Download Immune System Barriers Skin Outer surface is dry and oily, most

Immune System
Barriers
A.
Skin
1.
Outer surface is dry and oily, most organisms cannot obtain the water and nutrients they need
2.
Sweat and oil secretions contain acids and antibiotics that inhibit growth of bacteria and fungi
B.
Mucous Membrane
1.
Membranes of digestive and respiratory tract secret mucus that contains antibacterial enzymes which destroy bacterial cell walls
2.
Mucus physically traps microbes that enter the body, cilia sweep up mucus and microbes
3.
Specific antibody called IgA binds to surfaces of microbes and prevents the microbes from invading the cells of the mucous membranes
Nonspecific Internal Defenses
A.
Eaters and Killers
1.
Macrophages – white blood cells that ingest microbes by phagocytosis and “present” parts of the microbe to the other cells of the immune system
2.
Natural killer cells – white blood cells that strike at the body’s own cells that have been invaded by viruses; weapons are proteins that they secrete onto the membrane of the infected
cell; proteins insert themselves in target membrane and drills holes
B.
Inflammatory Response
1.
Damaged cells release histamine into wounded area which makes capillary walls leaky and relaxes smooth muscle surrounding arterioles, leading to increased blood flow
2.
Fluid seeps from leaky capillaries into tissues around the wound
3.
Other chemicals released by injured cells initiate blood clotting which “walls off” wounded area from rest of body, preventing microbes from escaping into bloodstream
4.
Pus consists of bacteria, tissue debris, and living and dead white blood cells
C.
Fever
1.
Certain white blood cells release hormones called endogenous pyrogens which travel in the bloodstream to the hypothalamus and raise the thermostat’s set point
2.
Fever and reduced iron in the blood combine to slow down microbial reproduction
3.
Cells invaded by viruses release a protein called interferon which travels to other cells and increases their resistance to viral attack
Specific Immune Response

Involves two types of lymphocytes: B and T cells
Plasma cells, which are descendants of B cells, secrete antibodies into the bloodstream, causing humoral immunity
Killer T cells destroy some microbes, cancer cells and virus-infected cells on contact, causing cell-mediated immunity
Helper T cells stimulate both the humoral and cell-mediated immune responses

Immune responses have three steps: recognition, attack, and memory
Recognition: diversity of antibodies arises from gene shuffling and mutation of antibody genes during immune cell development, each antibody has specific sites that bind one or a
few types of antigen, normally on foreign antigens are recognized by immune cells
Attack: B cells divide rapidly, producing plasma cells that synthesize massive quantities of antibodies which circulate and destroy antigens through direct neutralization, promotion
of phagocytosis by WBC, agglutination, and complement reactions. T cells divide rapidly to form killer T cells which bind to antigens on microbes, infected cells, or cancer cells
and kill the cells. Helper T cells stimulate and suppressor T cells turn off both the B and killer T cell responses.
A.
Antibodies
1.
Y-shaped proteins composed of two pairs of peptide chains: one heavy chain and one light chain on either side of the Y
2.
Both chains consist of a constant region (similar in all antibodies) and a variable region (different among antibodies).
3.
Arms are the specific binding sites of antigens and have their own peculiar shape and electric charge
4.
The stem determines the function of an antibody
5.
Antibodies are receptors.
a.
Stem attaches the antibody to the plasma membrane while the two arms protrude outward, sampling the blood for antigens
6.
Antibodies are effectors
a.
Certain descendants of B cells secrete antibodies into the bloodstream which neutralize poisonous antigens or promote destruction of microbes bearing antigens
b.
Small differences in the stem yield five types of true antibodies
B.
How can genes encode for millions of antibodies if there are fewer than a hundred thousand genes in the entire human genome?
1.
Genes encode parts of antibodies which join together during immune cell development to form complete genes for the light and the heavy chains of antibodies.
2.
Some genes are prone to mutation, constantly generating new antibody genes
C.
Distinguishing “Self” from “Non-self”
1.
Immune system retains only those immune cells that do not respond to the body’s own molecules
D.
Humoral Immunity
a.
When a microbial infection occurs, the antibodies born by a few B cells will bind to antigens on the microbe
b.
Antigen-antibody binding causes these B cells to divide rapidly into plasma cells and memory cells
i.
Plasma cells – enlarged and packed with endoplasmic reticulum, churning out huge quantities of that cell type’s specific antibody and releasing them into the
bloodstream
ii.
Memory cells – last for many years and will multiply rapidly, generating huge populations of plasma cells and killer cells
c.
Antibodies in the bloodstream may affect antigenic molecules and infected cells in four ways:
i.
Neutralization – the antibody may combine with or cover up the active site of a toxic antigen, thereby preventing the toxin from harming the body
ii.
Promotion of phagocytosis – the antibody may coat the surface of a microbe making it easier for phagocytic white blood cells to engulf the microbe
iii.
Agglutination – Binding sites may attach to antigens on two different microbes, holding them together which seems to enhance phagocytosis
iv.
Complement reactions – antibody-antigen complex on the surface of an invading cell may trigger a series of reactions with blood proteins called the complement
system; when these proteins bind to the antibody stems, they attract phagocytic white blood cells to the site, promoting phagocytosis of the foreign cells
d.
Activation of B cells
E.
Cell-Mediated Immunity
a. Destroy the body’s own cells when they have become cancerous or have been infected
b. When antigen binds to antibodies on a T cell, the cell divides rapidly, producing effector and memory cells
c. Effector T cells are helper, killer and suppressor cells
d. Activation of Killer T cells
Summary of Immune Cell Communication
1.
Macrophages bind to antigens, enguld them, and present them on their surfaces, along with “self-identification” MHC molecules, to helper T cells.
2.
When helper T cells recognize the antigen/MHC II complex, they multiply rapidly.
3.
Meanwhile, killer T cells and B cells recognize the same antigen.
4.
Hormones released by helper T cells stimulate cell division and maturation of only those killer T cells and B cells that have also been stimulated by antigen binding.
5.
The stimulated killer T cells and B cells then provide cell-mediated and humoral immunity, respectively.
Medicine and Immune Response

