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 Cells Excitable cells are a class 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 steps: A stimulus is received by the dendrites of a nerve cell. This causes the Na+ channels to 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 .

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