On the fine structure of the nerve terminals in the human myocardium

Quantification of the intrinsic neural plexus of the heart - The missing link in histological tissue analysis

BACKGROUND AND OBJECTIVE

The heart is under strict regulation of the autonomic nervous system, during which, in a healthy state, the effects of sympathetic and parasympathetic branches are balanced. In recent years, there has been increasing interest in pathological remodeling and outgrowth of cardiac autonomic nerves in relation to arrhythmogenesis. However, the small size of the cardiac nerves in relatively large tissues renders research using histological quantification of these nerves extremely challenging and usually relies on quantification of the nerve density in selected regions of interest only. Our aim was to develop a method to be able to quantify the histological nerve density in transmural tissue sections.

METHODS

Here we describe a novel workflow that enables visualization and quantification of variable innervation types and their heterogeneity within transmural myocardial tissue sections. A custom semiautomatic workflow for the quantification of cardiac nerves involving Python, MATLAB and ImageJ is provided and described in this protocol in a stepwise and detailed manner.

REPRESENTATIVE RESULTS

The results of two example tissue sections are represented in this paper. An example tissue section taken from the infarction core with a high heterogeneity value of 0.20, 63.3% normal innervation, 12.2% hyperinnervation, 3.6% hypoinnervation and 21.0% denervation. The second example tissue section taken from an area of the left ventricle remote from the infarction showed a low heterogeneity value of 0.02, 95.3% normal innervation, 3.8% hyperinnervation, 0.5% hypoinnervation and 0.5% denervation.

CONCLUSIONS

This approach has the potential to be broadly applied to any research involving high-resolution imaging of nerves in large tissues.

Development, maturation, and transdifferentiation of cardiac sympathetic nerves

The heart is electrically and mechanically controlled as a syncytium by the autonomic nervous system. The cardiac nervous system comprises the sympathetic, parasympathetic, and sensory nervous systems that together regulate heart function on demand. Sympathetic electric activation was initially considered the main regulator of cardiac function; however, modern molecular biotechnological approaches have provided a new dimension to our understanding of the mechanisms controlling the cardiac nervous system. The heart is extensively innervated, although the innervation density is not uniform within the heart, being high in the subepicardium and the special conduction system. We and others showed previously that the balance between neural chemoattractants and chemorepellents determine cardiac nervous development, with both factors expressed in heart. Nerve growth factor is a potent chemoattractant synthesized by cardiomyocytes, whereas Sema3a is a neural chemorepellent expressed specifically in the subendocardium. Disruption of this well-organized molecular balance and innervation density can induce sudden cardiac death due to lethal arrhythmias. In diseased hearts, various causes and mechanisms underlie cardiac sympathetic abnormalities, although their detailed pathology and significance remain contentious. We reported that cardiac sympathetic rejuvenation occurs in cardiac hypertrophy and, moreover, interleukin-6 cytokines secreted from the failing myocardium induce cholinergic transdifferentiation of the cardiac sympathetic system via a gp130 signaling pathway, affecting cardiac performance and prognosis. In this review, we summarize the molecular mechanisms involved in sympathetic development, maturation, and transdifferentiation, and propose their investigation as new therapeutic targets for heart disease.

Pseudoischemic electrocardiogram in myasthenia gravis with thymoma: reversibility after thymectomy

Abnormal ST T-wave changes can be found at presentation in various noncoronary disorders; misinterpretation of these patterns as ischemic heart disease can lead to erroneous diagnosis and treatment. Here we present a case of myasthenia gravis (MG) with thymoma, in which the resting electrocardiogram (ECG) led to a misleading diagnosis of myocardial ischemia. After thymectomy, the ECG resumed a normal pattern. Myasthenia gravis is not usually considered in the differential diagnosis of conditions associated with an abnormal ECG. The combination of dysphagia, dyspnoea, ECG changes, and creatine kinase (CK) elevations may easily bring to mind an erroneous and possibly deleterious diagnosis of myocardial ischemia.

Evidence for Increased Atrial Sympathetic Innervation in Persistent Human Atrial Fibrillation

OBJECTIVE

In this study, we aimed to compare the level of atrial sympathetic innervation in human atrial fibrillation (AF) to that in sinus rhythm (SR).

BACKGROUND

Histological studies of atrial tissue obtained from animals with experimentally induced AF indicate that sympathetic hyperinnervation could play a role in the pathogenesis of AF.

