Po l2e ch34 lecture neurons, sense organs, and nervous systems edited sphs

Neurons, Sense Organs, and
Nervous Systems
34

Chapter 34 Neurons, Sense Organs, and Nervous Systems
Key Concepts
34.1 Nervous Systems Are Composed of Neurons and Glial
Cells
34.2 Neurons Generate Electric Signals by Controlling Ion
Distributions
34.3 Neurons Communicate with Other Cells
at Synapses
34.4 Sensory Processes Provide Information on an
Animal’s External Environment and Internal Status
34.5 Neurons Are Organized into Nervous Systems

Chapter 34 Opening Question
How might the star-nosed mole’s brain be
specialized to process information from its
nose?

34.1
Nervous Systems are Composed of
Neurons and Glial Cells

Concept 34.1 Nervous Systems Are Composed of Neurons and
Glial Cells
Animals need a way to transmit signals at high speeds
from place to place within their bodies (e.g., to avoid
danger).
Mammalian neurons transmit signals at 20–100 meters
per second.

Glial Cells
Neurons are excitable cells—they can generate and
transmit electrical signals, called action potentials.
Cell membranes ordinarily have electrical polarity:
the outside is more positive than the inside.
An impulse, or action potential, is a state of
reversed polarity.

Glial Cells
In an excitable cell, an action potential generated at
one point propagates over the whole membrane.
The region of depolarization moves along the cell
membrane, and the membrane is said to “conduct”
the impulse.

Glial Cells

Glial Cells
Neurons (nerve cells) are specially adapted to
generate electric signals, usually in the form of action
potentials.
Neurons must make contact with target cells.
Synapse: cell-to-cell contact point specialized for
signal transmission
A signal arrives at the synapse by way of the
presynaptic cell and leaves by way of the
postsynaptic cell.

Glial Cells
Most neurons have four regions:
• Dendrites—carry signals to
the cell body
• Cell body—contains nucleus
and most organelles
• Axon—conducts action
potentials away from the cell
body; can be very long
• Presynaptic axon terminals
—make contact with other
cells

Figure 34.2 A Generalized Neuron

Glial Cells
A neuron is said to innervate (provide neural imput)
the cells that the axon terminals contact.
Axons of many neurons often travel together in
bundles called nerves.
Nerve refers only to axon bundles outside the brain
and spinal cord.
In the brain or spinal cord they are called tracts.

Glial Cells
Glial cells, or glia, are not excitable. They have
several functions:
• Help orient neurons toward their target cells
during embryonic development
• Provide metabolic support for neurons
• Help regulate composition of extracellular
fluids and perform immune functions
• Assist signal transmission across synapses

Glial Cells
Oligodendrocytes are glia that
insulate axons in the brain and
spinal cord.
Schwann cells insulate axons in
nerves outside of these areas.
The glial membranes form a
nonconductive sheath called
myelin.
Myelin-coated axons are white
matter and areas of cell bodies
are gray matter.

Figure 34.3 Electrically Insulating an Axon

34.2
Neurons Generate Electric Signals by
Controlling Ion Distributions

Click-and-Learn (HHMI)
View this Click-and-Learn about Neurons
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Concept 34.2 Neurons Generate Electric Signals by Controlling
Ion Distributions
Current: flow of electric charges from place to
place; in cells, current is based on flow of ions
such as Na+
Voltage, or electrical potential difference exists
if positive charges are concentrated in one place
and negative charges are concentrated in a
different place.
Voltages produce currents because opposite
charges attract and will move toward one another
if given a chance.

Ion Distributions
No voltage
differences exist
within open
solutions such
as the intracellular
fluids.
Voltage
differences exist
only across
membranes such
as the cell
membrane.

Ion Distributions

Ion Distributions
The voltage across a membrane is called membrane
potential and is easily measured.
Resting neuron: membrane potential is the resting
potential, typically –60 to –70 millivolts (mV)
• Negative sign means the inside of the cell is
electrically negative relative to the outside.

Figure 34.4 Measuring the Membrane Potential

Ion Distributions
Membrane potential can change rapidly, and only
relatively small numbers of positive charges need to
move through the membrane for this change of
membrane potential to occur.
Composition of the bulk solutions (the intra- and
extracellular fluids) does not change.

