Electrical properties and acetylcholine sensitivity of singly and multiply innervated avian muscle fibers

Structure and distribution of fibre types in the external eye muscles of the rat

Rat extraocular muscles are composed of two layers differing in muscle fibre diameter. In both layers multiply innervated fibres are found besides focally innervated ones. Significant differences in internal structure thus demand further subtyping. The global layer (predominantly larger fibres) contains 10" 'clear' fibres (multiple innervation, medium size) and a spectrum of focally innervated fibres, from small 'dark' fibres (30%, abundant mitochondria) over 'intermediate' fibres (30%) to large 'pale' fibres (30 %, few mitochondria). The orbital layer (exclusively small fibres) contains 80% focally innervated 'dark' fibres and 20% extremely small 'clear' fibres with multiple innervation. An ultrastructural characterization of the fibre types is given and possible functional implications are discussed.

Spatial and temporal patterns of myosin heavy chain expression in developing rat extraocular muscle

The present study describes transitions in myosin heavy chain expression in the extraocular muscles of rats between the ages of E17 and adult. The unique phenotype of the extraocular muscle is reflected in its fibre type composition, which is comprised by six distinct profiles, each defined by location (orbital versus global layer) and innervation pattern (single versus multiple terminals). During extraocular muscle myogenesis, developmental myosin heavy chains were expressed in both primary and secondary fibres from embryonic day E17 through the first postnatal week. At this time, the downregulation of developmental myosin heavy chain isoforms began in the global layer in a fibre type-specific manner, reaching completion only after the first postnatal month. By contrast, developmental isoforms were retained in the overwhelming majority of orbital layer fibres into adulthood and expressed differentially along the length of these fibres. Fast myosin heavy chain was detected pre- and postnatally in developing secondary fibres and in all of the singly innervated fibre types and one of the multiply innervated fibre types in the adult. As many as four fast isoforms were detected in maturing extraocular muscle, including the extraocular muscle-specific myosin heavy chain. Slow myosin heavy chain was expressed in primary fibres throughout development and in one of the multiply innervated fibre types in the adult. In contrast, the pure fast-twitch retractor bulbi initially expressed slow myosin heavy chain in fibres destined to switch to the fast myosin heavy chain developmental programme. Based upon spatial and temporal patterns of myosin heavy chain isoform transitions, we suggest that epigenetic influences, rather than purely myogenic stage-specific factors, are critical in determining the unique extraocular muscle phenotype.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

The morphological basis of folded-wing posture in the American kestrel, Falco sparverius

Gross dissection and histochemical analysis of the shoulder musculature of the American kestrel, Falco sparverius, revealed that four muscles are specialized for slow contraction and may function in the postural control of the folded wing. Mm. latissimus dorsi pars cranialis, scapulohumeralis cranialis, and brachialis were found to contain greater than 95% tonic fibers, whereas M. deltoideus minor was found to possess a relatively even mix of fast-twitch and tonic muscle fibers. M. latissimus dorsi pars cranialis, M. scapulohumeralis cranialis, and M. deltoideus minor all cross the shoulder joint caudally to the articulation, and M. brachialis crosses the elbow joint on the ventral surface of the forearm. This paper suggests postural muscles have largely been ignored in studies of avian musculature, and the need to consider a variety of possible muscle functions when analyzing locomotor muscle functions.

Neural cell adhesion molecule (N-CAM) is elevated in adult avian slow muscle fibers with multiple terminals

Many adult avian muscles contain two types of muscle fiber: those that receive innervation at single focal terminals and those with multiple terminals. The muscles of the syrinx, the vocal organ of birds, are such mixed muscles. To study this heterogeneity of fiber type and innervation, we combined immunocytochemistry to classify muscle fibers with techniques to visualize neuromuscular junctions. One monoclonal antibody, S58, directed against a slow class of myosin, labels only fibers that have multiple terminals. We also examined the distribution of immunoreactivity for neural cell adhesion molecule (N-CAM), which has been suggested to play a role in innervation of muscle and formation of neuromuscular junctions. S58-positive fibers have elevated N-CAM staining, indicating that multiple innervation of a fiber is correlated with the fiber's expression of high levels of N-CAM immunoreactivity. Most, and perhaps all, fibers that have multiple terminals also contain abundant N-CAM immunoreactivity. This suggests that N-CAM may play a role in the maintenance of multiterminal innervation in adult innervated muscle.