METHODS

In 24 patients (12 in SR and 12 in AF) undergoing bypass surgery, we collected right atrial appendage tissue. In AF patients, left atrial appendage tissue was also acquired. The degree of sympathetic innervation was quantified by measuring the amount of staining for tyrosine hydroxylase (TH) and tissue norepinephrine (NE). In conjunction, nerve growth factor (NGF) mRNA expression was assessed by real-time polymerase chain reaction (PCR). Growth-associated protein 43 (GAP43) immunostaining was used to assess degree of new neural growth.

RESULTS

When corrected for differences in tissue fibrosis, the expression of both TH (AF 0.45 +/- 0.1%, SR 0.09 +/- 0.03%, P = 0.02) and tissue NE (AF 358 +/- 49 pg/mg, SR 225 +/- 39 pg/mg, P = 0.04) was greater in atrial tissue of the AF cohort. The degree of atrial TH staining (P = 0.01) and NE content (P < 0.001) was also significantly greater in the right compared with left atrial samples in the AF cohort. There were no differences in NGF mRNA expression or GAP43 staining.

CONCLUSION

This study provides evidence for the presence of heightened atrial sympathetic innervation in patients with persistent AF, suggesting autonomic remodeling may be part of atrial substrate for AF.

Electron microscopic study of intrinsic cardiac ganglia in the adult human

The aim of the present study was to describe in detail the ultrastructure of intrinsic cardiac ganglionic cells in the healthy human as these cells appear to be directly involved in the development of tachycardia, atrioventricular block, ventricular fibrillation, and sudden cardiac death. Tissues examined in this study were obtained from hearts of 10 adult humans of either sex aged 22-80 years at autopsy performed no more than 8 h after death. The examined human intrinsic cardiac nerve cells were in most respects typical autonomic neurons surrounded by a sheath of satellite cells that was either uni- or multilayered. In addition to regular unmyelinated axons, prominent large axon terminals containing lamellated dense bodies, mitochondria and vesicles in the cytoplasm were observed in the ganglion neuropil. Synaptic profiles were more common in the ganglion neuropil than on neuronal somata. According to axon terminal contents, synaptic profiles were of three types. The most common Type 1 synaptic profiles contained a predominance of small clear, with a few larger dense-cored vesicles and mitochondria. Type 2 synaptic profiles, in addition to the same components as in Type 1, had glycogen-like particles. Type 3 vesicle-containing profiles clearly differed from both the previous ones as they were the largest in diameter and included plentifiul large clear pleomorphic or dense-cored vesicles together with small clear and larger dense-cored vesicles, mitochondria, dense and multivesicular bodies. Independently of age of the human, the most frequent neuronal abnormality was an abundant accumulation of inclusions inside of somata and dendrites that, in profile, appeared like circular membranous or fine granular bodies variable in electron density. In addition to inclusions, some neuronal somata and dendrites had strongly swollen mitochondria filled up with granular material in spite of their close association with normal looking ganglionic neurons. Structures resembling an axon growth cone in profile were revealed inside of cardiac ganglia derived from an 80 year old man. In conclusion, the present results provide baseline information on the normal ultrastructure of intracardiac ganglia in healthy humans which may be useful for assessing and interpreting the degree of damage of ganglionic cells both in autonomic and sensory neuropathies of the human heart.

Morphology of human intracardiac nerves: an electron microscope study

Since many human heart diseases involve both the intrinsic cardiac neurons and nerves, their detailed normal ultrastructure was examined in material from autopsy cases without cardiac complications obtained no more than 8 h after death. Many intracardiac nerves were covered by epineurium, the thickness of which was related to nerve diameter. The perineurial sheath varied from nerve to nerve and, depending on nerve diameter, contained up to 12 layers of perineurial cells. The sheaths of the intracardiac nerves therefore become progressively attenuated during their course in the heart. The intraneural capillaries of the human heart differ from those in animals in possessing an increased number of endothelial cells. A proportion of the intraneural capillaries were fenestrated. The number of unmyelinated axons within unmyelinated nerve fibres was related to nerve diameter, thin cardiac nerves possessing fewer axons. The most distinctive feature was the presence of stacks of laminated Schwann cell processes unassociated with axons that were more frequent in older subjects. Most unmyelinated and myelinated nerve fibres showed normal ultrastructure, although a number of profiles displayed a variety of different axoplasmic contents. Collectively, the data provide baseline information on the normal structure of intracardiac nerves in healthy humans which may be useful for assessing the degree of nerve damage both in autonomic and sensory neuropathies in the human heart.