Ion Distributions
Ion redistribution occurs through membrane channel
proteins and ion transporters in the membrane.
Sodium–potassium pump—uses energy from ATP
to move 3 Na+
ions to the outside and 2 K+
to the
inside; establishes concentration gradients of these
ions

Ion Distributions
Potassium channels (K+
) are open in the resting
membrane.
K+
ions diffuse out of the cell through leak channels
and leave behind negative charges within the cell.
K+
ions diffuse back into the cell because of the
negative electrical potential.
At this equilibrium point, there is no net movement of
K+
; called the equilibrium potential of K+
.

Ion Distributions
Diffusion of ions is controlled by concentration effect and
electrical effect. When they are equal,
electrochemical equilibrium is reached.

Ion Distributions
Electrochemical equilibrium is called the
equilibrium potential of the ion, calculated by the
Nernst equation:

Ion Distributions
Most ion channels are “gated”—they open and close
under certain conditions. Most are closed in a
resting neuron, which is why K+
leak channels
determine resting membrane potential.
• Voltage-gated channels open or close in
response to changes in membrane potential
• Stretch-gated channels respond to tension
applied to cell membrane
• Ligand-gated channels open or close when a
specific chemical (ligand) binds to the channel
protein.

Figure 34.5 Three Types of Gated Ion Channels (Part 1)
• Voltage-gated channels open or close in response
to changes in membrane potential

• Stretch-gated channels respond to tension applied
to cell membrane

• Ligand-gated channels open or close when a
specific chemical (ligand) binds to the channel
protein.

Concept 34.2 Neurons Generate Electric Signals by Controlling Ion
Distributions
Opening and closing gated channels can alter
membrane potential.
If Na+
channels open, Na+
diffuses into the neuron
because it is more concentrated outside the cell,
and the cell membrane is more negative on the
inside.
When membrane becomes less negative on the
inside, the membrane is depolarized.
The membrane is hyperpolarized if the charge on
the inside becomes more negative.

Distributions
Membrane potential can be graded or all-or-none.
Graded membrane potentials are changes from the
resting potential that are less than the threshold of –50
mV.
• Graded means any value of the membrane potential
is possible
• Caused by various ion channels opening or closing
• Only spread a short distance

Distributions
Graded membrane potentials spread only a short
distance.

Ion Distributions
If neuron depolarizes to
the –50 mV threshold,
an all-or-none event
occurs: an action
potential is generated.
Action potentials are not
graded (always the
same size) and do not
become smaller, they
stay the same in size as
they propagate along the
cell membrane.

Figure 34.6 Graded and All-or-None Changes in Membrane Potential
A membrane
potential of
-50 mV or greater
isneeded to start
and action
potential.
Action potentials
stay thesamesize
asthey move
along the
membrane.

Ion Distributions
Graded changes can give rise to all-or-
none changes by being summed
together; provides a mechanism for
integrating signals.
A key area for this integration is the
axon hillock, where action potentials
are most often generated.
Graded changes resulting from
multiple signals reaching the
dendrites, spread to the axon hillock,
where all the depolarizations and
hyperpolarizations sum (add
together).

Ion Distributions
An action potential (nerve impulse) is a rapid, large
change in membrane potential that reverses
membrane polarity.
The membrane depolarizes from –65 mV at rest to about
+40 mV (depolarization).
It is localized and brief but is propagated with no loss of
size—an action potential at one location causes
currents to flow that depolarize neighboring regions.

Ion Distributions
When membrane potential reaches threshold, many
voltage-gated Na+
channels open quickly, and Na+
rushes into the axon.
The influx of positive ions (Na+
) causes more
depolarization, and an action potential occurs.

Figure 34.7 Production of an Action Potential (Part 3)

Ion Distributions
The axon quickly returns to resting potential:
• Voltage-gated Na+
channels close
• Voltage-gated K+
channels open slowly and stay
open longer—K+
moves out

Ion Distributions
Positive feedback during depolarization:
• When the membrane is partially depolarized, some
Na+
channels open; as Na+
starts to diffuse into the
cell, more depolarization occurs, opening more
channels.
• This continues until all voltage-gated Na+
channels
open and maximum depolarization occurs.

Figure 34.8 Positive Feedback Plays a Key Role in the Generation of an Action Potential
Positive Feedback!

