Contents 1. Structural Organization of the Human Body (2 Lectures) 1. 3. 1 Tissues Tissue: groups of structurally similar cells that have perform common/related function Tissues cooperate within an organ for function of organ as a whole, different issues = division of labor 1. 3. 2 4 Types of Tissue: 1. Muscle Tissue: movement 2. Epithelial Tissue: covering 3. Nervous Tissue: control (regulation) 4. Connective Tissue: support Minimum two present in organ, usually takes 4 to make organs like kidney, heart, etc.
*histology: study of tissues and their cellular organization Epithelial tissue Sheet of cells covering a body surface or lines body cavity 1.
Covering and lining epithelium: e. g. skin, trachea, lung, urinary lining 2. Glandular Epithelium Functions of various epithelia 1. protection (mechanical, chemical, infectious) – skin, 2. absorption – GI tract, 3. filtration – kidney, 4. excretion – kidney, 5. secretion – glands, 6. sensory reception-taste buds, olfactory membranes 1. 3. 4 Seven(7) Special Characteristics 1. cellularity: specific cell shapes (e.
g. squamous, cuboidal…, dense, little fluid in between cells) 2. specialized contacts: tight, gap junctions and desmosomes 3. Polarity: Apical and Basal surfaces Apical faces outwards to exterior of cell, Specializations: microvilli in intestines to increase SA Cilia in lungs to propel mucus Basal surface attaches to underlying tissues (CT) 4.
Basal Lamina: noncellular, glycoprotein sheet supporting the epithelium Functions Attachment Filter Mobility of epithelia It is a secreted material found right under epithelium
5. Basement Membrane = basal lamina + reticular lamina (reticular CT) Cancer cells perforate this membrane and move into deeper tissue 6. Innvervated (has nerve fibers) but avascular (no blood vessels) Nourished from diffusion of substances coming from blood vessels of underlying connective tissue 7. Regeneration: constantly damaged so must be constantly replaced. 1. 3. 5 Classification of epithelial cells 1. 3. 6 Types of simple Epithelia 1) Simple Squamous: thin permeable, -filtration, and diffusion.
e. g. endothelium (lining of lymphatic and cardiovascular vessels, organs, capillaries consists only of endothelium) and kidney, lungs 2) Simple Cuboidal: secretion + absorption, eg: kidney tubules, small glands 3) Simple Columnar: also digestion, secretion-eg. digestive tract 4) Pseudo stratified columnar: a single layer of cells that vary in height, all cells rest on basement membrane but only tallest reach free surface, cell nuclei lie at different positions giving false impression of stratification.
Secretion, absorption, ciliated version found in respiratory tract, where it secretes mucus and cilia propel sheets of dust-trapping mucus. Stratified epithelia 2 or more layers Regen from below (basal cells divide and replace apical), replace dying cells on surface More durable Protection is major role, but not only role Transitional Epithelium (NOT SIMPLE! ) Stratified, lines the bladder and found in organs that have to fill up Basal layers are columnar to cuboidal, apical layers become squamous-like as filling occurs 1. 3. 7 Stratified Squamous Epithelium (Skin)
Most abundant of stratified epithelia Found in external part of skin and extends a short distance into every body opening that is directly continuous with our skin. Skin (epidermis) is keratinized, keratin = tough, protective protein Wear and tear 1. 3. 8 Glandular Epethelia Glands consist of one or more cells that make and secrete an aqueous product. 2 types: Endocrine: (ductless) internal secretion, produce hormones Exocrine: exteral secretion, secrete substances (mucus, sweat, oil, saliva, bile)via specialized ducts which then deliver it to body surfaces or into body cavities.
