BIOL 422 - Lecture (Unit 2)

• broadened perspective and understanding of evolution and its results

speciation

• speciation: the evolution of 2+ distinct descendant species from a single ancestral species
  • in a phylogeny, the root is the ancestral species, the latest nodes are descendant species
• species: two notable definitions necessary for course; definitions are not clear-cut
  1. biological species concept: a group of populations that can interbreed with one another; members that breed and produce viable reproductive species; they cannot reproduce with members outside of their species
     • … ? shark and stingray
     • most applied, clear-cut concept in class; works well for most animals
     • does not work well with fossil record species: variation in one species or similar but different species?
     • does not work well with asexual unicellular organisms; must be applied to sexually reproductive species
     • works well with mechanisms of speciation

2. phylogenetic species concept: patterns of shared ancestry and descent; phylogenies; the most terminal, finer points of branches that cannot be subdivided are species; the smallest monophyletic groups on the phylogeny are species - not limited to reproduction to define a species; use molecular, morphological information, etc - fairly modern way of defining a species

• a general process/formula is followed under speciation
• gene pool of the ancestral species is subdivided into two new species
  • results in reproductive isolation
  • genetic divergence follows; changes which follow are only shared within one group
  • eventually, if they become so different that they cannot interbreed, this results in speciation by the biological species concept
    • genetics and phenotypes differ
• different pathways to speciation or form

allopatric speciation

• allopatry:
• relevant to allopatric populations — non-overlapping geographic ranges; both species are in separate ranges physically; no access to reproduction
• straightforward form of reproductive isolation
• forms of allopatric speciation; how does the ancestral population subdivide into the subgroups
  1. by vicariance
     • vicariance event
     • geological forces or other forces have physically divided the overall habitat/region where the ancestral population lived
       • slow gradualist mountain barrier formation (plate tectonics)
     • genetic divergence occurs due to different factors (microevolutionary and macroevolutionary factors):
       • natural selection: evolution is occurring because of new changes in the environments of each population: different communities, predators, environment
       • genetic drift, allelic frequency alterations by chance; these chances are unlikely to be identical between populations
       • chance mutations over thousands of generations which differ between populations
  2. by founder event
     • original population is in one range and the founders have moved to an expanded range outside of the original population’s boundaries
     • Hawai’ian Honeycreepers; common ancestor was an Asian rose finch
       • 4-5mya: Kuaii forms, eventually populated by species into habitable region; small number of rose finches disperse from Asia onto Kuaii and start a founder event
       • reproductive isolation via physical barriers
       • genetic divergence occurs similarly to by vicariance, but speciation starts instantly because of founder event because of genetic drift and natural selection due to a change in environments/different environmental pressures
         • mutations may occur, but are rarer; still will add divergence in both populations
       • further dispersion created when migrating to new islands (Kuaii → Oahu); still affected by genetic drift and some effect of natural selection due to different communities between islands
         • if dispersion occurs after the next species has diverged, this will cause another founder event on the same island
       • 

sympatric speciation

• related to geographical range
• overlapping range in sympatric speciation; must be partial or complete overlap
  • not parapatric (adjacent regions)
• harder to identify how reproductive isolation occurred due to sharing physical space
• occurs due to specialization towards different resources within the same habitat (hawthorn maggot vs apple maggot flies; Rhagoletis pomonella)
  • mutations occurred causing flies to shift to apple life cycle source vs. hawthorn; genetic mutation basis
  • non-random mating; positive assortative mating for hawthorn and apple maggot flies
  • impedes gene flow between populations → causes reproductive isolation → genetic divergence between populations
  • specialization is so strong that they positively assortatively mate towards entirely different phenotypes
• allopatric speciation into sympatric speciation subdivisions within new habitats

hybridization

• uncommon method of speciation for animals; more often found in plants
• still occurs
• what if two species still can interbreed and produce viable offspring for generations? → hybridization of species C
• projected infinite future of generational reproduction
• different isolating mechanism; reproductive isolation mechanisms, prezygotic won’t often go far

