S-D Curve & properties of excitable tissue

Nerve Physiology

• resting membrane potential

• the neuronal membrane creates & maintains concentration gradient of certain ions

• charge on inside is negative, outside is positive

  • produces electrical potential

• Na+ / K+ ATPase pump moves 2 K+ ions into cell, pumps 3 Na+ ions out

  • results in cell polarization

• Resting membrane potential

  • -70mV in nerves

  • -90mV in skeletal muscle

  • -85mV in cardiac myocytes

resting membrane potential

potential of a cell while at rest

Action potential

• stimulus is applied to a neuron

• Na+ channels open, allowing influx and rapid+ change in membrane potential = depolarization

• if depolarization > action potential threshold (-55 mV) produces full action potential spike (all or none)

• K+ ions flow out of cell to cause repolarization

• undershoot /hyperpolarization — K+ channels close

• Summation

Summation

subsequent action potentials needed

The Role of Myelin

• allows rapid transmission of electrical signals along the nerve

• what happens if myelin is damaged?

myelin

lipid rich insulator that surrounds nerve cell axons

Physiological response to electrical current

chemical, thermal, or physical

Electrochemical Effects

• heat is a product of energy exchange

• the amount of heat produced in biological tissues depend on:

  • amount of current flowing

  • resistance to the current

  • duration the current flows

• accumulation of heat at the skin

• redness should resolve within hours

• based on the stimulation, or depolarization of cells

• as current flows from the electrodes, the concentration of electrons at the cathode induces depolarization of the cell membrane

• this depolarizes the cell if the critical threshold is reached

• application of ES depolarizes the nerve first (not muscle fibers)

• then propagates the stimulus along the motor axon, across the neuromuscular junction, and across the sarcolemma

Anodes

attracts ACID

Cathode

attracts BASE

Clinical Application

the effectiveness of a current to target specific tissue is dependent on 3 factors:

1. adequate intensity to reach threshold (i.e. amplitude)

2. current onset fast enough to reduce accommodation (i.e. rise time)

3. Duration long enough to exceed capacitance of the tissue (i.e. phase duration)

Response of excitable tissue to stimulation

• for a stimulus to depolarize the cell and elicit an action potential, it must be of sufficient strength and duration

• there is a range of stimulus combinations that will work to depolarize the cell

• *plot these combinations as a strength-duration curve (S-D curve)

Strength Duration Curve

Rheobase:

• minimum strength (mA) that is capable of eliciting a minimally detectable motor response

Chronaxie:

• the duration (microseconds or msec) of stimulus 2x the rheobase strength capable of eliciting a minimally detectable motor response

*It is ALL or NOTHING*

Levels of Response

• sensory: excitation of A-beta fibers in the superficial dermis

  • pins and needles feeling

• Motor: excitation A-alpha fibers

  • magnitude of response proportional to the intensity of the stimulus

  • varies from small twitches to full contraction

• Noxious: excitation of the A-delta and C fibers

  • elicits the perception of pain

Henneman Size Principal

• describes progressive recruitment of alpha motor neuron cell bodies

  • slow, small motor units to fast, large motor units

• some theories suggest electrical stimulation may recruit motor units in the opposite order of what is expected (debated)

• estim recruits muscle fibers in a nonselective, spatially fixed, and temporally synchronous pattern