Neuron Physiology and Spinal Reflexes

Transmembrane Potential

Resting Potential - at this transmembrane potential an equilibrium exists between the electrochemical forces and the sodium-potassium exchange pump

Membrane Channels

Passive Channels - always open and permit leakage of ions

Gated Channels - open or close in response to specific stimuli

Chemically Regulated Channels - open or close in response to binding to specific extracellular chemicals

Voltage Regulated Channels - open or close in response to changes in the transmembrane potential


Generation of an action potential

An action potential occurs in response to a graded potential that leads to a threshold depolarization of the cell membrane, about -60 mV to -55 mV in an axon

activation of voltage regulated sodium channels

the threshold depolarization opens the sodium channels permits sodium ions to enter the cytoplasm, decreasing the transmembrane potential to about +30 mV

sodium channel inactivation

the decrease in transmembrane potential closes the sodium channels

activation of voltage regulated potassium channel

the decease in transmembrane potential opens the potassium channels and permits potassium to leave the cytoplasm, increasing the transmembrane potential toward -70 mV

return to normal permeability

repolarization of the membrane returns the sodium channels to their normal state (closed and capable of opening) and closes the potassium channels, leading to a brief hyperpolarization


Withdrawal reflexes (flexor reflex)

Lead to withdrawal of limb from a source of stimulation

Detection of stimulation by sensory neurons

triggered most prominently by painful stimuli

Stimulation of interneurons by the sensory neurons

excitatory interneurons and inhibitory neurons are stimulated by the sensory neurons

Excitation and Inhibition of motor neurons

flexor neurons are stimulated by the excitatory interneurons, leading to contraction of the flexor muscles (agonists)

extensor neurons are inhibited by the inhibitory interneurons, leading to relaxation of the extensor muscles (antagonists)


David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006