Monday, July 03, 2006

Exam 2

Chapter 6

Extracellular environment(33% water weight):
  • includes all constituents of the body located outside of the cells.
  • The cells of our body must receive nourishment from, and release their waster products into the extracellular environment.
  • the different cells of a tissue, the cells of different tissues within an organ, and the cells of different organs interact with each other through chemical regulators secreted into the extracellular environment.
  • Extracellular environment includes: 1) interstitial fluid 2) plasma( 20% of the extracellular fluid)
Intracellular compartment( 67% water weight)

Blood Plasma: (20% of the extracellular fluid)
  • the blood transport oxygen from the lung to the body cells
  • transport carbon dioxide from the body cells to the lung.
  • transport nutrients derived from food in the intestine to the body cells.
  • transport other nutrients between organs (glucose from the liver to the brain, or lactic acid from muscles to the liver.)
  • transport metabolic wastes from the body cells to the liver and kidneys for elimination in the bile and urine.
  • transport regulatory molecules called hormones from endocrine glands to the cells of their target organs.
Interstitial fluid (80% of the extracellular fluid)--a gel like extracellular matrix contains 1) protein 2) salt 3) water
  • Interstitial fluid is formed continuously from blood plasma, and it continuously returns to the blood plasma.
  • Oxygen, nutrients, and regulatory molecules traveling in the blood must first pass into the interstitial fluid before reaching the body cells.
  • wates products and hormone secretions from the cells must first pass into the interstitial fluid before reaching blood.
Extracellular matrix
  • the cells that comprise the organs of the body are embedded within the extracellular material of connective tissues---extracellular matrix. it consists of the protein fibers 1) collagen 2) elastin 3) ground substance ( the interestitial fluid referred to the hydrated gel of the ground substance)
Collagen 胶原质 and elastin弹性蛋白: they provide structural strength to the connective tissues. 1 type of collagen consitute the basal lasmina underlying epithelial membranes.


Ground substance
  • highly functionaly complex organization of molecules chemically linked to the extracellular protein fibers of the collagen and elastin as well as the carbohydrates that cover the outside surface of the cell's plasma membrane.
  • The gel: is composed of 1) glycoproteins糖蛋白类 (sugar proteins) 2) proteoglycan蛋白多糖. They are all composed primarily polysaccharides and have a high content of water molecules.
  • By forming chemical bonds between the carbohydrates on the outside surface of the plasma membrane of the epithelial cells, and the glycoproteins and proteoglycans of the matrix in the connective tissuesm and basal lamina helps to wed the epithelium to its underlying connective tissues.
Mucosaccharide黏多糖 : long unbranched polysaccharides consisting of a repeating disaccharide unit.form an important component of connective tissues. GAG chains may be covalently linked to a protein to form proteoglycans.

Integrin整联蛋白
  • a class of glycoproteins th at extend from the cytoskeleton within a cell, through its plasma membrances, and into the extracellular matrix.
  • By bind ot the components within the matrix, integrin serve as "glue" between cells and the extracellular matrix. Integrins couple the ECM outside a cell to the cytoskeleton (in particular the microfilaments) inside the cell.
  • signal transduction: Connection with ECM molecules can cause a signal to be relayed into the cell through protein kinases that are connected with the intracellular end of the integrin molecule.
Plasma membrane selectively permeability
  • not permeable to : 1) proteins 2) nucleic acids 3) other molecules needed for the structure and function of the cell 4) polar molecules 5) big molecules
  • Selectively Permeable to 1) certain ions. to permits electrochemical currents across the membrane used for the production of impulses in nerve and muscle cells
The mechanisms in the transport of molecules and ions
  1. Carrier-mediated transport: transport that requires the action of specific carrier proteins in the membrane.
  2. Simple diffusion of ions, lipid-soluable molecules, and water through membrane.Transport that is not carrier-mediated(do not use carrier proteins)
Carrier-mediated transport:
  1. Facilitated diffusion
  2. Active transport.
Osmosis: the net diffusion of solvent through a membrane.

Membrane transport processes
  1. Passive transport: the net movement of molecules and ions across a membrane from higher to lower concentration down a concentration gradient. It does not require metabolic energy. 1) simple diffusion 2) osmosis 3) facillitated diffusion.
  2. Active transport: the net movement across a membrane that occurs against a concentraion gradients( to the region of higher concentration) using metabolic energy ATP. It involves specific carrier proteins.
Diffusion and osmosis

Solution: consists of 1) solvent 2) water 3) solute molecule that are dissolved in the water.