reproductive isolative mechanisms

• they create and maintain reproductive isolation
• prezygotic or postzygotic mechanisms
  • zygote: unicellular fertilized egg which undergoes cleavage cell divisions to later become a multicellular organism
• pre-zygotic isolating mechanisms
  1. geographic isolation (allopatry)
  2. ecological isolation (sympatry exclusive)
     • species stay separate due to subdivision of the habitat (specialization of resources within the same habitat and positive assortative mating)
     • no hybridization
  3. temporal isolation; they never reproduce at the same time even if sympatric species (different seasons, migration; coral reef fishes who breed at different times)
     • broadcast spawning
  4. behavioral isolation; blue and red foot boobies, fruit flies who mate/attract differently
  5. mechanical isolation; copulation/sex; dioecious individuals must have complementary sexual structures
  6. gametic isolation; dioecious gametes must be able to fuse
  • geographic isolation is allopatric; other mechanisms usually sympatric
• post-zygotic isolation mechanism
  1. hybrid inviability: a zygote is made, but the embryo dies — miscarriages, stillbirths; “liepard”
  2. hybrid sterility: a living offspring is made, but it is infertile; mules; gene pool cannot be extended
  3. hybrid breakdown: a living offspring is made that is fertile and lasts for several generations, but the hybrid descendants are eventually unable to reproduce and unable to form a lineage
     • copepods, wild cat
     • 
• tends to be a combination of reproductive isolating mechanisms which prevent hybridization between species, not just one

stasis

• an alternate outcome to speciation
• no substantial change over a long period of time
• most do not experience stasis; evolution → speciation or extinction common
• “living fossils” layman term
• alive today but resemble old fossil records
• ex. Solenodon and aardvarks; unchanged for 55my; aquatic beetles and beetles in general; horseshoe crabs

extinction

• an alternate outcome to speciation
  • the ultimate, expected end for each particular species
• 99% species on earth extinct
• results from
  • environmental changes
  • low genetic variation which fails in natural selection and adaptation
  • environment changes too fast for adaptation via natural selection

background extinction route

• fossil records
• standard rate of extinction
• 1 species goes extinct per million per year (~10-100 species/yr)
• average species lifetime
  • invertebrate: 5-10my
  • mammals: 1-2my

mass extinction

• substantial extinctions in a relatively short period of time (ex. more than 60% species extinct within 1my)
• 
• five major extinctions: ordovician, devonian, permo-triassic, end-triassic, cretaceous-tertiary extinctions
  • cretaceous: 65 mya; 70% went extinct including dinos
• extinction rate is low but constant, then spikes during mass extinction events
• current risk of 6th mass extinction due to human activity; current rate extraordinarily higher (1000-10000x higher than bg rate)

macro-evolution

• microevolution is evolutionary change within a species; below the species level; changes in allelic frequency within a population across generations
  • shorter timescale; small generations, maybe hundreds or thousands
  • major factors are natural selection and genetic drift; as well as gene flow and non-random mating
• macroevolution is evolutionary change above the species level; genus or higher
  • changing to an entirely different species and genus
  • longer time scale
  • associated with major physiological or morphological innovations — evolutionary novelties; “game changers”
  • ex. origin of animals, jaws, tetrapodal limbs
• macroevolution includes adaptive radiation
• adaptive radiation: rapid speciation events that form a cluster of closely related descendant taxa; “diversification of an ancestral species into many descendant taxa”; high diversity in taxa
  • hawai’ian shortswords, honeycreepers, mammals
    • mammals: result of occupying vacated niches from dinosaurs in an ecological opportunity after their mass extinction
• do microevolutionary changes cumulate into macroevolutionary changes? allelic frequency changes → evolutionary novelties? → not typically; micro and macroevolutionary changes are related, but allelic frequency accumulations alone do not cumulate in macroevolution; need other additional processes
• unusual processes need to occur which start macroevolution (evolutionary novelty), which is then followed by microevolutionary processes after the novelty arises
  • remember natural selection does not invent a novelty, it decides it is the most advantageous one then promotes it
  • macroevolution needs to result in a major novelty, not a minor one
• two rare processes contributing to macroevolution
  1. mutation, especially certain kinds
     • very rare: 1/1 million zygotes per locus, and must occur in a germ cell; further, not all mutations are relevant to macroevolution
     • most important ones are those that reorganize body plans or the timing of development; produce dramatic changes in phenotypical form
       1. mutations that result in novel spatial organization of the body plans
       2. homeotic gene mutations
     • mutations are the only way to create/introduce a new variation/function which can be affected by microevolutionary processes
  2. exaptation: taking an extant structure and using it in a new way; repurposing/reusing