Alpha 1-adrenoceptors in the conduction system of rat hearts

1. We have characterized alpha 1-adrenoceptor in the conduction systems of the rat heart by quantitative autoradiography. 2. Consecutive 20 micron thick sections from a single rat heart containing the sinoatrial (SA) node and atrioventricular (AV) node were incubated with increasing concentrations of [3H]-prazosin with or without 10 microM phentolamine. After exposure to 3H-Ultrofilm, optical densities corresponding to the SA node and AV node were determined by computerized densitometry after comparison with 3H standards. 3. The SA node and AV node were stained heavily for cholinesterase and they contained a higher concentration of alpha 1-adrenoceptors than the adjacent myocardium without a significant change in the affinity. 4. These results support the hypothesis that alpha 1-adrenoceptors may play an important role not only in inotropism but also in chronotropism of rat hearts.

Sympathetic and parasympathetic neuromuscular junctions in the guinea-pig sino-atrial node

The structure and organization of cholinergic and adrenergic varicosities in the sino-atrial node of the guinea-pig heart was determined by electron microscopy. When random sections of tissue were examined, some varicosities were found in close proximity (less than 90 nm) to a muscle cell, while others appeared to be some distance (greater than 90 nm) from the nearest muscle cell. When the organization of individual varicosities and their relationships with nearby cardiac muscle cells were determined by examining serial sections of tissue, it was found that most varicosities which lost all or part of their Schwann cell wrap formed close appositions with one or more cardiac muscle cells. At the regions of close apposition, the neuromuscular clefts were filled with a single layer of basal lamina, giving neuromuscular separations of about 80 nm. Although evidence of pre-synaptic or post-synaptic thickenings was not found, there was an accumulation of synaptic vesicles towards the regions of close apposition. These observations are discussed in relation to the idea that in a number of different tissues, most autonomic varicosities which lose part of their Schwann cell wrap, form organized neuromuscular junctions and that these junctions may be the sites of neuromuscular transmission.

Fine structure of A: autonomic nerve fibers and terminals in human myocardium; and B: myocardial changes in congenital heart disease

In a histological and fine structural study of right atrial biopsy specimens from 31 patients with rheumatic heart disease (RHD), aged 7 to 46 years, and 11 patients with congenital heart disease (CHD), aged 3 to 36 years, nerve fibers or endings were seen by electron microscopy in 11 specimens. There was concurrence of ordinary axons along with terminals bearing pale cholinergic or dark adrenergic synaptic vesicles. Smaller and denser cholinergic vesicles suggested proliferation followed by exhaustion of such nerve endings. The closest proximity of nerve terminal to muscle fiber was about 100 nm. In one RHD specimen a "specific terminal cell" was present between a nerve ending and muscle fiber; in another a possible neuromuscular contact was developing at the surface of a regenerating small muscle fiber with a few myofilaments. Unmyelinated axons amidst increased subendocardial and subepicardial collagen, with prominent fibroblasts and depleted muscle fibers, were seen more frequently in specimens of CHD. Loss of myofibrils and accumulation of mitochondria, with infrequent formation of lipofuscin bodies, characterized degenerating muscle fibers in CHD also, although to a lesser degree than in RHD (reported earlier, 1985). The myocardial blood vessels in CHD tended to have pale swollen endothelial cells and narrowed lumen. The most severely affected cases of CHD were those with (1) a very wide atrial septal defect (ASD), (2) ventricular septal defect (VSD) with vegetations near the defect, (3) infundibular pulmonary stenosis, and (4) Fallot's tetralogy.

Immunohistochemical and ultrastructural localisation of peptide-containing nerves and myocardial cells in the human atrial appendage

