Wednesday, June 20, 2012

Biological Science 6 (Specialized Eukaryotic Cells and Tissues)

Specialized Eukaryotic Cells and Tissues

Day 11:

Specialized Eukaryotic Cells and Tissues

Being complex organisms humans and other eukaryotes have not one multiple cells, but the cells are specialized and vary greatly in size, shape, color, ability and function (like pikmen). Specialized cells and tissue are very interesting. Did you know, you grow crystals in your ears? They are called otoconia or otoliths, problems with them make you dizzy. 

Dislodge ear rocks cause 20% of vertigo.
Pikmen Remind me of the Cone Cells of the Eye

Eyes have cones (to see color) and rods (to see outlines and motion).

A. Nerve Cell/Neural

Beautiful Nerve Cells
Thanks Huskona Hljod

We have more neurons in our bodies (1,000 billion), then there are stars in the milky way.

Camillo Golgi (left) and Santiago Cajal (right) were the people responsible for beginning to figure out what nerve cells (neurons) are and how they work together.

History of Neuroscience Video -->

Nerve cells are beautiful flowers, the roots are dendrites (they feel around their area for stimulus), the nucleus is in the cell body, the cell body is where the roots join each other, axons are the stalks, nodes of ranvier are areas where the stalk shows through, myelin sheaths cells cover the stalk (axon) everywhere except the nodes of ranvier. Interestingly the signals sent from the roots (dendrites) pass through the stalk only in the nodes of ranvier, magically jumping (saltatory conduction) past all the myelinated areas all the way to the axon terminal bundle. 

Stimulus triggers an electric discharge (like a capacitor) producing an electrical pulse of 50-70 milivolts called an action potential. Nerves are super slow their signals go tens of meters per second, not the speed of light like electricity on wires. At the end of the line the neuron secretes neurotransmiters. The axon is 40 μm wide (40 thousandth of a millimeter).

The dendrites of a neuron are like roots of a flower, the axon is like a stem (covered with alternating fat globs, which is weird) the axon terminal bundle is the flower which releases pollen (neurotransmitters). Put your head all the way on your right shoulder and look at the above picture if you don't see it. If you put your head on the left shoulder it is a spooky tree.

1. Cell Body (Site of Nucleus and Organelles)

The cell body is a "normal cell" it has a nucleus, mitochondria and other organelles.

2. Axon (Structure, Function)
The axon (as well as the dendrites) is what makes a neuron a neuron. It is the wire that transmits information in humans (our AV cables, our HDMI cables our DVI cables). All the feelings of pain or comfort, heat or cold must pass through an axon.

3. Dendrites (Structure, Function)
Dendrites are the receptors of the neuron, dendrites "feel" a stimulus first and may or may not send a signal to the brain via the axon.

4. Myelin Sheath, Schwann Cells, Oligodendrocytes, Insulation of Axon
Glial cells are the buddies of neurons, they do many different things to support the neurons. The fat one of the glial group is the oligodendrocyte. Oligodendrocytes make myelin sheaths in the central nervous system. The peripheral nervous system is the Schwann Cell's turf. Each oligodendrocytes myelinates (covers with fat) multiple axons.

5. Nodes of Ranvier (Role in Propagation of Nerve Impulse Along Axon)
Gaps where there is no fat on an axon, allowing the electrical signal to travel (since fat/myelin sheaths block the signal).

6. Synapse (Site of Impulse Propagation Between Cells)
Santiago Cajal noticed the gaps in between neurons. They don't meet connect with each other, they have wifi (in the form of neurotransmitters). Neurotransmitters released by one neuron stimulate the next neuron to propagate the signal.

7. Synaptic Activity
Exocytosis of chemicals (neurotransmitters) at the axon terminal bundle occurs, then the next neuron or the intended muscle receives the chemicals through receptors and either the signal is passed on or the muscle responds.

The synapse occurs between the axon terminal and the dendrite of the next nerve.

A. Transmitter Molecules
Different molecules go between the synaptic cleft, these are transmitter molecules. The transmitter molecules bind to and trigger receptors on the post synaptic molecule. The distance they travel is 20 nm, the width of the synaptic cleft. 

Common Transmitters:
Acetylcholine (ACh)     Raspberry Flavor
Norepinephrine (NE)   Blueberry Flavor
Dopamine                      Strawberry Flavor
Serotonin                       Banana Flavor
Histamine                      Watermellon Flavor
ATP                                Lime Flavor (these flavors just help me remember)
B. Synaptic Knobs
The name of the disk on the end of an axonal branch that borders the cleft on the presynaptic side. 

When the transmitters leave the synaptic knobs (due to calcium influx) and travel 20 nm to the other side they open or close ion channels. Opening ion channels depolarizes (excitatory transmitters) or repolarizes (inhibitory transmitters) the cell membrane of the post synaptic neuron.

C. Fatigue
Fatigue is when the transmitters run out, so the presynaptic neuron can not send signals to the postsynaptic neuron until they re-uptake more transmitters. 

