Chapter 27. Nervous System

This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/[e-mail] is licensed under a Creative Commons Attribution License 4.0 license.

Episode Transcript

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Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is chapter twenty seven,
Nervous System. All hyperlinks, imagesand sources can be found at the

(00:24):
link to the book. In thedescription, we can trace the development of
a nervous system and correlate with itthe parallel phenomena of sensation and thought.
We see with undoubting certainty that theygo hand in hand, but we try
to soar in a vacuum the momentwe seek to comprehend the connection between them.

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Man the object is separated by animpassable gulf from Man the subject.
John Tyndall, British physicist, inFragments of Science for Unscientific People, a
series of detached essays, lectures andreviews, eighteen ninety two. The distinction
between the brain and the mind,as described by Tindal, is but one
of many questions that have fascinated scientistsregarding the human nervous system. Several Nobel

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Prizes have been awarded to scientists whohave helped elucidate the workings of nerves and
nervous systems, usually with the aidof studies in non human organisms. While
you are reading this, your nervoussystem is performing several functions simultaneously. The
visual system is processing what is seenon the page. The motor system controls
the turn of the pages or clickof the mouse. The prefrontal cortex maintains

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attention. Even fundamental functions like breathingand regulation of body temperature are controlled by
the nervous system. A nervous systemis an organism's control center. It processes
sensory information from outside and inside thebody and controls all behaviors, from eating
to sleeping, to studying to findinga mate. Diversity of nervous systems Nervous

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systems throughout the animal kingdom vary instructure and complexity, as illustrated by the
variety of animals shown in figure.Some organisms, like cea sponges, lack
a true nervous system. Others,like jellyfish, lack a true brain and
instead have a system of separate butconnected nerve cells neurons called the nerve net

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it. Chinoderms such as sea starshave nerve cells that are bundled into fibers
called nerves. Flatworms of the phylumplat Helmets have both a central nervous system
CNS, made up of a smallbrain and two nerve cords, and a
peripheral nervous system PNS, containing asystem of nerves that extend throughout the body.
The insect nervous system is more complex, but also fairly decentralized. It

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contains a brain, ventral nerve cord, and ganglia clusters of connected neurons.
These ganglia can control movements and behaviorswithout input from the brain. Octopi may
have the most complicated of invertebrate nervoussystems. They have neurons that are organized
in specialized lobes and eyes that arestructurally similar to vertebrate species. Illustration A

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shows the nerve net of a hydra, which resembles a fish net surrounding the
body. Illustration B shows the nervoussystem of a c star. A nerve
ring is present in the center ofthe body. Radiating out from this ring
into the five arms are radial nerves. Illustration C shows the nervous system of
a planarian or flat worm. Theflat worm has centralized ganglia or brains around

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each eye in the anterior end,and two nerve cords that run along the
sides of the body. Transverse nervesconnect the nerve cords together. Illustration D
shows the nervous system of a bThe central ganglia or brain is located in
the head. The ventral nerve cordruns along the lower part part of the

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body. Bumps of nerve cell bodiescalled peripheral ganglia occur periodically along the nerve
cord. Illustration E shows the nervoussystem of the octopus, which consists of
a large brain located between the twoeyes, and nerves that run into the
body and arms. Two large gangliaexist in the nerves located in the body.

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Illustration F shows the nervous system ofa human, which consists of a
central nervous system composed of the brainand spinal cord, and a peripheral nervous
system composed of the nerves running intothe rest of the body. Nervous systems
vary in structure and complexity. Ina Nigerians, nerve cells form a decentralized

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nerve net n b echinoderms, nervecells are bundled into fibers called nerves.
In animals exhibiting bilateral symmetry, suchas c planarians, neurons cluster into an
anterior brain that processes information. Inaddition to a brain, d arthropods have
clusters of nerve cell bodies called peripheralganglia located along the ventral nerve cord.

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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

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Vecchione, Clyde F. E.Roper and Michael J. Sweeney Noah 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 sharea basic structure, a CNS that contains

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a brain and spinal cord, anda PNS made up of peripheral sensory and
motor nerves. One interesting difference betweenthe nervous systems of invertebrates and vertebrates is
that the nerve chords of many invertebratesare located ventrally, whereas the vertebrate spinal
cords are located dorsally. There isdebate among evolutionary biologists as to whether these

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different nervous system plans evolved separately,or whether the invertebrate body plant arrangement somehow
flipped during the evolution of vertebrates neuronsand glial cells. The nervous system is
made up of neurons, specialized cellsthat can receive and transmit chemical or electrical
signals, and glia cells that providesupport functions for the neurons by playing an

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information processing role that is complementary toneurons. A neuron can be compared to
an electrical wire it transmits a signalfrom one place to another. Glia can
be compared to the workers that theelectric company who make sure wires go to
the right places, maintain the wires, and take down wires that are broken.

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This analogy might be oversimplified, However, recent evidence suggect that glial cells
also usurp some of the signaling functionsof neurons. The nervous system of the
common laboratory fly Drosophila melanogaster contains aroundone hundred thousand neurons, the same number
as a lobster. This number comparesto seventy five million in the mouse and

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three hundred million in the octopus.A human brain contains around eighty six billion
neurons. Despite these very different numbers, the nervous systems of these animals control
many of the same behaviors, frombasic reflexes to more complicated behaviors like finding
food and courting mates. The abilityof neurons to communicate with each other,

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as well as with other types ofcells, underlies all of these behaviors.
There is great diversity in the typesof neurons and glia that are present in
different parts of the nervous system.There are three major functional types of neurons
and many different morphological types, andthey share several important cellular components, but
neurons are also highly specialized. Differenttypes of neurons have different sizes and shapes

