ANATOMY & PHYSIOLOGY: TEST 3 REVIEWLecture 13-14Transmembrane Potential:• Ion Movements and Electrical Signals• All plasma (cell) membranes produce electrical signals by ion movements• Transmembrane potential is particularly important to neurons• Five Main Membrane Processes in Neural Activities1. Resting potential • The transmembrane potential of resting cell2. Graded potential • Temporary, localized change in resting potential• Caused by stimulus to the cell3. Action potential • An electrical impulse produced by graded potential• Propagates along surface of axon to the synapse4. Synaptic activity • Releases neurotransmitters at presynaptic membrane• Produces graded potentials in postsynaptic membrane 5. Information processing • Response (integration of stimuli) of postsynaptic cellThe Transmembrane Potential- Three important concepts:1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition• Concentration gradient of ions (Na+, K+) 2. Cells have selectively permeable membranes3. Membrane permeability varies by ion• Due to the presence of passive (leak) channelsPassive Forces acting Across the Plasma Membrane1. Chemical gradients• Concentration gradients (chemical gradient) of ions (Na+, K+) 2. Electrical gradients• Separate charges of positive and negative ions• Result in potential differenceElectrical Currents and Resistance• Electrical currento Movement of charges to eliminate potential difference (opposite charges attract)• Resistanceo The amount of current a membrane restricts The Electrochemical Gradient• For a particular ion (Na+, K+), the electrochemical gradient is:o The sum of chemical and electrical forces acting on the ion across a plasma membrane This is a form of potential energy (Since sodium is positive and low in the cell (cell negative inside), sodium wants to go inside. Opposing forces for potassium, chemical is a stronger force so potassium will leak out)Active Forces across the Membrane• Sodium–potassium ATPase (exchange pump) o Powered by ATP, it carries 3 Na+ out and 2 K+ in Balances passive forces of diffusion (through passive channels) Maintains resting potential (–70 mV)The Resting Potential• Because the plasma membrane is highly permeable to potassium ions• The electrochemical gradient for sodium ions is very large, but the membrane’s permeability to these ions is very low• The sodium–potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it bringsinto the cello It serves to stabilize the resting potential when the ratio of Na+ entry to K+ loss through passive channels is 3:2o (Depolarization: moving toward 0)• At the normal resting potential, these passive and active mechanisms are in balanceo The resting potential varies widely with the type of cello A typical neuron has a resting potential of approximately –70 mVChanges in the Transmembrane Potential• Transmembrane potential rises or fallso In response to temporary changes in membrane permeability from opening or closing specific membrane channelsSodium and Potassium Channels• Membrane permeability to Na+ and K+ determines transmembrane potential• These channels are either passive or activeo Passive channels - Are always open, permeability changes with conditions (flow is from greater to lesser)o Active channels - Open and close in response to stimuli At resting potential, most of these gated channels are closed. Three States of Gated Channels1. Closed, but capable of opening2. Open (activated)3. Closed, not capable of opening (inactivated)Three Classes of Gated Channels1. Chemically gated channels• Open in presence of specific chemicals (e.g., Ach-the neurotransmitter acetylcholine) at a binding site• Found on neuron cell body and dendrites 2. Voltage-gated channels• Respond to changes in transmembrane potentialo A change in membrane potential causes channels to open• Have activation gates (open) and inactivation gates (close)• Characteristic of excitable membrane• Found in neural axons, skeletal muscle sarcolemma, cardiac muscle3. Mechanically gated channels• Respond to membrane distortion • Found in sensory receptors (touch, pressure, vibration) (Remember Merkel Cells, hair root plexus in the integumentary system?)Transmembrane Potential Exists Across Plasma Membrane • Because: Cytosol and extracellular fluid have different chemical/ionic balance (inside cell has negative charged proteins, not usually found outside of the cell) The plasma membrane is selectively permeable Transmembrane Potential• Changes with plasma membrane permeability in response to chemical or physical stimuliTransmembrane Graded Potential:Graded Potentials (local potentials)• Are changes in transmembrane potential that cannot spread far from site of stimulation• Includes any stimulus that opens a gated channel which produces a graded potential• The resting state Opening sodium channel produces graded potential. Events include:- Resting membrane exposed to chemical- Sodium channel opens- Sodium ions enter the cell- Transmembrane potential rises- Depolarization occurs• Depolarization - A shift in transmembrane potential toward 0 mV. Events include: 1. Movement of Na+ through channel 2. Produces local current 3. Depolarizes nearby plasma membrane (graded potential)4. Note: change in potential is proportional to stimulus. • Whether the potential is depolarizing or hyperpolarizing, share four basic characteristics1. The transmembrane potential is most changed at the site of stimulation, and the effect decreases with distance (Ex. Ripples in a pool)2. The effect spreads passively, due to local currents. The spread is in all directions, not just in one direction. (Ex. Creating ripples when dropping a pebble into a pool of water). 3. The graded change in transmembrane potential may involve either depolarization or hyperpolarization The properties and distribution of the membrane channels involved determine the nature of the change- For example, in a resting membrane, the opening of sodium channels causes depolarization, whereas the opening of potassium channels causes hyperpolarizationo The change in transmembrane potential reflects whether positive changes enter or leave the cell.4. The stronger the stimulus, the greater the change in the transmembrane potential and the larger the area affected• Repolarization - occurs when the stimulus is
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