D. Propagation Between Cells Without Resistance Loss
The potential of one neuron is the same as the next (both all or nothing), so there is no resistance loss between cells.
8. Resting Potential (Electrochemical Gradient)
3Na+ out and 2K+ in, leaves - charge inside, + charge outside. K+ can leak out, but Na+ cannot leak in supporting the - charge inside, + charge outside. The normal resting potential is -70 mV. The electrochemical gradient refers to sodium and negative charge outside the cell (blood and body fluids are salty, and negative...), potassium and positive charge inside the cell (cells insides are banana flavored -not really- and positive...).

Resting Potential of the Cell (-70 mV Normally) Changes to Action Potenital

9. Action Potential

  • Resting: cell at rest, sodium potassium pump on, -70 mV, sodium outside, potassium inside, ion channels are closed.
  • Depolarization: sodium channels open, positive sodium rushes inside, membrane potential becomes +30 mV, lots of sodium and potassium inside.
  • Repolarization: potassium channels open (positive potassium rushes outside), sodium channels close (lots of sodium inside), this is opposite of the resting state.
  • Hyperpolarization: membrane potential drops below normal level.
  • Refractory Period: the sodium potassium pump reestablishes the resting state, the neuron can not generate another action potential until the cell goes back to the normal -70 mV.

Action Potential: In Action

A. Threshold, All-or-None
All-or-None response means that if a stimulus meets the minimum threshold it will cause the same response as any greater stimulus.

B. Sodium–Potassium Pump
The sodium-potassium pump moves 3 sodium outside and 2 potassium inside keeping the cell negative on the inside (negative membrane potential).

10. Excitatory and Inhibitory Nerve Fibers (Summation, Frequency of Firing)
Excitatory nerves cause nerves to send a signal (membrane -), inhibitory nerve fibers prevent the signal (membrane +).

This was the nobel prize of physiology in 1963 for Sir John Eccles, Alan L. Hodgkin and Andrew F. Huxley. The nobel prize organization describes the phenomena:
"There are two kinds of synapses, one excitatory, the other inhibitory. If the arriving impulse is connected to excitatory synapses the response of the cell is yes, i. e.excitability increasesvice versa the inhibitory synapses make the cell respond with a no, a diminution of excitability. Eccles has shown how excitation and inhibition are expressed by changes of membrane potential.

When the response is sufficiently strong to cause excitation, the membrane potential decreases until a value is reached at which the cell fires off an impulse, the sodium impulse we have spoken of. This impulse travels through the nerve fibre of the cell and in our example causes contraction in a muscle. Obviously a cell may also send impulses to another cell at whose membrane the synaptic processes repeat themselves with plus or minus sign, as the case may be.

A cell engaged in activity may be influenced by impulses reaching inhibitory synapses. In this case the membrane potential increases and, as a consequence, the impulse discharge is inhibited. Thus excitation and inhibition correspond to ionic currents which push the membrane potential in opposite directions.

The nerve cells are provided with thousands of synapses which correspond to terminals of fibres originating in sense organs or other nerve cells. The sum total of synaptic processes determines the state of balance between excitation and inhibition in which the integrated messages of nerve cells find expression and the code of impulses its interpretation."

So a neuron has many synaptic processes, both inhibitory and excitatory. Neurons hear I'm too cold, I'm too warm, I'm okay, I hurt all at the same time and the total balance is what is conveyed along that neuron. Which is why massage takes away pain, since sending the feelings of pressure and rhythms gets in the way of sending the signals for pain.

B. Muscle Cell/Contractile

Muscle contraction occurs by the filament sliding past each other and shortening each cell. The cell folds up like an accordion. The filaments that cause the motion are green below. 

Muscle Cells
Thank you Evelyn Ralston

1. Abundant Mitochondria in Red Muscle Cells (ATP Source)
Red muscle is for endurance activities, the color is myoglobin that prevents fatigue in maintaining posture. Abundant mitochondria produce ATP to be used for endurance activities. White muscle has few mitochondria due to glycolysis being predominant.

2. Organization of Contractile Elements
(Actin and Myosin Filaments, Cross Bridges, Sliding Filament Model)
Thin actin (lighter in picture below) and thick myosin (darker in picture below) are parallel to each other along the length of the cell. Actin has troponin and tropomyosis on it.

Sarcomeres Made of Actin and Myosin

Myosin Cross Bridge (Moving Myosin Heads Bind to Actin)

Sliding Filament Model (Narrated by a Romanian Robot - It Sounds Like...)

Sliding Filament Model in summary, the myosin is able to hook onto the actin and then pull it, shortening the cell. It is like pressing the tips of your fingers together facing you, then letting them slide past each other into the spaces of the opposite hand. ATP provides the energy for the de-powerstroke, to unbend the myosin head. The power stroke occurs without ATP, as myosin head bend in the direction of the M line (causing the muscle to contract). 

Rigor mortis is caused by a lack of ATP, without ATP the muscles cannot unbind to relax.
Troponin moves tropomyosin out of the way of myosis heads when Ca+ is high allowing cross bridges to form.

