Two types of static gamma-axon in cat muscle spindles.

The action of single static gamma-axons in 'single fibre' ventral root filaments was tested on the responses of from two to eight spindles in the same tenuissimus muscle in anaesthetized cats. The type of intrafusal fibre activated in each spindle by a particular axon was diagnosed from its effect on the discharge of primary and secondary sensory endings, frequently in the same spindle. One or two spindles were subsequently isolated in most experiments and the type of intrafusal fibre activated by the axon was observed directly. Except in the case of a few weak actions which could not be interpreted, the diagnosis made from the afferent recordings was always correct. Twenty-four static gamma-axons activated the chain fibres in every spindle they supplied on which their action was tested; the static bag2 fibre was also involved in about 20% of the spindles though its contraction was quite often weak. Eleven static gamma-axons activated the static bag2 fibre in every spindle they supplied on which their action was tested; some chain fibres were activated as well in about 20% of the spindles. There were only two possible exceptions to the general rule that a static gamma-axon activates either the chain fibres or the static bag2 fibre in every spindle it supplies and never activates only the chain fibres in one spindle, and only the static bag2 fibre in another spindle. Direct observation of isolated spindles provided preliminary evidence that some non-selective connexions of static gamma-axons have a pronounced physiological action by way of their terminals on one type of intrafusal fibre while the terminal(s) on the other type of fibre produce such a weak contraction that the sensory endings are not affected. Other non-selective connexions do, however, have a significant physiological effect by way of their terminals on both the static bag2 fibre and the chain fibres. It is proposed that there are two types of static gamma-motoneurone. 'Static bag gamma-motoneurones', through their terminals on the static bag2 fibre in every spindle, bias the discharge from primary endings (and to a lesser extent from some secondary endings) but leave their length sensitivity largely unaltered; connexions to chain fibres in some spindles have a weak action on primary endings but may greatly increase the length sensitivity of secondary endings.(ABSTRACT TRUNCATED AT 400 WORDS)

The ultrastructure of cat fusimotor endings and their relationship to foci of sarcomere convergence in intrafusal fibres.

1. Six muscle spindle poles, five from experiments in which foci of sarcomere convergence had been observed during stimulation of fusimotor axons, were serially sectioned for light and electron microscopy. Every somatic motor terminal was studied in ultrathin sections at several levels.2. In all six poles static gamma axons, or presumed static gamma axons, supplying the static bag(2) fibre and/or chain fibres had no terminations on the dynamic bag(1) fibre. In five poles, the dynamic bag(1) fibre was selectively innervated by dynamic gamma or beta axons save in one case where a dynamic gamma axon also innervated one chain fibre.3. Seventy-seven motor endings were of four distinct ultrastructural types: ;m(a) plates' lay superficially on the surface of static bag(2) or chain fibres; ;m(b) plates' were deeply indented into dynamic bag(1) fibres; in ;m(c) plates', found on chain fibres only, the muscle surface was thrown into projecting fingers between which the axon terminals were embedded; one type ;m(d) plate' was found, fully indented into a long chain fibre. A few plates of intermediate form (m(ab)) were variants of m(a) and m(b) plates.4. The muscle membrane beneath both m(a) and m(b) plates was smooth, or had a few wide, shallow folds; m(c) plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the m(d) plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag(1) and static bag(2) fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag(1) fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag(2) fibres; contraction in these fibres is typically mid-polar.6. In some poles local contraction of chain fibres centred on the location of m(c) plates. In others, very localized contraction occurred distal to the sites of m(a) plates. Both m(a) and m(c) plates were never found on the same pole of a chain fibre.7. Dynamic gamma or beta axons end in m(b) plates, probably equivalent to p(2) plates. The concept of distinctly different p(1) and p(2) plates on dynamic bag(1) fibres, supplied by dynamic beta and gamma axons, respectively, is not supported by ultrastructural evidence.8. Some static gamma axons end in multiple m(a) plates which correspond with ;trail endings', or in single large m(a) plates, on static bag(2) or chain fibres. The m(c) plates are the terminations of other static gamma, or occasionally dynamic gamma, axons on chain fibres. Static beta axons probably end in m(d) plates on long chain fibres which may correspond with p(1) plates.9. It is proposed that there are two types of static gamma motoneurone, one terminating in m(a) plates and the other in m(c) plates, possibly directed preferentially towards static bag(2) fibres and chain fibres, respectively.

