Download The Nervous System

BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Communication and Control:
The Nervous and Endocrine Systems
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27.1 Structure of the Nervous System
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• The nervous system includes all the nervous
tissue in the body plus the body’s sensory
organs, such as the eyes and ears.
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The Nervous System
• Nervous tissue is composed of two kinds of
cells:
– Neurons, which transmit nervous system messages.
– Glial cells, which support neurons and modify their
signaling.
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The Nervous System
• The two major divisions of the human nervous
system are:
– The central nervous system (CNS), consisting of
the brain and spinal cord.
– The peripheral nervous system (PNS), which
includes all the neural tissue outside the CNS plus
the sensory organs.
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Divisions of the Nervous System
(a) The nervous system has two (b) How these two components interact
components
Central nervous
Central nervous system (CNS)
system
information
brain
processing
spinal cord
Peripheral nervous
system (PNS)
sensory information travels
in afferent
division
motor information
travels in
efferent division
which includes...
somatic autonomic
nervous nervous
system
system
Sensory
receptors
in eyes nose,
etc.
Peripheral
nervous
system
sympathetic
division
parasympathetic
division
cardiac
muscle,
smooth muscle
glands
effectors
skeletal
muscle
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Figure 27.1
Divisions of the Nervous System
• The PNS has an afferent division, which
brings sensory information to the CNS; and an
efferent division, which carries action (motor)
commands to the body’s “effectors”—muscles
and glands.
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Divisions of the Nervous System
• Within the PNS’s efferent division are two
subsystems:
– The somatic nervous system, which provides
voluntary control over skeletal muscles.
– The autonomic nervous system, which provides
involuntary regulation of smooth muscle, cardiac
muscle, and glands.
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Divisions of the Nervous System
• The autonomic system is further divided into
the sympathetic division, which generally has
stimulatory effects; and the parasympathetic
division, which generally has relaxing effects.
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27.2 Cells of the Nervous System
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Cells of the Nervous System
• There are three types of neurons:
– sensory neurons
– motor neurons
– interneurons
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Cells of the Nervous System
• Sensory neurons sense conditions inside and
outside the body and convey information about
these conditions to neurons inside the CNS.
• Motor neurons carry instructions from the
CNS to such structures as muscles or glands.
• Interneurons are located entirely within the
CNS and which interconnect other neurons.
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Cells of the Nervous System
Three types of neurons
sensory neuron
afferent
neuron
interneuron
neuron within
efferent neuron
central nervous system
motor neuron
effector
(muscle)
Anatomy of a neuron
axon
synaptic
terminals
dendrites
cell body
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Figure 27.2
Cells of the Nervous System
• Each neuron has extensions called dendrites
that receive signals coming to the neuron cell
body and a single large extension, called an
axon, that carries signals away from the cell
body.
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Cells of the Nervous System
• The support that glial cells provide for neurons
includes the production of a fat-rich wrapping,
called myelin, that can surround neuronal axons
and that increases the speed of neural signals.
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Cells of the Nervous System
A myelinated axon
myelin nodes
glial cells
glial cell
nucleus
myelin
covering
axon
glial cell
cytoplasm
Anatomy of a nerve
nerve
blood
vessels
connective
tissue
axons
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Figure 27.3
Cells of the Nervous System
• Recent research indicates that glial cells also
modify communication among neurons by, for
example, increasing the level of signaling that
goes on between differing sets of neurons.
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Cells of the Nervous System
• A nerve is a bundle of axons in the PNS that
transmits information to or from the CNS.
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27.3 How Nervous System
Communication Works
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Nervous System Communication
•
Nervous system communication can be
conceptualized as working through a two-step
process:
1. signal movement down a neuron’s axon
2. signal movement from this axon to a second cell
across a structure known as a synapse
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Nervous System Communication
• An electrical charge difference, called a
membrane potential, exists across the plasma
membrane of neurons because the inside of the
neuron is negatively charged relative to the
outside.