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that relate to their functional roles.There are also several types of glial cells.
Australia oligodendrocytes, Schwann cells, etcetera, each with different functions.
Neurons parts of a neuron. Likeother cells, Each neuron has a cell
body or soma that contains a nucleus, smooth and rough endoplasmic reticulum, gaugy

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apparatus, mitochondria, and other cellularcomponents. Neurons also contain unique structures illustrated
in figure for receiving and sending theelectrical signals that make neural communication possible.
Dendrites are tree like structures that extendaway from the cell body to receive messages
from other neurons at specialized junctions calledsynapses. Although some neurons do not have

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any dendrites, some types of neuronshave multiple dendrites. Once a signal is
received by the dendrite, it thentravels to the cell body. The cell
body contains a specialized structure, theaxon hillock, that integrates signals from multiple
synapses and serves as a junction betweenthe cell body and an axon. An

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axon is a tube like structure thatpropagates the integrated signal to specialized endings called
axon terminals. These terminals in turnsynapse on other neurons, muscle or target
organs. Chemicals known as neurotransmitters releasedat axon terminals allows signals to be communicated
to these other cells. Neurons usuallyhave one or two axons, but some

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neurons, like amicron cells in theretina, do not contain any axons.
Some axons are covered with myelin,a product of the glial cells, which
acts as an insulator and greatly increasesthe speed of conduction along the axon.
There are periodic gaps in the myelinsheath. These gaps are called nodes of

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ronviller and are sites where the signalis recharged as it travels along the axon.
It is important to note that asingle neuron does not act alone.
Neural communication depends on the connections thatneurons make with one another as well as
with other cells like muscle cells.Dendrites from a single neuron may receive synaptic

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contact from many other neurons. Forexample, dendrites from a perkingy cell in
the cerebellum are thought to receive contactfrom as many as two hundred thousand other
neurons. Illustration shows a neuron themain part of the cell body called the
soma contains the nucleus. Branch likedendrites project from three sides of the soma

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along thin axon projects from the fourthside. The axon branches at the end.
The tip of the axon is inclose proximity to dendrites of an adjacent
nerve cell. The narrow space betweenthe axon and dendrites is called the synapse.
Cells called oligodendrocytes are located next tothe axon. Projections from the oligodendrocytes

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wrap around the axon, forming amileon sheath. The mileon sheath is not
continuous, and gaps where the axonis exposed are called nodes of ronvia.
Neurons contain organelles common to many othercells, such as a nucleus and mitochondria.
They also have more specialized structures,including dendrites and axons types of neurons.

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There are different types of neurons,and the functional role of a given
neuron is intimately dependent on its structure. Although there are only three functional types
of neurons figure, an amazing diversityof neuron shapes and sizes can found in
different parts of the nervous system anda cross species neuron types diagram of sensory,

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interdash and motor neurons. The threegeneral classes of neurons all have an
zone receptor endings dendrites and or thecell body, an axon a cell body,
and an output zone axon terminals.Sensory neurons have receptor endings at one
end that are sensitive to various stimulie g. Heat, pressure, light,

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et cetera, or relatively long axonand axon terminals that forms in napses
with dendrites at the other end.B interneurons receive signals from sensory neurons via
their dendrites at one end, havea relatively short axon and pass signals to
another neuron axon terminals at the otherend. C Motor neurons receive signals via

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dendrites at one end, have along axon, and transmit signals to muscles
or glands at the other end.Image by Eva Horn. While there are
many defined neuron cell shapes, neuronsare broadly divided into three basic types,
sensory, interneuron and motor neuron.In general, sensory neurons detect information either

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from the external environment or from internalsources. Examples of sensory neurons include the
pain receptors in your skin and thephotoreceptors in your retina. When activated by
the signal to which they are attuned, they send information via an action potential
to an interneuron. Interneurons both receivesignals from other neurons and transmit signals to

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other neurons. The majority of thecells in your brain and spinal cord are
interneurons, communicating only with other neurons. Interneurons can also send a signal to
motor neurons, which control muscles andendocrine glands glya. While glya are often
thought of as the supporting cast ofthe nervous system, the number of glial

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cells in the human brain actually outnumbersthe number of neurons by a factor of
ten. Neurons would be unable tofunction without the vital roles that are fulfilled
by these glial cells. Glyagide,developing neurons to their destinations, buffer ions
and chemicals that would otherwise harm neurons, contribute to the formation of cerebrospinal fluid,

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and provide myelin sheets around axons.Scientists have recently discovered that They also
play a role in responding to nerveactivity and modulating communication between nerve cells.
When glia do not function properly,the result can be disastrous. Most brain
tumors are caused by mutations in glia. How neurons communicate. All functions performed

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by the nervous system, from asimple 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 languageto communicate, neurons use electrical and chemical
signals. Just like a person ina committee. One neuron usually receives and

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synthesizes messages from multiple other neurons beforemaking the decision to send the message on
to other neurons. Nerve impulse transmissionwithin a neuron. For the nervous system
to function, neurons must be ableto send and receive signals. These signals
are possible because each neuron has acharged cellular membrane of voltage difference between the

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inside and the outside, and thecharge of this membrane can change in response
to neurotransmitter molecules released from other neuronsand environmental stimuli. To understand how neurons
communicate, one must first understand thebasis of the baseline or resting membrane charge
neuronyl charged membranes. The lipid bilayermembrane that surrounds a neuron is impermeable to

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charged molecules or ions. To enteror exit the neuron, ions must pass
through special proteins called ion channels thatspan the membrane. Ion channels have different
configurations open, closed, and inactive, as illustrated in figure. Some ion
channels need to be activated in orderto open and allow ions to pass into