macroevolution via mutation

homeotic genes

• homeotic genes, aka “master regulatory genes”
• hierarchical control of other genes; homeotic genes have greater levels of control, especially over development regulation; placement and spatial organization of body parts
• observed in animals, plants, and fungi, incl. unicellular fungi; very impt. in multicellular eukaryotic cellular orgs
• produce protein products that regulate genetic expression for underlying/subgroup genes
  • ex. they make proteins that activate for a specific region; hierarchical levels of control and production
• mutations can have dramatic effects; one might either mutate
  • the gene itself: change dna and the specific gene → inherited gene is completely different due to the mutation
  • the expression: dna is not changed and gene is not changed, but gene expression is changed (ex. how/when/how strongly/how much of the gene is produced)
• animal homeotic genes are specifically called Hox genes vs. plant homeotic genes
  • provide developmental positional info, specifically for that along anterior/posterior axis, the limb buds, and the genitals
  • genetic homology in animals
  • appears in clusters along chromosomes; number of hox genes differs
  • duplication was likely key in animal evolution; original hox gene alongside a new one that took on novel purposes
  • mutation example: hoxd13 (regulation of limb digits) → synpolydactyly; drosophila hox → multiple wings without a haltere, limbs and antennae swapping places
  • tetrapodal evolution and homoeotic genetic mutations:

heterochrony

• heterochronic genes: genes that change when development occurs
• ex. stripe development; timing of gene expression
• paedomorphosis: a subset of heterochrony in which adults of the descendant taxon retain juvenile traits; they closely resemble the juveniles, not the adults of the ancestral taxon
  • axolotl’s gills in adulthood
  • timelines of development in amphibian ancestral species:
    • body morphology: egg → juvenile tadpole from hatching → metamorphoses into adult
    • sexual maturity: sexual immaturity → sexual maturity in adulthood
    • if mutations change heterochronic genes, the outcome may differ; ie hatching or metamorphosis occurs at a different time than depicted
• paedomorphosis occurs when mutations cause:
  • the morphology timeline to slow while the sexual maturity timeline remains constant → sexually mature adult who has grown to the adult stage, but resembles a juvenile (ex. did not undergo metamorphosis)
    • axolotl undergoes this form of paedomorphic heterochrony
    • human skull shape
      • 
  • the sexual maturity timeline to speed up while the morphology timeline remains constant → halts before reaching stages, smaller size (juvenile size), but sexually mature

exaptation

• exaptation: a structure that evolved in an ancestral taxon for a given function, inherited by a descendant taxon, but used in a new way; an ancestral, inherited structure co-opted by a descendant taxon
• feathers on birds:
  • therapod dinosaurs’ feathers contained melanosomes, and were evolved for heat — a “heat shield” and for insulation; also used for communication and for behavioral displays, which was also inherited as a secondary use for birds
  • co-opted for flight in avian descendant and then refined through natural selection (microevolution)

physiology

fuck.

