Action Potential: Electricity within a cell

The Action Potential is the elctrical signal that accompanies the mechanical contraction of a cell when stimulated by an electrical current.(neural or external). It is caused by the flow of sodium cation, pottasium cation, chloride anion and other ions accross the cell membrane. The action potential is the basic component of all bioelectrical signals. It provides information on the nature of physiological activity at the single cell level. Recording of an Action Potential requires the isolation of a single cell, and microelectrodes with tips of the order of a few micrometers to stimulate the cell and record the response.
Resting Potential:  Nerve and Muscle cells are encased in a semipermeable membrane that permits selected substances to pass through while others are kept out. Body fluids surrounding cells are conductive solutions containing charged atoms known as ions. In their resting state, membranes of excitable state readily permit the entry of K+ and Cl- ions, but effectively block the entry of Na+ ions (the permeability for K+ is 50-100 times that of Na+). Various ions seek to establish a balance between the inside and the outside of a cell according to charge and concentration. The inability of Na+ to penetrate a cell membrane results in the following:

• Na+ concentration inside the cell is far less than the outside.
• The outside of the cell is more positive than the inside.
• To balance the charge, additional K+ ions enter the cell, causing higher K+ concentration inside the cell than outside.
• Charge balance cannot be reached due to differences in membranes permeability for the various ions
• A state of equillibrium is established with a potential difference, with the inside of the cell being negative with respect to the outside.

A cell in its resting state is said to be polarized. Most cells maintains a resting potential of the order of -60 to -100mV until some disturbance or stimulus upsets the equilibrium.

Depolarization: Whwn a cell is exicted by ionic currents or an external stimulus, the membrane changes its characteristics and begin to allow Na+ ions constitutes an ionic current, which further reduces the membrane barrier to Na+ ions. This leads to an avalanche effect: N+ ions rush into the cell. K+ ions try to leave the cell as they were in higher concentration inside the cell in the preceding resulting state, but cannot move as fast as the Na+ ions. The net result is that the inside of the cell becomes positive with respect to the outside due to an imbalance of K+ ions. A new state of equillibrium is reached after the rush of Na+ ions stops. The change represents the beginning  of the action potential, with a peak value of about 20+ mV for most of the cells. An excited cell displaying an action potential is said to be depolarized,; the process is called Depolarization.

Repolarization: After a certain period of being in the depolarized state the cell becomes polarized again and  returns to its potential via a process known as repolarization. Repolarization occurs by processes that are analogous to those the  of depolarization, except that instead of Na+ ions, the principal ion involved for repolarization are K+ ions. Membrane depolarization , while increasing the permeability for Na+ ions, also increases the permeability of the membrane for K+   ions via a specific class of ion channels known as voltage dependent K+ channels. Although this may appear paradoxial at first glance, the key to mechanism for repolarization lies in the time dependence and voltage dependence of the membrane permeability changes for K+ ions compared with that for Na+ ions. The permeability changes for K+ during depolarization occurs considerably more slowly than those for NA+ ions, hence the initial depolarization is caused by an inrush of Na+ ions. However, the membrane permeability changes for Na+ spontaneously decrease near the peak of depolarization, whereas those for K+ions are beginning to increase. Hence, during repolarization, the predominant membrane permeability is for K+ ions. Because K+ concentration is much higher  inside the cell than outside there is a net efflux of K+ from the cell, which makes the inside more negative thereby effecting repolarization back to the resulting potential.
It should be noted that the voltage-dependent K+ permeability change is due to a distinctly different class of ion channels than those that are responsible for setting the resting potential. A mechanism known as Na+-K+ pump extrudes Na+ ions in exchange for transporting K+ back into cell. However, this transport mechanism carry very little current in comparison with ion channels, and therefore makes a minor contribution the repolarization process. The Na+-K+ pump is essential for resetting the Na+-K+ balance of the cell, but the process occurs on a longer scale than the duration of an action potential.
Nerve and muscle cells repolarize rapidly, with an action potential duration of about one ms. Heart muscle repolarize slowly with an action potention of duration of 150-300 ms.
The action potential is always same for a given cell, regardless of the methods of excitation or the intensity of stimulus beyond a threshold: this is known as the all-or-none or all-for-nothing phenomenon. After an action potential, there is a period which a cell cannot respond to any new stimulus, known as refractory period(about 1 ms in nerve cells). This is followed by a relative refractory period(sevral ms in nerve cells), when another action potential may be triggered by a much stronger stimulus than in the normal situation.

