Neuro Lecture B3 Flashcards
Graded Potential
A local change of membrane potential, the amplitude of which increases with the amplitude of the stimulus
Length Constant
the distant over which a localized graded potential declines to 1/e (or 37%) of its original size in an axon or skeletal muscle fiber.
Threshold
A level of the membrane potential that will trigger an action potential
Impulse
Action Potential ( or spike)
Receptive ending of neurons respond electrically to what specific stimuli
Primary afferent (touch, sound, light) or other neurons (chemical messengers released by other cells)
Sensory receptors express????
receptor potentials
Other neurons express???
Postsynaptic potentials
stimulation of the sensory ending of an afferent neuron produces what kind of response?
A local depolarization (due to the receptors) producing a local response to that sensory ending. adjacent regions are decremental depolarized by the spreading of positive charges
Larger-diameter axons tend to have greater or shorter length constants?
greater
The length constant of a cellular cable increases or decreases with electrical resistance of the membrane?
Increases
The length constant of a cellular cable increases or decreases with electrical resistance of the cytoplasm?
decreases
Length constant?
distance over which a graded electrical potential decays in a cable like conductor (dendrite or axon) to 37% of its max amplitude
Are action potential considered graded or decremental?
no, action potentials are often described as all or none.
Where is the trigger zone (dense population of depolarization-sensitive sodium channels) in multipolar neurons?
The axon Hillock
Where is the impulse initiation zone in pseudo unipolar and bipolar afferent neurons?
sensory endings (hence why they are AFFERENT neurons)
What is the definition of threshold?
voltage required to open masses of channels
The rising phase of an action potential thus reflects what?
the membrane depolarizing towards Ena
The falling phase of the impulse represents what?
efflux of K ions down their electrochemical gradient eventually repolarizes the membrane
Voltage-gated Na, K and Ca channels form a superfamily of structurally similar channels. Each channel has four identical subunits clustered around a central pore. What does each subunit contain?
Six membrane spanning segments
A charged voltage sensor compartment
A pore loop that confers ion selectivity
Describe the ball and chain model of inactivation
Voltage sensitive part of the Na channel mediates its later inactivation during depolarization. So the positively charged ball moves into the pore during depolarization closing the Na channel
What is the absolute refractory period?
reflects the inactivation of virtually all Na channels
what is the relative refractory period?
a new impulse can be evoked by a larger stimulus, reflects the incomplete return of inactivated Na channels to a normal closed state plus residual K currents.
Where are Na channels found?
Axon initial segment (initiation zone), at nodes of Ranvier and presynaptic terminals.
K channels found?
paranodal and juxtaparanodal portions of both peripheral and central myelinated axons, in presynaptic terminals, as well as at axon initial segment.
Does myelinated or unmyelinated axons produce rapid conduction?
myelinated
In both unmyelinated and myelinated axons, action potentials propagate with or without decrement (reduction)?
without
In unmyelinated axons, the impulse in one region causes positive charge to flow passively within the cell, depolarizing the adjacent Na channel-rich region beyond threshold. The process recurs down the length of the axon to yield???
Continous conduction
In myelinated axons, positive charge entering each node of Ranvier during an impulse moves internally down the electrical gradient towards the next node. The impulse that jumps from one node to the next ensure rapid, ??
saltatory conduction
Myelinated or Unmyelinated axons conduct relatively slow?
Unmyelinated axons
What factors limit conduction velocity?
small diameters (limits intracellular current spread) and lack of sequential driving of all axonal segments.
Myelinated axons support impulses only at what?
Nodes with minimal time lost driving currents between successive nodes
what increases conduction velocity
larger length constant, large diameter axons (due to less resistance), increased intermodal distances
Uncharged lidocaine and procaine readily cross the membrane, subsequently gaining a charge and blocking what?
the voltage-gated Na channel, therefore a suppression of action potentials is seen