Identification of sarcolemma-associated antigens with differential distributions on fast and slow skeletal muscle fibers

We have identified three sarcolemma-associated antigens, including two antigens that are differentially distributed on skeletal muscle fibers of the fast, fast/slow, and slow types. Monoclonal antibodies were prepared using partially purified membranes of adult chicken skeletal muscles as immunogens and were used to characterize three antigens associated with the sarcolemma of muscle fibers. Immunofluorescence staining of cryosections of adult and embryonic chicken muscles showed that two of the three antigens differed in expression by fibers depending on developmental age and whether the fibers were of the fast, fast/slow, or slow type. Fiber type was assigned by determining the content of fast and slow myosin heavy chain. MSA-55 was expressed equally by fibers of all types. In contrast, MSA-slow and MSA-140 differed in their expression by muscle fibers depending on fiber type. MSA-slow was detected exclusively at the periphery of fast/slow and slow fibers, but was not detected on fast fibers. MSA-140 was detected on all fibers but fast/slow and slow fibers stained more intensely suggesting that these fiber types contain more MSA-140 than fast fibers. These sarcolemma-associated antigens were developmentally regulated in ovo and in vitro. MSA-55 and MSA-140 were detected on all primary muscle fibers by day 8 in ovo of embryonic development, whereas MSA-slow was first detected on muscle fibers just before hatching. Those antigens expressed by fast fibers (MSA-55 and MSA-140) were expressed only after myoblasts differentiated into myotubes, but were not expressed by fibroblasts in cell culture. Each antigen was also detected in one or more nonskeletal muscle cell types: MSA-55 and MSA-slow in cardiac myocytes and smooth muscle of gizzard (but not vascular structures) and MSA-140 in cardiac myocytes and smooth muscle of vascular structures. MSA-55 was identified as an Mr 55,000, nonglycosylated, detergent-soluble protein, and MSA-140 was an Mr 140,000, cell surface protein. The Mr of MSA-slow could not be determined by immunoblotting or immunoprecipitation techniques. These findings indicate that muscle fibers of different physiological function differ in the components associated with the sarcolemma. While the function of these sarcolemma-associated antigens is unknown, their regulated appearance during development in ovo and as myoblasts differentiate in culture suggests that they may be important in the formation, maturation, and function of fast, fast/slow, and slow muscle fibers.

Tetraethylammonium-methohexitone induced contractures and miniature end-plate potential discharge in the chick biventer cervicis muscle

End-plate and miniature end-plate potentials (EPP, MEPP) were recorded in the chick biventer cervicis muscle in vitro. The control frequency of discharge of MEPPs was 5.97 +/- 1.04 min-1, n = 10. Methohexitone (8.8 X 10(-5) M) did not affect the frequency of MEPP discharge, but tetraethylammonium (TEA, 2.4-9.5 mM), reduced the frequency to 32.9 +/- 8.6% of the control. In the presence of neostigmine (3.3 X 10(-6) M) methohexitone (8.8 X 10(-5) M) reduced MEPP frequency to 38.7 +/- 5.4% of the control and TEA (2.4-9.5 mM) further reduced the frequency to 8.7 +/- 2.1% of the control. After a transient facilitation of the EPP, TEA (2.4-9.5 mM) in the presence of methohexitone (8.8 X 10(-5) M) and neostigmine (3.3 X 10(-6) M) markedly reduced EPP amplitude. The contractures which TEA produced in the presence of methohexitone were not due to an increase in the spontaneous release of vesicular ACh.

Vinculin in subsarcolemmal densities in chicken skeletal muscle: localization and relationship to intracellular and extracellular structures

Using immunocytochemical methods we have studied the distribution of vinculin in the anterior and posterior latissimus dorsi skeletal (ALD and PLD, respectively) muscles of the adult chicken. The ALD muscle is made up of both tonic (85%) and twitch (15%) myofibers, and the PLD muscle is made up entirely of twitch myofibers. In indirect immunofluorescence, antivinculin antibodies stained specific regions adjacent to the sarcolemma of the ALD and PLD muscles. In the central and myotendinous regions of the ALD, staining of the tonic fibers was intense all around the fiber periphery. Staining of the twitch fibers of both ALD and PLD muscles was intense only at neuromuscular junctions and myotendinous regions. Electron microscopy revealed subsarcolemmal, electron-dense plaques associated with the membrane only in those regions where vinculin was localized by immunofluorescence. Using antivinculin antibody and protein A conjugated to colloidal gold, we found that the electron-dense subsarcolemmal densities in the tonic fibers of the ALD contain vinculin; no other structures were labeled. The basal lamina overlying the densities appeared to be connected to the sarcolemma by fine, filamentous structures, more enriched at these sites than elsewhere along the muscle fiber. Increased amounts of endomysial connective tissue were often found just outside the basal lamina near the densities. In tonic ALD muscle fibers, the subsarcolemmal densities were present preferentially over the I-bands. In partially contracted ALD muscle, subsarcolemmal densities adjacent to the Z-disk appeared to be connected to that structure by short filaments. We propose that in the ALD muscle, through their association with the extracellular matrix, the densities stabilize the muscle membrane and perhaps assist in force transmission.

