Chapter 48 Nervous Systems
Overview: Command and Control Center
• The human brain contains an estimated 1011 (100 billion) neurons.
• Each neuron may communicate with thousands of other
neurons in complex information-processing circuits.
• Recently developed technologies can record brain activity from
outside the skull.
• One technique is functional magnetic resonance imaging
(fMRI), which reconstructs a 3-D map of the subject’s brain
• The results of brain imaging and other research methods show
that groups of neurons function in specialized circuits dedicated
to different tasks.
The ability of cells to respond to the environment has evolved over
billions of years.
• The ability to sense and react originated billions of years ago with
prokaryotes that could detect changes in their environment and
respond in ways that enhanced their survival and reproductive
• Such cells could locate food sources by chemotaxis.
• Later, modification of this simple process provided multicellular
organisms with a mechanism for communication between cells of the
• By the time of the Cambrian explosion, systems of neurons that
allowed animals to sense and move rapidly had evolved in essentially
Concept 48.1 Nervous systems consist of circuits of neurons and
Nervous systems show diverse patterns of organization. • All animals except sponges have some type of nervous system.
• What distinguishes nervous systems of different animal groups is how
the neurons are organized into circuits.
• Cnidarians have radially symmetrical bodies organized around a
• In hydras, neurons controlling the contraction and expansion of
the gastrovascular cavity are arranged in diffuse nerve nets.
• The nervous systems of more complex animals contain nerve nets,
as well as nerves, which are bundles of fiberlike extensions of
• With cephalization come more complex nervous systems.
• Neurons are clustered in a brain near the anterior end in
animals with elongated, bilaterally symmetrical bodies.
• In simple cephalized animals such as the planarian, a small brain and
longitudinal nerve cords form a simple central nervous system (CNS).
• In more complex invertebrates, such as annelids and arthropods,
behavior is regulated by more complicated brains and ventral nerve
cords containing segmentally arranged clusters of neurons called
• Nerves that connect the CNS with the rest of the animal’s body
make up the peripheral nervous system (PNS).
• The nervous systems of molluscs correlate with lifestyle.
• Clams and chitons have little or no cephalization and simple
• Squids and octopuses have the most sophisticated nervous
systems of any invertebrates, rivaling those of some
• The large brain and image-forming eyes of cephalopods
support an active, predatory lifestyle.
Nervous systems consist of circuits of neurons and supporting cells. • In general, there are three stages in the processing of information by
nervous systems: sensory input, integration, and motor output.
• Sensory neurons transmit information from sensors that detect
external stimuli (light, heat, touch) and internal conditions (blood
pressure, muscle tension).
• Sensory input is conveyed to the CNS, where interneurons
integrate the sensory input.
• Motor output leaves the CNS via motor neurons, which communicate
with effector cells (muscle or endocrine cells).
• Effector cells carry out the body’s response to a stimulus.
• The stages of sensory input, integration, and motor output are easy to
study in the simple nerve circuits that produce reflexes, the body’s
automatic responses to stimuli.
Networks of neurons with intricate connections form nervous
• The neuron is the structural and functional unit of the nervous
• The neuron’s nucleus is located in the cell body.
• Arising from the cell body are two types of extensions: numerous
dendrites and a single axon.
• Dendrites are highly branched extensions that receive signals
from other neurons.
• An axon is a longer extension that transmits signals to neurons
or effector cells.
• The axon joins the cell body at the axon hillock, where
signals that travel down the axon are generated.
• Many axons are enclosed in a myelin sheath.
• Near its end, axons divide into several branches, each of which
ends in a synaptic terminal. • The site of communication between a synaptic terminal and another
cell is called a synapse.
• At most synapses, information is passed from the transmitting
neuron (the presynaptic cell) to the receiving cell (the
postsynaptic cell) by means of chemical messengers called
• Glia are supporting cells that are essential for the structural integrity
of the nervous system and for the normal functioning of neurons.
• There are several types of glia in the brain and spinal cord.
• Astrocytes are found within the CNS.
• They provide structural support for neurons and regulate
the extracellular concentrations of ions and
• Some astrocytes respond to activity in neighboring
neurons by facilitating information transfer at those
• By inducing the formation of tight junctions between
capillary cells, astrocytes help form the blood-brain
barrier, which restricts the passage of substances into the
• In an embryo, radial glia form tracks along which newly formed
neurons migrate from the neural tube.
