THE NERVOUS SYSTEM
CHAPTER 28
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• The nervous system consists of neurons and
supporting cells.
• Association neurons (or interneurons) are located
in the brain and spinal cord of vertebrates,
together called the central nervous system (CNS).
• They help provide more complex reflexes and,
in the case of the brain, higher associative
functions, including learning and memory.
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• Sensory neurons that carry
impulses from sensory
receptors to the CNS.
• Motor neurons that carry
impulses away from the
the CNS to effectors
(muscles and glands).
• Together, the motor and
sensory neurons constitute
the peripheral nervous
system (PNS) of
vertebrates.
Association
neuron
Cell
body
Cell body
Axon
2
Dendrites
3
Cell body
Axon
Motor
neuron
Axon
1
Sensory
neuron
Direction of
conduction
Dendrites
ORGANIZATION OF THE VERTEBRATE
NERVOUS SYSTEM
Central
nervous system
“Integration center’’
(Association
neurons of brain and
spinal cord)
Motor
nervous system
Sensory
nervous system
(Efferent
nerves)
(Afferent
nerves)
Muscles
Organs
Glands
Sensory receptors
Peripheral
nervous
system
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• Sponges are the only major phylum of
animals that lack nerves.
• Sponges respond minimally to stimuli and do not
send messages from one part of the body to
another.
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• The simplest nervous
systems occur among
cnidarians.
• Cnidarian neurons are
linked to one another in a
web, or nerve net.
• There is no associative
activity and little
coordination.
• Any motion that results is
called a reflex because it
is an automatic
consequence of
stimulation.
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• The first associative
activity in nervous systems
is seen in free living
flatworms, phylum
Platyhelminthes.
• These animals have a
ladderlike nervous system
with two nerve cords.
• The two cords converge at
an enlarged association
area that functions as a
primitive brain.
EVOLUTION OF THE ANIMAL
NERVOUS SYSTEM
• A series of evolutionary
changes lead from the
flatworms to the vertebrate
nervous system:
• More sophisticated sensory
mechanisms.
• Differentiation into central and
peripheral nervous systems.
• Differentiation of sensory and
motor areas.
• Increased complexity of
association.
• Elaboration of the brain.
Nerve
net
1 Cnidarian
Associative
neurons
Nerve
cords
2 Flatworm
Central nervous
system
Brain
Peripheral
nerves
3 Earthworm
Brain
Ventral
nerve
cords
Giant
axon
5 Arthropod
4 Mollusk
NEURONS GENERATE NERVE
IMPULSES
• All neurons have the same basic structure:
cell body, dendrites, and axon.
Cell body
Nucleus
Dendrites
Schwann
cell
Schwann
cell
Axon
Nucleus
Axon
(a)
Myelin
sheath
Node of
Ranvier
Schwann
cell
Axon
(b)
Myelin sheath
NEURONS GENERATE NERVE
IMPULSES
• Most neurons depend upon support from
neuroglial cells.
• Schwann cells and oligodendrocytes are
important supporting cells that envelop the axons
of many neurons with a sheath of fatty material
called myelin.
• Myelin acts as an electrical insulator.
NEURONS GENERATE NERVE
IMPULSES
• Schwann cells produce myelin in the PNS
while oligodendrocytes produce myelin in
the CNS.
• The myelin is wrapped around the axon as a
myelin sheath comprised of multiple layers.
• The myelin sheath is interrupted at intervals,
leaving unmyelinated gaps called nodes of
Ranvier.
• Nerve impulses jump from node to node, greatly
increasing the speed of transmission.
• In multiple sclerosis and Tay-Sachs disease, the myelin
sheath degenerates.
NEURONS GENERATE NERVE
IMPULSES
• When a neuron is “at rest,” active transport
channels (sodium-potassium pumps) in the
neuron transport Na+ out of the cell and K+
ions in.
• The result is to make the outside of the membrane
more positive than the inside, a condition called
the resting membrane potential.
NEURONS GENERATE NERVE
IMPULSES
• Neurons are constantly expending energy to
maintain the resting membrane potential.
• The voltage difference between the neuron
interior and exterior is –70 millivolts.
• The resting potential is the starting point for a
nerve impulse.
NEURONS GENERATE NERVE
IMPULSES
• A nerve impulse travels along the axon and
dendrites as electrical current caused by
ions moving in and out of the neuron
through voltage-gated channels.
• These membrane channels open and close in
response to electrical voltage changes.
• The impulse starts when pressure or other
sensory inputs disturb a neuron’s plasma
membrane, causing Na+ channels to open.
NEURONS GENERATE NERVE
IMPULSES
• When Na+ channels open, Na+ floods into
the neuron from the outside.
• For a brief moment, the inside of the neuron is
“depolarized,” becoming more positive.
