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

Summary The morphogenesis of neuromuscular junctions (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 terminals 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.     

Reduction of multiaxonal innervation at the neuromuscular junction of the rat during development.

1. A study was made of the functional and structural changes that occur during the decrease in multiaxonal innervation of neonate rat muscle fibres 2-14 days after birth. 2. In day 8 to day 14 animals there was a constant daily loss in the average number of functionally transmitting axons/muscle fibre measured electrophysiologically. An investigation of synaptic transmission during this period revealed that the loss of functional contact from the supernumerary axons was not preceded by any sign of failing terminal conduction or a gradual decrease in transmission efficacy but rather appeared to occur abruptly. 3. Neonate end-plates showing signs of abnormal ultrastructure were observed during the period of synapse elimination. Some axon terminals had a high cytoplasmic density and condensation of synaptic vesicles. Signs of Schwann cell encroachment into the synaptic cleft were readily found and large areas of post-junctional membrane apposed only by Schwann cell were evident. 4. It is suggested that the mechanics of the process of synapse elimination in neonates is similar to that occurring during degeneration in the denervated adult. Transmission failure occurs abruptly at the supernumerary endings and they are disposed of by the Schwann cell.     

Morphogenesis of motor endplates along the proximodistal axis of the mouse hindlimb

The morphogenesis of motor endplates along the proximodistal hindlimb axis is described for the mouse using nonspecific cholinesterase histochemistry and electron microscopy. There is a two day lag in relative stages of development between a proximal muscle (rectus femoris, RF) and a distal muscle (flexor hallucis brevis, FHB). Cholinesterase activity first appears in the RF on embryonic day 15 and the FHB on embryonic day 17. In the following days, faint wisps of reaction product thicken, form small ovals on myotubes, and finally enlarge with internal ramifications as the muscle fibers increase in diameter. Axons first enter the RF between embryonic days 12 and 13, and contact both embryonic cells (most likely myoblasts) and cells assumed to be Schwann cells. Myotubes are present in the RF the following day. The first signs of synapse formation—appearance of symmetrical electron opaque membrane patches, and dense cored and synaptic vesicles—occur between axons and myotubes in the RF on embryonic day 15. During the following days basal lamina material accumulates in the synaptic cleft, coated vesicles and postjunctional folds appear in the myotubes, and synaptic vesicles accumulate in the axon terminals. By postnatal day 42 the axon terminals lay in primary gutters opposite deep secondary postjunctional folds, and are separated and capped by Schwann cell processes.     

Effacement and regeneration of tactile lamellar corpuscles of rat after postnatal nerve crush

The development of Meissner-like lamellar corpuscles was studied in rat toe pads under normal conditions and after crushing the sciatic nerve in 1- to 15-day-old animals. During normal development, rat lamellar corpuscles begin to differentiate first by postnatal day 8. By this time, sensory axons have grown up to the apex of dermal papillae and form axon terminals beneath epidermis. The terminals are ensheathed by lamellar cells derived from Schwann cells. First thin lamellae are formed around the terminals 8-12 days after birth, and the number of lamellar layers increases until the corpuscles become structurally mature by 20 days after birth. A mature corpuscle consists of two or more terminals, each surrounded by approximately 10 lamellae, all components being enclosed by an incomplete capsule. No lamellar corpuscles develop in toe pads after crushing the sciatic nerve in newborn rats, and only occasional corpuscles regenerate after nerve crush at 5 days of age. The corpuscles fail to develop because dermal papillae remain permanently denervated after crushing the nerve early postnatally. After nerve crush in 10-day-old rats, lamellar corpuscles regenerate by 1 month after the operation, but they remain underdeveloped: their number and size are smaller than normal even 1 year after injury, and their terminals are encircled only by 1-3 lamellar layers. After nerve crush in 15-day-old rats, the corpuscles recover upon reinnervation and their size and lamellation become almost normal.     

