Topic 6.5
Option H1


Nervous systems are the most intricately
organized data-processing systems on Earth.
Neuron
Nerve cells; your brain contains an estimated 100 billion
 Specialized for carrying signals from one location in the
body to another.
 consists of a cell body, containing the nucleus and cell
organelles, and long, think extensions called neuron
fibers that convey signals.
 Each neuron may communicate with thousands of others,
forming networks that enable us to learn, remember,
percieve our surroundings, and move.


Nervous system has two main divisions:

Central nervous system (CNS)
 Consists of the brain and, in vertebrates, the spinal cord

Peripheral nervous system (PNS)
 Made up mostly of communication lines called nerves
that carry signals into and out of the CNS
 A nerve is cable-like bundle of neuron extensions
tightly wrapped in connective tissue.
 In addition to nerves, the PNS also has ganglia, clusters
of neuron cell bodies.

A nervous system has three interconnected
functions:

Sensory input
 The conduction of signals from sensory receptors, such
as light-detecting cells of the eye, to integration centers.

Integration
 The interpretation of the sensory signals and the
formulation of responses.

Motor output
 The conduction of signals from the integration centers
to effector cells, such as muscle cells or gland cells,
which perform the body’s responses.


The relationship between neurons and the nervous system
structure and function is easiest to see in the relatively
simply circuits that produce reflexes, or automatic responses
to stimuli
Three functional types of neurons correspond to a nervous
system’s three main functions:
 Sensory neurons
 Convey signals (information) from sensory receptors into the
CNS

Interneurons
 Located entirely within the CNS.
 Integrate data and then relay appropriate signals to other
interneurons or to motor neurons

Motor neurons
 Convey signals from the CNS to effector cells

Knee-jerk reflex (when the knee is tapped)




1. a sensory receptor detects a stretch in the tendon
2. a sensory neuron conveys this information into the
CNS (spinal cord)
3. In the CNS, the information goes to a motor
neuron and to an interneuron.
4. One set of muscles (quadriceps) responds to motor
signals conveyed by a motor neuron by contracting,
jerking the lower leg forward.
 Simultaneously, another motor neuron, responding to
signals from the interneuron, inhibits the flexor
muscles, making them relax and not resist the action of
quadriceps.





The ability of neurons to receive and transmit
information depends on their structure.
Most of a neuron’s organelles, including its nucleus, are
located in the cell body.
Arising from the cell body are two types of extensions:
numerous dendrites and a single axon
Dendrite
 Highly branched extensions that receive signals from
other neurons and convey this information toward the
cell body
 Often short
Axon


Typically a much longer extension that transmits signals to other
cells, which may be other neurons or effector cells.
Some axons, like the ones that reach from your spinal cord to
your feet, can be over a meter long.


Neuron only make up part of a nervous system
Outnumbering neurons by as many as 50 to 1
are supporting cells, or glia, that are essential for
the structural integrity and normal functioning
of the nervous system.

Schwann cell:
 One kind of supporting cell found in PNS
 Forms beads along axon, and is insulated by Myelin
sheath
 Myelin sheath is essentially a chain of Schwann cells, each
wrapped many times around the axon.
 Spaces between Schwann cells are called nodes of Ranvier;
the only points on the axon where signals can be
transmitted.


Everywhere else besides the nodes of Ranvier,
the myelin sheath insulates the axon,
preventing signals from passing along it.
When a signal travels along a myelinated axon,
it jumps from node to node:



By jumping along the axon, the signal travels much
faster than it could if it had to take the long route
along the whole length of the axon.
In human nervous system, signals can travel along a
myelinated axon about 150 m/sec (over 330mi/hr!),
which means that a command from your brain can
make your fingers move in just a few milliseconds.
Without myelin sheaths, the signals could go only
about 5 m/sec


A typical axon has hundreds or thousands of
these branches, each with a synaptic terminal
at the very end.
The site of communication between a synaptic
terminal and another cell is called a synapse.



MS demonstrates the importance of myelin
sheaths.
MS leads to a gradual destruction of myelin
sheaths by the individual’s own immune
system.
The result is a progressive loss of signal
conduction, muscle control, and brain function.