Distributions
Action potential travels in only one direction:
• After the action potential, Na+
channels cannot
open again for a brief period (refractory period)
and cannot depolarize.
• Thus the action potential can only propagate in the
direction of the axon terminals.
Why can action potentials only travel in one direction?

Distributions
Action potentials travel faster in larger diameter axons.
Myelination by glial cells also increases speed of action
potentials.
The nodes of Ranvier are gaps where the axon is not
covered by myelin.
Action potentials are generated only at the nodes and
jump from node to node (saltatory conduction).

Concept 34.3 Neurons Communicate with Other Cells at
Synapses
Neurons communicate with other neurons or target cells
at synapses.
Chemical synapse: a very narrow space between cells
(synaptic cleft) that an action potential cannot cross
• When an action potential arrives at the end of the
presynaptic cell, a neurotransmitter is released that
diffuses across the space.

Synapses

Synapses
Neurotransmitters diffuse across the synaptic cleft very
rapidly (short distance).
They bind to receptors on the postsynaptic cell
membrane, which generates another action
potential or other change.
Neurotransmitters are quickly removed from the cleft—
to end signal transmission—by enzymatic breakdown,
uptake by other neurons or glial cells, or reuptake by
the presynaptic cell.

Synapses
Electrical synapse: cells are joined by gap junctions
where the cytoplasm is continuous; signals cross
with essentially no delay
• They occur where very fast, invariant signal
transmission is needed, such as neurons that
control escape swimming in some fish.
• Also occur where many cells must be stimulated
to act together, such as fish electric organs.

Synapses
Neuromuscular
junctions: chemical
synapses between
motor neurons and
skeletal muscle cells.
The axon of the
presynaptic cell
branches close to the
muscle cell, creating
several axon
terminals (boutons)
that synapse with the
muscle cell.

Figure 34.9 Synaptic Transmission at a Vertebrate Neuromuscular Junction (Part 1)

Synapses
An action potential causes voltage-gated
Ca+
channels to open in the
presynaptic membrane, allowing Ca+
to
flow in.
This induces release of the
neurotransmitter acetylcholine (ACh)
(one of the most common
neurotransmitters in vertebrates and
invertebrates – can be inhibitory or
excitatory):
• ACh is stored in vesicles that fuse
with the cell membrane to release
ACh into the cleft by exocytosis.

Synapses
ACh diffuses across the cleft and binds to receptors on
the postsynaptic cell.
These receptors allow Na+
and K+
to flow through, and
the increase in Na+
depolarizes the membrane.
If it reaches threshold, more Na+
voltage-gated channels
are activated and an action potential is generated.

Synapses
Three categories of neurotransmitters:
• Amino acids—glutamate, glycine, and
γ-aminobutyric acid (GABA)
• Biogenic amines include acetylcholine, dopamine,
epinephrine, norepinephrine, and serotonin
 Active in the CNS and PNS
• A variety of peptides (strings of amino acids)
 Active in the brain
There are also gases such as nitric oxide and carbon
monoxide
 Are local regulators in the PNS

Synapses
In the brain, a postsynaptic neuron
may have chemical synapses with
hundreds or thousands of
presynaptic neurons, which may use
different neurotransmitters.
Receptors for a given neurotransmitter
on the postsynaptic cell may be of
different types with different actions.
This complexity in synapse function
helps explain the complexity of brain
function.

Figure 34.10 Synapses on a Single Postsynaptic Neuron in the Brain of a Mouse

Synapses
Neurotransmitter receptors:
• Ionotropic receptors are ligand-gated ion channels
—cause changes in ion movement; response is fast
and short-lived.
• Metabotropic receptors are G protein-linked
receptors that produce second messengers that
induce signaling cascades; responses are slower
and longer-lived.

Synapses
Synapses
Excitatory synapses produce graded membrane
depolarizations called excitatory postsynaptic
potentials (EPSPs); shift membrane potential
towards threshold.
Inhibitory synapses shift membrane potential
away from threshold; produce graded membrane
hyperpolarizations called inhibitory
postsynaptic potentials (IPSPs).

Synapses
Each EPSP or IPSP is usually less than 1 mV, and
disappears in 10–20 milliseconds.
They are graded potentials, typically produced at
synapses on dendrites and the cell body.
They affect membrane potential at the axon hillock,
where action potentials are generated.
Summation of the graded potentials is both temporal
(must be present at the same time), and spatial.