Diffusion: random thermal motion
  • this random motion tends to eliminate the concentration difference as the molecules become more diffusely spread out.
  • Increasing entropy: in terms of the second law of thermodynamics the concentration difference represent an unstable state of high organization( low entropy) that changes to produce an uniformly distributed solution with maximum disorganization (high entropy)
  • Net diffusion: molecule moves from high concentration to low concentration. however, molecules do move from the low concentration to high concentration but not as frequently. As a result, a net movement from the region of higher concentration to the region of lower concentration until the concentraion difference no longer exists.
  • property of Net diffusion 1) a physical process that orccurs whenever there is a concentration difference across a membrane and 2) the membrane is permeable to the diffusing substance.
Diffusion through the membrane
  • non-polar, lipid soluable substance can easily pass from one side to another side. O2 and steroid hormone.
  • Polar unchaged convalent bonds like CO2, ethanol, urea. are also able to penetrate the phospholipid bilayer.
Oxygen
  • the oxygen concentration is relatively high in the extracellular fluid because O2 is carried from the lungs to the body tissues by the blood.
  • Oxygen concentration is low in the inside cell because O2 is combined with hydrogen to form water in aerobic cell respiration.
CO2
  • The CO2 concentration is relatively low in the extracellular environment and high inside of the cell.
  • Gas exchange : O2 high concentration diffuse from the extracellular to the inside of the cell and cell then use O2 to produce CO2 in the cell respiration and CO2 then diffuse out of the cell.
Water
  • not lipid soluable, but it can pass the membrane to a limited degree because of their 1) small size 2) lack of net charge 3) in certain membrane is can be aided by specific channels for the physiological regulation.
  • Osmosis: the net diffusion of water molecules( the solvent) acrosee the membrane.
  • Osmosis is the simple diffusion of solvent instead of solute.
Glucose
  • Large, polar molecule can not pass through the membrane-----thus require special carrier protein.
Inorganic ions
  • the phospholipid portion of the membrane is impermeable to charged ions, such as Na +, K+ .
  • They can pass the membrane by ion channels.
Carrier protein
Ion channel************check wikipedia for more
  • Permit passage of ions
  • Some channels are always open and the transport is an ongoing process. Many are closed ----gated ion chnnels.
  • Gated ion channels can be opened by stimuli : 1) chemical or electrical signals, 2) temperature, 3) mechanical force, depending on the variety of channel.
Rate of diffusion depend on
  • the magnitude of the concentration difference across the membrane.
  • the permeability of the membrane to the diffusing substances
  • the temperature of the solution
  • the surface area of the membrane throug which the substances are diffusing.
The magnitude of the concentration difference----"driving force" for diffusion.

1) Diffusion will not occur if the membrane is not permeable to the substance. In a resting neuron, the plasma membrane is about2) 20 times more permeable to K+ than to Na+. K+ diffuse much more rapidly than Na+. 3) Changes in protein structure of the membrane channels can change the permeability of the membrane. When Na+ channels are opened and allow Na+ diffuse more than K+

In areas of the body that need rapid diffusion, the surface area of the cell membranes may be increased by numerous foldes to let more absorption and diffusion. -----in small intestine which the digestion acroos the epithelial membranes, surface area is increaed by microvilli( also presented in kidney tubule epithelium, which must reabsorb various molecules that are filtered out of the blood. )

Osmosis
For the osmosis to occure: 1) the membrane must be selectively permeable, that is, it must be more permeable to water molecule than at least one species of solute. 2) there must be a difference in the concentraion of a solute on the two sides of a selectively permeable membrane 3) the membrane must be relatively impermeable to the solute. Glucose and water.

Osmotivally active:--- solutes that cannot freely pass through the membrane.

Aquaporin: the water channels on certain membrane of cells that allow water to move through membrane more rapidly. In kidney, the aquaporin chnnels are inserted into the plasma membrane in response to regulator molecules.

Osmotic Pressure: the measure of the pulling power for water molecule. ---the greater the solute concentration of a solution, the greater its osmotic pressure.

Molar: Relating to or designating a solution that contains one mole of solute per liter of solution.

Molarity:The molar concentration of a solution, usually expressed as the number of moles of solute per liter of solution.

Osmolarity: The osmotic concentration of a solution expressed as osmoles of solute per liter of solution.

Tonicity: The osmotic pressure or tension of a solution. the effect of a solution on the osmotic movement of water.

Isotonicity: the concentration of solutes entering and leaving the cell occurs at the same rate.

A solution may be isosmotic but not isotonic-----because whenever the solute in the isomotic solution can freely penetrate the membrane. A 0.3 m urea is isomostic to red blood cell. but it is not isotonic because the cell membrane is permeable to urea. So, when red blood cells are placed in a 0.3 m urea solution, the urea diffues into the cells until its concentration on bothe sides of the cell membreanes becomes equal. Meanwhile, the solute within the cells that cannot exit-- and which are there fore osmotcially active-- to cause the osmosis of water into the cells. So, red blood cells placed in 0.3 m urea will eventually burst.

Hypertonic: a cell whose rate of solutes leaving it is greater than its rate of solutes entering it. A solutions that has a higher total concentration solutes than that of plasma, therefor a higher osmotic pressure. a red blood cell placed in a hypertonic solution will shrink.

Crenation: is the contraction of cells within animals in a hypertonic solution, due to the loss of water through osmosis.

Hyptotonic: a cell whose rate of solutes entering it is greater than its rate of solutes leaving it. This may cause the red blood cell to expand or burst. Solutions that have a lower total solutes concentraion than that of plasma.

Hemolysis: red bllod cells placed in a hypotonic solutions gain water and may burst.

Intravenous fluids 静脉注射液: must be isotonic to blood in oder to maintain the correct osmotic pressure and prevent cells from either expanding or shrinking from the gain or loos of water.
Common fluid used for intravenous fluids sare 0.9% normal saline and 5% dextrose葡萄糖 , Ringer's lactate (contains glucose and lactic acid and numbers of different salts) which have the same osmalarity as normal plasma (300mOsm).

Regulation of blood osmolarity

Dehydration: the removal of water from an object.
  • the blood becomes more concentrated as the total blood volume is reduced.
  • The increased blood osmolarity and osmotic pressure stimulate osmoreceptors (neurons located in hypothalamus of brain.)
  • The osmoreceptors in the hypothalamus stimulate a tract of axons that terminate in the posterior pituitary]垂体 to cause the release of antidiuretic hormone (ADH) into the blood.
  • ADH acts on kidney to promote water retention 保持力. so that a lower volume of more concentrated urine is excreted.
  • Thirsty.
  • Drinking increases water intake.
  • --------------------------------therefore dehydrated person drinks more and urine less----a negative feedback loop, which acts to maitain hoeostasis of the plasma concentration and helps to maintain a proper blood volume.
A person with normal blood volume who eats salty food will get thirsty, and more ADH will be released from the posterior pituitary.------drinking and exreting less------more diluted normal blood concentration but a higher blood volumve.

