crest (13). The cells from the latter move to eventually create different types of cells such as neurons and glial; it is important to note that wherever they go will affect the type of cell they will become (14). The neural tube will develop into the brain and the spinal cord (15). The neural plate folds itself in order to create the neural tube. There are several points of view in order to study the brain in terms of orientation. The first one is rostral, which refers to a frontal view, caudate, which refers to a rear perspective, dorsal, which refers to looking at things from above, and ventral, which refers to looking at things from a lower perspective (16). The rostral part of the neural tube becomes the cerebral hemispheres. A developmental problem in the neural tube is anencephaly, which occurs when the neural tube does not close all the way (17). The caudal part of the tube becomes the spinal cord. A similar developmental problem in the caudal part of the neural tube is spina bifida. This is when the spinal cord does not close fully (18).


The Neural Tube and Its Vesicles

Image result for three vesicle stage
The neural tube develops three major vesicles at a certain stage. I will first provide the developmental names, but I will also include names that are easier to remember. They are the prosencephalon, also known as the forebrain, the mesencephalon, also known as the midbrain, and the rhombencephalon, also known as the hindbrain (19). The prosencephalon has two subdivisions. They are the telencephalon, which later develops into the cerebral hemispheres, and the diencephalon. The mesencephalon, which is the midbrain, has no subdivision. And the rhombencephalon, which is the hindbrain, has like the forebrain two subdivisions. They are the metencephalon, which later develops into the pons and cerebellum, and the myelencephalon, which later becomes the medulla oblongata. The part of the neural tube without the vesicles later becomes the spinal cord.


Side Note

I want to go into more detail about how the same body parts have different names depending whether they are in the CNS or PNS. For example, a collection of cells in the PNS is known as a ganglion, but it is called a nucleus in the central nervous system. However, there are exceptions like the basal ganglia, which is located in the central nervous system. Another example would be tract and nerve. They are basically the same thing, nevertheless, the name of the former is different because it is located in the CNS, while the nerve is located in the PNS. Keep in mind that this happens with other parts.

The Spinal Cord

Image result for developing spinal cord
Once the neural tube closes, the spinal cord starts to form. Then, a process called lamination starts to occur. It has this name because it obtains three layers. They are the marginal, the mantle, and the ependymal layer. The first one is filled with white matter, it looks like that because of the collection of axons present there. The second mantle has a darker appearance and it is known as gray matter. The last layer is the ependymal one. This is a layer made up of epithelial cells, which are glial cells that create the epithelial lining of the "brain and the central canal of the spinal cord"(21).

Then, when the neural crest stops developing, we are left with the peripheral nervous system. Like mentioned before this includes the Schwann cells, which are the glial cells that provide the myelin sheath to neurons (remember that the ones in the CNS are called oligodendrocytes). This also includes things outside the spinal cord like the dorsal root ganglion cell, but we will cover more of this later. 

In the mantle layer, which can be observed in the picture above, there are two zones where there is a rapid increase of cells. These two zones are called the alar and basal plate (both can also be observed in the picture above). Both plates become the dorsal and ventral horn, respectively. The horns can be seen in the picture down below. The horns are in charge of moving information. The dorsal horn moves sensory information up to the brain, and the ventral horn moves motor info down the spinal cord. The sulcus limitans is a groove that separates the basal and the alar plates.


Image result for dorsal and ventral horns




Resources

1. "Neuroanatomy Text and Atlas" by John H. Martin

2. http://www.ncbi.nlm.nih.gov/books/NBK10869/

3. http://www.dictionary.com/browse/phagocytize

4. http://www.thefreedictionary.com/phago-

5. http://medical-dictionary.thefreedictionary.com/-cyte

6. http://www.dictionary.com/browse/peripheral

7. http://medical-dictionary.thefreedictionary.com/somatic+nervous+system

8. http://www.merriam-webster.com/dictionary/soma

9. http://www.merriam-webster.com/dictionary/autonomic%20nervous%20system

10. http://www.medicinenet.com/script/main/art.asp?articlekey=3286

11. https://www.britannica.com/science/human-nervous-system/The-autonomic-nervous-system#ref942317

12. https://www.sciencedaily.com/terms/parasympathetic_nervous_system.htm

13. http://medical-dictionary.thefreedictionary.com/neural+plate

14. http://www.ncbi.nlm.nih.gov/books/NBK10065/

15. http://www.ncbi.nlm.nih.gov/books/NBK10080/

16. http://www.columbia.edu/cu/psychology/courses/1010/mangels/neuro/navigation/navigation.html

17. http://www.cdc.gov/ncbddd/birthdefects/anencephaly.html

18. http://www.cdc.gov/ncbddd/spinabifida/facts.html

19. http://www.med.umich.edu/lrc/coursepages/m1/embryology/embryo/08nervoussystem.htm

20. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2799634/

21. https://www.britannica.com/science/ependymal-cell

Saturday, September 3, 2016

Introduction to Neuroanatomy - Part 1 (The Nervous System)

The brain, the spinal cord, and everything else.


To learn more about neuroanatomy, please check out: "Neuroanatomy Text and Atlas," by John H. Martin.


