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The Nervous System
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System

Passage of information occurs in 2 ways:

Nerves :process and send information fast (eg.
Stepping on a tack)

Hormones: process & send information slowly
(eg. Growth hormone)
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Neurons (Nerve Cells)

Conduct messages in the form of nerve impulses

They number in the billions (much higher in anatomy
teachers)

Have extreme longevity

Most cannot divide (hippocampus is a rare exception; it is
involved in memory).

Have a high metabolic rate; require mucho oxygen and
glucose

3 basic regions: dendrites, cell body, and axons

Impulses travel from dendrites to cell body to axons
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Neurons (Nerve Cells)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.4b
Neuroglial Cells

Nervous tissue cells that aid and protect components of
nervous system by functioning like connective tissue.
Various types of neuroglial cells in the CNS:
1.
Microglia- function as phagocytes by engulfing foreign
invaders.
2.
Oligodendrocytes- connect thick neuronal fibers and
produce an important insulating material called the myelin
sheath.
3.
Astrocytes-star shaped cells that connect neurons together
and to their blood supply.
4.
Ependymal- (epithelial-like) provide a barrier between brain
and spinal fluid.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System

The master controlling and communicating system
of the body

Functions

Sensory input – monitoring stimuli

Integration – interpretation of sensory input

Motor output – response to stimuli
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Nervous System
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.1
Organization of the Nervous System


Central nervous system (CNS)

Brain and spinal cord

Integration and command center
Peripheral nervous system (PNS)

Paired spinal and cranial nerves

Carries messages to and from the spinal cord and
brain
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Involuntary
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Voluntary
Motor Division: Two Main Parts

Somatic nervous system


Conscious control of skeletal muscles
Autonomic nervous system (ANS)


Regulates smooth muscle, cardiac muscle, and
glands
Divisions – sympathetic and parasympathetic
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Differences Between Sympathetic & Parasympathetic
Systems


Parasympathetic- “resting and digesting system”

Most active in nonstressful situations

Keeps energy use low and maintains vital
housekeeping activities running.
Sympathetic division- “fight or flight” division

Exercise, excitement, emergency, and
embarrassment division

Prepares the body for action
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Differences Between Sympathetic & Parasympathetic
Systems

Characteristic
Sympathetic
Parasympathetic
When functioning?
Emergencies
Normal/Everyday
Digestive System
Inhibits/slows
Promotes
Pupil
Dialates
Constricts
Heartbeat/breathing rate
Accelerates
Retards
Neurotransmitter
Norepinephrine
Acetylcholine
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Concept Check

What is the difference between peripheral and
central nervous systems?

Explain the difference between somatic and
autonomic nervous systems…

What are the two divisions of the PNS?
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurons

A neuron has a cell body with mitochondria, lysosomes,
golgi apparatus, rough endoplasmic reticulum, and
neurofibrils.

Nerve fibers include a solitary axon and numerous
dendrites

Brnaching dendrites carry impulses from other neurons (or
from receptors) toward the cell body.

The axon transmits the impulse away from the cell body
and may give off side branches

Larger axons are enclosed by sheaths of myelin provided by
Schwann cells and are myelinated fibers.
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Neurons Continued. . .

The outer layer of myelin is surrounded by a sheath made
up of the cytoplasm and nuclei of the Schwann cell.

Narrow gaps in the myelin sheath between Schwann cells
are called Nodes of Ranvier.

The smallest axons lack a myelin sheath.

White matter vs. gray matter

White brain matter in the CNS is myelinated.

Gray brain matter in the CNS is unmylinated

Meniges are membranse that wrap & protect the CNS
(brain & spinal cord)
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheath

Whitish, fatty (protein-lipoid), segmented sheath
around most long axons

It functions to:

Increase the speed of nerve impulse transmission
because the nerve impulse jumps from one Node of
Ranvier to the next.

Protect the axon by insulating to prevent the nerve
impulse form “shorting” out.

Electrically insulate fibers from one another
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Histology of Nerve Tissue

The two principal cell types of the nervous system
are:


Neurons – excitable cells that transmit electrical
signals
Supporting cells – cells that surround and wrap
neurons
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Microglia and Ependymal Cells

Microglia – small, ovoid cells with spiny processes


Phagocytes that monitor the health of neurons
Ependymal cells – range in shape from squamous
to columnar

They line the central cavities of the brain and
spinal column
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Microglia and Ependymal Cells
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Figure 11.3b, c
Oligodendrocytes, Schwann Cells, and
Satellite Cells
 Oligodendrocytes – branched cells that wrap CNS
nerve fibers