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or out of the cell. Theseion channels are sensitive to the environment and
can change their shape accordingly. Ionchannels that change their structure in response to
voltage changes are called voltage gated ionchannels. Voltage gated ion channels regulate the
relative concentrations of different ions inside andoutside the cell. The difference in total

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charge between the inside and outside ofthe cell is called the membrane potential.
The first image shows a voltage gatedsodium channel that is closed at the resting
potential. In response to a nerveimpulse, the channel opens, allowing sodium
to enter the cell. After theimpulse, the channel enters an inactive state,

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The channel closes by a different mechanismand for a brief period does not
reopen in response to a near nerveimpulse voltage. Gated ion channels open in
response to changes in membrane voltage.After activation, they become inactivated for a
brief period and will no longer openin response to a signal. Resting membrane

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potential and neuron at rest is negativelycharged. The inside of a cell is
approximately seventy millibles more negative than theoutside minus seventy m v. Note that
this number varies by neuron type andby species. This voltage is called the
resting membrane potential. It is generatedby differences in the concentrations of ions inside
and outside the cell. If themembrane were equally permeable to all ions,

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each type of ion would flow acrossthe membrane and the system would reach equilibrium.
Because ions cannot freely cross the membrane, and because enzymes can pump ions
into or out of a cell,there are different concentrations of several ions inside
and outside the cell, as shownin table. The difference in the number
of positively charged potassium ions K plusinside and outside the cell dominates the resting

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membrane potential. Figure. When themembrane is at rest, k plus ions
accumulate inside the cell. The negativeresting membrane potential is created and maintained by
increasing the concentration of catitions outside thecell in the extracellular fluid relative to inside
the cell. In the cytoplasm,with more positive ions outside than inside,

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the inside of the cell is negativelycharged dash seventy m v compared to the
extracellular space. The negative charge withinthe cell is created by the plasma membrane
being more permeable to potassium ion movementthan sodium ion movement. In neurons,
potassium ions are maintained at high concentrationswithin the cell, while sodium ions are

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maintained at high concentrations outside of thecell. The cell possesses potassium and sodium
leakage channels that allow the two catitionsto diffuse down their concentration gradient. However,
the neurons have far more potassium leakagechannels than sodium leakage channels. There
or, potassium diffuses out of thecell at a much faster rate than sodium

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leaks in. Because more catitions areleaving the cell than are entering. This
causes the interior of the cell tobe negatively charged relative to the outside of
the cell. The actions of thesodium potassium pump helped to maintain the resting
potential once established. Recall that sodiumpotassium pumps brings two k plus ions into

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the cell while removing three nooplus ionsper atp consumed. As more catitions are
expelled from the cell than taken in, the inside of the cell remains negatively
charged relative to the extracellular fluid.It should be noted that chloride ions cl
tend to accumulate outside of the cellbecause they are repelled by negatively charged proteins
within the cytoplasm. The resting membranepotential is a result of different concentrations inside

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and outside the cell ion concentration insideand outside neurons. The resting membrane potential
of minus seventy volts is maintained bya sodium slash potassium transporter that transports sodium
ions out of the cell and potassiumions in voltage gated sodium and potassium channels
are closed in response to a nerveimpulse. Some sodium channels open, allowing

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sodium iions to enter the cell.The membrane starts to depolarize. In other
words, the charge across the membranelessons. If the membrane potential increases to
the threshold of excitation, all thesodium channels open. At the peak action
potential, potassium channels open and potassiumiions leave the cell. The membrane eventually

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becomes hyperpolarized. The a resting membranepotential is a result of different concentrations of
nauplus and K plus ions inside andoutside the cell. A nerve impulse causes
naoplus to rapidly enter the cell,resulting in b depolarization. At the peak
action potential, K plus channels openand the cell becomes c hyperpolarized action potential.

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A neuron can receive input from otherneurons and if this input is strong
enough, send the signal to downstreamneurons. Transmission of a signal between neurons
is generally dependent on chemicals we callneurotransmitters, which move between nerve cells and
their targets. Transmission of a signalwithin a neuron from dendrite to axon terminal

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is initiated by a brief reversal ofthe resting membrane potential called an action potential.
When neurro transmitter molecules bind to receptorslocated on a neurons dendrites, ion
channels open at excitatory synapses. Thisopening allows positive ions to enter the neuron
and results in depolarization of the membrane, a decrease in the difference in voltage

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between the inside and outside of theneuron. A stimulus from a sensory cell
or another neuron depolarizes the target neuronto its threshold potential minus fifty five mv.
Non plus channels in the axon hillicopen, allowing positive ions to enter
the cell figure and figure. Oncethe sodium channels open, the neuron completely

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depolarizes to a membrane potential of aboutplus forty mv. Action potentials are considered
an all or nothing event, inthat once the threshold potential is reached,
the neuron always completely depolarizes. Oncedepolarization is complete, the cell must now
reset its membrane voltage back to theresting potential. To accomplish this, the

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nonplus channels close and cannot be opened. This begins the neuron's refractory period,
in which it cannot produce another actionpotential because its sodium channels will not open.
At the same time, voltage gatedK plus channels open, allowing K
plus to leave the cell. AsK plus ions leave the cell, the

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membrane potential once again becomes negative.The diffusion of K plus out of the
cell actually hyperpolarizes the cell and thatthe membrane potential becomes more negative than the
cell's normal resting potential. At thispoint, the sodium channels will return to
their resting state, meaning they areready to open again if the membrane potential
again exceeds the threshold potential. Graphplots membrane potential in millivolts versus time.