• The open Na+ channels in the small patch of
depolarized membrane remain open for only one
half of a millisecond.
• If the voltage change of the depolarization is
great enough, it causes nearby voltage-gated
Na+ and K+ channels to open.
NEURONS GENERATE NERVE
IMPULSES
1
Na+ channel
• The Na+ channels open first,
which starts a wave of
depolarization moving down
the neuron.
• This moving local reversal of
voltage is called an action
potential.
• An action potential follows an allor-none law: a large enough
depolarization produces either a
full action potential or none at all.
K+
Voltage-gatedK+ channel
channels
+ + + +
– – – –
Na+
+ + +
– – –
– Na–K
pump– – – – –
+ Na+ + + + + +
Na+
At the resting membrane potential, the inside of the axon is
negatively charged because the sodium-potassium pump keeps
a higher concentration of Na+ outside. Voltage-gated ion channels
are closed, but there is some leakage of K+.
2
K+
– – – + + + + + +
+ + + – – – – – –
Na+
+ + + – – – – – –
– – – + + + + + +
Na+
In response to a stimulus, the membrane depolarizes: Voltagegated Na+ channels open, Na+ flows into the cell, and the inside
becomes more positive.
3
K+
K+
+ + – – – + + + +
– – + + + – – – –
Na+
Na+
– – + + + – – – –
+ + –+ – – + + + +
Na+
K
The local change in voltage opens adjacent voltage-gated Na+
channels, and an action potential is produced.
4
K+
K+
K+
+ + + + – – – + +
– – – – + + + – –
Na+
– – – – + + + – –
+ + + + – – – + +
K+
As the action potential travels farther down the axon, voltage-gated
Na+ channels close and K+ channels open, allowing K+ to flow out
of the cell and restoring the negative charge inside the cell.
Ultimately, the sodium-potassium pump restores the resting
membrane potential.
NEURONS GENERATE NERVE
IMPULSES
• The K+ voltage-gated channels open after a
slight delay, causing K+ to flow out of the cell
• this makes the interior of the neuron more
negative, causing the voltage-gated Na+
channels to close.
• The period of time after an action potential
has passed but before the resting potential
is restored is called the refractory period.
http://youtu.be/XdCrZm_JAp0
THE SYNAPSE
• Axons do not actually make direct contact
with other neurons or with other cells.
• A narrow gap called the synaptic cleft, separates
the axon tip and the target neuron or tissue.
• This junction of an axon with another cell is called
a synapse.
• The membrane on the axon side is called the
presynaptic membrane.
• The membrane on the receiving side is called
the postsynaptic membrane.
A SYNAPSE BETWEEN TWO NEURONS
Synaptic
cleft
Axon of
presynaptic
cell
Postsynaptic
cell
Presynaptic
vesicles
Presynaptic
membrane
Postsynaptic
membrane
© Dennis Kunkel/Phototake
THE SYNAPSE
• Signals from an axon are carried across the
synapse by chemical messengers called
neurotransmitters.
• These chemicals are packaged in tiny sacs, or
vesicles, at the tip of the axon.
• When a nerve impulse reaches the end of an
axon, it causes the vesicles to release the
neurotransmitters into the synaptic cleft.
• Chemically-gated channels on the postsynaptic membrane then respond by allowing
ions to enter.
EVENTS AT THE SYNAPSE
Neurotransmitter
Gate closed
Axon
terminal
Chemically-gated
ion channel
Mitochondria
Synaptic
vesicles
Synaptic
cleft
Postsynaptic
cell skeletal
muscle)
(a)
(b)
THE SYNAPSE
• The vertebrate nervous system uses dozens
of different kinds of neurotransmitters.
• They fall into two classes, depending on
whether they excite or inhibit the
postsynaptic cell.
• In an excitatory synapse, the chemically-gated
channel is usually a Na+ channel.
• This can lead to an action potential.
• In an inhibitory synapse, the chemically-gated
channel is usually a K+ or Cl– channel.
• This prevents an action potential.
THE SYNAPSE
• An individual nerve cell can possess both
excitatory and inhibitory synapses.
• Integration must occur in which the various
excitatory and inhibitory electrical effects tend to
cancel or reinforce one another.
• If the result of the integration is a large enough
depolarization, an action potential will fire.
INTEGRATION
Axon hillock
Axon
(right): © E.R. Lewis, YY Zeevi, T.E. Everhart, U. of California/Biological Photo Service
A single motor neuron in the spinal cord may have as
many as 50,000 synapses!
THE SYNAPSE
• Examples of neurotransmitters:
• Acetylcholine (Ach)
• Used at the synapse between a nerve and a
muscle fiber, called a neuromuscular junction.
• It is used in excitatory synapses in skeletal
muscle and in inhibitory synapses in cardiac
muscle.