Figure 34.11 A Postsynaptic Neuron Carries Out Spatial Summation

Synapses
The postsynaptic cell sums the excitatory and
inhibitory input.
Summation determines whether the postsynaptic cell
produces action potentials.
If the sum of EPSPs and IPSPs at the axon hillock
is great enough to reach threshold, an action
potential is produced.

Synapses
Synaptic plasticity: synapses in an individual can
undergo long-term changes in functional properties
and physical shape during the individual’s lifetime.
This may be one of the major mechanisms of
learning.
• Experiences at one time in life produce long-term
changes in synapses, so that future experiences
are processed by the nervous system in altered
ways.

Synapses
Sea hares (mollusks) pull their gills inside
when certain parts of the body are touched:

Synapses
They withdraw their gills more vigorously if they
have previously been exposed to a noxious
agent (sensitization).
The synapses between the sensory neurons
and the motor neurons for gill withdrawal are
functionally strengthened—more
neurotransmitter is released per impulse.
The postsynaptic cell is thus excited to a
greater degree.

Synapses
In mammals, the hippocampus is associated
with spatial learning and memory formation.

Synapses
In studies of mice brains, when a circuit is
repeatedly stimulated, the postsynaptic
structures physically grow and the synapses
strengthen functionally.
The postsynaptic receptor molecules increase,
increasing response.
Synaptic plasticity has been shown to depend
on second messengers, altered protein
synthesis, and altered gene transcription.

Concept 34.4 Sensory Processes Provide Information on an
Animals need information about their external
environments to move, locate food, find mates,
and avoid danger.
They also need information regarding internal
conditions, such as partial pressure of O2 and CO2
in the blood, and tension in contracting muscles.

Sensory receptor cells: neurons specialized for
sensory transduction—transforming the energy of a
stimulus into an electric signal.
The electric signal generates action potentials. Action
potentials convey the sensory information to the
brain or other areas of the nervous system.
Sensory receptor cells are highly specific in the
stimuli to which they respond.

Sensory receptor proteins: membrane proteins in
sensory receptor cells that initially detect a stimulus
They produce graded membrane potentials (receptor
potentials).
Receptor cell membranes are often modified to have
a large surface area, such as microvilli, cilia, or
folding. This allows more receptor molecules and
greater sensitivity.

Cone cells in vertebrate eyes have highly folded
membranes with great numbers of photoreceptor
molecules.

Ionotropic receptor cells:
receptor proteins are typically
stimulus-gated Na+
channels.
Stimulus opens the channel, Na+
moves in, and receptor potential
is generated.
Metabotropic receptor cells are
typically G protein-linked
receptors; activation leads to
change in a second messenger.
This can directly or indirectly
produce a receptor potential.

Figure 34.12 Sensory Receptor Cells Are Ionotropic or Metabotropic

In the brain, different regions receive and process
information from different sensory systems.
Examples:
• Axons from the eyes travel to the visual cortex
• Axons from the inner ear travel to the auditory
cortex
Intensity of sensations is coded by the frequency of
the action potentials.

Mechanoreceptors respond to mechanical distortion
of the cell membrane; most are ionotropic.
Stretch receptor cells in muscles respond when the
muscle contracts.

Stretch receptors are
specialized neurons.
When cell membranes of
the dendrites are
stretched, Na+
channels
open and produce a
graded receptor potential
that spreads to the axon
hillock.
Action potentials are
generated if
depolarization is above
threshold.

Figure 34.13 Stimulating a Crayfish Stretch Receptor Cell Produces Electric Signals

Stretch receptors in the biceps help adjust the muscle’s
strength of contraction to match the load the muscle
must sustain.
Receptors in the muscle detect how much the muscle is
being lengthened by a load; this information is used to
keep the arm stable as the load increases.

Figure 34.14 Negative Feedback: Sensory Signals from Stretch Receptors Enable the
Nervous System to Control Muscle Contraction

Smell
The sense of smell (olfaction) involves metabotropic
receptors.
Chemicals detected by smell are odorants. The sensory
cells are chemoreceptors.
In mammals, the receptors are in the lining of the nasal
cavity. Axons extend to the olfactory bulb in the brain.
Odorants must be dissolved in the liquid mucus to be
detected.