Neuroanatomy, and psychology in general, can be studied with different perspectives, I will cover some of them in the following bullet points.


  • Anatomical Perspective: This refers to the structure of body parts
  • Cytoarchitectonic Perspective:This refers to the study of the tissue in terms of cellular structure (1).
  • Phylogenetic Perspective: This refers to the comparative study of living organisms in terms of their evolutionary history (2).
  • Functional Perspective: This refers to the point of view that studies a specific body part, in this case, the brain, in terms of functions.
  • Ontogenetic Perspective: This refers to the study of a part of the brain in terms of its development (3).

Using these perspectives, or other ones that are not mentioned here, we can look at the brain in terms of its different organizations. One of them would be a hierarchical approach, which refers to how the body parts are "connected" so they can interact with each. For example, when you see something, let's say a soccer ball, coming your way. The light reaches the receptors in your eyes. Then, a process known as transduction occurs. This is when a type of information, usually sensory info, gets "translated" or converted into another type of info, usually electric potentials for the brain (4). The signal then reaches the thalamus, which then sends a signal to the occipital lobe, which is the primary visual cortex (5). So as you can see this point of view follows how each part interacts with each other in a chronological sequence of events.


Image result for eyes to the occipital lobe   

Cells of the Nervous System

There are two main cells in the nervous system. They are the neuron and the glia cell (6). I'm going to use the same material I used to explain these cells in my old psychopharmacology post (if you want to check that out, please check it out by clicking here: http://hbookreviews.blogspot.com/2016/07/introduction-to-psychopharmacology-part_17.html). The nervous system has two main divisions: The central nervous system and the peripheral nervous system. The former is composed of the brain and the spinal cord, the peripheral includes everything else. Each division has subsets, but we will cover each of them later. Neurons are located in the central nervous system (CNS) and their main function is to process information (7). As you can see some of the parts of the neurons are in the picture to the right, we will cover them later.


The physiology of neurons depends on their specialized function. An example of this would be the difference between sensory neurons and the motor ones. The former receives info from places such as the skin and sends it to the CNS (8), the latter sends information to the muscles so they can move. There are other types, but I don't want to cover them fully, an example of this would be interneurons, which are located between a sensory and a motor neuron (9).


Fun Fact: It is estimated that there are around 100 billion neurons in the brain (9)

Image result for synapseLet us go back to neurons and talk about their parts. The structure of a neuron depends on its function, however, there are similitudes between all of them. These include the soma, which is the cell body of a neuron, the dendrites, which look like tree branches and receive information, an axon, which is the part of the neuron that sends info. There is another special structure called the synapse. It is composed of three parts: The synaptic cleft, which is the space between the terminal button and a dendrite of a neuron, the presynaptic cell, which is the neuron  before the synaptic cleft, and the postsynaptic cell, which is the neuron after the cleft. Think of a neuron functioning as a telephone, the dendrites is where you hear the person speak to you (although there are some exceptions where the dendrites send signals, but this is uncommon) and the terminal button is where the microphone is for you to speak is located. They communicate with each other with chemicals known as neurotransmitters, we will learn more about them later.





Types of neurons

The most common type of neuron is called multipolar (10). This has one axon and many dendrites attached to its soma. A bipolar neuron has an axon from one side and a dendrite from the other.



Bipolar Neuron

Another type of cell is the unipolar neuron. This cell has only one stalk that divides itself into an axon and the dendrites. Bipolar cells are usually found in sensory systems such as vision. Unipolar neurons usually work with somatosensory functions such as feeling pain and temperature. Both of them receive information from the physical world and send it into the CNS.

Like it was stated before, neurons communicate with each other using neurotransmitters. These are released by the terminal buttons when an action potential happens (We will cover this later). A neurotransmitter is a chemical that has an effect on another neuron. Remember how the synaptic cleft is the space between a terminal button and a dendrite? Well, when the neurotransmitter is released it travels in this space in search of a receptor, which can usually be found on a dendrite of another cell either by inhibiting or exciting it. This is how neurons communicate.

Specialized Cells

Neurons are considered specialized cells for three reasons. The first one is that they are polarized. This refers to the fact that neurons are negatively charged on the inside. This allows them to communicate via action potentials (We will talk specifically about this later). The second reason is that they are excitable. This means that neurons are able to change their charge. (Our brain works on electrical activity generated by chemicals). The third reason is that they are modulatory. Each neuron is able to communicate with many other neurons. In other words, every thought we have, every movement we make, every behavior, can be observed in the brain in the form of electric activity across neurons. 

   
Resources 

1. http://medical-dictionary.thefreedictionary.com/cytoarchitectonic

2. http://www.merriam-webster.com/dictionary/phylogeny

3. http://www.merriam-webster.com/dictionary/ontogeny

4. http://psychologydictionary.org/transduction/

5. http://webspace.ship.edu/cgboer/lobes.html

6. http://www.columbia.edu/cu/psychology/courses/1010/mangels/neuro/neurocells/neurocells.html

7. http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php

8. http://www.britannica.com/science/sensory-neuron

9. http://neuroscience.uth.tmc.edu/s2/chapter02.html

10. Physiology of Behavior by Neil Carson