2Graded Potentials Action Potential Objectives: Student should know1. Graded potential2. Types of graded potentials3. Action potential4. Three stages of action potential5. Types of action potential6. Generation of action potential7. Properties of action potentials8. Differences – graded vs action potential
3Graded PotentialsA. Subthreshold electrical stimuli that do not produce a true action potential but do generate electrical signalsB. Stimuli may be electrical, chemical, or mechanicalC. Stimuli produce two types of physiochemical disturbances
4Graded Potentials1. Local, graded, non propagated potentials called receptor or generator potentials, synaptic potentials or electrotonic potentials2. Action potentials (complete depolarization) or nerve impulses which are propagated down the axon to cause the release of neurotransmitters
8Graded Potentials Local Response A. Subthreshold responseB. Characteristics of graded potentials1. It is Local - changes in membrane potential are confined to relatively small regions of the plasma membrane2. It is graded - Refers to the magnitude of the potential change and that the signal can be reinforced.
9Graded Potentials Local Response A. Magnitude can vary (is graded) with the magnitude of the stimulusB. Graded events can be hypopolarizing (depolarizing - decrease in potential difference) or hyperpolarizing
10Graded Potentials Local Response 3. Graded potentials are conducted with decrement. (conduction magnitude falls off the further you get from the point of origin)
11Graded 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 distancesC. Graded potentials die out in mm of the origin
12Types of Graded Potentials 1. Characteristics of Graded Potentialsa. Only type of communication by some neuronsb. Play an important role in the initiation and integration of long distance signals by neurons and other cells
13Types of Graded Potentials 2. Specific types of graded potentialsa. Receptor (Generator) potentials1) Sensory receptors respond to stimuli from mechanoreceptors, thermoreceptors, nociceptors (pain), chemoreceptors, and electromagnetic receptors (vision)a) Graded potential from stimuli is called receptor potentialb) If graded potential reached threshold an action potential is generated and sensory information is sent to the spinal cord and brain
14Types of Graded Potentials b. Pacemaker potential - heart1) Specialized coronary muscle cells in the cardiac pacemaker region (SA node) have “leaky” ion channels graded potentials can potentially induce a true cardiac action potential2) Graded potential is responsible for cardiac automaticity
15Types of Graded Potentials c. Postsynaptic membrane potentials1) 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
16Types of Graded Potentials D. EPP End Plate PotentialPost 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.
17Action Potential Generation Graded Potentials which reach threshold generate action potentials1. Much larger response - Membrane polarity reverses (complete depolarization)2. AP are propagated without decrementa. Size and shape of AP are constant along nerve fiber
18Action Potential Generation All or None Response - Size and shape of AP are not influenced by the size of the stimulusAction Potential - Rapid but transient change in a membrane potential - Change in local membrane polarity -Polarized___Depolarized___Polarized
19Action Potential Generation The Action Potential
20Action Potential Generation The Action Potential Characteristics of action potentials1. Requires specific voltage- gated ion channels2, AP are the result of rapid changes in ion conductance3. AP occur only on regions of cell membranes that are electrically excitable4. AP generally are a standard size and shape for a specific cell type5. All or none - when membrane reaches threshold an AP is generated (Not-Graded)
21Action 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
22Action Potential Generation Importance of Action PotentialsNerve 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 brainThreshold stimuli (Graded Potential) cause the.generation of an action potential
23Action Potential Generation Three Stages of the Action Potential1. 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 mV2. Depolarization stage - Sodium ions (Na+) flow into the cell as the threshold for voltage gated Na+ channels are exceeded.
24Action 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.
25Action Potential Generation The Action Potential Three Stages1. Resting Stage(Polarized State)2. Depolarization3. Repolarization1
30Action Potential Generation Components of an Action Potential1. Threshold - Membrane potential at which voltage gated channels will open2. 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
31Action 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 repolarization6. Threshold - As the membrane potential passes back through threshold the voltage gated channels reset (both the Na+ and K+ channels)
32Action 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
35Action 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.
36Action Potential Generation Properties Absolute RefractoryDuring this period the voltage gated channels responsible for the action potential have not reset and therefore, do not respond to stimulation.
37Action Potential Generation Properties Relative RefractoryThis 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.
38Action 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.