Unicellular (goblet cells) One cell, no ducts, all produce mucin Multicellular An epithelium derived duct and a secretory unit Surround by supportive CT giving it blood vessels, nerves Secretory methods: Merocrine (or eccrine): exocytosis , most common type Holocrine: accumulate products until cell rupture, only sebaceous gland Apocrine: accumulate products but only just beneath free surface, cell apex pinches off releasing the secretory granules and a small amount of cytoplasm, cell repairs and repeats 1. 3. 9 Types of Connective Tissue (CT) 5 types: Mesenchyme CT proper
Bone Cartilage Blood Functions: Binding and support (e. g. reticular CT, ligament , tendons) Protection (e. g. skull) Insulation (e. g. adipose tissue) Transportation (e. g. blood) 1. 3. 10 Structural Elements of CT 1) Structural Elements a) Ground substance: composed of interstitial (tissue) fluid, cell adhesion proteins(fibronectin, laminin which help cells attach to CT elements ) and proteoglycans: (molecular sieve, reserve of fluid, water, nutrients, sugar) b) Fibers: i. Collagen: high tensile strength ii. Elastic: long, thin, protein: elastin allows stretch & recoil iii.
Reticular fibers: thin collagen protein; fine network to support blood vessels, soft tissues c) Cells: immature=blast, mature = cyte i. Blasts are actively dividing/synthesizing, growth/repair ii. Cytes: mature, maintenance cells 2) Types of CT: a) Mesenchyme: source of all other CTS b) CT Proper: 2 subclasses… i. Loose CT (areolar, adipose, reticular) ii. Dense CT (dense irregular, dense regular, elastic) 1. 3. 11 Loose CT i. Areolar: Description: loose arrangement of fibers, high in ground substance, Edema: excess fluid accumulation?
water is pushed out of bloodstream into areolar CT, ex. Ppl with high blood pressure will often have edema, pressure pushes water out Location: under epithelia e. g. lamina propria mucus membranes Function: , reservoir of water & salts, cushioning, immunity (macrophages) ii. Adipose Description: fat filled, nutrient storage, cells packed tight Location: under skin, kidneys, breasts, 18% of our weight Function: Fuel reservoir, insulation, support, protect iii. Reticular CT: Description: Only reticular fibers, similar to areolar Location: lymphoid organs
Function: network, soft internal skeleton which supports free blood cells 1. 3. 12 dense CT i. Dense Regular CT: Description: bundles of parallel collagen fibers Location: tendons(muscle to bone) , ligaments (bone to bone at joints) Function: attachment with strength ii. Dense Irregular CT Description: collegen bundles arranged irregularly , thicker Location: dermis, fibrous capsules of organs, joints Function: withstand multidirectional tension, strength iii. Elastic CT Like dense regular CT, but high elastic content fibers, found in very elastic ligaments 1. 3.
13 CARTILAGE, BONE, BLOOD 1) Cartilage : features between dense CT & bone ? tough, but flexible Avascular, devoid of nerve fibers, relies on diffusion so heals slowly, not thick Ground Substance contains lots of GAG’s, so quite firm Mostly collagen, some elastic Up to 80% water 2) Bone: well vascularized, innervated (we feel pain when it breaks) Osteoblasts: immature cells, growth repair Osteocytes: actual mature bone cells Osteoclasts: digestion of bone when blood needs higher calcium levels (triggered by parathyroid hormone, parathyroid gland), gives bone strength and density 3) Blood:
classified as CT because it has cells (RBC, WBC) surrounded by nonliving fluid matrix (blood plasma), develops from mesenchyme the “fibers” are soluble proteins, which are visible during blood clotting 2. Cellular Physiology of Nerve and Muscle (5 Lectures) 2. 1 Membrane Transport 2. 1. 1 Fluid Mosaic Model 9999 Depicts plasma membrane as thin double layer of lipid with several protein molecules dispersed in it, that are constantly changing and floating around, which makes it look like a “mosaic”. Oily, semifluid? it’s a compound of phospholipids, cholesterols and glycolipids Components: Phospholipid bilayer:
Phospholipid composed of 1 phosphate group(head) and 2 fatty acids (hydrocarbon tail) Tail is hydrophobic (does not like water)since the hydrocarbon is non-polar Head is hydrophilic (likes water) since the phosphate makes it polar Integral membrane proteins Span the full thickness of plasma membrane, most are transmembrane (protrude from both ends of PM) Can be carriers (carrier mediated) or channels All are ampiphilic (hydrophobic and hydrophilic regions), allowing them to interact with both the nonpolar tails of the phospholipids and the polar water in and outside the cell They can be used for:
Transport (channels and carriers) Hydrophilic pathways for ions Enzymes Binds to and catalyzes reactions Receptors for hormones or chemical messengers that relay messages to cell interior Allows specific molecules to bind to cell and allows appropriate response Intercellular joining Junctions (desmosomes, tight junctions, gap junctions) Extracellular joining Attachment to ECM (extracellular matrix) which attaches to Basal Lamina Intracellular joining Attachment to cytoskeleton (inside cell) 2. 1. 1. 2 Components of plasma membrane (PM)
1. Peripheral Proteins: attached to integral proteins, are not embedded in the PM attachment function: include a network of filaments that support the membrane from inside the cell enzymes motor proteins: change cell shape link cells together 2. Cytoskeleton: Anchors to PM, also interacts with receptors such as ( eg??? ) Controls cell shape and mobility 3. Glycocalyx (“sugar-covering”) Ensemble of carbohydrates attached to the lipids and proteins on the extracellular face of the PM Also contains glycoplipids and glycoporteins
Every cell type has a different patter of sugars in the glycocalyx, this provides a highly specific biological marker by which approaching cells recognize eachother ex. Sperm recognizes an ovum due to the ovum’s unique glycocalyx 4. Cholesterol Role: reduces the general membrane fluidity & stabalizes its structure , an excess of cholesterol will cause membrane to lose its flexibility A balance of cholesterol needed ( some to provide rigidity but not too much to cause loss of all fluidity and flexibility) 5. Lipid Rafts Forms 20% of outer membrane surface
Tightly packed saturated phospholipid Concentrating platforms for certain receptor molecules or protein molecules needed for cell signaling 2. 1. 1. 3 Junctions (pg. 67) a) Tight Junctions Series of integral protein molecules in the PM of adjacen molecules fuse together foming an impermeable junction that holds the plasma membrane tight to the cell Ex. Tight Junctions in the epilithial cells lining the GI keep digestive enzumes and microogranisms found in the digestive tract from seeping intothe bloodstream. Some Tight Junctions may be leaky though and allow certain ions to pass
b) Desmosomes (‘binding bodies’) Anchoring junctions: molecular linking of cells, prevent cells from separating from eachother Plaques are buttonlike thickenings on the cytoplasmic (interior) face of each membrane and thin protein filaments (cadherins) extend from these plaques and fit together in the intercellular space, linking the two cells Keratin filaments, which are part of the cytoskeleton , extend across the cell and link to the plaque on the other side of the cell, creating an internal network of strong wires to support the cell Location: esophagus, GI tract, heart:
In the heart, during heavy contractions, desmosomes hold cells in place and attached firmly to one another c) Gap Junctions (or nexus) Molecular channels connecting the cytoplasm two adjacent cells allowing passage of cytoplasmic molecules; water filled Allows passage of ions and molecules for intercellular communication Location: Present in electrically excitable tissues like the heart and smooth muscle, where ion passage between cells help synchronize their electrical activity and contraction. Connexons: the hollow cylinder that makes the channel
Connexins: the individual proteins that make the connexons, one connexon is composed of 6 connexin proteins, two connexons make 1 channel The different types of connexins vary the selectivity of the gap junction channels 2. 1. 1. 4 Functions of the Plasma Membrane (RECAP) Barrier between intracellular and extracellular fluids Environment is different outside the cell than inside Ex. Ion concentrations inside and outside of a cell need a barrier to maintain the right concentration Selectively Permeable Responds to Extracellular Change
Site of cell-cell interaction and recognition 2. 1. 2 Transport Across The Plasma Membrane The PM is a selectively permeable barrier between the interstitial fluid and the cytoplasm Allows some substances to pass while others cannot, keeps undesirable substances out, keeping valuable cell protiens and other substances in, but can also excrete waste regulates when and how much the cell extracts the substances it needs from the nutrient rich interstitial fluid so that cell remains healthy Hydrophobic: water soluble substances require carriers to get through the membrane
Interstitial fluid: our cells are bathed in this, a rich nutritious blood filtrate (derived from the blood) which is like a soup , containing the thousands of ingredients( salts, sugars, amino acids, vitamins, hormones, metabolites, gases such as O2 and CO2 , neurotransmitters, etc. ) To remain healthy cell extracts exact amounts of substances it needs from this fluid at specific times HOMEOSTASIS: to maintain homeostasis and function normally, a cell must extract needed items , keep valuable materials inside & discard wastes 2.