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Nervous System Communication
• The membrane potential represents a form of
potential energy that is put to use when special
protein channels in the neuron’s membrane
open up on stimulation, thereby allowing
charged particles called ions to flow into the
neuron.
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Nervous System Communication
• This influx of ions at an initial point on the
axon triggers reactions that cause the adjacent
portion of the axonal membrane to initiate the
same influx of ions.
• Thus, a conducted nerve impulse, called an
action potential, moves down the entire axon in
a set of linked reactions.
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Nervous System Communication
sodium ion
Resting potential
Electrical energy is stored across the plasma
membrane of a resting neuron. There are more
negatively charged compounds just inside the
membrane than outside of it. As a result, the inside
of the cell is negatively charged relative to the
outside. The charge difference creates a form of
stored energy called a membrane potential. Protein
channels (shown in green) that can allow the
movement of electrically charged ions across the
membrane remain closed in a resting cell, thus
maintaining the membrane potential.
Cell exterior is
positively charged
relative to interior
outside
cell
resting
membrane
potential
plasma
membrane
inside
cell
potassium ion
Action potential
Nerve signal transmission begins
when, upon stimulation, some protein
channels open up, allowing a movement of positively charged sodium ions
(Na+) into the cell. For a brief time, the
interior of the cell becomes positively
charged at this location. On either side
of this location, however, the interior of
the cell remains negatively charged.
Attracted by this negative charge, the
Na+ ions move laterally in both directions from their point of entry.
Cell interior becomes
positively charged
sodium ions
potassium ions
Cell interior becomes
positively charged again
Na+
The
gates close and the gates for
positively charged potassium ions (K+)
open up, allowing a movement of K+
out of the cell. With this, there is once
again a net positive charge outside the
membrane. Meanwhile, the arrival of
the charged Na+ ions “downstream”
from their original point of entry triggers an influx of Na+ ions at the next
Na+ channel.
Through repetition of this process, the
nerve signal is then propagated one
way along the axon. Given that Na+
ions move laterally in both directions
from their original point of entry, why
isn’t the signal propagated in both
directions? Because once an Na+ channel has opened, it enters a brief period
in which it cannot respond to any additional stimulus. Thus, each “upstream”
Na+ channel remains briefly closed,
while each “downstream” channel is
opened in succession.
action potential
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Figure 27.4
Nervous System Communication
• A nerve signal moves from one neuron to
another (or from a neuron to a muscle or gland
cell) across a synapse.
• This includes a “sending” neuron, a “receiving”
cell, and a tiny gap between the two cells,
called a synaptic cleft.
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How Neurons Work
Suggested Media Enhancement:
How Neurons Work
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Nervous System Communication
• A chemical called a neurotransmitter diffuses
across the synaptic cleft from the sending
neuron to the receiving neuron.
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Nervous System Communication
• It then binds with receptors on the receiving
neuron, thus keeping the signal going.
• Neurotransmitters can be degraded in synaptic
clefts by enzymes or taken back into a sending
cell in the process called reuptake.
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Nervous System Communication
sending cell
receiving cell
synaptic
cleft
synaptic
terminal
arrival of
nerve impulse
initiation of
new impulse
mitochondrion
vesicles containing
neurotransmitter
molecules (such as
acetylcholine)
neurotransmitter
receptors
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Figure 27.5
Nervous System Communication
PLAY
Animation 27.1: The Nervous System
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27.4 The Spinal Cord
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The Spinal Cord
• The spinal cord can act as a nervous system
communication center, receiving input from
sensory neurons and directing motor neurons
with no input from the brain.
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The Spinal Cord
• The spinal cord also channels sensory impulses
to the brain.
• In cross section, the spinal cord has a darker, Hshaped central area, composed mostly of the
cell bodies of neurons; and a lighter peripheral
area, composed mostly of axons.
• These two areas are the gray matter and white
matter of the spinal cord, respectively.
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The Spinal Cord
• The central canal of the spinal cord is filled
with cerebrospinal fluid, which provides the
spinal cord with nutrients.
