Chapter 6



1. uses millions of sensory receptors to monitor
changes occurring inside and outside the body
2. processes and interprets the sensory input
and makes decisions on what should be done
(integration)
3.effects a response by activating muscles or
glands (motor output)

Has 2 subdivisions

Central nervous system (CNS)
 Brain and spinal cord
 Occupy the dorsal body cavity
 Are the integrating and command centers

Peripheral nervous system (PNS)
 Part that is outside the CNS
 Nerves outside the brain and spinal cord (spinal and
cranial nerves)
 Communication lines from sensory receptors to CNS
then to glands / muscles

For the PNS only: there are two subdivisions

Sensory (afferent) division
 Convey impulses TO the CNS from sensory receptors
 Somatic impulses come from skin, skeletal muscle, and
joints
 Visceral impulses come from organs

Motor (efferent) division
 Carries impulses from the CNS to effector organs
(muscles and glands)
 They activate (get a response) from muscles and glands

Motor Division has 2 subdivisions

Somatic nervous system
 Allows conscious or voluntary control on skeletal
muscles
 Reflexes are NOT voluntary but are carried by the same
fibers

Autonomic nervous system
 Regulates events that are involuntary (glands, organs)
 2 parts (bring about opposite effects)
 Sympathetic
 parasympathetic

Two types of cells in nervous tissue

Supporting cells
 Grouped as “neuroglia” or “glia”
 Many cells that support, insulate, and protect neurons

Neurons
 Or nerve cells, specialized to transmit messages
 All have common features
 Cell body (with nucleus and organelles)
 One or more slender processes extending from the cell

Astrocytes
Star-shaped
 Cling to neurons, anchoring them to nutrient
supplies (capillaries)
 Help make exchanges between neurons and blood
supply
 In the brain, pick up excess ions and recapture
neurotransmitters


Microglia

Spiderlike phagocytes; dispose of debris (dead cells
and bacteria)

Ependymal cells



Line the cavities of brain and spinal cord
Cilia circulate cerebrospinal fluid
Oligodendrocytes


Wrap extensions around nerve fibers
Produce fatty insulation (myelin sheath)


Resemble neurons but do NOT transmit nerve
impulses
Never lose the ability to divide (mitosis)


Most brain tumors are “gliomas” formed by glial
cells
In the PNS

Schwann cells
 Form myelin sheaths

Satellite cells
 Protective, cushioning cells


Also called nerve cells
Highly specialized to transmit messages from
one area of the body to another



Metabolic center of the neuron
Contains the organelles (but no centrioles)
Rough ER (Nissl substance) and neurofibrils
are abundant




Vary in length (microscopic to 3-4 feet)
Dendrites – convey incoming messages to the
cell body
Axons – convey nerve impulses away from cell
body
Neurons may have a few to hundreds of
dendrites, but will have only one axon.



Arise from the axon hillock on the cell body
Some give off collateral branches
All branch profusely at the terminal end
forming hundreds of axon terminals


Terminals contain the vesicles full of
neurotransmitters
When nerve impulses reach the axon terminals,
neurotransmitters are released to extracellular space


Synaptic cleft separates the axon terminal from
the next neuron (neurons do NOT touch each
other)
This functional junction is the synapse


Long nerve fibers are covered with myelin
(fatty material that protects and insulates) that
increases the rate of impulse transmission
Axons outside the CNS are myelinated with
Schwann cells that wrap around the axon to
produce the myelin sheath


Neurilemma-cytoplasm of the Schwann cells just
beneath the plasma membrane (external to the
myelin)
Schwann cells are individual so there are gaps
between the cells called nodes of Ranvier



In the CNS, myelinated fibers (myelin) is
produced by the oligodendrocytes
Schwann cells myelinate only a short segment
of nerve fiber; oligodendrocytes can myelinate
as many as 60 fibers at the same time
CNS myelin sheaths do not have the
neurilemma (so there is no regenerative
properties for the CNS)





