Morphology Of Nerve Terminals Research Articles

Biochemistry of neurotransmitter release.

tcalcium channels transiently. The brief intlux of calcium rapidly stimulates release of quanta of transmitter, which in turn produce postsynaptic change in conductance. Anatomical studies of the morphology of nerve terminals have correlated the quantum of transmitter release with exocyto­ sis of the contents of a single synaptic vesicle. The exocytotic event occurs at morphologically specialized regions of the nerve terminal. By comparison with the wealth of physiological and anatomical data, our knowledge of the molecular events that occur at the nerve terminal is meager indeed. How­ ever, recent developments in membrane biochemistry, in our understanding of non-neuronal secretory systems, and in the purification and characteriza­ tion of nerve terminal components encourage the hope that in the next few years the biochemical basis of transmitter release will be unraveled. We review here what is presently known of the molecular interactions that take place during the release of neurotransmitter from nerve terminals. We discuss 1. some of the more recent anatomical and electrophysiolog ical observations on neurotransmitter release that either suggest molecular mechanisms or place restrictions on what molecular models may be con­ structed; 2. the various models suggested by the anatomical and electro­ physiological observations, in the light of our current knowledge of non-neural secretory cells and model membrane systems; and 3. the extent

Morphology of motor nerve terminals on rat soleus muscle fibers reinnervated by the original and by a "foreign" nerve.

Motor nerve terminals in normal “slow” soleus and “fast” peroneus brevis muscles and reinnervated soleus muscles of rats were stained with methylene blue. Reinnervation was induced by cutting the original tibial nerve innervation, and was established either by the tibial nerve within the original innervation band or by the previously transposed “foreign” superficial peroneal nerve outside the innervation band. Terminals were classified as morphological types A, B, or C according to previously defined criteria, and their sizes were measured. In the normal muscles, type B terminals predominate in the soleus muscle, whereas type A ones predominate in the peroneus brevis muscle. In soleus muscles the size of the terminals increases with increasing animal size and age. In the reinnervated muscles the three types of terminals can be recognized. In the self-reinnervated soleus muscles the new terminals after 16 and 54 weeks conform with those occurring normally in soleus muscles. The first identifiable “foreign” terminals ten days after denervation resemble those in normal soleus muscles (type B). With increasing postdenervation time, however, terminals resembling type A ones increase in number at the expense of type B ones. Thus, 3 and 14 weeks after induced reinnervation, the frequency distribution of types of “foreign” terminals is changed and resembles that in normal peroneus brevis muscle. The development continues, however, and after a year most “foreign” terminals resemble type A ones, rendering the frequency distribution consistently different from that in normal soleus as well as peroneus brevis muscles. The size of the terminals steadily increases during this period, an increase beyond the normal increase with age. The results indicate that morphological features of the new motor nerve terminals formed in the experimental models used mainly depend on the type of motor axons reinnervating a muscle, modified however by the type of muscle fibers being reinnervated.

Influence of 5- and 6-hydroxydopamine on adrenergic transmission and nerve terminal morphology in the canine pulmonary vascular bed.

We studied the effects of 5- and 6-hydroxydopamine on adrenergic neurotransmission, fluorescence histochemistry, and nerve terminal ultrastructure in the canine pulmonary vascular bed. Fluorescence histochemistry on stretched preparations and sections of intrapulmonary artery and vein demonstrated that these vessels are well supplied with adrenergic nerves electron microscopy revealed adrenergic terminals in the adventitia and outer third of the media in the artery, but only in the adventitia in the vein. Adrenergic terminals in artery and vein contained many small and a few large dense-core vesicles. At least 20% of the terminals in the artery contained many small agranular vesicles and a few large opaque vesicles; this suggests that they were of the cholinergic type; Such terminals were not found in intrapulmonary veins. Under conditions of controlled blood flow, stimulation of the sympathetic nerves to the lung and intralobar injection of norepinephrine increased pressure in the perfused lobar artery and small intrapulmonary vein in a stimulus-related manner. The rise in pressure in the lobar artery and vein in response to nerve stimulation was blocked after administration of either 5- and 6-hydroxydopamine; Neither agent modified the response of the pulmonary vascular bed to norepinephrine; In contrast, the rise in pressure in the lobar artery and vein in response to both norepinephrine and to nerve stimulation was blocked by phenoxybenzamine, an alpha-receptor blocking agent. The attenuated neurogenic vasoconstrictor response in dogs treated with 5- and 6-hydroxydopamine was associated with a marked decrease in intensity of fluorescence of the abundant adrenergic innervation in both intrapulmonary artery and vein, and with the appearance of an osmiophilic material in dense-core vesicles of adrenergic terminals in artery and vein. We believe that these data suggest that 5- and 6-hydroxydopamine interfere with adrenergic transmission in intrapulmonary vessels by depleting norepinephrine from adrenergic terminals. Furthermore, we conclude from hemodynamic, histochemical, and ultrastructural studies that vasomotor tone in the pulmonary vascular bed can be regulated by the sympathetic nervous system.