Membrane electrical properties of developing fast-twitch and slow-tonic muscle fibres of the chick

Isolated single fibres from the anterior (a.l.d.) and the posterior (p.l.d.) lattissimus dorsi muscles of embryonic and young chicks were used to study in vivo development of membrane electrical properties. Isolated fibres were obtained by an enzymatic dissociation procedure. Intracellular micro-electrode recordings from isolated fibres and from fibres in intact muscles showed that the dissociation procedure did not significantly alter resting membrane potentials, input resistances or membrane time constants (tau m). The 14 day embryonic fibres of a.l.d. and p.l.d. did not have a measurable resting conductance to Cl-. At hatching, about 70% of the resting conductance in p.l.d. fibres was due to Cl-. Membrane electrical properties were estimated from the analysis of voltage responses to intracellular injection of rectangular pulses of current. At 14 days in ovo, membrane resistance (Rm) was approximately 20 k omega cm2 and membrane capacitance (Cm) was 1-2 microF/cm2 for both a.l.d. and p.l.d. The mean membrane length constants (lambda) were 1.7 mm for a.l.d. and 1.5 mm for p.l.d. For p.l.d., the values of Rm, tau m and lambda decreased as development proceeded. For a.l.d., there was no change in these values by the time of hatching (21 days). The decreases in the electrical constants for p.l.d. fibres were partly explained by the appearance of a resting Cl- conductance during the last week of embryonic development.

Fibre types in chicken skeletal muscles and their changes in muscular dystrophy

1. Five major fibre types in chicken skeletal muscles are recognized, based upon their histochemical and morphological characteristics. A classification of these which is readily related to a commonly used classification of mammalian muscle fibre types is given.2. Seven muscles of the chicken were analysed in recognizing this range of fibre types. The proportions of the different types in each of these were determined. In some cases a gradient of fibre type composition exists across a single muscle.3. Measurements of muscle contraction were used in defining tonic muscles, which contain two fibre types. It was shown that in addition to the anterior latissimus dorsi (a.l.d.), previously well known to be a tonic muscle, two other muscles, the plantaris and the adductor profundus, are of the same class, but differ subtly from the a.l.d. in certain features. Gross red colouration is not a useful diagnostic feature of slow muscles, since the tonic adductor profundus, for example, is white.4. Fibres similar histochemically to mammalian type I (slow-twitch) occur in some of the avian twitch muscles investigated. These are oxidative in character, and despite the fact that they are multiply innervated we suggest that these are avian slow-twitch fibres.5. The patterns of cholinesterases found in a skeletal muscle correspond to its fibre type composition, with regard to both the concentrations and the proportions of the multiple forms of enzyme present. The distinctive patterns of those forms of acetylcholinesterase in the different fibre types are described.6. The fibre type composition is changed by inherited muscular dystrophy in a characteristic manner. This change has so far been found (at the earlier stages of the disease) only in the muscles with a predominance of type II B fibres in the normal chicken. Pathological changes within the fibres occur selectively in the type II B fibres, but there are exceptions to this and the effect can be greatly modified by the type of neighbouring fibres.

The tensor tympani muscle of cat and dog contains IIM and slow-tonic fibres: an unusual combination of fibre types

Using recently developed highly specific antisera to the full range of known adult mammalian skeletal muscle myosins, an immunohistochemical and histochemical examination was made of the middle ear muscle tensor tympani in the dog and cat. Approximately half the fibres were of the IIM type and there was a substantial population of apparently slow-tonic fibres, both these types being rare in mammals. In addition, some type I but no IIA nor IIB fibres were detected. Moreover, as no multiple end-plate innervation, thought to be typical of slow-tonic fibres, could be demonstrated in this muscle by acetylcholinesterase staining or by Ruffini gold impregnation, it is suggested that in tensor tympani the slow-tonic fibres are focally innervated. The very short length of the fibres, only 1-2 mm, is probably sufficient to permit adequate depolarization of a whole fibre by a single centrally situated end-plate. The functional implications of this combination of very rare fibre types in tensor tympani are unclear at present.