In salt deprivation: lower plasma osmolarity will not stimulate as much osmoreceptors and less ADH released. then more water is excreted in the urine to again restore the proper trange os plasma concentration, but with lower blood volume and pressure(fatal stage)

Rehydration: is the replenishment of water and electrolytes lost through dehydration

Characteristic of carrier protein
  1. Specificity: GLUT for glucose
  2. Competition
  3. Saturation: as the conentration of a transported molecule is increased its rate of transport will also be increased only up to a maximum----Transport maximum (Tm)
Disbetes mellitus
  • The kidney transport a number of molecules from the blood filtrate back into the blood. Glucose, is normally reabsorbed so that urine is normally free of glucose.
  1. Hyperglycemia: a condition in which an excessive amount of glucose circulates in the blood plasma.
  2. Hypoglycemia: the plasma glucose concentration is abnormally low.may be caused by overdose of insulin.
  3. Glycosuria: It leads to excretion of glucose in the urine. Due to a lack of the hormone insulin, plasma glucose levels are above normal or too much sugar consumption. This leads to saturation of receptors in the kidneys, and occurs at plasma glucose levels above 11 mmol/L.
Facilitated diffusion: is a process of diffusion, a form of passive transport, via which molecules diffuse across membranes, with the assistance of carrier proteins.

Carrier protein
  • Protein characteristic : specificity, competition, saturation
  • GLUT: the transport carrier for the facilitative diffusion of glucose.
  • Isoenzyme of GLUT: GLUT4 --- carrier protein for glucose in skeletal muscales.
  • 1) in unstimulated muscles, the GLUT4 protein are within the membrane enclosing cytoplasmic vesicles. 2) exercise and stimulation by insulin--cause these vesicles to fuse with the membrane so that the transport carriers are inserted into the plasma membrane. 3) During sercise and insulin stimulation, therefore, more glucose is able to enter the skeletal muscle cells from the blood plasma.
  • Passisve transport:from high to low without energy.
Active transport: is the movement of molecules and ions against their concentration gradients from lower to higher concentrations with requirement of energy from ATP.
  • the epitherlial linings of the small intestine and kidney tubules move glucose from the side of lower to the side of higher concentration--from the space within the tube(umen) to the blood.
  • Cells extrude Ca2+ into the extracellular envronment and mtaintain an intracellular Ca2+ that is 1000 to 10000 times lower than extracellular. so this steep concentration gradients of Ca2+ make it a regulatory signals. ------- the opening of membrane Ca2+ channels, and the rapid diffusion of Ca2+ channeles provides a signal for neurotransmitter release, muscle contraction and other.
  • If the cell is poisoned with cyanide(which inhibits oxidative phosphorylation), active transport will stop,.
Primary active transport
  • directly uses energy to transport molecules across a membrane when hydrolysis of ATP is required.
  • these carrier are transmembrance integral protein.
  • Most of the enzymes that perform this type of transport are transmembrane ATPases.
  • A primary ATPase universal to all cellular life is the sodium-potassium pump, which helps maintain the cell potential.
  • 1) the molecules or ion to be transported binds to a specific" recognition site" on one side of the carrier protein. 2) the bonding stimulates the carrier proteins 3) as a result of phophorylation, the carrier protein undergoes a conformational changes 4) a hingelike motion of the carrier protein releases the transported molecules or ion on the opposite side of the membrane.
Sodium-Potassium pump
  • Pump: primary active transport carriers are reffered as as pump. some transport only one molecules or ion at a time, others exchanges one for another.
  • Na+/K+ pump: 1) an ATPase enzyme that converts ATP to ADP and P1. 2) extrude 3 Na+ from the cell / 2 K+ into the cell. Both ions are against their concentration gradients.
  • 200 Na+/K+ pump in red blood cell, 35,000 per white blood cell. several million per cell in the kidney tubules.
  • 1) the steep Na+ gradient is used to provide energy for the "coupled transport" of another molecules.
    2) the activity of the pump can be adjusted primarily by thyroid hormones to regulate the resting clories expenditure and basal metabolic rate of the body 3) the gradients for Na+ and K+ concentrations of nerve and muscle cells are used to produce electrochemical impulses needed for functions of the nerve and muscles, including the heart muscle. 4) the active extrusion of Na+ is important for osmotic resons: if the pump stop, the increased Na+ concentraions within the cells promote the osmotic inflow of water--damaging the cell.
  • It helps maintain cell potential and regulate cellular volume.
  • it contributing to the negative intracellular charge. and adds 3mV to the membrane potential.
Secondary active transport (couple transport)
  • the energy need for the "uphill" movement of a molecule or ion is obtained from the "downhill" transport of Na+ into the cell.
  • Hydrolysis of the ATP by the action of the Na+/K+ pumps is required indirectly, in order to maintain low intracellular Na+ concentration.
  • The diffusion of Na+ down its concentration gradient into the cell can then power the movement of a different ion or molecule against its concentration gradient.
  • Cotransport. Symport: if the other molecule of ion is moved in the same direction as Na+,(into the cell) 1) cotransport of glucose and Na+ from the extracellular fluid into the epithelial cells of the small intestine and kidney tubules. 2)a carrier protein binds to glucose and Na+ in the extracellular fluid 3) the downhill transport of Na+ into the cell furnishes the energy for the uphill transport of glucose. 4) then a steep gradient for Na+ must be restored by Na+/K+ pump
  • Countertransport/antiport: if the other molecule or ion is moved in the opposite direction (out of the cell). 1) an example of countertransport is the uphill extrusion of Ca2+ from a cell by type of pump that is coupled to the passive diffusion of Na+ into the cell. 2) ATP is used to maintain the steep of Na+ concentration. 1) another example: diffusion of bicarbonate (HCO3-) out of the cell powers the entry of the chloride. ??????
Transport across epithelial membrane
  • in order for a molecule or ion to move from the external ecvironment into the blood, it must 1) first pass through an epitheliam membrane
  • Absorption: the transport of digestion productions( such as glucose ) across the intestinal epithelium into the blood.
  • Reabsorption: the transport of molecules out of the urinary filtrate back into the blood.
  • 1) the cotransport of Na+ and glucose are located in the apical plasma membrane of the epithelial cells. which face the lumen of the intestine or kidney tubules. 2) the Na+/K+ pump is locaed on the opposite side of the epithelial cells. 3) thus, the transport is from the lumen of small intestine of kidney tubule into the epithelial cells then to the blood. then to the muscles for energy .
  • Transcellular transport: the membrane transport mechanism move materials through the cytoplasma of the epithelial cell.
  • Paracellular transport: diffusion and osmosis occur to a limited extent in the tiny space between epithelial cells.
Transport between cells: (paracellular transport)
  • is limited by junctional complexes: 1) a structure within a tissue of a multicellular organism. 2)e especially abundant in epithelial tissues. 3) They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they built up the paracellular barrier of epithelia and control the paracellular transport.
  • 1) tight junction: where the plasma membranes of two cells physically join together and proteins penetrate the membrane to bridge the cytoskeleton actin fibers of the two cells. forming a virtual impermeable barrier to fluid.
  • 2) adherens junction: the plasma membrane of the two cells come very close together and are "glued" by the interactions between proteins that span each membrane and coonet the cytoskeleton of each cells.
  • 3) desmosome: the plasma membranes of the two cells are "buttoned together" by the interactions between particular desmosomal proteins.
Tight junction:
  • They hold cells together
  • They block the movement of integral membrane proteins between the apical and basolateral surfaces of the cell. Thus the specialized functions of each surface, for example
    • receptor-mediated endocytosis at the apical surface
    • exocytosis at the basolateral surfacecan be preserved.
  • They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides control over what substances are allowed through. (Tight junctions play this role in maintaining the blood-brain barrier.)
Adherens junction
  • serve as a bridge connecting the actin cytoskeleton of neighboring cells
  • the characteristic of anchoring cells through their cytoplasmic actin filaments.
Desmosomes :are molecular complexes of cell adhesion proteins and linking proteins that attach the cell surface adhesion proteins to intracellular keratin cytoskeletal filaments.