Antibiotics kill microbes or slow down their reproduction, thus allowing the immune system more time to respond and exterminate the invaders

Vaccinations are injections of antigens from disease organisms, often the weakened or dead microbes themselves

Allergies are immune responses to normally harmless foreign substances

Autoimmune diseases arise when the immune system destroys some of the body’s own cells

Immune deficiency diseases occur when the immune system cannot respond strongly enough to ward off normally minor diseases
AIDS

Acquired immunodeficiency syndrome

Caused by one of two viruses called HIV 1 and 2

These viruses invade helper T cells and destroy them

Without helper T cells to stimulate the immune responses of B cells and killer T cells, the AIDS victim is extremely susceptible to a wide assortment of diseases
Cancer

A cancer is a population of cells that has escaped from normal regulatory processes and grows without control

Oncogenes – genes that cause cancer
A potentially dangerous oncogene may be present in all cells, but only causes cancer when activated by some external trigger
A harmless gene may mutate into an oncogene
Nervous System
Functions of Neurons
1.
Receive information from the internal or external environment or from other neurons
2.
Integrate the information and produce and appropriate output signal
3.
Conduct the signal to its terminal ending
4.
Transmit the signal to other nerve cells, glands, or muscles
5.
Coordinate the metabolic activities that maintain the integrity of the cell
Structure of Neurons
Dendrites

Receives information

Respond to these stimuli by converting them into electrical signals

Dendrites that receive signals from other neurons have protein receptors in their membrane that are specialized to receive a chemical released by another neuron
Cell Body

Serves as an integration center

Adds up the various signals from the dendrites and “decides” whether to produce an action potential (the electrical output signal of the neuron)

Synthesizes proteins, lipids, carbohydrates, and coordinates the metabolic activities of the cell
Axon

Long, thin fiber that extends outward from cell body

Carry action potentials from the cell body to the synaptic terminals

Usually bundled together into nerves
Synaptic Terminals

Signals are transmitted

Contain a neurotransmitter that is released in response to a signal traveling down the axon

May communicate with a gland, muscle, or dendrites or cell body of a second neuron
Mechanisms of Neural Activity

Inactive neurons maintain a constant electric difference, or potential, across their cell membranes

This potential, called the resting potential, is always negative inside and cell and ranges from -40 to -90 millivolts

If a neuron is stimulated, the negative potential inside can be altered

If the potential is made sufficiently less negative, it reaches a level called threshold at which an action potential is triggered

During the action potential, the neuron suddenly becomes 20 to 50 millivolts positive inside but last only a few milliseconds
The Origin of the Resting Potential

Cell membrane of a neuron encloses cytoplasm with various ions dissolved in it

Neuron itself is immersed in a salt solution (extracellular fluid)

Ions of the cytoplasm consist mainly of positively charged potassium ions and large, negatively charged organic molecules such as proteins and molecules of the citric acid
cycle

Outside the cell, the EC fluid contains mostly positively charged sodium ions and negatively charged chloride ions.