Innervation of avian latissimus dorsi muscles and axonal outgrowth pattern in the posterior latissimus dorsi motor nerve during embryonic development

The distribution of the innervation to the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of the chicken are described on the day of hatching and 6 weeks later using electron microscopy. In the ALD muscle, there are 5,000 muscle fibres and 374,000 endplates supplied by about 169 skeletomotor axons; in the PLD muscle, there are 12,000 focally innervated muscle fibers supplied by about 20 skeletomotor axons. On the cell surface of the muscle fibers the mean total subsynaptic area contacted by each motor axon is comparable in the ALD and PLD muscles. The growth pattern of the axons in the PLD motor nerve was described from the ninth day in ovo up to 6 weeks after hatching. The axons arrive in the PLD muscle in two successive waves: first, the large somatic axons which are already present before the ninth day in ovo and second, the small autonomic axons which continue to accumulate until hatching. The total number of somatic axons decreases from the ninth day until the hatching day when it reaches its definitive value. This decrease takes place during a period when the numbers of myofibers and of endplates dramatically increase, and it coincides with the axonal segregation by the Schwann cells. The myelination of the axons starts on the 15th day in ovo and is essentially complete upon hatching. Despite the decreasing number of somatic axons in the PLD nerve, the decrease in number of nerve endings per PLD endplate and the increasing number of PLD endplates per PLD muscle, it was found that between the 16th day in ovo and 6 weeks after hatching the mean number of axonal branches per PLD motor axon does not decrease.

Histochemical properties and innervation pattern of fast and slow-tonic fibre types of the anterior latissimus dorsi muscle of the chick

The anterior latissimus dorsi muscle of the chick is largely composed of slow-tonic fibres but contains a few fast fibres defined by their acid-labile, alkali-stable myofibrillar ATPase activity. These fibres are referred to as alpha fibres. Differing from the slow-tonic (alpha' and beta') fibres which are multiply innervated with en grappe endings, the alpha (fast) fibres are innervated by typical en plaque endings. The innervation of thirty-two alpha fibres were studied closely and it was concluded that about a half of the alpha fibres were focally innervated whereas a half were innervated in two distinct loci by en plaque endings. In only one case, a fast fibre with three widely spaced en plaque endings was observed. The mean intersynaptic length was significantly larger on alpha fibres than on alpha' and beta' fibres. No variation in the histochemical properties of myofibrillar ATPase was observed either along the entire length of singly and dually innervated alpha fibres, or along alpha' and beta' fibres. It is concluded that the three extrafusal fibre types of the anterior latissimus dorsi muscle maintain uniform histochemical characteristics along their length in spite of a possible innervation by several motoneurons.

Contractures in normal and denervated inferior oblique muscle of the rabbit

Isometric contracture responses of normal and denervated inferior oblique muscles (IO) of the rabbit have been investigated in vitro at 35 degrees C. The threshold concentration for eliciting potassium contractures was about 20 mM K+. In normal IU low potassium concentrations up to about 50 mM K+ evoked only sustained contractures, higher concentrations were responded by contractures with an initial transient component. The transient tension development was maximal at about 100 mM K+ the sustained component at 80 mM K+. After denervation the characteristic time course of the contractures was not changed, but the tension output of the preparation was diminished and long-term denervated IO have a somewhat lowered threshold. In normal IO acetylcholine (ACh), succinylcholine (SCh) and choline (Ch) caused also sustained contractures, the threshold doses were about 5 microM for ACh and SCh and 500 microM for Ch. The ACh sensitivity of the preparations was increased by physostigmine and decreased or abolished by d-tubocurarine. Denervation increased the drug sensitivity but the shape of the contractures was hardly influenced. The properties of slow tonic muscle fibres in mammalian extraocular muscles (EOM) probably responsible for sustained contractures and their changes after denervation are discussed.

Potassium and caffeine contractures in fast and slow muscles of the chicken

1 K+ contractures, caffeine contractures and electrical properties were studied in slow (posterior latissimus dorsi; p.l.d.) and fast (anterior latissimus dorsi; a.l.d.) chicken muscles. 2. P.l.d. K+ contractures show a transient increase of tension that relaxes spontaneously. Contractures in a.l.d. show an initial component followed by a maintained tension. 3. A.l.d. K+ contractures of similar amplitude and time course were reproduced at 4 min intervals. In p.l.d., the interval needed for full recovery is about 30 min. In Cl-free saline p.l.d. and a.l.d. K+ contractures can be reproduced at 4 min intervals. 4. The time course of repolarization after a short exposure to 160 mM-KCl was much slower in p.l.d. than in a.l.d. In Cl-free saline the time course of repolarization becomes faster in p.l.d. 5. The membrane resistance was not modified in a.l.d. and was increased in p.l.d. by Cl-free saline. The calculated Cl- conductance in p.l.d. was about 70% of the total membrane conductance. 6. In a.l.d., Mn2+, D600 and external Ca2+ reduction greatly diminishes the maintained phase of the K+ contracture leaving the initial phase almost unmodified. Under similar conditions p.l.d. K+ contractures were slightly reduced. 7. P.l.d. caffeine contractures (10-40 mM) were not maintained and they were not modified by Ca-free saline, Cd2+, Co2+, Mn2+ and D600. 8. A.l.d. caffeine contractures (2-15 mM) were maintained and were highly dependent on external Ca2+. In addition they were greatly reduced by Cd2+, Co2+, Mn2 and D600. 9. It is suggested that caffeine contractures of a.l.d. are elicited by a Ca2+ entry into the muscle from the external fluid.