Bulk transport: bulk means many molecules are moved at the same time.
  1. Exocytosis: many cells secrete molecules such as hormone or neurotransmitter by the excytosis. this invovles the fusion of a membrane bound vesicle that contains these cellular products with the membrane so that the membrane become continuous.
  2. Endocytosis: specific molecules such as protein-bound cholesterol can be taken into the cell because of the interaction between the cholesterol transport protein and a protein receptor on the plasma membrane. thus cholesterol is removed from the blood by the liver and by the walls of blood vessels through this. 1) molecules taken into a cell by endocytosis are still separated from the cytoplasma by the membrane of the endocytotic vesicle. 2) some, such as membrane receptors will be moved back to the plasma membrane, while the rest will end up in lysosomes.
Definite direction / Polarity: to transport in epithelial cells.
  • exocytosis vesicle that bud from the Golgi complex fuse with the plasma membrane at its apical or top surface. while the nucleus and endoplasmic reticulum are located more toward the bottom of the cell nearer to the basement membrane.
Voltage: the separation of charges.

Equilibrium potential : a particular ion is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other.
  • fact that the net ion flux at the voltage is zero K+ -90mV. Na+ : +60mV
  • at equilibrium potential, the force of electrical attaction and of the diffusion gradient are equal and opposite.
Resting membrane potential
  • is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential.
  • In most cells the resting potential has a negative value, which by convention means that there is excess negative charge inside compared to outside.
  • The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane
  • the resting membrane potential for most cells in the body ranges from -65mV to -85mV. in neurons it averages -70mV.
The actual value of the resting membrane potential depends on
  1. the ratio of the concentration (Xout side/ X inside) of each ion on the two sides of the plasma membrane.
  2. The specific permeability of the membrane to each difference ion.
Two implication
  1. for any gicen ion, a change in its concentration in the extracellular fluid will change the resting membrane potential--but only to the extent that the membrane is permeable to that ion. Because resting membrane is most permeable to K+, a change in the extracellular concentration of K+ has the greatest effect on the resting membrane potential.
  2. A change in the membrane permeability to any given ion will change the membrane potential.
Cell signaling: the way cell communicate with each other it governs basic cellular activities and coordinates cell actions
  • The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue homeostasis.
  • Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity, and diabetes.
Type of cell signalling
  1. Some cell-to-cell communication requires direct cell-cell contact. Some cells can form gap junctions that connect their cytoplasm to the cytoplasm of adjacent cells. In cardiac muscle, gap juctions between adjacent cells allows for action potential propagation from the cardiac pacemaker region of the heart to spread and coordinately cause contraction of the heart.
  2. Many cell signals are carried by molecules that are released by one cell and move to make contact with another cell. 1) paracrine signaling 2) synaptic signaling 3)endocrine signaling.
Paracrine signaling
  • cells within an organ secrete regulatory molecules that diffuse through the extracellular matrix to nearby target cells.
  • It is local because it involves the cells of a particular organ.
  • signal chemical is broken down too quickly to be carried to other parts of the body
  • the nearby cells take up the signal at a very high rate, leaving little signal free to travel further
  • the signal gets stuck in the extracellular-matrix, or structure surrounding the signal releasing cell, and thus the signal is unable to travel far from the signal releasing cell.
  • growth factor and clotting factors.
Synaptic signaling: refers to the means by which neurons regulate their target cells.
  1. the axone of a neuron innervate its target organ through a functional connection or synapse between the axon ending and the target cell. and chemical neurotransmitters are released by the axon ending.
Endocrine signaling: the cells of endocrine glands secrete chemical regulators called hormones into the extracellular fluid(blood ) (ductless)
  • the hormones enter the blood and are carried by the blood to all the cells in the body. only the target cells fro a particular hormone can respsond to the hormone.
Receptor protein
  • in order for a target cell to respond to a hormone, neurtransmitters or paracrine regulator, it must have specific receptor proteins for these molecules.
  • some receptor protein may be on the outer surface of the membrane
  • or in the cytoplasma or nucleus
  • the location of the receptor proteins depends on whether the regulatory molecule can penetrate the plasma of the target cell.