When an odorant binds to a receptor protein, it
activates a G protein, which activates a second
messenger.
The second messenger opens ion channels and an
influx of Na+
depolarizes the olfactory neuron.

Different receptor cells have different G proteins that
bind different odorants.
With tens of thousands of olfactory receptor cells—each
with one type of receptor
protein—a wide range of odorants can
be detected.
The combination of olfactory cells stimulated
by a particular odorant is unique to that odorant. The
brain can interpret the pattern
of signals as pointing to a particular smell.

Some animals have extremely
specific olfactory receptor cells that
are extremely sensitive.
Some male moths have thousands
of receptors on their antennae that
respond to pheromones emitted by
the females.
They can detect a female when the
pheromone level is as low as
0.000000000000001% of the air
molecules.

Hearing
Auditory systems use mechanoreceptors to sense
sound pressure waves.
Many hearing organs have a membrane that
moves in and out when sound pressure waves hit
it.
In mammals, this is the tympanic membrane (ear
drum).

The middle ear is an air-filled
cavity with three bones
(ossicles)—the malleus, incus,
stapes.
They transmit vibrations of the
tympanic membrane to another
membrane, the oval window.
The oval window connects to the
cochlea, a coiled, fluid-filled
tube where sound energy is
transduced into electric signals.

Figure 34.15 The Human Ear (Part 1)

The cochlea has three parallel canals
separated by membranes.
The basilar membrane changes in width and
stiffness over its length.
Different pitches, or frequency of vibration,
cause different regions of the basilar
membrane to oscillate.

Figure 34.16 Frequencies in Sound Pressure Waves Are Transduced into Signals from
Distinctive Sets of Hair Cells (Part 1)

Figure 34.16 Frequencies in Sound Pressure Waves Are Transduced into Signals from
Distinctive Sets of Hair Cells (Part 2)

The organ of Corti, on the basilar membrane, has
mechanoreceptor cells called hair cells.
Hair cells have projections called stereocilia that
project into the semi-rigid tectorial membrane. When
the basilar membrane moves, the stereocilia are
bent.
Hair cells transduce the bending motions of the
stereocilia into electric signals and synapse with
neurons that produce action potentials.

Photoreceptors—receptor cells sensitive to
light
Photoreceptors are metabotropic; the receptor
proteins are all in the family of pigments
called rhodopsins, which act as G protein–
linked receptors.
Rhodopsin consists of the protein opsin and a
light-absorbing group, 11-cis-retinal.

When 11-cis-retinal absorbs light, it changes to
the isomer all-trans-retinal, which changes the
conformation of opsin.

An advantage of metabotropic control is that signals
can be amplified.
With large numbers of rhodopsin molecules and
strong amplification, some photoreceptor cells
undergo a measurable change in membrane
potential in response to just a single photon of light.

Vertebrates have image-forming eyes, like a camera.

Connective tissue forms the transparent cornea
on front of eye.
Iris (pigmented)—controls amount of light
reaching photoreceptors; opening is the pupil
Lens—crystalline protein, focuses image, can
change shape for focusing
Retina—photosensitive layer at the back of the
eye

The retina has several types of neurons including the
photoreceptors (rods and cones).
Rods and cones have large membrane surface area
and many rhodopsin molecules. In humans, rods
outnumber cones.
Rods are more sensitive to light; important in dim light
situations. Cones provide color vision.

Humans have three types of cone cells with slightly
different opsin molecules—they absorb different
wavelengths of light.
This allows the brain to interpret input from the
different cones as a full range of color.

Rod and cone cells only produce graded
membrane potentials (not action potentials).
When stimulated, they hyperpolarize—the
opposite of other sensory cells responding to
stimuli.
In darkness, Na+
channels are open and Na+
continually enters the cells. When stimulated,
the 2nd
messenger closes the channels, and
inside of membrane becomes more negative.

Figure 34.18 A Rod Cell Responds to Light (Part 1)

Figure 34.18 A Rod Cell Responds to Light (Part 2)

The effect of the hyperpolarization is graded,
depending on light intensity.
Rod and cone cells synapse with other neurons
in the retina and relay signals to them by
graded neurotransmitter release.

The retina has four types of integrating neurons
arranged in layers.
Rods and cones synapse with bipolar cells,
which synapse with ganglion cells.
Horizontal cells and amacrine cells
communicate laterally within the retina.
Ganglion cells are the only ones that produce
action potentials. Their axons converge to
form the optic nerves.