The influence of innervation on differentiating tonic and twitch muscle fibres of the chicken

1. The anterior (ALD) and posterior (PLD) latissimus dorsi muscles of adult chickens were denervated by section of their motor nerves. Four weeks later the contractile and membrane properties of these muscles were studied in vitro at room temperature.2. Although the time course of the PLD muscle twitch was slightly prolonged, the qualitative difference in contractile characteristics of the slow ALD and the fast PLD muscles was maintained after denervation.3. The difference in the passive membrane characteristics of the ALD and PLD muscle fibres was not lost after denervation, although the membrane resistance (R(m)) and space constant (lambda) of the denervated muscles fell. The membrane resistance, space (lambda) and time (tau(m)) constants of the ALD muscle remained significantly greater than for the PLD muscle fibres. The absolute values of tau(m) in both muscles increased, implying that in the case of the ALD the membrane capacitance (C(m)) was increased above normal after denervation. This is discussed in terms of the ultrastructural changes in this muscle after denervation.4. The ALD muscle was cut into small pieces and replaced in the bed of the PLD muscle, which in turn was minced and placed into the bed of the ALD muscle. These muscles regenerated and became reinnervated by the PLD and ALD nerves respectively. They aligned themselves in the muscle bed and adopted the former shape of the muscle that they replaced. The passive cable properties of the regenerated ALD muscle fibres innervated by the PLD nerve resembled the control PLD fibres and the regenerated PLD fibres reinnervated by the ALD nerve resembled those of the control ALD. Regenerated ALD and PLD reinnervated by their own nerves had contractile and membrane properties similar to those of control muscles.5. The results show that the fundamental differences, between slow and fast muscles once established, persist even when they are deprived of their innervation. The properties of developing muscle fibres however are determined by the motor nerves even in the adult animal.

Development of neuromuscular junctions of fast and slow muscles in the chick embryo: a light and electron microscopic study

The morphogenesis of neuromuscular injections (NMJ) was studied by electron microscopy in fast posterior and slow anterior latissimus dorsi muscles (PLD and ALD) of chick embryos. In 8 day embryos, the NMJ is already established in both types. In PLD at this stage, individual axons completely ensheathed by Schwann cell processes form NMJs with myotubes, whereas in ALD axon terminal lie adjacent to (and not separated by Schwann cell processes from) naked axons which are components of a nerve bundle. At 11-15 days, the number of profiles of axon terminals at each endplate increases in both PLD and ALD. In PLD, individual axon terminals are ensheathed by Schwann cells and often branch. In ALD, several axon terminals become ensheathed as a group by processes of a Schwann cell, along with a small number of adjacent naked axons. The individual axon terminals were confirmed by analysis of serial sections to originate from different preterminal axons. Thus, the increase in number of axon terminals in PLD may be due to extensive terminal branching, whereas in ALD it may be due to the arrival of other nerve fibres. From 16 days, each axon terminal in an endplate of ALD becomes individually ensheathed by Schwann cell processes. However, the property of polyneuronal innervation in each endplate is retained even in the adult muscle. The junctional sarcoplasm protrudes to separate individual axon terminals at certain developmental stages: the protuberances are thinner and more numerous in ALD than in PLD at 15-16 days. It is concluded that NMJ morphogenesis differs between PLD and ALD and that the differences reside mainly on the neuronal side.

Differentiation of electrical and contractile properties of slow and fast muscle fibres

1. The development of the electrical and contractile properties of slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of the chick was studied. 2. At the earliest ages studied (14-16 days of incubation), the membrane resistance was similar in both muscles. Subsequently the membrane resistance of PLD decreased, and that of ALD increased. Electrical differentiation continued after hatching. 3. The contractile speeds of ALD and PLD muscles were similar in embryos at 14-16 days of incubation, and suddenly differentiated at 17-18 days. Unlike the electrical properties, contractile speeds changed little after hatching. 4. It is suggested that the different electrical properties of the membrane of the two types of muscle fibre develop as a result of the particular type of excitation imposed by their nerves.

Neurotrophic effects on the electrical properties of cultured muscle produced by conditioned medium from spinal cord explants

The passive electrical properties of cultured chick skeletal muscle are significantly altered when the muscle is co-cultured with spinal cord explants. A reduced transverse membrane resistance appears to be responsible for the smaller values of input resistance, space constant, and time constant observed in co-cultures relative to those observed in pure muscle cultures. In this report, we show that neuromuscular junctions are not required in order to observe this neurotrophic effect because medium from spinal cord explant cultures is capable of producing the same reduction in transverse membrane resistance as the co-culturing of spinal cord explants with muscle. Control medium from liver explant cultures has no effect on muscle passive electrical properties. These results indicate that a trophic substance which is capable of regulating the electrical properties of excitable cells is released into the culture medium by spinal cord explants.

