Concept 49.1
NERVOUS SYSTEMS CONSIST OF
CIRCUITS OR NEURONS AND
SUPPORTING CELLS
Mary Kathryn Pearson, Susan Sabatier, and
Olivia Johnson
SIMPLE NERVOUS SYSTEMS
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In most animals with nervous systems, clusters of neurons
perform specialized functions.
Such clustering is absent in the cnidarians, the simplest
animals with nervous systems.
In cnidarians, a series of interconnected nerve cells form a
diffuse nerve net that controls the contraction and
expansion of the gastrovascular cavity.
NERVOUS SYSTEM ORGANIZATION
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In more complex animals, the axons of multiple nerve cells
may be bundled to form nerves which channel and organize
information that flows along specific routes through the
nervous system
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Animals with elongated, bilaterally symmetrical bodies have
more specialized nervous systems.
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Example- Sea stars have a set of radial nerves connecting to a central
nerve ring. Within each arm, the radial nerve is linked to a nerve net from
which it receives input and to which it sends signals controlling motor
activity.
Such animals exhibit cephalization, the clustering of sensory neurons and
interneurons at the anterior end. One or more nerve cords extending
toward the posterior end connect these structures with nerves elsewhere in
the body.
In nonsegmented worms like the planarian, a small brain and
longitudinal nerve cords make up the simplest clearly defined
central nervous system (CNS).
NERVOUS SYSTEM ORGANIZATION CONT.
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In more complex invertebrates, such as annelids and
arthropods, the number of neurons is much greater.
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Within an animal group, nervous system organization often
correlates with lifestyle.
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Behavior is regulated by more complicated brains and by ventral
nerve cords containing ganglia, segmentally arranged clusters of
neurons.
Sessile and slow-moving molluscs have relatively simple sense
organs and little or no cephalization.
Active predatory molluscs have the most sophisticated nervous
systems of any invertebrate.
In vertebrates, the brain and the spinal cord form the CNS;
the nerves and ganglia make up the peripheral nervous
system (PNS).
Eyespot
Brain
Radial
nerve
Nerve
cords
Nerve
ring
Transverse
nerve
Nerve net
Brain
Ventral
nerve
cord
Segmental
ganglia
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
(c) Planarian (flatworm)
(d) Leech (annelid)
Brain
Brain
Ventral
nerve cord
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
Ganglia
(f) Chiton (mollusc)
(g) Squid (mollusc)
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglia
Segmental
ganglia
(e) Insect (arthropod)
(h) Salamander (vertebrate)
ORGANIZATION OF THE VERTEBRATE
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Brain and spinal cord of the vertebrate CNS are
tightly coordinated
Brain provides the integrative power that
underlies the complex behavior of vertebrates
Spinal cord conveys information to and from the
brain and generates basic patterns of locomotion
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Acts independently of the brain as a part of the
simple nerve circuits that produce reflexes
REFLEXES
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Protects the body by triggering a rapid,
involuntary response to a particular stimulus
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Ex- touching a hot burner, picking up a heavy object
Quadriceps
muscle
Cell body of
sensory neuron in
dorsal root
ganglion
Gray
matter
White matter
Hamstring
muscle
Spinal cord
(cross section)
Sensory neuron
Motor neuron
Interneuron
SPINAL CORD
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Located on the
dorsal side of the
body
Does not contain
segmental ganglia
but there is ganglia
present just outside
the spinal cord
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Cranial
nerves
Spinal cord
Ganglia
outside
CNS
Spinal
nerves
SPINAL CORD CONT.
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Brain and spinal cord of vertabrates are derived from the
dorsal embryonic nerve cord which is hollow
In development, the hollow cavity is transformed into the
narrow central canal of spinal cord and ventricles of the
brain
Both are filled with cerebrospinal fluid
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Formed by filtration of arterial blood in brain
This fluid circulates slowly through central canal and
ventricles then drains into the veins
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Supplies brain with nutrients and hormones
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Carries away wastes
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Cushions the brain and spinal cord by circulating between layers
of connective tissue that surround the CNS
GRAY MATTER AND WHITE MATTER
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Gray matter- consists of neuron cell bodies,
dendrites, and unmyelinated axons
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White matter- bundled axons that have myelin
sheaths
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Gives the axons a whitish appearance
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White matter in spinal cord on the outside
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Links the CNS to sensory and motor neurons of the
PNS
WHITE MATTER IN BRAIN
On the inside
 Reflecting the role of signaling between neurons of the
brain in learning, feeling emotions, processing sensory
information, and generating commands
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Gray
matter
White
matter
Ventricles
PERIPHERAL NERVOUS SYSTEM
 Afferent
neurons: sensory neurons
 Efferent
neurons
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Motor system
Autonomic nervous system
EFFERENT NEURONS
 Motor
system (locomotion): controls
skeletal muscles (mainly in response to
external stimuli)
 Autonomic nervous system: controls
smooth and cardiac muscles ad the organs
of the digestive, cardiovascular, excretory
and endocrine systems.
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Sympathetic division
Parasympathetic division
Enteric division
EFFERENT NEURONS (CONT’D)
Autonomic Nervous System:
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Sympathetic division
Corresponds to arousal and energy generation (“fightor-flight” response)
 Ex. Gas exchange
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Parasympathetic division
Causes opposite responses that promote calming and
a return to self-maintenance functions
 Ex. Circulation
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Enteric division
Controls secretion and the smooth muscle in the
digestive tract, pancreas and gallbladder
 Ex. Digestion
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