• Spinal nerves extend from the spinal cord to
most areas of the body.
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The Spinal Cord
brain
cervical
spinal
nerves
thoracic
spinal
nerves
lumbar
spinal
nerves
gray
matter
sacral
spinal
nerves
white matter ventral root
spinal
nerve
dorsal root
ganglion central canaldorsal root
tip of
spinal cord
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Figure 27.6
The Spinal Cord
• Spinal cord motor neurons, which are sending
motor commands to muscles and other
effectors, have cell bodies that lie within the
gray matter of the spinal cord.
• The axons of these neurons leave the spinal
cord through its ventral roots.
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The Spinal Cord
• Sensory neurons, which transmit information to
the spinal cord, have their cell bodies outside
the spinal cord, in the dorsal root ganglia.
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The Spinal Cord
• Sensory information comes to the spinal cord
through a dorsal root, while motor commands
leave the spinal cord through a ventral root.
• Dorsal and ventral roots come together, like
fibers being joined in a single cable, to form a
given spinal nerve.
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Reflexes
• Reflexes are automatic nervous system
responses, triggered by specific stimuli, that
help us avoid danger or preserve a stable
physical state.
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Reflexes
• The neural wiring of a single reflex, called a
reflex arc, begins with a sensory receptor, runs
through the spinal cord to a motor neuron, and
proceeds back out to an effector such as a
muscle or gland.
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Reflex Arc
Stimulus (tapping) arrives
afferent
and receptor is activated. signal
receptor
reflex
arc
stimulus
effector
response
The signal from the receptor reaches a
sensory neuron cell body in the dorsal
root ganglion.
spinal
cord
motor
efferent neuron
signal
The signal arrives at a sensory
neuron/motor neuron synapse
in the spinal cord. Information
processing takes place prompting
a signal to be sent through the
motor neuron.
The motor neuron signal stimulates
the effector (the quadriceps muscles)
to contract. Note that CNS
processing for this reaction was
handled entirely in the spinal cord;
the brain was not involved.
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Figure 27.7
27.5 The Autonomic Nervous System
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The Autonomic Nervous System
• The sympathetic division of the autonomic
nervous system is often called the fight-orflight system because it generally activates
bodily functions.
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The Autonomic Nervous System
• The parasympathetic division is often called the
rest-and-digest system because it conserves
energy and promotes digestive activities.
• Most organs receive input from both systems.
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The Autonomic Nervous System
Parasympathetic division (rest and digest)
Sympathetic division (fight or flight)
constricts
pupil
dilates
pupil
stimulates
salivation
inhibits
salivation
cranial
nerves
slows
heart
accelerates
heart
cervical
nerves
facilitates
breathing
constricts
breathing
stimulates
digestion
thoracic
nerves
inhibits
digestion
stimulates
release of
glucose
stimulates
gallbladder
lumbar
nerves
contracts
bladder
sacral
nerves
secretes
adrenaline and
noradrenaline
relaxes
bladder
inhibits sex
organs
stimulates
sex organs
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Figure 27.8
27.6 The Human Brain
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The Human Brain
• The brain contains almost 98 percent of the
human body’s neural tissue.
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The Human Brain
• There are six major regions in the adult brain:
–
–
–
–
–
–
the cerebrum
thalamus and hypothalamus
midbrain
pons
cerebellum
medulla oblongata
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The Human Brain
• The cerebrum is divided into right and left
cerebral hemispheres and is the seat of our
higher thinking and processing.
• The cerebrum also has a thin outer layer, the
cerebral cortex, that is the site of our highest
thinking.
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The Human Brain
• The brainstem is a collective term for three
brain areas—the midbrain, pons, and medulla
oblongata.
• These brainstem structures are active in:
– Controlling involuntary bodily activities (such as
breathing and digesting).
– Relaying information.
– Processing sensory information.
