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Transcript
PHYSIOLOGY OF THE NERVOUS SYSTEM Neurons are IRRITABLE • Ability to respond to a stimulus! • (What’s a stimulus?) How do neurons respond to stimuli? • By generating an ACTION POTENTIAL! • Action Potential = “Nerve Impulse” • Reminder: Which part of the neuron generates impulses? • NOTE: – A stimulus must be strong enough (above the threshold level) to generate an Action Potential – Examples… So how does an Action Potential get generated??? • Involves: 1. Ions 2. Ion Channels in Neuron Membranes 3. Electricity! (Currents, Voltage) 1. Ions Involved: • • • • • K+ Na+ ClCa++ Various Anions (A- ) 2. Ion Channels • Transport Proteins embedded in Phospholipid Membrane of Axon! • 2 Major Types: – Passive (Leakage) Channels – Active (Gated) Channels: • Voltage-Gated Channels • Chemically-Gated Channels • Mechanically-Gated Channels The Big Idea AN ELECTRICAL IMPULSE IS GENERATED WHEN IONS MOVE ACROSS THE AXON MEMBRANE THROUGH OPEN CHANNELS!! 3. ELECTRICITY • Overall, – The body is Electrically Neutral! • ( # + charges = # - charges) Voltage Measures the Potential Energy due to separation of charges across a membrane! Current Flow of electrical charge from one point to another. Can be used to do WORK!! (Kinetic Energy) When Charges are Separated Across the Neuron Membrane… • Neuron is “AT REST” –( Potential Energy, Kinetic Energy) • RESTING MEMBRANE POTENTIAL: – Separation of Charge across membrane… – More (-) charges inside than (+) outside – Voltage Reading = –70 millivolts – Membrane is POLARIZED ! Resting Membrane Potential The Polarized State What Maintains the Resting Membrane Potential? Maintaining the differences in [ion] between the intracellular and extracellular fluids!! Distribution of Ions during Resting Potential State • • • • K+ : inside membrane Na+ : outside membrane Cl- : outside membrane Various Anions (A- ) : inside membrane Permeability Across the Membrane • K+ • Na+ • High Permeability (Leakage Channels) • Low Permeability • Cl- • High Permeability • Anions- • IMPERMEABLE!!! The Sodium Potassium Pump • K+ constantly leaks across membrane to outside! ( Why?) • K+ pumped back into cell! • Na+ pumped out of cell! • Ratio : 3 Na+ OUT / 2 K+ IN • ACTIVE TRANSPORT MECHANISM!! (What does that mean??) Let the Fun Begin… Generation of a Nerve Impulse 1. Signal is detected/received and relayed to axon membrane. 2. RESTING MEMBRANE POTENTIAL ACTION POTENTIAL!! (aka: Nerve Impulse!) Action Potentials • Caused by a shift in membrane potentials along the axon! • Detection of STIMULUS Shift in Membrane Permeabilities! So what’s the deal with Action Potentials? • An Action Potential (AP) is: a wave of positive charges that travel undiminished in strength along the axon The Chain of Events 1. 2. 3. 4. Stimulus is detected Gated Ion Channels open Na+ rushes into the cell Charge on cell membrane becomes DEPOLARIZED ( -70 mv 0 mv) 5. If stimulus is strong enough (above THRESHOLD LEVEL), MORE Na+ GATES OPEN!! 6. Complete DEPOLARIZATION occurs! (-70mv 0mv +30mv) 7. Na+ channels open further down the neuron 8. AP travels down length of neuron! Action Potentials are ALL OR NOTHING !!! • IF A STIMULUS DOES NOT REACH THRESHOLD LEVEL…. NO AP! • IF A STIMULUS REACHES THRESHOLD LEVEL, AP IS GENERATED AND TRAVELS DOWN LENGTH OF THE AXON!!! http://www.stolaf.edu/people/giannini/fla shanimat/transport/secondary%20active %20transport.swf