Electrical Signals in the Body
Electrical signals are carried about the body by nerve cells. All cells have a membrane potential because on the inside of the membrane there are more potassium ions, and on the outside there are more sodium ions. The potential is maintained by a biological mechanism called the sodium potassium pump.
Image from Wikimedia Commons. https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface
The membrane potential difference is about 70 mV, with the outside being regarded as being at 0 and the inside being at -70 mV. The negative ions are carried on large organic ions that cannot cross the cell membrane. When a cell membrane on a nerve cell is stimulated,
it suddenly becomes permeable to sodium ions which diffuse through, attracted by the negative charge..
The potential rises initially to 0 millivolts (depolarisation)
and then to +30 mV (reverse polarisation).
Then the membrane becomes impermeable to sodium ions and they are trapped within the nerve cell.
Potassium ions diffuse out of the membrane which restores the potential (repolarisation).
The whole process takes about 2 ms.
Then the potassium ions are pumped out, a process taking about 50 ms.
The depolarisation is called an action potential.
The action potential propagates along the cell membrane at a rate of about 100 m s-1. The nerve cell has a structure like this:
The measurement of electrical activity in the body can be done in two ways:
A concentric needle electrode which can be used to detect electrical activity in a very small area. This is invasive which means the body has to be penetrated. It can involve risk of infection.
Surface electrodes which are placed on the body. A good electrical contact is made with a saline (sodium chloride) gel. This is non-invasive, but cannot be used in very precise work.
Both types of electrode are connected to a high gain amplifier, and a pen recorder. The pen recorder makes a voltage time graph. Nowadays electrical activity can be logged into a computer.
Nerve cells make muscles contract, and the electrical behaviour of muscles can be followed in a similar way. The analysis of electrical signals is very useful for looking at:
Activity of the heart.
Brain activity, which can help the doctor to diagnose brain conditions such as epilepsy.
The heart is a muscular bag that consists of four chambers:
The right atrium which receives deoxygenated blood from the body
The right ventricle that pumps the deoxygenated blood to the lungs. The pulmonary artery is the only artery that carries deoxygenated blood.
The left atrium that receives oxygenated blood from the lungs. The pulmonary vein is the only vein that carries oxygenated blood.
The left ventricle that pumps blood to the body through the aorta, a very large artery. We can think of it like a manifold that distributes blood to arteries that convey the blood to the whole body.
Image from Wikimedia Commons. Author not stated
The heart is a double pump. The right hand side pumps blood at a low pressure to the lungs.
Why must the blood leaving the heart for the lungs be at a much lower pressure than the blood leaving the heart through the aorta?
The left side of the pump is thicker and provides a higher pressure to get the blood around the rest of body.
The heart is a muscular back controlled by nerve cells. It has beat in a coordinated way, otherwise it will end up just twitching and not pumping blood at all, which is not very good for the health of the patient. The regular pumping action is controlled by a special set of cells called the sino-atrial node located on the right atrium. It produces an electrical stimulus about 70 times a minute, but higher in times of exercise.
The atria contract, forcing blood into the ventricles through the valves.
Then the ventricles contract; the valves from the atria are closed and the valves into the arteries are opened.
Click HERE to see an animation of the heart. You will need Flash Player to be enabled.
The action potential is shown:
Note that the action potential for the heart is rather slower than the action potential in a nerve cell.
The electrocardiogram (ECG) allows doctors to look at the electrical behaviour of the heart. The conducting nature of body fluids transmits some of the electrical activity to the surface. The signals are much reduced in size, and have amplitudes of about 1 mV. To get a good ECG the patient must be relaxed.
Why does the patient need to be relaxed?
A typical ECG is shown:
The important features are referred to by the letters shown on the graph:
P-wave is due to the depolarisation and contraction of the atria;
The QRS-wave is due to the depolarisation and contraction of the ventricles;
The T wave is due to the re-polarisation and relaxation of the ventricles.
Note that the ECG is not the same as the heart's action potential. The whole process of a heart beat is set off using the action potential of the sino-atrial node. It is a group of cells on the right atrium of the heart. It is connected to the autonomic nervous system which detects increased demand for oxygen. Therefore the heart rate goes up within three to five seconds of starting exercise. The brain is not involved.
To obtain an ECG, electrodes are placed on the arms and legs as well as the chest:
The right leg is not used because it's furthest from the heart. The leads are named from a convention dating back to Einthoven who devised the first ECG machine in 1903. Nowadays the ECG is connected either to a chart recorder, or a computer. The general layout of the machine is shown.
The machine has these features:
A voltage gain of about 1000. Compared with other voltage gains, this does not seem that high. Any higher and spurious signals from other muscular activity will be picked up. The frequency range is not very high, 0 - 20 Hz. The patient can pick up mains hum at 50 Hz, which would interfere with the signals from the heart.
The frequency response must be even, so that the trace is not distorted.
A high input resistance is essential, otherwise the p.d. will be reduced because of the body resistance and a certain amount of capacitance. Contact resistance is reduced by using conducting gel between the skin and the electrodes. The circuit behaves rather like a potential divider.
High signal to noise ratio. Random signals can be given off in electronic circuits. It can be heard as "white noise" in an audio amplifier. These signals could mask small changes.
The ECG can be used to diagnose problems with the heart which include:
arythmia, an irregular pumping pattern which is quite common in young people, increased by exercise.
blockage of part of the blood supply for the heart. This can lead to the heart muscle getting tired, or in extreme cases death of the heart muscle, myocardial infarction (heart attack).
fibrillation in which there is no co-ordinated pumping activity, which will lead to death if not treated very quickly.
Defibrillation is a dramatic intervention. Paddles are placed over the heart and a brief massive electric shock is given, causing a major contraction of all the heart muscles. This is often gets the heart beating in a regular way again.
The operator using a defibrillator often shouts "stand clear" before operating the machine. Why?
The role of the sino-atrial node can be taken over by an artificial pacemaker, which produces about 70 beats a minute. This can give a heart patient a good quality of life.
When I was a young student, we did a practical in the physiology lab that involved hooking ourselves up to an ECG machine. My trace was sufficiently odd to get the lecturers to consult the professor (an expert cardiologist). A dead student in the lab would not give the university good publicity. However my heart did not pack in. The cardiologist suggested that it lies on its side, rather than upright. It has since reliably driven me across many kilometres of rowing races, and fell runs. And I don't intend to drop dead yet, despite the heartfelt wish of one or two of my students...