• animal characteristics: multicellular eukaryotes who lack cell walls, are heterotrophic, are capable of movement and environmental responses (muscle/nervous tissue), and have a specific development process that includes the development of a blastula
  • note that not all animals have tissues, but the majority
  • blastula; special development process with different intricacies, but commonality is blastula

animal development

• cleavage: an orderly sequence of cell division by mitosis immediately following fertilization of a zygote
  • orderly; each performs mitosis (division into 2s)
  • cell size is reduced to a normal size from a cell (largest type of cell)
  • end result: a hollow ball of cells (blastula)
    • 128-cell ball in humans; takes 4 days after conception to develop
    • blastocoel: fluid-filled cavity
      • note that coel is a root word for cavity (coelom, coelomate, spongocoel)
  • all animals develop to the blastula stage, but may do so in two forms of cleavage
    • radial cleavage: stacked on top of one another; unrecognizable in a top-down
    • spiral cleavage: appears from a top down view
    • these are all visible at the 8-cell stage of the blastula
    • only difference is in position of topmost cells relative to bottommost cells; however, difference does signify developmental homologies and phylogenetic relationships; common ancestry of cleavage sequences
• embryonic germ layers
  • all animals develop to the blastula stage, but proceeding after that differs
  • how the blastula is changed affects how embryonic germ layers form
  • no germ layers: blastula → adult stage
    • no tissues will develop
    • sponges (porifera)
  • all other animals go through gastrulation
    • blastula goes through an invagination in which an endoderm and ectoderm germ layer form
    • diploblastic
    • cnidaria remains diploblastic
      • gastrocoel: cavity lined by an invaginated endoderm
      • blastopore: opening into the gastrocoel/endoderm invaginated cavity
  • all remaining animals develop a third germ layer → triploblastic
    • endoderm, ectoderm, mesoderm
      • ectoderm: skin, nervous system, eyes
        • nervous system and eyes fold in from ectoderm tissue and become organs
      • endoderm: digestive structures; digestive tract an lung lining, liver, pancreas, gall bladder
      • mesoderm: remaining organs
    • mesoderm fills in the gastrocoel
• coelom: fluid-filled body cavity with complete lining on sides
  • coelom is within the mesoderm region
  • true coelomate has a completely lined cavity which is lined by the mesoderm fully
  • acoelomate animals have no cavity; completely filled in by mesoderm tissue (Platyhelminthes)
  • pseudocoelomate animals have a cavity which is partially not lined by the mesoderm (ie lined by part mesoderm, part by endoderm)
• protostome v. deuterostome
  • two clades based on developmental homologies
  • primarily for triploblastic animals (not porifera/cnidaria)
  • protostomes
    • “first mouth”: the first opening/pore to form becomes the mouth, and the second becomes the anus
    • platyhelminthes, mollusca, annelida, arthropoda
      • platyhelminthes had reversal-of-state evolutions for the complete gut and the body cavity
    • primarily (excl. platyhelminthes) triploblastic coelomates
    • spiral cleavage sequence
    • adhere to determinate (mosaic) cleavage
      • adherence is noted extremely early (around 2-cell stage)
    • blastopore becomes the mouth (like diploblastic cnidaria)
    • coelom forms through schizocoely
      • mesoderm forms one solid mass between the two germ layers; cells near the center of the mesoderm perform apoptosis and leave a cavity
      • gut (gastrocoel) extends further and further until breaking through the other end of the ectoderm, forming a complete gut
      • mesoderm fills in-between ectoderm and endoderm; then cells die off
  • deuterostomes
    • “second mouth”
    • echinodermata, chordata
    • radial cleavage sequence with indeterminate cleavage
      • cell fate is not fixed until a later period of development (~4-cell stage)
    • enterocoely formation

homeostasis

• maintenance of a “steady internal state” with regard to a specific variable (osmolarity, temperature)
• conformation vs regulation
  • conformer: internal state changes alongside the environment state
  • regulator: internal state is controlled regardless of the environment state
• three required components
  • sensor organ: monitors body’s status for the specific variable
  • control sensor: compares sensor input to homeostasis value; usually brain
  • effector: changes variable; always a muscle or gland cell
• negative feedback system counters previous changes in the body; compare to a positive feedback system
  • negative: thermoregulation
  • positive: hormonal endocrines, uterus contraction and stimulation/oxytocin release