Different names in CNS and PNS
CNS – clusters are called nuclei are well
protected by the skull and vertebral column; if
the cell dies, it is NOT replaced
PNS – clusters are called ganglia
Bundles of nerve fibers (CNS – tracts; PNS –
nerves)
Gray matter and white matter refers to the
presence or absence of myelin (white matter is
highly myelinated)

Functional classification


Groups neurons according to the direction the nerve
impulse is traveling relative to the CNS
Structural classification

Based on the number of processes extending from
the cell body

Sensory (afferent)



Neurons carrying impulses from sensory receptors
to the CNS
Cell bodies located in ganglia outside CNS
Give information about the outside and inside of
body
 Dendrites have receptors activated by changes
 Cutaneous sense organs are in skin
 Pain receptors are bare dendrite endings
 Proprioceptors are in muscles and tendons
 Detect stretch or tension and send messages to maintain
balance and posture

Motor (efferent)



Carry impulses from CNS to the viscera (muscles
and glands)
Cell bodies inside CNS
Association neurons (interneurons)


Connect motor and sensory neurons in neural
pathways
Cell bodies inside CNS

Multipolar neurons




Bipolar neurons




Several processes coming off cell body
Most common
Includes motor and association neurons
Have one axon and one dendrite
Rare in adults
Found only in special sense organs (eye, ear) for sensory
reception
Unipolar neurons




A single process extending from the cell body
Divides into proximal and distal fibers
Axon conducts impulses toward and away from cell body
Found in sensory organs of PNS

Two major functions of neurons

Irritability
 Ability to respond to a stimulus and convert it to a
nerve impulse

Conductivity
 Ability to transmit the impulse to other neurons,
muscles, glands

Plasma membrane of a resting (inactive)
neuron is polarized
Fewer positive ions inside the cell than outside
 K+ is inside; Na+ is outside
 Inside must be more negative than the outside (so
there are less K+ inside, but more Na+ outside)





Neuron must be stimulated which causes the
sodium gates to open
Na+ ions rush into the cell (making inside more
+)
If strong enough, depolarization activates the
neuron to initiate and transmit an action
potential or nerve impulse
This is “all or none”; impulse either happens
completely or it doesn’t




Immediately after Na+ rushes in, membrane
permeability changes (so now it’s impermeable
to Na+)
Na+ cannot go into the cell but K+ rushes out
to restore the electrical conditions of the cell
(repolarization)
Neurons cannot conduct another impulse until
repolarization occurs
Na+-K+ pump restores initial concentrations of
Na+ (outside) and K+ (inside) using ATP



This is polarization, depolarization, and
repolarization
Slower on unmyelinated axons
Those neurons with myelinated axons carry
impulses much faster since the impulse can
jump from node to node (between the myelin
sheaths)




Carrying the impulse from one neuron to the
next
When the impulse reaches the axon terminals,
neurotransmitters are released
Neurotransmitters diffuse across the synapse to
bind to receptors on the membrane of the next
neuron
If enough neurotransmitter is released,
irritability of the next neuron occurs

This is an electrochemical event


Transmission down the axon of one neuron is
electrical
Stimulating the next neuron is chemical (sending
neurotransmitters across the synapse)



Reflexes – rapid, predictable, and involuntary
responses to stimuli
Once it begins, it goes in the same direction
Reflexes occur over reflex arcs

Autonomic reflexes



Regulate activity of smooth muscles, heart, glands
Regulate body functions like digestion, elimination,
blood pressure, sweating
Somatic reflexes

Reflexes that stimulate skeletal muscles

All reflexes have 5 elements




Sensory receptor – reacts to a stimulus
Effector organ – muscle or gland stimulated
Afferent and efferent neurons to connect the sensory
and effector
CNS integration center - synapse between the
afferent and efferent neurons




Knee-jerk reflex
two-neuron reflex
Quadriceps muscle attached to the hit tendon is
stretched
Tested to determine general health of motor
portion of nervous system



