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Neural Control
Chapter 33 Part 1
Impacts, Issues
In Pursuit of Ecstasy
 Neural controls maintain life; drugs like Ecstasy
flood the brain with signaling molecules and
saturate receptors, disrupting these controls
Fig. 33-1a, p. 552
Fig. 33-1b, p. 552
Fig. 33-1c, p. 552
33.1 Evolution of Nervous Systems
 Interacting neurons allow animals 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
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 Cnidarian Nerve Net
 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
Bilateral, Cephalized Nervous System
 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
• 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
Simple Nervous Systems
Fig. 33-2a, p. 554
a nerve net
(highlighted
in purple)
controls the
contractile
cells in the
epithelium
Hydra, a cnidarian
Fig. 33-2a, p. 554
Fig. 33-2b, p. 554
pair of
ganglia
pair of nerve
cords crossconnected
by lateral
nerves
Planarian, a flatworm
Fig. 33-2b, p. 554
Fig. 33-2c, p. 554
rudimentary
brain
ventral
nerve cord
ganglion
c Earthworm, an annelid
Fig. 33-2c, p. 554
Fig. 33-2 (d-e), p. 554
brain
brain
optic lobe
(one pair, for
visual stimuli)
branching
nerves
paired ventral
nerve cords
ganglion
Crayfish, a crustacean
(a type of arthropod)
Grasshopper, an insect
(a type of arthropod)
Fig. 33-2 (d-e), p. 554
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
Functional Divisions
of the Vertebrate Nervous System
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
(cranial and spinal nerves)
Autonomic Nerves
Nerves that carry signals
to and from smooth muscle,
cardiac muscle, and glands
Somatic Nerves
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
Fig. 33-3, p. 555
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
Fig. 33-3, p. 555
Major Nerves
of the Human Nervous System
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)
Fig. 33-4, p. 555
33.1 Key Concepts
How Animal Nervous Tissue is Organized
 In radially symmetrical animals, excitable
neurons interconnect as a nerve net
 Most animals are bilaterally symmetrical with a
nervous system that has a concentration of
neurons at the anterior end and one or more
nerve cords running the length of the body
33.2 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
A Motor Neuron
dendrites
input zone
cell body
trigger zone
conducting zone
axon
output zone
axon terminals
Fig. 33-5, p. 556
Animation: Neuron structure and
function
Direction of Information Flow
receptor peripheral cell axon axon
endings
axon
body
terminal
cell
body
axon
cell axon
body
axon
terminals
dendrites
dendrites
a sensory neuron
b interneuron
c motor neuron
Fig. 33-6, p. 556
receptor peripheral cell axon axon
endings
axon
body
terminal
cell
body
axon
cell axon
body
axon
terminals
dendrites
dendrites
a sensory neuron
b interneuron
c motor neuron
Stepped Art
Fig. 33-6, p. 556
33.3 Membrane Potentials
 Resting membrane potential
• The interior of a resting neuron is more negative
than the fluid outside the cell (-70 mV)
• Negatively charged proteins and active transport
of Na+ and K+ ions maintain the resting potential
150 Na+
interstitial
fluid
5 K+
plasma
membrane
15 Na+
150 K+
65
neuron’s
cytoplasm
p. 557
Action Potentials
 Action potential
• An abrupt reversal in the electric gradient across
the plasma membrane
• When properly stimulated, voltage-gated
channels open, ions flow through, and the
membrane potential briefly reverses
Membrane Proteins: Pumps,
Transporters, and Gated Channels
interstitial fluid
neuron cytoplasm
A Sodium–potassium
pumps actively transport
3 Na+ out of a neuron for
every 2 K+ they pump in.
B Passive transporters
allow K+ ions to leak
across the plasma
membrane, down their
concentration gradient.
C In a resting neuron,
gates of voltage-sensitive
channels are shut (left).
During action potentials,
the gates open (right),
allowing Na+ or K+ to flow
through them.