Crustacean Neuromuscular Mechanisms: Functional Morphology of Nerve Terminals and the Mechanism of Facilitation

Two aspects of crustacean neuromuscular physiology are discussed: (1) the ultrastructural identification of the excitatory and inhibitory nerve terminals, and (2) the characteristics of, and the possible mechanisms for, facilitation. The first problem was studied in crayfish opener muscle which has one excitatory and one inhibitory axon. One of the nerves was stimulated in the presence of DNP until synaptic transmission failed; the preparations were then fixed for electron microscopy. Whenever the excitatory nerve was stimulated, the terminals with round synaptic vesicles were depleted while nearby terminals with smaller elongate vesicles were normal. When the inhibitory nerve was stimulated, the converse was true. The possible reasons for the diversity in crustacean neuromuscular properties are discussed. Large EPSP's with a high quantal content ( m ), appear to be produced by terminals which are invaded by a propagated spike. Small EPSP's (small m ) appear to be produced by terminals which don't spike and which are depolarized by a decrementally conducted potential. There is an inverse relationship between m and the amount of facilitation. The physiological basis for facilitation is discussed; previous hypotheses are found wanting and a new one is proposed, that of slow depolarization.

The mechanism of selective reinnervation of fish eye muscle. II. Evidence from electronmicroscopy of nerve endings

1. (1) The electronmicroscopic appearance of nerve endings in fish extraocular muscles after nerve section and regeneration is described. 2. (2) Muscles reinnervated by their own or foreign eye muscle nerves contain endings indistinguishable from those in normal muscles. 3. (3) All nerve endings in doubly innervated muscles, which from measurement of ocular reflexes are known to contain some functional and some non-functional nerves, also appear normal. 4. (4) We infer that a correct nerve may take command of a muscle and suppress the function of incorrect nerves without altering the morphology of nerve terminals in any noticeable way.

Morphology of motor nerve terminals subjected to polarizing currents.

THE electric charge carried by synaptic vesicles is of considerable physiological interest. Strong support has recently been provided for the “vesicle hypothesis”, which equates these vesicles with the quantal units of synaptic transmission1,2. The proposal that vesicles are charged has been used to explain excitation-release coupling (ref. 3 and unpublished results of Bass and Moore) and the effects of presynaptic polarization when transmitter is released by nerve impulses4. Electrophoretic behaviour of vesicles, however, has only been investigated with subcellular brain fractions5, and we therefore thought it would be interesting to investigate the problem in intact motor nerve terminals.

NERVE ENDINGS IN MAMMALIAN SKIN

SUMMARY To most authors the study of nerve terminations in mammalian skin has been an exercise in descriptive morphology. A single histological technique was usually considered adequate and presumably impeccable, for observations resulting from different techniques were not compared and the possibility of artefacts was not considered. The literature is thus excessively large and full of controversies over histological minutiae. It also contains theories concerning the organization of the nervous system which are at variance with the results of physiological observations. The work of a few authors is in striking contrast, but their observations have either been ignored or discounted. The recent discovery that nerve fibres can be seen in fresh specimens of cornea under phase‐contrast conditions has led to the development of new neuro‐histological techniques. These display nerve fibres and their terminals selectively in skin. They appear in an undistorted state and are virtually free from artefacts. The use of new techniques has shown that it is characteristic of all nerves entering mammalian skin to terminate in an arborization of fine (< 1μ in diameter), naked, axoplasmic filaments which probably end freely. Ensheathed stem fibres give rise to unencapsulated nerve endings in all skin strata, and terminals from neighbouring stem fibres overlap and interdigitate extensively. Axoplasmic filaments terminate in the cellular layers of the epidermis and in the dermis but in no specific relation to the capillaries. They are found in relation to the myoepithelial and gland cells of sweat glands, and in relation to the adventitia and media of blood vessels. In hairy skin there are in addition to the unencapsulated nerve endings nerves which end specifically in relation to hair follicles. Ensheathed myelinated stem axons give rise to two distinct and separate series of arborizations of fine, naked, axoplasmic filaments which lie at right angles one within the other. The outer series encircle the hair lying among the cells of the middle layer of the dermal coat. The inner series lie among the cells of the outer root sheath parallel to the hair shaft. No encapsulated nerve endings are seen in hairy skin. In glabrous skin and mucous membranes, such as the lip, anus and glans penis, there are, in addition to the unencapsulated nerve endings, numerous encapsulated nerve endings. They are of different sizes and shapes, and the ensheathed myelinated stem fibre or fibres which enter the capsule pursue a more or less tortuous course. They then give rise in all cases to an arborization of fine, naked, axoplasmic filaments which end freely among the capsular cells in planes roughly parallel to the surface layer of cells. In the light of these findings a re‐analysis of the observations and, in particular, the diagrams in the literature, forces one to the conclusion that the actual facts which have been reported by various authors have more in common than would have been supposed from a perusal of their summaries and conclusions. As the result of comparing observations on the innervation of skin following the use of numerous different techniques including those involving the use of hyaluronidase, it is now possible to reinterpret the observations in the literature. A standard for histological comparison now exists and it is also possible to determine what is and what is not an artefact, and what a distorted and disrupted nerve fibre or terminal really looks like. If the literature is reinterpreted in this way, the points concerning the innervation of mammalian skin which emerge correspond both to those described by a few authors whose work is usually ignored or discounted, and to those described by Weddell et al. (1954) using the improved neurohistological techniques. If this is so, then we are forced to the conclusion that there are not and never have been any purely histological grounds on which to erect theories of cutaneous sensibility based on the existence of four primary modalities, touch, warmth, cold and pain, operating within the ‘law of specific nervous energies’. Furthermore, there is not and never really has been convincing histological evidence for the commonly accepted statement that morphologically specific nerve endings subserve each of the primary modalities of cutaneous sensibility. Our observations suggest that the morphology of nerve terminals in mammalian skin should be studied from the point of view of their functions as transducers of stimuli (mechanical, thermal or chemical) into propagated action potentials rather than taxonomically. This work was made possible by a grant from the Rockefeller Foundation which is gratefully acknowledged. Our best thanks are also due to Miss C. Court for her careful and accurate execution of the illustrations.