action potential

• Most action potentials originate in the initial segment (axon hillock)
• Depolarization, the first phase of the action potential, is characterized by the membrane potential changing from a negative value to a positive value
• Repolarization, the second phase of the action potential, occurs when the membrane depolarizes to a peak value of +30 mV and then repolarizes to its negative resting value of -70 mV
• The generation of an action potential is triggered by the membrane potential depolarizing from the resting voltage of -70 mV to a threshold value of -55 mV
• Voltage-gated Na+ channels change shape and their activation gates open in response to a threshold stimulus
• An action potential is initially generated at the axon hillock in the neuron
• The depolarization phase of an action potential results from the opening of voltage-gated Na+ channels, allowing Na+ to rush into the cell causing depolarization
• The repolarization phase of an action potential results from the opening of voltage-gated K+ channels, allowing K+ to rush out of the cell and causing the membrane potential to become more negative on the inside, thus repolarizing the cell
• Hyperpolarization results from the slow closing of voltage-gated K+ channels
• The magnitude (amplitude) of an action potential is 100 mV
• The resting membrane potential is a result of uneven distribution of ions across the cell membrane and differences in membrane permeability to ions
• Potassium ions are higher in concentration inside the cell compared to outside
• Ion channel inactivation is the closing of the channel even when the stimulus continues
• The absolute refractory period of an action potential ensures one-way travel down an axon, allows a neuron to ignore a second signal sent that closely follows the first, and prevents summation of action potentials
• Voltage-gated channels are located in the membranes of dendrites, in the membranes of axons, and on the neuron cell body
• The sodium-potassium exchange pump requires ATP to function
• If voltage-gated K+ channels of a resting neuron open, K+ leaves the neuron
• The all-or-none principle states that all stimuli great enough to bring the membrane to threshold will produce action potentials of identical magnitude
• If voltage-gated Na+ channels of a resting neuron open, Na+ enters the neuron and the neuron depolarizes
• Ion concentrations are first significantly affected after a few thousand action potentials
• In the membrane of a resting nerve cell, when chemically gated Cl- channels open, Cl- ions enter the cell
• Action potentials are mainly associated with the membranes of neurons
• Hyperkalemia specifically indicates too much potassium in the plasma fluid compartment
• Most graded potentials originate in dendrites
• If the sodium-potassium pumps in the cell membrane of a neuron fail to function, over time the intracellular concentration of sodium ion will increase, the extracellular concentration of potassium ion will increase, and the membrane resting potential will become more positive than normal
• If voltage-gated sodium channels remained inactivated, the absolute refractory period would be longer than normal