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Functional Organization
of Nervous System
Chapter 7; 20-22.10.2010
Overview of the Nervous System
 One of the body’s homeostatic control systems
 Contains sensors, integrating centers, and output
pathways
 More interneurons in a pathways  greater number
of interconnections and ability to integrate
information
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VERTEBRATE NERVOUS SYSTEM
Process information
Action potentials
Changes in the
membrane potential
Changes in behaviour
or physiological
processes
Fig.7.1 An overview of the nervous system
The nervous system contains sensors, integrating center, and output pathways.
CELLS OF NERVOUS SYSTEM
NEURONS
Functional units composed of:
dendrites
cell body
axon
Excitable cells (+ muscle cells)
Conduct electrical signal
Cause neurotransmitter release
SUPPORTING CELLS
NOT excitable
Electrical insulation
oligodendrocytes (CNS)
Schwann cells (PNS)
Protective
astrocytes (blood-brain barrier)
microglia
Fig.4.1
Cnidarians
 Most nervous systems are organized into three
functional divisions
 Afferent sensory, integrating, and efferent motor
 Cnidarians are an exception
 Their nervous system is an interconnected web or
nerve net
 Neurons are not specialized into different divisions
 Neurons carry action potentials in both directions
 Neurons are not specifically sensory or motor
 Organism can still perform complex behaviors
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Cnidarians
Cutaway view of a sea anemone
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Figure 7.2a
Nervous System Terms
 Bilaterally symmetrical – right and left side are
mirror images
 Cephalization – sense organs are concentrated at
the anterior end
 Ganglia – groups of neuronal cell bodies
 Nuclei – groups of neuronal cell bodies within the
brain
 Brain – an integrating center made up of clusters of
nuclei
 Tracts – bundles of many axons within the CNS
 Nerve – a bundle of many axons outside of the
CNS
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Structure of a Nerve
 Bundles of myelinated and unmyelinated axons
enclosed in three layers of connective tissue
 Endoneurium – wraps each axon
 Perineurium – wraps a bundle (fascicle) of axons
 Epineurium – wraps the entire nerve
 Mixed nerves – contain both afferent and efferent
neurons
 Each neuron is either afferent (sensory) or efferent
(motor)
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Structure of a Nerve
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Figure 7.3
Nervous Systems Across Animal Groups
 Cephalization occurs in most animals and is more
apparent in more complex nervous systems
 Cnidarians and Echinoderms (sea cucumber) are
exceptions in that because they lack cephalization
 Organisms with more complex nervous systems
have more neurons; and therefore, more synapses
 Increased numbers of synapses allow for more
integration of information, and more complex
behaviors
 Since memories are stored in synapses, a complex
nervous system also allows for a greater potential for
learning
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Nervous Systems Across Animal Groups
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Figure 7.4
The Vertebrate Central Nervous System
 High degree of cephalization
 Unique in having a hollow dorsal nerve cord
(spinal cord)
 Part of the nervous system is encased within
cartilage or bone
 Central nervous system (CNS) – brain and spinal cord
 Part of the nervous system extends to the periphery
of the body
 Peripheral nervous system (PNS) – nerves outside of
the CNS
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Spinal nerves
The Vertebrate Central Nervous System
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Figure 7.5a
Cranial Nerves
Cranial nerves
 Exit directly from skull
 12/13 pairs (labeled with roman numerals)
 Some afferent, some efferent, some mixed
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Cranial Nerves
Table 7.1
“0” - “Terminal”, function unclear. Probably neuromodulatory, regulating
olfactory sensitivity and reproductive behaviour. Absent in cyclostomes, birds,
and humans.