• Glycine and GABA
• These are inhibitory neurotransmitters, especially
in neural controls of body movements.
THE SYNAPSE
• Biogenic amines are a group of
neurotransmitters including:
• Dopamine, important in controlling body
movements.
• Norepinephrine and epinephrine, which are
involved in the autonomic nervous system.
• Serotonin, which is involved in sleep regulation
and other emotional states.
ADDICTIVE DRUGS ACT ON
CHEMICAL SYNAPSES
• Emotional states (mood,
pleasure, pain, etc.) are
determined by particular
groups of neurons that use
special sets of
Serotonin
neurotransmitters and
neuromodulators.
• Many researchers think that
depression results from a
Prozac
shortage of serotonin.
blocks
• Prozac, an antidepressant, reabsorption
inhibits the reabsorption of
serotonin.
Receptor
ADDICTIVE DRUGS ACT ON
CHEMICAL SYNAPSES
• Nerve cells are particularly prone to the loss
of sensitivity when exposed to a chemical
signal for a long time.
• If receptor proteins within synapses are exposed
to high levels of neurotransmitters for prolonged
periods, the nerve cell often responds by inserting
fewer receptor proteins into the membrane.
ADDICTIVE DRUGS ACT ON
CHEMICAL SYNAPSES
• When receptor proteins in the pleasure
pathways of the brain are exposed to high
levels of dopamine due to cocaine, for
example, the nerve cells respond by
lowering the number of receptor proteins.
• With so few receptors, the drug user needs the
drug to maintain even normal nerve activity
levels.
• This is addiction, the physiological adaptation of
the nervous system due to drug abuse.
KEY BIOLOGICAL PROCESS: DRUG
ADDICTION
1
Neurotransmitter
2
3
4
Transporter
protein
Synapse
Receptor protein
At a normal synapse, neurotransmitters are quickly
recycled by transporter
proteins, so the firing rate of
receptor proteins stays low.
Drug molecule
Drug molecules like cocaine
bind to the transporters and
block recycling, so the level
of neurotransmitters rises,
and the firing rate increases.
The receiving neuron “turns
down the volume” by lowering
the number of receptors,
so the firing rate returns to
normal.
If the cocaine is removed,
the level of neurotransmitters
falls to normal, too low to
fire the reduced number of
receptors.
EVOLUTION OF THE VERTEBRATE
BRAIN
• The brain is the most complex vertebrate
organ ever to evolve, and it can perform a
bewildering variety of complex functions.
EVOLUTION OF THE VERTEBRATE
BRAIN
• The basic organization of the vertebrate
brain can be seen in the brains of primitive
fishes.
Thalamus
Cerebrum
Optic tectum
Cerebellum
Olfactory bulb
Spinal cord
Optic chiasm
Medulla oblongata
Hindbrain
(Rhombencephalon)
Pituitary
Hypothalamus
Midbrain
(Mesencephalon)
Forebrain
(Prosencephalon)
EVOLUTION OF THE
VERTEBRATE BRAIN
• The brain is divided into three regions that
are found in differing proportions in all
vertebrates:
• 1) The hindbrain, or rhombencephalon - The
largest portion of the brain in fishes.
• 2) The midbrain, or mesencephalon - Devoted
primarily to processing visual information in fishes.
• 3) The forebrain, or prosencephalon - Concerned
mainly with olfaction (smell) in fishes.
• Plays a far more dominant role in neural
processing in terrestrial vertebrates than in
fishes.
THE EVOLUTION OF THE VERTEBRATE
BRAIN
Shark
Frog
Crocodile
Cat
Spinal cord
Medulla oblongata
Human
Cerebellum
Optic tectum
Midbrain
Cerebrum
Olfactory tract
Bird
EVOLUTION OF THE VERTEBRATE
BRAIN
• In sharks and other fishes, the hindbrain is
predominant, and the rest of the brain
serves primarily to process sensory
information.
• In amphibians and reptiles, the forebrain is
far larger, and it contains a larger cerebrum
devoted to associative activity.
• In birds, which evolved from reptiles, the
cerebrum is even more pronounced.
• In mammals, the cerebrum is the largest
portion of the brains.
HOW THE BRAIN WORKS
• The cerebrum is the center
for thought and association
and makes up about 85% of
the weight of the brain.
• The cerebrum is divided into
right and left halves, called
cerebral hemispheres.
• Much of the neural activity of
the cerebrum occurs within a
thin, gray outer layer called
the cerebral cortex.
Thalamus
Corpus striatum
Cerebral
cortex
Corpus
callosum
Skull
Pineal
gland
Pons
Hypothalamus
Pituitary
gland
Reticular
formation
Medulla
oblongata
Cerebellum
Spinal
cord
HOW THE BRAIN WORKS
• The cerebral cortex is gray because it is
densely packed with cell bodies.