thermoregulation

• thermoregulation is not synonymous with homeostasis; many animals use homeostasis to maintain thermoregulation, and many animals do not
• reviewing how heat flows into animal bodies (heat transfer)
  • radiation: heat loss and heat gain
    • all objects with a temperature greater than absolute zero radiate electromagnetic waves, causing heat loss
    • heat radiates into other objects, cycle continues
  • conduction: objects directly contact one another and transfer heat between each other
    • water is a 50-100x more effective conductor than air, so marine animals lose a lot of their body heat to water
  • convection: water or air in motion (a liquid or gas state in motion) more effectively conducts heat; “windshield factor”
  • evaporation: evaporative cooling; overheating measure
• cold-blooded vs. warm-blooded are inaccurate terms: do not account for the variability of internal temperature
• instead, account for variability and sources of heat for internal temperature
  • heterothermic: variable temperature
    • synonymous with poikilothermic
  • homeothermic: constant temperature
  • endothermic: high metabolic rate; primary source of heat derives from self
    • endotherms tend to be homeotherms; organisms will have a constant rate of heat regardless of how the temperature changes
  • ectothermic: low metabolic rate; primary source of heat derives from environment
    • ectotherms tend to be heterotherms; a varied environment means an organism is constantly never generating enough heat to survive
• common groups
  • heterothermic ectotherms: most invertebrates and several chordates (many fishes and amphibians, non-bird reptiles)
    • low energy requirements
    • activity is dependent on environment temperature; physical processes occur faster at higher temperatures
    • must live where areas are warm and hot
  • homeothermic endotherm: mammals at ~97-100*F, birds at ~103-105*F
    • constant activity
    • requires a lot of food
    • must live where food is abundant (no extreme cold environments)
• behavioral thermoregulation: behaviors that influence thermoregulation
  • “shuttling behavior”: shuttling between environments of different temperature to regulate heat based on the environment/ambient temperature
    • heterothermic ectotherms; keeps temperature constant similar to homeothermic endotherms
    • only available at a point when sun/heat is available
    • also seen in endotherms to regulate heat
  • folding/expanding wings; expanding or folding surface area for absorption
• vasodilation/vasoconstriction: the loss of heat through the reshaping of blood vessels
  • smooth muscle contractions and relaxation
  • dilation enlarges the vessel; constriction shrinks the vessel
  • physiological response; involuntary
• counter-current heat exchanger: two currents and the resulting effects on gradients based on their flow direction
  • human circulatory system: warm blood circulates in a vessel flowing close to vessel flow towards the heart; creates an efficient setup where a temperature gradient is maintained through the entire system
  • arctic wolf legs, gray whale tongues (high surface area + baleen filter feeding), bluefin tuna and active large predatory fishes (high body and head temperatures compared to water; required for speed and reaction), human testes
• endotherms
  • have specific adaptations evolved as a result of their dependence on high metabolic rates and high heat
  • requires an efficient way to retain heat
  • insulation through fur or feathers; fat
    • piloerection: “goosebumps”
    • fat layers; thicker fat layers with feathers
  • endotherms do not live in thermophilic environments; they need enough food to maintain heat
  • reduction of metabolic cost in thermophilic environments
    • hibernation
      • long term: weeks to months long
      • prevents metabolic rate from exponentially increasing in extreme periods
      • heart rate and body temperature
    • torpor
      • short term: hours to days
      • only a few animals on earth
      • hummingbirds and bats
        • day/night cycle of metabolism rates

