Conduction Of Action Potentials
The action potential described in Chapter 5 is a local event that can occur in all excitable cells. This local event is an all-or-nothing response, leading to abolishion and then reversal of the polarity from negative (−70 mV) to positive (+40 mV) on the inside of the cell with respect to the outside for a short time during the course of the action potential.
Local currents are set up around the action potential because the positive charges from the membrane ahead of the action potential are drawn towards the area of negativity surrounding the action potential (current sink). This decreases the polarity of the membrane ahead of the action potential.
This electronic depolarization initiates a local response that causes the opening of the voltage-gated ion channels (Na+ followed by K+); when the threshold for firing of the action potential is reached, it propagates the action potential and this, in turn, leads to the local depolarization of the next area, and so on. Once initiated, an action potential does not depolarize the area behind it sufficiently to initiate another action potential because the area is refractory (Chapter 5).
This successive depolarization moves along each segment of an unmyelinated nerve until it reaches the end. It is all-or-nothing and does not decrease in size (Fig. 6a).
Conduction in myelinated axons depends on a similar pattern of current flows. However, because myelin is an insulator and because the membrane below it cannot be depolarized, the only areas of the myelinated axon that can be depolarized are those that are devoid of any myelin, i.e. at the nodes of Ranvier. The depolarization jumps from one node to another and is called saltatory, from the Latin saltare (to jump) (Fig. 6b). Saltatory conduction is rapid and can be up to 50 times faster than in the fastest unmyelinated fibres.
Saltatory conduction not only increases the velocity of impulse transmission by causing the depolarization process to jump from one node to the next, but also conserves energy for the axon because depolarization only occurs at the nodes and not along the whole length of the nerve fibre, as in unmyelinated fibres. This leads to up to 100 times less movement of ions than would otherwise be necessary, therefore conserving the energy required to re-establish the Na+ and K+ concentration differences across the membranes following a series of action potentials being propagated along the fibre.
All nerve fibres are capable of conducting impulses in either direction if stimulated in the middle of their axon; however, normally they conduct impulses in one direction only (orthodromically), from either the receptor to the axon terminal or from the synaptic junction to the axon terminal. Antidromic conduction does not normally occur.
Fibre diameters and conduction velocities Some information needs to be transmitted to and from the central nervous system very rapidly, whereas other information does not. Nerve fibres are able to cover both of these extremes and any in between by virtue of their size, and therefore conduction velocity, and whether or not they are myelinated. Nerve fibres come in all sizes, from 0.5 to 20 μm in diameter, with the smallest diameter unmyelinated fibres being the slowest conducting and the largest myelinated fibres the fastest conducting.
Classification of nerve fibres
Unfortunately, there are two classifications of nerve fibres. One, originally described by Erlanger and Gasser, and often referred to as the general classification, uses the letters A, B and C, with A further subdivided into α, β, γ and δ. The second, originally described by Lloyd and Hunt, and often referred to as the sensory or afferent clas- sification, uses the Roman numerals I, II, III and IV, with I further subdivided into A and B. The groups are subdivided differently in the two classifications and so, unfortunately, it is not possible to rely on only one of the classifications for the description of nerve fibres. The fibres of groups A and B and also of groups I, II and III are all myelinated, and those of group C and IV are unmyelinated. These classifications, conduction velocities, fibre diameters and examples of their functions are shown in Figure 6c. A word of caution is necessary concerning the average conduction velocities of the larger myelinated fibres: in reality, although there may be a few larger diameter fibres in the human body that do indeed conduct impulses as fast as 120/ms, a more common observation is that the fastest proprioceptive (sensory) fibres conduct at below 100 m/s, with the average being closer to 80 m/s. The same applies to the α-motor neurones in that the conduction velocities rarely exceed 90 m/s, with the average being closer to 60 m/s.
Compound action potentials
Peripheral nerves in most animals comprise a number of axons bound together by a fibrous tissue called the epineurium. When extracellular recording electrodes are placed close to a peripheral nerve, the recorded voltage signal, when an action potential is initiated in the bundle, is much smaller (microvolts) than that recorded by an electrode inserted directly into the axon (millivolts). The extracellular recorded signal is made up of the electrical events occurring in all of the active fibres within the nerve bundle. If all the nerve fibres in a nerve bundle are synchronously stimulated at one end of a nerve, and recording electrodes are placed at a number of locations along the length of the bundle, a compound action potential is recorded at each electrode. The waveform recorded from each of the electrodes will differ due to the different conduction velocities of each group of fibres that makes up the bundle. Theoretically, if the nerve bundle were to contain examples of all classifications of nerve fibres (i.e. Aα, Aβ, Aγ, Aδ, B and C), the recorded compound action potential would be seen as a multi-peaked display, as the action potentials in the fastest conducting fibres (Aα) would reach the electrode before those in the slowest conducting fibres (C). Action potentials in the fibres with conduction velocities between these two extremes would arrive between these two times (Fig. 6d).