Schwann cells (neurolemmocytes) – surround
fibers of the PNS
Satellite cells surround neuron cell bodies with
ganglia
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Oligodendrocytes, Schwann Cells, and
Satellite Cells
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Figure 11.3d, e
Concept Check

What is the difference between Schwann Cells and
Oligodendrocytes?
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Dendrites of Motor Neurons

Short, tapering, and diffusely branched processes

They are the receptive, or input, regions of the
neuron

Electrical signals are conveyed as graded
potentials (not action potentials)
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Axons: Function

Generate and transmit action potentials

Secrete neurotransmitters from the axonal
terminals

Movement along axons occurs in two ways

Anterograde — toward axonal terminal

Retrograde — away from axonal terminal
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Myelin Sheath and Neurilemma: Formation

Formed by Schwann cells in the PNS

A Schwann cell:


Envelopes an axon in a trough

Encloses the axon with its plasma membrane

Has concentric layers of membrane that make up
the myelin sheath
Neurilemma – remaining nucleus and cytoplasm of
a Schwann cell
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Myelin Sheath and Neurilemma: Formation
PLAY
InterActive Physiology ®:
Nervous System I, Anatomy Review, page 10
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.5a–c
Nodes of Ranvier (Neurofibral Nodes)

Gaps in the myelin sheath between adjacent
Schwann cells

They are the sites where axon collaterals can
emerge
PLAY
InterActive Physiology ®:
Nervous System I, Anatomy Review, page 11
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Unmyelinated Axons

A Schwann cell surrounds nerve fibers but coiling
does not take place

Schwann cells partially enclose 15 or more axons
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Axons of the CNS

Both myelinated and unmyelinated fibers are
present

Myelin sheaths are formed by oligodendrocytes

Nodes of Ranvier are widely spaced

There is no neurilemma
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Regions of the Brain and Spinal Cord


White matter – dense collections of myelinated
fibers
Gray matter – mostly soma (body) and
unmyelinated fibers
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Concept Check

Why would there be unmyelinated neurons in the
brain?
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Neuron Classification

Functional:



Sensory (afferent) — transmit impulses toward the
CNS
Motor (efferent) — carry impulses away from the
CNS
Interneurons (association neurons) — shuttle
signals through CNS pathways
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Neurophysiology

Neurons are highly irritable

Action potentials, or nerve impulses, are:

Electrical impulses carried along the length of
axons

Always the same regardless of stimulus

The underlying functional feature of the nervous
system
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Electricity Definitions



Voltage (V) – measure of potential energy generated by
separated charge
Potential difference – voltage measured between two
points
Current (I) – the flow of electrical charge between two
points

Resistance (R) – hindrance to charge flow

Insulator – substance with high electrical resistance

Conductor – substance with low electrical resistance
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Concept Check

Use the following in a sentence to explain what
they mean:

Resistance, current, insulator

Voltage, potential difference

Conductor, current, voltage
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Electrical Current and the Body

Reflects the flow of ions rather than electrons

There is a potential on either side of membranes
when:

The number of ions is different across the
membrane

The membrane provides a resistance to ion flow
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Role of Ion Channels

Types of plasma membrane ion channels:




PLAY
Passive, or leakage, channels – always open
Chemically gated channels – open with binding of a specific
neurotransmitter
Voltage-gated channels – open and close in response to membrane
potential
Mechanically gated channels – open and close in response to physical
deformation of receptors
InterActive Physiology ®:
Nervous System I: Ion Channels
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Operation of a Gated Channel

Example: Na+-K+ gated channel

Closed when a neurotransmitter is not bound to the
extracellular receptor


Na+ cannot enter the cell and K+ cannot exit the
cell
Open when a neurotransmitter is attached to the
receptor

Na+ enters the cell and K+ exits the cell
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Operation of a Gated Channel
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Figure 11.6a
Operation of a Voltage-Gated Channel

Example: Na+ channel

Closed when the intracellular environment is
negative


Na+ cannot enter the cell
Open when the intracellular environment is
positive

Na+ can enter the cell
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Operation of a Voltage-Gated Channel
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Figure 11.6b
Gated Channels

When gated channels are open:

Ions move quickly across the membrane

Movement is along their electrochemical gradients

An electrical current is created

Voltage changes across the membrane
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Electrochemical Gradient

Ions flow along their chemical gradient when they
move from an area of high concentration to an area
of low concentration

Ions flow along their electrical gradient when they
move toward an area of opposite charge

Electrochemical gradient – the electrical and
chemical gradients taken together
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Resting Membrane Potential (Vr)

The potential difference (–70 mV) across the
membrane of a resting neuron

It is generated by different concentrations of Na+,
K+, Cl, and protein anions (A)

Ionic differences are the consequence of:

Differential permeability of the neurilemma to Na+
and K+

Operation of the sodium-potassium pump
PLAY
InterActive Physiology ®:
Nervous System I: Membrane Potential
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Measuring Mebrane Potential
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Figure 11.7
Resting Membrane Potential (Vr)
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Figure 11.8
Membrane Potentials: Signals

Used to integrate, send, and receive information

Membrane potential changes are produced by:


Changes in membrane permeability to ions

Alterations of ion concentrations across the
membrane
Types of signals – graded potentials and action
potentials
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Changes in Membrane Potential

Changes are caused by three events



Depolarization – the inside of the membrane
becomes less negative
Repolarization – the membrane returns to its
resting membrane potential
Hyperpolarization – the inside of the membrane
becomes more negative than the resting potential
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Changes in Membrane Potential
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Figure 11.9
Action Potential:
Role of the Sodium-Potassium Pump
 Repolarization


Restores the resting electrical conditions of the
neuron

Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is
restored by the sodium-potassium pump
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Phases of the Action Potential




1 – resting state
2 – depolarization
phase
3 – repolarization
phase
4 – hyperpolarization
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Figure 11.12
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Saltatory Conduction
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Figure 11.16
Synapses



A junction that mediates information transfer from
one neuron:

To another neuron

To an effector cell
Presynaptic neuron – conducts impulses toward the
synapse
Postsynaptic neuron – transmits impulses away
from the synapse
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Synapses
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Figure 11.17
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Chemical Synapses

Specialized for the release and reception of
neurotransmitters

Typically composed of two parts:

Axonal terminal of the presynaptic neuron, which
contains synaptic vesicles

Receptor region on the dendrite(s) or soma of the
postsynaptic neuron
PLAY
InterActive Physiology ®:
Nervous System II: Anatomy Review, page 7
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Cleft

Fluid-filled space separating the presynaptic and
postsynaptic neurons

Prevents nerve impulses from directly passing
from one neuron to the next

Transmission across the synaptic cleft:

Is a chemical event (as opposed to an electrical
one)

Ensures unidirectional communication between
neurons
PLAY
InterActive Physiology ®:
Nervous System II: Anatomy Review, page 8
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Cleft: Information Transfer

Nerve impulses reach the axonal terminal of the
presynaptic neuron and open Ca2+ channels

Neurotransmitter is released into the synaptic cleft
via exocytosis in response to synaptotagmin

Neurotransmitter crosses the synaptic cleft and
binds to receptors on the postsynaptic neuron

Postsynaptic membrane permeability changes,
causing an excitatory or inhibitory effect
PLAY
InterActive Physiology ®:
Nervous System II: Synaptic Transmission, pages 3–6
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Neurotransmitters

Chemicals used for neuronal communication with
the body and the brain

50 different neurotransmitters have been identified

Classified chemically and functionally
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Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
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Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Axon of
presynaptic
neuron
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Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Mitochondrion
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules
2
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Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Mitochondrion
Postsynaptic
membrane
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules
2
Synaptic
cleft
Ion channel
(closed)
3
Ion channel (open)
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Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel (open)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
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Figure 11.18
Basic Pattern of the Central Nervous System


Spinal Cord

Central cavity surrounded by a gray matter core

External to which is white matter composed of
myelinated fiber tracts
Brain

Similar to spinal cord but with additional areas of
gray matter

Cerebellum has gray matter in nuclei

Cerebrum has nuclei and additional gray matter in
the cortex
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Functional Areas of the Cerebral Cortex

The three types of functional areas are:

Motor areas – control voluntary movement

Sensory areas – conscious awareness of sensation

Association areas – integrate diverse information
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Functional Areas of the Cerebral Cortex
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Figure 12.8a
Functional Areas of the Cerebral Cortex
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Figure 12.8b
Lateralization of Cortical Function




Lateralization – each hemisphere has abilities not
shared with its partner
Cerebral dominance – designates the hemisphere
dominant for language
Left hemisphere – controls language, math, and
logic
Right hemisphere – controls visual-spatial skills,
emotion, and artistic skills
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Cerebral White Matter

Consists of deep myelinated fibers and their tracts

It is responsible for communication between:

The cerebral cortex and lower CNS center, and
areas of the cerebrum
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Cerebral White Matter

Types include:



Commissures – connect corresponding gray areas
of the two hemispheres
Association fibers – connect different parts of the
same hemisphere
Projection fibers – enter the hemispheres from
lower brain or cord centers
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Fiber Tracts in White Matter
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Figure 12.10a
Fiber Tracts in White Matter
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Figure 12.10b
Brain Stem