To understand nerve signals, we must first study a
resting neuron, one that is not transmitting a signal.
Membrane potential





A resting neuron has potential energy, or membrane potential,
that can be put to work to send signals from one part of the body
to another.
Resides in an electrical charge difference across the neuron’s
plasma membrane.
The cytoplasm just inside the membrane is negative in charge,
and the fluid outside the membrane is positive.
Since opposite charges tend to move toward each other, a
membrane stores energy by holding opposite charges apart, like a
battery.
A voltmeter measures the strength (voltage) of a neuron’s stored
energy

Membrane potential (continued)
 The resting potential is the voltage across the plasma membrane of a
resting neuron.
 About -70 millivolts (the minus sign indicates that the inside of the cell
is negative relative to the outside)
 Exists because of differences in ionic composition of the fluids inside
and outside the cell.
 Plasma membrane surrounding the neuron has channels and pumps,
made of proteins, that regulate the passage of inorganic ions
 One result is that a resting membrane allows much more K+ than
Na+ to diffuse inward, making Na+ much more concentrated
outside the cell than inside.
-Na+ channels allow very little to diffuse in.
 However, K+, which is more concentrated inside, can freely flow
out, leaving behind an excess of negative charge.
 Also, membrane proteins called the sodium-potassium pumps:
 These pumps actively transport Na+ out of the cells and K+ in,
thereby helping keep the concentration of Na+ low in the cell and
K+ high.

Membrane potential (continued)


The ionic gradient (high K+/low Na+ concentrations
inside coupling with low K+/high Na+
concentrations outside) produces an electrical
potential difference, or voltage, across the membrane
–the resting potential
The membrane potential can change from its resting
value if the membrane’s permeability to particular
ions changes  basis of nearly all electrical signals in
the nervous system.



The disovery of axons in squids (up to 1mm in diameter)
gave researchers their first chance to study how stimuli
trigger signals in a living neuron.
 From microelectrode studies with squid neurons,
British biologists worked out the details of nerve
signal transmission in the 1940s.
Stimulating a neuron’s plasma membrane can trigger the
release and use of the membrane’s potential energy to
generate a nerve signal.
 Similar to turning on a flashlight using the potential
energy stored in a battery to create light.
A stimulus is any factor that causes a nerve signal to be
generated.
 Examples include light, sound, a tap on the knee, or a
chemical signal from another neuron.

Action potential


Graphing electrical changes in neuron membranes was
the first step in discovering how nerve signals are
generated.
An action potential is a nerve signal that carries
information along an axon
The Action Potential (Figure 28.4)
1.
The graph starts out at the membrane’s resting potential (-70mV)
2.
The stimulus is applied. If it is strong enough, the voltage rises to
what is called the threshold
1.
The difference between the threshold and the resting potential
is the minimum change in the membrane’s voltage that must
occur to generate the action potential.
3.
Once threshold is reached, the action potential is triggered.
1.
The membrane polarity reverse abruptly, with the interior of
the cell becoming positive with respect to the outside.
4.
The membrane then rapidly repolarizes as the voltage drops back
down.
5.
The resting potential undershoots.
6.
Resting potential returns

What causes the electrical changes of the action
potential?

The rapid flip-flop of the membrane potential results
from the rapid movements of ions across the
membrane at Na+ and K+ channels.
 Called voltage-gated channels because they have special
gates that open and close, depending on the changes in
membrane potential

Causes of action potential (Figure 28.4):


1. the resting membrane is positively charged on the
outside, and the cytoplasm just inside the membrane
is negatively charged.
2. A stimulus triggers the opening of a few Na+
channels in the membrane, and a tiny amount of Na+
enters the axon.
 Makes the inside surface of the membrane slightly less
negative
 If the stimulus is strong enough, a sufficient number of
Na+ channels open to raise the voltage to the threshold.

3.Once the threshold is reached, more Na+ channels
open, allowing even more Na+ to diffuse into the cell.
 As more and more Na+ moves in, the voltage soars to its
peak.

Causes of action potential (continued…)

4. The peak voltage triggers closing and inactivation
of the Na+ channels.
 Meanwhile, the K+ channels open, allowing K+ to
diffuse rapidly out.
 These changes produce the downswing on the graph.



5. A very brief undershoot of the resting potential
results because the K+ channels close slowly.
6. The membrane then returns to its resting potential.
**In a typical neuron, this entire process is very
brief—only about 1-2 msec in duration.