1. 2. 1 Passive Transport MEchanisms Two types: Diffusion and Filtration. For now we will discuss only diffusion in detail. 2. 1. 2. 2 Diffusion Tendency of molecules or ions to scatter evenly throughout the environment. Goes from state of high concentration to low concentration: along their concentration gradient Molecules have kinetic energy: the constant random and high speed motion of molecules and ions results in collision, each collision causing the particles to ricochet off each other and scatter throughout the environment.
Diffusion occur faster the greater the difference in concentration between the two areas because more collisions occur. DIFFUSION RATE: because the driving force for diffusion is Ek the speed of diffusion is affected by: 1. Molecule size (smaller =faster) 2. Temperature (warmer= faster) 3. Gradient slope , as mentioned above (steeper slope =faster) PM is hydrophobic, to traverse a molecule must be (atleast one) 1. Lipid soluble 2. Very small 3. Carried (carrier-mediated) 2. 1. 2.
3 Simple Diffusion For non-polar or lipid soluble substances ( oxygen, CO2 , fats, urea, alcohol) NO ATP REQ’D O2 concentration is always higher in blood than in cell so it diffuses constantly into tissue cell CO2 is in higher [ ] in the cells so it diffuses out constantly from tissue cells to blood 2. 1. 2. 4 Facilitated Diffusion Lipid-insoluble molecules like sugars and amino acids and some ions move through the membrane passively even though they are unable to pass through the lipid bilayer.
They are facilitated by either a substance that (1) binds to the protein and carries it across -carrier- or (2) ferried across a water-filled protein channel. A. Carrier-Mediated Carriers are trans membrane integral proteins Proteins are specific to the molecule being carried Shields substance from non-polar regions No ATP required Can be inhibited Substance is still moving down its concentration gradient Limited by number of protein carriers present (when all carriers are engaged, they are called “saturated”) Glucose most common substance transported by carrier mediated facilitated diffusion.
Glucose is in higher concentrations in blood, than in cell where it rapidly used up for ATP synthesis B. Channel-Mediated Trans membrane proteins specific for the transpo of usually ions and water, through aqueous channels, ions selected by size and charge They are selective due to pore size and based on the charge of amino acids, lining the channel Can be inhibited and can become saturated just like carriers Rate of facilitated diffusion controllable by regulating activity or # of carriers/channels, Simple diffusion rate cannot be regulated 2 types:
i. Leakage Channels: always open to allow ions or water to move according to concentration gradients. ii. Gated Channels: controlled (open/close) by chemical or electrical signals. Open in response to hormone (chemical) or charge (volted channel such as in neurotransmission) 2. 1. 2. 4A filtration not much info hydrostatic pressure; blood pressure pushes the water and solutes through the cappillaries most substances can get through the pores of capillaires into the interstitial space exceptions large things like red blood cells and
protein 2. 1. 2. 5 Types of Active Transport (Primary vs. Secondary) Bulk Transport: movement of vesicles requires ATP endocytosis, exoctysosis, pinocytosis, receptor mediated endo, etc. Active Transport: Moving something against concentration gradient using ATP because: 1) too large for pores 2) lipid insoluble 3) moving against concentration gradient Uses a carrier that combines specifically and reversibly with a substance, and pumps substances against concentration gradients Many AT systems are coupled:
symport: substances move in same direction (ex. Na+ and glucose) through cotransporter protein, charge balances maintained because all ions (positive or negative) move in same direction. antiport: moves substances in opposite directions (ex. Na+ – K+ pump or using Na+ to expel hydrogen ions to maintain intracellular pH) Primary Active Transport Inside cell: high levels of K+ Outside cell: high levels of Na+ The cell has to maintain this gradient to have normal cell function/responsiveness/volume This gradient is challenged by
Constant leakage of K+ and Na+ through leakage channels along their concentration gradient Stimulation of nerve, muscle cells creates action potential which causes influx of Na+ and K+ outflux Cell uses Na+/K+ATPase to actively pump the ions across the membrane against concentration gradients to maintain the right levels of Na+ and K+ on either side of the cell membrane and have good cell responsiveness Secondary Active Transport Transport of a solute using energy derived from primary active transport Ex. Sodium Glucose Symport