Protease inhibitors reduce the loss of nerve terminals induced by activity and calcium in developing rat soleus muscles in vitro

The end-plate of a mammalian skeletal muscle fibre is innervated by several axons at the time of birth but by only one axon in the adult. In the rat soleus muscle the transition from polyneuronal to single innervation occurs during the first 2–3 weeks after birth. While it is evident that the loss of the excess nerve terminals depends to some extent on neuromuscular activity, the mechanism involved is not known. In the present experiments neonatal rat soleus muscles were stimulated in vitro in the presence of a variety of combinations of calcium, the cholinesterase inhibitor edrophonium and the proteolytic enzyme inhibitors leupeptin, pepstatin and Ep-475. Electron microscopical examination revealed that stimulation alone had little effect on the morphology of the end-plate region but stimulation in the presence of raised levels of calcium caused severe disruption of the nerve terminals and a marked reduction in the number of intact nerve terminal profiles contacting each end-plate. Contraction measurements showed that, in spite of this, the muscles were not functionally denervated to any large extent. The addition of edrophonium potentiated the morphological alterations but caused no further reduction in the number of profiles. Conversely, the protease inhibitors wholly or partially (in the case of Ep-475) prevented the effects of stimulation and calcium on the nerve terminals.These results are consistent with the idea that neuromuscular activity induces the secretion of proteolytic enzymes into the end-plate region, where they digest the immature nerve terminals. The importance of calcium suggests that the calcium-dependent neutral protease may be involved and is also consistent with a secretory mechanism. The possibility that the nerve terminals are digested by their own proteases is also discussed.     

Cytological organization of the dorsal lateral geniculate nuclei in mutant anophthalmic and postnatally enucleated mice

Summary The dorsal lateral geniculate nuclei (dLGN) of anophthalmic and early postnatally enucleated mice were studied to determine the role retinal fibres play in the differentiation of postsynaptic target structures. Cell counts indicate that retinal fibres are necessary for the development and maintenance of the normal number of dLGN neurons and glia. This retinal fibre dependence is greater for animals enucleated on postnatal day 3 than for animals with a congenital absence of optic axons. Golgi analysis reveals, however, that the lack of retinal fibres does not preclude the development of the thalamo-cortical and intrinsic types of dLGN neuron.Ultrastructural analysis reveals that in anophthalmic and early postnatally enucleated mice, dLGN synaptic sites normally occupied by optic axon terminals become innervated by large terminals containing round synaptic vesicles and mitochondria with an electron dense matrix. Significantly, the formation of these replacement terminals does not seem to depend upon either the previous existence of retinal fibres or the early postnatal stage at which retinal fibres are removed. The possibility that some of these large replacement terminals originate from cortical or recurrent collateral axons is considered.     

Ultrastructural observations on synapse elimination in neonatal rabbit skeletal muscle

Summary Electron microscopic techniques were used to investigate two main questions about mammalian neuromuscular development. One, does neonatal synapse elimination proceed by the degeneration of synaptic terminals and preterminal axons, or are the terminals retracted into the parent axon, in a process analogous to the resorption of axonal growth cones? Two, is there any discernible relationship between the elimination of supernumerary synapses and the myelination of preterminal axons? Examination of several hundred sections through endplates fixed at the peak time of synapse elimination revealed no signs of degeneration. This result is not consistent with the proposal that the major mechanism of synapse elimination is terminal degeneration, according to calculations based on the time course of terminal degeneration following neonatal nerve transection.Serial and semi-serial reconstruction of terminals and preterminal axons suggest that myelination of intramuscular axons lags behind synapse elimination and that elimination can proceed while axons bear an immature relationship to Schwann cells. In addition, reconstruction of serial sections through neonatal synapses revealed that their three-dimensional configuration is more complex than that of mature neuromuscular synapses; this feature may be indicative of a dynamic relationship between nerve and muscle at early stages.     

The development of Golgi tendon organs

Summary The fine structural development of Golgi tendon organs (GTOs) was studied in leg muscles of the rat from day 18 gestation until 2 months after birth. Small axons were found at the aponeuroses on day 18, but GTOs were first identified on day 20–21 gestation. Prenatally, they appear as discrete islets in the aponeurosis in which a single Ib axon branches and terminates among fibroblasts, collagen bundles and on myotubes inserting into the tendinous tissue.The receptor body is formed during week 1 postnatal as a result of several concurrent processes: (1) New terminals arise and form numerous contacts with 5–9 myotubes inserting into the GTO area. The innervated myotube tips gradually recede from the aponeurosis. (2) Schwann cells and fibroblasts proliferate and form longitudinal strands adjoining the receding myotubes and converging towards the attachment to the aponeurosis. (3) Axonal branches ensheathed by Schwann cells form terminal enlargements and small-sized terminals. Terminal enlargements are predominantly filled with mitochondria, whereas their peripheral projections and small-sized terminals mainly contain clear and dense core vesicles. (4) Concomitantly collagen fibrils assemble between Schwann cell processes and the developing terminals, and interconnect with collagen bundles, linking the muscle fibre tips to the aponeurosis. Thus collagen bundles of the GTO, which eventually spiral around the terminals and bring about their depolarization when stretched, are formed concurrently with the developing terminals. (5) The GTO becomes encapsulated from day 2 postnatal onwards. (6) Axon terminals loosen their contacts with muscle fibres and are only found among collagen bundles of the receptor from day 5 postnatal onwards. (7) Muscle-fibre tips recede from the GTO lumen by day 7 after birth.Subsequently, the GTO grows further in length, acquires its fusiform shape by tightening of the capsule at both poles, and becomes structurally mature 2–3 weeks after birth.The Ib sensory axon together with Schwann cells appear to be the main formative agents in the GTO development, the Ib terminals presumably inducing the retraction of myotubes from the aponeurosis, and the Schwann cells affecting the assembly of collagen fibrils and bundles surrounding the terminals.     