  • If the molecule is non-polar, it can enter the target cell: such as steroid hormones, thyroid hormone, and nitric oxide gas( a paracrine regulator)---------receptors are intracellular.
  • Regulatory molecules that are large or polar such as epinephrine(an amine hormone) and acetylcholine(an amine neurotransmitter) and insulin( a polypeptide hormone) cannot enter the cell, ------receptor protein must located on the outer surface of the membrane.
Chapter 7

Nervous system : central nervous system and peripheral nervous system.

CNS
PNS
The nervous system is composed of two types of cells: 1) neurons 2) supporting cells

Neuron
  • cells in the nervous system.
  • sometimes called nerve cells, many neurons do not form nerves.
  • In vertebrates, neurons are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.
  • soma, or 'cell body', is the central part of the cell, where the nucleus is located and where most protein synthesis occurs. The cell body also contains densely staining areas of rough endoplasmic reticulum -------Nissle bodies, that are not found in the dendrites aor axon.
  • The cell body in the CNS cluster into groups called Nuclei. Cell bodies in the PNS in cluster called ganglia.
  • dendrite, is a branching arbor of cellular extensions. Most neurons have several dendrites with profuse dendritic branches.and is traditionally thought to be the main information receiving network for the neuron. However, information outflow (i.e. from dendrites to other neurons) can also occur.
  • axon, is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length.The axon carries nerve signals away from the soma (and carry some types of information in the other direction also). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells.
  • The part of the axon where it emerges from the soma is called the 'axon hillock'. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. Thus it has the most hyperpolarized action potential threshold of any part of the neuron. In other words, it is the most easily-excited part of the neuron, and thus serves as the spike initiation zone for the axon. While the axon and axon hillock are generally considered places of information outflow, this region can receive input from other neurons as well.
  • Axon collateral may extend from the axon.
  • Can not divide by mitosis
Transport in neuron
  1. Axoplasmic flow: the slower transport result from rhythmic waves of contraction tht push the cytoplasm from the axon hillock to the nerve endings.
  2. Axonal transport: employs microtubules and ir more rapid and more selective, may occure in reverse ( retrograde) direction as well as in a forward (orthograde) direction.
  3. Retrograde transport, may be responsible for the movement of the herpesvirus, rabies virus and tetanus toxin from the nerve terminals into the cell bodies.
Classification of neurons and nerves--the functional classification is based on the direction in which they conduct impulses--------------Sensory neuron, Motor neuron, Interneurons (located entirly in the CNS and serve the associative or integrative functions of the nervous system.
  • Two type of motor neurons: 1) Somatic 2) autonomic.
  • Somatic motor neurons are for both reflex and voluntary control of skeletal muscles.
  • Autonomic motor neurons send axons to the involuntary effectos ---smooth muscle, cardia muscle, and gland. The cell bodies of the autonomic neurons that innervate these organs are located outside the CNS in autonomic ganglia.
  • Two subdivision os autonomic neurons: 1) sympathetic 2) parasympathetic
The structural classification of neuron is based on the number of processe that extend from the cell body of the neuron.
  • Pseudounipolar neuron: have a single short process that branches like a T to form a paire of longer processes.
  • Bipolar neurons : two processes, one at the eigher end, found in the retina of the eye.
  • Multipolar neurons: several dendrites and one axon extending from the cell body. motor neuron.
Nerve:
  • is a bundle of axons located outside the CNS. most nerves are composed of bother moter and sensory fibers and are thus called mixed nerves.
  • Some of the cranial nerves, however, contains sensory fibers only, they are the nerves that serve the special senses of sight, hearing, taste, and smell.
Supporting cells----two types of supporting cells in the peripheral nervous system
  1. Schwann cells: form myeline sheaths around the peripheral axons.
  2. Satellite cell or ganglioni gliocytes, support neuron cell bodies within the ganglia of the ONS.
Four kinds of supporting cells: called glial cells, in the CNS
  1. oligodendrocytes: form myelin sheaths around axons of the CNS.
  2. Microglia: mirgrate through the CNS and phagocytose foreign and degenerated material.
  3. Astrocytes: help to regulate the external environment of neurons in the CNS.
  4. Ependymal cell: line the ventricles (cavities) of the brain and central canal of the spinal cord.
neural stem cells----------ependymal cells and astrocyte can divide and their progeny can differentiate along different lines, to become new neurons and neuroglial cells.

