Nervous System 2 - Super 7 List.docx

Nervous System II – Super 7 Resting Membrane Potential: (YOU CAN PROBBALY IGNORE THIS CRAP) Exists because of small buildup of (-) ions in cytosol along inside of membrane & equal buildup of (+) ions in ECF along outside surface of membrane only Separation of (+) and (-) charges is form of pote...

Nervous System II – Super 7 Resting Membrane Potential: (YOU CAN PROBBALY IGNORE THIS CRAP) Exists because of small buildup of (-) ions in cytosol along inside of membrane & equal buildup of (+) ions in ECF along outside surface of membrane only Separation of (+) and (-) charges is form of potential energy, measured in volts or millivolts (1 mV = 0.0001 V) Greater difference in charge across membrane, larger membrane potential (voltage) Cytosol or ECF elsewhere in cell contains equal numbers of (+) and (-) charges and is electrically neutral Potential difference in resting neuron is called resting membrane potential (anywhere from -40 mV to -90 mV in different types of neurons, a typical value is -70 mV), minus sign indicates inside is negative relative to outside; membrane is said to be polarized Differences in ionic makeup on ICF and ECF: Cell cytosol contains low concentrations of Na+ and high concentrations of K+ than ECF (-) charged proteins help to balance (+) charges of intracellular cations (mainly K+ ◊ plays important role in generating membrane potential); in ECF, (+) charges of Na+ and other cations are balanced by Cl- ions Differential permeability of membrane to various ions: At rest, membrane is impermeable to large anionic (-) cytoplasmic proteins, is very slightly permeable to Na+, ~75x more permeable to K+ than to Na+, and freely permeable to Cl- ions Reflect properties of leakage ion channels in membrane K+ ions diffuse out of cell along concentration gradient more easily than Na+ ions can enter cell along theirs (K+ flowing out of cell causes cell to be more (-) inside, while Na+ trickling in cells makes cell slightly (+) ATP-driven sodium-potassium pump ejects three Na+ from cell and then transports two K+ back into cell, stabilizing resting membrane by maintaining concentration gradients Electrical properties of neurons and synapse structure Neurons are highly irritable or electrically excitable. Two types of electrical signals: Graded potentials: Short-lived, localized changes in membrane potential that can be either depolarization (membrane less polarized or inside less neg) or hyperpolarization (membrane more polarized or inside more neg) Changes cause current flows that decrease in magnitude with distance Magnitude varies directly with stimulus – stronger the stimulus, more the voltage changes and farther current flows Triggered by change (stimulus) in neuron’s environment causing gated ion channels to open Different names dependent on where they occur and functions performed Postsynaptic potential: when neurotransmitter released by another neuron because it is released into fluid-filled gap called synapse and influences neuron beyond (post) synapse When small area of neuron’s plasma membrane has been depolarized by stimulus, ions will flow on both sides of membrane between depolarized membrane area and adjacent polarized (resting) area, the direction of cation movement is direction of current flow and (-) ions simultaneously move toward more (+) areas ◊ in that small area, (+) ions like K+ inside cell move away from depolarized area and accumulate where they neutralize (-) ions, while (+) ions on outer membrane face move toward area of reversed membrane polarity (depolarized region, momentarily less (+); depolarization spreads as neighbouring membrane is depolarized Plasma membrane permeable and therefore most of charge is quickly lost through leakage channels, thus current dies out within few mm of origin (decremental conduction) Summation: process by which graded potentials add together. If two depolarizing graded potentials summate, net result is larger depolarizing graded potential; if two hyperpolarizing graded potentials summate, net result is larger hyperpolarizing graded potential; if two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), they cancel each other out and graded potential disappears. Synapse Structure (Chemical) Action potential arrives at axon terminal Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal Ca2+ entry causes neurotransmitter-containing synaptic vesicles to release their contents by exocytosis Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane Binding of neurotransmitter opens ion channels, resulting in graded potentials6. Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse Neurotransmitters, their roles, and their receptors – ACh, NE, epinephrine, Dopamine, catecholamines, some amino acids, endorphins (Reference Pg. 419 Marieb & Hoehn) Action potentials (AP) including threshold: Brief reversal of membrane potential with total amplitude (change in voltage) of ~100 mV (from =70 mV to +30 mV) Way neurons send signals over long distances, does not decrease in strength with distance Only neuron and muscle cells can generate AP, typically in axons Depolarization phase (neg membrane potential becomes less negative/reaches zero) followed by repolarization phase (membrane potential restored to resting state of -70 mv) and often short period of hyperpolarization (membrane potential temp becomes more neg than resting level) Stimulus changes permeability of neuron’s membrane by opening specific voltage-gated channels on axon, open and close in response to changes in membrane potential and activated by local currents (graded potential) that spread toward axon