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.     

The fine structure of the Golgi tendon organ

Summary The Golgi tendon organ (GTO) has a capsule composed of cells confluent with the perineural epithelial sheath surrounding the Ib afferent nerve. Fluids of the capsule lumen are isolated from extra-capsular fluids by the capsule wall; its tight-fitting collars form seals through which collagen bundles enter and leave the proximal and distal ends of the fusiform capsule. The capsule lumen between the sealed capsule openings, is divided into longitudinal compartments by delicate processes of septal cells. Collagen bundles spiral down through the longitudinal axis of the GTO. As they descend through the lumen, the bundles are distinctly separated from each other by fluid-filled spaces. These collagen bundles divide, twist, and regroup over short longtiudinal distances and unmyelinated axons interwine among them. Compartments which are densely filled with collagen are not innervated. Axon profiles appear in four forms: (I) myelinated branches of Ib axon, (II) unmyelinated axons surrounded by Schwann cell processes and basal lamina, (III) axons covered only by basal lamina, and (IV) axons with bare surfaces. We postulate that increased tensile forces on the collagen bundles caused by muscle contraction tighten the braided collagen bundles thereby squeezing and distorting the axon terminals. The proposed mechanical events are consistent with known discharge characteristics of Ib axons.     

The structural relations between nerve fibres and muscle cells in the urinary bladder of the rat

Summary Intramuscular nerve fibres in the bladder of adult female rats were investigated by means of serial sections. The following observations were made. (1) Upon penetrating into the musculature the nerve bundles branch repeatedly, and almost all turn into single fibres; their axons become varicose, the Schwann cell sheath is attenuated, incomplete or absent, and the separation between axonal membrane and muscle cell membrane is reduced to tens of nanometres. (2) All single axons, and some of those within bundles, are varicose, but the characteristic of being varicose is expressed by degrees, and is not an all-or-none state. (3) Varicosities contain vesicles (mostly of the agranular type), microtubules (with little connection with the axolemma or the vesicles), some neurofilaments (scarce or absent in the best developed varicosities), mitochondria (whose size is on average smaller than those of the perikaryon, and a minute amount of endoplasmic reticulum. (4) Terminal varicosities, the true anatomical ending of an axon, are often devoid of Schwann cell sheath, are packed with vesicles, rarely contain microtubules or neurofilaments, and lie close to a muscle cell: the gap is often reduced to 10 nm. (5) Schwann cells accompany the axons within the muscle strands. Unlike the area of the axonal profiles, the area of glial sheath changes little along the length of the nerve fibre, except towards its end. (6) The Schwann cell sheath around a varicosity is often incomplete; the area of the axolemma thus exposed is covered by the basal lamina, and is here referred to as a window. While some varicosities have a window only a few tens of nanometres in width, others have more than one window, and some are devoid of Schwann cell altogether, so that their entire axolemma is in contact with the basal lamina. The Schwann cell never extends beyond the axon, whereas very often (and possibly always) the axon extends beyond the Schwann cell. (7) Intervaricose segments vary in length and diameter, the narrowest ones accompanying the more clear-cut varicosities. Some intervaricose segments are as small as 50 nm in diameter, contain a single microtubule and lack a Schwann cell sheath. Others, sheathed by a Schwann cell, contain a single neurofilament or no organelles at all. (8) Specialized contacts between an axon and a muscle cell (neuro-muscular junctions) are abundant and are identified by four features: the axon is a varicosity packed with vesicles; the axolemma is exposed (presence of a window); the distance between the two membranes ranges between 10 and 100 nm, mostly 30–50 nm; and the intercellular gap excludes fibrils, such as collagen, but is occupied by a single basal lamina. Any of these parameters, however, can also occur uncoupled (windows on intervaricose segments; varicosities without a window; exposed axolemma far from a muscle cell). (9) There are no direct contacts between axons. Even when they run close to each other within a bundle, they are always separated by a Schwann cell process. (10) The muscle cell membrane is concave beneath the varicosities; however, the muscle cell ultrastructural features in the region of the neuro-muscular junction are not different from those in other regions of the cell. (11) On average there is more than one neuro-muscular junction per muscle cell, and examples of muscle cells receiving multiple nerve endings from one or from two axons are picked up by the serial sections. (12) A striking feature of the bladder innervation is the variability of its ultrastructural parameters. The bladder innervation does not appear to be built on a rigid structural plan, and the notion of loose-patterned innervation is presented.     