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The membrane remains at the resting potentialof minus seventy millivolts until a nerve impulse
occurs. In step one. Somesodium channels open, and the potential begins
to rapidly climb past the threshold ofexcitation of minus fifty five millivolts, at
which point all the sodium channels open. At the peak action potential, the

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potential begins to rapidly drop as potassiumchannels open and sodium channels close. As
a result, the membrane repolarizes pastthe resting membrane potential and becomes hi were
polarized. The membrane potential then graduallyreturns to normal. The formation of an
action potential can be divided into fivesteps. One, a stimulus from a

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sensory cell or another neuron causes thetarget cell to depolarize toward the threshold potential.
Two, If the threshold of excitationis reached, allnoplus channels open and
the membrane depolarizes. Three at thepeak action potential, K plus channels open
and K plus begins to leave thecell. At the same time, nooplus

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channels close. Four, the membranebecomes hyperpolarized. As K plus ions continue
to leave the cell. The hyperpolarizedmembrane is in a refractory period and cannot
fire. Five the K plus channelsclose, and thenoplus slash K plus transporter
restores the resting potential. The actionpotential travels from the soma down the axon

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to the axon terminal. The axonpotential is initiated when a signal from the
psoma causes the soma end of theaxon membrane to depolarize. The depolarization spreads
down the axon. Meanwhile, themembrane at the start of the axon repolarizes.
Because potassium channels are open, themembrane cannot depolarize again, the action

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potential continues to spread down the axon. This way, the action potential is
conducted down the axon as the axonmembrane depolarizes then repolarizes. Synaptic transmission the
synapse word gap is the place whereinformation is transmitted from one neuron to another.
Synapses usually form between axon terminals anddendritic spines, but this is not

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universally true. There are also axonto axon, dendrite to dendrite, and
axon to sell body synapses. Theneuron transmitting the signal is called the presynaptic
neuron, and the neuron receiving thesignal is called the post synaptic neuron.
Note that these designations are relative toa particular synapse. Most neurons are both

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presynaptic and postsynaptic. There are twotypes of synapses, chemical and electrical chemical
synapse. When an action potential reachesthe axon terminal, it depolarizes the membrane
and opens voltage gatednaplus channels. Nooplusions enter the cell, further depolarizing the

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presynaptic membrane. This depolarization causes voltagegated CO two plus channels to open.
Calcium ions entering the cell initiate asignaling cascade that causes small membrane bound vesicles
called synaptic vesicles containing neurotransmitter molecules tofuse with the presynaptic membrane. Synaptic vesicles

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are shown in figure, which isan image from a scanning electron microscope.
The axon terminal is spherical. Asection is sliced off, revealing small blue
and orange vesicles just inside. Thispseudocolored image taken with a scanning electron microscope
shows an axon terminal that was brokenopen to reveal synaptic vesicles blue and orange

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inside the neuron credit modification of workby Tina Carvallo and nih nigms scale bar
Detta from Matt Russell. Fusion ofa vesicle with the presynaptic neural plasma membrane
causes neurotransmitter to be released into thesynaptic cleft, the extracellular space between the
presynaptic and post synaptic cells. Asillustrated in figure. The neurotransmitter diffuses across

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the synaptic cleft and binds to receptorproteins on the post synaptic cell's plasma membrane.
Illustration shows a narrow axon of apresynaptic cell widening into a bulb like
axon terminal. A narrow synaptic cleftseparates the axon terminal of the presynaptic cell
from the post synaptic cell. Instep one, an action potential arrives at

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the axon terminal. In step two, the action potential causes voltage gated calcium
channels in the axon terminal open,allowing calcium to enter. In step three,
calcium influx causes neurotransmitter containing synaptic vesiclesto fuse with the plasma membrane.
Contents of the vesicles are released intothe synaptic cleft by exocytosis. In step

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four, neurotransmitter diffuses across the synapticcleft and binds ligan gated ion channels on
the post synaptic membrane, causing thechannels to open. In step five,
the open channels cause ion movement intwo or out of the cell, resulting
in a localized change in membrane potentialin step six reuptake by the presynaptic neuron.

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Enzymatic degradation and diffusion reduce neuro transmitterlevels, terminating the signal communication at
chemical synapses re wire's release of neurotransmitters. When the presynaptic membrane is depolarized,
voltage gated CA two plus channels openand allow CA two plus to enter the
cell. The calcium entry causes synapticvesicles to fuse with the membrane and release

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neurotransmitter molecules into the synaptic cleft.The neurotransmitter diffuses across the synaptic cleft and
binds to ligand gated ion channels inthe post synaptic membrane, resulting in a
localized depolarization or hyperpolarization of the postsynaptic neuron. The binding of a specific
neurotransmitter causes particular ion channels on thepost synaptic membrane to open. Unlike the

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sodium channels, which respond to achange in the membrane voltage voltage gated channels,
these ion channels are classified as ligandgatedsince they open the gates in response
to binding of the ligand neurotransmitter.Neurotransmitters can either have excitatory or inhibitory effects
on the post synaptictic membrane, asdetailed in table. For example, when

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acetyl coline is released at the synapsebetween a nerve and muscle called the neuromuscular
junction by a presynaptic neuron, itcauses post synaptic NOA plus channels to open.
Now plus enters the post synaptic celland causes the postsynaptic membrane to depolarize.
Once neurotransmission has occurred, the neurotransmittermust be removed from the synaptic cleft

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so the postsynaptic membrane can reset andbe ready to receive another signal. This
can be accomplished in three ways.The neurotransmitter can diffuse away from the synaptic
cleft, it can be degraded byenzymes in the synaptic cleft, or it
can be recycled sometimes called reuptake,by the presynaptic neuron. Several drugs act

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at this step of neurotransmission. Forexample, cocaine acts to inhibit reuptake of
neurotransmitters, which acts to prolong theexcitatory stimulus initiated by those neurotransmitters. Many
of the well known antidepressant drugs workin the same way neurotransmitter function and location