All axons in the PNS (melinate and unmelinated) are surrounded by a continuous, liveing sheath of Schwann cells, ---neurilemma of the sheath of Schwann. The axonds of the CNS, lack a neurilemma ( Schwanna cell are only found in the PNS). This is signigicant in terms of regeneration of damaged axons.








Glial cells
  • commonly called neuroglia or simply glial cells, are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by as much as 50 to 1.
  • Some glia function primarily as physical support for neurons.
  • Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and provide nutrition to nerve cells.
  • Glia have important developmental roles, guiding migration of neurons in early development, and producing molecules that modify the growth of axons and dendrites.
  • active participants in synaptic transmission, regulating clearance of neurotransmitter from the synaptic cleft, releasing factors such as ATP which modulate presynaptic function, and even releasing neurotransmitters themselves.
  • can divide by mitosis.----this explains the brain tumors in adults are usually composed of glial cells rather than of neurons.
Axoplasmic flow
  • axonal transport, is responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other cell parts to and from a neuron's cell body through the cytoplasm of its axon (the axoplasm).
  • . Axons, which can be 1,000 or 10,000 times the length of the cell body, or soma, contain no ribosomes or means of producing proteins, and so rely on axoplasmic transport for all their protein needs
  • also responsible for moving molecules destined for degradation from the axon to lysosomes to be broken down
  • Mitochondria that are transported along microtubules continue making ATP, which can be used by motor proteins carrying them
Sensory neuron
  • are nerve cells within the nervous system responsible for converting external stimuli from the organism's environment into internal electrical motor reflex loops and several forms of involuntary behavior, including pain avoidance. In humans, such reflex circuits are commonly located in the spinal cord.
  • sensory neurons relay their information to the central nervous system or in less complex organisms, such as the hydra, directly to motor neurons and sensory neurons also transmit information to the brain, where it can be further processed and acted upon.
  • sensory receptors located on the cell membrane of sensory neurons are responsible for the conversion of stimuli into electrical impulses.
Supporting cells in PNS
  • Schwann cells
  • Satellite cells or ganglionic gliocytes.
Resting membrane potential
  • is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential.
  • most cells the resting potential has a negative value, which by convention means that there is excess negative charge inside compared to outside
  • The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane.
  • the amount of certain potassium channels is most important for control of the resting potential (see below). Some ion pumps such as the Na+/K+ATPase are electrogenic, that is, they produce charge imbalance across the cell membrane and can also contribute to the membrane potential.
Excitability
  • the ability of alterring membrane potential in response to stimulation thus produce and conduct changes in the membrane potential.
Electrochemical gradient
  • the electrical and chemical properties across a membrane
  • In biological processes the direction an ion will move by diffusion or active transport across membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation.
  • In generic terms, electrochemical potential is the mechanical work done in bringing 1 mole of an ion from a standard state to a specified concentration and electrical potential.
  • First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favourable direction for an ions movement across a membrane.
  • the inclined tendency of an electrically charged solute, such as a potassium ion, to move across the membrane is decided by the difference in its electrochemical potential on either side of the membrane, which arises from three factors:
  • the difference in the concentration of the solute between the two sides of the membrane
  • the charge or "valence" of the solute molecule
  • the difference in voltage between the two sides of the membrane (i.e. the transmembrane potential).
Ion current

Depolarization(excitatory)
  • Elimination or neutralization of polarity, as in nerve cells.
  • The rising phases of an action potential
  • membrane potential change in the depolarizing direction from the resting potential
Repolarization
  • the change in membrane potential that returns the membrane potential to a negative value after the depolarization phase of an action potential has just previously changed the membrane potential to a positive value.
  • Repolarization results from the movement of positively charged potassium ions out of the cell. Typically the repolarization phase of an action potential results in hyperpolarization, attainment of a membrane potential that is more negative than the resting potential.
Hyperpolarization(inhibitory)
Voltage-gated ion channels are a class of trans-membrane ion channels that are activated by the surrounding potential difference near the channel (or near the cell, neuron or synapse).
  • crucial role in excitable tissues such as nerve and muscle, since they allow a rapid and co-ordinated depolarisation in response to a triggering voltage change.
  • the sodium and potassium voltage-gated channels of nerve and muscle, and the voltage-gated calcium channels that play a role in neurotransmitter release in pre-synaptic nerve endings.
Chemical gated channel or ligand-gated ion channel
Threshold: an action potential is initiated if the membrane potential is depolarized to the threshold potential.