Withdrawal reflex
Three-neuron reflex arc
Has 5 elements






Receptor
Afferent neuron
Association neuron
Efferent neuron
effector
Always a delay at the synapses (takes time for the
neurotransmitters to diffuse across)

The more synapses involved, the longer it takes for the
reflex to occur


Involve only spinal cord neurons (no brain
involvement)
Some need brain involvement to “interpret”
the information so the appropriate reflex
occurs, such as contracting the pupils in bright
light

Embryonic development



First appears as a neural tube extending down the
dorsal median plane
By 4th week, anterior expansion (brain development)
begins; the rest becomes spinal cord
Central canal of the neural tube enlarges into 4
regions /chambers called ventricles





“two fist-fulls” of gray tissue all wrinkled
Texture of cold oatmeal
Weighs a little over 3 pounds
Largest and most complex mass of nervous
tissue
Discussed in terms of the 4 major regions




Cerebral hemispheres
Diencephalon
Brain stem
Cerebellum



Most superior part of the brain
Much larger than other 3 brain regions
Enclose and obscure most of brain stem





Surface has gyri (ridges) separated by sulci
(grooves)
Has less numerous fissures that separate larger
regions
Fissures and gyri serve as “landmarks”
2 hemispheres separated by the longitudinal
fissure
Other fissures/gyri separate the hemispheres
into smaller lobes (named for the cranial bones
over them)

Control
Speech
 Memory
 Logical and emotional response
 Consciousness
 Interpretation of sensation
 Voluntary movement




Has somatic sensory area (posterior to the central
sulcus)
Interprets impulses from sensory receptors
Recognize
Pain
 Coldness
 Light touch



Lips/fingertips send impulses to neurons that
occupy most of sensory cortex
Sensory pathways are crossed

Left lobe receives info from the right side of the body;
right lobe receives info from left side of the body


Interprets impulses from the eyes
Visual area is in the posterior portion




Interprets impulses from the ears
Auditory area borders the lateral sulcus
Olfactory area (smell) is deep inside this lobe
Also involved in complex memories







Primary motor area; anterior to central sulcus
Conscious movement of the skeletal muscles
Axons form major voluntary motor tract –
pyramidal or corticospinal tract
Pathways are crossed
Most neurons control fine motor areas: face,
mouth, hands
Anterior portion controls higher intellectual
reasoning
Also involved in complex memories and language
comprehension




Controls ability to speak
Base of the precentral gyrus (anterior to central
sulcus)
Usually located in left hemisphere
Damage prevents ability to say words properly



Junction of the temporal, parietal, and occipital
lobes
Allows one to sound out words
Usually only in one hemisphere



Gray matter
Contains the cell bodies of neurons involved in
speech, motor skills, responding to the senses
Highly ridged and convoluted (more surface
area=more neurons)



Deeper tissue (below gray matter)
Composed of fiber tracts carrying impulses to
and from the cortex
Corpus callosum – large fiber tract that
connects the two hemispheres and allows them
to communicate


Islands of gray matter deep within the white
matter
Regulate voluntary motor activities by
modifying instructions sent to the skeletal
muscles by primary motor cortex



Also called “interbrain”
Located on top of the brain stem and enclosed
by the hemispheres
Contains



Thalamus
Hypothalamus
epithalamus



Encloses the third ventricle of the brain (a relay
station for sensory impulses going to the
sensory cortex)
Gives a crude recognition of sensation (a forewarning of liking or disliking the response)
Localization and interpretation of the sensation
is done in the sensory cortex





Floor of the diencephalon
Autonomic nervous system center
Regulates body temperature, water balance,
metabolism
Center for emotions
Part of the limbic system (emotional-visceral
brain)


Thirst, appetite, sex, pain, and pleasure
Regulates the pituitary gland

Pituitary gland




Endocrine organ
Releases hormones
Hangs from the floor of the hypothalamus
Mammillary bodies



Reflex centers for olfaction
Bulge from floor of hypothalamus
Posterior to the pituitary