nervous system

• each nervous system is comprised of specific neurons
  • peripheral nervous system
    • sensory and motor neurons
      • eyes, muscles
  • central nervous system
    • interneurons of brain and spiral cord
      • brain and spinal cord
  • normal pathway: a response to stimuli which passes through the CNS (brain)
    • sensory receptor sends signals to sensory neuron (PNS)
    • → sensory neuron sends electrical messages to interneurons (PNS → CNS); action potentials
    • → interneurons → brain → interneurons; processes sensory stimuli
    • → interneurons of CNS send messages to motor neurons (CNS → PNS)
    • → motor neurons communicate to effector cells (remember: muscle or gland cells)
  • reflex pathway (reflex arc): a response to stimuli which bypasses the brain
    • fractions of a second, minimization of tissue damage
    • sensory receptor → sensory neuron (PNS)
    • → sensory neuron → interneuron (PNS → CNS)
    • → interneuron → motor neuron (CNS → PNS)
    • → motor → effector cells
    • short communication
  • neuron structure
    • cell body with nucleus and organelles: remember a neuron is a kind of cell found in the nervous system
    • dendrites: branches off of the cell body
    • axon: a long branch that derives from the cell body; the longest branch
      • axon intersection with the cell body; triangular region: axon hillock
        • beginning of action potential as a neuron communicates
    • a neuron signal always begins from the dendrites, travels through the cell body, through the axon, and onto the branches of the axon
      • synaptic terminals: endpoint branches of the axon; terminal branches; they communicate with the next neuron in the communication sequence, transmitting the signal between neurons
        • synapse: a transmission/interface between neuron and a communicating cell; a point of interlocking communication between cells
          • compare pre-synaptic cell and post-synaptic cell
  • glial cells: supportive nervous system cells outside of the neurons; miscellaneous, non-neuron cells that maintain the ion environment, maintain the function of neurons, etc.
    • Schwann cells: insulative cells for axons; maintain the electrical signal, maintain security of axon and electrical signal; found in the PNS
    • oligodendrocytes: insulative cells for axons; found in the CNS
    • Schwann and oligodendrocytes form the myelin sheath: a layer of fat that insulate the axons
      • myelin sheath grows and covers most the axon; however, there are gaps in the covering known as the nodes of Ranvier
      • Schwann cells contain large amounts of myelin adipose, hence the name of myelin sheath, which increase communication efficiency
• nervous system has different functions
  • somatic nervous system: voluntary; controls skeletal muscles in responses to external stimuli
  • automatic nervous system: involuntary; internal environment of the organism; response to internal stimuli, physiology, involuntary responses — digestion, heart rate, blood pressure

potentials

• different forms of potential: a voltage (charge) difference across the cell membrane
  • remember that cells have partial polarity, so all cells have membrane potential
  • usually this membrane potential is static; however, neurons and muscle cells can change this membrane potential
• remember ions move across the membrane through diffusion or active transport through the sodium-potassium pump
  • in either transport, ions transfer via ion-specific channels
  • in passive (diffusion), the channels may be ungated or gated: ungated channels are unregulated, free-flowing channels, and gated regulate ion facilitation, affecting membrane permeability
    • some gated channels are gated by changes in voltage; others are gated by ligand binding (lock-and-key molecules)
• important gradients for diffusion
  • chemical gradients (concentration)
  • electrical gradients (ionic charge)
  • remember gradients always diffuse from high to low
  • consider both gradients simultaneously; consider an electrochemical gradient
    • different directions of gradients may cause equilibrium

resting potential

• a neuron at rest has a potential -70mV compared to the environment
  • a millivolt is 1/1k of a volt
• resting potential is determined by ungated channels, and specifically, there is a higher concentration of potassium electrochemical ungated channels; potassium is more permeable and able to diffuse
  • many ungated potassium channels and some ungated sodium channels
  • large amount of potassium inside and small amount outside; large amount of sodium outside and small amount inside
  • concentration gradient moves potassium out while an electrical gradient moves potassium in, creating an equilibrium where the voltage of potassium is ~-90mV
  • an opposing gradient with sodium is also created; the potassium gradient is partially balanced out, but since there are fewer sodium solutes, the actual resting potential is altered to ~-70 mV
• ungated sodium channel creates instability because it is not at equilibrium, while potassium is; sodium gradient changed voltage
  • action potentials occurring without actual stimuli
• resting potential is maintained by active transport via the sodium-potassium pump
  • 2 potassium in, 3 sodium out
• all gated channels are closed