along dendritic and cell body membranes Resting state: all gated Na+ and K+ channels closed. Leakage channels open Each Na+ channel has two gates: voltage-sensitive activation gate (closed at rest), responds to depolarization by opening; & inactivation gate blocks channel once opened; both gates must be open for Na+ to enter but either gate closing will close other Each active K+ channel has single voltage-sensitive gate closed in resting and opens slowly in response to depolarization Depolarizing phase: Na+ channels open. Voltage-gated Na+ channels open and Na+ rushes into cell Influx of (=) charge depolarizes local area of membrane opening more Na+ channels so that cell interior becomes less (-) When depolarization reaches threshold, it becomes self-generating, urged on by positive feedback As more Na+ enters, membrane depolarizes further and opens more channels until all Na+ channels open; Na+ permeability is 1000x greater than in resting neuron Therefore, membrane potential is less and less (-) and then overshoots to about +30 Mv as Na+ rushes in Rapid depolarization and polarity reversal produces sharply upward spike of action potential Repolarizing phase: Na+ channels inactivated and K+ channels open. Rising phase of AP persist for ~ 1ms, self-limiting d/t slow inactivation gates of Na+ channels begin to close – membrane permeability of Na+ stops completely and AP spike stops As decline of Na+ enters, slow voltage-gated K+ channels open and K+ rushes out of cell, following electrochemical gradient Abrupt decline of Na+ permeability and increased permeability of K+ contribute to repolarization (restoring internal negativity of resting neuron) Hyperpolarization: some K+ channels remain open and Na+ channels reset. Result of excessive K+ efflux, this after-hyperpolarization (undershoot) seen on AP curve as slight dip following spike (before K+ gates close), Na+ channels begin to reset by changing shape to reopen inactivation gates and close activation gates Propagation of AP: If serving as neuron’s signaling device, AP must be propagated (sent/transmitted) along axon’s entire length Na+ channels inactivated and no new AP generated because area where AP originated had just generate AP; AP propagates away from point of its origin If isolated axon stimulated by electrode, nerve impulses will move away from point of stimulus in all directions along membrane Once initiated, AP is self-propagating and continues along axon at constant velocity Occurs on unmyelinated axons (on myelinated axons, called salutatory conduction) AP is regenerated anew at each membrane patch, and every subsequent AP is identical to one generated initially Threshold: is when action potential occurs in membrane of axon when depolarization reaches a certain level (between -55 and -50 mV) Action potential will not occur in response to subthreshold stimulus, one that is weak depolarization that can’t bring membrane potential to threshold Depolarization status represents unstable equilibrium state, if one more Na+ enters, further depolarization occurs opening more Na+ channels and allowing more Na+ entry But if one more K+ leaves, membrane potential driven away from threshold, Na+ channels close, and K+ continues to diffuse until potential returns to resting value Critical factor is total amount of current that flows through membrane during stimulus (electrical charge x time) ◊ strong stimuli depolarize membrane to threshold quickly, weaker stimuli must be applied for longer periods to provide crucial amount of current flow All-or-none phenomenon: either happens completely or not at all Coding for Stimulus Intensity: Stimulus intensity coded for be number of impulses per second (by frequency of AP), rather than by increases in strength (amplitude) of individual Aps Refractory Periods: When patch of neuron membrane generates AP and voltage-gated Na+ channels are open, neuron cannot respond to another stimulus regardless of strength Absolute refractory period: A period from opening of Na+ channels until Na+ channels begin to reset to their original resting state, ensures each AP is separate, all-or-none event and enforces one-way transmission Relative refractory period: interval following absolute refractory period, when most Na+ channels returned to their resting state, some K+ channels still open and repolarization is occurring – axon’s threshold for AP generation is substantially elevated and thus stimulus that would normally generate AP no longer suffice, but exceptionally strong stimulus can reopen Na+ channels that have returned to resting state (strong stimuli cause more frequent generation of APs by intruding into relative refractory period) Conduction Velocity: Nerve fibers transmitting impulses more rapidly (100 m/s or more) found in neural pathways where speed’s essential (those that mediate some postural reflexes) Axons conduct impulses more slowly typically serve internal organs (gut, glands, blood vessels) Rate of impulse propagation depends largely on two factors: Axon diameter: Axons vary in diameter and the larger the axon’s diameter, the faster it conducts impulses - larger axons conduct more rapidly because less resistance to flow of local currents and adjacent areas of membrane can more quickly come to threshold Degree of myelination: On unmyelinated axons, channels immediately adjacent to each other & conduction is relatively slow, type of AP propagation called continuous conduction Presence of myelin sheath dramatically increases rate of AP propagation because myelin acts as insulator Current can only pass through membrane of myelinated axon only at nodes of Ranvier