As sodium leaks back into the cell along its concentration gradient, it carries glucose against ITS concentration gradient into the cell Channel allows for both to go through if we didn’t use ATP in the Na-K pump to make sure Na concentration is maintained outside the cell, we wouldn’t be able to drive this secondary active transport, if glucose wasn’t transported, we wouldn’t have ATP to run the Na-K pump, basically a continuous cycle Active Transport MEchanisms requires ATP Exocytosis secretion of hormones, ejection waste,
substance is enclosed in a vesicle, vesicle moves to PM, fuses with PM, ruptures, releasing contents outside of cell Vesicle Docking: 1) Vesicle travels to PM 2) Proteins on the vesicle membrane (v-snares) bind with proteins on the membrane (t-snares) 3) Fusing of both membranes, pore opens 4) Vesicle contents released What is the net result of repeated vesicle docking?? : add more and more membrane, but if somewhere else in the cell we are using endocytosis, it balances out Endocytosis Allows items to enter cell, requires ATP
taking up large amount of substance without concerning what it is takign up (will consume other substances) -clathrin (protein): causing the vesicle to form and dip inwards into cell once inside, vesicles are either transported across cell (transcytosis) or its contents are digested by a lysosome Types of Endocytosis Receptor mediated: Substances will bind to specific receptor proteins, which will then cause the formation (clathrin proteins) of the vesicle and dip it into cell , sort of like a pit that goes into cell.
Phagocytosis “Cell Eating” Cell enguls particle in a pseudopod Encloses a membrane around it and brings it into cell , where it is digested by lysosome Has receptors that bind to solid particles/microorgansims Pinocytosis “Cell Drinking” (or Fluid Phase Endocytosis) Gulps drops of extracellular fluid Non specific: no rceptors used Routing activity escpecially in cells that line absportion areas like the intestines
IN general, the membrane used in endcytosis activities is recycled back to the PM during exocytosis, so the pm surface area remains Very constant. 2. 1. 3 Osmosis Definition: unassisted diffusion of water from area of low to one of high solute concentration across a semipermeable membrane Water molecule is polar, but small enough to pass through most pores Movement of water due to concentration gradient of solute ( ex. Wherever salt goes, water follows), water goes to wherever there is more solute Ex.
Dehydrated: high solute concentration in body due to lack of water consumption, water will come out of cell into bloodstream to fix it You don’t give dehydrated person water quickly at once, you can cause water to rush into cell and rupture RBC’s Osmolarity: the total concentration of solute particles in a solution (the type of particle doesn’t matter Ex. NaCL =2 osmoles/L Each ion counts as a solute particle Glucose=1 osmole (its one molecule, not two ions like NaCl) Tonicity: concentration of nonpenetrating solute particles in a solution = ability of solution to change the shape of a cell bathed by that solution – why??
: If a cell is surrounded by a solution with less solute (hypotonic Phy solution) water will move inside the cell cause there is a higher concentration of solute in the cell , this might rupture cell If a cell is surrounded by a solution with more solute (hypertonic solution) water will move out of the cell cause there is a higher concentration of solute in the water, causing cell to shrink Rule: HYPO makes HIPPO Applications: 1. Hypertonic solutions for edema: pull water out of swollen tissues. 2. Hypotonic solutions to carefully rehydrate dehydrated patients. 2. 2 Physiology of the Neuron 2. 2.
1 Regions of the Neuron and their functions Neuron: structural units of NS: conduct electrical impulses from one body part to another Very specialized cells If one neuron is damaged, it cannot be replaced thru simple mitosis, our brain develops a number of neurons as we age and make connections in our brain, behaviours, memories Extreme longevity: Once they are formed, have to last for the rest of our lives, neurons can’t handle brain heating up, change in pH, but if undamaged, can last up to 100 years Amitotic: cannot reprod through mitosis, if we damage neuron beyond repair, we lose them Ex.