A quantitative ultrastructural investigation of tyrosine hydroxylase-immunoreactive axons in the hairy skin of the guinea pig

Besides its thermoregulatory role, the sympathetic innervation of the skin is involved in a modulation of sensory processing and trophic functions that has not been fully characterized. To investigate possible sites at which such sympathosensory interactions might occur, a quantitative ultrastructural study of the sympathetic innervation of the skin was attempted. The hairy skin of the guinea pig was studied because the sympathetic and sensory nerve axons in this species can easily be discriminated by the presence of immunoreactivity to the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH). The thermoregulatory role of the sympathetic skin innervation was highlighted by the almost exclusive sympathetic innervation of piloarrector muscles which contained 62% (n = 195) of randomly selected TH-immunoreactive (TH-IR) axon profiles. Of TH-IR pilomotor axons, 53% were filled with vesicles. Vesicle-containing axonal profiles were equally frequent around dermal arterial blood vessels (partly associated with mast cells), hair follicles, and within nerve fibre bundles surrounded by a perineural sheath, in each case accounting for about 3% of all dermal TH-IR axonal profiles. In contrast to piloarrector muscles, at these locations TH-IR (sympathetic) and non-reactive (sensory) axons were found in close association. These findings are in line with the previously reported inhibitory influence of sympathetic stimulation upon hair follicle afferents and perivascular sensory nerve terminals. In addition, they point to a yet underestimated target of sympathetic axon terminals, i. e. preterminal nerve fibre bundles.     

Structure of rat aortic baroreceptors and their relationship to connective tissue

Summary The ultrastructure of fibres and sensory terminals of the aortic nerve innervating the aorta between the left common carotid and left subclavian arteries was investigated in the rat. This is the region from which most baroreceptor responses are recorded electrophysiologically. The fibres of the aortic nerve enter the adventitia and separate into bundles generally containing one myelinated fibre and four or five unmyelinated fibres of various sizes. The bundles pursue a roughly helical course through the adventitia; when they are close to the aortic media, the myelinated fibre loses its myelin sheath. A complex sensory terminal region is formed, as both the unmyelinated and premyelinated axons become irregularly varicose. The concentration of mitochondria becomes very dense and cytoplasmic deposits of glycogen are observed. Both unmyelinated and premyelinated axons branch, and the unmyelinated axons wind irregularly around the premyelinated axon. The latter may have several loops and small holes. The terminal regions of both types of axon contain clusters of clear 40 nm vesicles. Part of the surface of each terminal region is ensheathed by Schwann cells, but the rest of the axolemma is directly exposed to extracellular connective tissue. There are often several layers of basal lamina around the sensory terminals and parts of the axolemma and Schwann cell membranes are attached to it by fine fibrillar material. The basal laminae are also attached to fibroblasts, fibroblast-like perineurial cells and elastic laminae, and the whole cellular and extracellular system appears to be tightly bound together. No differences between baroreceptors of spontaneously hypertensive and normal rats were found.     

