Module 1

Lesson 3

by Dr. Irena O’Brien, PhD

Brain Anatomy

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Welcome to part 3 of module 1 where we look at how the nervous system is organized.

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The nervous system consists of the brain, spinal cord, and a complex network of nerves. This system is responsible for sending, receiving, and interpreting information from all parts of the body. The nervous system monitors and coordinates internal organ function and responds to changes in the external environment. This system can be divided into two parts: the central nervous system and the peripheral nervous system.

Image by Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=10187018

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The central nervous system (CNS) is the processing centre for the nervous system. It receives information from and sends information to the peripheral nervous system. The two main organs of the CNS are the brain and spinal cord. The brain processes and interprets sensory information sent from the spinal cord.

The spinal cord is a cylindrical shaped bundle of nerve fibres that is connected to the brain. The spinal cord runs down the centre of the spinal column extending from the neck to the lower back. Spinal cord nerves transmit information from body organs and external stimuli to the brain and send information from the brain to other areas of the body. The nerves of the spinal cord are grouped into bundles of nerve fibres that travel in two pathways. Ascending - or afferent - nerve tracts carry sensory information from the body to the brain. Descending - or efferent - nerve tracts send information pertaining to motor function from the brain to the rest of the body. This is a simplified illustration of the central nervous system, but it well illustrates the components of the central nervous system.

Image by CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=389748

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The peripheral nervous system consists of nerves and ganglia - ganglia are nerve cell clusters - on the outside of the brain and spinal cord. The main function of the peripheral nervous system (PNS) is to connect the central nervous system (CNS) to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and spinal cord with the rest of the body.

Image by: By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

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Here is an illustration of the how the peripheral nervous system is connected to the central nervous system.

The afferent, or ascending, nerve cells, from the skin for example , send information to the spinal cord . They send information from both external stimuli, such as touch, and from internal organs.

The efferent, or descending, nerves send information from the spinal cord to the motor cells, in this case muscles . That’s how we have movement.

Image by Pearson Scott Foresman - Archives of Pearson Scott Foresman, donated to the Wikimedia Foundation→This file has been extracted from another file: Afferent (PSF).jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2499895

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The peripheral nervous system is divided into the somatic, autonomic, and enteric nervous systems.

The somatic nervous system controls skeletal muscle as well as external sensory organs such as the skin. This system is said to be voluntary because the responses can be controlled consciously. Reflex reactions of skeletal muscle however are an exception. These are involuntary reactions to external stimuli.

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The autonomic nervous system controls bodily functions not consciously directed, such as breathing, the heartbeat, and digestive processes. This system is also called the involuntary nervous system. The autonomic nervous system can further be divided into the parasympathetic and sympathetic divisions.

The sympathetic nervous system is often considered the "fight or flight" system, while the parasympathetic nervous system is often considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it.

Image by Geo-Science-International - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=47377075

The parasympathetic division controls various functions which include inhibiting heart rate, constricting pupils, and contracting the bladder. The nerves of the sympathetic division often have an opposite effect when they are located within the same organs as parasympathetic nerves. Nerves of the sympathetic division speed up heart rate, dilate pupils, and relax the bladder.

The sympathetic and parasympathetic systems are independent to each other, which allows activation of certain parts of the body, while others remain rested.

Unlike the sympathetic system, humans have some voluntary controls in the parasympathetic system. The most prominent examples of this control are urination and defecation.

The autonomic nervous system is always activated but is either in the sympathetic or parasympathetic state. Depending on the situation, one state can overshadow the other. In sexual activity and orgasm, both the sympathetic and parasympathetic systems are at play. One of the reasons why it’s important to know about the sympathetic and parasympathetic nervous systems is because they control the stress response.

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The enteric nervous system (ENS) is another one of the main divisions of the nervous system and consists of a system of neurons that governs the function of the gastrointestinal system. It is now usually referred to as separate from the autonomic nervous system since it has its own independent reflex activity.

The enteric nervous system consists of approximately 500 million neurons, which is one two-hundredth of the number of neurons in the brain, 5 times as many as the one hundred million neurons in the spinal cord.

The enteric nervous system is embedded in the lining of the gastrointestinal system. The ENS is capable of autonomous functions such as the coordination of reflexes; although it receives considerable innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord. The enteric nervous system has been described as a "second brain" for several reasons. The enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems.

The enteric nervous system includes efferent neurons (from the brain), afferent neurons (to the brain), and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating centre without CNS input. The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine.

Image by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30147911

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If that seemed complicated, here’s a diagram of the nervous system that will bring some clarity.

The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of the nerves. It communicates with the spinal cord via afferent, or ascending sensory nerves, and efferent, or descending motor nerves.

Image by The original uploader was Fuzzform at English Wikipedia - http://commons.wikimedia.org/wiki/File:NSdiagram.png, GFDL, https://commons.wikimedia.org/w/index.php?curid=22092840

The peripheral nervous system can be further divided into the autonomic and somatic nervous systems. The somatic nervous system controls skeletal muscles and external sensory organs such as the skin.

The autonomic nervous system controls bodily functions not consciously directed, such as breathing, the heartbeat, and digestion . It can further be divided into the parasympathetic and sympathetic divisions. The sympathetic division mobilizes the fight or flight response and the parasympathetic division controls the reset or relaxation response.

The enteric nervous system, not shown in this diagram, controls digestive functions. It communicates with the central nervous system but can operate autonomously, hence it’s often called a second brain.

There you have it: a whistle-stop tour through the brain. I realize that, for many of you, this is complicated. However, it’s important to have a basic understanding of the brain and nervous system going forward so you can appreciate the the next modules. And in a course on neuroscience, I would be remiss not to provide you with a foundation in

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neuroanatomy.

You can watch this video as often as you like. You can ask questions in the Facebook group. And we will be having a Q & A call in a few weeks where you will also be able to ask questions. And I will be posting further resources if you’d like to dive deeper.

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