Each ganglion cell has a receptive field—a
defined, circular field formed by the cells from
which it receives signals.
Different ganglion cells are excited by light
falling on the center versus the periphery of
the receptive field.
This enables ganglion cells to communicate
information to the brain about visual patterns
such as spots, edges, and areas of contrast.

About 1–2% of ganglion cells are photosensitive. The
receptor protein is believed to be melanopsin.
They provide information on the presence of light and
its intensity.
This information is relayed to the suprachiasmatic
nuclei, where it is used to entrain the master circadian
biological clock.
It also regulates pupil size so they are large in dim light
and small in bright light.

Arthropods have compound eyes consisting of
units called ommatidia.
Each ommatidium has a lens to focus light onto
photoreceptor cells containing rhodopsin.
Each ommatidium points in a slightly different
direction. The more ommatidia, the higher the
image resolution. Fast-flying predators such
as dragonflies have up to 30,000.

Figure 34.20 Ommatidia: The Functional Units of Arthropod Eyes

Some animals have evolved unusual sensory
abilities to sense information such as electric
fields or Earth’s magnetic field.
Some animals perceive their environment very
differently than humans do, for example,
hearing sound frequencies that we cannot
hear, or seeing electromagnetic wavelengths
that we cannot see.

Our eyes can see wavelengths between 0.4
and 0.7 μm.
Infrared wavelengths—longer than 0.7 μm
Ultraviolet wavelengths—shorter than 0.4 μm
Some snakes can see infrared wavelengths,
detecting warmth in darkness, which helps
them find prey.

Pit vipers have infrared sensing organs in pits
near the eyes.

Many animals can see ultraviolet wave-
lengths—bees, some birds.
Male and female cedar waxwings look alike to us, but
researchers have discovered that the birds
themselves can tell the difference, with UV
wavelengths.

Some animals can sense electric fields.
Some fish produce weak electric fields and detect
distortions in these fields caused by objects in their
environment.
The fish can perceive objects even in complete darkness.

Concept 34.5 Neurons Are Organized into Nervous Systems
The cnidarians have nerve nets, the most
simple type of nervous system.

Evolution of nervous systems followed two major
trends:
• Centralization—integrating neurons became
clustered together in centralized organs (e.g.,
brain and spinal cord).
• Cephalization—major integrating areas
became concentrated toward the anterior end of
the animal (head).
The anterior end meets the environment first;
sensory organs are also concentrated there.

Figure 34.21 The Organization of Vertebrate and Arthropod Nervous Systems

Organization of the
Nervous System

Central nervous system
(CNS)—brain and spinal cord
Composed mostly of
integrating neurons and glial
cells; it must interact with
sensors and effectors.
Effectors are cells or tissues
that perform actions, that
“carry out orders,” such as
muscle cells.

Peripheral nervous
system (PNS)—
neurons outside the
CNS
Sensors bring sensory
information from sense
organs to the CNS and
carry orders from the
CNS to effectors.
Nerves are bundles of
axons in the PNS.

Kinds of Neurons
Interneurons—neurons confined to the CNS
Sensory neurons—sensory receptor cells or neurons
that carry signals from sensory cells to the CNS
(afferent neurons)
Efferent neurons—convey signals from the CNS to
muscles or other effectors
Motor neurons—carry signals to skeletal muscles

Autonomic nervous system (ANS)—controls
effectors other than skeletal muscles (autonomic
effectors)
Controls smooth muscle in organs, exocrine glands,
some endocrine glands, acid secreting cells in the
stomach, and other effectors.
ANS has 3 divisions:
• Enteric
• Sympathetic
• Parasympathetic

Vertebrate ANS (Autonomic
nervous system) has three
divisions:
• Enteric division—nerve
cells internal to the gut
wall
• Sympathetic division
prepares the body for
emergencies—“fight or
flight”
• Parasympathetic
division slows the heart,
lowers blood pressure and
increases digestion

Figure 34.22 The Autonomic Nervous System

Autonomic efferent pathways begin with preganglionic
neurons with cell bodies in the CNS.
Axons of preganglionic neurons synapse on a second
neuron outside the CNS in a collection of nerve cell
bodies called a ganglion.
The second neuron is postganglionic—its axon leaves
the ganglion and synapses in the target organs.