• Both radial glia and astrocytes can also act as stem cells,
generating neurons and other glia.
• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS)
are glia that form myelin sheaths around the axons of
• These sheaths provide electrical insulation of the axon.
• In multiple sclerosis, myelin sheaths gradually deteriorate,
resulting in a progressive loss of body function due to the
disruption of nerve signal transmission. Concept 48.2 Ion pumps and ion channels maintain the resting
potential of a neuron
Every cell has a voltage, or membrane potential, across its plasma
• All cells have an electrical potential difference (voltage) across their
• This voltage is called the membrane potential.
• In neurons, the membrane potential is typically between ?60
and ?80 mV when the cell is not transmitting signals.
• The membrane potential of a neuron that is not transmitting signals is
called the resting potential.
• In all neurons, the resting potential depends on the ionic
gradients that exist across the plasma membrane.
• In mammals, the extracellular fluid has a Na+ concentration of
150 millimolar (mM) and a K+ of 5 mM.
• In the cytosol, Na+ concentration is 15 mM, and K+
concentration is 150 mM.
• These gradients are maintained by the sodium-potassium
• The magnitude of the membrane voltage at equilibrium, called the
equilibrium potential (Eion), is given by a formula called the Nernst
• For an ion with a net charge of +1, the Nernst equation is:
• Eion = 62mV (log [ion]outside/[ion]inside)
• The Nernst equation applies to any membrane that is
permeable to a single type of ion.
• In our model, the membrane is only permeable to K+, and the
Nernst equation can be used to calculate EK, the equilibrium
potential for K+. • With this K+ concentration gradient, K+ is at equilibrium
when the inside of the membrane is 92 mV more negative
than the outside.
• Assume that the membrane is only permeable to Na+.
• ENa, the equilibrium potential for Na+, is +62 mV,
indicating that, with this Na+ concentration gradient, Na+
is at equilibrium when the inside of the membrane is 62
mV more positive than the outside.
• How does a real mammalian neuron differ from these model
• The plasma membrane of a real neuron at rest has many open
potassium channels, but it also has a relatively small number of open
• Consequently, the resting potential is around ?60 to ?80 mV,
between EK and ENa.
• Neither K+ nor Na + is at equilibrium, and there is a net flow of
each ion (a current) across the membrane at rest.
• The resting membrane potential remains steady, which means that
the K+ and Na+ currents are equal and opposite.
• The reason the resting potential is closer to EK than to ENa is that
the membrane is more permeable to K+ than to Na+.
• If something causes the membrane’s permeability to Na+ to increase,
the membrane potential will move toward ENa and away from EK.
• This is the basis of nearly all electrical signals in the nervous system.
• The membrane potential can change from its resting value when the
membrane’s permeability to particular ions changes.
• Sodium and potassium play major roles, but there are also important
roles for chloride and calcium ions.
• The resting potential results from the diffusion of K+ and Na+ through
ion channels that are always open. • These channels are ungated.
• Neurons also have gated ion channels, which open or close in
response to one of three types of stimuli.
• Stretch-gated ion channels are found in cells that sense stretch,
and open when the membrane is mechanically deformed.
• Ligand-gated ion channels are found at synapses and open or
close when a specific chemical, such as a neurotransmitter,
binds to the channel.
• Voltage-gated ion channels are found in axons (and in the
dendrites and cell bodies of some neurons, as well as in some
other types of cells) and open or close in response to a change
in membrane potential.
Concept 48.3 Action potentials are the signals conducted by axons
• Gated ion channels are responsible for generating the signals of the
• If a cell has gated ion channels, its membrane potential may
change in response to stimuli that open or close those
• Some stimuli trigger a hyperpolarization, an increase in the
magnitude of the membrane potential.
• Gated K+ channels open, K+ diffuses out of the cell, and the
inside of the membrane becomes more negative.
• Other stimuli trigger a depolarization, a reduction in the magnitude of
the membrane potential.
• Gated Na+ channels open, Na+ diffuses into the cell, and the
inside of the membrane becomes less negative.
• These changes in membrane potential are called graded potentials
because the magnitude of the change—either hyperpolarization or
depolarization—varies with the strength of the stimulus.
• A larger stimulus causes a larger change in membrane
permeability and, thus, a larger change in membrane potential. • In most neurons, depolarizations are graded only up to a certain
membrane voltage, called the threshold.
