Cell Membrane Potential Chapter 9: Nervous System Unit 3: Integration and Coordination Neuron Communication Neurons communicate with one another through lots of nerve action potentials (nerve impulses) Generation of action potentials depends on two features of the cell membrane 1. A resting membrane potential 2. Existence of ion channels Cell Membrane Potential A Membrane Potential is when body cells exhibit a difference in the amount of electrical charge on the inside and outside of the membrane. Cells with a membrane potential = polarized Like voltage stored in a battery! Polarization occurs due to an unequal distribution of positive and negative ions between sides of the membrane. Resting membrane potential = neurons at “rest”, not conducting action potentials Flow of Ions Connecting the terminals of a battery with a piece of metal to a radio an electrical current flows from the battery allowing you to listen to music In living tissues, a flow of ions generates this electrical current Ions can flow across the membrane through ion channels. Some channels are always open, and others can be opened and closed. Channels can also be selective. What are Ion Channels? When open, ion channels allow specific ions to diffuse across the membrane (high to low) Also, + charged ions will move towards a - charged area and vice versa There are 2 Types of ion channels 1. Leakage Channels 2. Gated Channels Types of Ion Channels Leakage channels - allow a slow but steady stream of ions to leak across the membrane Example - Sodium (Na +), Potassium (K+), Cl- Gated channels - open and close on command Example - Voltage-gated channels open in response to a change in membrane potential Resting Potential Two main ions are involved in potentials, K+ and Na+. Resting potential arises from the unequal distributions of various ions in cytosol and interstitial fluid. In a resting neuron, the outside of the membrane has a + charge and inside the membrane has a - charge. This creates potential energy, -70mV Potential Changes Nerve cells exhibit electrical excitability. Changes (stimuli) usually affect the resting potential in a particular region of a nerve cell membrane. When a 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 Depolarized. Changes in Membrane Potential Changes in the resting potential of a membrane have two main phases: 1. Depolarizing Phase - reversal of charge inside to outside 2. Repolarizing Phase - membrane polarization is restored to resting state Changes 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. Action Potentials An action potential (AP) is a sequence of rapidly occurring events that decrease and reverse the membrane potential before eventually restoring it to the resting state Generation of Action Potential Many subthreshold potential changes must combine to reach threshold and create an Action Potential. At the threshold potential, permeability suddenly changes at the region of the cell membrane being stimulated. Channels highly selective for sodium ions open and allow sodium to diffuse freely inward. As sodium ions diffuse inward, the membrane loses its negative electrical charge and becomes Depolarized. Action Potential Continued As sodium ions diffuse inward, the membrane loses its negative electrical charge and becomes depolarized. At almost the same time, potassium ions diffuse outward, and the inside of the membrane becomes negatively charged once more. The membrane become Repolarized, and it remains in this state until stimulated again. Major Events of Action Potential When the membrane reaches threshold, sodium channels open, some sodium diffuses in, and the membrane is depolarized. Soon afterward, potassium channels open. Potassium ions diffuse out, and the membrane is repolarized. Nerve Impulse Axons are capable of AP’s, dendrites and the cell body are not. AP’s in one region of a nerve cell membrane cause 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. Events 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. 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. Impulse Conduction A myelinated 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, and jump from node to node. This is called saltatory conduction. Speed of Nerve Impulses The speed of nerve impulse conduction is proportional to the diameter of the axon. The greater the diameter, the faster the impulse. An impulse on a thick, myelinated motor neuron of skeletal muscle and travel 120 meters/second! A thin, unmyelinated skin neuron might be 0.5 meters/second. All-or-None Response Nerve impulse 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.
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