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electrical synapses. While electrical synapses arefewer in numbered than chemical synapses, they
are found in all nervous systems andplay important and unique roles. The mode
of neurotransmission in electrical synapses is quitedifferent from that in chemical synapses. In
an electrical synapse, the presynaptic andpost synaptic membranes are very close together and

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are actually physically connected by channel proteinsforming gap junctions. Gap junctions allow current
to pass directly from one cell tothe next. In addition to the ions
that carry this current, other moleculesEGATP or signaling ions like caaplus plus can
diffuse through the large gap junction pores. There are key differences between chemical and

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electrical synapses because chemical synapse depend onthe release of neurotransmitter molecules from synaptic vesicles
to pass on their signal. Thereis an approximately one millisecond delay between when
the axon potential reaches the presynaptic terminaland when the neurotransmitter leads to opening of
post synaptic ion channels. Additionally,this signaling is unidirectional. Signaling in electrical

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synapses, in contrast, is virtuallyinstantaneous, which is important for synapses involved
in key reflexes, and some electricalsynapses are bidirectional. Electrical synapses are also
more reliable as they are less likelyto be blocked, and they are important
for synchronizing the electrical activity of agroup of neurons. For example, electrical

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synapses and the thalamus are thought toregulate slow wave sleep, and disruption of
these synapses can cause seizures. Centralnervous system The central nervous system CNS is
made up of the brain, apart of which is shown in figure in
spinal cord and is covered with threelayers of protective coverings called meningies, from
the Greek word for membrane. Theoutermost layer is the diuramater Latin four hard

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mother. As the Latin suggests,the primary function for this thick layer is
to protect the brain and spinal cord. The diruramater also contains vein like structures
that carry blood from the brain backto the heart. The middle layer is
the weblike arachnoid mater. The lastlayer is the piamater Latin four soft mother,

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which directly contacts and covers the brainand spinal cord like plastic wrap.
The space between the arachnoid and pamatersis filled with cerebro spinal fluid CSF.
CSF is produced by a tissue calledcory plexus in fluid filled compartments and the
cns called ventricles. The brain floatsin CSF, which acts as a cushion

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and shock absorber and makes the brainneutrally buoyant. CSF also of functions to
circulate chemical substances throughout the brain andinto the spinal cord. The entire brain
contains only about eight point five tablespoonsof CSF, but CSF is constantly produced
in the ventricles. This creates aproblem. When a ventricle is blocked,

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the CSF builds up and creates swellingand the brain is pushed against the skull.
This swelling condition is called hydrocephalous waterhead and can cause seizures, cognitive
problems, and even death if ashunt is not inserted to remove the fluid
and pressure. Illustration shows the threemeningies that protect the brain. The outermost

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layer, just beneath the skull isthe durham mater The durham mater is the
thickest meninga and blood vessels run throughit. Beneath the durham mater is the
arachnoid mater, and beneath this isthe pam mater. The cerebral cortex is
covered by three layers of miningjies thedura, arachnoid, and piamaters credit modification

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of work by Gray's anatomy. Brain. The brain is the part of the
central nervous system that is contained inthe cranial cavity of the skull. It
includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus,
and cerebellum. There are three differentways that a brain can be sectioned in

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order to view internal structures. Asatival section cuts the brain left to right,
as shown in Figure B. Acoronal section cuts the brain front to
back as shown in Figure A,and a horizontal section cuts the brain top
to bottom. Cerebral cortex, theoutermost part of the brain, is a
thick piece of nervous system tissue calledthe cerebral cortex, which is folded into

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hills called gyri, singular gyris,and valleys called sulki singular sulcus. The
cortex is made up of two hemispheres, right and left, which are separated
by a large sulcus. A thickfiber bundle called the corpus colosum Latin tough
body, connects the two hemispheres andallows information to be passed from one side

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to the other. Although there aresome brain functions that are localized more to
one hemisphere than the other, thefunctions of the two hemispheres are largely redundant.
In fact, sometimes very rarely,an entire hemisphere is removed to treat
severe epilepsy. While patients do suffersome deficits following the surgery, they can

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have surprisingly few problems, especially whenthe surgery is performed on children who have
very immature nervous systems. Illustration showscoronal front and sagial side sections of a
human brain. In the coronal section, the large upper part of the brain,
called the cerebral cortex, is dividedinto left and right hemispheres. A

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cavity resembling butterfly wings exists between theleft and right cortex. The corpus colossum
is a band that connects the twohemispheres together. Just above this cavity.
The surface of the cerebral cortex containsbumpy protrusions called gyri. The cerebral cortex
is anchored by the brainstem, whichconnects with the spinal cord on either side

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of the brain stem. Tucked beneaththe cerebral cortex is the cerebellum. The
surface of the cerebellum is bumpy,but not as bumpy as the cerebral cortex.
The sagival section reveals that the cerebralcortex makes up the front and top
part of the brain, while thebrainstem and cerebellum make up the lower back
part. The oval thalamus sits inthe cavity in the middle of the cerebral

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cortex. The corpus colossum wraps aroundthe top part thalamus. The basal ganglia
wraps around the corpus colossum, startingat the lower front part of the brain
and continuing three quarters of the wayaround, so the back end almost meets
the front end. The basal gangliais separated into segments that are connected along

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the top and bottom. The amygdalais a spherical structure at the end of
the basal ganglia. These illustrations showthe a coronal end b sagial sections of
the human brain. Each cortical hemispherecontains regions called lobes that are involved in
different functions. Scientists use various techniquesto determine what brain areas are involved in