• The wrinkles on the surface of the cerebral cortex
increase its surface area (and number of cell
bodies) threefold.
• Underneath the cortex is a solid white region
of myelinated nerve fibers that shuttle
information between the cortex and the rest
of the brain.
HOW THE BRAIN WORKS
• Different areas of the cerebral cortex
control different body activities.
Frontal
lobe
Visual association area
Cerebrum
Vision
Occipital
lobe
Smell
Temporal
lobe
Medulla oblongata
Balance
and
coordination
Primary visual area
Cerebellum
Spinal cord
HOW THE BRAIN WORKS
• The right and left cerebral hemispheres are
linked by bundles of neurons called tracts.
• The tracts serve as information highways telling
each half of the brain what the other half is
doing.
• These tracts cross over so that each half of the
brain controls muscles and glands on the
opposite side of the body.
HOW THE BRAIN WORKS
• Beneath the cerebrum
are the thalamus and
hypothalamus.
Hypothalamus
• The thalamus is the
major site of sensory
processing in the brain.
• The hypothalamus
Amygdala
integrates internal
activities.
Hippocampus
• It controls centers in the brain stem that regulate the
internal environment of the body, such as heartbeat,
temperature, blood pressure, and respiration rate.
• It also directs secretions of the pituitary gland.
Thalamu
HOW THE BRAIN WORKS
• Extending back from the base of the brain is
a structure known as the cerebellum.
• It controls balance, posture, and muscular
coordination.
• It is best developed in birds to control the
complicated balance and coordination
associated with flight.
HOW THE BRAIN WORKS
• The brain stem connects the rest of the brain
to the spinal cord and includes:
• the midbrain
• pons
• medulla oblongata
• The brain stem contains nerves that control
functions, such as breathing, swallowing,
digestion, heartbeat, and diameter of blood
vessels.
HOW THE BRAIN WORKS
• The two hemispheres of the cerebrum are
responsible for different activities.
• Language is lateralized to one dominant
hemisphere, usually the left.
• The nondominant hemisphere is adept at spatial
reasoning.
DIFFERENT BRAIN REGIONS CONTROL
VARIOUS ACTIVITIES
THE SPINAL CORD
• The spinal cord is a cable of neurons
extending from the brain down through the
backbone.
• The spinal cord is surrounded and protected by a
series of bones called the vertebrae.
• Spinal nerves pass out to the body from between
the vertebrae.
THE VERTEBRATE NERVOUS SYSTEM
Cerebrum
Grey
matter
White
matter
Femoral
nerve
Sciatic
nerve
Tibial
nerve
Sacral
nerves
Lumbar
nerves
Thoracic
nerves
Cerebellum
Cervical
nerves
Thalamusand
hypothalamus
(surroundedby
the cerebrum)
Spinal
cord
VOLUNTARY AND AUTONOMIC
NERVOUS SYSTEMS
• The motor pathways of the PNS can be
further divided into:
• The somatic (voluntary) nervous system
• Relays commands to the skeletal muscles
• The autonomic (involuntary) nervous system
• Relays commands to the smooth muscles
of the body and to cardiac muscle
THE DIVISIONS OF THE VERTEBRATE
NERVOUS SYSTEM
Nervous system
Central
nervous
system
Brain
Peripheral
nervous
system
Spinal
cord
Sensory
pathways
Somatic
(voluntary)
nervous system
Sympathetic
division
Motor
pathways
Autonomic
(involuntary)
nervous system
Parasympathetic
division
VOLUNTARY AND AUTONOMIC
NERVOUS SYSTEMS
• Motor neurons of the voluntary system
stimulate muscles to contract in two ways.
• First, muscles are stimulated to contract in
response to conscious commands.
• Second, muscles can be stimulated as part of
reflexes (does not require conscious control.
VOLUNTARY AND AUTONOMIC
NERVOUS SYSTEMS
SPINAL CORD
White
Gray
matter matter
• A reflex produces a
rapid motor response
to a stimulus because
a sensory neuron
passes information Monosynaptic
synapse
directly to a motor
neuron.
Motor
• Useful for the body to
neuron
react particularly quicklyQuadriceps
muscle
in time of danger.
(effector)
• Many reflexes never
reach the brain.
Dorsalroot
ganglion
Sensory
neuron
Stretch
receptor
Patella
Patellar
tendon
VOLUNTARY AND AUTONOMIC
NERVOUS SYSTEMS
• Neurons of the autonomic nervous system
carry messages from the CNS that keep the
body going even when it is not active.
• The CNS uses this system to maintain the body’s
homeostasis.
• It is comprised of two elements that act in
opposition to one another:
• Sympathetic nervous system that dominates in
times of stress.
• Parasympathetic nervous system that does the
opposite and conserves energy.