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4: Basics of the Nervous System - Biology

4: Basics of the Nervous System - Biology


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  • 4.1: Prelude to the Nervous System
    When you’re reading this book, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page; the motor system controls the turn of the pages (or click of the mouse); the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system.
  • 4.2: Neurons and Glial Cells
    Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 35.1.1. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves.
  • 4.3: How Neurons Communicate
    All functions performed by the nervous system—from a simple motor reflex to more advanced functions like making a memory or a decision—require neurons to communicate with one another. While humans use words and body language to communicate, neurons use electrical and chemical signals. Just like a person in a committee, one neuron usually receives and synthesizes messages from multiple other neurons before “making the decision” to send the message on to other neurons.

Basic Structure and Function of the Nervous System

The picture you have in your mind of the nervous system probably includes the brain , the nervous tissue contained within the cranium, and the spinal cord , the extension of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.


Diversity of Nervous Systems

Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 1. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves.

Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.

Figure 1. Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA credit f: modification of work by NIH)

Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.

Watch this video of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate evolution.


Central Nervous System:

The central nervous system serves as the processing system for the nerve impulses received from the peripheral nervous system. All of our sense organs send information to the spinal chord and the brain. Both of these areas are responsible for complex task management. The spinal-chord does less processing, but is the root for most of our reflexes (automatic responses to stimuli). The brain on the other hand, is divided up into many regions, which control different parts of the body.

The Brain: Thinking Headquarters

Almost all nerve processing takes place in the major regions of the brain – the cerebrum, cerebellum, and brain stem. Each of these regions controls a different part of the body. But the brain does more than just command the ever changing body. It also changes with time depending on environmental conditions. We’ve outlined the major regions of the brain below. Scroll over each section to learn about what it does.

The Spinal Chord:

The spinal-chord is the main communication link between the brain and the rest of the body. The spinal-chord is different from the vertebral column. It resides inside and is protected by the bony vertebral column. Because the spinal-chord acts to combine nerves from several regions around the body, damage to the spinal-chord can result in loss of communication from the brain to that bodily region. This is often what happens when someone becomes paralyzed.


Examples of the Sympathetic Nervous System Response

The classic case of SNS response is a physical danger, especially with a potential predator, and the preparation of the body for either fight-or-flight. The overall system is designed to enhance voluntary muscle activity while shutting down all non-essential functions.

Arterioles and veins in most vascular beds constrict, reducing blood flow to the skin and digestive organs. Sphincters in the digestive organs also constrict, to control the flow of food from one organ to the next. However, coronary blood vessels, pulmonary circulation and parts of the respiratory tree respond with dilation, to enhance cardiac output. The heart beats with greater contractile force and at a higher frequency as well. Vasoconstriction in many parts of the body also increases blood pressure.

For instance, when faced with a charging elephant or bull, the body quickly primes itself to run quickly, and for a prolonged period. All the changes to the cardiovascular system are designed to sustain survival. Interestingly, though, the body responds in a similar fashion even when the threat is merely being observed rather than experienced, such as watching a scary movie.


Introduction

When you’re reading this book, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page the motor system controls the turn of the pages (or click of the mouse) the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors—from eating to sleeping to finding a mate.


Axonal Guidance

Neuronal survival, synapse formation and refinement
Junctions between nerve cells and muscle cells in vertebrates are neuromuscular junctions.
When motor neuron axons reach their targets, interactions between the neuron and the muscle fibre form the neuromuscular junction.

Interactions between nerve and muscle form the neuromuscular junction.
The axon branches end in large contacts with the endplate of the muscle fibre.
The neuromuscular junction (or synapse) is where the muscle and axon's plasma membranes are separated by the narrow synaptic cleft containing basal lamina (ECM) secreted by both cells.
The electrical impulse propagated down the axon is converted into a chemical signal which diffuse across the cleft to interact with receptors in the muscle cell to cause contraction.
Aggregation of acetylcholine receptors is aided by agrin activation of the Musk receptor and by neuregulin induction of localized acetylcholine receptor synthesis.

Neurotrophic factors promote neuronal survival.
Neurons that do not connect with their target undergo apoptosis.
20,000 motor neurons are formed in the spinal cord of the chick but

half die
Survival may depend upon establishing a functional synapse with a muscle cell.
Even after neuromuscular connections are made, some are eliminated until each muscle fibre is innervated by only one motor neuron.

NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor),
NT-3 (neurotrophin-3) and NT-4/5 (neurotrophin-4/5) are neurotrophic factors that neuronal survival depends upon.
Trk proteins, receptor tyrosine kinases, are the neurotrophin receptors and act in the specificity of neuron type survival.


Basics of autonomic nervous system function

The autonomic nervous system has widespread innervation to nearly every organ system in the body. In order to understand the basics of autonomic function, knowledge of the neuroanatomy of the autonomic nervous system is necessary. Frequently considered to control the "fight or flight" and "rest and digest" functions, the autonomic nervous system has an intricate network of connections to finely tune the systemic response to nearly any situation. Although traditionally considered two discrete systems (sympathetic and parasympathetic), the enteric nervous system is now considered a third component of the autonomic nervous system. This chapter reviews the background of the neuroanatomical distribution of the autonomic nervous system in order to facilitate understanding the basics of autonomic function.

Keywords: Autonomic nervous system Autonomic neuroanatomy Enteric nervous system Parasympathetic nervous system Sympathetic nervous system.


Neurodegenerative Disorders

Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are usually caused by neuronal death. These diseases generally worsen over time as more and more neurons die. The symptoms of a particular neurodegenerative disease are related to where in the nervous system the death of neurons occurs. Spinocerebellar ataxia, for example, leads to neuronal death in the cerebellum. The death of these neurons causes problems in balance and walking. Neurodegenerative disorders include Huntington’s disease, amyotrophic lateral sclerosis, Alzheimer’s disease and other types of dementia disorders, and Parkinson’s disease. Here, Alzheimer’s and Parkinson’s disease will be discussed in more depth.

Alzheimer’s Disease

Alzheimer’s disease is the most common cause of dementia in the elderly. In 2012, an estimated 5.4 million Americans suffered from Alzheimer’s disease, and payments for their care are estimated at $200 billion. Roughly one in every eight people age 65 or older has the disease. Due to the aging of the baby-boomer generation, there are projected to be as many as 13 million Alzheimer’s patients in the United States in the year 2050.

Symptoms of Alzheimer’s disease include disruptive memory loss, confusion about time or place, difficulty planning or executing tasks, poor judgment, and personality changes. Problems smelling certain scents can also be indicative of Alzheimer’s disease and may serve as an early warning sign. Many of these symptoms are also common in people who are aging normally, so it is the severity and longevity of the symptoms that determine whether a person is suffering from Alzheimer’s.

Alzheimer’s disease was named for Alois Alzheimer, a German psychiatrist who published a report in 1911 about a woman who showed severe dementia symptoms. Along with his colleagues, he examined the woman’s brain following her death and reported the presence of abnormal clumps, which are now called amyloid plaques, along with tangled brain fibers called neurofibrillary tangles. Amyloid plaques, neurofibrillary tangles, and an overall shrinking of brain volume are commonly seen in the brains of Alzheimer’s patients. Loss of neurons in the hippocampus is especially severe in advanced Alzheimer’s patients. Figure 1 compares a normal brain to the brain of an Alzheimer’s patient. Many research groups are examining the causes of these hallmarks of the disease.

One form of the disease is usually caused by mutations in one of three known genes. This rare form of early onset Alzheimer’s disease affects fewer than five percent of patients with the disease and causes dementia beginning between the ages of 30 and 60. The more prevalent, late-onset form of the disease likely also has a genetic component. One particular gene, apolipoprotein E (APOE) has a variant (E4) that increases a carrier’s likelihood of getting the disease. Many other genes have been identified that might be involved in the pathology.

Link to Learning

Visit this website for video links discussing genetics and Alzheimer’s disease.