For an action potential to function as a longdistance signal, it must travel along the axon
from the cell body to the synaptic terminals.
It does so by regenerating itself along the axon


A nerve signal starts out as an action potential
generated in the axon near the cell body of the
neuron.
The effect of this action potential is like tipping the
first row of standing dominoes: the first domino does
not travel along the row, but its fall is relayed to the
end of the row, one domino at a time.

Propagation of the action potential along an axon (Figure 28.5):
 1.When the region of the axon closest to the cell body has its Na+
channels open, Na+ rushes inward and an action potential is
generated.
 This corresponds to the upswing of the curve in step 2 in
Figure 28.4.
 2. Soon, the K+ channels in the same region open, allowing K+ to
diffuse out of the axon;
 at this time, its Na+ channels are closed and inactivated
 we would see the downswing of the action potential.
 3.a short time later, we would see no signs of an action potential
at this spot on the axon (closest to the cell body) , because the
axon membrane here has returned to its resting potential.

“Domino Effect” of a nerve signal:



1. In step 1 of Figure 28.5, local spreading of the
electrical changes caused by the inflowing Na+
associates with the first action potential
2. This triggers the opening of Na+ channels in the
membrane just to the right of the action potential.
resulting in a 2nd action potential
In the same way, a third action potential is generated
in step 3, and each action potential generates another
all the way down the axon.

So why are action potentials propagated in only one direction
along the axon?
 Local electrical changes do spread in both directions in the axon.
 However, these changes cannot open Na+ channels and
generate an action potential when the Na+ channels are
inactivated.
 Thus, an action potential cannot be generated in the regions
where K+ is leaving the axon and Na+ channels are still
inactivated.
 Consequently, the inward flow of Na+ that depolarizes the axon
membrane ahead of the action potential cannot produce another
action potential behind it.
 Once an action potential starts, it moves only in the one
direction, toward the synaptic terminals.

Example:


If you tap your finger on a desk, the contact is a
stimulus that triggers action potentials in the tips of
the sensory neurons in your skin.
The action potentials propagate along the axons,
carrying the information into your central nervous
system.


Action potentials are “all-or-none” events; that
is they are the same no matter how strong or
weak the stimulus that triggers them.
How, then, do action potentials relay different
intensities of information to your central
nervous system?


It is the frequency of action potentials that changes
with the intensity of stimuli
For example, if you tap your finger hard against the
desk, your CNS receives many more action
potentials per millisecond than after a soft tap.



When the action potential reaches the end of
an axon, it generally stops there.
In most cases, action potentials are not
transmitted from cell to cell.
However, information is transmitted, and this
transmission occurs at synapses– the regions
of communication between a synaptic
terminal and another cell.

Synapses come in two varieties:
 1. electrical
 Electrical current passes directly from one neuron
to the next.
 The receiving neuron is quickly and at the same
frequency of action potentials as the sending
neuron.
 In the human body, electrical synapses are found
in the heart and digestive tract, where nerve
signals maintain steady, rhythmic muscle
contractions.
Electrical Synapse

2. chemical
 Prevalent in most other organs, including the CNS, where
signaling among neurons is more complex and varied.
 Have a narrow gap, called the synaptic cleft, separating a
synaptic terminal of the sending neuron from the receiving
neuron.
 The cleft prevents the action potential in the sending neuron
from spreading directly to the receiving neuron.
 Instead, the action potential (electrical signal) is first
converted to a chemical signal.
 The chemical signal, consisting of molecules of
neurotransmitter, may then generate an action potential in the
receiving cell.
 Neurotransmitter is contained in synaptic vesicles in the
sending neuron’s synaptic terminals.

Events that occur at a chemical synapse (Figure 28.6)
 1. An action potential arrives at the synaptic terminal.
 2. The action potential triggers chemical changes that cause some
synaptic vesicles to fuse with the plasma membrane of the sending
cell.
 3. The fused vesicles release their neurotransmitter molecules by
exocytosis into the synaptic cleft, and the neurotransmitter diffuses
across the cleft.
 4. (4-6 varies among different types of chemical synapses) In one
common type, the released neurotransmitter binds to receptor
molecules on the receiving cell’s plasma membrane.
 5. the binding of neurotransmitter to receptor opens chemically
gated ion channels in the receiving cell’s membrane
 With the channels open, ions can diffuse into the receiving cell and
trigger new action potentials.
6. The neurotransmitter is broken down by an enzyme or transported
back into the signaling cell, and the ion channels close.
*Ensures that the neurotransmitter’s effect is brief and
precise.
Chemical Synapse