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The Human Brain
cerebral
cortex
cerebrum
cerebellum
thalamus
hypothalamus
pituitary
gland
midbrain
brainstem
pons
medulla
oblongata
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Figure 27.9
The Human Brain
• Most of the body’s sensory perceptions are
channeled through the thalamus before going to
the cerebral cortex.
• The hypothalamus is important in sensing
internal conditions and in maintaining stability
or homeostasis in the body, largely through its
control of many of the body’s hormones.
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Routing of Sensory Information
somatosensory cortex
thalamus
gustatory
cortex
auditory
cortex
olfactory
cortex
visual
cortex
vision
hippocampus
amygdala
smell
hypothalamus
brainstem
taste
hearing
touch senses
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Figure 27.10
27.7 The Nervous System in Action:
Our Senses
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Our Senses
• Human beings have more sensory capabilities
than the famous five of vision, touch, smell,
taste, and hearing.
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Our Senses
• Each sense employs cells called sensory
receptors that do two things:
– Respond to stimuli (such as vibration in sound).
– Transform these responses into the language of the
nervous system—electrical signals that travel
through action potentials.
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Our Senses
• Signals from every sense except smell are
routed through the brain’s thalamus and then to
specific areas of the cerebral cortex.
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27.8 Our Senses of Touch
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Our Senses of Touch
• Touch works through a variety of sensory
receptors that distinguish among such qualities
as light or heavy pressure and new or ongoing
contact.
• In some sensory cells, the stretching of their
outer membrane prompts an influx of ions that
results in the initiation of a nerve signal.
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Our Senses of Touch
Touch receptors in the skin
hair
tactile receptor
free nerve endings
(pain, temperature)
epidermis
tactile receptor
dermis
hair follicle
receptor
tactile receptor
Pacinian
corpuscle
How one touch receptor works
interior of nerve ending
pressure
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Figure 27.11
27.9 Our Sense of Smell
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Our Sense of Smell
• Our sense of smell, or olfaction, works through
a set of sensory receptors whose dendrites
extend into the nasal passages.
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Our Sense of Smell
• “Odorants,” which are molecules that have
identifiable smells, bind with hair-like
extensions of these dendrites, resulting in a
nerve signal to the brain.
• The higher processing centers of the brain
distinguish one odorant from another by
sensing unique groups of neurons that fire in
connection with given odorants.
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Our Sense of Smell
olfactory
bulb
olfactory
bulb
to olfactory cortex,
amygdala,
and hypothalamus
olfactory
tract
supporting
cells
olfactory
receptor
cell
olfactory
epithelium
odorants
odorants
mucous
layer
cilia
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Figure 27.12
27.10 Our Sense of Taste
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Our Sense of Taste
• Our sense of taste works through a group of
taste cells, located in taste buds near the surface
of the tongue, which have receptors that bind to
“tastants,” or molecules of food that elicit
different tastes.
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Our Sense of Taste
• A given taste cell can respond through any of
four (or perhaps five) chemical signaling routes
that correspond to the basic tastes of sweet,
sour, salty, and bitter and a possible taste of
umami.
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Our Sense of Taste
papilla
taste buds
connective
tissue
salivary glands
muscle layer
papillae
taste bud
taste pore
microvilli taste connective dendrites
cell
tissue
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Figure 27.13
Our Sense of Taste
• The neurons that receive input from taste cells
vary in their response to different tastants.
• The brain makes sense of the pattern of input it
gets from these neurons, thus yielding the large
number of tastes we experience.
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27.11 Our Sense of Hearing
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Our Sense of Hearing
• Our sense of hearing is based on the fact that
vibrations result in “waves” of air molecules
that are, by turns, more and less compressed
than the ambient air around them.
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Our Sense of Hearing
• These waves of compression bump up against
our eardrums (or tympanic membranes), which
in turn vibrate.
• This initiates a chain of vibration that ends in
the fluid-filled cochlea of the inner ear.
• “Hair cells” in the cochlea have ion channels
that open and close in response to this
vibration, resulting in nerve signals to the brain.