Quantitative study of the non-circularity of myelinated peripheral nerve fibres in the cat.

1. One hind limb of each of four cats was either chronically de-efferentated, or chronically de-afferentated, and perfused with buffered glutaraldehyde fixative. Up to three different muscle nerves were dissected from each limb, post-fixed in osmium tetroxide and embedded in Epon. Ultrathin transverse sections were mounted on Formvar-coated single-hole specimen grids so that all the fibres in each nerve could be examined individually by electron microscopy.2. Non-circularity was expressed as the ratio (ø): [Formula: see text] The degree of non-circularity of all the afferent axons, or all the efferent axons, in each muscle nerve was determined. The proportion of fibres cut through the paranodal region, or through the Schwann cell nucleus, was as expected for group I afferent and for alpha and gamma efferent fibres, but hardly any typical paranodal sections of group II or III afferent fibres were encountered which suggests that their paranodal arrangement differs from that of other groups. In a quantitative comparison of noncircularity in different functional groups, fibres cut through paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were rejected.3. All the gamma efferent fibres in one nerve were studied in a series of sections cut at 25 mum intervals. The degree of non-circularity was found to be relatively constant along the internode of most fibres when the values at paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were ignored.4. The value of ø varied widely from 1.0 (circular) to 0.5 or less from fibre to fibre within every functional group. However, the mean value of ø was less for gamma axons (0.68) than for alpha axons (0.78), and less for group III axons (0.79) than for axons in groups I and II (both 0.84). When the results for all the nerves were aggregated, these differences were statistically very highly significant, as was the difference in ø between group I and alpha fibres. If values of ø < 0.5 were rejected, the difference between the mean ø for group III and group II was then of doubtful significance whereas that between alpha and gamma fibres was still very highly significant.5. The external perimeter (S) of a non-circular fibre differs from pi times the diameter of a circle just enclosing the fibre (D). It is shown that S = 0.95 pi D for group I and II fibres, S = 0.90 piD for alpha and group III fibres, and S = 0.85 piD for gamma fibres.6. The myelin period, or interperiod repeat distance, varied from 14.1 to 15.6 nm in different cats, implying radial shrinkage of the myelin sheath from 15 to 23%. The myelin period in a particular cat was the same for several nerves, and the same for fibres in different functional groups.7. The possibility that repetitive firing of axons during fixation contributed to the varying degree of non-circularity is considered but rejected as unlikely.8. It is deduced that about 10% radial shrinkage of the myelin sheath, but little or no osmotic shrinkage of the axon, occurred during fixation and rinsing. Further radial shrinkage of about 8% in all components of the fibre probably occurred as a result of subsequent histological processing. It is concluded that the non-circularity of all axons, and the greater non-circularity of small axons, is unlikely to have been due to histological processing.9. It is concluded that axons are non-circular in vivo. The hypothesis that non-circularity allows axons to accommodate swelling during repetitive activity is discussed. Suggestions are made as to why gamma axons may be more non-circular than alpha or group III axons in an anaesthetized cat immediately prior to fixation, and why alpha axons may be more non-circular than axons in groups I and II.

Ultrastructural dimensions of myelinated peripheral nerve fibres in the cat and their relation to conduction velocity.

