Biomedical Signals Flashcards
What is resting potential?
In excitable cells like a neuron or muscle cell
- RP is the electrical potential difference (voltage) between the inside and outside of the cell.
- semipermeable membrane (=some ions can move in and out, some can not depending on the state of the cell and the voltage-gated ion channel.
- body fluids: conductive solutions containing ions
- RP ~ -60 mV to -100 mV
Describe the state of equilibrium of a cell.
In the resting state, the membrane of excitable cells
- permit entry of K+ and Cl-
- block Na+ ions
- equilibrium acc. to charge and concentration –> with a potential difference (RP)
- inside of the cell is negative (-) w.r.t the outside
- until some disturbance or stimulus upsets the equilibrium
What is an action potential?
- the basic component of all bioelectrical signals
- an electrical signal that accompanies the mechanical contraction of a single cell when stimulated by an electrical current (neural or external)
- Depolarization: Na+ influx, K+ outflux –>net (+), peak +20mV (most cells), +40mV (heart muscle cells)
- Repolarization: net efflux of K from the cell, inside more negative –> RP
- Nerve and muscle cells repolarize rapidly ~ 1ms
- Heart muscle cells repolarize slowly 150 ms - 300 ms
What is the absolute/relative refractory period?
The absolute refractory period is a period during which a cell cannot respond to any new stimulus after an action potential. (in nerve cell ~1ms)
This is followed by a relative refractory period, during which another AP may be triggered by a much stronger stimulus than a normal situation. (in nerve cells 3-5ms)
What is saltatory conduction?
The propagation of AP as a current flows by jumping from one node to the next node of Ranvier along myelinated nerve fibers.
What can ENG be used to measure?
the velocity of propagation (or conduction velocity) of a stimulus or AP in a nerve.
What is ENG?
Electroneurogram
the electrical signal observed as
- a stimulus
- and the associated nerve AP propagates over the length of a nerve.
How can we measure ENG?
- concentric needle electrodes
- or silver-silver-chloride electrodes (Ag-AgCl)
- at the surface of the body
How can peripheral nerve conduction velocity be measured?
- stimulating a motor nerve
- measuring the related activity at 2 points that are a known distance apart along its course.
- stimulus 100V, 100-300 microseconds
- ENG amplitude 10 microVolt
- amplifier: gain 2000, bandwidth 10 Hz - 10000 Hz
How much is the typical conduction velocity in nerve fibers?
45-70 m/s
Give an example of using ENG to measure conduction velocity.
- experimental limb: forearm
- recording sites: from the elbow to the wrist
- note: relaxed posture –> minimize muscle contraction and other undesired effects
- short stimulus ~ 100V, in 100-300 microseconds duration
- record the difference in the latencies of the ENGs –> conduction time
- repeat measurement 3 times (or more)
- the known distance between the recording sites
- conduction velocity (m/s) = distance / time
What is EMG?
- electromyography
- measures muscle response or electrical activity in response to a nerve´s stimulation of the muscle.
- recorded using needle electrodes or Ag/AgCl electrodes at the body surface
How does skeletal muscle work?
- Skeletal muscle fibers are twitch fibers.
- produce a mechanical twitch response for a single stimulus
- generate a propagated AP
What is a motor unit?
- motor unit = MU
- the smallest muscle unit that can be activated by volitional effort
- consisting of :
- an anterior horn cell ( or motoneuron, from a spinal cord cross-section)
- its axon
- all muscle fibers innervated by that axon
–> the constituent fibers of a MU are activated synchronously
–> extend lengthwise in loose bundles along the muscle
–>interspered with the fibers of other MUs
What is SMUAP?
- single motor unit action potential
- upon stimulation,
- each MU contracts –> causes an electrical signal
- summation of the APs of all its constituent fiber cells
=> mechanical output (contraction) = net result of stimulation and contraction of several of its MUs - normal SMUAPs usually have biphasis or triphasic waveform
How can we measure a single motor unit?
–> measure muscle fibers at the interested point correlating to an interested movement
What are the factors that can affect the shape of recorded SMUAP?
- needle electrode type used
- its positioning w.r.t the active MU
- the projection of the electrical field of the activity on the electrodes
e.g. SMUAP trains recorded simultaneously from 3 needle electrode channels at different locations –> 3 different MUs are active –> the same SMUAP shows different shapes due to the projection onto the 3 channel axes
What is the innervation ratio?
- number of muscle fibers per motor nerve fibers / MU
- e.g.
large muscle –>gross movement: 100s of fibers/MU
small muscle –>precise movement: fewer fibers /MU
+ platysma (neck) muscle: IR of 25
+ first dorsal interosseus (finger)muscle: IR of 340
+ medail gastrocnemius (leg) muscle: IR of 1934
Describe a physiological model for EMG signal generation.
- Each motoneuron from the spinal cord branches each axon to muscle fibers through nerve fibers.
- Each motor neuron fires and can be represented by an impulse train.
- Firing activates MU represented by system hi(t)
- MU generates a train of SMUAPs
- Net EMG is a sum of several SMUAP trains represented as mp(t,F)
–> the observed EMG is a function of time t and muscular force produced F –> m(t,F)
How is the shape of SMUAP affected by diseases?
- higher rate (at low-to-medium levels of effort)
- polyphasic
- large amplitude
How does neuropathy affect the shape of SMUAPs?
Neuropathy due to:
+ damage to nerves
+ slow conduction of fibers in 1 MU
+ desynchronized activation of fibers within 1 MU
+ polyphasic SMUAP within an amplitude larger than normal
=> the same MU may fire at higher rates than normal before more MUs are recruited
How does myopathy affect the shape of SMUAPs?
Myopathy due to:
+ loss of muscle fibers in MUs, neurons intact
+ patchy destruction of fibers (muscular dystrophy)
+ size of MU is reduced
+ asynchrony activation
+ splintering of SMUAPs occurs / polyphasic SMUAPs
=> more MUs than normal recruited at low levels of effort
What is Gradation (gradation of muscular force)?
- control of muscular contraction levels
- 2 neural mechanisms responsible
+ recruitment = spatial summation of activated MUs –> increasing Nr. of MUs activated/recruited –> increasing amount of force required during movement
+ rate coding = temporal summation of activated MUs —> higher frequencies of discharge (firing rate) of each MU with increasing effort –> greater tensions
During contraction, are the responsible MUs activated at the same times/frequencies)
No
- MUs activated at different times & frequencies = asynchronous contraction
- Timing:
+ twitches of individual MUs sum and fuse to form tetanic (sustained) contraction and increased force
- Rate/Frequency
+ weak volitional effort: 5-15 pps (pulses per second)
+ at greater tension: 25-50 pps