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Our Sense of Hearing
Anatomy of the ear
incus
malleus stapes
oval window
tympanic
membrane
cochlea
nerve
ear canal
outer ear
middle ear
inner ear
From air vibration to nerve signal
The tympanic membrane
vibrates the three bones of
the middle ear; the
malleus, incus
2
and stapes.
Sound waves enter
through the ear
sound
1
canal and vibrate
the tympanic
membrane.
The vibration of the stapes
focuses the sound-wave
vibration on the membrane
of the oval window.
3
4
The oval window’s vibrations
cause fluid vibrations within the
coiled, tubular cochlea (shown
elongated here for illustrative
purposes).
perception
of sound
5
These fluid vibrations cause
cells within the cochlea to
release a neurotransmitter
which triggers a nerve signal
to the brain.
How fluid triggers nerve signal
tectorial membrane
tectorial
membrane
hair
cells
vestibular
duct
cochlear
duct
tympanic
duct
K+
nerve
nucleus
basilar membrane
Ca2+
Seen in cross-section, the
cochlea has vestibular
and tympanic ducts, in
which fluid is vibrating.
This vibration shakes the
basilar membrane, pushing
hair cells on it up against the
overlying tectorial membrane.
Ca2+
As the hair cells contact the
tectorial membrane, cilia on
them bend. This change in
position causes “trap door”
channels in the hair cells to
open, which allows
potassium ions (K+) to flow
into them.
This influx triggers an influx
of calcium ions (Ca2+) at the
base of the hair cells, which
in turn causes the cells to
release a neurotransmitter.
The neurotransmitter is
received by adjacent
dendrites and a nerve
signal is sent to the brain.
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Figure 27.14
27.12 Our Sense of Vision
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Our Sense of Vision
• The human visual system must accomplish
three central tasks.
– Gather and focus light reflected by objects in the
outside world.
– Convert light signals into nervous system signals.
– Make sense of the visual information it has
received.
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Our Sense of Vision
• Light first enters the eye through the cornea and
then passes through the lens and various
materials on its way to a layer of tissue called
the retina at the back of the eye.
• Light is bent or refracted by the cornea and the
lens in such a way that it ends up as a tiny,
sharply focused image on the retina.
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Our Sense of Vision
vitreous
body
retina
cornea
iris
pupil
lens
optic nerve
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Figure 27.15
Our Sense of Vision
• Light signals are converted to nervous system
signals by cells in the retina called
photoreceptors, which come in two varieties:
rods and cones.
• Rods function in dim light but are not sensitive
to color.
• Cones function best in bright light but are
sensitive to color.
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Our Sense of Vision
Normal vision
light rays converge
on the retina
Farsighted vision
light rays converge
behind the retina
Nearsighted vision
light rays converge
in front of the retina
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Figure 27.16
Our Sense of Vision
• These photoreceptors have pigments, or lightabsorbing molecules, embedded in membranes
within them.
• When light strikes a pigment, it changes shape
in a way that prompts a cascade of chemical
reactions that results in neurotransmitter release
being inhibited between the rod or cone and its
adjoining connecting cell.
• This lack of release sends the signal,
“Photoreceptor stimulated here.”
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Our Sense of Vision
• Vision signals travel from photoreceptors
through two sets of adjoining cells, the latter of
which have axons that come together to form
the body’s optic nerves.
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Our Sense of Vision
• The brain does not passively record visual
information. Rather, it constructs images as
much as it records them.
• The visual system operates through a series of
genetically based “rules” that allow us to
quickly make sense of what we perceive.
• Evolution shaped our vision in this way to be
maximally useful to us in survival and
reproduction.
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27.13 The Endocrine System
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The Endocrine System
• The endocrine system functions in the control
and regulation of bodily processes.
• It works through a group of chemical
messengers called hormones: substances
secreted by one group of cells that travel
through the bloodstream and affect the
activities of other cells.
• Hormones are distinct from other signaling
molecules that do not travel through the
bloodstream.
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The Endocrine System
• Molecules called paracrines diffuse from one or
more cells to a nearby group of cells, causing a
metabolic change in them.