Action potential
  • a wave of electrical discharge that travels along the membrane of a cell.
  • rapidly carrying information within and between tissues.
  • The action potential does not dwell in one location of the cell's membrane, but travels along the membrane. It can travel along an axon for long distances, for example to carry signals from the spinal cord to the muscles of the foot. In large animals, such as giraffes and whales, the distance traveled can be many meters.
  • At resting potential some potassium leak channels are open but the voltage-gated sodium channels are closed. Potassium diffusing down the potassium concentration gradient creates a negative-inside membrane potential.
  • A local membrane depolarization caused by an excitatory stimulus causes some voltage-gated sodium channels in the neuron cell surface membrane to open and therefore sodium ions diffuse in through the channels along their electrochemical gradient. Being positively charged, they begin a reversal in the potential difference across the membrane from negaitve-inside to positive-inside. Initially, the inward movement of sodium ions is also favored by the negative-inside membrane potential.
  • As sodium ions enter and the membrane potential becomes less negative, more sodium channels open, causing an even greater influx of sodium ions. This is an example of positive feedback. As more sodium channels open, the sodium current dominates over the potassium leak current and the membrane potential becomes positive inside.
  • Once a membrane potential of around +40 mV has been established, voltage-sensitive inactivation gates of the sodium channels, sensitive to the now positive membrane potential gradient, close (so further influx of sodium is prevented). While this occurs, the voltage-sensitive activation gates on the voltage-gated potassium channels begin to open.
  • As voltage-gated potassium channels open and there is a large outward movement of potassium ions driven by the potassium concentration gradient and initially favored by the positive-inside electrical gradient. As potassium ions diffuse out, this movement of positive charge causes a reversal of the membrane potential to negative-inside and repolarisation of the neuron back towards the large negative-inside resting potential.
  • The large outward current of potassium ions through the voltage-gated potssium channels causes the temporary overshoot of the electrical gradient, with the inside of the neuron being even more negative (relative to the outside) than the usual resting potential. This is called hyperpolarisation (hyperpolarization). The voltage-sensitive inactivation gates on the potassium channels now close and the continual movement of potassium through potassium leak channels again dominates the membrane potential. Sodium-potassium pumps continue to pump sodium ions out and potassium ions in, preventing any long-term loss of the ion gradients. The resting potential of -70 mV is re-established and the neuron is said to be repolarised.
All or none-------if a Neuron fires at all, it will be propagated all the way from the beginning to the end of the axonal process.

Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal.

FM: one or more discrete frequencies correspond to each desired significant condition of a code.
The code for stimulus strength is the frequency modulated.

Recruitment: as the intensity of stimulation increases, more and more axons will become activated.. this represent another mechanims by which the nervous system can code for stimulus.

Refractory periods
  • the amount of time it takes for an excitable membrane to be stimulated and then be ready for stimulus again.
  • The refractory period is due to the inactivation property of voltage-gated sodium channels. Voltage-gated sodium channels have two gating mechanisms, one that opens the channel with depolarization and the inactivation mechanism that closes the channel with depolarization.
  • Channel opening with depolarization is faster than inactivation, thereby allowing an initial entry of sodium ions in to the cell. But eventually, all the sodium channels will close with sustained depolarization. The only way to de-inactivate voltage-gated sodium channels is to hyperpolarize the membrane for a sustained period of time.
  • The time between one action potential and when not enough of the voltage-gated sodium channels are de-inactivated and able to generate a new action potential in response to stimulus is called the absolute refractory period.
  • The time between absolute refractory period and when all the sodium channels are de-inactivated is called the relative refractory period.
Cable property of neuron
  • refers to the ability of a neuron to transmit charges through its cytoplasm.
Conducted without decrement-----the action potential produced at the last region of the axon has the same smplitude as the action potential produced at the first region.

Conduction in a myelinated Axon
  • the myeline sheath provide insulation for the axon, preventing movements of Na+ and K+ through the membrane.
  • Nodes of Ranvier.
  • Saltatory跳跃的 conduction : action potentials, occur only ar the nodes of Ranvier and leap from node to node.
Synapse
  • specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands. A chemical synapse between a motor neuron and a muscle cell is called a neuromuscular junction.
  • The functional connection between a neuron and a second cell.
  • axodentritic, axosomatic,axoaxonic
  • Synaptic cleft
Neurotransmitter
  • are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell.
  • It is synthesized endogenously, that is, within the presynaptic neuron;
  • It is available in sufficient quantity in the presynaptic neuron to exert an effect on the postsynaptic neuron;
  • Externally administered, it must mimic the endogenously-released substance; and
  • A biochemical mechanism for inactivation must be present.

  • Within the cells, small-molecule neurotransmitter molecules are usually packaged in vesicles. When an action potential travels to the synapse, the rapid depolarization causes calcium ion channels to open. Calcium then stimulates the transport of vesicles to the synaptic membrane; the vesicle and cell membrane fuse, leading to the release of the packaged neurotransmitter, a mechanism called exocytosis.

    The neurotransmitters then diffuse across the synaptic cleft to bind to receptors. The receptors are broadly classified into ionotropic and metabotropic receptors. Ionotropic receptors are ligand-gated ion channels that open or close through neurotransmitter binding. Metabotropic receptors, which can have a diverse range of effects on a cell, transduct the signal by secondary messenger systems, or G-proteins.

  • A neurotransmitter's effect is determined by its receptor. For example, GABA can act on both rapid or slow inhibitory receptors (the GABA-A and GABA-B receptor respectively). Many other neurotransmitters, however, may have excitatory or inhibitory actions depending on which receptor they bind to.
  • Neurotransmitters may cause either excitatory or inhibitory post-synaptic potentials. That is, they may help the initiation of a nerve impulse in the receiving neuron, or they may discourage such an impulse by modifying the local membrane voltage potential. In the central nervous system, combined input from several synapses is usually required to trigger an action potential. Glutamate is the most prominent of excitatory transmitters; GABA and glycine are well-known inhibitory neurotransmitters.
  • Many neurotransmitters are removed from the synaptic cleft by a process called reuptake (or often simply uptake). Without reuptake, the molecules might continue to stimulate or inhibit the firing of the postsynaptic neuron. Another mechanism for removal of a neurotransmitter is digestion by an enzyme. For example, at cholinergic synapses (where acetylcholine is the neurotransmitter), the enzyme acetylcholinesterase breaks down the acetylcholine. Neuroactive peptides are often removed from the cleft by diffusion, and eventual broken down by proteases.
Terminal boutons, presynaptic axon endings where transmission across the synapse in the nervous system is one-way.