Roof of the third ventricle
Contain the


Pineal body (part of the endocrine system)
Choroid plexus (knots of capillaries that form
cerebral spinal fluid)


Thumb size in diameter and about 3 inches
long
Contains
Midbrain
 pons
 Medulla oblongata



Provides a pathway for ascending and
descending tracts
Has small gray matter areas (which controls
activities such as breathing, blood pressure)





Small part of brain stem
Extends from mammillary bodies to pons
Contains the cerebral aqueduct (canal through
the midbrain from third ventricle to fourth
ventricle)
Cerebral peduncles (anterior portion) convey
ascending and descending impulses
Corpora quadrigemina (dorsal) are reflex
centers for vision and hearing




Most inferior of the midbrain
Merges into spinal cord
Regulates vital visceral activities (heart rate,
blood pressure, breathing, swallowing,
vomiting, etc.)
Fourth ventricle is posterior to pons and
medulla and anterior to cerebellum





Extends entire length of brain stem
Large mass of gray matter
Involved in motor control of visceral organs
Reticular activating system (RAS) has a role in
consciousness and awake/sleep cycles
Damage can result in coma








Dorsal under the occipital lobe
2 hemispheres and convoluted surface
Has outer cortex of gray matter; inner region of
white matter
Provides precise timing for skeletal muscle activity
Controls balance and equilibrium
Smooth body movements
Receives fibers from inner ear, eye, proprioceptors
of skeletal muscles and tendons
Monitors body position and tension




Bone enclosures (skull and vertebrae)
Membranes (meninges)
Watery cushion (cerebrospinal fluid)
Blood-brain barrier


3 connective tissue membranes
Outer most layer is dura mater – double
layered membrane around brain
Outer layer is the periosteum connected to the skull
 Inner layer is the meningeal layer forming the
outermost layer of the brain and continues as dura
mater of spinal cord
 Two layers are fused except at dural sinuses where
venous blood is collected


Two folds of the meningeal layer


Falx cerebri extends inward to attach the brain to the
cranial cavity
Tentorium cerebelli separates the cerebellum from
the cerebrum

Arachnoid mater – middle meningeal layer


Pia mater


Clings to the surfaces of brain and spinal cord
Subarachnoid space


Threadlike extensions attach to pia mater
Filled with cerebrospinal fluid
Arachnoid villi
Part of arachnoid mater
 Protrude through dura mater
 Absorb cerebrospinal fluid into venous blood of the
dural sinuses






Formed from blood plasma (has less protein,
more vitamin C, different ion composition)
Continually formed from blood by choroid
plexus (capillaries in brain ventricles)
Continually circulates
Forms and drains at a constant rate to retain
normal pressure and volume
Changes may indicate meningitis or other brain
pathologies


Brain must have a constant internal environment
(some ions (Na+ and K+) are needed for impulses)
Blood-brain barrier separates the brain from bloodborne substances
Made of the least permeable capillaries
only water, glucose, and essential amino acids pass
through (all are water soluble)
 Metabolic wastes, toxins, proteins, and most drugs cannot
pass into brain tissue
 Nonessential amino acids and K+ are pumped out of
brain tissue into the blood
 Cannot stop the passage fats, respiratory gases and fatsoluble molecules


 So blood-borne alcohol, nicotine, anesthetics can pass easily

Concussion




Brain injury is slight
Dizziness, “see stars”, lose consciousness briefly
No permanent damage
Contusion



Marked tissue destruction
Damage to cerebral cortex may not cause
unconsciousness
Damage to brain stem results in coma (lasting hours
to lifetime)

Cerebral edema



Intercranial hemorrhage (bleeding)
Swelling due to inflammatory response
Both compress vital brain tissue




Commonly called strokes
Third leading cause of death in US
Occur when blood circulation to the brain is
blocked (clot or ruptured vessel)
Vital brain tissue dies


Can result in paralysis
Can result in aphasia (loss of or impaired speech)