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different functions. They examine patients whohave had injuries or diseases that affect specific
areas and see how those areas arerelated to functional deficits. They also conduct
animal studies where they stimulate brain areasand see if there are any behavioral changes.
They use a technique called transmagnetic stimulationTMS to temporarily deactivate specific parts of

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the cortex using strong magnets placed outsidethe hat, and they use functional magnetic
resonance imaging fMRI to look at changesin oxygenated blood flow in particular brain regions
that correlate with specific behavioral tasks.These techniques and others have given great insight
into the functions of different brain regions, but have also showed that any given

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brain area can be involved in morethan one behavior or process, and any
given behavior or process generally involves neuronsin multiple brain areas. That being said,
each hemisphere of the mammalian cerebral cortexcan be broken down into four functionally
and spatially defined lobes frontal, parietal, temporal, and occipital. Figure illustrates

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these four lobes of the human cerebralcortex. Sagittal or side view of the
human brain shows the different lobes ofthe cerebral cortex. The frontal lobe is
at the front center of the brain, The parietal lobe is at the top
back part of the brain, Theoccipital lobe is at the back of the

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brain, and the temporal lobe isat the bottom center of the brain.
The motor cortex is the back ofthe frontal lobe, and the olfactory bulb
is the bombom part. The somatosensorycortex is the front part of the parietal
lobe. The brainstem is beneath thetemporal lobe, and the cerebellum is beneath
the occipital lobe. The human cerebralcortex includes the frontal, parietal, temporal,

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and occipital lobes. The frontal lobeis located at the front of the
brain over the eyes. This lobecontains the olfactory bulb, which processes odors.
The frontal lobe also contains the motorcortex, which is important for planning
and implementing movement. Areas within themotor cortex map to different muscle groups,

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and there is some organization to thismap, as shown in figure. For
example, the neurons that control movementof the fingers are next to the neurons
that control movement of the hand.Neurons in the frontal lobe also control cognitive
functions like maintaining attention, speech,and decision making. Studies of humans who

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have damaged their frontal lobes show thatparts of this area are involved in personality,
socialization, and assessing risk. Diagramshows the location of motor control for
various muscle groups on the right hemispherecerebral cortex, from the top middle of
the motor cortex to the bottom right. The order of areas controlled is toes,

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ankles, knees, hips, trunk, shoulders, elbows, wrists,
hands, fingers, thumbs, neck, eyebrows and eyelids, eyeballs, face,
lips, jaw, tongue, salivation, chewing, and swallowing. Different
parts of the motor cortex control differentmuscle groups. Muscle groups that are neighbors

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in the body are generally controlled byneighboring regions of the motor cortex as well.
For example, the neurons that controlfinger movement are near the neurons that
control hand movement. The parietal lobeis located at the top of the brain.
Neurons in the parietal lobe are involvedin speech and also reading. Two

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of the parietal lobe's main functions areprocessing somatosensation touch sensations like pressure, pain,
heat, cold, and processing proprioception, the sense of how parts of
the body are oriented in space.The parietal lobe contains a somatosensory map of
the body, similar to the motorcortex. The occipital lobe is located at

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the back of the brain. Itis primarily involved in vision, seeing,
recognizing, and identifying the visual world. The temporal lobe is located at the
base of the brain by your earsand is primarily involved in processing and interpreting
sounds. It also contains the hippocampusGreek four sea horse, a structure that

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processes memory formation. The hippocampus isillustrated in link. The role of the
hippocampus in memory was partially determined bystudying one famous epileptic patient, HM who
had both sides of his hippocampus removedin an attempt to cure his epilepsy.
His seizures went away, but hecould no longer form new memories, although

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he could remember some facts from beforehis surgery and could learn new motor tasks.
Hypothalamus. One small but critically importantpart of the brain is the hypothalamus
shown in link. The hypothalamus controlsthe endocrine system by sending signals to the
pituitary gland, a pea sized endocrinegland that releases several different hormones that affect

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other glands as well as other cells. This relationship means that the hypothalamus regulates
many important behaviors that are controlled bythese hormones. The hypothalamus is the body's
thermostat. It makes sure key functionslike food and water intake, energy expenditure,
and body temperature are kept at appropriatelevels. Neurons within the hypothalamus also

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regulate circadian rhythms times called sleep cycles. Cerebellum The cerebellum Latin four little brain
shown in figure sits at the baseof the brain on top of the brain
stem. The cerebellum controls balance andaids in coordinating movement and learning new motor
tasks. Brain stem. The brainstemillustrated in figure, connects the rest of

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the brain with the spinal cord.It consists of the midbrain medulla oblongata,
and the ponds. Motor and sensoryneurons extend through the brainstem, allowing for
the relay of signals between the brainand spinal cord. Ascending neural pathways cross
in the section of the brain,allowing the left hemisphere of the cerebrum to

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control the right side of the bodyand vice versa. The brainstem coordinates motor
control signals sent from the brain tothe body. The brainstem controls several important
functions of the body, including alertness, arousal, breathing, blood pressure,
digestion, heart rate, swallowing,walking, and sensory and motor information integration.

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Spinal cord connecting to the brain stemand extending down the body through the
spinal column is the spinal cord shownin figure. The spinal cord is a
thick bundle of nerve tissue that carriesinformation about the body to the brain and
from the brain to the body.The spinal cord is contained within the bones
of the vertebrate column, but isable to communicate signals to and from the

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body through its connections with spinal nerves, part of the peripheral nervous system.
A cross section of the spinal cordlooks like a white oval containing a gray
butterfly shape, as illustrated in figure. Myolinated axons make up the white matter,
and neuron and glial cell bodies makeup the gray matter. Gray matter
is also composed of inter neurons,which connect two neurons, each located in

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different parts of the body. Axonsand cell bodies, and the dorsal facing
the back of the animal, spinechord convey mostly sensory information from the body
to the brain. Axons and cellbodies in the ventral facing the front of
the animal spinal cord primarily transmit signalscontrolling movement from the brain to the body.