In an unstimulated neuron, only potassium ions can cross the membrane, traveling through specific proteins called potassium channels while sodium channels remain closed

Since potassium ions are most concentrated inside the cell, they will diffuse out of the cell, leaving the large, negatively charged organic ions behind

At some point, the diffusion of potassium ions out of the neuron due to concentration differences will be balanced by the electrical attraction tending to pull them back inside.
At this point, there is no more net movement of potassium ions and the cell reaches a stable resting potential, negative inside.
Action Potentials: Long-Distance Messages

Nervous information is encoded in changes in potential in nerve cells. Two examples of changes in electrical potential in neurons are action potentials and postsynaptic
potentials

Action potential – wave of positive charge that travels along the axon to the synaptic terminal

Usually initiated at the point where the axon leaves the cell body

Immediately after the action potential passes any point along the axon, the negative resting potential is restored within the axon

At threshold, sodium channels open and positively charged sodium ions flood into the cell making it momentarily positive

Potassium ions are then driven out by the diffusion gradient and by electrical repulsion from the positive sodium ions that recently entered and inside becomes negative again

Action potentials are all-or-none – they do not vary in magnitude. If the neuron does not reach threshold, there will be no action potential at all, but if threshold is reached,
then a full-sized action potential will occur and travel the entire length of the axon
Role of Sodium Potassium-Pump

Set of active transport molecules

Uses energy from ATP to pump sodium out and potassium in, maintaining the concentration gradients of these ions across the cell membrane
Conduction of the Action Potential

Must be transmitted along the axon to cell specialized to receive the message

If the neuron is to conduct an action potential along its axon to its synaptic terminal, the action potential must not diminish in magnitude and die out along the way

Cell maintains the magnitude of the action potential by renewing it at each successive point along the axon

Action potential begins when threshold is reached, sodium channels open, and sodium ions enter the cell, making the cell positive

Although much of this positive charge leaks back out, some spreads passively and almost instantaneously along the inside of the axon, making the adjacent region less
negative

When the adjacent region of the membrane reaches threshold, it repeats the cycle, continuing the entire length of the axon, while sodium channels at the site of the original
action potential close and resting potential is reestablished there
Saltatory Conduction

In vertebrates, axons that need to conduct rapidly are wrapped with insulating layers called myelin, interrupted at intervals with naked areas called nodes of Ranvier

These myelinated axons transmit signals extremely rapidly because ion channels are concentrated only at the nodes

When an action potential occurs in a myelinated axon, the positive charge cant leak back out through the myelin but instead travels to the next node where channels open and
a new action potential is initiated

Myelinated axons maintain the size of the signal by initiating new action potentials at each node

Transmission of an action potential along a myelinated axon is called salutatory conduction

Myelin is formed from specialized non-neural cells that flatten and wrap themselves around the axon
Synapse

When an electric signal reaches the synaptic terminal of the axon, it encounters the synapse, where two neurons are close together but do not touch

A miniscule gap, the synaptic cleft, separates the synaptic terminal of the first, or presynaptic, neuron from the dendrite or cell body of the second, or postsynaptic neuron

When an action potential reaches a synaptic terminal, the inside of the synaptic terminal becomes positively charged which triggers the synaptic terminal to release a
chemical neurotransmitter into the synaptic cleft

Neurotransmitter molecules rapidly diffuse across the gap and bind to receptors (specialized proteins in the membrane of the dendrites)
Postsynaptic Potentials

Bind to a specific type of neurotransmitter

Receptor causes specific types of ion channels in the membrane of the photosynaptic neuron to open

Flow of ions in postsynaptic neuron causes a postsynaptic potential in the dendrites or cell body where the synapse occurs

Excitatory postsynaptic potential – neuron is less negative inside and more likely to fire an action potential

Inhibitory postsynaptic potential – More negative and less likely to fire
Integration of Synaptic Potentials

Postsynaptic potentials produced by different presynaptic neurons are then integrated in the cell body of the postsynaptic neuron which will produce an action potential only
if the excitatory and inhibitory potentials, when added together, raise the electrical potential inside the neuron above threshold
The Fate of Neurotransmitters

A few are destroyed by enzymes

Others are removed by active transport back into the presynaptic neuron

Or diffuses away into the extracellular fluid
Building and Operating a Nervous System
Four operations:
1.
Signal intensity of a stimulus

Signaled by the frequency of action potentials in a single neuron

Number of similar neurons firing at the same time
Determine the type of Stimulus

Nervous system monitors which neurons are firing action potentials
3.
Integrate Information from Many Sources

Nervous systems integrate information through convergence where many neurons funnel their signals to fewer neurons
4.
Initiate and direct the response

