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Chapter 32
Neural Control
Sections 1-6
Albia Dugger • Miami Dade College
32.1 In Pursuit of Ecstasy
• Ecstasy (MDMA) is a psychoactive drug, similar in structure to
methamphetamine
• Drugs like MDMA flood the brain with signaling molecules and
saturate receptors, disrupting neural controls
• Repeated doses of MDMA may alter and even kill neurons in
the brain
• A bad reaction to MDMA can cause death
Meth and Ecstasy
methamphetamine
Ecstasy (MDMA)
Effect of Ecstasy
32.2 Evolution of Nervous Systems
• Interacting neurons give animals a capacity to respond to
stimuli in the environment and inside their body
• Neuron
• A cell that can relay electrical signals along its plasma
membrane and can communicate with other cells by
specific chemical messages
• Neuroglia
• Support neurons functionally and structurally
Nerve Nets
• Cnidarians are the simplest animals that have neurons, which
are arranged as a nerve net
• Nerve net
• A mesh of interconnecting neurons with no centralized
controlling organ
Cnidarian Nerve Net
A nerve net
(highlighted
in purple)
controls the
contractile
cells in the
epithelium.
Bilateral, Cephalized Invertebrates
• Flatworms are the simplest animals with a bilateral,
cephalized nervous system
• Cephalization
• The concentration of neurons that detect and process
information at the body’s head end
• Ganglion
• A cluster of neuron cell bodies that functions as an
integrating center
Nerve Cords
• Annelids and arthropods have paired ventral nerve cords
that connect to a simple brain, and a pair of ganglia in each
segment for local control
• Chordates have a single, dorsal nerve cord; vertebrates have
a brain at the anterior region of the nerve cord
Flatworm Cephalization
pair of
ganglia
pair of nerve cords connected
by lateral nerves
Insect with a Simple Brain
brain
nerve cords with ganglia
ANIMATED FIGURE: Bilateral nervous
systems
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Three Types of Neurons
• Sensory neurons detect stimuli and signal interneurons or
motor neurons
• Interneurons process information from sensory neurons and
send signals to motor neurons
• Motor neurons control muscles and glands
The Vertebrate Nervous System
• Central nervous system (CNS)
• Brain and spinal cord (mostly interneurons)
• Peripheral nervous system (PNS)
• Nerves from the CNS to the rest of the body (efferent) and
from the body to CNS (afferent)
• Autonomic nerves and somatic nerves control different
organs of the body
Nerves
• A nerve consists of nerve fibers bundled inside a sheath of
connective tissue
• Peripheral nerves are divided into two functional categories
•
Autonomic nerves regulate the body’s internal state; they
control smooth muscle, cardiac muscle, and glands
• Somatic nerves monitor body’s position and external
conditions; they control skeletal muscle
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
(cranial and spinal nerves)
Autonomic Nerves
Somatic Nerves
Nerves that carry signals
to and from smooth muscle,
cardiac muscle, and glands
Nerves that carry signals
to and from skeletal muscle,
tendons, and the skin
Sympathetic Parasympathetic
Division
Division
Two sets of nerves that often
signal the same effectors and
have opposing effects
Stepped Art
Figure 32-3 p543
Sensory stimuli, as from
the nose, eyes, and ears
Temporary storage in
the cerebral cortex
Input forgotten
SHORT-TERM MEMORY
Recall of
stored
input
Emotional state, having time
to repeat (or rehearse) input,
and associating the input with
stored categories of memory
influence transfer to long-term
storage
LONG-TERM MEMORY
Input irretrievable
Stepped Art
Figure 32-25 p559
Brain
cranial nerves
(twelve pairs)
cervical nerves
(eight pairs)
Spinal Cord
thoracic nerves
(twelve pairs)
ulnar nerve
(one in
each arm)
sciatic nerve
(one in each leg)
lumbar nerves
(five pairs)
sacral nerves
(five pairs)
coccygeal nerves
(one pair)
Figure 32-4 p543
Take-Home Message: What
are the features of
animal nervous systems?
• Cnidarians and echinoderms have a simple nervous system,
a nerve net with no central integrating organ.
• Bilateral animals have three types of neurons: sensory
neurons, interneurons, and motor neurons.
• Flatworms have paired ganglia that serve as an integrating
center. Other invertebrates have more complex brains.
• Bilateral invertebrates usually have a pair of ventral nerve
cords. In contrast, the chordates have a dorsal nerve cord.
• The vertebrate nervous system includes a well-developed
brain, a spinal cord, and peripheral nerves.
