Anaesthesia, 1976, Volume 3 1, pages 494-503 REVIEW ARTICLE
Classification of peripheral nerve fibres An historical perspective
J. G. WHITWAM
This review provides an historical perspective to the present system of classification of nerve fibres and makes suggestions for a more uniform method of description in the future. The composition of nerves was first studied by Remak' and subsequently by Sherrington' who observed that muscle nerves contained larger afferent fibres than cutaneous nerves. In the same year Westpha13 reported that the peripheral spinal nerves in the newborn infant were thinner and more difficult to stain than in the adult. Unmyelinated fibres were first described by Remak' and defined histologically by R a n ~ o nwho ~ ' ~modified the Cajal stain for this purpose. In order to describe, with clarity, the results of studies in clinical neurophysiology, and into chemical block of nerve conduction, reference has to be made to afferent and efferent nerve fibres of different sizes, conduction velocity and sensory modality. In the anaesthetic literature there is a tendency to use different terms for the same type of nerve fibre and this causes confusion. For example, the terms C and group IV are both used to describe unmyelinated fibres; a and group I are often used interchangeably to describe fibres of the highest velocity, whilst and group I1 describe fibres of medium conduction velocity. Confusion also arises because the term B refers to preganglionic sympathetic nerve fibres, whilst y refers to fibres innervating muscle spindles, even though both these groups of fibres are myelinated and may have similar conduction velocities, most of which lie in the range normally ascribed to 6 fibres. Anatomical and physiological considerations
Growth of nerve fibres in animals and man Nerves enlarge during foetal and post embryonal development of the animal and myelination of the axon takes place as the fibre enlarges above a critical diameter in the developing animal.6 A number of studies have shown that during maturation of the peripheral nervous J. G. Whitwarn, MB, ChB, PhD, MRCP, FFARCS, Reader in Clinical Anaesthesia, Department of Anaesthetics, Royal Postgraduate Medical School, D u a n e Road, London, W12 OHS.
494
Classification of peripheral nerve fibres
495
system there is a progressive increase in the conduction velocity and diameter of fibres in the peripheral nerves which is most marked in early life. For example in 4-day-old kittens the maximum velocity at which impulses are conducted in the saphenous nerve is 11 m/sec as compared with 80-90 m/sec in full grown cats,7 Skoglund* and Ekholm’ have confirmed these findings in the cat and have shown that maturation of nerve conduction in the kitten is complete 34-4 months after birth. The number of nodes of Ranvier stays constant throughout development and hence, as the nerve fibres lengthen during growth, not only does the myelin sheath increase in thickness but also the distance between the nodes of Ranvier
lengthen^.^.' O S 1 In man myelination in the foetus starts about the 4th month of intra-uterine life, first in the spinal motor roots and subsequently in the dorsal root^.''"^ M yelination of the peripheral nerves increases with age and starts p r ~ x i m a l l y ’ ~so ~ ’ that ~ the peripheral nerves may mature later than the spinal roots. For this reason nerves in the newborn have a relatively high threshold for stimulation and Holmes’6 found that the excitability of nerves fell to adult levels between 3 and 6 years. The work of Eccles & Sherrington” and Nystrom & Skoglund’’ suggests that from birth to adult life the fibre diameter in motor nerves (both large and small fibres) approximately doubles, and Rexed” concluded that the maturation of ventral roots was virtually complete between 2 and 5 years. In parallel with the increase in fibre size, the maximum conduction velocities in peripheral nerve fibres increase after birth, at first rapidly and then The subject more slowly, so that between 3 and 5 years adult values are a~hieved.’~-’~ of nerve development in the neonate has been reviewed relatively re~ently.’~*’~ It is interesting to speculate that the alert but ‘floppy’ neonate, sometimes seen after maternal epidural analgesia, may be the result of the effects of low concentrations of analgesic drug on the immature, poorly myelinated, peripheral nerve fibres of the newborn infant. In view of the work on the development of nerve fibres outlined above, the observations which follow apply only to mature adult mammalian nerve fibres. Inevitably, most of the investigations have been made on experimental animals. Size and myelination
