Neurophysiology is the study of how nerve cells (neurones) receive and transmit information. Two types of phenomena are involved in processing nerve signals: electrical and chemical. Electrical events propagate a signal within a neurone, and chemical processes transmit the signal from one neurone to another neurone, or to a muscle cell.
A neurone is a long cell that has a thick central body containing the nucleus. It also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites receive impulses form other neurones (the main exceptions are sensory neurones, such as those that transmit information about temperature or touch, in which the signal is generated by specialised receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the end of the axon the signal is chemically transmitted to an adjacent neurone or muscle cell.
Live neurones are normally polarised at rest, ie, they have a negative electrical charge inside the cell membrane. This is due to the free movement of positively- charged potassium ions through the cell membrane and, at the same time, the retention of large, negatively-charged molecules within the cell. Positively-charged sodium ions are kept outside the cell by an active metabolic process. All cells have this charge difference, but when a suitable stimulus is applied to a nerve cell, a unique sequence of events occurs. First, potassium ions flow into the cell, reducing the negative charge (depolarisation). At a certain point (as a result of this) the properties of the membrane change and the cell becomes permeable to sodium, which rapidly enters the cell and causes a net positive charge inside the neurone. This is called the action potential.
Once this potential is reached at one area in the neurone, it travels down the axon by ion exchange at specific points, called Nodes of Ranvier. The size of the action- potential is self-limiting, because a high internal sodium concentration causes the pumping out first of potassium and then of sodium ions, restoring the negative charge inside the cell membrane, ie, the neurone is repolarised. This whole process takes less than one-thousandth of a second. After a very brief period, called the refractory period, the neurone can repeat this process.
When the above electrical signal reaches the end of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neuro-transmitters, which are released into the sub-microscopic space between neurones (the synaptic cleft). These neuro-transmitters attach to specialised receptors on the surface of the adjacent neurone. This stimulus causes the adjacent cell to depolarise and propagate an action potential of its own. The duration of the stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and their re-uptake by the neurone that produced them.
All of the above neural processes result in electrical signals being generated. Although the voltages are typically small (they are often at only a micro-volt level), it is possible with neurophysiology techniques to study brain, peripheral-nerve and muscle disorders by recording and analysing these electrical signals. This is achieved by using either electrodes on the surface of the skin or by inserting a fine needle into selected muscles. The resulting studies are called electroencephalograms, nerve conduction studies and electromyograms, all of which are summarised below.
Common Neurophysiological Investigations
Nerve Conduction Study (NCS)
A nerve conduction study is used for the diagnosis of a variety of peripheral-nerve problems. This study can give an overall impression of nerve function, but it can also be used for diagnosing localised nerve-entrapments. Nerve Conduction Studies are typically performed by using an externally-generated electrical pulse to cause a nearby nerve to discharge, resulting in propagation of a signal proximally or distally along the nerve in question. These signals are then recorded by skin or muscle electrodes which are placed further along the nerve being tested. Nerve conduction studies are typically performed for symptoms such as tingling and numbness, weakness and muscle wasting. This type of testing is often performed in conjunction with EMG studies.
Electromyography (EMG) Studies
EMG studies are used to test the nerve supply to a muscle or group of muscles. Fine needles are inserted into the muscle being tested and the patient is asked to contract and relax that muscle. This type of testing is used in the diagnosis of nerve-root compressions, myopathies, and acute and chronic denervation following injury or disease. Typical symptoms requiring EMG studies are muscle-weakness and wasting, abnormal muscle twitches, and abnormal movements. This type of testing is often performed in conjunction with nerve conduction studies.