Ultrastructure of motor end-plates during pharmacologically-induced degeneration and subsequent regeneration of skeletal muscle

Summary A local anaesthetic, methyl-bupivacaine was injected into the planta of adult mice, and the ultra-structure of motor end-plates was studied during the degenerative and regenerative cycle induced in lumbrical muscles. Muscle degeneration took place during the first day after drug administration. The postsynaptic part of the neuromuscular junction completely degenerated as did the whole injured muscle fibre. Nerve terminals, however, remained unaffected. By the second day after muscle injury, axon terminals were enclosed within Schwann cell cytoplasm and thus became separated from the residual sarcolemmal tube. One to three days later, when myotubes were formed by fusion of the surviving myoblasts, the layer of Schwann cell cytoplasm on nerve terminals was discontinuous. Subsequently nerve terminals approached the regenerating muscle cell and the subneural apparatus began to differentiate. Slight depressions and furrows appeared on the myotube surface below the nerve ending and the myotube membrane, covered with basement membrane, became undercoated by dense material in this region. Where the distance between nerve ending and myotube was reduced to that found in the normal neuromuscular junction, i.e. to about 500 Å, junctional folds were formed. Fourteen days after drug administration, newly formed end-plates were indistinguishable from those in normal control lumbrical muscles.     

Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. III. Origins and frequency of occurrence of the terminals

Summary The cell bodies of the layer II/III pyramidal cells in rat visual cortex receive three morphologically distinct types of axon terminals. These axon terminals all form symmetric synapses and have been termed large, medium-sized, and dense axon terminals. The present study shows that each of these different kinds of axon terminals contains gamma-aminobutyric acid (GABA) which suggests that they are inhibitory. From an analysis of the profiles of 50 cell bodies it is calculated that the average layer II/III pyramidal cell has 65 axosomatic synapses, of which 43 are formed by medium-sized terminals, 10 by large terminals/and 12 by dense terminals. Comparison of these different kinds of axon terminals with labelled axon terminals of known origin suggests that the medium-sized terminals are derived from smooth multipolar cells with unmyelinated axons, and that at least some of the dense terminals originate from bipolar cells that contain vasoactive intestinal polypeptides. The source of the large axon terminals is not known, but it is suggested that they originate from multipolar non-pyramidal cells with myelinated axons.Since the initial axon segments of these same neurons receive GABAergic axon terminals from chandelier cells, at least four different types of neurons provide inhibition to the cell bodies and axons of layer II/III pyramidal cells. This serves as an illustration; of the complexity of the neuronal circuits in which pyramidal cells are involved.     

Precision of reinnervation of original postsynaptic sites in frog muscle after a nerve crush

Summary Regenerating neuromuscular junctions in the cutaneous pectoris muscle of the frog were examined by light and electron microscopy up to three months after crushing the motor nerve. The aim was to determine the precision of reinnervation of the original synaptic sites. More than 95% of the original postsynaptic membrane is recovered by nerve terminals and little, if any, synaptic contact is made on other portions of the muscle fibre surface. Even after prolonged denervation when the Schwann cells have retracted from 70–80% of the postsynaptic membrane, regenerating terminals return to and cover a large fraction of it. Although synapses are confined to the original synaptic sites, the pattern of innervation of muscle fibres is altered in several ways: (a) regenerating axon terminals can fail to branch leaving small stretches of postsynaptic membrane uncovered; (b) two terminal branches can lie side by side over a stretch of postsynaptic membrane normally occupied by one terminal; and (c) after growing along a stretch of postsynaptic membrane on one muscle fibre, terminals can leave it to end either in extracellular space or on the postsynaptic membrane of another fibre. Altogether the results demonstrate a strong and specific affinity between the original synaptic sites and regenerating nerve terminals.     

Fine structural localization of non-specific cholinesterase activity in rat tendon organs

Electron microscopical localization of non-specific cholinesterase (nChE) activity was studied in tendon organs of the rat hindlimb muscles. The comparison between neurotendinous part (with high nChE activity) and purely collagenous compartment(s) (with very low nChE activity) demonstrated that Schwann cells are the fundamental source of this enzyme in rat tendon organs. Although particles of the nChE reaction product were also found in and around fibroblasts in both neurotendinous and purely collagenous compartments, their contribution to the overall nChE was not significant. nChE activity in rat tendon organs displayed heterogeneity along the Ib sensory axon; the highest activity was related to the Schwann cell investment of the unmyelinated part of Ib axon, lower activity to sensory terminals covered only by basement membrane and negligible activity to the myelinated part of sensory axons. Particles of the non-specific cholinesterase reaction product persisted in the basement membrane of Schwann cells 20 d after degeneration of Ib sensory axons and their terminals. The function of non-specific cholinesterase in sensory receptors is still not clear. It is suggested that this enzyme may be involved in the maintenance of the ionic milieu around sensory axon terminals during or after functional activity.     