1. The ultrastructure of all the afferent fibres, or all the efferent fibres, was studied in selected nerves from chronically de-afferentated or de-efferentated cat hind limbs perfusion-fixed with glutaraldehyde.2. The following parameters were measured: number of lamellae in the myelin sheath (n), axon perimeter (s), external fibre perimeter (S), axon cross-sectional area (A). Fibres were allocated to afferent groups I, II, III or efferent groups alpha and gamma according to the number of lamellae in the myelin sheath.3. The thickness of the myelin sheath (m) was linearly related to axon perimeter within the range s = 4 mum to s = 20 mum (groups II, III and gamma). The relation m = 0.103 s - 0.26 provided a good fit for all afferent and efferent axons in this range in several different anatomical muscle nerves in three cats. The myelin sheaths were thinner in a fourth, presumably younger, cat.4. The myelin sheaths were relatively thinner for large fibres in groups I and alpha (s = 20-50 mum). The results are interpreted in one of three ways. Either m tends to a limit of about 2.2 mum, or m is linearly related to s such that for large fibres m = 0.032 s + 1.11.5. Alternatively, m may be considered to be proportional to log(10)s for all sizes of axon so that m = 2.58 log(10) S - 1.73. The interpretation that there are two separate linear relations for large and small fibres is favoured.6. The ratio of axon to external fibre perimeter (g) falls from about 0.70 for group III and small gamma fibres in the cat to about 0.62 for group II and large gamma fibres and then rises again to 0.70, or even 0.75 for group I and alpha axons.7. The above relations between m and s are combined with the observations of Boyd & Kalu (1979) that Theta = 5.7 D for groups I and alpha and Theta = 4.6 D for groups II, III and gamma. It is shown that Theta = 2.5 s approximately for all sizes of axon (s from material fixed for electron microscopy) in rat, cat and man. The accuracy of this equation may be improved by deducting 3 m/sec in the case of small fibres. This conclusion is compatible with experimental observations of the relation between l and D (Hursh, 1939; Lubinska, 1960; Coppin, 1973) and between l and Theta (Coppin & Jack, 1972).8. From the theoretical analyses of Rushton (1951) and others Theta should be proportional to the external dimensions of the fibre rather than to axon size. It is shown that the thinning of the myelin sheath ought to affect Theta substantially. Thus some other factors must compensate for the thinning of the sheath.9. Small fibres are significantly more non-circular than large fibres. From the quantitative data of Arbuthnott et al. (1980) it is concluded that non-circularity may contribute to the fact that Theta proportional, variant s rather than Theta proportional, variant S, but cannot wholly account for it. Other possibilities considered are that axoplasmic resistivity or specific nodal conductance may differ for large and small fibres.10. It is suggested that myelinated peripheral nerve fibres may fall into two distinct classes with different properties, one comprising groups I and alpha and the other groups II, III and gamma. The conclusion predicted from theory may apply to each of these classes separately so that Theta = 2.0 S for the large-fibre class and Theta = 1.6 S for the small-fibre class.

Scaling factor relating conduction velocity and diameter for myelinated afferent nerve fibres in the cat hind limb.

1. Compound action potentials were recorded from certain muscle and cutaneous nerves in normal and chronically de-efferentated hind limbs of cats during stimulation of the appropriate dorsal spinal roots, 2. The peaks for groups I, II and III in the compound action potential were correlated with the corresponding peaks in the fibre-diameter histograms of the same de-efferentated nerve after processing it for light microscopy. 3. The scaling factor (ratio of conduction velocity in m/sec to total diameter in micrometer) was not constant for all sizes of fibre nor did it increase progressively with fibre size. Evidence is presented that a logarithmic relation between conduction velocity and fibre diameter is not appropriate. 4. In muscle nerves the scaling factor for fibres fixed by glutaraldehyde perfusion and embedded in Epon was 5.7 for group I afferent fibres and 4.6 for myelinated fibres in both group II and group III. 5. In cutaneous nerves the scaling factor was 5.6 for large fibres (group I or Abeta) and 4.6 for small fibres (group III or Adelta). 6. The scaling factor for group I fibres is the same as was found previously for alpha-efferent fibres, and that for groups II and III is the same as for gamma-efferent fibres (Boyd & Davey, 1968). 7. The possibility that there is a clear discontinuity in scaling factor between fibres in groups I and alpha, and those in other functional groups, is discussed. 8. It is concluded that there must be some structural feature of alpha and group I fibres which differs from that of smaller myelinated fibres. It is likely that a difference in the relative thickness of the myelin sheath is involved and possibly also in the conductances responsible for generating the action potential.

Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels.

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic gamma axons, dynamic beta axons, or static gamma axons in ventral root filaments was observed and recorded on still and moving film. 2. Most spindles were controlled by one dynamic gamma axon (sometimes a beta axon) and three static gamma axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only. 3. Dynamic gamma and beta axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static gamma axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'. 4. Static gamma axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'. 5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static gamma axons, frequently by static gamma axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic gamma axons and on one occasion only by a static gamma axon controlling a nuclear bag fibre alone. 6. The conduction velocities of dynamic gamma and static gamma axons overlapped extensively, though dynamic gamma axons were absent from the lower end, and static gamma axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities. 7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972). 8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

The response of fast and slow nuclear bag fibres and nuclear chain fibres in isolated cat muscle spindles to fusimotor stimulation, and the effect of intrafusal contraction on the sensory endings.

