CHAPTER 7
THE NERVOUS SYSTEM
Introduction
The Nervous System (NS) is the master
controlling and communicating system
of the body.
Functions of the NS:
It uses sensory receptors to monitor
changes occurring inside and outside
the body.
It processes and interprets the sensory
input and makes decisions about what
should be done.
It activates muscles or glands.
Organization of the
Nervous System
Structural Classification – Includes ALL
nervous system organs
Central NS (CNS) – consists of the brain and
spinal cord
 Peripheral NS (PNS) – consists of the nerves
that extend from the brain or spinal cord
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Organization of the
Nervous System
Functional Classification
Sensory (Afferent) Division – consist of nerve
fibers that carry impulses to the CNS from
sensory receptors; helps keep the CNS
constantly informed of events going on both
inside and outside the body.
 Motor (Efferent) Division – carries impulses from
the CNS to the organs, muscles, and glands to
activate them; has 2 smaller subdivisions:

Somatic NS – allows us to consciously control our
skeletal muscles (voluntary)
 Autonomic NS – regulates events that are involuntary
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Nervous Tissue:
Structure & Function
The NS is made up of 2 types of cells – supporting
cells and neurons
Supporting Cells - Lumped together in the CNS as
NEUROGLIA, which means “nerve glue.” Each
different type of neuroglia is called GLIA. The CNS
glia include:
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Astrocytes – star shaped cells that help make exchanges
between the neurons and the capillaries; MAKES UP
NEARLY ½ OF THE NERUAL TISSUE
Microglia – spider shaped cells that dispose of debris,
including bacteria and dead brain cells
Ependymal Cells – line the cavities of the brain and
spinal cord; have cilia to circulate cerebrospinal fluid
Oligodendrocytes – provide insulation to the nerve fibers
Nervous Tissue:
Structure & Function
Neurons – Anatomy:
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Also called NERVE CELLS – specialized to transmit
messages from one part of the body to another.
All neurons have a cell body and one or more slender
processes extending from the cell body.
Parts of the Neuron:
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Cell Body – the metabolic center of the neuron; contains organelles
Nucleus – center of the cell
Mitochondrion – gives the cell its energy
Nissl Substance – the rough ER that maintains the shape of the cell
Dendrites – convey incoming messages TOWARD the cell body
Axons – convey incoming messages AWAY from the cell body
Axonal Terminals – where the axons end
Schwann Cells – cells that wrap around the axon
Nodes of Ranvier – the gaps or indentions between the Schwann
Cells
Nervous Tissue:
Structure & Function
Neurons – Classification:
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Functional Classification – groups neurons according to
the direction the nerve impulse is traveling; 3groups:
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Sensory – carry impulses from sensory receptors to the CNS
Motor – carry impulses from the CNS to the muscles and glands
Association – they connect the motor and sensory neurons in a
pathway
Structural – groups neurons according to the number of
processes extending from the cell body; 3 groups:
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Multipolar – has several processes; includes all motor and
association neurons
Bipolar – has 2 processes (axon and dendrite); found only in the
eye and ear
Unipolar – have one process; includes sensory neurons in the
PNS
Nervous Tissue:
Structure & Function
Physiology
Nerve Impulse – an electrochemical event,
initiated by stimuli, that transmits to other
neurons, muscle, or glands.
 Reflexes – rapid, predictable, and involuntary
responses to stimuli; 2 types:

Autonomic Reflex – secretion of saliva and changes
in the size of the pupil
 Somatic Reflex – pulling your hand away from a hot
object
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Reflex Arc
Central Nervous System
Functional Anatomy of the Brain:
Size = about 2 good fistfuls of pinkish gray
matter
 Weight = a little over 3 pounds
 4 major parts of the brain:
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Cerebrum – largest part
 Cerebellum
 Brain stem
 Diencephalon
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Parts of the Brain
Cerebrum
Structure:
Consists of 2 large masses called CEREBRAL
HEMISPHERES – mirror images of each other
 4 lobes of the cerebral hemispheres:
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Frontal Lobe = anterior portion of each cerebral
hemisphere
 Parietal Lobe = posterior to the frontal lobe
 Temporal Lobe = lies below the frontal and parietal
lobes
 Occipital Lobe = posterior portion of each cerebral
hemisphere
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Cerebral Cortex – outermost portion of the
cerebrum
Structure of the Cerebrum
Cerebrum
Functions
The cerebrum is concerned with higher brain
functions.
 Three Functional Regions
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Motor Areas – the motor area of the right cerebral
hemisphere controls skeletal muscles on the left side of
the body and vice versa
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Frontal Lobe = PRIMARY MOTOR AREA – controls speech,
movement of the eyes, and writing
Sensory Areas – involves several lobes
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Parietal Lobe = sensations from all parts of the skin
Occipital Lobe = vision
Temporal Lobe = hearing
Cerebrum
Functions
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Three Functional Regions continued…
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Association Areas – function in the analysis and interpretation of
sensory experiences and are involved with memory, reasoning,
verbalizing, judgement, and emotional feelings
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Frontal Lobe = concentrating, planning, problem solving
Parietal Lobe = understanding speech and choosing the words
needed to express thoughts and feelings
Temporal Lobe = understanding speech and reading printed words,
memory of visual scenes and music
Occipital Lobe = analyzing visual patterns and recognizing another
person or an object
Both cerebral hemispheres participate in basic functions.