As you read this PowerPoint…
 Action potentials carrying information about the words on the
SmartBoard are streaming along sensory neurons from your
eyes to your brain.
 Arriving at synapses with receiving cells (interneurons in the
brain), the action potentials are triggering the release of
neurotransmitters at the ends of sensory neurons.
 The neurotransmitters are diffusing across synaptic clefts and
triggering changes in some of your interneurons—changes that
lead to integration of the signals and ultimately to what the
signals actually mean (in this case, the meaning of words and
sentences)
 Next, motor neurons in your brain will send out action
potentials to muscle cells in your fingers, telling them to write
down this information in just the right way.

One neuron may interact with many others.



In fact, a neuron may receive information via
neurotransmitters from hundreds of other neurons
via thousands of synaptic terminals.
Inputs can be highly varied because each sending
neuron may secrete a different quantity or kind of
neurotransmitter.
If the signals are strong enough to raise the
membrane potential to threshold, an action potential
will be generated in the receiving cell.
 That neuron then passes signals along its axon to other
cells at a rate that represents a summation of all the
information it has received.
 ***Signal frequency is key because action potentials are allor-none events***

What do neurotransmitters actually do to receiving neurons?
 The binding of a neurotransmitter to a receptor may open ion
channels in the receiving cell’s plasma membrane or trigger a
signal transduction pathway that does so.
 The more neurotransmitter molecules that bind to receptors on
the receiving cell and the closer the synapse is to the base of the
receiving cell’s axon, the stronger the effect.
 For example, neurotransmitters that open Na+ channels may
trigger action potentials in the receiving cell.
 These neurotransmitters and synapses from which they are
released are referred to as “excitatory”
 In contrast, many neurotransmitters open membrane channels
for ions that decrease the tendency to develop action potentials
in the receiving cell—such as channels that release K+
 These neurotransmitters and synapses from which they are released
are referred to as “inhibitory.”



The propagation of nerve signals across
chemical synapses depends on
neurotransmitters.
Many are small, nitrogen-containing organic
molecules.
Acetylcholine



Important in the brain and at synapses between
motor neurons and muscle cells.
Depending on the kind of receptors on receiving
cells, it may be excitatory or inhibitory.
For example, it makes our skeletal muscles contract
but slows the rate of contraction of cardiac muscles.

Biogenic amines





Derived from amino acids; nitrogen-containing.
Include epinephrine, norepinephrine, serotonin, and dopamine,
all of which also function as hormones.
Important in the CNS.
Serotonin and dopamine affect sleep, mood, attention, and
learning.
Imbalances of biogenic amines are associated with various
disorders.
 Parkinson’s disease is associated with a lack of dopamine,
and an excess of dopamine is linked to schizophrenia.
 Reduced levels of norepinephrine and serotonin seem to be
linked with some types of depression.
 Some psychoactive drugs, including LSD, apparently produce
their hallucinatory effects by binding to serotonin and
dopamine receptors in the brain

Aspartate, glutamate, glycine, and GABA
(gamma aminobutyric acid)


Amino acids that are known to be important in the
CNS
Aspartate and glutamate are excitatory; glycine and
GABA are inhibitory.

Several peptides, relatively short chains of
amino acids:


One called substance P, is an excitatory
neurotransmitter that mediates our perception of
pain.
Also, endorphins function as both neurotransmitters
and hormones, decreasing our perception of pain
during times of physical or emotional stress.

Dissolved gases
Nitric oxide (NO); during sexual arousal in human
males, certain neurons release NO into the erectile
tissue of the penis, and the NO triggers an erection.
 Neurons produce NO molecules on demand, rather
than storing them in synaptic vesicles.
 The dissolved gas diffuses into neighboring cells,
produces a change, and is quickly broken down.
 **The erectile dysfunction drug Viagra promotes this
effect of NO**



Drugs are used both medicinally and
recreationally.
While they have the ability to increase alertness
and sense of well-being or to reduce physical
and emotional pain, they also have the
potential to disrupt the brain’s finely tuned
neural pathways, altering the chemical
balances that are the product of millions of
years of evolution.