1. The mechanical behaviour of intrafusal muscle fibres during fusimotor stimulation and passive stretch was observed directly in muscle spindles isolated from the cat tenuissimus muscle. 2. Mammalian intrafusal muscle fibres are of three functional types. Most spindles contain one slow nuclear bag fibre, one fast nuclear bag fibre, and four or five nuclear chain fibres. 3. Contraction in slow nuclear bag fibres is characterized by a long latency and very slow initial velocity, whereas the latency for the other intrafusal fibres is short and the inital velocity rapid. The mean time for maximum contraction (at 75 Hz to 100 Hz) and relaxation is significantly longer for slow nuclear bag fibres (0-8s) than for other intrafusal fibres (0-5 s). The contraction time of fast nuclear bag fibres is sometimes longer than that of nuclear chain fibres but the mean values are not significantly different; a difference in the time to attain 90% contraction is more obvious. 4. At low stimulation frequencies (10 Hz) contraction in slow nuclear bag fibres and in most fast nuclear bag fibres is smooth whereas nuclear chain fibres exhibit marked oscillations. Single stimuli elicit small local twitches in nuclear chain fibres and occasionally in fast nuclear bag fibres but produce no visible effect in slow nuclear bag fibres. 5. Maximum contraction of slow and fast nuclear bag fibres at body temperature is attained at a stimulation frequency of 75 Hz to 100 Hz, whereas a frequency of 150 Hz or more is required for maximum contraction of nuclear chain fibres. At 50 Hz at body temperature contraction in nuclear bag fibres is at least half the maximum, whereas in many spindles nuclear chain fibres show only a very small contraction at this frequency. 6. Contraction in slow nuclear bag fibres occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Weak contraction extends the primary sensory spiral by a small amount (2%-8%) at a low velocity (5%-10%s-1). When the fibre is passively stretched the spiral opens and then creeps back to about 75% of the extension at the end of the stretch due to yielding in the poles of fibre; creep is complete in 0-5s to 2-5s. 7. Contraction in fast nuclear bag fibres also occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Shortening of sarcomeres at the foci and extension of the sensory spiral are, however, up to eight times greater (up to 25%) than in slow nuclear bag fibres, and the velocity of stretch of the spiral is three to eight times greater (25%-40%s-1). Fast nuclear bag fibres exhibit little or no creep following passive stretch. 8. Contraction in the nuclear chain fibre bundle is localized to the intracapsular region, centered on a point in the intracapsular region between 0-9 mm and 1-6 mm from the spindle equator. Maximal contraction stretches primary and secondary sensory endings by 15% to 20%, at 30% to 40% s-1...

Motor control of nuclear bag and nuclear chain intrafusal fibres in isolated living muscle spindles from the cat.

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles was studied by direct observation during repetitive stimulation of the muscle nerve at different stimulus strengths. Contraction of intrafusal fibres and stretch of sensory endings was recorded on film. 2. Tenuissimus spindles are usually operated by a total of four or five fusimotor axons, and the individual action of all of them was studied in many cases. 3. The great majority of fusimotor axons produce activity at one spindle pole only. 4. In about 60% of spindles nuclear bag and nuclear chain intrafusal fibres are selectively controlled by different fusimotor axons, while in one third of these spindles the individual nuclear bag fibres are themselves controlled independently. The remaining 40% of spindles, in addition to some selective innervation, receive one non-selective axon which operates both nuclear chain and nuclear bag fibres though usually only one of the nuclear bag fibres is involved. Selective control is demonstrated in photographs. 5. The thresholds of fusimotor axons selectively innervating nuclear bag and nuclear chain fibres, and of non-selective fusimotor axons are not significantly different. 6. It is suggested that in spindles in which the nuclear bag fibres are controlled by the same axon, it is a 'dynamic' gamma, or occasionally beta, axon. Where one nuclear bag fibre is operated along with the nuclear chain fibres it is controlled by 'static' gamma axon(s), and the other nuclear bag fibre is selectively controlled by 'dynamic' gamma, and perhaps beta, axon(s). Where two nuclear bag fibres are separately operated one may be controlled by 'dynamic' axon(s) and the other by 'static' gamma axon(s). Nuclear chain fibres are always controlled by 'static' gamma axons.