Parasympathetic preganglionic neurons exit the
CNS from the brain and sacral region of the spinal
cord.
• The ganglia are near the target organs.
Sympathetic preganglionic neurons exit the CNS at
the thoracic and lumbar regions of the spinal cord.
• Most of the ganglia lie next to the spinal cord.

Sympathetic postganglionic neurons use
norepinephrine as the neurotransmitter.
Parasympathetic postganglionic neurons use
acetylcholine.
In organs that receive both inputs, the target cells
usually respond in opposite ways.

The sympathetic and parasympathetic divisions often
work in opposition; acting together, they can adjust
effector functions up or down as needed.
Example: Sympathetic stimulation of the pace maker
causes the heart rate to increase, and
parasympathetic stimulation causes it to decrease.

The fight-or-flight response is an effect of the
sympathetic division.
When activated, it increases the heart rate, force of
contraction, and cardiac output; it dilates lung
passageways and increases release of glucose from
the liver.
At the same time, it reduces less urgent activities such
as digestion.

Many neurons that control skeletal muscles enter or
leave the CNS in spinal nerves.
Spinal nerves have both sensory and motor neurons.

Figure 34.23 Spinal Nerves Connect to the Spinal Cord at Regular Intervals

Spinal reflex—afferent
information converts to efferent
activity without going through
the brain
The knee-jerk reflex:
• Stretch receptors in the
patellar tendon send action
potentials to the spinal cord.
• The sensory neuron
synapses with a motor
neuron, sending an action
potential to the leg muscle.

Figure 34.24 The Spinal Cord Coordinates the Knee-Jerk Reflex

Vertebrate brains have three
main regions: forebrain,
midbrain, and hindbrain.
The brain and spinal cord
must pass through the
medulla oblongata, the
most posterior part of the
hindbrain.
• This area has changed
little over the course of
vertebrate evolution.

Figure 34.25 Evolution of the Vertebrate Brain

In contrast, the cerebral hemispheres have
undergone dramatic changes.
• They are important in carrying out high-
order sensory, motor, and integrative
functions.
The evolution of enhanced functionality in
mammals and birds has gone hand in hand
with large increases in the numbers of
neurons and larger brain size.

However, it is important to note that some animals
with small brains exhibit stunning behavioral
capacities.
In humans, all available evidence indicates that
individual intelligence is not correlated with individual
brain size.

Cerebral cortex—outer-most layer of the
cerebral hemispheres, with many cell bodies
• It is folded into convolutions, which
increases its size.
The left side of the body is served mostly by
the right side of the brain, and vice versa.
In each cerebral hemisphere, specific regions
are specialized to carry out specific sensory
and motor functions.

Figure 34.26 The Human Left Cerebral Hemisphere (Part 1)

Figure 34.26 The Human Left Cerebral Hemisphere (Part 2)

Combined sensory and
motor functions often
occur in localized brain
areas.
Imaging methods such
as PET allow us to
visualize areas where
neurons exhibit
increased electrical
activity correlated with
specific activities, such
as language functions.

Figure 34.27 Imaging Techniques Reveal Active Parts of the Brain

Functional magnetic
resonance imaging (fMRI)
is another technique to
pinpoint brain activity.
Example: In a person
experiencing fear,
increased activity is seen
in the amygdala in the
forebrain.
Even memories of
frightening situations can
activate the amygdala.

Figure 34.28 Source of the Fear Response

Parts of the brain that serve various anatomical
regions of the body are physically related to
each other in ways that mirror the rest of the
body.
Example: Map of the somatosensory (“body
sensing”) part of the cerebral cortex
• The size of each body part in the drawing
reflects the amount of cortical area devoted
to the part.

Figure 34.29 The Body Is Represented in Maplike Ways in the Primary Somatosensory and
Primary Motor Parts of the Cerebral Cortex

Answer to Opening Question
Large areas of a star-nosed mole’s
cerebral cortex are devoted to
processing sensory information
from the tentacles of its nose.
A map of the cortical region
receiving sensory input from all 11
tentacles shows that it exactly
mirrors the arrangement of the
tentacles.
How might the star-nosed mole’s brain be
specialized to process information from its
nose?

Figure 34.30 Representation of the Nose of the Star-nosed Mole in the Cerebral Cortex

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