In adult mammals the largest myelinated fibres have diameters up to 20 pm whereas those for unmyelinated fibres are in the range 0.3-1.35 pm. In fully grown animals, the ratio between the diameter of the axon and the outside diameter of the fibre is approximately 0.726for fibres above 8 pm diameter, but the ratio declines progressively for smaller fibres until a value of 0.55 is reached for nerves having diameters below 3 However myelinated fibres lose the myelin sheath at their terminals. Also in the autonomic nervous system, for example in the vagus nerves, it is not uncommon for fibres to be myelinated for one part of their course and to be virtually unmyelinated for another part.’* It can be appreciated that, for any individual nerve fibre, a particular characterisation in terms of size and myelination may only be expected to apply for part of the length of the nerve fibre. Conduction velocity
Myelination and the diameter of nerve fibres are both important in determining the
496
J. G . Whitwam
speed at which impulses are conducted. There is a linear relationship between the conduction velocity and both the fibre diameter7s27s29-33 and the thickness of the myelin sheath.29i34It has also been suggested that the conduction velocity is proportional to the internodal length7 but this has not been ~onfirmed.~’
Excitability Another property related to fibre size which is very important in experimental work is the threshold of electrical excitation. In general, the threshold is lowest in the largest fibres which also have the highest conduction v e l ~ c i t i e s ,and ~ ~ the threshold for electrical excitation of unmyelinated fibres has been shown to be more than 20 times that of the fastest myelinated fibres.29 These points are illustrated in Fig. 1, which shows stimulus intensity duration curves for the three principal groups of fibres in the radial nerve of the dog. Thus, while using stimuli of very low intensity, it is possible to activate selectively the fastest myelinated fibres, but a stimulus of sufficient intensity to
0
0.1
0.2
0.3
0.4
05
Stimulus duration (msec)
Fig. 1. Stimulus intensity duration curves at threshold for the compound potential of the three principal fibre groups in the lateral branch of the superficial branch of the radial nerve of the dog. 0-0 Group 11 fibres; x - x Group Ill fibres; 0-0 Group IV fibres. (Ordinate logarithmic scale.)
activate the smallest, most slowly conducting fibres will also stimulate the larger fibres in the nerve. However, Gasser37 has suggested that there is a group of myelinated fibres, the ‘delta 3’ group with conduction velocities between 5 and 8 m/sec which have a higher threshold to electrical excitation than the most excitable unmyelinated fibres. These fibres cannot therefore be identified solely on the basis of electrical excitability. Koll ef al.’* have also defined a group of fibres with conduction velocities between 4 and 10 m/sec whose properties are different to other small myelinated fibres and which they referred to as the ‘post 6 group’. Whereas there is a close correlation between conduction velocity and electrical excitability in myelinated fibres, no such correlation occurs within the unmyelinated fibre population. At threshold, electrical stimuli excite not only the most rapidly conducting unmyelinated fibres but also those belonging to the more slowly conducting subgroups.39 Another property related to fibre size is the absolute refractory period which is longer in smaller
Classijication of peripheral nerve fibres
497
Historical view of the classification of mature nerve fibres The classification of nerve fibres by conduction velocity has been based either on studies of the evoked compound action potentials in peripheral nerves or observations of the conduction velocities of single fibres. Classification by evoked compound action potentials
In 1924, Erlanger, Gasser & Bishop” described the compound nature of the con~’ peripheral ducted potential in various nerves and Erlanger & G a ~ s e r ~ ”classified nerve fibres into three main groups. Group A comprised myelinated somatic afferent and efferent fibres. Group B consisted of preganglionic autonomic fibres, and group C were non-myelinated somatic afferent fibres and post ganglionic autonomic fibres. From observations of the waves in the compound action potential in a large mixed peripheral nerve Erlanger & further subdivided group A into c[, /3, y and 6 fibres. In 1930 Eccles & Sherrington” constructed histograms of the fibre sizes in the nerves to several muscles and showed that there were distinct groups of large and small motor nerve fibres. In 1945 Lekse114’ showed that these motor fibres contributed to the tl and y elevations in the compound action potential. More recently Boyd and his associates43 have further subdivided y efferent fibres into two groups, a fast relatively heavily myelinated group and slow thinly myelinated fibres which have relatively slow conduction velocities in the ranges of 6 and group I11 fibres (see below). Classijication of afferent fibres
