Introduction to Nervous System and Spinal Nerves

7/27/2019 Introduction to Nervous System and Spinal Nerves

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The Nervous SystemThe Nervous System

Alex Forrest

Associate Professor of Forensic OdontologyForensic Science Research & Innovation Centre, Griffith University

Consultant Forensic Odontologist,

Queensland Health Forensic and Scientific Services,

39 Kessels Rd, Coopers Plains, Queensland, Australia 4108

Oral Biology


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Do not remove this notice.


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Learning ObjectivesLearning Objectives

1. You should be able to construct a concept map of the

nervous system.

2. You should understand and be able to explain the

basic structural organization of the human nervous

system.

3. You should be able to explain the major structural and

functional characteristics of the somatic components ofthe nervous system.


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Intercellular CommunicationIntercellular Communication

The body depends not only on proper structure of its billions of

cells to function, but also on communication between those

cells.

Sometimes this is automatic, and sometimes it is under

conscious control.


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There are two major communication systems in the body.

These are the nervous system and the endocrine system.

While both systems are designed to transmit information from

one area of the body to others, they do it in quite different ways.

Intersystem CommunicationIntersystem Communication


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The nervous system is specialized to transmit selected

information very rapidly from one part of the body to another.

This information is often quite specific, and it can be very

accurately targeted.



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The endocrine system, by contrast, works far more slowly.

It operates by secreting hormones from ductless glands into theblood-vascular system, and these are then circulated to other

organs and parts of the body.



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These two systems operate to control and integrate the body'sfunctions, enabling the body to look after itself and take care of

its needs.



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Nervous SystemNervous System

Today we examine aspects of the anatomy of the human

nervous system.

We will begin by discussing the basic layout of the nervous

system and its various divisions.

Then, we shall discuss the voluntary or somatic nervous

structures, and examine the mechanisms by which they

communicate with the brain through the spinal cord.


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The organs of the nervous system include the brain and the

spinal cord as well as the various nerves of the body.

Also included are the specialised sense organs such as the

ears and eyes, and the tiny sense organs found in the skin and

in other organs of the body.



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We can divide thenervous system into

two major parts:

Central Nervous System

Peripheral Nervous System

Copyright Alex Forrest 2013


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The central nervous

system comprisesthe brain and spinal

cord.

It is called the CNSbecause it occupies

a central midline

position in the body.



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The peripheral

nervous system (PNS)

comprises the afferent

(sensory) fibres, whichconnect the sensory

end organs to the

central nervous

system, and theefferent (motor) fibres,

which connect the

central nervoussystem to the effector

organs.



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It can be divided into a

somatic or voluntarycomponent, and a

visceral, or autonomic

or involuntary part.

The visceral system is

further subdivided into

a sympathetic and a

parasympathetic part.



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The peripheral nervous system includes the 31 pairs of spinal

nerves and the 12 cranial nerves.

The sympathetic trunks with their ganglia and branches also

belong to this system.


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Individual spinal nerves can contain fibres of both somatic and

visceral nerves, and there is a very clear pattern of distribution

of such fibres throughout the peripheral nervous system, as we

shall see.

They contain both afferent (incoming sensory) and efferent

(outgoing motor) fibres.



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Sensory fibres connect with general sensory endings, which

are scattered profusely throughout the body.

These are biological transducers in which physical stimulicreate action potentials in nerve endings.

On reaching the central nervous system, the resulting nerve

impulse gives rise to reflex response, awareness of the

stimulus, or both.



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Sensory Nerve EndingsSensory Nerve Endings

General sensory nerve endings may be divided into:

Exteroceptors

Proprioceptors

Interoceptors


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Cutaneous endings are called exteroceptors and are sensitive

to stimuli for pain, temperature, touch and pressure.

Proprioceptors in muscles, tendons and joints provide data for

reflex adjustments of muscle action, and for awareness of

position and movement.

Interoceptors arise within the viscera. Central conduction is

through sensory visceral neurons, and will be discussed in thenext session.

Sensory Nerve EndingsSensory Nerve Endings


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GangliaGanglia

Nerve ganglia (singular = ganglion) are aggregations of nerve

cell bodies found on some peripheral nerves.

They contain nerve cell bodies and their nerve fibres (axons),

as well as fibres derived from cells elsewhere which pass right

through or terminate within the ganglion.