Many psychoactive drugs, even common ones
such as caffeine, nicotine, and alcohol, affect the
action of neurotransmitters in the brain’s
billions of synapses.
Caffeine keeps us awake by countering the effects of
inhibitory neurotransmitters.
 Nicotine acts as a stimulant by binding to and
activating acetylcholine receptors.
 Alcohol is a strong depressant

 Its precise effect on the nervous system is not yet
known, but its seems to increase the inhibitory effects of
the neurotransmitter GABA.

Prescription drugs
Used to treat psychological disorders by altering the
effects of neurotransmitters.
 Most popular class of antidepressant medication
works by affecting the action of serotonin.
 Selective serotonin reuptake inhibitors (SSRIs), block
the removal of serotonin from synapses, increasing
the amount of time this mood-altering
neurotransmitter is available to receiving cells.
 Tranquilizers such as diazepam (Valium) and
alprazolam (Xanax) activate the receptors for GABA,
increasing the effect of this inhibitory
neurotransmitter.


Prescription drugs


Antipsychotic drugs are used to treat schizophrenia
by blocking dopamine receptors
Drug used to treat attention deficit hyperactivity
disorder (ADHD), such as methphenidate (Ritalin)
are chemically similar to dopamine and
norepinephrine.
 ADHD medications are believed to block the reuptake
of these neurotransmitters, but their precise actions are
poorly understood.

Illegal drugs
 Stimulants such as amphetamines and cocaine increase the
release and availability of norepinephrine and dopamine at
synapses.
 Abuse of these drugs can produce symptoms resembling
schizophrenia.
 The active ingredient in marijuana (tetrahydrocannabinol, or
THC) binds to brain receptors normally used by another
neurotransmitter (anandamide) that seems to play a role in
pain, depression, appetite, memory, and fertility.
 Opiates bind to endorphin receptors, reducing pain and
producing euphoria.
 Morphine, codeine, and heroin.
 Can be used medicinally for pain relief
 However, abuse may permenantly change the brain’s chemical
synapses and reduce normal synthesis of neurotransmitters.



Hormone: a chemical signal that is carried by the circulatory
system (usually in the blood) and that communicates regulatory
messages within the body.
Endocrine glands are the organs that make and secrete hormones.
Endocrine system contain all of an animal’s hormone-secreting
cells and serves as the body’s main chemical-regulating system.
 Because most hormones are carried in the blood, they reach all
parts of the body, and the endocrine system is thus especially
important in controlling whole-body activities.
 For example, hormones coordinate responses to stimuli such as
stress, dehydration, or low blood glucose levels.
 Hormones also regulate long-term developmental processes,
such as growth, maturation, and reproduction.

Hormones may travel to all parts of the body,
but only certain types of cells, called target cells,
are equipped to respond.



A single hormone molecule may dramatically alter a
target cell’s metabolism by turning on or off the
production of a number of enzymes.
A tiny amount of a hormone can govern the activities
of enormous numbers of target cells in a variety of
organs.
A hormone is ignored by other types of cells
(nontarget cells)



Hormones are the body’s long-distance
chemical regulators and convey information via
the bloodstream to target cells throughout the
body.
Local regulators, other chemical signals, are
secreted into the interstitial fluid and affect only
nearby target cells.
Phermones, another chemical signal, carry
messages between different individuals of a
species, as in mate attraction.

The endocrine system often collaborates with
the nervous system.

The nervous system transmits electrical signals via
nerve cells rapid messages control split-second
responses.
 Ex. Jerk of your hand away from a flame result from
high-speed nerve signals.

Endocrine system coordinates slower but longerlasting responses
 Takes hours or even days to act, partly because of the
time it takes for hormones to be made and transported
to all their target organs.

The lines between the nervous system and
endocrine system are in reality blurred, although
we distinguish them as separate systems.



For example, certain specialized nerve cells called
neurosecretory cells perform functions of both
systems.
Like all nerve cells, neurosecretory cells conduct nerve
signals, but they also make and secrete hormones in
the blood.
Some chemicals serve both as hormones in the
endocrine system and as chemical signals in the
nervous system
 For example, epinephrine (adrenaline), functions as the
“fight or flight” hormone and as a neurotransmitter.
 **note that only hormones travel in the blood stream, not
neurotransmitters

Three major classes of molecules function as
hormones in vertebrates: proteins and
peptides, amines, and steroids.