In 1943 Lloyd44introduced a new terminology to refer to aferentfibres in both muscle and cutaneous nerves. As originally proposed this classification was based entirely on fibre size. He found that myelinated afferent fibres fell into 3 broad groups with diameters in the ranges 12-20 pm, 6-12 pm and 1-6 pm for which he used the terms group I, I1 and I11 respectively. Unmyelinated fibres he referred to as group IV. In 1948 Lloyd & Chang4’ published histograms of afferent fibres from a number of nerves and showed that most nerves contain all three myelinated groups of fibres as relatively distinct populations. In the original classification of Lloyd44 afferent fibres from muscle spindles were referred to as group Ia, and those from tendon organs as s~~ that the group I compound action group Ib. In 1953 Bradley & E c ~ l e suggested potential had fast and slow components produced by fibres of groups Ia and Ib respectively. More recently Jack & MacLennan4’ have challenged this observation and have shown that the conduction velocities of afferent fibres from muscle spindles and tendons show considerable overlap and that both groups are represented throughout the whole range of conduction velocities. Some of the observed conduction * velocities of afferent fibres from tendon organs can be as low as 62.5 r n / ~ e c . ~Therefore the division of these fibres into groups Ia and Ib cannot be based on conduction velocity but is an anatomical classification depending on their source. Classification of 1965
At the first Nobel Symposium on ‘Muscle afferents and motor control’ in Stockholm in 196549it was suggested that the terms a, p, y and 6 should be confined to groups of
498
J. G. Whitwam
myelinated efferent fibres. The terminology I, 11,111 and IV should refer to groups of afferent fibres whether in muscle, cutaneous or autonomic nerves. If these suggestions are accepted the conduction velocities of efferent myelinated fibres would be as follows: a 70-120 m/sec, /3 and y 30-70 m/sec. An arbitrary division at 50 m/sec is made to distinguish between /3 and y, whilst 6 fibres have velocities of less than 30 m/sec. However, any classification is to some extent arbitrary and, for example, Casey & Blick” referred to fibres in cutaneous nerves of the cat with conduction velocities of less than 24 m/sec as A 6 fibres. This illustrates one of the difficulties in the application of this classification. Efferent motor fibres have high conduction velocities and would be referred to as a fibres. However a study of the size and inferred conduction velocity of motor fibres reveals many with conduction velocities below 70 m / ~ e and c ~ therefore ~ in the /3 range. There are very few comprehensive studies of the range of conduction velocities of nerve fibres subserving various afferent and efferent functions of the type mentioned It follows that the maximum possible ranges of conduction velocities can not be regarded as defined with the accuracy required as a basis for classification. Coppin, Jack & MacLennan4’ defined the division between group I and I1 fibres as 0.65 x the conduction velocity of the fastest velocity recorded in the particular nerve (i.e. the velocity of the fibres contributing to the first deflection in the recorded compound action potential), Jack & MacLennan4’ found that this method gave a value of 72.8 m/sec as the division between fibres on groups I and I1 in their study. The conventional view5’ is to classify the conduction velocities of afferent fibres as follows. Group I, 70-120 m/sec; group 11, 30-70 m/sec; Group IIIc30 m/sec. It is customary to regard the upper limit of conduction velocities in unmyelinated mammalian nerve fibres as 2.5 m/sec. However Paintal” in a study of conduction velocities of single fibres in the aortic nerve of the cat, observed a gap in the range of conduction velocities below 4.5 m/sec which he considered to be the lowest conduction velocity in the myelinated fibres. Most of the other fibres were found to have conduction velocities below 2.5 m/sec (i.e. unmyelinated), but he found several fibres with conduction velocities between 2.5 and 3 m/sec. Paintal” thought it unlikely that such fibres would be myelinated and suggested that the accepted upper range of conduction velocities in unmyelinated fibres should be increased to 3 m/sec. Classificationby single fibre (spike)potentials
The principal features of the action potential of a single fibre, recorded monophasically, are illustrated in Fig. 2.52 The spike has a rising phase associated with depolarisation and sodium movement Spike potential
t
W t i v e after-potential
Fig. 2. Principal features of the monophasic action potential of a peripheral nerve fibre.