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They are present in the dorsal roots of the spinal nerves and

the sensory roots of V, VII, VIII, IX, X.

They also occur in association with visceral (or autonomic)

nerves.

GangliaGanglia


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Cells of the Nervous SystemCells of the Nervous System

The two types of cell found in the nervous system are called

neurons (or nerve cells), and glia (specialised connective tissuesupporting cells).


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Each neuron comprises

three parts:

NeuronsNeurons

https://reader008.{domain}/reader008/html5/0417/5ad58ff55fb98/

A cell body

One or more branchingprojections called dendrites

An axon


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Sensory neurons (afferent neurons) transmit information to the

brain and spinal cord from the various parts of the body.

Motor neurons (efferent neurons) transmit information awayfrom the brain and out to various glands and muscles.

Interneurons connect sensory and motor neurons and transmit

information between the two.

NeuronsNeurons


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Some neurons are myelinated. This means the axon is

surrounded by a whitish material called myelin.

NeuronsNeurons

http://ccins.camosun.bc.ca/~tonks/courses/psyc110/neuron.jpg


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Myelin issecreted by a

special cell

called a

Schwann cell,

which wraps

around the

axons of somecells in the PNS.

NeuronsNeurons

https://reader008.{domain}/reader008/html5/0417/5ad58ff55fb98/5ad59008e6ad


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Nodes occur in betweenmyelinated segments of axon,

and these are termed the

Nodes of Ranvier.

The presence of myelin tends

to speed up conduction in an

axon.

NeuronsNeurons

http://www.bmb.psu.edu/courses/bisci004a/nerve/ner

veb4.htm


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GliaGlia

Glial cells, also known as neuroglia, are cells in the nervous

system that are not specialised for transmitting electrical

signals.

http://www.bmb.psu.edu/courses/bisci004a/nerve/nerveb4.htm


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GliaGlia

Glial cells are actually a

specialised type of

connective tissue cell,

and they hold thefunctioning neurons

together and protect

them.

They includeAstrocytes,

Oligodendrocytes (which

produce the CNS myelinhttp://www.bmb.psu.edu/courses/bisci004a/nerve/nerveb4.htm

sheath), Microglia (CNS macrophages) and Ependymal cells inthe CNS, and the Schwann cells in the PNS.

They are implicated in some of the most serious central

nervous system neoplasia (cancers).


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Central Nervous SystemCentral Nervous System


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BrainBrain


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CNS - BrainCNS - Brain

The cerebrum is the largest and most superior part of the brain

in man.

It features numerous gyri and sulci (or folds and valleys).

http://projects.edtech.sandi.net/brooklyn/humanbody/images/brain.gif


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It is divided by a large longitudinal fissure into right and left

cerebral hemispheres.

http://www.neuroscience.pomona.edu/Menu/Brain.JPG


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These hemispheres are connected by the corpus callosum which

comprises vast bundles of connections (myelinated axons).

https://reader008.{domain}/reader008/html5/0417/5ad58ff55fb98/5ad5900e564a0.jpg


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http://www.ai.mit.ed

u/people/ekm/homer

-brain.jpg


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The surface of the cerebrum is madeof gray matter. It is called the cerebral

cortex.

Gray matter comprises nerve cell

bodies and dendrites with some glial

cells.

Inside the cerebrum lies white matter,

which is made up of bundles ofmyelinated nerve axons or tracts.


http://www.csit.fsu.edu/~beason/ph

oto2002/hw06/brain.jpg


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http://faculty.washington.edu/chudler/functional.html

Specific areas of the cerebral cortex have quite specificfunctions.

For instance, the areas for vision lie in the occipital lobe.

Damage to this area can impair or destroy the ability to see.


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The areas for auditory function lie in the temporal lobes.

General sensory and motor areas are mapped out on the post-

central and pre-central gyri respectively.


http://www.ling.upenn.edu/undergrad/brain_regions.gif


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http://sds1.files.wordpress.com/2007/10/male_brain.gif


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Gary Larsen,

from:

http://www.th.physik.un

i-

frankfurt.de/~jr/gif/carto

on/brain.jpg


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http://www.astro.univie.ac.at/~dsn/gerald/brain.jpg


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Spinal CordSpinal Cord

CNS S i l C dCNS S i l C d


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CNS Spinal CordCNS Spinal Cord

The adult spinal cord is about half

a metre long, quite short when

you think of the overall length of

the body trunk.