• A stimulus strong enough to produce a depolarization that reaches
the threshold triggers a different type of response, called an action
• An action potential is an all-or-none phenomenon.
• Once triggered, it has a magnitude that is independent of the
strength of the triggering stimulus.
• Action potentials of neurons are very brief—only 1–2 milliseconds in
• This allows a neuron to produce them at high frequency.
• Both voltage-gated Na+ channels and voltage-gated K+ channels are
involved in the production of an action potential.
• Both types of channels are opened by depolarizing the
membrane, but they respond independently and sequentially:
Na+ channels open before K+ channels.
• Each voltage-gated Na+ channel has two gates, an activation gate
and an inactivation gate, and both must be open for Na+ to diffuse
through the channel.
• At the resting potential, the activation gate is closed and the
inactivation gate is open on most Na+ channels.
• Depolarization of the membrane rapidly opens the activation
gate and slowly closes the inactivation gate.
• Each voltage-gated K+ channel has just one gate, an activation gate.
• At the resting potential, the activation gate on most K+ channels
• Depolarization of the membrane slowly opens the K+ channel’s
• How do these channel properties contribute to the production of an
action potential? • When a stimulus depolarizes the membrane, the activation
gates on some Na+ channels open, allowing more Na+ to
diffuse into the cell.
• The Na+ influx causes further depolarization, which opens the
activation gates on still more Na+ channels, and so on.
• Once the threshold is crossed, this positive-feedback cycle rapidly
brings the membrane potential close to ENa during the rising phase.
• However, two events prevent the membrane potential from actually
• The inactivation gates on most Na+ channels close, halting Na+
• The activation gates on most K+ channels open, causing a
rapid efflux of K+.
• Both events quickly bring the membrane potential back toward EK
during the falling phase.
• In fact, in the final phase of an action potential, called the
undershoot, the membrane’s permeability to K+ is higher than
at rest, so the membrane potential is closer to EK than it is at
the resting potential.
• The K+ channels’ activation gates eventually close, and the
membrane potential returns to the resting potential.
• The Na+ channels’ inactivation gates remain closed during the falling
phase and the early part of the undershoot.
• As a result, if a second depolarizing stimulus occurs during this
refractory period, it will be unable to trigger an action potential.
Nerve impulses propagate themselves along an axon.
• The action potential is repeatedly regenerated along the length of the
• An action potential achieved at one region of the membrane is
sufficient to depolarize a neighboring region above the
threshold level, thus triggering a new action potential. • Immediately behind the traveling zone of depolarization due to Na+
influx is a zone of repolarization due to K+ efflux.
• In the repolarized zone, the activation gates of Na+ channels
are still closed.
• Consequently, the inward current that depolarizes the axon
membrane ahead of the action potential cannot produce
another action potential behind it.
• Once an action potential starts, it normally moves in only one
direction—toward the synaptic terminals.
• Several factors affect the speed at which action potentials are
conducted along an axon.
• One factor is the diameter of the axon: the larger the axon’s
diameter, the faster the conduction.
• In the myelinated neurons of vertebrates, voltage-gated Na+ and K+
channels are concentrated at gaps in the myelin sheath called nodes
• Only these unmyelinated regions of the axon depolarize.
• Thus, the impulse moves faster than in unmyelinated neurons.
• This mechanism is called saltatory conduction.
Concept 48.4 Neurons communicate with other cells at synapses
• When an action potential reaches the terminal of the axon, it
generally stops there.
• However, information is transmitted from a neuron to another
cell at the synapse.
• Some synapses, called electrical synapses, contain gap junctions
that do allow electrical current to flow directly from cell to cell.
• Action potentials travel directly from the presynaptic to the
postsynaptic cell. • In both vertebrates and invertebrates, electrical synapses
synchronize the activity of neurons responsible for rapid,
• The vast majority of synapses are chemical synapses, which involve
the release of chemical neurotransmitter by the presynaptic neuron.
• The presynaptic neuron synthesizes the neurotransmitter and
packages it in synaptic vesicles, which are stored in the
neuron’s synaptic terminals.
• When an action potential reaches a terminal, it depolarizes the
terminal membrane, opening voltage-gated calcium channels in
• Calcium ions (Ca2+) then diffuse into the terminal, and the rise in
Ca2+ concentration in the terminal causes some of the synaptic
vesicles to fuse with the terminal membrane, releasing the
neurotransmitter by exocytosis.