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Spinal Nerves
Spinal nerves
 Branch from spinal cord
 Enter and exit between adjacent vertebrae
 Named based on region of vertebral column from
which they emerge
 Cervical, thoracic, lumbar, sacral, and coccygeal
 Mixed nerves
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Gray and White Matter
Brain and spinal cord contain two types of tissue
 Gray matter – neuronal cell bodies
 White matter – tracts of axons and their myelin
sheaths
 Spinal chord: white matter on surface, gray matter
inside
 Cerebral cortex: gray matter on surface, white
matter inside
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Gray and White Matter
Cross section of a mammalian spinal cord
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Figure 7.5b
Gray and White Matter
A coronal section through the human cerebrum
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Figure 7.10
The CNS Is Isolated and Protected
 Meninges
 Layers of connective tissue that surround brain and
spinal cord
 Number of meninges vary across taxa (fish have one,
mammals have three)
 Cerebral spinal fluid (CSF)
 Fills spaces within the CNS and acts as shock absorber
 Blood-brain barrier
 Tight junctions in brain capillary endothelium limit
passage of solutes from bloodstream into the CSF
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The CNS Is Isolated and Protected
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Figure 7.6
The Vertebrate Brain
 The brain is an extension of the spinal cord
 Nerve tracts extend between brain and spinal cord
 It has several cavities called ventricles that contain CSF
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The Vertebrate Brain
 Three main regions
 Rhombencephalon (hindbrain)
 Reflexes and involuntary behaviors
 Mesencephalon (midbrain)
 Coordination of sensory information
 Relay center in mammals
 Prosencephalon (forebrain)
 Integration of olfactory information with other senses
 Regulation of body temperature, reproduction, eating, emotion
 Learning and memory in mammals
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The Vertebrate Brain
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Figure 7.7
Brain Size and Morphology
Most groups of vertebrates have the same major brain structures,
although these structures vary in relative size
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Figure 7.9
Brain Size
 Much of the variation due to body size
 Birds and mammals have larger brains than other
vertebrates of same size
 Animals with large brains have more neurons
 More complex integrating centers and more behaviors
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Figure 7.8
The Parts of the Mammalian Brain
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Table 7.2
Hindbrain (Rhombencephalon)
Three regions
 Pons – located above medulla
 Pathway between medulla, cerebellum, and forebrain
 Controls alertness, initiates sleep and dreaming
 Cerebellum – two hemispheres at back of brain
 Responsible for motor coordination
 Contains half of the neurons in the brain
 Medulla oblongata – located at top of spinal cord
 Regulates breathing, heart rate, diameter of blood
vessels, blood pressure
 Contains pathways between spinal cord and brain
 Many cross over (e.g., left to right)
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Midbrain (Mesencephalon)
 Primary center for coordinating and initiating
behavioral responses in fish and amphibians
 Size and function reduced in mammals
 Primarily serves as relay center between spinal cord
and forebrain
 Sometimes grouped with the pons and medulla and
termed the brainstem
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Forebrain (Prosencephalon)
Involved in processing and integrating sensory
information, and in coordinating behavior
Main regions




Cerebrum
Thalamus
Epithalamus
Hypothalamus
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Cerebrum
Outer layer is the cortex
Divided into two cerebral hemispheres
 Left side controls right side of body
 Right side controls left side of body
 Neurons pass between the two sides via the corpus
callosum
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Figure 7.10
Hypothalamus
 Located at base of forebrain
 Under the thalamus
 Helps maintain homeostasis
 Body temperature, thirst, hunger, reproduction, etc.
 Interacts with autonomic nervous system
 Regulates secretion of pituitary hormones
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Limbic System
 A network of connected structures that lie between
the cortex and the rest of the brain
 Influences emotions, motivation, memory
 Sometimes called the “emotional brain”
 Includes hypothalamus and other parts:
 Amygdala – aggression and fear responses
 Hippocampus – converts short-term memory to longterm memory
 Olfactory bulbs – sense of smell
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Limbic System
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Figure 7.11
Thalamus and Epithalamus
 Thalamus
 Large grouping of gray matter above hypothalamus
 Part of the reticular formation (a poorly-differentiated
area of the brain stem)
 Receives input from limbic system and all senses
except olfaction
 Relays information to cortex
 Acts as a filter by blocking some afferent signals
 Epithalamus
 Located above the thalamus
 Habenular nuclei – communicates with the tegmentum
(forms the floor of the midbrain)
 Pineal complex – Establishes circadian rhythms and
secretes melatonin
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Cortex
 Integrates and interprets sensory information and
initiates voluntary movements
 Has taken over many of the midbrain functions of
lower vertebrates
 Isocortex (outer layer) necessary for cognition and
higher brain functions
 More folded in more advanced mammals
 Gyri (singular: gyrus) = folds
 Sulci (singular: sulcus) = grooves
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Cortex
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Figure 7.12b
Cortical Layers
 Six anatomically distinct layers
 Differ in shape and density of neurons
 Variable number of connections within each layer
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Figure 7.13
Cortical Lobes
Lobes named according
to their function or
overlying bones of the
skull
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Figure 7.14a,b
Topology of the Cerebral Cortex
 Each region of the cortex corresponds to a specific
part of the body that it controls by motor output, or
from which it receives sensory input
 Size of the brain region devoted to different parts of
the body varies widely
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Figure 7.?