It lies inside the vertebral column

in the spinal canal.

The core of the cord consists of

gray matter, and the outside is

white matter, the opposite of whatwe find in the cerebrum.

http://www.back.com/anatomy-lumbar.html



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The white matter comprises bundles of myelinated nerve fibres- spinal tracts - which provide conduction paths to and from the

brain.

Ascending tracts send information to the brain (sensory), and

descending tracts carry information from the brain (motor).

The spinal cord provides the link between the CNS and the rest

of the body through the PNS. It also provides reflex arcs for

rapid control.




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The spinal cord andthe dorsal root ganglia

are directly

responsible for

innervation of thebody, except for most

of the head and

viscera, which are

innervated by CranialNerves.


Grays Anatomy, London, Longman, 37th Ed, 1989, p.924



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Afferent (sensory)fibres enter the cord

through the dorsal

roots of the spinal

nerves, while efferent

(motor) fibres leave by

way of the ventral

roots.This is known as the

Bell-Magendie Law.





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Grays Anatomy, London, Longman, 37th

Ed, 1989, p. 1088


The spinal cord is a

cylindrical structure, slightlyflattened dorso-ventrally. It

occupies the spinal canal in

the vertebral column.

Protection is provided by the

vertebrae and their ligaments,

and by the meninges and a

cushion of cerebrospinal fluid.

C S S CC S S C


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The segmented nature of the

spinal cord is demonstrated

by the presence of 31 pairs of

spinal nerves.There is, however, little

evidence of segmentation in

its internal structure.


http://www.bmb.psu.edu/courses/bis

ci004a/nerve/fig17-1.jpg



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In the spinal cord,

the white matter ofthe cord lies on the

outside, and

comprises bundles

of axons ascendingto the brain and

brainstem, or

descending from the

brain and

brainstem.





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A tract occurs in the central nervous system, and may be

thought of as a group of axons that serve a similar function, thatare grouped together.

It is almost the CNS equivalent of a PNS nerve.


Spinal Cord Grey MatterSpinal Cord Grey Matter


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Spinal Cord Grey MatterSpinal Cord Grey Matter

As seen in transversesection, the grey matter

has a roughly butterfly-

shaped outline. It contains

a small central canal.

Dorsal and ventral grey

commissures surround the

central canal.

Modified from: Grays Anatomy, London, Longman, 37th Ed,

1989, p.928



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The cells of the dorsal horn and the intermediate zone consist

mainly of tract cells, and internuncial neurons which receive

incoming sensory connections (afferents) from the dorsal root

fibres, and connect either on motor cells (for reflex loops), oron tract cells for communication with higher centres. Note

particularly, however, the substantia gelatinosa (see next

slide).

The ventral horn consists mostly of motor cell bodies.



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Modified from: Grays Anatomy, London, Longman, 37th

Ed, 1989, p.928

Dorsal HornDorsal Horn


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Dorsal HornDorsal Horn

The dorsal horn is where many of the peripheral sensory fibres

terminate to connect with other neurons that either ascend to

the brain/brainstem or form reflex loops for rapid reactions.

As we said previously, the substantia gelatinosa is found here,

and it plays a major role in the perception of pain.

Ventral HornVentral Horn


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e t a oe t a o

The efferent, or motor, part of the grey matter is found in theventral horn.

White MatterWhite Matter


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The white matter consists of nerve fibres, neuroglia and blood

vessels.

Its whiteness is due to the large proportion of myelinated

nerve fibres present.



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Within each

funiculus are tractsof fibres, some of

which may be

ascending, that is

taking data tosupraspinal levels,

descending, or

carrying efferent

data from higherlevels, or Modified from: Grays Anatomy, London, Longman, 37

th Ed, 1989, p.930

intersegmental, which is to say that they consist of tracts of

fibres connecting a number of segments.



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A fibre tract is defined as a bundle of fibres having the same

origin, termination and function.

The origin is the locus of the cell bodies, and the termination is

the point at which the fibres synapse with the cell bodies of the

next neurons in the chain.



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Afferent fibres

from the dorsalroot enter the

spinal cord in

an area known

as thedorsolateral

tract because

of its

relationship tothe dorsolateral

sulcus.