• The neurotransmitter diffuses across the narrow gap, called the
synaptic cleft, which separates the presynaptic neuron from the
• The effect of the neurotransmitter on the postsynaptic cell may
be direct or indirect.
• Information transfer at the synapse can be modified in response
to environmental conditions.
• Such modification may form the basis for learning or memory.
Neural integration occurs at the cellular level.
• At many chemical synapses, ligand-gated ion channels capable of
binding to the neurotransmitter are clustered in the membrane of the
postsynaptic cell, directly opposite the synaptic terminal.
• Binding of the neurotransmitter to the receptor opens the channel and
allows specific ions to diffuse across the postsynaptic membrane.
• This mechanism of information transfer is called direct synaptic
transmission. • The result is generally a postsynaptic potential, a change in the
membrane potential of the postsynaptic cell.
• Excitatory postsynaptic potentials (EPSPs) depolarize the
• The binding of neurotransmitter to postsynaptic receptors
opens gated channels that allow Na+ to diffuse into and K+ to
diffuse out of the cell.
• Inhibitory postsynaptic potential (IPSP) hyperpolarizes the
• The binding of neurotransmitter to postsynaptic receptors open
gated channels that allow K+ to diffuse out of the cell and/or Cl?
to diffuse into the cell.
• Various mechanisms end the effect of neurotransmitters on
• The neurotransmitter may simply diffuse out of the synaptic
• The neurotransmitter may be taken up by the presynaptic
neuron through active transport and repackaged into synaptic
• Glia actively take up the neurotransmitter at some synapses
and metabolize it as fuel.
• The neurotransmitter acetylcholine is degraded by
acetylcholinesterase, an enzyme in the synaptic cleft.
• Postsynaptic potentials are graded; their magnitude varies with a
number of factors, including the amount of neurotransmitter released
by the presynaptic neuron.
• Postsynaptic potentials do not regenerate but diminish with
distance from the synapse.
• Most synapses on a neuron are located on its dendrites or cell
body, whereas action potentials are generally initiated at the
axon hillock. • Therefore, a single EPSP is usually too small to trigger an
action potential in a postsynaptic neuron.
• Graded potentials (EPSPs and IPSPs) are summed to either
depolarize or hyperpolarize a postsynaptic neuron.
• Two EPSPs produced in rapid succession at the same synapse
can be added in an effect called temporal summation.
• Two EPSPs produced nearly simultaneously by different
synapses on the same postsynaptic neuron can be added, in
an effect called spatial summation.
• Summation also applies to IPSPs.
• This interplay between multiple excitatory and inhibitory inputs is the
essence of integration in the nervous system.
• The axon hillock is the neuron’s integrating center, where the
membrane potential at any instant represents the summed
effect of all EPSPs and IPSPs.
• Whenever the membrane potential at the axon hillock reaches
the threshold, an action potential is generated and travels along
the axon to its synaptic terminals.
• In indirect synaptic transmission, a neurotransmitter binds to a
receptor that is not part of an ion channel.
• This binding activates a signal transduction pathway involving a
second messenger in the postsynaptic cell.
• This form of transmission has a slower onset, but its effects
have a longer duration.
• cAMP acts as a secondary messenger in indirect synaptic
• When the neurotransmitter norepinephrine binds to its receptor,
the neurotransmitter-receptor complex activates a G-protein,
which in turn activates adenylyl cyclase, the enzyme that
converts ATP to cAMP. • cAMP activates protein kinase A, which phosphorylates specific
channel proteins in the postsynaptic membrane, causing them
to open or close.
• Because of the amplifying effect of the signal transduction
pathway, the binding of a neurotransmitter to a single receptor
can open or close many channels.
The same neurotransmitter can produce different effects on different
types of cells.
• Each of the known neurotransmitters binds to a specific group of
• Some neurotransmitters have a dozen or more receptors, which
can produce very different effects in postsynaptic cells.
• Acetylcholine is one of the most common neurotransmitters in both
invertebrates and vertebrates.
• In the vertebrate CNS, it can be inhibitory or excitatory,
depending on the type of receptor.
• At the vertebrate neuromuscular junction, acetylcholine
released by the motor neuron binds to receptors on ligand-
gated channels in the muscle cell, producing an EPSP via
direct synaptic transmission.
• Nicotine binds to the same receptors.
• Acetylcholine is inhibitory to cardiac muscle cell contraction.