Propagation of an action potential: An action potential propagates along a muscle fiber or anunmyelinated nerve fiber as follows : Once initiated by a stimulus, the action potential propagates along the whole length of the fibre without decresein amplitude by progressive depolarization of membrane. Current flows from a depolarized region through the intra-cellular fluid to adjacent inactive regions, thereby depolarizing them. Current also flows through the extra-cellular fluids, through the depolarized membrane, and back into intracellular space, completeing the local circuit. The energy required in conduction is supplied by the fiber itself.

BioMedical Signals: A Sparkful Electrical Introduction

Are We Really Robots???

You know the most common thing between robots and Humans are, they both run on electricity. Robot requires it to run due its motors while we require to run it our organs. You know our brain consumes so much electric current a day that it can be used to light a low watt bulb. Your brain contains about 100 billion microscopic cells, the count so large that it would take thousands of years to count, they are called neurons. Whenever you dream, laugh, think, see, or move, it’s electrical signals(some other things like chemicals) that are racing between these neurons along billions of tiny neuron highways. Believe it or not, the activity in your brain never stops. Countless messages zip around inside it every second like a supercharged pinball machine. Your neurons create and send more messages than all the phones in the entire world. Even our heart it requires electric charge to pump. Our muscles even require electric Signals to move. So what this electric signals are really are? Can we measure them? Can it tells anything about our body.  Lets make a list of what kinds of Electrical Signals are flowing through our body:

1. Electroneurograms aka ENG
2. Electromyogram aka EMG
3. Electroencephalogram aka EEG
4. Electrocardiogram aka ECG
5. Electrogastrogram aka EGG
6. Event Related Potential aka ERP

These are not only the signals that flows through our body. There are many more signals like Phonocardiogram(PCG) also known as heart sound, Photoplethysmogram (PPG), Vibromyogram (VMG), Vibroarthogram (VAG) and lots more. I am not going to discuss it here as they are not electric in nature and upon discussing it would violate the subject of the topic. We will have a seprate Post regarding it. So, Lets have a deeper look to the electric current flowing through our body.

1. Electroneurogram aka ENG

Firing of Electrical Signals in Neurons

 

Electroneurogram are electrical activity in the neurons throughout the body and may be observed as a stimulus and associated nerve potential throughout the nerve. Conduction velocity also known as velocity of propagation of stimulus or Action potential is measured using ENG by placing concentric needle electrodes or silver-silver chloride electrodes at the surface of a body. ENG are susceptible to power line interference and instrumentation noise  and have amplitude of order 10 microVolts generally.

An electroneurogram is usually obtained by placing an electrode in the neural tissue. By stimulating a motor nerve at a known distant and mesuring the related activity one can get the conduction velocity in a peripheral nerve. The stimulus must be short and strong in nature. Output of the electrodes are transmitted to an acquisition system used to visualize the signal. The activity of a single neuron or the general activity both can be measured and is dependent on the precision of the electrodes. 

As per the researchers the propagation rate or nerve conduction velocity are 45-50m/s in nerve fibre, 0.2-0.4 m/s in heart muscle and 0.03-0.05m/s in fibers between artia and ventricles. And if the conduction velocity is found to be less than specified, the individual may be suffering from some kind of neural disease.

2.  Electromyogram aka EMG

 

 Electromyogram (EMG) are the electrical activity produced by skeletal muscle as a response for a single stimuli. There is something known as motor-unit (collection of a motor neuron, its axon and all muscle fibres innervated by that axon.) in our skeletal muscle which can be activated by volitional effort and when stimulated by a neuron, it contracts leading to a electrical signal which is equivalent to sum of  action potential of all it's constituent cells called single motor unit action potential or simply SMUAP. Shape of the SMUAP is dependent upon the type of electrode needle used, the electrical field activity onto the electrodes and varies with respect to position of the active motor unit.
Motor units are activated at different frequencies and varing time causing an asynchronous contraction.  The twitches of individual  motor units sum and fuseto form tetanic contraction and increased force. Weak volitional effort causes firing of 5-15pps(pulses per second). As the tension is developed an interference pattern EMG is obtained in which the constituent active motor units fires at a rate of 25-50pps. We can simply understand it as Spatio-temporal summation of the MUAPs of all active motor units give rise to EMG of the muscle.
Surface electrodes are used to record the complex EMG signals including interference patterns of several MUAP trains and are difficult to analyze. Simply, the EMG tells the level of activity in the muscle and can be used to diagnose neuromuscular diseases such as neuropathy and myopathy.