Topology of the Cerebral Cortex
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Figure 7.15
Association Areas of the Cerebrum
 Receive information from adjacent areas and
further process and integrate the information
 Size of these areas is larger in animals with more
complex behaviors
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Peripheral Nervous System Divisions
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Figure 7.16
Autonomic Nervous System
“Involuntary nervous system”
Involved in homeostasis
Branches of the Autonomic Nervous System
 Sympathetic
 Most active during periods of stress or physical activity
 “Fight-or-flight” system
 Parasympathetic
 Most active during periods of rest
 “Resting and digesting” system
 Enteric
 Independent of other two systems
 Affects digestion by innervating the organs of the alimentary
canal
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Maintaining Homeostasis
Balancing of the sympathetic and parasympathetic
systems
Three mechanisms for regulating autonomic function
 Dual innervation
 Most organs receive input from both systems
 Antagonistic action
 One system stimulates while the other inhibits
 Basal tone
 Even under resting conditions autonomic neurons carry
APs
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Dual Innervation
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Figure 7.17
Antagonistic Action
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Table 7.3
Similarities in Autonomic Pathways
 Pathways contain two neurons in series
 Preganglionic
 May synapse with many postganglionic neurons and
intrinsic neurons
 Postganglionic
 Neurotransmitter is released at the effector organ from
varicosities
 The pre- and postganglionic neurons synapse with
each other in the autonomic ganglia
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Similarities in Autonomic Pathways
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Figure 7.18
Differences in Autonomic Pathways
Differences between the sympathetic (S) and
parasympathetic (PS) branches
 Preganglionic cell body location
 S – thoracic and lumbar regions of spinal cord
 PS – hindbrain and sacral region of spinal cord
 Ganglia location
 S – chain that runs close to spinal cord
 PS – close to the effector
 Number of postganglionic neurons that synapse with a
single preganglionic neuron
 S – ten or more
 P – three or fewer
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Differences in Autonomic Pathways
Neurotransmitter released at the effector organ
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Figure 7.19
Only Sympathetic Innervation
Some effectors receive
only sympathetic
innervation
 Adrenal medulla
 Collection of modified
postganglionic neurons
 Sweat glands
 Arrector pili muscles in
the skin
 Kidneys
 Most blood vessels
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Figure 7.20
Sympathetic vs. Parasympathetic Systems
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Table 7.4
Regulation of the Autonomic System
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Figure 7.21
Autonomic Reflex Arcs
 Most autonomic
changes occur via
simple neural circuits
that do not involve
conscious centers of
the brain
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Figure 7.22
Somatic Motor Pathways
“Voluntary nervous system”
 Control of skeletal muscles
 Usually under conscious control
 Cerebrum
 Some pathways are not under conscious control, for
example, knee-jerk reflex
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Somatic Motor Pathway Characteristics
 Control only one type of effector, skeletal muscle
 Cell bodies of motor neurons are located in the
CNS
 Monosynaptic
 Axons are very long, and extend all the way to the
muscle
 Axon splits into a cluster of axon terminals at the
neuromuscular junction
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Somatic Motor Pathway Characteristics
 Release the neurotransmitter acetylcholine
 Synaptic cleft between the motor neuron and the
muscle is very narrow
 Effect on the muscle cell always excitatory
 For example, causes depolarization and contraction
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Fig. 5.25
Animal Behaviors
Three categories
 Reflex behaviors
 Involuntary and simple
 Rhythmic behaviors
 Underlie locomotion, breathing, and the function of the
heart
 Voluntary behaviors
 Most complex and diverse
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Reflex Arcs
 Least complex integrated responses
 Can involve as few as two neurons (monosynaptic)
or more than two (polysynaptic)
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Figure 7.23 and Figure 7.25
Convergence and Divergence in Reflex Arcs
Neurons in reflex arcs can
be arranged in two ways:
 Convergence – allows
spatial summation
 Divergence – can
amplify signals
 Some reflex arcs have
both convergence and
divergence
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Figure 7.24
Rhythmic Behaviors
 Governed by pattern generators
 Groups of neurons that produce self-sustaining,
rhythmic depolarizations
 Two types of pattern generators
 Pacemaker cell
 A cell generates spontaneous depolarizations that control
the firing of all the cells in the network
 Emergent property of the network
 Rhythmic depolarization occurs because of the
organization of neurons in the network
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Swimming Behavior in the Leech
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Figure 7.26
Tetrapod Locomotion
 Involves pattern generators and reflexes
 The brainstem initiates the process and regulates speed
 The spinal cord acts as a pattern generator
 Afferent signals are sent back to the CNS
 The cortex is involved with guiding locomotion in
complex environments
 The cerebellum coordinates locomotion
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Tetrapod Locomotion
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Figure 7.27
Voluntary Movements
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Figure 7.28
Learning and Memory
 Most animals can learn and form memories due to
the plasticity of the nervous system
 Learning
 Process of acquiring new information
 Memory
 Retention and retrieval of information
 Plasticity
 Changes in synaptic and neuronal function in response to
stimuli
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Invertebrate Learning and Memory
 Well studied in the sea slug, Aplysia (~20,000
neurons)
 Habituation
 Decline in response to a stimulus after repeated
exposure
 Allows animal to ignore unimportant stimuli and focus
on novel stimuli
 Caused by changes in the presynaptic axon terminal
 Inactivation of Ca2+ channels   neurotransmitter
release
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Invertebrate Learning and Memory
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Figure 7.29
Invertebrate Learning and Memory
 Sensitization
 Increase in the response to a gentle stimulus after
exposure to a strong stimulus
 Caused by changes in the presynaptic axon terminal
 Involves a secondary circuit
 Serotonin released by facilitating interneuron 
 Binds to receptors 
 Activation of G-proteins 
 Inactivation of K+ channels 
  AP duration 
  Ca2+ influx 
  neurotransmitter release by sensory neuron
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Invertebrate Learning and Memory
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Figure 7.30
Mechanism of Serotonin’s Effects
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Figure 7.31
Memory in Mammals
 Hippocampus involved in long-term memory, but
memories are “stored” in cerebrum
 Memories are “stored” by increasing the efficiency of
the synapse between two neurons
 Long-term potentiation (LTP) – repetitive
stimulation of hippocampal tissue leads to an
increase in the response of the postsynaptic neuron
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Long-term Potentiation
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Figure 7.32