Lesson 13 Membrane Potential and the Action Potential Describe the main membrane processes in neurons The processes in neural activities are the resting potential graded potential action potential synaptic activity and information processing The resting potential is the membrane potential difference between the inside outside of a resting cell The graded potential as a temporary localized change in resting potential that make the inside more positive it s caused by a stimulus The action potential is an electrical impulse that retains it s strength strength of the stimulus as it travels down the neuron The action potential is produced by a graded potential if the stimulus is strong enough and then it propagates along the surface of the axon to the synapse Synaptic activity is the release of neurotransmittes at the presynaptic membrane Neurotransmitters diffuse across the synaptic cleft and bind to postsynaptic cell receptors which produces a graded potential Information processing is the response of the postsynaptic cell Explain the transmembrane potential The transmembrane potential is the difference between electrical potential from the inside of membrane to outside the membrane Why is it important The extracellular fluid ECF and intracellular fluid cytosol differ greatly in ionic composition o Concentration gradient of ions Na K o More Na outside more K inside Cells have selectively permeable membranes o Remember water soluble substance including ions cannot pass through hydrophobic region of lipid bilayer need channel carrier protein 1 Membrane permeability varies by ion o Due to the presence of specific passive leak channels What are the main ions molecules that generate this potential Sodium and Potassium have more potassium ion channels Explain the electrochemical gradient The electrochemical gradient is a form of potential energy It s the sum of chemical electrical forces acting on a particular ion Na K An example is the sodium potassium ATPase exchange pump Why is it important It s important because it maintains a resting potential of 70 mV Explain the role of sodium and potassium ions on the membrane potential Sodium and potassium balance the passive forces of diffusion by carrying 3 Na out and 2 K in Do they enter leave through active or passive channels Passive channels Explain how gated channels function Gated channels open and close in response to stimuli and are usually closed at resting potential What are the different types of gated channels Chemically gated channels voltage gated channels and mechanically gated channels Chemically Gated Channels o Open in presence of specific chemicals e g Ach acetylcholine at a binding site o Found on neuron cell body and dendrites Voltage gated Channels o Respond to changes in membrane potential 2 A change in membrane potential causes the channels to open close o Have activation gates open and inactivation gates close Na has two gates while K only has one gate o Characteristic of excitable membrane o Found in neural axons skeletal muscle sarcolemma cardiac muscle Mechanically Gated Channels o Respond to membrane distortion o Found in sensory receptors touch pressure vibration What are the different states to a gated channel 3 1 Closed capable of opening 2 open activated and 3 closed not capable of opening inactivated Describe the difference between action potentials and graded potentials Graded potentials Decay over time Used for signaling over short distances All cells can produce GPs Vary in strength of stimulus Action potentials Persist the entire length Used for signaling for long distances Only excitable cells can produce APs Follow the all or none principle Are they always depolarizing or hyperpolarizing Graded potentials can be depolarizing or hyperpolarizing Explain how graded potentials can generate action potentials AP s Graded potentials depolarize the axon hillock to threshold to generate an action potential Which region of the neuron generates the AP The initial segment What is the all or none principle When a stimulus exceeds threshold and triggers an AP the AP is the same no matter how large the stimulus Basically an AP either occurs or doesn t Describe the steps to the generation of an action potential Step 1 Depolarization to threshold Step 2 Activation of Na channels Step 3 Inactivation of Na channels and activation of K channels Step 4 Return to normal permeability 4 What is the difference between a closed gate and an inactivated gate Closed gates are closed but capable of opening and inactivated gates are closed an incapable of opening 5 Explain the refractory period The refractory period is the time period from the beginning of an action potential to return to a resting state during which the membrane will not response normally to additional stimuli What is the difference between the absolute refractory period and the relative refractory period During the absolute refractory period NO action potential is possible and the Na channels are open or inactivated During the relative refractory period an action potential CAN be generated by a large stimulus which causes more Na entry than K exit Explain why an action potential only travels along the axon in one direction generation It can only travel in one direction because the segment in the opposing direction of travel has already been depolarized Describe the role of the sodium potassium pump in action potential The Na gated channels must be opened for the membrane potential to depolarize to threshold and generate an action potential The K gated channels is required to be open to return the membrane potential back to the resting membrane potential Will cessation of this pump cause an immediate halt to action potential generation No the concentration gradient would disappear very slowy and would causes neurons to stop functioning Describe the two forms of action potential propagation The two forms are continuous and salatory propagation Continuous propagation happens in unmyelinated axons and the AP has to be generated during the entire length of the axon affecting one segment at a time 6 Local current in segment 2 travels to segment 3 and 4 and also to segment 1 Segment 3 depolarizes to threshold and AP is formed Segment 1 depolarizes but no AP formed since channels there are in refractory period Salatory propagation is the jumping between nodes of Ranvier in myelinated axons 7 Which form is faster Salatory propagation is faster Why The myelin wrapped around
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