ClassiJication of peripheral nerve fibres
499
into the fibre and a falling phase, associated with potassium movement out of the fibre. T h v e is a negative after potential when the membrane is still partially depolarised (the cause of this potential is not fully understood) and a positive after potential during which time the sodium pump is active and the membrane and ion equilibria return to normal. Various groups of nerve fibres overlap with regard to properties such as velocity, diameter, and duration of their absolute refractory period. The action potentials of the various groups of fibres also show fundamental similarities and these are summarked in Table 1. Two aspects merit discussion: BJibres. These were originally described in the sciatic nerve of the frog, cat and dog41 and in sympathetic nerves of the cat53and the rabbit.29 Their diameters are less than 3 pm and their conduction velocities are less than 15 Table 1. Some properties of mammalian nerve fibres (modified from Brazier”) Group C Group A
Diameter (pm) Conduction velocity (mlsec) Spike duration (msec) Negative after potential % Spike height Duration (msec) Positive after potential % Spike height Duration (msec)
20-1 120-5
Group B
Sympathetic
Classification of peripheral nerve fibres An historical perspective
J. G. WHITWAM
This review provides an historical perspective to the present system of classification of nerve fibres and makes suggestions for a more uniform method of description in the future. The composition of nerves was first studied by Remak' and subsequently by Sherrington' who observed that muscle nerves contained larger afferent fibres than cutaneous nerves. In the same year Westpha13 reported that the peripheral spinal nerves in the newborn infant were thinner and more difficult to stain than in the adult. Unmyelinated fibres were first described by Remak' and defined histologically by R a n ~ o nwho ~ ' ~modified the Cajal stain for this purpose. In order to describe, with clarity, the results of studies in clinical neurophysiology, and into chemical block of nerve conduction, reference has to be made to afferent and efferent nerve fibres of different sizes, conduction velocity and sensory modality. In the anaesthetic literature there is a tendency to use different terms for the same type of nerve fibre and this causes confusion. For example, the terms C and group IV are both used to describe unmyelinated fibres; a and group I are often used interchangeably to describe fibres of the highest velocity, whilst and group I1 describe fibres of medium conduction velocity. Confusion also arises because the term B refers to preganglionic sympathetic nerve fibres, whilst y refers to fibres innervating muscle spindles, even though both these groups of fibres are myelinated and may have similar conduction velocities, most of which lie in the range normally ascribed to 6 fibres. Anatomical and physiological considerations
Growth of nerve fibres in animals and man Nerves enlarge during foetal and post embryonal development of the animal and myelination of the axon takes place as the fibre enlarges above a critical diameter in the developing animal.6 A number of studies have shown that during maturation of the peripheral nervous J. G. Whitwarn, MB, ChB, PhD, MRCP, FFARCS, Reader in Clinical Anaesthesia, Department of Anaesthetics, Royal Postgraduate Medical School, D u a n e Road, London, W12 OHS.