CAMs---Call adhersion molecule---the protein that physiolocally stablized pre- post synaptic membrane at the chemical synapse.

Calmodulin: a Ca2+-binding protein that activate enzyme called protein kinase. Protein kinase phosphorylation synapsin in the membrane of the synaptive vesicle. this aids the fusion of synaptic vesicles with the membrane.

EPSP: excitatory postsynaptic potential (EPSP) is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell.
  • A postsynaptic potential is defined as excitatory if it makes it easier for the neuron to fire an action potential. EPSPs can also result from a decrease in outgoing positive charges,
  • When multiple EPSPs occur on a single patch of postsynaptic membrane, their combined effect is the sum of the individual EPSPs. Larger EPSPs result in greater membrane depolarization and thus increase the likelihood that the postsynaptic cell reaches the threshold for firing an action potential.
  • EPSPs in living cells are caused chemically. When an active presynaptic cell releases neurotransmitters into the synapse, some of them bind to receptors on the postsynaptic cell. Many of these receptors contain an ion channel capable of passing positively-charged ions either into or out of the cell (such receptors are called ionotropic receptors). At excitatory synapses, the ion channel typically allows sodium into the cell, generating an excitatory postsynaptic current. This depolarizing current causes an increase in membrane potential, the EPSP
  • The neurotransmitter most often associated with EPSPs is the amino acid glutamate, and is the main excitatory neurotransmitter in the central nervous system Its ubiquity at excitatory synapses has led to it being called the excitatory neurotransmitter. In the neuromuscular junction, EPSPs (called end-plate potentials, EPP) are mediated by the neurotransmitter acetylcholine.
IPSP: is the change in membrane voltage of a postsynaptic neuron which results from synaptic activation of inhibitory neurotransmitter receptors.
  • The most common inhibitory neurotransmitters in the nervous system are GABA and glycine.
  • is considered inhibitory, when the resulting change in membrane voltage makes it more difficult for the cell to fire an action potential, lowering the firing rate of the neuron. They are the opposite of excitatory postsynaptic potentials (EPSPs), which result from the flow of positive ions into the cell.
  • At a typical inhibitory synapse the postsynaptic neural membrane permeability increases for positive potassium (K+) ions and negative chloride (Cl-) ions but not sodium (Na+) ions. This generally causes an influx of Cl- ions and efflux of K+ ions, thereby bringing the membrane potential closer to the equilibrium potential of these ions.


Types of neurotransmitter
Acetylcholine (ACh)
Nicotinic ACh receptor

IV Summary

In the patellar reflex experiment, the normal response for subject A is 30cm and B for 19 cm. When clench fist is employed, A’s reflex distance decreased to 18 cm and B to 17cm. When both subject were asked to look away, the reflex distance for A is 9cm, B is 17cm. After add columns of figures, A’s response is 9cm and B’s is 20cm. When both them were asked to watch the experimenter performing, A’s response is 31cm and B’s response is 10cm.

In the cilio-spinal reflex experiment, after pinching the skin on the nape of the neck of the volunteer, both of her pupils were significantly enlarged.

After performing photo-pupil reflex and observing, both of the eyes of volunteer constricted in unison. The diameter of her right eye constricted from 3.5mm to 3.0mm. Her left eye 3.0mm to 2.5mm.

V Conclusion

The patellar reflex is the reflex employing three neurons. Striking the patellar ligament below the patella stretches the quadriceps tendon. Neural activity at other sites in the body may influence the patellar reflex response. The experiment suggests that the magnitude of the patellar reflex can increased when the contractile tones of the quadriceps muscles is increased. Mental activity, which increased muscle tone, may also increase the magnitude of the response. Decreased metal activity can decrease the magnitude of the reflex while physical activity and metal stress may make the reflex less sensitive.

In the papillary reflex, the papillary skin reflex (cilio-spinal reflex) is the increase of pupil size in response to the skin pinch of the neck spinal nerve. The papillary light reflex is the reduction of pupil size in response to light. Abnormal response on those two exams can lead to possible damage of the spinal and brain stem function.

VI Food for Thought

1 In what ways might these reflexes be adaptive to the organism? That is, how might they help to increase the chances of survival of the individual or of the species?

Reflex is a response to a perturbing stimulus that acts to return the body to homeostasis. Reflexes require a minimum of two neurons, a sensory neuron and a motor neuron. The sensory neuron detects the stimulus and sends a signal towards the CNS. And a motor neuron innervates the effector’s tissue. More complex reflexes may have more neurons involved as well as interneuron and their integration center may be in the spinal cord, in the brain stem or in the cerebrum where the thoughts are initiated.

This mechanism is crucial for the survival of human being because it can help to detect any deviation from the normal range of an ideal living environment and readjust body to either adapt new situation or migrate to other location for survival.

2 Describe the physiological value of an athlete tensing his muscles in preparation for the start of play.

When an athlete tensing his muscles, this involves events which are results of in the sensation of muscle stretch. Muscle spindles are innervated by sensory neurons and are arranged in parallel to normal muscle fibers. Muscle fibers response to tension by depolarizing a sensory neuron. The sensory neuron synapses with a motor neuron in the spinal cord that innervates a normal muscle fiber, decreasing stimulation to neuron.

3 What kinds of response should the physician look for as indicators of pathological condition?

No response of patellar reflex may relate to damage of muscle spindles and neurons. No enlargement of pupil in the cilio-spinal exam may results in damage of spinal cord. No constrictions or the constrictions performed in a separate matter in pupils-photo exam may indicate damage of cranial nerves, optic nerve and oculomotor nerve.

posted by C-T @ 5:52 PM

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