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The spinal cord also controls motor reflexes. These reflexes are quick, unconscious
movements, like automatically removing a handfrom a hot object. Reflexes are so
fast because they involve local synaptic connections. For example, the knee reflex that
a doctor tests during a routine physicalis controlled by a single synapse between a

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sensory neuron and a motor neuron,while a reflex may only require the involvement
of one or two synapses. Synapseswith inter neurons and the spinal column transmit
information to the brain to convey whathappened the knee jerked or the hand was
hot. In the United States,there are around ten thousand spinal cord injuries

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each year. Because the spinal cordis the information superhighway connecting the brain with
the body, damage to the spinalcord can lead to paralysis. The extent
of the paralysis depends on the locationof the injury along the spinal cord and
whether the spinal cord was completely severed. For example, if the spinal cord

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is damaged at the level of theneck, it can cause paralysis from the
neck down, whereas damage to thespinal column further down may limit paralysis to
the legs. Spinal cord injuries arenotoriously difficult to treat because spinal nerves do
not regenerate, although ongoing research suggeststhat stem cell transplants may be able to
act as a bridge to reconnect severednerves. Researchers are also looking at ways

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to prevent the inflammation that worsens nervedamage after injury. One such treatment is
to pump the body with cold salineto induce hypothermia. This cooling can prevent
swelling and other processes that are thoughtto worsen spinal cord injuries. In the
cross section, the gray matter formsan X inside the oval white matter.

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The legs of the X are thickerthan the arms. Each leg is called
a ventral horn and each arm iscalled a dorsal horn. A cross section
of the spinal cord shows gray mattercontaining cell bodies and interneurons, and white
matter containing axons. Peripheral nervous system. The peripheral nervous system PNS is the

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connection between the central nervous system andthe rest of the body. The CNS
is like the power plant of thenervous system. It creates the signals that
control the functions of the body.The PNS is like the wires that go
to individual houses. Without those wires, the signals produced by the CNS could

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not control the body, and theCNS would not be able to receive sensory
information from the body either. ThePNS consists of the sensory division AFFRONT,
which consists of sensory neurons, andthe motor division EFFRONT, which consists of
motor neurons. The sensory division conveysinformation to the CNS to be processed,

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and the motor division conveys the responseof the CNS to muscles, glands,
and organs. The motor component ofthe PNS is even more complex and can
be divided into the autonomic division andthe somatic division. The autonomic motor division,
as the name implies, is notcontrolled or initiated by the conscious thought

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of an individual. The somatic motordivision is consciously controlled by the individual and
usually affects scalatal muscles autonomic motor division. The autonomic division of the motor division
is divided into sympathetic and parasympathetic systems. In the sympathetic system, the soma

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of the preganglionic neurons is usually locatedin the spine, while in the parasympathetic
system, the soma is usually inthe brainstem or sacral the bottom of the
spine. In both systems, thepreganglionic neuron releases the neurotransmitter acetylcholine into the
synapse postganglionic neurons of the sympathetic systemhave solmas in a sympathetic ganglion located next

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to the spinal cord. Postganglionic neuronsof the parasympathetic system have solmas in ganglions
near the target organ. Postganglionic neuronsof the sympathetic system release nora epinephrin into
the synapse, while postganglionic neurons ofthe parasympathetic system release acetylcholine or nitric oxide.

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In the autonomic motor division of theperipheral nervous system, a preganglionic neuron
of the CNS synapses with a postganglionicneuron of the PNS. The postganglionic neuron,
in turn acts on a target organ. Autonomic responses are mediated by the
sympathetic and the parasympathetic systems, whichare antagonistic to one another. The sympathetic

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system activates the fight or flight response, while the parasympathetic system activates the rest
in digest response. The autonomic motordivision serves as the relay between the CNS
and the internal organs. It controlsthe lungs, the heart, smooth muscle,
and exocrine and endocrine glands. Theautonomic motor division controls these organs largely

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without conscious control. It can continuouslymonitor the conditions of these different systems and
implement changes as needed. Signaling tothe target tissue usually involves two synapses.
A preganglionic neuron originating in the conssynapses to a neuron in a ganglion that
in turn synapses on the target organAs illustrated in figure. There are two

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divisions of the autonomic motor division thatoften have opposing effects, the sympathetic division
and the parasympathetic division. Sympathetic division. The sympathetic divis is responsible for the
fight or flight response that occurs whenan animal encounters a dangerous situation. One

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way to remember this is to thinkof the surprise a person feels when encountering
a snake. Snake and sympathetic bothbegin with s. Examples of functions controlled
by the sympathetic division include an acceleratedheart rate and inhibited digestion. These functions
help prepare an organism's body for thephysical strain required to escape a potentially dangerous

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situation or to fend off a predator. Illustration shows the effects of the sympathetic
and parasympathetic systems on target organs andthe placement of the preganglionic neurons that mediate
these effects. The parasympathetic system causespupils and bronchi to constrict, slows the
heart rate, and stimulates salivation,digestion, and bile secretion. Preganglionic neurons

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that mediate these effects are all locatedin the brain stem. Preganglionic neuron of
the parasympathetic system that are located inthe sacral cause the bladder to contract.
The sympathetic system causes pupils and bronchito dilate, increases heart rate, inhibits
digestion, stimulates the breakdown of glycogenand the secretion of adrenaline and nora adrenaline,

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and inhibits contraction of the bladder.The preganglionic neurons that mediate these effects
are all located in the spine.The sympathetic and parasympathetic divisions often have a
posing effects on target organs. Mostpreganglionic neurons in the sympathetic division originate in

