Topic 6.5Option 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 threshold1. 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 long-distance 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 all-or-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

longer-lasting 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 water-soluble.

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 HormoneFor 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.

http://sciencelauncher.com/images/steroidsignal.gif

Lipid Soluble Hormones

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 year.

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.