where myelin sheath is interrupted and axon bare which all voltage-gated Na+ channels are concentrated at nodes When AP generated in myelinated fiber, current maintained and moves rapidly to next node, distance of ~1mm where another AP is triggered APs are triggered only at nodes, type of conduction called salutatory conduction because electrical signal jumps from node to n ode along axon, is 30x faster than continuous conduction Types of membrane channels, different types of stimuli Ion Channels: Selective as to type of ions it allows to pass Membrane channels are large proteins whose amino acid chains snake back and forth across membrane Ion channels open and close d/t presence of “gates” – changes shape to open and close the channel in response to specific signals When gated channels are open, ions diffuse quickly across membrane following electrochemical gradients creates electrical currents & voltage changes Move along chemical concentration gradient when diffusing passively (area of high to area of low concentration); and along electrical gradient when move toward area of opposite electrical charge – together constitute electrochemical gradient Electrical signals produced by neurons and muscle fibers rely on 4 types of ion channels: Leakage or non-gated channels: some are always open while some alternate randomly between open and closed positions Ligand-gated (chemical) channels: open when appropriate chemical (neurotransmitter) binds Mechanically gated channels: open in response to physical deformation of receptor (force distorts channel from resting position, opening gate) – sensory receptors for touch and pressure, form of vibration (sound waves), or tissue stretching Voltage-gated channels: open and close in response to changes in membrane potential, participate in generation and conduction of action potentials Graded potentials including summation types Graded potentials are short lived, localized changes in membrane potential, usually in dendrites or the cell body. They can be depolarized or hyperpolarized. These changes cause current flows that decrease in magnitude with distance They are called graded because their magnitude varies directly with stimulus strength The stronger the stimulus, the more voltage changes and the farther the current flows They are trigged by a change in the neurons environment that opens gated ion channels Neurotransmitters, their roles, and their receptors – Ach, NE, Epinephrine, Dopamine, Catecholamines, some amino acids, Endorphins Neurotransmitter Roles Receptor Acetylcholine Prolonged effects can lead to tetanic muscle spams. Ach levels decreased in certain brain areas in Alzheimer’s disease. Binding of nicotine to nicotinic receptors in the brain enhances dopamine release, which may account for the behavioural effects of nicotine smokers - At nicotinic ACh receptors (on skeletal muscles, autonomic ganglia, and in the CNS: direct action)- At muscarinic Ach receptors (on visceral effectors and in the CNS: indirect action via second messengers) Norepinephrine A “feeling good” neurotransmitter. Helps control alertness and arousal; too little can depress mood. Excitatory or inhibitory. Indirect action via second messengers Dopamine Involved in reward, motivation, and motor control. Too little = Parkinson’s. Excitatory or inhibitory. Indirect action via second messengers Epinephrine Activates a sympathetic nervous system by making the heart beat faster, stopping digestion, enlarging pupils, sending sugar into the bloodstream, preparing a blood clot faster Excitatory or inhibitory depending on receptor type bound Serotonin Affects hunger, sleep, arousal, and mood. Appears in lower-than-normal levels in depressed persons Mainly inhibitory. Indirect action via second messengers; direct action at 5-HT3 receptors Histamine Released in excess during allergic reactions causing swelling and inflammation of tissues. Excitatory or inhibitory depending on receptor type bound. Indirect action via second messengers GABA A major inhibitory neurotransmitter. Lessens the ability of the nerve cell to receive, create or send chemical messages to other nerve cells. Produces calming effect. Generally inhibitory Direct and indirect actions via second messengers Glutamate A major excitatory neurotransmitter involved in information transmission throughout the brain; during brain trauma there is excessive release of this Generally excitatory Direct action Glycine A principle inhibitory neurotransmitter of the spinal cord. When glycerine receptors are blocked it can result in uncontrolled convulsions and respiratory arrest Generally inhibitory Direct action Endorphins (ex. Dynorphin, Enkephalins) “Morphine within”- natural, opiatelike neurotransmitters linked to pain control and to pleasure Generally inhibitory Indirect action via second messengers 12 cranial nerves (name, type – sensory/motor/both, function) Cranial Name Major Functions I Olfactory Smell II Optic Vision III Oculomotor Eyelid and eyeball movement IV Trochlear Innervates superior oblique Turns eye downward and laterally V Trigeminal Chewing Face and mouth (touch & pain) VI Abducens Turns eye laterally VII Facial Controls most facial expressions Secretion of tears and saliva Taste VIII Vestibulocochlear (auditory) Hearing Equilibrium sensation (vertigo) XI Glossopharyngeal Taste Senses carotid blood pressure X Vagus Sense aortic blood pressure Slows heart rate Stimulates digestive organs Taste XI Spinal Accessory Controls trapezius & sternocleidomastoid Controls swallowing movements XII Hypoglossal Controls tongue movements See key concepts for Gastrointestinal System also!