ANIMATION: Vertebrate nervous system
divisions
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32.3 Neurons: The Great Communicators
• Neurons have special cytoplasmic extensions for receiving
and sending messages
• Dendrites receive information from other cells
• Axons send chemical signals to other cells
• Sensory neurons have an axon with one end that responds to
stimuli; the other sends signals
• Interneurons and motor neurons have many dendrites and
one axon
Three Types of Neurons
receptor
endings
peripheral cell axon axon
axon
body
terminals
cell
body
axon
cell
body
axon
dendrites
dendrites
axon
terminals
A Motor Neuron
3
Conducting zone
axon
2
Trigger zone
1
Input zone
cell body
dendrites
4
Output zone
axon terminals
ANIMATED FIGURE: Neuron structure and
function
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Properties of the Neuron Plasma Membrane
• Neurons have electrical and concentration gradients across
their plasma membrane – their cytoplasm is more negatively
charged than the interstitial fluid outside the cell
• Negatively charged proteins and active transport of Na+ and
K+ ions maintain voltage difference across a cell membrane,
called the membrane potential
• An unstimulated neuron has a resting membrane potential
of about –70 mV
Resting Membrane Potential
150
Na+
5
interstitial
fluid
K+
plasma
membrane
15 Na+
150 K+
65
neuron’s
cytoplasm
Transport Proteins in a Neuron Membrane
interstitial fluid
3
Na+
2
K+
ADP + Pi
cytoplasm
A Sodium–potassium
cotransporters actively
transport three Na+
out of a neuron for
every two K+ they
pump in.
B Passive transporters
allow K+ ions to move
across the plasma
membrane, down
their concentration
gradient.
c Voltage-gated
channels for Na+ or
K+ are closed in a
neuron at rest (left),
but open when it is
excited (right).
Take-Home Message: How does a neuron’s
structure affect its function?
• Sensory neurons have an axon with one end that responds to
a specific stimulus and another that signals other cells.
• Interneurons and motor neurons have many signal-receiving
dendrites and one signal-sending axon.
• Transport proteins in the neuron plasma membrane set up
electrical and concentration gradients across the membrane
of a resting neuron.
• A neuron’s axon has special voltage-gated channel proteins
that function in the transmission of electrical signals along the
axon.
ANIMATION: Measuring membrane
potential
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32.4 The Action Potential
• When stimulated, neurons and muscle cells undergo an
action potential – a brief reversal in the electric gradient
across the plasma membrane
• During an action potential, membrane potential rises from its
resting potential (–70 mV) to a peak of +30 mV, then declines
to resting potential
Graded Potentials and Reaching Threshold
• Stimulation of a neuron’s input zone causes a local, graded
potential – a slight shift in the voltage difference across the
neuron’s membrane
• When stimulus in the neuron’s trigger zone reaches a
threshold potential, gated sodium channels open
• Voltage difference decreases and starts the action potential
An All-or-Nothing Spike
• Diffusion of sodium into the neuron has a positive feedback
effect – gated sodium channels open in an accelerating way
after threshold is reached
• Once threshold level is reached, membrane potential always
rises to the same level as an action potential peak (all-ornothing response)
• Outward diffusion of K+ causes membrane potential to decline
to a bit below its resting value in a small area
Propagation of an Action Potential
• An action potential is self-propagating – sodium ions diffuse to
the adjoining region of the axon, triggering sodium gates one
after another
• The action potential can only move one way, toward axon
terminals – a brief refractory period after sodium gates close
prevents the signal from moving backwards
Action Potential Membrane Potential
+30
Membrane potential (millivolts)
action potential
3
threshold
level
2
-60
resting
level
4
-70
1
0
1
2
3
Time (milliseconds)
4
5
6
Neuron at Rest
voltage-gated
ion channels
Threshold
Na+
Na+
Na+
Na+
Na+
Na+
K+ Channels Open
K+
K+
K+
Na+
Na+
Na+
K+ Channels Close
K+
K+
K+
Na+
Na+
ANIMATED FIGURE: Action potential
propagation
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Take-Home Message: What happens during an
action potential?
• An action potential begins in the neuron’s trigger zone. A
strong stimulus decreases the voltage difference across the
membrane. This causes gated sodium channels to open, and
the voltage difference reverses.
• The action potential travels along an axon as consecutive
patches of membrane undergo reversals in membrane
potential.
Take-Home Message (cont.)
• At each patch of membrane, an action potential ends as
sodium channels close and potassium channels open.
Potassium ions flow out of the neuron and restore the voltage
difference across the membrane.