However, in most persons one side acts as a dominant
hemisphere for certain functions.
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In 90% of the population, the left hemisphere is dominant for
speech, writing, and reading.
Cerebellum
The cerebellum is a large mass of tissue located
below the occipital lobes of the cerebrum and
posterior to the pons and medulla oblongata of the
brainstem.
Functions:
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It communicates with other parts of the CNS
It transmits sensory information concerning the position
of the limbs and joints
It stimulates skeletal muscles to cause the desired body
movement
It helps maintain posture
Damage to the cerebellum can result in tremors,
inaccurate movements of voluntary muscles, the
loss of muscle tone, and the loss of equilibrium
Brainstem
Brain Stem – a bundle of nervous tissue
that connects the cerebrum to the spinal
cord
3 parts:
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Midbrain
A short section of the brain stem located between
the diencephalons and pons
 Serves as a reflex center
 Responsible for moving the eyes to view
something as the head is turned
 It contains the auditory reflex centers that operate
when a person needs to move his/her head in
order to hear sounds more distinctly
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Brainstem
3 Parts Continued:
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Pons
Appears as a rounded bulge on the underside of
the brain stem
 Relays impulses to and from the medulla oblongata
to the cerebrum
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Medulla Oblongata
An enlarged continuation of the spinal cord
extending from the pons to the skull
 The nerve fibers that connect the brain and spinal
cord MUST pass through the medulla olongata
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Brain Stem
Diencephalon
Located between the cerebral hemispheres
and above the midbrain
Contains the thalamus – central relay
station for sensory impulses ascending
from other parts of the NS to the cerebral
cortex
Contains the hypothalamus – plays a key
role in maintaining homeostasis by
regulating a variety of activities such as
blood pressure, body temperature, body
weight, sleep, and hunger
Protection of the CNS
Nervous tissue is very soft and delicate
Meninges - 3 connective tissue membranes
covering and protecting the CNS structures
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Dura Mater – outermost layer; very tough and leathery
Arachnoid Mater – the middle layer; weblike
Pia Mater – innermost layer; clings tightly to the surface
of the brain and spinal cord
Cerebrospinal Fluid
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Provides a watery cushion around the brain and spinal
cord
Any significant change in CSF may be a sign of
meningitis
Brain Dysfunctions
Traumatic Brain Injuries
Concussion – occurs when brain injury is
slight; The victim may be dizzy, “see stars,” or
lose consciousness briefly; no permanent brain
damage
 Contusion – the result of marked tissue
destruction; can result in coma lasting from
hours to a lifetime
 Cerebral Edema – swelling of the brain due to
inflammatory response to injury; can result in
death
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Spinal Cord
Structure – a cylindrical shaped structure
which is a continuation of the brain stem
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31 pairs of spinal nerves arise from the cord
and exit from the vertebral column to serve the
body area close by
Function – provides a 2-way conduction
pathway to and from the brain
Peripheral Nervous System
Consist of nerves found outside the CNS
Structure of a Nerve
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Nerve – a bundle of neuron fibers found outside
the CNS.
Nerves are classified according to the direction in
which they transmit impulses:
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Mixed Nerves – nerves carrying both sensory and motor
fibers
Sensory Nerves – nerves carrying impulses toward the
CNS
Motor Nerves – nerves carrying impulses away from the
CNS
Cranial Nerves - 12 pairs that arise from the
brain and serve the head and neck
Spinal Nerves – 31 pairs that arise from the
spinal cord and serve the limbs
Peripheral Nervous System
Autonomic Nervous System
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Structure:
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The motor subdivision of the PNS that controls body activities
automatically
Also known as the INVOLUNTARY NERVOUS SYSTEM
Autonomic Functioning – 2 divisions:
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Sympathetic Division
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Referred to as the “fight-or-flight” system
Its activity is evident when we are excited or find ourselves in
emergency or threatening situations
Signs of Activity = pounding heart, deepbreathing, sweaty skin
It allows the body to cope rapidly with situations that threaten
homeostasis
Parasympathetic Division
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Most active when the body is at rest and not threatened in any way.
Concerned with normal digestion and elimination of wastes and
conserving body energy
Example: Relaxing after a meal
Developmental Aspects of
the Nervous System
Because the NS is formed during the first month of
embryonic development, any maternal infection
early in pregnancy can have harmful effects on the
fetal NS.
One of the last areas of the CNS to mature is the
hypothalamus, which regulates body temperature.
This is why premature babies usually have to be
monitored closely and put under a heating
element.
The brain reaches its maximum weight in the
young adult
Neurons die throughout life and are not replaced –
brain mass declines with age