Unfortunately, there is no cure for Alzheimer’s disease. Current treatments focus on managing the symptoms of the disease. Because decrease in the activity of cholinergic neurons (neurons that use the neurotransmitter acetylcholine) is common in Alzheimer’s disease, several drugs used to treat the disease work by increasing acetylcholine neurotransmission, often by inhibiting the enzyme that breaks down acetylcholine in the synaptic cleft. Other clinical interventions focus on behavioral therapies like psychotherapy, sensory therapy, and cognitive exercises. Since Alzheimer’s disease appears to hijack the normal aging process, research into prevention is prevalent. Smoking, obesity, and cardiovascular problems may be risk factors for the disease, so treatments for those may also help to prevent Alzheimer’s disease. Some studies have shown that people who remain intellectually active by playing games, reading, playing musical instruments, and being socially active in later life have a reduced risk of developing the disease.

Figure 1. Compared to a normal brain (left), the brain from a patient with Alzheimer’s disease (right) shows a dramatic neurodegeneration, particularly within the ventricles and hippocampus. (credit: modification of work by “Garrando”/Wikimedia Commons based on original images by ADEAR: “Alzheimer’s Disease Education and Referral Center, a service of the National Institute on Aging”)

Parkinson’s Disease

Like Alzheimer’s disease, Parkinson’s disease is a neurodegenerative disease. It was first characterized by James Parkinson in 1817. Each year, 50,000-60,000 people in the United States are diagnosed with the disease. Parkinson’s disease causes the loss of dopamine neurons in the substantia nigra, a midbrain structure that regulates movement. Loss of these neurons causes many symptoms including tremor (shaking of fingers or a limb), slowed movement, speech changes, balance and posture problems, and rigid muscles. The combination of these symptoms often causes a characteristic slow hunched shuffling walk, illustrated in Figure 2. Patients with Parkinson’s disease can also exhibit psychological symptoms, such as dementia or emotional problems.

Figure 2. Parkinson’s patients often have a characteristic hunched walk.

Although some patients have a form of the disease known to be caused by a single mutation, for most patients the exact causes of Parkinson’s disease remain unknown: the disease likely results from a combination of genetic and environmental factors (similar to Alzheimer’s disease). Post-mortem analysis of brains from Parkinson’s patients shows the presence of Lewy bodies—abnormal protein clumps—in dopaminergic neurons. The prevalence of these Lewy bodies often correlates with the severity of the disease.

There is no cure for Parkinson’s disease, and treatment is focused on easing symptoms. One of the most commonly prescribed drugs for Parkinson’s is L-DOPA, which is a chemical that is converted into dopamine by neurons in the brain. This conversion increases the overall level of dopamine neurotransmission and can help compensate for the loss of dopaminergic neurons in the substantia nigra. Other drugs work by inhibiting the enzyme that breaks down dopamine.


Nervous System

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The nervous system is a network of neurons that send signals to different parts of an organism's body to coordinate the actions of the organisms. Most animals have two parts to their nervous systems - the central nervous system includes the brain and spinal cord while the peripheral nervous system includes sensory neurons and nerves. The nervous system is different from the endocrine system because its messages travel more quickly and don't last as long.

One of my favorite systems to teach about is the Nervous System and this is because it's the voice talking inside your head it's what's watching me right now and the Nervous System is the one of the two major control systems of the body it works with the endocrine system to help control and regulate the activities of the body and maintain homeostasis. Now when studying the Nervous System you got to be kind of careful because it's been something that's been just inspiring scientists to dive into the research for years and years and so each scientist tries to organize it and they've organized it in many different ways and so sometimes kids get confused which ones is what? Well there's one way of organizing things based on essentially it's like Geography it's just where are they continent and that's saying the central nervous system versus the peripheral nervous system.

The central nervous system is the brain and spinal cord alright? The stuff in the middle, whereas the peripheral nervous system is all of the nerves coming off of the brain called cranial nerves and all the nerves coming out from the spinal column the spinal cord called the spinal nerves alright? Now within the peripheral nervous system they'll make the distinction between the somatic nervous system which is the part of your nervous system that controls your skeletal muscles in other words it's my somatic nervous system that I actually or activate in order to do this or that why I don't know but autonomic nervous system controls everything else now that's a really broad category because its really a lot of things. It's the autonomic nervous system that helps you regulate how open or close blood vessels are two different parts of your body your autonomic nervous system adjust the size of the pupil the hole in your eyelids that allows more or less lighting based on how much light you're looking at.