graded potential

• a local event on the neuron membrane (dendrite or cell body; not axon)
  • a local event occurs in the same area (local) in which the stimulus was received. change from resting potential to graded potential only within that area of the membrane, not the entirety
• reception causes one of two outcomes
  • picture a graph: y=local membrane potential (mV), and x=time (mSec)
  1. hyperpolarization: the stimulus causes the local potential to become even more negative (even more polarized)
  2. depolarization: the stimulus caused the local potential to increase (become less polarized)
• local events last only a few milliseconds but are incredibly important; eventually they are balanced back to the resting potential
• only depolarization can increase the likelihood of action potential stimulation
  • action potential is the actual electrical signal
• hyperpolarization halts a message/response to a stimulus
• graded refers to how the magnitude (change) of the response is proportional to the magnitude of the stimulus
• result of neurotransmitter bindings, opening of gated channels and very specific transmission of potential
• summated, cumulative events; think of wave interference
• gated channels open and allow certain ions to move across the membrane, allowing a change in membrane potential

action potential

• a local event on the neuron axon membrane
• action potentials are not graded nor summated; “all-or-none” events
• threshold potential: -55 mV
  • when a depolarization stimulus is received, if the local potential reaches -55 mV (the threshold), action potential occurs
  • if not, no action potential occurs
  • static graph; will happen regardless
  • rapid, less than 4 mSec; each phase only takes 1-2mSec
• all gated channels for action potential are voltage-regulated; types include
  • voltage-regulated potassium ion (K^+) channels: open/close system, single-gate
  • voltage-regulated sodium ion (Na^+) channels
    • double-gated; both must be open for sodium ions to move
    • one activation gate closed at rest; opens via small depolarization stimuli
    • one inactivation gate open at rest; close via large depolarization stimuli
• action potentials have a refractory period
  • the affected local spot cannot respond to another stimulus because of the occurrence of the action potential
  • refractory period lasts from initial stimulus (depolarization phase) until new resting phase

1. resting phase: local membrane potential = resting potential (-70 mV); maintained by the sodium-transport pump (active transport, ATP)
   • all voltage-regulated gated potassium ion channels are closed
   • all voltage-regulated gated sodium ion channels:
     • activation gate is closed
     • inactivation gate is open
2. depolarization phase: internal local membrane potential becomes positive value
   • reception of initial stimulus (graded potential)
   • some activation gates begin to open
   • creation of strong electrochemical gradient: an influx of sodium ions diffuse into the axon membrane
   • cumulates (summates?) once reaching threshold potential; more activation gates open
     • positive feedback system: the addition of sodium ions creates its own individual depolarization potential; the potential skyrockets
     • peaks at +40 mV; inactivation gates close
3. repolarization phase: rapid decrease of potential value
   • huge depolarization stimulus opens potassium ion channel gates
   • massive rapid potassium efflux
   • reaches -70 mV
4. hyperpolarization phase: potential reaches lower than resting potential value before stabilizing
   • potassium ion channel gates begin to close slowly due to slow fulfillment of electrochemical equilibrium
   • brief hyperpolarization period
   • reaches -75 mV
   • resets to resting potential once peaking at -75 mV
     • potassium gates fully close
     • gated sodium channels: inactivation gates open, activation gates close; reverted to normal resting positions
     • ungated channels begin to matter again; resting potential generation
     • small amounts of depolarization from sodium flowing through ungated channels restores resting potential value
       • Sodium-Potassium pump reinforces resting potential and prevents new action potential event

• ungated channel change is so insignificant during graded/action potential in comparison to gated channel changes and ion transmission