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Cell membrane potential
- 1. Neuron Membranes &the Action Potential Chapter 9: Nervous System Unit 3: Integration and Coordination
- 2. Cell Membrane PotentialThe surface of the cell membrane is usually polarized (charged), with respect to the inside. This polarization arises from an unequal distribution of positive and negative ions between sides of the membrane. This polarization is particularly important in the conduction of muscle and nerve impulses.
- 3. Distribution of IonsThe distribution of ions inside and outside cell membranes is determined in part by pores or channels in those membranes. Some channels are always open, and others can be opened and closed. Most channels are selective and only allow one type of ion/molecule through.
- 4. Resting Membrane PotentialActive transport creates a concentration gradient across the cell membrane of sodium and potassium ions. Na+/K+ pumps work to move Na+ out of the neuron and K+ into the neuron. Uses ATP as energy source ATPase breaks down ATP into ADP + P Similar effects as those we saw in the ATPase of myosin heads in muscle fibers
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- 6. Resting Membrane PotentialThe Na+/K+ pump creates: High concentration of sodium outside High concentration of potassium inside Negative amino acids are found in abundance inside the cell Would make the intracellular fluid more negative, but… Chlorine ions (Cl-) are found in abundance outside the cell Counteract the negative of the amino acids
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- 8. Resting Membrane PotentialIf these were the only factors involved, the neuron would be neutral However, K+ leaks out of the cell through leakage channels that are always open Positives leaving the intracellular space means the overall charge is negative inside (-70mV)
- 9. Potential Changes Whenneurons are excited (stimulated) Affect the resting potential in a particular region of a nerve cell membrane. If the membrane’s resting potential decreases (as the inside of the membrane becomes less negative when compared to the outside), the membrane is said to be Depolarizing .
- 10. Potential Changes Changesin potential are directly proportional to the intensity of the stimulation. If additional stimulation arrives before the effect of previous stimulation subsides, summation takes place. As a result of summated potentials, a level called Threshold Potential may be reached.
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- 12. Action Potential An action potential can be thought of as the “firing” of the neuron. Action potentials will propagate down the length of a neuron’s axon Action potentials are the electrical signals that move down a neuron
- 13. Action Potential Many subthreshold potential changes must combine to reach threshold, and once threshold is achieved, an event called Action Potential occurs. At the threshold potential, permeability to ions changes suddenly at the region of the cell membrane being stimulated. This is due to the presence of voltage-gated ion channels Channels that respond to changes in membrane potential (voltage) There are VGICs that are permeable to only K+ and others that are permeable to only Na+
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- 16. Action Potential When threshold is reached: 1. VGICs that are permeable to Na+ open Na+ diffuses into neuron Neuron’s membrane potential rises from -70mV to +40mV ( depolarizes ) 2. Na+ channels close as K+ VGICs open K+ diffuses out of neuron Neuron’s membrane potential repolarizes , going from +40mV to nearly -85mV
- 17. Action Potential When threshold is reached: 3. The cell is now in what is called the refractory period (it’s too negative) Neuron will not respond to further stimulation at this time This is due to K+ channels being open a bit too long 4. All VGICs return to normal and the neuron is ready to “fire” again
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- 19. Nerve Impulse Whenan action potential occurs in one region of a Neuron membrane, it causes a bioelectric current to flow to adjacent portions of the membrane. This Local Current stimulates the adjacent membrane to its threshold level and triggers another action potential. A wave of action potentials move down the axon to the end. This propagation of action potentials along a nerve axon constitutes a Nerve Impulse . Animation
- 20. Events Leading tothe Conduction of a Nerve Impulse 1. Neuron membrane maintains resting potential. 2. Threshold stimulus is received. 3. Sodium channels in a local region of the membrane open. 4. Sodium ions diffuse inward, depolarizing the membrane.
- 21. Events Continued 5.Potassium channels in the membrane open. 6. Potassium ions diffuse outward, repolarizing the membrane. 7. The resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane. 8. Wave of action potentials travels the length of the axon as a nerve impulse.
- 22. Impulse Conduction Amyelinated axon functions as an insulator and prevents almost all ion flow through the membrane it encloses. Nodes of Ranvier between adjacent Schwann cells interrupt the sheath. Action potentials occur at these nodes, where the exposed axon membrane contains sodium and potassium channels. Nerve impulses jump from node to node, and are many times faster than conduction on an unmyelinated axon.
- 23. Speed of NerveImpulses The speed of nerve impulse conduction is proportional to the diameter of the axon. The greater the diameter, the faster the impulse.
- 24. All-or-None Response Nerveimpulse conduction is an all-or-none response. If a neuron responds at all, it responds completely. A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon, and all impulses carried on that axon are of the same strength. A greater intensity of stimulation does not produce a stronger impulse, but more impulses per second.