Proteins and peptides (small polypeptides
containing 3 to 30 amino acids) and amines
(molecules derived from amino acids) are watersoluble.
Steroid hormones are lipid-soluble.
*Both water-soluble and lipid-soluble hormones
elicit cellular responses differently.

Regardless of their chemical structure, however, signaling by any of
these molecules involves three key events: reception, signal
transduction, and response.
 Reception
 Occurs when a hormone binds to a specific receptor protein on
or in the target cell.
 Each signal molecule has a specific shape that can be
recognized by its target cell receptors.
 *cells without receptors for a particular chemical signal do not
respond to that signal.
 Signal Transduction
 The binding of a signal molecule to a receptor protein triggers
events within the target cell
 Response
 Result of signal transduction

Water soluble hormones

The receptors for these hormones are embedded in the plasma
membrane of target cells and project outward from the cell
surface.
 1.A hormone molecule binds to the receptor protein, activating it.
 2.This initiates a signal transduction pathway, a series of changes in
cellular proteins (relay molecules) that convert an extra-cellular
chemical signal to a specific intracellular response.
 3. The final relay molecule activates a protein that carries out the
cell’s response, either in the cytoplasm or in the nucleus.

One hormone may trigger a variety of responses in target cells
because the cells may contain different receptors for that
hormone, diverse signal transduction pathways, or several
proteins that can carry out the response.
Water-Soluble Hormone
For example, Epinephrine
is used to activate
glycogen breakdown

Lipid-Soluble Hormone
 Steroid hormones bind to receptors inside the cell, and act by
turning genes on or off.
 Includes sex hormones such as testosterone and estrogen, which
are lipids made from cholesterol.
 Small, nonpolar molecules that can diffuse through the
phospholipid membranes of cells.
 1. A steroid hormone enters a cell directly through the cell
membrane
 2. If the cell is a target cell, the hormone binds to a receptor
protein in the cytoplasm or nucleus. Rather than triggering a
signal transduction pathway, the hormone-receptor complex
itself usually carries out the transduction of the hormonal signal:
The complex acts as a transcription factor—a gene activator
 3.The hormone-receptor complex attaches to specific sites on the
cell’s DNA in the nucleus.
 4. The binding of the hormone-receptor complex to DNA
stimulates transcription of certain genes into RNA, which is
translated into new proteins.
Lipid Soluble Hormones

http://sciencelauncher.com/images/steroidsi
gnal.gif


Because a hormone can bind to a variety of
receptors in various kinds of target cells,
different kinds of cells can respond differently
to the same hormone.
For example, the main effect of epinephrine on
heart muscle cells is cellular contraction, which
speeds up the heartbeat; its main effect on liver
and muscle cells is glycogen breakdown,
providing glucose to body cells.

The vertebrate endocrine system consists of
more than a dozen major glands.
Some of these such as the thyroid and pituitary
gland serve as endocrine specialists, secreting
hormones into the blood.
 Several other glands serve as both endocrine and
nonendocrine functions, such as the pancreas, which
secretes hormones that influence the level of glucose
in the blood and also secretes digestive enzymes into
the intestine via ducts.
 Other organs are primarily nonendocrine, like the
stomach, but have some cells that secrete hormones.


There are three chemical classes of hormone



Protein/peptides – water soluble
Amines –water soluble
Steroids –lipid soluble
 Only the sex organs and cortex of the adrenal gland
produce steroid hormones, the main type of hormone
that actually enters target cells

Hormones have a wide range of targets.



Some, like the sex hormones, promote male and female
characteristics which affect most of the tissues of the body.
Other hormones, like glucagon from the pancreas, have only a
few kinds of target cells (liver and fat cells for glucagon)
Some hormones have other endocrine glands as their targets,
like thyroid-stimulating hormone from the pituitary gland,
promotes activity of the thyroid gland.
**Refer to table 26.3 on p. 532 for a list of the major human
endocrine glands and their hormones***


The distinction between the endocrine system and the
nervous system often blurs, especially when we
consider the diverse roles of the hypothalamus and its
intricate association with the pituitary gland.
Hypothalamus:




As part of the brain, the hypothalumus receives information
from nerves about the internal condition of the body and about
the external environment.
It then responds to these conditions by sending out appropriate
nervous or endocrine signals.
Its hormonal signals directly control the pituitary gland, which
in turn secretes hormones that influence numerous body
functions.
Thus, the hypothalamus exerts master control over the endocrine
system by using the pituitary gland to relay directives to other
glands.