• Action potentials can move in one direction, toward axon
terminals, because gated sodium channels are briefly
inactivated after they close
32.5 How Neurons
Send Messages to Other Cells
• An action potential travels along a neuron’s axon to a
terminal at the tip
• Terminal sends chemical signals to a neuron, muscle fiber, or
gland cell across a synapse
Chemical Synapses
• A synapse is the region where an axon terminal (presynaptic
cell) send chemical signals to a neuron, muscle fiber or gland
cell (postsynaptic cell)
• The synapse between a motor neuron and a skeletal muscle
fiber is called a neuromuscular junction
Chemical Synapses
• Action potentials trigger release of signaling molecules
(neurotransmitters) from vesicles in the presynaptic terminal
into the synaptic cleft
• A motor neuron in a neuromuscular junction releases the
neurotransmitter acetylcholine (ACh)
Chemical Synapses
• Release of neurotransmitters from presynaptic vesicles
requires an influx of calcium ions, Ca++
• Postsynaptic membrane receptors bind the neurotransmitter
and initiate the response
• The neurotransmitter must be cleared from the synapse after
the signal is transmitted
A Neuromuscular Junction
axon of
a motor
neuron
neuromuscular
junction
Figure 32-9a p548
axon terminal of
motor neuron
plama membrane
of muscle fiber
synaptic
vesicle
Ca++
2
3
4
synaptic cleft
Figure 32-9b p548
binding site for
neurotransmitter
(no neurotransmitter
bound)
ion channel closed
Figure 32-9d p548
neurotransmitter
ion flows through
now-open channel
Figure 32-9d p548
3D ANIMATION:
Neurons: Synaptic Transmissions
Synaptic Integration
• A neurotransmitter may have excitatory or inhibitory effects on
a postsynaptic cell
• Typically, a postsynaptic cell receives messages from many
neurons at the same time
• Through synaptic integration a neuron sums all excitatory
and inhibitory signals arriving at a postsynaptic cell at the
same time
Synaptic Density
Synaptic Integration
Take-Home Message: How does information
pass between cells at a synapse?
• Action potentials travel to a neuron’s output zone. There they
stimulate release of neurotransmitters—chemical signals that
affect another cell.
• Neurotransmitters are signaling molecules secreted into a
synaptic cleft from a neuron’s output zone. They may have
excitatory or inhibitory effects on a postsynaptic cell.
Take-Home Message (cont.)
• Synaptic integration is the summation of all excitatory and
inhibitory signals arriving at a postsynaptic cell’s input zone
at the same time.
• For a synapse to function properly, neurotransmitter must be
cleared from the synaptic cleft after the chemical signal has
served its purpose.
ANIMATION: Chemical synapse
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33.6 A Smorgasbord of Signals
• There are a variety of neurotransmitters
• Neurological disorders and psychoactive drugs interfere with
their action
Neurotransmitter Discovery
• In the early 1920s, Otto Loewi discovered that the
neurotransmitter ACh controls heart rate
• ACh also acts on skeletal muscle, smooth Muscle, many
glands, and the brain
• Each type of tissue has a different kind of ACh receptor, so
ACh elicits different responses in different cells
Neurotransmitter Diversity
• The body produces many kinds of neurotransmitters:
• Norepinephrine and epinephrine (adrenaline) prepare the
body for stress or excitement
• Dopamine influences reward-based learning and acts in
fine motor control
• Serotonin influences mood and memory
• Glutamate excitates the central nervous system
• GABA (gamma aminobutyric acid) has a general inhibitory
effect on release of other neurotransmitters
Table 32-1 p550
Neuromodulators
• Neuromodulators
• Neuropeptides made by some neurons that influence the
effects of neurotransmitters
• Substance P enhances pain
• Enkephalins and endorphins are pain killers
Disrupted Signaling
• Many disorders of the nervous system involve disruption of
signaling at synapses:
• Alzheimer’s disease (dementia) involves damage to
neurons and lowered levels of ACh in the brain
• Parkinson’s disease involves dopamine-secreting
neurons in the motor-control part of the brain
• Attention deficit hyperactivity disorder (ADHD) also
involves low levels of dopamine
• Depression and anxiety disorders may involve low levels
of several neurotransmitters
Battling Parkinson’s Disease
Psychoactive Drugs
• Psychoactive drugs exert their effects by interfering with the
action of neurotransmitters
• Stimulants (nicotine, caffeine, cocaine, amphetamines)
• Depressants (alcohol, barbiturates)
• Analgesics (narcotics, ketamine, PCP)
• Hallucinogens (LSD, THC)
Take-Home Message: How do disorders and
drugs affect the nervous system?
• Neurological disorders lower the amount of a neurotransmitter
or the balance among neurotransmitters.
• Psychoactive drugs act by stimulating release, inhibiting
breakdown, or mimicking the action of natural
neurotransmitters. Many are addicting, and using them can
alter the body’s ability to produce neurotransmitter.
Video: Exploring Neurotransmitters