Fig. 33-7, p. 557
Animation: Ion concentrations
33.4 A Closer Look at Action Potentials
 An action potential begins
• Stimulation of a neuron’s input zone causes a
local, graded potential
• When stimulus in the neuron’s trigger zone
reaches threshold potential, gated sodium
channels open
• Voltage difference decreases and starts the
action potential
An All-or-Nothing Spike
 Once threshold level is reached, membrane
potential always rises to the same level as an
action potential peak (all-or-nothing response)
An All-or-Nothing Spike
action potential
threshold
level
resting
level
Fig. 33-10, p. 559
Animation: Measuring membrane
potential
Direction of Propagation
 An action potential is self-propagating
• Sodium ions diffuse to adjoining region of axon,
triggering sodium gates one after another
 An action potential can only move one way,
toward axon terminals
• Brief refractory period after sodium gates close
Propagation of an Action Potential
Propagation of an Action Potential
Propagation of an Action Potential
Propagation of an Action Potential
interstitial fluid
with high Na+,
low K+
Na+–K+
pump
voltage-gated
ion channels
cytoplasm with
low Na+, high K+
A Close-up of the trigger zone of a neuron. One sodium–potassium pump and some of
the voltage-gated ion channels are shown. At this point, the membrane is at rest and
the voltage-gated channels are closed. The cytoplasm’s charge is negative relative to
interstitial fluid.
Fig. 33-8a, p. 558
Na+
Na+
Na+
Na+
Na+
Na+
B Arrival of a sufficiently large signal in the trigger zone raises the
membrane potential to threshold level. Gated sodium channels open and
sodium (Na+) flows down its concentration gradient into the cytoplasm.
Sodium inflow reverses the voltage across the membrane.
Fig. 33-8b, p. 558
K+
K+
K+
Na+
Na+
Na+
C The charge reversal makes gated Na+ channels shut and gated K+
channels open. The K+ outflow restores the voltage difference across
the membrane. The action potential is propagated along the axon as
positive charges spreading from one region push the next region to
threshold.
Fig. 33-8c, p. 559
Na+–K+
pump
K+
K+ K+
Na+
Na+
Na+
K+
D After an action potential, gated Na+ channels are briefly inactivated, so the
action potential moves one way only, toward axon terminals. Na+ and K+
gradients disrupted by action potentials are restored by diffusion of ions that
were put into place by activity of sodium–potassium pumps.
Fig. 33-8d, p. 559
Animation: Action potential propagation
33.5 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
 Synapse
• The region where an axon terminal (presynaptic
cell) send chemical signals to a neuron, muscle
fiber or gland cell (postsynaptic cell)
 Action potentials trigger release of signaling
molecules (neurotransmitters) from vesicles in
the presynaptic terminal into the synaptic cleft
Neurotransmitter Action
 Release of neurotransmitters from presynaptic
vesicles requires an influx of calcium ions, Ca++
 Postsynaptic membrane receptors bind the
neurotransmitter and initiate the response
 Example: A neuromuscular junction and the
neurotransmitter acetylcholine (ACh)
A Neuromuscular Junction
Fig. 33-11 (a-b), p. 560
Neuromuscular junctions
A An action potential B The action potential
propagates along a
reaches axon terminals that
motor neuron.
lie close to muscle fibers.
muscle
fiber
axon of
a motor
neuron
axon
terminal
muscle
fiber
Fig. 33-11 (a-b), p. 560
Fig. 33-11 (c-d), p. 560
Close-up of a neuromuscular junction (a type of synapse)
C Arrival of the action
potential causes
calcium ions (Ca++) to
enter an axon terminal.
one axon terminal
of the presynaptic
cell (motor neuron)
plasma membrane
of the postsynaptic
cell (muscle cell)
Ca++
D
causes
vesicles with
signaling molecule
(neurotransmitter) to
move to the plasma
membrane and
release their
contents by
exocytosis.
synaptic
vesicle
receptor protein
in membrane of
post-synaptic cell
synaptic cleft (gap
between pre- and
postsynaptic cells)
Fig. 33-11 (c-d), p. 560
Fig. 33-11 (e-f), p. 560
Close-up of neurotransmitter receptor proteins in the plasma
membrane of the postsynaptic cell
binding site for
neurotransmitter
is vacant
channel through
interior is closed
E When neurotransmitter is not
present, the channel through the
receptor protein is shut, and ions
cannot flow through it.
neurotransmitter
in binding site
ion crossing
plasma membrane
through the nowopen channel
F Neurotransmitter diffuses across
the synaptic cleft and binds to the
receptor protein. The ion channel
opens, and ions flow passively into
the postsynaptic cell.
Fig. 33-11 (e-f), p. 560
Animation: Chemical synapse
Receiving the Signal
 A neurotransmitter may have excitatory or
inhibitory effects on a postsynaptic cell
 Synaptic integration
• Summation of all excitatory and inhibitory signals
arriving at a postsynaptic cell at the same time
 The neurotransmitter must be cleared from the
synapse after the signal is transmitted
Synaptic Integration
Animation: Bilateral nervous systems
Animation: Comparison of nervous
systems
Animation: Nerve net
Animation: Vertebrate nervous system
divisions