• Biogenic amines are neurotransmitters derived from amino acids. v
• One group, known as catecholamines, consists of neurotransmitters
produced from the amino acid tyrosine.
• This group includes epinephrine and norepinephrine and a closely
related compound called dopamine.
• Another biogenic amine, serotonin, is synthesized from the amino
acid tryptophan. • The biogenic amines are usually involved in indirect synaptic
transmission, most commonly in the CNS.
• Dopamine and serotonin affect sleep, mood, attention, and learning.
• Imbalances in these neurotransmitters are associated with several
• Parkinson’s disease is associated with a lack of dopamine in
• LSD and mescaline produce hallucinations by binding to brain
receptors for serotonin and dopamine.
• Depression is treated with drugs that increase the brain
concentrations of biogenic amines such as norepinephrine and
• Prozac inhibits the uptake of serotonin after its release,
increasing its effect.
• Four amino acids function as neurotransmitters in the CNS: gamma
aminobutyric acid (GABA), glycine, glutamate, and aspartate.
• GABA is the neurotransmitter at most inhibitory synapses in the
brain, where it produces IPSPs.
• Several neuropeptides, relatively short chains of amino acids, serve
• Most neurons release one or more neuropeptides as well as a
• Neuropeptides usually operate via signal transduction
• The neuropeptide substance P is a key excitatory
neurotransmitter that mediates our perception of pain.
• Other neuropeptides, endorphins, act as natural
• Opiates such as morphine and heroin bind to receptors
on brain neurons by mimicking endorphins, which are produced in the brain under times of physical or emotional
• Some neurons of the vertebrate PNS and CNS release dissolved
gases, especially nitric oxide and carbon monoxide, which act as
• During male sexual arousal, certain neurons release NO into
the erectile tissue of the penis.
• In response, smooth muscle cells in the blood vessel walls of
the erectile tissue relax, allowing the blood vessels to dilate and
fill the spongy erectile tissue with blood, producing an erection.
• Viagra inhibits an enzyme that slows the muscle-releasing
effects of NO.
• Carbon monoxide is synthesized by the enzyme heme oxygenase.
• In the brain, CO regulates the release of hypothalamic
• In the PNS, it acts as an inhibitory neurotransmitter that
hyperpolarizes intestinal smooth muscle cells.
• NO and CO are synthesized by cells as needed.
• They diffuse into neighboring target cells, produce an effect,
and are broken down, all within a few seconds.
Concept 48.5 The vertebrate nervous system is regionally specialized
Vertebrate nervous systems have central and peripheral components.
• In all vertebrates, the nervous system shows a high degree of
cephalization and has distinct CNS and PNS components.
• The brain provides integrative power that underlies the complex
behavior of vertebrates.
• The spinal cord integrates simple responses to certain kinds of
stimuli and conveys information to and from the brain.
• The vertebrate CNS is derived from the dorsal embryonic nerve cord,
which is hollow. • In the adult, this feature persists as the narrow central canal of
the spinal cord and the four ventricles of the brain.
• Both the canal and the ventricles are filled with cerebrospinal
fluid, which is formed in the brain by filtration of the blood.
• Cerebrospinal fluid circulates through the central canal and
ventricles and then drains into the veins, assisting in the supply
of nutrients and hormone and the removal of wastes.
• In mammals, the fluid cushions the brain and spinal cord by
circulating between two of the meninges, layers of connective
tissue that surround the CNS.
• White matter of the CNS is composed of bundles of myelinated
• Gray matter consists of unmyelinated axons, nuclei, and
The divisions of the peripheral nervous system interact in maintaining
• The PNS transmits information to and from the CNS and plays an
important role in regulating the movement and internal environment of
• The vertebrate PNS consists of left-right pairs of cranial and
spinal nerves and their associated ganglia.
• Paired cranial nerves originate in the brain and innervate the
head and upper body.
• Paired spinal nerves originate in the spinal cord and innervate
the entire body.
• The PNS can be divided into two functional components: the somatic
nervous system and the autonomic nervous system.
• The somatic nervous system carries signals to and from skeletal
muscle, mainly in response to external stimuli. • It is subject to conscious control, but much skeletal muscle
activity is actually controlled by reflexes mediated by the spinal
cord or the brainstem.
• The autonomic nervous system regulates the internal environment by
controlling smooth and cardiac muscles and the organs of the
digestive, cardiovascular, excretory, and endocrine systems.
• Three divisions make up the autonomic nervous syst