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Chapter 49
Nervous Systems
Nervous systems consist of circuits of
neurons and supporting cells
• The simplest animals with nervous
systems, the cnidarians, have neurons
arranged in nerve nets
• A nerve net is a series of
interconnected nerve cells
• More complex animals have nerves
• Nerves are bundles that consist of the
axons of multiple nerve cells
Fig. 49-2a
Nerve nets
Radial
nerve
Nerve
ring
Nerve net
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
Characteristic of animals with radial symmetry.
• Bilaterally symmetrical animals
exhibit cephalization
• Cephalization is the clustering of
sensory organs at the front end of the
body
• Relatively simple cephalized animals,
such as flatworms, have a central
nervous system (CNS)
• The CNS consists of a brain and
longitudinal nerve cords
Fig. 49-2
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)
• Nervous system
organization usually
correlates with lifestyle
• Sessile molluscs (e.g.,
clams and chitons) have
simple systems, whereas
more complex molluscs
(e.g., octopuses and
squids) have more
sophisticated systems
• In vertebrates
–The CNS is composed of the brain
and spinal cord
–The peripheral nervous system
(PNS) is composed of nerves and
ganglia
Fig. 49-4
Central nervous
system (CNS)
Brain
Spinal
cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Organization of the Vertebrate
Nervous System
• The spinal cord conveys information from
the brain to the PNS
• The spinal cord also produces reflexes
independently of the brain
• A reflex is the body’s automatic response to
a stimulus
– For example, a doctor uses a mallet to trigger
a knee-jerk reflex
Fig. 49-3
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
• The central canal of the spinal cord and
the ventricles of the brain are hollow and
filled with cerebrospinal fluid
• The cerebrospinal fluid is filtered from
blood and functions to cushion the brain
and spinal cord
• The brain and spinal cord contain
–Gray matter, which consists of
neuron cell bodies, dendrites, and
unmyelinated axons
–White matter, which consists of
bundles of myelinated axons
Fig. 49-5
Gray matter
White
matter
Ventricles
The Peripheral Nervous System
• The PNS transmits information to
and from the CNS and regulates
movement and the internal
environment
• In the PNS, afferent neurons
transmit information to the CNS and
efferent neurons transmit
information away from the CNS
• Cranial nerves
originate in the
brain and mostly
terminate in organs
of the head and
upper body
• Spinal nerves
originate in the
spinal cord and
extend to parts of
the body below the
head
Fig. 49-7-2
PNS
Afferent
(sensory) neurons
Efferent
neurons
Autonomic
nervous system
Motor
system
Locomotion
Sympathetic
division
Parasympathetic
division
Hormone
Gas exchange Circulation action
Hearing
Enteric
division
Digestion
• The PNS has two functional
components: the motor system and
the autonomic nervous system
• The motor system carries signals to
skeletal muscles and is voluntary
• The autonomic nervous system
regulates the internal environment
in an involuntary manner
• The autonomic nervous system has
sympathetic, parasympathetic, and
enteric divisions
• The sympathetic and
parasympathetic divisions have
antagonistic effects on target
organs
• The sympathetic division correlates
with the “fight-or-flight” response
• The parasympathetic division
promotes a return to “rest and digest”
• The enteric division controls activity
of the digestive tract, pancreas, and
gallbladder
Fig. 49-8a
Parasympathetic division
Sympathetic division
Action on target organs:
Action on target organs:
Constricts pupil
of eye
Dilates pupil
of eye
Stimulates salivary
gland secretion
Inhibits salivary
gland secretion
Constricts
bronchi in lungs
Slows heart
Stimulates activity
of stomach and
intestines
Stimulates activity
of pancreas
Stimulates
gallbladder
Cervical
Sympathetic
ganglia
Fig. 49-8b
Sympathetic division
Parasympathetic division
Relaxes bronchi
in lungs
Accelerates heart
Thoracic
Inhibits activity
of stomach and
intestines
STRESS
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla
Promotes emptying
of bladder
Promotes erection
of genitals
Inhibits emptying
of bladder
Sacral
Synapse
Promotes ejaculation and
vaginal contractions
Neurotransmitters
• Parasympathetic – acetylcholine
• Sympathetic – norepinephrine or
acetylcholine depending on location of
ganglia
• Cerebrum – integrates sensory and motor
information, thinking (cortex)
• Brainstem – regulates involuntary
responses (breathing, heart rate,
digestion)
• Cerebellum – balance and movement
• Thalamus – sorts and relays information to
cerebrum
• Hypothalamus – homeostatic regulation,
secretes hormones
• Pituitary gland – secretes hormones
What is lateralization of the
cerebrum?
• Lateralization means that some
functions are carried out exclusively on
one side of the brain (e.g., speech,
which is on the left side of the brain in
most people).
• Left side of cerebrum controls right
side of body and vice versa.
Left Hemisphere
Right Hemisphere
Speech
Music and art appreciation,
drawing ability
Movement of the right side of
the body
Movement of the left side of the
body
Sensation on the right side of
the body
Sensation on the left side of the
body
Vision in the right half of the
"visual field"
Vision in the left half of the
"visual field
The corpus callosum in involved with
communication between the
hemispheres.
Fig. 49-15
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Somatosensory
association
area
Taste
Reading
Speech
Hearing
Smell
Auditory
association
area
Visual
association
area
Vision
Temporal lobe
Occipital lobe
Studying the Brain
http://video.nationalgeographic.com/video/science/he
alth-human-body-sci/human-body/brain-bank-sci/
Phineas Gage: A Rod Went
Through His Skull
Memory and Learning
• Learning can occur when neurons
make new connections or when the
strength of existing neural
connections changes
• Glutamate is often involved.
Fig. 49-20a
Ca2+
Na+
Glutamate
NMDA receptor
(open)
Mg2+
Stored
AMPA
receptor
(a) Synapse prior to long-term potentiation (LTP)
NMDA
receptor
(closed)
Fig. 49-20b
1
3
2
(b) Establishing LTP
Fig. 49-20c
3
4
1
2
(c) Synapse exhibiting LTP
Nervous system disorders can be
explained in molecular terms
• Disorders of the nervous system include
schizophrenia, depression, addiction,
Alzheimer’s disease, and Parkinson’s
disease
• Genetic and environmental factors
contribute to diseases of the nervous
system
Drug Addiction and the Brain
Reward System
• The brain’s reward system rewards
motivation with pleasure
• Some drugs are addictive because they
increase activity of the brain’s reward
system
• These drugs include cocaine, amphetamine,
heroin, alcohol, and tobacco
• Drug addiction is characterized by
compulsive consumption and an inability to
control intake
• Addictive drugs enhance the activity of the
dopamine pathway
• Drug addiction leads to long-lasting
changes in the reward circuitry that cause
craving for the drug
Fig. 49-22
Nicotine
stimulates
dopaminereleasing
VTA neuron.
Opium and heroin
decrease activity
of inhibitory
neuron.
Cocaine and
amphetamines
block removal
of dopamine.
Cerebral
neuron of
reward
pathway
Reward
system
response
Stem Cell–Based Therapy
• Unlike the PNS, the CNS cannot fully repair
itself
• However, it was recently discovered that
the adult human brain contains stem cells
that can differentiate into mature neurons
• Induction of stem cell differentiation and
transplantation of cultured stem cells are
potential methods for replacing neurons
lost to trauma or disease