CHAPTER 11 GRADED VS ACTION POTENTIALS
REFRACTORY PERIOD
A time during which a neuron cannot respond to another stimulus.
ACTION POTENTIAL location of event
Axon hillock and axon; also called a *nerve impulse*
GRADED POTENTIAL repolarization
Voltage independent; occurs when stimulus is no longer present
GRADED POTENTIAL location of event
cell body and dendrites, typically
ACTION POTENTIAL peak membrane potential
+30 to +50 mV
DEPOLARIZATION
A decrease in membrane potential. The inside of the membrane becomes *less negative* (moves closer to zero) than the resting potential. [-70mV to -65 mV is a depolarization] Also includes events that reverse and move above zero to become positive.
GRADED POTENTIAL positive feed back cycle
Absent
ACTION POTENTIAL amplitude
Always the same size (all-or-none); does not decay with distance. [change in voltage from -70 mV to +30 mV or a total amplitude of about 100 mV]
HYPERPOLARIZATION
An increase in membrane potential - the inside of the membrane becomes *more negative* (moves further from zero) than the resting potential [-70mV to -75mV]
GRADED POTENTIAL stimulus for ion channels
Chemical (neurotransmitter) or sensory stimulus (light, pressure, temperature)
diameter, myelination
Conduction velocity (rate of impulse propogation) depends on two things: the axon's _____________ and the degree of ________________.
SALTATORY CONDUCTION
Current passes through a myelinated axon only at the nodes of Ranvier. Voltage-gated sodium chanels concentrated at those nodes. Action potentials triggered only at the nodes and jump from one node to the next. Much faster than unmyelinated axon conduction.
GRADED POTENTIAL peak membrane potential EPSP
Depolarizes; moves toward 0 mV
ACTION POTENTIAL summation
Does not occur; all-or-none phenomenon
ABSOLUTE REFRACTORY PERIOD
Ensures each action potential is a separate, all-or-none event and enforces one-way transmission of the action potential.
GRADED POTENTIAL function [postsynaptic (EPSP) potential]
Excitatory --short-distance signaling; depolarization that spreads to axon hillock; moves membrane potential toward threshold for generating an action potential
RECEPTOR POTENTIAL
Generator potential -- when the receptor of a sensory neuron is excited by some form of energy (heat, light, etc) the graded potential is called this.
GRADED POTENTIAL peak membrane potential IPSP
Hyperpolarizes; moves toward -90 mV
Inhibitory Postsynaptic Potentials (IPSPs)
Hyperpolarizing changes in potential (caused by binding of neurotransmitters at inhibitory synapses) reduce a postsynaptic neurons' ability to generate an action potential.
DEPOLARIZATION
Increases the probability of producing nerve impulses
GRADED POTENTIAL function [postsynaptic (IPSP) potential]
Inhibitory -- short-distance signaling; hyperpolarization that spreads to axon hillock; moves membrane potential away from threshold of generating an AP
Excitatory Postsynaptic Potentials (EPSPs)
Local graded depolarization events that occur at excitatory postsynaptic membranes
ACTION POTENTIAL distance traveled
Long distance - from trigger zone at axon hillock through entire length of axon (a few mm to over a meter). APs do not decay with distance.
ACTION POTENTIAL function
Long-distance signaling; constitutes the nerve impulse
GRADED POTENTIAL
Magnitude varies with strength of stimulus. Stronger stimulus=more voltage changes=further current flow
CONTINUOUS CONDUCTION
Occurs on unmyelinated axons only. Action potentials are generated at sites immediately adjacent to each other. Conduction is relatively slow.
GRADED POTENTIAL initial effect of stimulus IPSP
Opens chemically gated Potassium or Chlorine channels
GRADED POTENTIAL initial effect of stimulus EPSP
Opens chemically gated channels that allow simultaneous Sodium and Potassium fluxes
ACTION POTENTIAL initial effect of stimulus
Opens voltage-gated channels; first opens Sodium channels, then Potassium channels
ACTION POTENTIAL positive feed back cycle
Present
HYPERPOLARIZATION
Reduces the probability of producing nerve impulses
GRADED POTENTIAL distance traveled
Short distance, typically within cell body to axon hillock (0.1-1.0 mm) Because current is lost through the "leaky" plasma membrane, voltage declines with distance from the stimulus; the voltage is *decremental.*
Frequency of action potentals
Stimulus intensity is measured by number of impulses per second, or _____________, rather than by increases in the strength (amplitude) of the individual action potentials.
GRADED POTENTIAL summation
Stimulus responses can summate to increae amplitude of graded potential. Temporal (increased frequency of stimuli) Spatial (stimuli from multiple sources)
More often
Strong stimuli generate nerve impulses *more often or less often* in a given time interval than do weak stimuli?
Propogation of a nerve impulse
This means that the action potential is regenerated anew at each membrane patch, and every subsequent action potentail is identical to the one that was initially generated.
Function of an EPSP
To help trigger an action potential distally at the axon hillock of the postsynaptic neuron
PROPAGATING
Transmitting the action potentials
GRADED POTENTIAL amplitude
Various sizes (graded); decays with distance
ACTION POTENTIAL stimulus for ion channels
Voltage (depolarization, triggered by graded potential reaching threshold)
ACTION POTENTIAL repolarization
Voltage regulated; occurs when Sodium channels inactivate and Potassium channels open
The neuron cannot respond to another stimulus no matter how strong it is.
What happens when a patch o neuron membrane is generating an AP and its voltage-gated sodium channels are open?
THEY ARE CELLS WITH EXCITABLE MEMBRANES
What is special about neurons and muscle cells that they can generate action potentials?
RELATIVE REFRACTORY PERIOD
What is the interval following the absolute refractory period when most sodium channels have returned to their resting state and some potassium channels are still open, but repolarization is occurring.
POSTSYNAPTIC POTENTIAL
When the stimulus is a neurotransmitter released by another neuron, the graded potential is called this.