Pituitary gland:
Pituitary gland consists of two distinct parts: a posterior and an anterior
lobe, both situated in a pocket of skull bone at the base of the
hypothalamus.
 Posterior Pituitary
 Composed of nervous tissue and is actually an extension of the
hypothalamus.
 It stores and secretes two hormones made in the hypothalamus.
(oxytocin and ADH)
 Anterior Pituitary
 Composed of endocrine cells that synthesize and secrete numerous
hormones directly into the blood.
 Several of these hormones control the activity of other endocrine
glands.
 Hypothalamus controls the anterior pituitary by secreting two kinds
of hormones:

 Releasing hormones
 Stimulate the anterior pituitary to secrete hormones
 Inhibiting hormones
 Induce the anterior pituitary to stop secreting hormones.

Oxytocin and ADH (antidiuretic hormone)




Produced by a set of neurosecretory cells that extends from the
hypothalamus into the posterior pituitary gland.
These hormones are channeled along the neurosecretory cells
into the posterior pituitary gland
When released into the blood from the posterior pituitary gland,
oxytocin causes uterine muscles to contract during childbirth
and mammary glands to eject milk during nursing
ADH helps cells of the kidney tubules to reabsorb water, thus
decreasing urine voluve when the body needs to retain water.
 When the body has too much water, the hypothalamus
responds to negative feedback, slowing the release of ADH
from the posterior pituitary.

A second set of neurosecretory cells in the
hypothalamus secrete releasing and inhibiting
hormones that control the anterior pituitary gland.


A system of small blood vessels carries these hormones from the
hypothalamus to the anterior pituitary.
In response to hypothalamic releasing hormones, the anterior
pituitary synthesizes and releases many different peptide and
protein hormones, which influence a broad range of body
activities.
 Thyroid-stimulating hormone (TSH), adrenocorticotropic
hormone (ACTH), follicle-stimulating hormone (FSH), and
luteinizing hormone (LH), all activate other endocrine glands.
 Feedback mechanisms control the secretion of these hormones
by the anterior pituitary gland.

The hypothalamus operates through the anterior
pituitary to direct the activity of the thyroid gland.


The hypothalamus secretes a releasing hormone known as TRH
(TSH-releasing hormone)
 In turn, TRH stimulates the anterior pituitary to produce
TSH.
 Under the influence of TSH, the thyroid secretes the hormone
thyroxine in the blood, which increases the metabolic rate of
most body cells, warming the body as a result.
The precise regulation of TRH-TSH-thyroxine system keeps the
hormones at levels that maintain homeostasis.
 The hypothalamus takes cues from the environment; cold
temperatures tend to increase its secretion of TRH.
 In addition, negative-feedback mechanisms control the
secretion of thryroxine.
 When thyroxine increases in the blood, it acts on the
hypothalamus and anterior pituitary, inhibiting TRH and TSH
secretion and consequently thyroxine.

Growth hormone (GH)

Secreted from anterior pituitary

Promotes protein synthesis and the use of body fat for energy
metabolism in a wide variety of target cells.
In young mammals, GH promotes the development and
enlargement of all parts of the body.
Abnormal production of GH can result in several human
disorders;
 Too much GH during childhood, usually during a pituitary
tumor, can lead to gigantism. Excessive production of GH
during adulthood can lead to a condition known as acromegaly,
stimulates bony growth in the face, hands, and feet.
 Too little GH in childhood can lead to pituitary dwarfism;
administering GH to children with deficiency can successfully
prevent dwarfism
 Can be abused to bulk muscles in athletes, which can cause
disfigurement, heart failure, and multiple cancers.



Prolactin



Another anterior pituitary hormone
Produces very different effects in different species.
In mammals, it stimulates mammary glands to produce
milk

Endorphins
 Hormones produced by the anterior pituitary as well as the brain
 Serve as the body’s natural painkillers.
 These chemical signals bind to receptors in the brain and dull the
perception of pain.
 Effect on the nervous system is similar to that of the drug
morphine, earning endorphins the nickname “natural opiates”
 Some researches speculate that the so-called “runner’s high” results
partly from the release of endorphins when stress and pain in the
body reach critical levels.
 It has also been suggested that endorphins may be released during
deep meditation, by acupuncture treatments, or even by eating very
spicy foods.