Actions directed by the brain may involve many parts of the body and require divergence, the flow of electrical signals from a relatively small number of decisionmaking cells onto many different neurons controlling muscle of glandular activity
Neuron Networks
1.
Sensory neurons respond to a stimulus
2.
Association neurons decide what to do based on input from many sensory neurons, stores memories, hormonal states and other factors
3.
Motor neurons receive instructions from the association neurons and activate the muscles
4.
Effectors, usually muscles or glands, perform the behavior
From Neuron to Behavior

Reflex – relatively involuntary movement of a body part in response to a stimulus
Nervous System Design

Diffuse nervous systems – hydra have a network of neurons often called a nerve net

Ganglion – cluster of neurons

Centralized nervous systems – found in more complex organisms
Human Nervous System
Peripheral Nervous System

Consists of nerves that link the brain and spinal cord to the rest of the body

Axons of sensory neurons that bring sensory information to the central nervous system from all parts of the body

Contain the axons of motor neurons that carry signals from the central nervous system to the organs and the muscles

Somatic nervous system – control voluntary movement

Autosomatic Nervous System – involuntary movement, synapse on the heart, smooth muscle and glands
Controlled by both medulla and hypothalamus of the brain
Sympathetic nervous system: acts on organs that prepare the body for stressful activity, axons found nerves that originate from middle and lower portions of spinal cord,
synapse occurs in ganglia that are near spinal cord
Parasympathetic nervous system: dominates during maintenance activities that are relaxing, promotes maintenance activities such as digestion, axons found in brain and base
of spinal cord, synapse occurs in smaller ganglia located at or very near each target organ
Central Nervous System

Consists of the brain and spinal cord

Sensory information is received and processed, thoughts are generated and responses are directed

Brain and spinal cord protected by skull, a triple layer of connective tissue called meninges, and between the meninges, the cerebrospinal fluiod which also fills
spaces within the brain called ventricles
Spinal Cord

Protected by vertebrae column

Between the vertebrae, nerves called dorsal roots and ventral roots arise to form spinal nerves

Neural pathways for reflexes and certain simple behaviors

Motor neurons control muscles involved in voluntary actions

Axons leading to and from brain
Brain

Hindbrain (medullar, pons and cerebellum) – automatic behaviors such as breathing and heart rate

Midbrain – controls vision, contains reticular activation formation, a filter and relay for sensory stimuli

Forebrain – largely dealt with sense of smell
1.
Thalamus – most of neural information that reaches cerebrum is channeled through here, shuttles sensory information to limbic system and cerebrum
2.
Limbic system – consists of groups of neurons that control instincts and emotions
i.
Hypothalamus – controls body temperature, hunger, sexual arousal
ii.
Amygdala – responsible for production of appropriate behavioral responses to stimuli
iii.
Hippocampus – Formation of long-term memory
3.
Cerebrum – brain region that receives sensory information, processes it, and initiates it
i.
Split into two parts: cerebral hemispheres, which communicate via a large band of axons, the corpus callosum
ii.
50-100 billion neurons packed into thin layer at surface called cerebral cortex
iii.
Cortex is thrown into folds called convolutions that greatly increase its area
iv.
Receive sensory information, process it, store some in memory and direct voluntary motor output
v.
Damage to this section results in problems in speech, difficulty reading, or the inability to sense or move specific parts of the body
Neurotransmitters (released in response to a signal traveling in axon)
Acetylcholine

The only transmitter found at the synapses between motor neurons and skeletal muscles where it’s always excitatory
Dopamine

Important in the brain where effects are largely inhibitory
Serotonin

Acts in brain and spinal cord

Can inhibit pain sensory neurons in spinal cord

Affects sleep and mood, too little may cause depression
Noradrenaline

Released by neurons of the sympathetic nervous system

Effects prepare the body to respond to stressful situations
Neuromodulators (substances released by neurons that modify the properties of synapses making them more or less effective and can cause long-term changes in the excitability of neurons)
Peptide Neuromodulators

Opoids (endorphin and enkephalim)
Bind to specific receptors on neurons of the central nervous system
Brain and Mind
Right Hemisphere – controls left side of body, input from left visual field, left ear, right nostril, centers for spatial perception, music creativity
Left Hemisphere –controls right side of body, input from right visual field, right ear, left nostril, centers for language, mathematics
Short-term memory is electrical, involving the repeated activity of a particular neural circuit
Long-term memory is structural, involving the formation of a permanent synaptic connection between specific neurons or strengthening of existing but weak synaptic connections
Learning, Memory, and Retrieval Sites in the Brain
Hippocampus is the site of learning, and responsible for transferring information from short-term into long-term memory
Retrieval of long-term memories is in the temporal lobes of the cerebral hemispheres
2.