PHYSIOLOGY 1 LECTURE 12 Graded Potentials Action Potential Generation Graded Potentials Action Potential Objectives: Student should know – 1. – 2. – 3. – 4. – 5. – 6. – 7. – 8. Graded potential Types of graded potentials Action potential Three stages of action potential Types of action potential Generation of action potential Properties of action potentials Differences – graded vs action potential Graded Potentials A. Subthreshold electrical stimuli that do not produce a true action potential but do generate electrical signals B. Stimuli may be electrical, chemical, or mechanical C. Stimuli produce two types of physiochemical disturbances Graded Potentials 1. Local, graded, non propagated potentials called receptor or generator potentials, synaptic potentials or electrotonic potentials 2. Action potentials (complete depolarization) or nerve impulses which are propagated down the axon to cause the release of neurotransmitters ACTION POTENTIAL Graded Potentials Graded Potentials Graded Potentials Local Response A. Subthreshold response B. Characteristics of graded potentials – 1. It is Local - changes in membrane potential are confined to relatively small regions of the plasma membrane – 2. It is graded - Refers to the magnitude of the potential change and that the signal can be reinforced. Graded Potentials Local Response A. Magnitude can vary (is graded) with the magnitude of the stimulus B. Graded events can be hypopolarizing (depolarizing decrease in potential difference) or hyperpolarizing Graded Potentials Local Response 3. Graded potentials are conducted with decrement. (conduction magnitude falls off the further you get from the point of origin) Graded Potentials Local Response A. Charge is lost across the membrane because of “leaky” channels and the magnitude of the potential decreases with distance from the site of origin (charge density falls). B. Graded potentials and the local current they generate can function as signals over very short distances C. Graded potentials die out in 1 - 2 mm of the origin Types of Graded Potentials 1. Characteristics of Graded Potentials – a. Only type of communication by some neurons – b. Play an important role in the initiation and integration of long distance signals by neurons and other cells Types of Graded Potentials 2. Specific types of graded potentials – a. Receptor (Generator) potentials 1) Sensory receptors respond to stimuli from mechanoreceptors, thermoreceptors, nociceptors (pain), chemoreceptors, and electromagnetic receptors (vision) – a) Graded potential from stimuli is called receptor potential – b) If graded potential reached threshold an action potential is generated and sensory information is sent to the spinal cord and brain Types of Graded Potentials – b. Pacemaker potential - heart 1) Specialized coronary muscle cells in the cardiac pacemaker region (SA node) have “leaky” ion channels graded potentials can potentially induce a true cardiac action potential 2) Graded potential is responsible for cardiac automaticity Types of Graded Potentials – c. Postsynaptic membrane potentials 1) Graded potentials that develop on the postsynaptic membrane during synaptic transmission (stimuli from other nerves - can be stimulatory or inhibitory) 2) If graded potentials reach threshold action potential develops Types of Graded Potentials – D. EPP End Plate Potential – Post synaptic graded potential that develops at the neuromuscular junction (always stimulatory and always reach threshold if generated by an action potential in the innervating alpha motor neuron). Postsynaptic membrane potentials are important in AP generation in nerve to nerve and nerve to muscle communication. Action Potential Generation Graded Potentials which reach threshold generate action potentials 1. Much larger response - Membrane polarity reverses (complete depolarization) 2. AP are propagated without decrement – a. Size and shape of AP are constant along nerve fiber Action Potential Generation – All or None Response - Size and shape of AP are not influenced by the size of the stimulus Action Potential - Rapid but transient change in a membrane potential Change in local membrane polarity Polarized___Depolarized___Polarized Action Potential Generation The Action Potential Action Potential Generation The Action Potential Characteristics of action potentials – 1. Requires specific voltage- gated ion channels – 2, AP are the result of rapid changes in ion conductance – 3. AP occur only on regions of cell membranes that are electrically excitable – 4. AP generally are a standard size and shape for a specific cell type – 5. All or none - when membrane reaches threshold an AP is generated (Not-Graded) Action Potential Generation The Action Potential – 6. Time - AP not only have a specific size and shape but also exists within a specific time frame , ave. 1 to 5 msec.- (ie time duration of the action potential is always the same for a specific tissue) – Specific to transport protein cycle times Action Potential Generation Importance of Action Potentials Nerve traffic, muscle contraction, hormone release, G.I. secretions, Cognitive thought, etc. Action Potentials are required for the senses Sight, hearing, and touch are all dependent on action potentials for transmission of information to the brain Threshold stimuli (Graded Potential) cause the.generation of an action potential Action Potential Generation Three Stages of the Action Potential – 1. Resting stage - Polarized stage - This is the normal resting membrane potential and varies with the cell type nerve = -90 mV, heart pacemaker = -60 mV, and skeletal muscle = -83 mV – 2. Depolarization stage - Sodium ions (Na+) flow into the cell as the threshold for voltage gated Na+ channels are exceeded. Action Potential Generation – 3. Repolarization stage - Potassium (K+) ions flow out of the cell as voltage gated K+ channels are opened and the cell membrane potential moves back toward the resting membrane potential. Action Potential Generation The Action Potential Three Stages 1. Resting Stage (Polarized State) 2. Depolarization 3. Repolarization 1 Action Potential Generation Components of an Action Potential – 1. Threshold - Membrane potential at which voltage gated channels will open – 2. Rising phase - as Na+ channels open membrane potential begins to shift toward the equilibrium potential for Na+ (Nernst Potential for Na+) – 3. Overshoot - The point at which the membrane potential becomes positive. The greater the overshoot potential the further the membrane will stay above threshold Action Potential Generation – 4. Peak - At the peak of the action potential the sodium conductance begins to fall (Closure of the slow gate) – 5. Repolarization - Inactivation of sodium channels and opening of the K+ channels (Opening of the K+ voltage channel slow gate) causes repolarization – 6. Threshold - As the membrane potential passes back through threshold the voltage gated channels reset (both the Na+ and K+ channels) Action Potential Generation – 7. After - hyperpolarization - The Na+ voltage gated channels have a fast gate and a slow gate passage of the membrane potential back through threshold causes the fast gate to close too rapidly for any Na+ ions to pass while the slow gate opens. The K+ voltage gate with it’s single slow gate begins to close slowly so for a period of time K+ still flows out of the cell hyperpolarizing the cell. Return to resting membrane voltage is due to Na+K+ATPase Action Potential Generation The Action Potential Types Action Potential Generation Properties Properties of Action Potentials Refractory periods are times when it is either impossible or more difficult than normal to generate a second action potential. Action Potential Generation Properties Absolute Refractory During this period the voltage gated channels responsible for the action potential have not reset and therefore, do not respond to stimulation. Action Potential Generation Properties Relative Refractory This period corresponds to the positive after potential period and due to the hyperpolarization of the cell it is more difficult to generate a second action potential. Action Potential Generation Properties Voltage Inactivation - If a cell membrane is maintained at a voltage potential above threshold than the voltage gated channels are not reset and, hence, inactivated and no action potentials can be generated. Accommodation to Slow Depolarization - If a slow depolarization occurs the voltage gated channels do not respond and no action potential occurs.