494
Classification of peripheral nerve fibres
495
system there is a progressive increase in the conduction velocity and diameter of fibres in the peripheral nerves which is most marked in early life. For example in 4-day-old kittens the maximum velocity at which impulses are conducted in the saphenous nerve is 11 m/sec as compared with 80-90 m/sec in full grown cats,7 Skoglund* and Ekholm’ have confirmed these findings in the cat and have shown that maturation of nerve conduction in the kitten is complete 34-4 months after birth. The number of nodes of Ranvier stays constant throughout development and hence, as the nerve fibres lengthen during growth, not only does the myelin sheath increase in thickness but also the distance between the nodes of Ranvier
lengthen^.^.' O S 1 In man myelination in the foetus starts about the 4th month of intra-uterine life, first in the spinal motor roots and subsequently in the dorsal root^.''"^ M yelination of the peripheral nerves increases with age and starts p r ~ x i m a l l y ’ ~so ~ ’ that ~ the peripheral nerves may mature later than the spinal roots. For this reason nerves in the newborn have a relatively high threshold for stimulation and Holmes’6 found that the excitability of nerves fell to adult levels between 3 and 6 years. The work of Eccles & Sherrington” and Nystrom & Skoglund’’ suggests that from birth to adult life the fibre diameter in motor nerves (both large and small fibres) approximately doubles, and Rexed” concluded that the maturation of ventral roots was virtually complete between 2 and 5 years. In parallel with the increase in fibre size, the maximum conduction velocities in peripheral nerve fibres increase after birth, at first rapidly and then The subject more slowly, so that between 3 and 5 years adult values are a~hieved.’~-’~ of nerve development in the neonate has been reviewed relatively re~ently.’~*’~ It is interesting to speculate that the alert but ‘floppy’ neonate, sometimes seen after maternal epidural analgesia, may be the result of the effects of low concentrations of analgesic drug on the immature, poorly myelinated, peripheral nerve fibres of the newborn infant. In view of the work on the development of nerve fibres outlined above, the observations which follow apply only to mature adult mammalian nerve fibres. Inevitably, most of the investigations have been made on experimental animals. Size and myelination
In adult mammals the largest myelinated fibres have diameters up to 20 pm whereas those for unmyelinated fibres are in the range 0.3-1.35 pm. In fully grown animals, the ratio between the diameter of the axon and the outside diameter of the fibre is approximately 0.726for fibres above 8 pm diameter, but the ratio declines progressively for smaller fibres until a value of 0.55 is reached for nerves having diameters below 3 However myelinated fibres lose the myelin sheath at their terminals. Also in the autonomic nervous system, for example in the vagus nerves, it is not uncommon for fibres to be myelinated for one part of their course and to be virtually unmyelinated for another part.’* It can be appreciated that, for any individual nerve fibre, a particular characterisation in terms of size and myelination may only be expected to apply for part of the length of the nerve fibre. Conduction velocity
Myelination and the diameter of nerve fibres are both important in determining the
496
J. G . Whitwam
speed at which impulses are conducted. There is a linear relationship between the conduction velocity and both the fibre diameter7s27s29-33 and the thickness of the myelin sheath.29i34It has also been suggested that the conduction velocity is proportional to the internodal length7 but this has not been ~onfirmed.~’
Excitability Another property related to fibre size which is very important in experimental work is the threshold of electrical excitation. In general, the threshold is lowest in the largest fibres which also have the highest conduction v e l ~ c i t i e s ,and ~ ~ the threshold for electrical excitation of unmyelinated fibres has been shown to be more than 20 times that of the fastest myelinated fibres.29 These points are illustrated in Fig. 1, which shows stimulus intensity duration curves for the three principal groups of fibres in the radial nerve of the dog. Thus, while using stimuli of very low intensity, it is possible to activate selectively the fastest myelinated fibres, but a stimulus of sufficient intensity to
0
0.1
0.2
0.3
0.4
05
Stimulus duration (msec)
Fig. 1. Stimulus intensity duration curves at threshold for the compound potential of the three principal fibre groups in the lateral branch of the superficial branch of the radial nerve of the dog. 0-0 Group 11 fibres; x - x Group Ill fibres; 0-0 Group IV fibres. (Ordinate logarithmic scale.)