About 17 inches long; extends from foramen
magnum (of skull) to the first or second lumbar
vertebra (it ends just below ribs)
Continuation of the brain stem
Provides two-way conduction pathway to and
from brain
Major reflex center
Enclosed inside the vertebral column
Cushioned / protected by meninges (which extend
past the end of the spinal cord inside the vertebral
canal)



Humans have 31 pairs of spinal nerves arising
from the cord that exit the vertebral column
Cord is the diameter of a thumb in most areas
except in cervical and lumbar areas (larger here
since nerves that serve the upper / lower body
areas arise here and exit the cord)
Cauda equina – spinal nerves that extend past
the lower part of the cord and travel through
the vertebral canal before exiting


Resembles a butterfly in cross section
Has two posterior (dorsal) horns and two
anterior (ventral) horns

Dorsal horns have interneurons
 Sensory neurons enter the cord by the dorsal root

Ventral horns have neurons of somatic nervous
system
 Send axons by the ventral root


Dorsal and ventral roots fuse to form spinal nerves
Surrounds the central canal of the cord and
contains cerebrospinal fluid

Composed of myelinated fiber tracts




Some run to higher centers
Some travel from brain to cord
Some send impulses from one side of cord to the
other
Divided into 3 regions



Posterior column-ascending tracts carrying sensory
impulses to brain
Lateral column-ascending and descending motor
tracts
Anterior column-ascending and descending motor
tracts

Nerves and groups of neuronal cell bodies
(ganglia) outside the CNS




Nerve-bundle of neuron fibers outside the CNS
Neuron fibers are wrapped in delicate
connective tissue (endoneurium)
Groups of fibers are wrapped in course
connective tissue (perineurium) to make
bundles (fascicles)
Fascicles are bound by tough sheaths
(epineurium) to form cord-like nerves

Nerves classified according to direction in
which they transmit impulses
Mixed nerves-carry both sensory and motor fibers
 Afferent nerves-carry impulses toward the CNS
 Efferent nerves-carry only motor fibers





12 pairs that serve the head and neck
One pair extends to the thoracic and abdominal
cavities (vagus nerves)
Most are mixed nerves
3 pairs (optic, olfactory, and vestibularcochlear)
are purely sensory




I. olfactory – purely sensory; carries impulses
for the sense of smell
II. Optic – purely sensory; carries impulses for
vision
III. Oculomotor – supplies motor fibers to four
of 6 muscles that direct the eyeball; eyelid;
internal eye muscles controlling lens shape and
pupil size
IV. Trochlear – supplies motor fibers for
external eye muscle



V. trigeminal – conducts sensory impulses
from skin of face and mucosa of nose and
mouth; contains motor fibers that activate the
chewing muscles
VI. Abducens – supplies motor fibers to lateral
rectus muscle which rolls the eye laterally
VII. Facial – activates the muscles of facial
expression and lacrimal and salivary glands;
carries sensory impulses from taste buds of
anterior tongue


VIII. Vestibulocochloear – purely sensory;
transmits impulses for sense of balance, and
hearing
IX. Glossopharyngeal – supplies motor fibers to
the pharynx that promote swallowing and
saliva production; carries sensory impulses
from taste buds of posterior tongue and from
pressure receptors of the carotid artery



X. Vagus – fibers carry sensory impulses from
and motor impulses to the pharynx, larynx,
and abdominal and thoracic viscera; most are
parasympathetic that promote digestive
activity and regulate heart activity
XI. Accessory – mostly motor fibers that
activate the sternocleidomastoid and trapezius
muscles
XII. Hypoglossal – motor fibers control tongue
movements; sensory fibers carry impulses from
the tongue



31 pairs
Formed by combination of the ventral and
dorsal roots of the spinal cord
Named for the region of the cord from which
they arise (cervical, thoracic, lumbar, sacral)




Spinal nerves divide into dorsal and ventral
rami
Rami contain both sensory and motor fibers
Damage to the spinal nerve or its rami can
result in loss of sensation and flaccid paralysis
Dorsal rami serve skin and muscles of posterior
body trunk