Modified from: Grays Anatomy, London, Longman, 37th Ed, 1989, p.930



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These fibres give off short ascending and descending

branches, and the entire fibre terminates in the substantia

gelatinosa of the segment of entry, or the next segmentcaudally or rostrally.

The dorsolateral tract is sometimes known as the Zone of

Lissauer.

MeningesMeninges


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The central nervous system is delicate and vulnerable todamage.

It is therefore surrounded and cushioned by a series of

membranes and fluid spaces which help absorb mechanicalstress.

These membranes are called the meninges, and they are, in

turn, surrounded by bone.

MeningesMeninges


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The meningeal layers

are, from superficial

to deep:

Dura mater

Arachnoid mater

Pia Mater

Cerebro-spinal fluid

CSF circulates in the

subarachnoid space.Grays Anatomy, London, Longman, 37th

Ed, 1989, p. 1088

MeningesMeninges


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CSF circulates in the subarachnoid space.

It also fills spaces in the brain that are termed ventricles. CSF is

filtered from the blood by the choroid plexus and passes into

the ventricles.

It then circulates around the spinal cord and brain before

returning to the blood system via the arachnoid granulations

into the cranial venous sinuses.


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Peripheral Nervous SystemPeripheral Nervous System


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Somatic or"voluntary" nerve

fibres are found in all

cranial and spinal

nerves.

They are motor in

function.

Sensory Somatic FibresSensory Somatic Fibres


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Nerve impulses from exteroceptors and proprioceptors are

conducted centrally by primary sensory neurons whose cellbodies are located in the dorsal root ganglia (or a cranial nerve

ganglion).



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On entering the spinal cord, the dorsal root fibres divide intoascending and descending branches.

These are distributed as necessary for reflex responses and for

transmission of sensory data to the brain.

They enter the dorsolateral tract of the segment of arrival, or

ascend or descend a number of segments before entering this

tract. They then synapse with either an internuncial neuron ordirectly with a tract cell.



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Note that the dorsal

root ganglia contain

no synapses.

They contain only the

bodies of the sensory

neurons, whoseaxons pass from the

sensory endings.

These cells arereferred to as

primary sensory

neurons. Grays Anatomy, London, Longman, 37th Ed, 1989, p.924

Motor Somatic FibresMotor Somatic Fibres


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The somatic efferent neurons for the innervation of skeletal

muscle are the axons of neurons in the anterior grey column of

the spinal cord.

Again, only one cell is required to reach the muscle and

terminate in a motor end-plate.



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A spinal nerve

therefore appears to

arise from the spinal

cord by two roots.

The ventral root

contains efferent or

motor fibres.




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The dorsal rootcontains afferent or

sensory fibres, and

is associated with a

dorsal root ganglionwhich contains the

cell bodies of the

primary sensoryneurons.




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Shortly after leaving

the spinal cord, the

two roots unite to

form a common

spinal nerve whichemerges through

the intervertebral

foramen and

immediately dividesinto a dorsal and a

ventral ramus.




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Each ramus will

therefore contain

both afferent andefferent somatic

fibres.




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The dorsal rami areusually smaller than

the ventral ones,

and turn posteriorly

to innervate

muscles and skin

over the posterior

regions of the neckand trunk.




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The ventral rami

form the major

nerve plexuses of

the body, andcontribute to the

nerve trunks seen in

most of the body

during dissection.




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There is no synapse along the length of a somatic efferent

peripheral nerve (which emerges in the ventral root), and

therefore no somatic ganglion exists on this root.


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We will discuss the

visceral division of

the Peripheral

Nervous System in alater session.

Cranial NervesCranial Nerves


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There are twelve cranial nerves.

They are traditionally numbered using Roman numerals, in the

order that they emerge from the brainstem.

Cranial NervesCranial Nerves


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I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Olfactory

Optic

Oculomotor

Trochlear

Trigeminal

Abducens

Facial

Vestibulocochlear

Glossopharyngeal

Vagus

Accessory

Hypoglossal

Smells

Sees

Moves eye

Moves eye

Eats, feels face

Moves eye

Moves face, salivates

Hears and balances

Swallows, feels throat, salivates

Speaks, swallows, digests, controls heart

Shrugs shoulders, turns head

Moves tongue


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Introduction to Nervous System and Spinal Nerves

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