In general people think of this somatic nervous system as being the voluntary part the consciously controlled part while the autonomic is the unconsciously controlled stuff. Now again there's some blowing going on here, when you walk very rarely do you actively sit there and think "how I'm I balancing? How do I need to adjust the muscles of my trunk my abdomen to keep me from falling over" even though it's technically that would fall under the somatic nervous system and autonomic stuff well you can have some input into that if you start working under those Buddhist Monks who'll site there and they can learn how to control blood flow and other really bizarre things.

Now within the autonomic nervous system there's a further subdivision, there's the sympathetic nervous division or nervous system and the parasympathetic nervous system or nervous division. Now the sympathetic division in general you can lump that together as it controls the fight or flight type responses in other words getting you ready for action whereas the parasympathetic nervous division in general gets you ready for resting and repairing now usually most body parts are being given signals by both of these and they're usually some kind of balance your very rarely all the way sympathetic and no parasympathetic or vice versa and when you're going through your normal life, there are just sitting there adjusting based on input on what your body needs and even your conscious mind can influence this.

Now to help you think through what would the sympathetic nervous division control? What kinds of body parts would it send blood to? What kinds of actions or effects would it have on your body and versus the parasympathetic, let's start thinking about hmm close your eyes and just kind of relax a little bit and in your head imagine its 3:00 a.m. you're in your bed oh you're just sleeping you're just doing that kind of surfing between consciousness and rest and you just feel so wonderfully relaxed, the room is dark it's perfectly quiet and all of a sudden you hear 'pop' and somebody licks your ear. Now how would your body react? Your heart would start increasing as the sympathetic division says "whoa we need to run away or kill the killer clown" you would you would open up the blood vessels to the large muscles of your arms your legs getting ready to run away or engage in your Ninja battles with the killer clown. The opening of your lungs the bronchi's and bronchioles will spasm open to allow more air flow because engaging in Ninja battles with killer clown takes a lot of oxygen so you need to start to breathing faster and deeper. Now what parts of your body don't need blood at that moment well you don't need your immune system to fight off the killer clown now you may get some diseases later but right now you need to focus on fighting killer clown that's why long term stress can cause problems with your immune system.

What else don't you need? Well you had a nice dinner few hours ago but you don't need to digest it right then so you shut off blood supply to your digestive system slow it down because you don't really need it and sometimes that's maybe why you go hmm if you've got too much food in your stomach when you all of a sudden go ah ah ah I need to fight your stomach may go, I can't handle this and reverse the pumps. Now unless you've really got a thing for killer clowns what other system don't you need at that moment? The reproductive system so the blood supply to there gets shut down and you just are ready for fighting and flighting. Now, what's an opposite kind of situation, Thanksgiving, you eat Thanksgiving dinner, you have your portion of the turkey, you have about 3 pounds of turkey, you have mashed potatoes you have everything else and then you sit on the lazy boy recliner and then your body says, we do not need to have heart-rate racing, we do not need excess air in the lungs, we do not need your muscles to be pumping and using lots of energy and instead it shuts it down. Digestive system gets a lot and everything is good.

Weirdly enough the sympathetic division does activate one activity of the reproductive system and that's either orgasm or it can actually activate labor which is why that stereotype of the woman giving birth and when she gets trapped in the elevator there actually is some legitimate see to these this means that if a lady is walking around she is looking pretty pregnant don't just jump up behind her and go aah and attack her with her with the chains that's not just nice to do.

Alright, now I can go through the structures of the spinal column or the spinal cord but what most people want to know about is the brain, so let's take a look at the brain. There's 4 major regions of the brain the brain stem, cerebellum, diencephalon and cerebrum. If we take a look at this diagram here, this portion here the light blue weirdly purple and light green thing there that is the brain stem. This the bottom of the brain and even though this is not quite accurate you could think of it as the part that evolved first and it deals with those basic needs this keeps your heart going your lungs breathing these are keeping you not dead kind of activities. Now there are some other things that are involved there besides just keeping you not dead there are some visually reflexes and some other things plus all the signals that are coming up the spinal column right at the bottom of this they're passing through the brain stem.