Consists of three regions – midbrain, pons, and
medulla oblongata

Similar to spinal cord but contains embedded
nuclei

Controls automatic behaviors necessary for
survival

Provides the pathway for tracts between higher and
lower brain centers

Associated with 10 of the 12 pairs of cranial nerves
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Brain Stem
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Figure 12.15a
Peripheral Nervous System (PNS)

PNS – all neural structures outside the brain and
spinal cord

Includes sensory receptors, peripheral nerves,
associated ganglia, and motor endings

Provides links to and from the external
environment
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PNS in the Nervous System
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Figure 13.1
Sensory Receptors

Structures specialized to respond to stimuli

Activation of sensory receptors results in
depolarizations that trigger impulses to the CNS

The realization of these stimuli, sensation and
perception, occur in the brain
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Receptor Classification by Stimulus Type





Mechanoreceptors – respond to touch, pressure,
vibration, stretch, and itch
Thermoreceptors – sensitive to changes in
temperature
Photoreceptors – respond to light energy (e.g.,
retina)
Chemoreceptors – respond to chemicals (e.g.,
smell, taste, changes in blood chemistry)
Nociceptors – sensitive to pain-causing stimuli
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.2
Main Aspects of Sensory Perception



Perceptual detection – detecting that a stimulus has
occurred and requires summation
Magnitude estimation – how much of a stimulus is
acting
Spatial discrimination – identifying the site or
pattern of the stimulus
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Main Aspects of Sensory Perception



Feature abstraction – used to identify a substance
that has specific texture or shape
Quality discrimination – the ability to identify
submodalities of a sensation (e.g., sweet or sour
tastes)
Pattern recognition – ability to recognize patterns
in stimuli (e.g., melody, familiar face)
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Structure of a Nerve


Nerve – cordlike organ of the PNS consisting of
peripheral axons enclosed by connective tissue
Connective tissue coverings include:



Endoneurium – loose connective tissue that
surrounds axons
Perineurium – coarse connective tissue that
bundles fibers into fascicles
Epineurium – tough fibrous sheath around a nerve
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Structure of a Nerve
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Figure 13.3b
Classification of Nerves

Sensory and motor divisions

Sensory (afferent) – carry impulse to the CNS

Motor (efferent) – carry impulses from CNS

Mixed – sensory and motor fibers carry impulses
to and from CNS; most common type of nerve
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Peripheral Nerves



Mixed nerves – carry somatic and autonomic
(visceral) impulses
The four types of mixed nerves are:

Somatic afferent and somatic efferent

Visceral afferent and visceral efferent
Peripheral nerves originate from the brain or spinal
column
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Regeneration of Nerve Fibers

Damage to nerve tissue is serious because mature
neurons are amitotic

If the soma of a damaged nerve remains intact,
damage can be repaired

Regeneration involves coordinated activity among:



Macrophages – remove debris
Schwann cells – form regeneration tube and secrete
growth factors
Axons – regenerate damaged part
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Regeneration of Nerve Fibers
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Figure 13.4
Regeneration of Nerve Fibers
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Figure 13.4
Cranial Nerves

Twelve pairs of cranial nerves arise from the brain

They have sensory, motor, or both sensory and
motor functions

Each nerve is identified by a number (I through
XII) and a name

Four cranial nerves carry parasympathetic fibers
that serve muscles and glands
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Cranial Nerves
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Figure 13.5a
Summary of Function of Cranial Nerves
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 13.5b
Cranial Nerve I: Olfactory

Arises from the olfactory epithelium

Passes through the cribriform plate of the ethmoid
bone

Fibers run through the olfactory bulb and terminate
in the primary olfactory cortex

Functions solely by carrying afferent impulses for
the sense of smell
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Cranial Nerve I: Olfactory
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure I from Table 13.2
Cranial Nerve II: Optic

Arises from the retina of the eye

Optic nerves pass through the optic canals and
converge at the optic chiasm

They continue to the thalamus where they synapse

From there, the optic radiation fibers run to the
visual cortex

Functions solely by carrying afferent impulses for
vision
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Cranial Nerve II: Optic
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure II from Table 13.2
Cranial Nerve III: Oculomotor

Fibers extend from the ventral midbrain, pass
through the superior orbital fissure, and go to the
extrinsic eye muscles

Functions in raising the eyelid, directing the
eyeball, constricting the iris, and controlling lens
shape

Parasympathetic cell bodies are in the ciliary
ganglia
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Cranial Nerve III: Oculomotor
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure III from Table 13.2
Cranial Nerve IV: Trochlear

Fibers emerge from the dorsal midbrain and enter
the orbits via the superior orbital fissures;
innervate the superior oblique muscle

Primarily a motor nerve that directs the eyeball
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Cranial Nerve IV: Trochlear
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure IV from Table 13.2