Basis Of Neural Control Flashcards
Communication
Occurs via electrical signals, through neurons
Dendrites to cell body to axon
Electrical signals
Action potentials
Receptor potentials and postsynaptic potentials
Action potentials
Or none
Initiated by summation of inputs
When depolarization is above threshold
Graded potentials
Sum of currents flowing
Sum to reach action potential threshold
Amplitude and duration of the stimulating current are important in reaching threshold
Chronaxie
Clinical ex of muscle disease
Used to ID appropriate current for chronic electrical stimulation to sustain muscle contraction and depolarization
Electrical stimulation can detect denervation associated changes
AP threshold
Rate of depolarization increases rapidly
Opening and closing of voltage sensitive channels in the cell membrane
Voltage sensitive Na channels
Action potential
Resting state, Na out and K in
Threshold which is Na coming in, K channels closed
Depolarization, more Na channels open
Repolarization, Na gates inactive and K active
Undershoot is only K gate active
Ion movement during the action potential
Na channels open and Na enters the cell once threshold
As reach depolarization K channels open and Na close
k continues to leave until after undershoot then excess K outside diffuses away restoring resting potential
Properties of an action potential
Na dependent depolarization
Blocked by specific Na channel blocker- TTX
AP overshoot
Inactivation/repolarization phase
Membrane permeability to Na is short lived
k permeability increases temporarily
AP
Na and K channel kinetics
Undershoot phase
After hyperpolarization
Increased K conductance which activates Na/K pump
Blocked by TEA
If no K conductance cannot hyperpolarization
Absolute refractory periods
Na channels that were active are inactivated after AP
Relative refractory periods
Early, some Na channels are still inactivated
Need a stronger than normal stimulus for AP to occur
K+ conductance elevated opposing depolarization of the membrane
Accommodation or depolarizing block
Higher threshold for activation
Reduced AP amplitude and fewer generated
Na channel inactivation and K conductance are longer lasting
Ca sensitivity
Increase in external Ca decreases excitability via threshold potential
Decrease in external Ca increases excitability
AP origination
At axon hillock
Nerve cells sum up signal from dendrites
Is depolarizing is greater than hyperpolarization than action potential
Propagation
Usually unidirectional precedes refractory zone
Flows passively, velocity, amplitude and duration are unchanged
Orthodromic
From cell body to the axon
AP artificially induced by electrical stimulation of axon
Antidromic AP
Propagates back to cell body, not refractory
AP conduction velocity
Dependent on axon membrane capacitance and internal resistance
Rate is inversely related to capacitance and axonal resistance
Reduced resistance increases velocity, resistance will decrease as axon size increases
Large diameter leads to increased velocity
Capacitance
Proportional to fiber diameter
Increases as axon diameter increases because membrane surface area increases
Increasing axon diameter decreases CR and velocity will increase
Problems with axon diameter
Metabolically expensive and nerves take up too much space
Myelin
Decreases C, conductance
Increases distance between charges
Saltatory conduction with myelin
Current from action potential at a node flows through myelinated axon
1 node to next is saltatory conduction
AP can move quickly across internodes to save metabolic energy
Myelinated vs. unmyelinated
Greater conductance velocity than unmyelinated axons 100 times greater in diameter
Decreased membrane capacitance which is more efficient
Increases membrane resistance which minimizes current loss across the membrane
Demyelination
Hinders signaling in NS and may lead to nerve death
In PNS demyelination diseases there are two modes of attack: primary and secondary
Primary demyelination
Mylin itself attacked and damaged or destroyed
Secondary demyelination
Violent inflammation phase which destroys axon itself
Nerve signal thus blocked and myelin degenerated