Now this weird lump the a reddish lump that's sitting behind the brain stem it's called the cerebellum which means little brain because it actually looks a lot like the bigger cerebrum that sits on top and you can even see it has it's white matter in this thing which means white or tree of life it's got its white matter on the inside just like the cerebrum has white matter in on the inside and grey matter on the outside. What does the cerebellum do? It does a number of things but some of the most important things that it does it helps regulate and coordinate motor control. Now when you wish to move your right or left arm you don't sit there and hope that your cerebellum does it that's what the cerebrum's involved with but the cerebellum is the thing that says "I wish" no the cerebellum says "I want lift my left arm" the cerebellum says "well okay but I know Newton's third law for every reaction there's an equal on opposite reaction if I lift this arm I'm going to fall forwards" so the cerebellum says, "tighten up trunk and keep me from flapping over" when the cerebrum carries out it's action and it does that kind of smoothing things out if this is the CEO of motor control, this is the middle management that makes the CEO's directives actually becomes a reality.

Now you may not realize how much of this actually regulates as I said it's involved a lot in balance and it helps coordinate actions and a lot times if it thinks the cerebrum's made a mistake it will override it this is why some times if you're trying to do a muscle activity like walking or climbing especially if there's something complicated if you start to think about what you're doing you may get screwed up. Now here's a weird little thing that you can do, stick out, sitting down stick out your right leg and start rotating it clockwise for you that we'd be rotating it like this then with your right hand in the air draw the number 6 what would happens to your leg? That's your cerebellum saying warning CEO stupid, stop that cerebrum and it overrides.

Alright sitting on top of the brain stem this weirdly yellow thing here is the diencephalon and its such a little property it's so important this is where a lot of relay relying of information starts happening. This part decides where things should go in the cerebrum plus it starts doing a lot of the initial analysis of the data that's coming in and out of the brain and it starts making some decisions. This is also where a lot of autonomic control the body is decided upon. You see this little dangly guy there that's the pituitary gland the master gland of the endocrine system and it's the diencephalon that controls the pituitary gland so this is involved in a wide range of things especially with this little part here called the hypothalamus you can be involved in deciding whether or not like something it's involved in thermoregulation lots of functions are located there.

Up here, is the cerebrum this is the perhaps most recently developed of the parts of the brain and this is what people think of as the brain. This is where your conscious thoughts probably mostly exists. Now it's divided into primarily 4 major lobes the frontal lobe in the front, the parietal lobe here, in the back you have the occipital lobe and to the side this is a kind of a view of the brain you have the temporal lobe. I often think of it kind of like a boxing glove where the thumb is the temporal lobe the fingers are the frontal lobe, the back of the palm is the parietal lobe and then it breaks down occipital lobe is the back here, weirdly enough vision analysis happens on occipital lobe I don't know why its not the front it make lot more sense. The temporal lobe which is right by the ear hey! That makes sense it's involved in the analysis of sound its also involved in analyzing things like smell, there's a number of other functions that go on in there involving there's some language stuff that goes on in there and memory is actually helped out by the temporal lobe. The parietal lobe does a lot of analysis of touch which you think of us touch and in fact right where you go from parietal lobe to frontal lobe you have this ridge called the primary somatic motor, primary somatic sensory area where you have the individual neurons that are listening to signals from different parts of your body and they've actually done things where they run along with electrodes somebody's that little ridge there and you the person will say "I'm reporting tick lings sensation or something" and it seems to run along their body and they can map it out and so you'll have lots of that brain portion dedicated to analyzing information from your hand but not so much to analyzing information from your elbow. Similarly the frontal lobe has right by that has a ridge that controls those muscles and again if you run your electrode along it you'll see the person won't feel anything but their body will start to move and that's called the primarily somatic motor area. The rest of the frontal lobe is involved in things like executive function and conscious decision making and speech so this is where you start thinking hmm what do I really want for my birthday now other parts of the brain may want may come up with I'm hungry but you may be thinking yes birthday cake is nice but I think I want a leather jacket. These portions of it right here they're involved in very long term judgment and this is the part that right now as a teenager if you're watching this, this is the part that this is fascinating to me, it's growing and developing right now which is why everyday as you age you're getting smatter and wiser and assuming that you don't do things to impede it's development, you're going to be a very wise and smart individual especially after watching this video.


Watch the video: Από το Κύτταρο στον Οργανισμό. Μέρος Α:Διαφοροποίηση-Επιθηλιακός Ιστός (November 2022).