activate the smallest, most slowly conducting fibres will also stimulate the larger fibres in the nerve. However, Gasser37 has suggested that there is a group of myelinated fibres, the ‘delta 3’ group with conduction velocities between 5 and 8 m/sec which have a higher threshold to electrical excitation than the most excitable unmyelinated fibres. These fibres cannot therefore be identified solely on the basis of electrical excitability. Koll ef al.’* have also defined a group of fibres with conduction velocities between 4 and 10 m/sec whose properties are different to other small myelinated fibres and which they referred to as the ‘post 6 group’. Whereas there is a close correlation between conduction velocity and electrical excitability in myelinated fibres, no such correlation occurs within the unmyelinated fibre population. At threshold, electrical stimuli excite not only the most rapidly conducting unmyelinated fibres but also those belonging to the more slowly conducting subgroups.39 Another property related to fibre size is the absolute refractory period which is longer in smaller
Classijication of peripheral nerve fibres
497
Historical view of the classification of mature nerve fibres The classification of nerve fibres by conduction velocity has been based either on studies of the evoked compound action potentials in peripheral nerves or observations of the conduction velocities of single fibres. Classification by evoked compound action potentials
In 1924, Erlanger, Gasser & Bishop” described the compound nature of the con~’ peripheral ducted potential in various nerves and Erlanger & G a ~ s e r ~ ”classified nerve fibres into three main groups. Group A comprised myelinated somatic afferent and efferent fibres. Group B consisted of preganglionic autonomic fibres, and group C were non-myelinated somatic afferent fibres and post ganglionic autonomic fibres. From observations of the waves in the compound action potential in a large mixed peripheral nerve Erlanger & further subdivided group A into c[, /3, y and 6 fibres. In 1930 Eccles & Sherrington” constructed histograms of the fibre sizes in the nerves to several muscles and showed that there were distinct groups of large and small motor nerve fibres. In 1945 Lekse114’ showed that these motor fibres contributed to the tl and y elevations in the compound action potential. More recently Boyd and his associates43 have further subdivided y efferent fibres into two groups, a fast relatively heavily myelinated group and slow thinly myelinated fibres which have relatively slow conduction velocities in the ranges of 6 and group I11 fibres (see below). Classijication of afferent fibres
In 1943 Lloyd44introduced a new terminology to refer to aferentfibres in both muscle and cutaneous nerves. As originally proposed this classification was based entirely on fibre size. He found that myelinated afferent fibres fell into 3 broad groups with diameters in the ranges 12-20 pm, 6-12 pm and 1-6 pm for which he used the terms group I, I1 and I11 respectively. Unmyelinated fibres he referred to as group IV. In 1948 Lloyd & Chang4’ published histograms of afferent fibres from a number of nerves and showed that most nerves contain all three myelinated groups of fibres as relatively distinct populations. In the original classification of Lloyd44 afferent fibres from muscle spindles were referred to as group Ia, and those from tendon organs as s~~ that the group I compound action group Ib. In 1953 Bradley & E c ~ l e suggested potential had fast and slow components produced by fibres of groups Ia and Ib respectively. More recently Jack & MacLennan4’ have challenged this observation and have shown that the conduction velocities of afferent fibres from muscle spindles and tendons show considerable overlap and that both groups are represented throughout the whole range of conduction velocities. Some of the observed conduction * velocities of afferent fibres from tendon organs can be as low as 62.5 r n / ~ e c . ~Therefore the division of these fibres into groups Ia and Ib cannot be based on conduction velocity but is an anatomical classification depending on their source. Classification of 1965
At the first Nobel Symposium on ‘Muscle afferents and motor control’ in Stockholm in 196549it was suggested that the terms a, p, y and 6 should be confined to groups of