 

3. Electroencephalogram aka EEG

 

Electroencephalography (EEG) is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain. In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from multiple electrodes placed on the scalp. Diagnostic applications generally focus on the spectral content of EEG, that is, the type of neural oscillations that can be observed in EEG signals.
EEG is most often used to diagnose epilepsy, which causes obvious abnormalities in EEG readings. It is also used to diagnose sleep disorders, coma, encephalopathies, and brain death. EEG used to be a first-line method of diagnosis for tumors, stroke and other focal brain disorders but this use has decreased with the advent of high-resolution anatomical imaging techniques such as MRI and CT. Despite limited spatial resolution, EEG continues to be a valuable tool for research and diagnosis, especially when millisecond-range temporal resolution (not possible with CT or MRI) is required.
Derivatives of the EEG technique include evoked potentials (EP), which involves averaging the EEG activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). Event-related potentials (ERPs) refer to averaged EEG responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research.

 

 4. Electrocardiogram aka ECG

Electrocardiography (ECG or EKG from Greek: kardia, meaning heart) is the recording of the electrical activity of the heart. Traditionally this is in the form of a transthoracic (across the thorax or chest) interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the surface of the skin and recorded or displayed by a device external to the body. The recording produced by this noninvasive procedure is termed an electrocardiogram (also ECG or EKG). It is possible to record ECGs invasively using an implantable loop recorder.
An ECG is used to measure the heart’s electrical conduction system.  It picks up electrical impulses generated by the polarization and depolarization of cardiac tissue and translates into a waveform. The waveform is then used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker.
Most ECGs are performed for diagnostic or research purposes on human hearts, but may also be performed on animals, usually for diagnosis of heart abnormalities or research.

 

 5. Electrogastrogram aka EGG

 

An electrogastrogram (EGG) is a graphic produced by an electrogastrograph, which records the electrical signals that travel through the stomach muscles and control the muscles' contractions. An electrogastroenterogram (or gastroenterogram) is a similar procedure, which writes down electric signals not only from the stomach, but also from intestines.
These names are made of different parts: electro, because it is related to electrical activity, gastro, Greek for stomach, entero, Greek for intestines, gram, a Greek root meaning "to write".
An electrogastrogram and a gastroenterogram are similar in principle to an electrocardiogram (ECG) in that sensors on the skin detect electrical signals indicative of muscular activity within. Where the electrocardiogram detects muscular activity in various regions of the heart, the electrogastrogram detects the wave-like contractions of the stomach (peristalsis).

 

 6. Event related Potential aka ERP

Whenever we touch, feel, see or do anything our brain acts differently so the electric impulses in it.
An Event-Related Potential (ERP) is a subset of the EEG and which is specifically the brain’s responses to a sequence of external stimuli which may be specific to a specific sensory, cognitive, or motor event.
ERPs are extracted from the brain’s background electrical activity by recording the EEGs during several stimulus-response repetitions.  The EEG waveforms from each repetition are then time-locked to the stimulus and averaged.  The portion of the EEG which is not time-locked to the stimulus averages to zero.  What is left after averaging is the ERP, the part of the EEG time-locked to the stimulus. By proper selection of the stimulus, sequence, timing, etc., often called an ERP paradigm, various cognitive domains can be targeted for investigation. Due to the high precision of ERP recording (milliseconds in the time domain and microvolts in the amplitude domain), many aspects of fundamental sensory and cognitive processing can be evaluated in detail.
ERPs are used in basic cognitive research, as diagnostic biomarkers of neurological disorders, and as measures of drug effect in therapeutic trials.