ANATOMY PHYSIOLOGY TEST 3 REVIEW Lecture 13 14 Transmembrane 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 Activities 1 Resting potential 2 Graded potential The transmembrane potential of resting cell Temporary localized change in resting potential Caused by stimulus to the cell 3 Action potential An electrical impulse produced by graded potential Propagates along surface of axon to the synapse 4 Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in postsynaptic membrane 5 Information processing Response integration of stimuli of postsynaptic cell The 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 membranes 3 Membrane permeability varies by ion Due to the presence of passive leak channels Passive Forces acting Across the Plasma Membrane 1 2 Chemical gradients Electrical gradients Electrical Currents and Resistance Concentration gradients chemical gradient of ions Na K Separate charges of positive and negative ions Result in potential difference Electrical current o Movement of charges to eliminate potential difference opposite charges attract Resistance o 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 brings into the cell o It serves to stabilize the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 o Depolarization moving toward 0 At the normal resting potential these passive and active mechanisms are in balance o The resting potential varies widely with the type of cell o A typical neuron has a resting potential of approximately 70 mV Changes in the Transmembrane Potential Transmembrane potential rises or falls o In response to temporary changes in membrane permeability from opening or closing specific membrane channels Sodium and Potassium Channels Membrane permeability to Na and K determines transmembrane potential These channels are either passive or active o 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 Channels 1 Closed but capable of opening 2 Open activated 3 Closed not capable of opening inactivated Three Classes of Gated Channels 1 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 potential o 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 muscle 3 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 Changes with plasma membrane permeability in response to chemical or physical stimuli Transmembrane Potential Transmembrane 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 characteristics 1 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 hyperpolarization o 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 removed the transmembrane potential returns to normal Hyperpolarization caused by increasing the negativity of the resting potential o Result of opening a potassium channel o Opposite effect of opening a sodium channel o NET Positive ions move out not into cell Transmembrane Potential Effects of graded potentials
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