The distribution of nodal sprouts in a paralysed or partly denervated mouse muscle

The right gluteus maximus muscles of young adult mice were paralysed with botulinum toxin for up to 21 days or partly denervated by spinal root section for up to 63 days; the intramuscular and extramuscular nerves were then examined in the electron microscope in thin sections of tissue fixed conventionally or stained with zinc iodide-osmium tetroxide. Contralateral muscles were also examined as controls. The distribution of nodal sprouts in the nerves of the paralysed or partly denervated muscles was determined by calculating the mean ratios of unmyelinated to myelinated axons in nerve profiles containing different numbers of myelinated axons (intact or degenerating) and comparing these with control ratios.In paralysed muscles there was a significantly higher proportion of nerve profiles containing one or two unmyelinated axons alongside a single myelinated axon. Nerve profiles containing two or more myelinated axons did not show any increase in the proportion of unmyelinated axons. Thus, there is probably nodal sprouting in paralysed muscles which is restricted to the most distal nodes of the intramuscular nerves.In muscles partly denervated for 8 days there were significant increases in the proportion of unmyelinated axons in nerve profiles which had contained up to 5–10 myelinated axons. After 21 days of partial denervation, similar increases may have occurred in the larger intramuscular nerve profiles and after 63 days there were large increases in the proportion of unmyelinated axons in the main intramuscular nerve branches and in the extramuscular nerve. Nodal sprouting in response to partial denervation is therefore localised initially to the smaller, more distal nerve branches; at later times, some sprouts probably grow slowly in a disto-proximal direction along denervated Schwann cell pathways.The existence of nodal sprouts in paralysed muscles and their restricted distribution in paralysed and partly denervated muscles suggest that the nodal sprouting stimulus is produced by the muscle and acts only at distal nodes.     

An ultrastructural study of the inner core of the Pacinian corpuscle

Summary Pacinian corpuscles from the hindfeet and mesentery of cats have been examined by light and electron microscopy. The study focuses on the inner core region which houses the single, non-myelinated terminal of the afferent axon. This region of the axon possesses specialized axon processes which enormously increase the surface area of the axolemma. The axon processes are long, branched, filiform structures, containing exclusively ~6 nm microfilaments, and are reminiscent of filopodia on the tips of growth cones of axons and dendrites. These axon processes emanate from the two poles of the elliptical terminal axon, from several sites in the transitional zone, and from the entire surface of the ultraterminal axon, the bulbous, branched, distal extremity of the nerve fibre. Each branch of an axon process articulates with an inner core cell hemilamella except at the distal end of the ultraterminal where axon processes may also approximate the inner edge of the outer core. The base of each axon process contains an elaborate array of organelles including clear and dense-core vesicles of synaptic vesicle size, all enmeshed in ~6 nm microfilaments. The structure and location of the axon processes appear to be eminently suitable for detecting pressure transients transmitted through the outer core of the corpuscle. It is suggested that the mechano-electrical transduction system has, as its morphological substrate, multiple units, each formed by a branched axon process and its specialized basal region. The existence of sympathetic axon terminals abutting the central axon is disputed but the presence of unusual, dense-centred and elongated vesicles within cellular profiles close to the axon is confirmed.     

Morphological identification and intrafusal distribution of the endings of static fusimotor axons in the cat

1. Tenuissimus muscles of the cat were prepared in which the motor innervation was reduced to a single gamma axon by cutting all the other motor axons and allowing them to degenerate during a period of 7-12 days. The function of the surviving gamma axon was then determined, and the distribution of its endings ascertained in teased, silver preparations.2. In the ten muscles successfully prepared the function of the surviving gamma axon was static and the motor innervation distributed to the spindles consisted of trail endings. The conduction velocities of the axons ranged from 33 to 48 m/sec.3. A detailed histological analysis was made of thirty spindles innervated by six of the surviving static axons.4. The six static axons distributed trail endings to both bag and chain muscle fibres in the poles of thirty spindles with about twice the frequency of supplying them to poles in which the distribution was restricted exclusively to one type of muscle fibre or the other.5. The density of trail innervation supplied to the bag fibres, in terms of the mean number of terminals per fibre, was typically from one and a half to twice that supplied to the chain fibres. On the other hand, whereas the number of bag fibres supplied with trail endings in a spindle pole was seldom more than one, the number of chain fibres innervated was usually two in a range of one to four.6. The possible effects that partial denervation might have had on the spindles are discussed, but it is concluded that they are unlikely to have affected the results.