Ventral rami (T1-T12) form intercostal nerves
that supply muscles between ribs and skin and
muscles of the anterior and lateral trunk
Ventral rami of other spinal nerves for plexuses
that serve motor and sensory needs of limbs



Motor subdivision of the PNS that controls
body activities automatically
Special neurons that regulate cardiac muscle,
smooth muscle, and glands
Also called the involuntary nervous system

Somatic system



Cell bodies of motor neurons are inside the CNS
Axons extend all the way to the skeletal muscles
they serve
Autonomic system




Uses 2 motor neurons
The first motor neuron is inside the CNS
Its preganglionic axon leaves the CNS to synapse
with the second motor neuron (outside the CNS)
The postganglionic axon extends to the organ it
serves

2 parts

Sympathetic
 Mobilizes the body during extreme stress

Parasympathetic
 Returns the body to normal

First neurons in the cranium (III, VII, IX, X)
vagus is most important; S2-S4 of spinal nerves


Axons of neurons serve the head and neck organs
Synapse with a second motor neuron in a terminal
ganglion; postganglionic axon extends a short
distance to the organ it serves

Sacral region


Preganglionic axons leave spinal cord and form the
pelvic nerves
These travel to the pelvic cavity to synapse with the
second motor neurons in terminal ganglia on the
organs they serve


Thoracolumbar division
First neurons are in the gray matter of the
spinal cord from T1 through L2



Preganglionic axons leave the cord in the ventral
root, enter the spinal nerve, and pass through a
ramus communicans to enter the sympathetic chain
ganglion to synapse with a second neuron
Sympathetic chain lies alongside the vertebral
column on each side
Postganglionic axon reenters the spinal nerve to
travel to the skin

If the first axon does not synapse in the
sympathetic chain ganglion, it will form part of
the splanchnic nerves
Travel to viscera to synapse with second neuron in
the collateral ganglion
 Major collateral ganglia (celiac, superior and inferior
mesenteric ganglia) supply abdominal and pelvic
organs
 Postganglionic axon leaves the collateral ganglion
and travels to serve a nearby visceral organ


Body organs receive fibers from both the
sympathetic and parasympathetic divisions


Except blood vessels, skin, some glands, adrenal
medulla
Causes antagonistic effects (due to different
neurotransmitters)


Parasympathetic (cholinergic fibers release
acetylcholine)
Sympathetic (adrenergic fibers release
norepinephrine)


“fight or flight” system
Activates systems in times of stress (emotional
or physical)

Increases
 Heart rate, blood pressure, blood glucose levels
 Dilation of bronchioles in lungs
 Dilation of blood vessels in skeletal muscles

Decreases
 Blood flow to digestive system



Most active when the body is at rest
“resting and digesting” system
Promotes normal digestion, elimination of
wastes, and conserving body energy


Formed during 1st month of embryonic
development (maternal infections can be
devastating during pregnancy)
Nervous tissue has a high metabolic rate and
requires large amounts of oxygen (maternal
use of cigarettes, drugs, and alcohol decrease
oxygen levels and can cause brain damage)


One last part to develop is hypothalamus
(regulates body temperature); premature
babies have difficulty regulating body
temperature
No new neurons are produced after birth but
growth and maturation continue due to
myelination that continues to occur during
early childhood




Brain reaches maximum weight in the young
adult
Over the next 60 years, neurons are damaged
and die
Unlimited number of neural pathways are
available and ready to develop
With age, sympathetic division becomes less
effecient

Nervous system deterioration caused by poor
circulation
Can be caused by heart disease
 Can be caused by hypertension
 Decreases oxygen levels
 Can lead to senility (forgetfulness, irritability,
difficulty concentrating or thinking clearly)


Reversible senility can be caused by
Some drugs
 Constipation
 Poor nutrition or dehydration
 Hormone imbalances


While reversible, this type of senility, most of
the time, is undiagnosed