• Likewise, a cell can be affected by its own
secreted chemical messenger, called an
autocrine.
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The Endocrine System
• Each hormone works only on specific cells—
the hormone’s target cells.
• Hormones bind to their target cells via
receptors on or in the target cells.
• This binding then spurs chemical reactions
within the target cells.
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The Endocrine System
Endocrine cells
release hormone.
Hormone enters
circulation.
Hormone is
carried throughout the body.
Hormone will not
bind to cells that
are not target cells
receptor
Binding occurs;
hormonal effects
take place.
target cell
(skeletal muscle)
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Figure 27.19
The Endocrine System
• Hormonal production and secretion take place
to a significant extent within endocrine glands,
meaning glands that secrete materials directly
into the bloodstream or into surrounding
tissues.
• Some hormones are secreted, however, not by
specialized glands, but by organs such as the
heart or kidneys.
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Major Hormone Secreting Glands
of the Body
hypothalamus
pineal gland
pituitary gland
thyroid gland
thymus gland
adrenal
glands
parathyroid
glands
(on posterior
surface of
thyroid gland)
cortex
medulla
pancreas
testes
ovaries
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Figure 27.20
The Endocrine System
• Once secreted, hormones can take from several
seconds to several hours to work, but then can
continue to have effects for extended periods of
time.
• Given these timeframes, hormones tend to
regulate processes that unfold over minutes,
hours, or even years.
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27.14 Types of Hormones
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Types of Hormones
• There are three principal classes of hormones:
– amino-acid-based hormones
– peptide hormones
– steroid hormones
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Types of Hormones
• Each amino acid-based hormone is derived
from a chemical modification of a single amino
acid.
• Peptide hormones are composed of amino acid
chains whose lengths can vary greatly.
• Steroid hormones are all constructed around the
chemical framework of the cholesterol
molecule.
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Types of Hormones
• Amino-acid-based and peptide hormones
generally link to their target cells via receptors
that protrude from the target cells’ outer
membranes.
• Some steroid hormones can bind in this
manner, but most pass through a cell’s plasma
membrane and bind with a receptor protein
inside the cell.
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Types of Hormones
• The combined steroid hormone/receptor
molecule then binds with the cell’s DNA, thus
turning on one or more cell genes, which results
in the production of one or more proteins.
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The Endocrine System
PLAY
Animation 27.2: The Endocrine System
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27.15 How is Hormone
Secretion Controlled?
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How is Hormone Secretion Controlled?
• Almost all hormone secretion is controlled by
negative feedback.
• With its many negative-feedback loops, the
endocrine system is important in preserving
homeostasis, meaning an organism’s tendency
to maintain a relatively stable internal
environment.
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How is Hormone Secretion Controlled?
• A part of the endocrine system can be viewed
as a hierarchy that has the brain’s hypothalamus
at the top.
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How is Hormone Secretion Controlled?
•
Although it ultimately is prompted to act via
negative feedback, the hypothalamus:
1. Acts as an endocrine organ, producing two
hormones that are released by the posterior
pituitary gland.
2. Exercises control, via the nervous system, over
the release of two hormones—adrenaline and
noradrenaline—that are produced by the adrenal
glands.
3. Controls release of six hormones secreted by the
anterior pituitary gland.
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How is Hormone Secretion Controlled?
• The anterior pituitary is known as the body’s
“master gland” because four of the hormones it
releases go on to affect the release of hormones
in other endocrine glands.
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How is Hormone Secretion Controlled?
• The hypothalamus exercises control over the
anterior pituitary through a set of releasing and
inhibiting hormones that it sends to the anterior
pituitary via a tiny set of blood vessels.