Differentiation of slow and fast muscles in chickens

1. The development of the characteristic histochemical appearance of the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) was studied in chickens during embryonic development as well as during regeneration of minced muscle. 2. During embryonic development the activity of the oxidative enzyme succinic dehydrogenase (SDH) is higher in the slow ALD muscle already at 16 days of incubation. At this time the fast PLD has a higher activity of the glycolytic enzyme, phosphorylase. Although the histochemical appearance of the two types of muscle is already different at 16 days, their contractile speeds are still similar. No difference in myosin ATP-ase was found in the two muscles in young embryos but in 20-day old embryos the two muscles became distinctly different when stained for this enzyme. 3. When PLD muscles in hatched chickens redeveloped during regeneration in place of ALD the histochemical characteristics of the regenerated muscle resembled ALD, and when ALD regenerated in place of PLD it resembled PLD. 4. It is concluded that the histochemical characteristics of slow and fast muscles become determined during early development, even before any difference in contractile properties can be detected and that they are determined by the nerve.

[Effect of curarization on sensitivity of chicken latissimus dorsi to electric current]

(1.) To test the sensitivity to electric current of the latissimus dorsi and, therefore, its ability to yield action potential, we have studied the effect of curarisation on the amplitude of the isometric tetanus during "massive" stimulations by alternative current. (2.) Curarisation causes a considerable but reversible reduction of the tetanic tension of the latissimus dorsi anterior (L.D.A.) but has no effect on the tetanus of the latissimus dorsi posterior (L.D.P.). It is concluded that a part of the L.D.A. is not sensitive to electric current and is thus unable to yield action potentials, unlike the opinion of some authors. (3.) Rising the temperature has an inverse effect on the speed of the tetanus ascending phase, depending on whether the L.D.A. has been curarised or not. This show the existence of two types of excitation processes of very different nature in this muscle. (4.) Direct current is much less efficient than alternative current on the curarised L.D.A.

Recovery of sensitivity to acetylcholine following desensitization in muscles of different vertebrate species

The recovery after desensitization (DS) of the frog sartorius muscle postsynaptic membrane has a constant rate with a half-time of about 6 sec at 22 degrees C. Several other avian and mammalian species were also examined to answer the question, whether this parameter differs in muscles of different function and acetylcholine receptor properties. The following muscles were studied using iontophoretic microapplication of ACh: frog m. cutaneus pectoris, chick anterior and posterior latissimus dorsi, innervated and denervated rat diaphragm, and diaphragm of awake and hibernating golden hamsters. In all cases, desensitization developed when series of 6-20 short ACh doses were applied to the end-plate region of the muscle fibre membrane without any DS potentiating drugs. Rate of DS onset was dependent on the dose and/or frequency of ACh application. The rate of recovery, however, differs very little in all the muscles studied, the half-times being in the range of 5-6 sec at 22 degrees C. A similar rate of DS recovery was thus found in muscles with different pharmacological properties of cholinoreceptors (e.g. different sensitivity of frog and rat innervated and denervated muscles to d-tubocurarine), or different ionic pathways during activation (hibernating golden hamster). These results support the assumption that desensitization is not of receptor origin and indicate that the system operating during DS does not undergo substantial changes during the functional differentiation of vertebrate muscle membrane.

Changes in chemosensitivity of developing chick muscle fibres in relation to endplate formation

1. The sensitivity to acetylcholine (ACh) of the fast posterior latissimus dorsi (PLD) and slow anterior latissimus dorsi (ALD) during embryonic development was studied and compared. The sensitivities were expressed as a ratio of the maximal tetanic tension and tension developed in response to ACh. 2. Up to the 17th day of incubation both muscles are sensitive to ACh to a similar extent; at the 18th day the sensitivity of the PLD muscle decreases and continues to do so until hatching and thereafter. 3. Since the decrease in sensitivity of PLD muscles takes place a few days after innervation, it is suggested that this is caused by activity of the motor nerve. To test this curare (dTc) and hemicholinium (HC-3), drugs that interfere with neuromuscular transmission, were injected into the yolk sac of the embryos when nervemuscle connections are usually established. In the curare and HC-3 treated embryos the desensitization of the PLD muscles did not take place. 4. The distribution of endplates on PLD muscles from drug treated 20-21 day old embryos was compared to that of untreated controls. Whereas control PLD muscles have only one band of endplates, muscles from curarized embryos and HC-3 treated embryos have several bands of endplates, and many muscle fibres with multiple innervation were found. 5. It is suggested that nerve fibres which make connections with PLD muscle fibres bring about a decline in chemosensitivity by releasing more transmitter, and thereby prevent further nerve muscle connections from being made along the same muscle fibre.