Pancreas produces two hormones that play a
large role in managing the body’s energy
supplies.
Clusters of endocrine cells, called islets of
Langerhans, are scattered throughout the
pancreas.
Each islet has a population of beta cells, which
produce the hormone insulin, and a population
of alpha cells, which produce another hormone
called glucagon.

Both are protein hormones that are secreted directly
into the blood.

Insulin and Glucagon
 Antagonistic hormones that regulate the concentration of glucose in
the blood.
 Two hormones counter each other in a feed-back circuit that
precisely manages the amount of circulating glucose available as
cellular fuel versus the amount of glucose stored as a polymer
glycogen in body cells.
 By negative feedback, the conc. of glucose in the blood determines
the relative amounts of insulin and glucago secreted by the islet
cells.
 High blood glucose stimulates the beta cells to secrete more
insulin.
 Insulin stimulates nearly all body cells to take up glucose from the
blood
 Liver (and skeletal muscle cells) take up much of the glucose and use
it to form glycogen, which they store.
 Insulin also stimulates cells to metabolize glucose for immediate
energy use, the storage of energy in fats, or the synthesis of proteins.
 Causes blood glucose level to drop, and the beta cells lose their
stimulus to secrete insulin

Low blood glucose stimulates the alpha cells to
secrete glucagon.
 Glucagon is a fuel mobilizer, signaling liver
cells to break glycogen down into glucose,
convert amino acids and fat-derived glycerol
to glucose, and release the glucose into the
blood.
 Then, when the blood glucose level returns to
the set point, the alpha cells slow their
secretion of glucagon.




Diabetes mellitus is a serious hormonal disease in which the body
cells are unable to absorb glucose from the blood.
 It affects about 18 million Americans—6%of the total
population—and an estimated 5 million of them do not respond
normally to blood insulin.
 Diabetes develops when there is not enough insulin in the blood
or when body cells do not respond normally to blood insulin.
 In either case, the cells cannot obtain enough glucose from the
blood, and thus, starved for fuel, they are forced to burn the
body’s supply of fats and proteins.
 Meanwhile, since the digestive system continues to absorb
glucose from the diet, the glucose conc. in the blood can become
extremely high—so high that measurable amounts of glucose are
excreted in urine (Normally, the kidney leaves no glucose in the
urine)
There are treatments for diabetes but no cure.
Left untreated, it can cause dehydration, blindness, cardiovascular
and kidney disease, and nerve damage
300,000 Americans die from the disease and its complications each

Type 1 (insulin-dependent) diabetes




An autoimmune disease, in which wbc’s (T cells) of
the body’s own immune system attack and destroy
pancreatic beta cells.
As a result, the pancreas does not produce enough
insulin, and glucose builds up in the blood.
Generally develops during childhood
Treatment consists of injections of human insulin,
produced by genetically engineered bacteria, several
times daily.

Type 2 (non-insulin-dependent)





Characterized either be a deficiency of insulin, or more
commonly by reduced responsiveness of target cells
due to some change in insulin receptors.
Almost always associated with being overweight and
underactive, although whether obesity causes diabetes
remains unknown.
Generally appears after age 40, but even young people
who are overweight and inactive can develop the
disease.
In the US, more than 90% of diabetics are type 2.
Many of them can manage their blood glucose with
regular exercise and a healthy diet high in soluble
fiber and low in fat and sodium; some require
medications.

Detection


Early signs of either type include a lack of energy, a
craving for sweets, frequent urination, and persistent
thirst.
The diagnostic test for diabetes is a glucose tolerance
test: the person swallows a sugar solution and then
has blood drawn at prescribed time intervals. Each
blood sample is tested for glucose.





Condition caused by hyperactive beta cells that put too
much insulin into the blood when sugar is eaten.
As a result, their blood glucose level can drop well
below normal
Usually occurs 2-4 hours after a meal and may be
accompanied by hunger, weakness, sweating, and
nervousness.
In severe cases, when the brain receives inadequate
amounts of glucose, a person may develop convulsions,
become unconcious, and even die.
Hypoglycemia is not common, and most forms of it can
be controlled by reducing sugar intake and eating more
frequently, in smaller amounts.