Spinal cord injuries are severe and irrepairable b/c of this High metabolic rate: require a lot of O2 and glucose to run Very large, complex cells: all have a cell body + one or more processes 3 Functional REgions 1) Receptive Region: dendrite 2) Conducting Region: axon ( axon hillock generates impulse) 3) Secretory Region: axon terminals Neuron cell Body Cell body = site of protein synthesis, biosynthetic center Extensive rough endoplasmic reticulum and ribosome clusters ( Nissi Bodies) , lots of mitochondria for ATP production CNS PNS Cluster of cell bodies Nucleus Ganglion
Bundle of nerve processes Tract Nerve Dendrites info arrives here, extensive branches for info coming in, lots of Surface Area, Dendritic spikes are thorny appendages that occur on highgly specialized dendrites. Convey info towards the cell body, in short distance signals called graded potentials Axon Axon Hillock: cone shaped area at beginning of axon that narrows evaluating strength of signal; prioritizes, not all signals are strong enough to depolarize axon hillock and create action potential, many outgoing branches to several other neurons long axons are called nerve fibers, i.
e the axons in the motor neurons controlling skeletal muscles of your big toe extend more than a meter 1 axon has usually 10000 telodendria (branches) which each end in axonal terminals Axon has same components as cell body, but no Nissi Bodies ( no elaborate Endoplasmic Reticulum), axons cannot repair themselves because they are so long and they lose contact with cell body when cut or damaged Directions of intracellular movement (items moved by cytoskeleton in the axon) Anterograde: from the cell body to axon and along neuron (items listed above in slide) Retrograde: back up towards the cell body (items that need to be wasted, recyceled, etc. ) see slide above Conduction velocities: (i) Thinner axons conduct info slower than thick (ii) Shorter axons will not be myelinated, unlike long ones, so they will have slower transmission speed Myelin Sheath Myelin sheath is the sheath covering the axon, the schwann cells are the individual nodes (“sausages”) that make up the myelin sheath 2. 2. 2. 1 The REsting MEmbrane Potential Neurons are excitable, they have a resting potential of around -70mV The PM creates this voltage (electric potential) by separation of oppositely-charged particles (ions) 2. 2. 2. 2 Na+/K+ ATP-ase
PM is slightly more permeable to K+ than to Na+, losing positive charge faster than it is gaining positive charge, build up of negative (absence of positive ) charge inside the cell = the resting membrane potential (-70mV) The Na+/K+ pump tries to maintain the appropriate levels of ion on each side of the PM ( pumps 3 Na+ for each 2K+), so it is maintaining this negative resting membrane potential Electrochemical Gradient Definition: Ions move along chemical concentration gradients when they diffuse passively from area of higher concentration to lower concentration. They move along electrical gradients when they move toward an area of opposite electrical charge. Together these two gradients from an electrochemical gradient. This gradient is the basis of all electrical events in the neuron. Ex. Electrochemical gradient: Na+ will go through channel due to negative charge on inside of cell, and due to chemical gradient 2. 2. 2. 4 Membrane IOn Channels Passive or Leakage channels: always open
Active or Gated channels: reacts to some sort of signal (chem or elec) a) Chemically gated (neurotransmitter opens channel by binding to protein (the gate) ) b) Voltage Gated: opens and closes in response to change in membrane potential Neurons and muscle cells communicate using changes in membrane potentials 2 Types of potentials 2. 2. 2. 5 Graded Potential: generated in the dendritic region, variable in intensity Short lived depolarizations or hyperpolarizations, the decremental movement of ions to either side of membrane propagates the signal for a short distance, the current decreases with distance travelled Magnitude determined by strength of stimulus Types:
a) Generator (receptor) potentials: sensory neuron excited by some stimulus, beginning of the potential b) Postsynaptic potential: when stimulus is a neurotransmitter released by another neuron (goes a cross synapse) to the dendrites of next neuron, created in a series of successive neurons in the pathway 2. 2. 2. 6 Action Potentials: A brief reversal of membrane potential; total amplitude = ~100 mV ( from -70 to +30) Transition from graded potential to AP created in the Axon hillock, always of the same magnitude (same charge change) Cells with excitable membranes (neurons and muscle cells), In neurons, only axons can generate action potentials AP also maintains the same amplitude
voltage-gated channels on axons open & close in response to local currents (graded potentials), enough will cause potential to reach axon hillock and fire an action potential It is an all-or-none phenomenon:either happens completely or not at all, local depolarizations (graded) must sum to reach threshold or no AP Depolarization: will make the resting membrane potential more positive (less negative) so it has less a distance to go to reach threshold, will quicken impulse, more probability of creating nerve impulses Hyperpolarization: channels opened will result in making the resting membrane potential even more negative (ex. K+ channels opening to let K+ ions out), less probability of producing nerve impulses, generation of an ACtion Potential 1.