The influence of innervation on the differentiation of contractile speeds of developing chick muscles

1. The role of innervation of the differentiation of contractile speeds was studied in the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscle of the chick. 2. These muscles become innervated during the 12th and 15th day of embryonic development. At this time both muscles contract and relax extremely slowly and their contractile speeds are very similar. By the 18th day their contraction and relaxation becomes more rapid. It is at this time that the contractile characteristics of both muscles also become very different from each other, ALD being about 3 times slower than PLD. Thus innervation percedes differentiation of contractile speeds by several days. 3. The influence of innervation on the contractile characteristics of developing slow and fast muscles was studied during muscle regeneration in adults. When a slow ALD muscle was minced and implanted in place of a fast PLD the newly regenerated ALD became innervated by a PLD nerve and resembled a fast PLD. Conversely, when PLD muscles were minced and replaced ALD the regenerated PLD was innervated by ALD nerve and became slow. 4. Histological examination revealed that the regenerated ALD became focally innervated, and the regenerated PLD multiply innervated. 5. Thus, the contractile speeds are not predetermined properties of the muscle fibre. Both contractile characteristic and the pattern of innervation of developing muscles are determined by the motor nerve.

Further evidence that extrinsic acetylcholine acts preferentially on extrajunctional receptors in the chick biventer cervicis muscle

The specificity of action of extrinsic acetylcholine on extrajunctional and junctional receptors in the chick biventer cervicis muscle was studied by determining its ability to protect the responses evoked by acetylcholine and by tetanic nerve stimulation from the blockade by alpha-bungarotoxin, an irreversible binding agent of acetylcholine receptors. At concentrations of 50-100 mug/ml, acetylcholine caused a desensitization to extrinsic acetylcholine but not to nerve stimulation and protected only the contractile response to extrinsic acetylcholine from the toxin blockade whereas neither the response to tetanic nerve stimulation nor the endplate potentials were protected. For the protection of the latter, higher concentrations of acetylcholine were needed. In the presence of physostigmine, a concentration of acetylcholine as low as 10 mug/ml protected the endplate potentials from the toxin blockade. By contrast, d-tubocurarine protected the tetanic contraction and the endplate potentials induced by nerve stimulation at a concentration which produced the same protection of acetylcholine-induced contraction as that produced by 50-100 mug/ml acetylcholine. These results indicate that in contrast to d-tubocurarine, extrinsic acetylcholine at low concentrations acts preferentially on the extrajunctional receptors in the absence of an anticholinesterase.

Acetylcholinesterase in singly and multiply innervated muscles of normal and dystrophic chickens. II. Effects of denervation

Neural regulation of mature normal fast twitch muscle of the chicken suppresses high activity, extrajunctional localization, and isozyme forms of acetylcholinesterase (AChE) characteristic of embryonic, denervated and dystrophic muscle. Normal adult slow tonic muscle ofthe chicken retains intermediate levels of activity and embryonic isozyme forms but not extrajunctional activity; it is not affected by muscular dystrophy. The hypothesis that neural regulation of the AChE system is lacking in slow tonic muscle and thus not affected by dystrophy was tested by denervating the fast twitch posterior latissimus dorsi and slow tonic anterior latissimus dorsi muscles of normal and dystrophic chickens. Extrajunctional AChE activity and embryonic isozyme forms increased, then declined, in both muscles. The results suggest that ocntrol of AChE is qualitatively similar in slow tonic and fast twitch muscle of the chicken.

Electrophysiological properties of biventer cervicis muscle fibers of normal and roller pigeons

Cable parameters, excitability characteristics, and contractile response to acetylcholine were measured in biventer cervicis muscles from Helmet pigeons, Racing Homer pigeons and Parlor (nonflying) Roller pigeons. Cable parameters for the three strains, were respectively: calculated diameter, 30.1, 42.5, and 37.3 mum; membrane resistance, 450, 556, and 386 omega-cm2; membrane capacitance, 4.2, 3.9, and 4.5 muF/cm2, and myoplasmic resistivity, 79, 185, and 116 omega-cm. Significant differences between excitability characteristics of Homer pigeon and Roller pigeon fibers were a 17% shorter maximal latency for spike initiation (P less than 0.025) and 24% lower rheobasic current (P less than 0.05) in Roller fibers. Dose-response curves of isolated biventer cervicis to acetylcholine revealed slight, but significant, differences between Helmets and Rollers. These are the first electrophysiological data from pigeon skeletal muscle and the first from any avian biventer cervicis. The biventer muscles of chickens contain mainly "slow" fibers, but our results show that pigeon biventer fibers have properties similar to the "fast" PLD fibers of the chicken. Furthermore, the existence of different myoplasmic resistivities for each strain of pigeons used in this study suggests the need for more careful determination of this parameter in electrophysiological investigations. Although our results show that Roller pigeon fibers differ from those of nonrolling pigeons in the respects described above, these differences are minor in comparison to the severe behavioral abnormalities of Roller pigeons. Some yet untested component of neuromuscular transmission may be directly involved in the rolling phenomenon, but the differences we report may simply be due to strain differences, muscle hypertrophy, or a more severe defect elsewhere in the nervous system.

