Medical Physics Tutorial 4 - Nerve Impulses and the Heart


Electrical Signals in the Body

Nerve Cells
The Heart

The Electrocardiogram


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.


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, 


The depolarisation is called an action potential.



Nerve Cells

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:


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:



The Heart

The heart is a muscular bag that consists of four chambers:


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.  


Question 1

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.

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

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.


Question 2

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:


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:


The ECG can be used to diagnose problems with the heart which include:


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.


Question 3

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...