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Hypothalamus: Pivotal Player
antidiuretic
hormone (ADH)
Controls retention
of water in the body
by the kidneys
hypothalamus
kidneys
1
3
anterior
pituitary
oxytocin
Stimulates contraction
of uterus muscles and
release of milk in
females; assists in
semen ejaculation in
males; possible social
role in both sexes
2
posterior
pituitary
medulla
cortex
adrenocorticotropic hormone (ACTH)
Stimulates adrenal cortex to secrete
glucocorticoids, which regulate energy use.
thyroid-stimulating hormone (TSH)
Triggers release of thyroid hormones
which increase metabolic rate
growth hormones (GH)
Stimulates growth by prompting liver’s
release of somatomedin hormones
adrenaline + noradrenaline
adrenal gland
prostate gland
in males
uterus and
mammary
glands in
females
glucocorticoids
thyroid hormones
thyroid gland
bone, muscle, other tissues
prolactin (PRL)
Stimulates mammary gland development
and production of milk
mammary glands
follicle-stimulating hormone
(FSH)
r
Male: promotes sperm production
Female: promotes egg development;
stimulates ovaries to produce estrogen
Luteinizing hormone (LH)
Female: produces ovulation; stimulates
ovaries to produce estrogen and progesterone
Male: stimulates testes to produce androgens
testosterone
testes
estrogen
progesterone
ovaries
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Figure 27.21
27.16 Hormones in Action: Four Examples
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Insulin and Glucagon
• The human body controls its blood levels of the
sugar glucose through the use of two hormones
secreted in the pancreas:
– insulin, a hormone that reduces blood levels of
glucose
– glucagon, a hormone that increases blood levels of
glucose
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Insulin and Glucagon
• Insulin is produced by the pancreas’ beta cells
while glucagon is produced by its alpha cells.
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Insulin and Glucagon
• After moving into circulation, insulin prompts
cells to create channels through which they take
in circulating glucose.
• The primary target cells for insulin are liver,
skeletal muscle, and fat cells.
• In liver and skeletal muscle cells, some of the
glucose that is taken in may be stored in the
form of a molecule called glycogen.
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Insulin and Glucagon
• Glucagon prompts the transformation of
glycogen back into glucose in skeletal muscle
and liver cells.
• Muscle cells use all of the glucose produced
within them through this process to power their
own activities.
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Insulin and Glucagon
• In contrast, much of the glucose produced in
the liver through this process is released into
general circulation.
• The disease diabetes results from a failure of
the body to move glucose into cells.
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The Control of Glucose Levels
After a meal, the role of insulin
glucose
glycogen
glycogen
insulin
beta cell
glucose
channel
glucose
muscle cell
Pancreas: Stimulated by high
levels of glucose in the bloodstream, beta cells in the Islets
of Langerhans produce insulin.
Other cells throughout the
body: Insulin enables glucose
to move from the bloodstream
into cells by triggering the formation of channels in the cell
membranes.
liver cells
Skeletal muscle cells and liver cells: With insulin’s help,
glucose can move into these cells and either be used right away
or stored in the form of glycogen molecules.
In between meals, the role of glucagon
glycogen
glucose
glycogen
glucose
glucose
glucagon
alpha cell
Pancreas: Stimulated by low
levels of glucose in the bloodstream, alpha cells in the
Islets of Langerhans produce
glucagon.
muscle cell
liver cells
Skeletal muscle cells and liver cells: With glucagon’s help,
glycogen is broken down into glucose. Muscle cells retain all
the glucose they derive from this process, using it to power
their own activities. Liver cells, meanwhile, move much of the
glucose they liberate into general circulation.
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Other cells throughout the
body: Glucose released by
the liver moves from the bloodstream into cells, supplying
them with energy.
Figure 27.23
Oxytocin
• The hormone oxytocin, released by the
posterior pituitary gland, prompts labor
contractions in childbirth and the release of
milk from nursing mothers.
• It also appears to stimulate various forms of
social contact among mammals and to affect
the levels of trust that human beings are willing
to place in one another.
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Cortisol
• The hormone cortisol, released by the adrenal
glands, helps bring energy stores into use but
also is a “stress” hormone that can have
negative effects if stress goes on for extended
periods.
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Hormones: Sources and Effects
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Table 27.1