An electrophysiological and morphological study of normal and denervated chicken latissimus dorsi muscles

1. Some electrophysiological and morphological properties of 'fast' (singly innervated) and 'slow' (multiply innervated) muscle fibres were studied in normal and denervated posterior and anterior latissimus dorsi muscles of the young chickens. 2. Normal singly and multiply innervated muscle fibres are capable of generating action potentials which in all qualitative respects are similar. 3. The action potentials of multiply innervated muscle fibres are of lower amplitude and slower maximum rate of rise than action potentials in singly innervated muscle fibres. 4. Denervation causes the resting membrane potential and the maximum rate of rise of the action potential to fall. The changes are greater in singly innervated than in multiply innervated fibres, but in neither case are as great as in mammalian skeletal muscle fibres after surgical denervation. 5. In neither singly nor multiply innervated muscle fibres do the action potentials generate any 'resistance' to tetrodotoxin as a result of denervation. 6. The diameter of multiply innervated fibres is increased after denervation, but it is reduced in singly innervated fibres. The number of myofilaments increases in multiply innervated fibres, but decreases in single innervated fibres. In both types of muscle fibre the volume fraction of myofibrils is decreased. 7. In the singly innervated muscle fibres there is an increase in the volume fraction of mitochondria. 8. In the singly innervated muscle fibres, there is some rearrangement of the membrane systems in that some of the transversely orientated triads are replaced by longitudinally orientated dyads.

The formation of synapses in reinnervated and cross-reinnervated adult avian muscle

1. A study has been made of the formation of synapses in spontaneously reinnervated and cross-reinnervated anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of adult fowls.2. Denervated ALD and PLD muscle fibres have a uniform and high sensitivity to iontophoretically applied acetylcholine (ACh). During early reinnervation the sensitivity distribution to ACh of the ALD muscle fibres begins to return to normal before synaptic potentials can be evoked. The normal ACh sensitivity distribution of PLD muscle fibres is also restored after reinnervation. After cross-reinnervation of the ALD and PLD muscles the ACh sensitivity distribution of many of the muscle fibres is again restored to normal.3. Reinnervating and cross-reinnervating ALD nerve terminals showed a greater than normal degree of facilitation of transmitter release when a test impulse was applied at various intervals after a conditioning impulse. Cross-reinnervating PLD nerve terminals showed facilitation of transmitter release rather than the normal depression in a conditioning-test impulse sequence.4. The distribution of nerve terminals over the surface of spontaneously reinnervated and cross-reinnervated ALD and PLD muscle fibres has been determined from an examination of the sensitivity distribution to applied ACh, the graded versus all-or-none nature of the evoked potential and the distribution of cholinesterase stained synapses.5. The results suggest that the innervation pattern of individual ALD and PLD muscle fibres is restored both after spontaneous reinnervation and cross-reinnervation.

A physiological study of chick myotubes grown in tissue culture

1. A study has been made of some passive and active membrane properties of myotubes of different ages obtained in culture from explants of chick embryo thigh muscle.2. After 3 days in vitro the mean values for the myotube resting membrane potential and input resistance were - 63.8 mV and 1.30 MOmega respectively. By 13 days these values had fallen to - 51.0 mV and 0.80 MOmega.3. Current/voltage relations were measured in the presence of tetrodotoxin. The relations were linear for membrane potentials between - 120 and - 35 mV. Further depolarization usually resulted in a delayed increase in conductance which inactivated with time.4. All myotubes tested using anodal break excitation were capable of generating action potentials. Action potentials were blocked by tetrodotoxin, saxitoxin and procaine.5. All myotubes were sensitive to iontophoretically applied ACh. The potential change produced by ACh reversed polarity at a membrane potential between 0 and + 10 mV. The depolarization produced by ACh was unaffected by anticholinesterases.6. The ACh response was blocked by cobra neurotoxin, D-tubocurarine and atropine.7. The electrical properties of the myotubes appear to resemble those of normal adult twitch-type skeletal muscle fibres.8. The pharmacological properties of the myotube cholinergic receptor have been compared with those of the neuromuscular junction and the denervated muscle fibre membrane.