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the spinal chord, as illustrated infigure. The axons of these neurons release
acetylcholine on postganglionic neurons within sympathetic ganglia. The sympathetic ganglia form a chain that
extends alongside the spinal cord. Theacetyl coline activates the post ganglionic neurons.
Postganglionic neurons then release nora epinephrin ontotarget organs. As anyone who has ever

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felt or rush before a big,tast speech, or athletic event can attest,
the effects of the sympathetic division arequite pervasive. This is both because
one preganglionic neuron synapses on multiple postganglionicneurons, amplifying the effect of the original
synapse, and because the adrenal glandalso releases norapinephrin and the closely related hormone

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epinephrin into the bloodstream. The physiologicaleffects of this noropinephrin release include dilating the
trachea and bronchi, making it easierfor the animal to breathe, increasing heart
rate, and moving blood from theskin to the heart, muscles, and
brain so the animal can think andrun. The strength and speed of the

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sympathetic response helps an organism avoid dangerparasympathetic division while the sympathetic division is activated
in stressful situations. The parasympathetic divisionallows an animal to rest and digest.
One way to remember this is tothink that during a RESTful situation like a
picnic, the parasympathetic division is incontrol. Picnic and parasympathetic both start with

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p parasympathetic preganglionic neurons have cell bodieslocated in the brainstem and in the sacral
toward the bottom spinal cord as shownin figure. The axons of the preganglionic
neurons release aceetyl cooline on the postganglionicneurons, which are generally located very near
the target organs. Most postganglionic neuronsrelease aceetyl coline onto target organs, although

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some release nitric oxide. The parasympatheticdivision resets organ function after the sympathetic division
is activated. The common adrenaline dumpyou feel after a fight or flight event.
Effects of acetyl coline release on targetorgans include slowing of heart rate,
lowered blood pressure, and stimulation ofdigestion. Somatic division. The somatic division

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of the peripheral nervous system is madeup of cranial and spinal nerves that contain
motor neurons. Motor neurons transmit messagesabout desired movement from the CNS to the
muscles to make them contract. Withoutits somatic division, an animal would be
unable to process any information about itsenvironment, what it sees, feels,

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hears, and so on, andcould not control motor movements. Unlike the
autonomic division, which has two synapsesbetween the CNS and the target organ,
motor neurons of the somatic division haveonly one synapse between the CNS and muscle
or organ. Acetyl Coline is themain neuro transmitter released at these synapses.

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Cranial and spinal nerves. Humans havetwelve cranial nerves nerves that emerge from or
enter the skull cranium, as opposedto the spinal nerves which emerge from the
vertebral column. Each cranial nerve isaccorded in name, which are detailed in
figure. Some cranial nerves transmit itonly sensory information. For example, the

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olfactory nerve transmits information about smells fromthe nose to the brain stem. Other
cranial nerves transmit almost solely motor information. For example, the oculomotor nerve controls
the opening and closing of the eyelidand smi movements. Other cranial nerves contain
a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has

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a roll in both taste, sensoryand swallowing motor. Illustration shows the underside
of the brain. The twelve cranialnerves cluster around the brain stem and are
symmetrically located on each side. Theolfactory nerve is short and lobeli and is
located closest to the front. Directlybehind this is the optic nerve than the

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oculomotor nerve. All these nerves arelocated in front of the brain stem.
The trigeminal nerve, which is thethickest, is located on either side of
the brain stem. It forms threebranches shortly after leaving the brain. The
trochlear nerve is a small nerve infront of the trigeminal nerve. Behind the

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brain stem are the smaller facial vestibulocoochleer, glossopharyngeal, and hypoglossal nerves.
The nerve furthest back is the accessorynerve. The human brain contains twelve cranial
nerves that receive sensory input and controlmotor output. For the head and neck,
spinal nerves transmit sensory and motor informationbetween the spinal cord and the rest

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of the body. Each of thethirty one spinal nerves in humans contains both
sensory and motor axons. The sensoryneuron cell bodies are grouped in structures called
dorsal root ganglia and are shown infigure. Each sensory neuron has one projection
with a sensory receptor ending in skin, muscle, or sensory organs, and

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another that synapses with a neuron inthe dorsal spinal cord. Motor neurons have
cell bodies in the ventral gray matterof the spinal cord that project to muscle
through the ventral root. These neuronsare usually stimulated by inter neurons within the
spinal cord, but are sometimes directlystimulated by sensory neurons. Illustration shows a

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cross section of the spinal cord.The gray matter forms an X inside the
white matter. A spinal nerve extendsfrom the left arm of the X,
and another extends from the left legof the X. The two nerves join
together to the left of the spine. The right arm and leg of the
X form a symmetrical nerve. Thepart of the nerve that exits from the

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leg of the X is called theventral root, and the part that exists
from the arm of the X iscalled the dorsal root. The ventral root
is on the belly side and thedorsal root is on the backside. The
dorsal root ganglion is a bull halfwaybetween where the nerve leaves the spine and
where the dorsal and ventral roots join. Sensory neuron soma's cluster in the dorsal

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root, motor neuron somo's cluster inthe gray matter in the leg of the
motor neuron axons are bundled in theventral root. Spinal nerves contain both sensory
and motor axons. The somas ofsensory neurons are located in dorsal root ganglia.
The somas of motor neurons are foundin the ventral portion of the gray

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matter of the spinal cord. Thispodcast will be released episodically and follow the
sections of the textbook in the description. For a deeper understanding, we encourage
you review the text version of thiswork voice by voicemaker dot I n.
This was produced by Brandon Casturo asa creative Common Sense production