Resting State: voltage-gated Na+ & K+ channels closed; normal leakage 2. Local Depolarization: Na+ voltage-gated (fast activation gates) channels opened, Na+ rushes into cell, making cell interior less negative. When we reach threshold (-55 to -50 mV) the AP becomes self-sustaining, urged on by positive feedback ( as more Na+ enters, membrane depolarizes further opening more channels until all channels are open: Na+ permeability is x1000 now, causing huge influx of Na+ membrane potential overshoots to 30 mV, causing rapid depolarization and polarity reversal: the spike. 3. Repolarization: slow inactivation gates close, causing Na+ permeabliltiy to decrease to resting levels, net influx of Na+ stops.
K+ slow voltage-gated activation gates open, and K+ rushes out of cell along its electrochemical gradient, restoring internal negativity. as membrane potential passes 0 mV, inside positivity resists further Na+ entry, Na+ gates begin to close; turning point in spike, cell will now begin to repolarize 4. Hyperpolarization: some K+ remain open while Na channels reset, causing a period of increased permeability to K+ , excess K+ efflux (out) causes hyperpolarization (the dip). Electrical conditions restored but ionic conditions must be restored via Na-K pump. 2. 2. 3. 1 Propogation of Action Potential AP must be transmitted to next neuron, depolarization of adjacent membranes away from origin Unidirectional because area where AP was just generated has inactivated Na+ gates.
AP generated at one end of axon and moves away from that point. Not conducted, but propagated- AP are not really conducted like a current in a wire (actually neurons are poor conductors, and charges leak through membrane causing decline in current flow) but actually they are regenerated anew at each membrane patch. All AP’ s are of same strength, stimulus intensity determined by frequency. Refractory Periods Absolute RP: period from opening of Na gates until they reset. Depolarization impossible. Relative RP: interval following absolute RP, most Na channels have reset, K+ gates still open, threshold for AP much higher, but possible 2. 2. 3. 2 Myelin Sheath:
Made of Schwann Cells (the sausages), minimal channels because myelin sheath is insulator, prevents leakage of charge and allow rapid membrane voltage change, Node of Ranvier: gaps in sheath, only place where current can leak through membrane of myelinated axon. Nearly all Na+ voltage gated located at these gaps. Saltatory Conduction: (saltare= leap) membrane regions are non-excitable , so AP moves rapidly from one node of ranvier to the next where another AP is generated. Rate of impulse propagation increased 150x Schwann cells: myelinate axons in the PNS only Oligodendrocytes: support structures for axons that can myelinate several axon cells at once in the CNS only. White matter = myelinated Grey Matter = unmyelinated 2. 2.
4 Synaptic Transmission Synapse: junction between 2 neurons, or neuron and effector Neurotransmitter moving across synapse by diffusion, the neurotransmitter encounters a channel with receptor and either opens/closes channel and influences how ions can move across membrane Unidirectional: no receptors on presynaptic neuron, so neurotransmitter cannot come back and bind to presynaptic neuron The action potential voltage gated calcium channels, the calcium promotes movement of vesicles to PM, exocytosis, neurotransmitters are relased into synaptic space, and that will carry signal to next neuron 2 types of synapse: 2. 2. 4. 1 Electrical Synapse i.
e in heart, allows for synchronous communication in muscular cells : cardiac muscle,smooth muscle, uterine muscle during labor Important during neural development, other than that we use chem synapses Like a gap junction, direct transfer of current from one cell to next via protein channels Rapid transmission (electrically coupled) allows for whole bunch of cells to respond as a synchronous unit (hearbeat) 2. 2. 4. 2 Chemical Synapse Neurotransmitters open or close ion channels that infulence membrane permeablility, and subsequently, mebrane potentilal, creates graded potential Travels from axonal terminals of presynaptic neuron to receptor region of postsynaptic neuron Mechanism of Synaptic Communication Initiation: 1. Ca++ gates open in presynaptic termin