3. Calcium Regulation of Contraction, Sarcoplasmic Reticulum
The smooth ER of muscle releases stores calcium (because an action potential from a nerve sent the signal down the sarcoplasmic reticulum via the T-tubules), calcium then binds with troponin, moving tropomyosin off the binding sites myosin heads use to link with actin in order to contract.

Calcium Causes Muscles to Contract by Moving Tropomyosin Out of the Way

4. Sarcomeres
(―I‖ and ―A‖ Bands, ―M‖ and ―Z‖ Lines, ―H‖ Zone—General Structure Only)
I bands (isotropic: constant, because it does not change length) are mostly actin, A bands (anisotropic: variable, because it shortens) are mostly myosin, M lines (mittelscheibe: middle split) are in the middle of the sarcomere, Z lines (zwischenscheibe: split between) are between sarcomeres, H zones (heller: brighter) are next to the middle and disappear when the muscle contracts.

The Sarcomere

5. Presence of Troponin and Tropomyosin
Troponin: binds to calcium, then moves tropomyosin that it is attached to, out of the way of the myosin cross bridges.
Tropomyosis: long spiral that blocks myosin from cross bridging. 

C. Other Specialized Cell Types

The four types of tissue in a human are connective, epithelial, muscle and neurons. Having covered nerves and muscle, connective (fat, bone, cartilage and goo) and epithelial tissue (skin) are featured in this section.

1. Epithelial Cells (Cell Types, Simple Epithelium, Stratified Epithelium)

Transitional Epithelial Cells
Thanks David Philips (he has a whole gallery of stunning biological art):

Epithelial cells are the outer surface of the body, they also line the internal surfaces and more. Skin, bladder, throat, nose, intestinal lining, lung aveoli... Usually pared with connective tissue to nourish the cells and remove their waste. Epithelial cells absorb food, absorb oxygen from the air, secrete mucus and protect our body from the outside world's cold, bacteria, radiation and other dangers. Epithelial cells have vastly different appearances (opposed to muscle and neurons that are always similar). Transitional (can change shape to expand), simple (one layer, to absorb faster), stratified (many layers, to shed and protect surfaces that get rubbed off by food or the environment) or  psuedostratified (tends to have goblet cells to secrete mucus), cells are columnar (column shaped cells have many talents, protection in the intestines, cornea, ear and nose and absorption and transportation of nutrients in the small intestines), cuboidal (cube shaped, mostly glands and ducts) or squamous (plate shaped, most skin cells). Simple columnar epithelial cells responsible in getting drunk, since the majority of alcohol (or nutrients) is absorbed in the small intestines... there is much more to say about these cells, but it may be extraneous.

Simple Squamous Epithelial Cells

Simple Cuboidal Epithelial Cells

Simple Columnar Epithelial Cells (Science Photo Library is a Great Photo Gallery)

Stratified Squamous Epithelial Cells

Stratified Cuboidal Epithelial Cells

Pseudostratified Columnar Epithelial Cells

2. Endothelial Cells
Lining on the inside of organs and blood vessels, mostly single layers since they don't face the dangers of the outside world and need to have rapid diffuse to remove waste CO2  and provide O2 for cellular respiration.

3. Connective Tissue Cells (Major Tissues and Cell Types, Fiber Types, Loose Versus Dense, Extracellular Matrix)
Connective tissue holds organs, tissues or cells in place. The extracellular matrix is the majority of the tissue, with some cells scattered in the matrix (like ships on the ocean). Fibers include collagenous fibers (collagen), elastic fibers (elastin, stretchy) and reticular fibers (join connective tissue to another type of tissue). Major tissue types include, fluid: lymph and blood. Supportive: bone (cancellous or compact) and cartilage (chondrin). Connective tissue proper: loose (adipose ie fat, areolar amd reticular) or dense (regular, irregular and elastic).

Fibroblasts cells create matrix
Chondroblasts cells create cartilage matrix 
Osteoblasts cells create bone matrix

Fibrous connective tissue may be dense regular (collagen fibers in the same direction, strong, in tendons, ligaments, dermis and organ capsules), elastic connective tissue (lots of elastic, squiggly appearance, allows stretching), dense irregular (collagen fibers are not running in the same direction).

Loose connective tissue may be areolar connective tissue (loose, fibers in many directions, packing between glands, muscles and nerves or in basement membranes), adipose (ring shaped cells, cushion) and reticular (some circle shaped cells, some branching fibers, kind of looks like cherry blossoms).

Cartilage includes elastic cartilage (elastic fibers, eyeball looking cells in the matrix, very flexible), hyaline cartilage (clear in living tissue, two chondrocytes per lacuna, support with some flexibility), fibrocartilage (collagen fibers in bundles, shock absorber).

Areolar Loose Connective Tissue

Adipose Loose Connective Tissue

Reticular Loose Connective Tissue

Dense Regular Connective Tissue

Dense Irregular Connective Tissue

Dense Regular Elastic Connective Tissue 

Compact Bone

Cancellous Bone

Hyaline Cartilage

 Elastic Cartilage


Blood (Now Created by Stem Cells: Link to Article Below)

Cells are specialized and vary greatly in size, shape, color, ability and function...

Pikmin all work together, like nerve cells (the raspberry is acetylcholine)...


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