Transcript Part 3 biosignals origin and measurement

Bio-signals
Origin of Bio-potentials
Bioelectric phenomena
Goals
Monitoring and Recording many forms of 
bioelectric phenomena
ECG (Electrocardiography)
EMG (Electromyography)
EEG (Electroencephalography)
ENG (electroneurography)
Bio-potentials
Certain systems
of the body create their own 
"monitoring" signals, which convey useful
information regarding the functions they represent.
These signals are the Bio-potentials “BP” 
associated with the conduction along the sensory
and motor nervous system, muscular contractions,
brain activity, heart contractions, etc.
These potentials are a result of the electrochemical 
activity occurring in certain classes of cells within
the body  Excitable Cells.
Measurements of these Bio-potentials can provide 
clinicians with invaluable diagnostic information
Cell Membrane Potentials
Cell membranes in general, and membranes of nerve cells 
in particular, maintain a small voltage or "potential" across
the membrane in its normal or resting state.
In the rest state, the inside of the nerve cell membrane is 
negative with respect to the outside (typically about -70
millivolts).
The voltage arises from differences in concentration of 
the electrolyte ions K+ and Na+.
There is a process which utilizes ATP to pump out three Na+ 
ions and pump in two K+ ions. The collective action of these
mechanisms leaves the interior of the membrane about -70
mV with respect to the outside.
If the equilibrium of the nerve cell is disturbed by the arrival 
of a suitable stimulus  dynamic changes in the
membrane potential in response to the stimulus is called an
Action Potential.
After the action potential the mechanisms described above 
bring the cell membrane back to its resting state.
Excitable
Excitable cells
are a class Cells
of cells that produce 
bioelectric potentials as a result of electrochemical
activity.
At any given time, these cells can exist in one of 
two states, resting and active.
Chemical and electrical stimuli can force an 
excitable cell from the resting to the active state.
While there are numerous ionic species present 
both inside and outside the cell, only three ions (for
which the cell membrane in its resting state is
permeable) play a key role in the behavior of these
cells: K+, Na+ and Cl-.
Active
State
If adequately stimulated, either electrically or 
chemically, the excitable cell will enter into the
active state.
The transmembrane potential varies with time and 
position within the cell in this state, and is called an
action potential.
The following sequence of events occurs when the 
cell enters the active state:
The chemical or electrical stimuli increases the 
permeability of the membrane to Na.
Na rushes into the cell due to the large concentration 
gradient.
Active
State
(cont.)
These positively
charged
ions entering
the cell cause 
the transmembrane potential to become less negative,
and eventually slightly positive. This change is often
referred to as a depolarization.
A short time ( tenths of microseconds) later the 
membranes permeability to K increases, which results in
an outflow of K.
The outflow of K causes the transmembrane potential to 
decrease. This decrease in potential causes the
membranes permeability to both Na, and eventually K,
to decrease to their resting levels
There is only a relatively small (immeasurable) net flow 
of ions across the membrane during an action potential.
The Na-K pump restores the concentrations (pumps Na
out and K in) of the ions to their resting levels.
The result of the transition from the
resting to the active state is the Action
Potential
In response to the appropriate stimulus, the cell 
membrane of a nerve cell goes through a sequence
of
depolarization from its rest state to the active state 
followed by
Repolarization to the rest state once again. 
The cell membrane actually reverses its normal 
polarity for a brief period before reestablishing the
rest potential.
The action potential sequence is essential for neural 
communication. The simplest action in response to
thought requires many such action potentials for its
communication and performance
The different phases a cell
membrane
The process involves several
A stimulus is received by the dendrites of a nerve cell. This
causes the Na+ channels tosteps:
open. If the opening is sufficient to
drive the interior potential from -70 mV up to -55 mV, the
process continues.
Having reached the action threshold, more Na+ channels
(sometimes called voltage-gated channels) open  The Na+
influx drives the interior of the cell membrane up to about +30
mV. The process to this point is called DEPOLARIZATION.
The Na+ channels close and the K+ channels open. Having
both Na+ and K+ channels open at the same time would drive
the system toward neutrality and prevent the creation of the
action potential.
With the K+ channels open, the membrane begins to
REPOLARIZE back toward its rest potential.
The repolarization typically overshoots the rest potential to
about -90 mV. This is called hyperpolarization.
Hyperpolarization prevents the neuron from receiving another
stimulus during this time.
After hyperpolarization, the Na+/K+ pumps eventually bring the
membrane back to its resting state of -70 mV .
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Absolute & Relative Refractory Period
ARP & RRP
During the initial portion of the Action potential  
membrane does not respond  Absolute
refractory period
During the Relative Refractory Period “RRP” the 
action potential takes action
The refractory period limits the frequency of a 
repetitive excitation procedure
e.g. ARP=1ms
→ upper limit of repetitive discharge 
< 1000 impulses/s
Absolute & Relative Refractory Period
ARP & RRP (cont.)
Nernst equil. Pot for Na
v: action pot.
Nernst equil. Pot for K
How the action is recorded?
The tip is
moved to until
the resting pot.
is recorded
A short time
later an
electrical
stimulus is
delivered for
the period L
until recording
Bioelectric Signal Measurement
Bioelectric measurements