498
J. G. Whitwam
myelinated efferent fibres. The terminology I, 11,111 and IV should refer to groups of afferent fibres whether in muscle, cutaneous or autonomic nerves. If these suggestions are accepted the conduction velocities of efferent myelinated fibres would be as follows: a 70-120 m/sec, /3 and y 30-70 m/sec. An arbitrary division at 50 m/sec is made to distinguish between /3 and y, whilst 6 fibres have velocities of less than 30 m/sec. However, any classification is to some extent arbitrary and, for example, Casey & Blick” referred to fibres in cutaneous nerves of the cat with conduction velocities of less than 24 m/sec as A 6 fibres. This illustrates one of the difficulties in the application of this classification. Efferent motor fibres have high conduction velocities and would be referred to as a fibres. However a study of the size and inferred conduction velocity of motor fibres reveals many with conduction velocities below 70 m / ~ e and c ~ therefore ~ in the /3 range. There are very few comprehensive studies of the range of conduction velocities of nerve fibres subserving various afferent and efferent functions of the type mentioned It follows that the maximum possible ranges of conduction velocities can not be regarded as defined with the accuracy required as a basis for classification. Coppin, Jack & MacLennan4’ defined the division between group I and I1 fibres as 0.65 x the conduction velocity of the fastest velocity recorded in the particular nerve (i.e. the velocity of the fibres contributing to the first deflection in the recorded compound action potential), Jack & MacLennan4’ found that this method gave a value of 72.8 m/sec as the division between fibres on groups I and I1 in their study. The conventional view5’ is to classify the conduction velocities of afferent fibres as follows. Group I, 70-120 m/sec; group 11, 30-70 m/sec; Group IIIc30 m/sec. It is customary to regard the upper limit of conduction velocities in unmyelinated mammalian nerve fibres as 2.5 m/sec. However Paintal” in a study of conduction velocities of single fibres in the aortic nerve of the cat, observed a gap in the range of conduction velocities below 4.5 m/sec which he considered to be the lowest conduction velocity in the myelinated fibres. Most of the other fibres were found to have conduction velocities below 2.5 m/sec (i.e. unmyelinated), but he found several fibres with conduction velocities between 2.5 and 3 m/sec. Paintal” thought it unlikely that such fibres would be myelinated and suggested that the accepted upper range of conduction velocities in unmyelinated fibres should be increased to 3 m/sec. Classificationby single fibre (spike)potentials
The principal features of the action potential of a single fibre, recorded monophasically, are illustrated in Fig. 2.52 The spike has a rising phase associated with depolarisation and sodium movement Spike potential
t
W t i v e after-potential
Fig. 2. Principal features of the monophasic action potential of a peripheral nerve fibre.
ClassiJication of peripheral nerve fibres
499
into the fibre and a falling phase, associated with potassium movement out of the fibre. T h v e is a negative after potential when the membrane is still partially depolarised (the cause of this potential is not fully understood) and a positive after potential during which time the sodium pump is active and the membrane and ion equilibria return to normal. Various groups of nerve fibres overlap with regard to properties such as velocity, diameter, and duration of their absolute refractory period. The action potentials of the various groups of fibres also show fundamental similarities and these are summarked in Table 1. Two aspects merit discussion: BJibres. These were originally described in the sciatic nerve of the frog, cat and dog41 and in sympathetic nerves of the cat53and the rabbit.29 Their diameters are less than 3 pm and their conduction velocities are less than 15 Table 1. Some properties of mammalian nerve fibres (modified from Brazier”) Group C Group A
Diameter (pm) Conduction velocity (mlsec) Spike duration (msec) Negative after potential % Spike height Duration (msec) Positive after potential % Spike height Duration (msec)
20-1 120-5
Group B
Sympathetic