The innervation and myocardial cells of the human atrial appendage were investigated by means of immunocytochemical and ultrastructural techniques using both tissue sections and whole mount preparations. A dense innervation of the myocardium, blood vessels and endocardium was revealed with antisera to general neuronal (protein gene product 9.5 and synaptophysin) and Schwann cell markers (S-100). The majority of nerve fibres possessed neuropeptide Y immunoreactivity and were found associated with myocardial cells, around small arteries and arterioles at the adventitial-medial border and forming a plexus in the endocardium. Subpopulations of nerve fibres displayed immunoreactivity for vasoactive intestinal polypeptide, somatostatin, substance P and calcitonin gene-related peptide. In whole-mount preparations of endocardium, substance P and calcitonin gene-related peptide immunoreactivities were found to coexist in the same varicose nerve terminals. Ultrastructural studies revealed the presence of numerous varicose terminals associated with myocardial, vascular smooth muscle and endothelial cells. Neuropeptide Y immunoreactivity was localised to large electron-dense secretory vesicles in nerve terminals which also contained numerous small vesicles. Atrial natriuretic peptide immunoreactivity occurred exclusively in myocardial cells where it was localised to large secretory vesicles. The human atrial appendage comprises a neuroendocrine complex of peptide-containing nerves and myocardial cells producing ANP.

Effects of vagotomy on the ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis)

The ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis) was studied after bilateral cervical vagotomy and survival times of 1, 3, 5, 7, 10, 21 and 28 days. During the first week after vagotomy, a few atrial cells showed a reduction in the sarcoplasm, crowding of the myofibrils, peripheral dispersion and reduced intercristal density of the mitochondria and increased sarcoplasmic reticulum and glycogen particles. In some profiles, there was increased electron density and granularity at the I bands and the intercalated discs. The number of such affected cells increased in the subsequent days such that by 21 to 28 days about 50% of the cells were estimated to be affected. During the latter stages further changes included, the degradation of the myofilaments and increased electron density, disorganisation and disintegration of the digital extensions at the intercalated discs. Throughout the experiments there was a leucocytic infiltration, more evident in the longer survival times.

Left ventricular receptors: physiological controllers or pathological curiosities?

Mechanically and chemically sensitive receptors in the ventricle have been described histologically and electrophysiologically. Early experiments documented the hypotension and bradycardia that resulted from the intracoronary administration of one of the veratrum alkaloids (the Bezold-Jarisch reflex). Mechanical distension of the ventricles also results in a reflex decrease in heart rate and a reduction in peripheral resistance. Skeletal muscle and coronary vascular resistance appear to be most prominently affected by stimulation of ventricular receptors. Coronary ischemia has also been shown to evoke reflex effects which are attributable to stimulation of ventricular receptors. The resultant bradycardia can be especially ominous in acute myocardial infarction. Changes in myocardial inotropic state have been shown to alter ventricular receptor discharge in experimental animals. This stimulus may evoke reflex changes in peripheral hemodynamics. A variety of humoral substances can alter ventricular receptor discharge and evoke Bezold-Jarisch like responses. These include bradykinin and prostaglandins. PGI2, when given intracoronary in small doses or intravenously in larger doses will lower blood pressure while inhibiting the baroreflex induced tachycardia. It has also been shown in some experiments that PGI2 and arachidonic acid can evoke overt bradycardia and hypotension via a reflex mechanism. The role of prostaglandins in cardiovascular reflex control may be important in pathophysiologic states such as coronary ischemia and heart failure. Ventricular receptors can interact centrally with the arterial baroreceptors to attenuate the baroreflex control of both heart rate and peripheral resistance. Finally, the stimulation of ventricular receptors can alter a variety of humoral substances which are important regulators of cardiovascular and fluid volume homeostasis. These include vasopressin, renin and catecholamines. Those studies which have been done within the last 10 years or so, especially in unanesthetized animals, have demonstrated that the Bezold-Jarisch reflex is more important to cardiovascular control than previously thought. Future work will be necessary to determine the precise role ventricular receptors play in various pathological situations.

Peptidergic innervation of human atrial myocardium: an electron microscopical and immunocytochemical study

Nerve terminals of human cardiac muscle were studied using an electron microscope. Substance P-, Leu-enkephalin- and vasoactive intestinal polypeptide-like (VIP) immunoreactive nerves were demonstrated by use of the light microscope. In addition, VIP- and substance P-like immunoreactive nerves were localized ultrastructurally by the peroxidase-antiperoxidase-method. Muscle specimens were obtained from right auricula of patients undergoing open-heart surgery. In the nerve fibres and terminals, which were situated close to the blood vessels and cardiac muscle cells several vesicle populations were identified. On the morphological basis the terminals could be tentatively categorized as cholinergic, mixed cholinergic-peptidergic, adrenergic, sensory or baroreceptor type, peptidergic and degenerating nerve endings. Substance P-, Leu-enkephalin- and VIP-like immunoreactive nerves were localized between cardiac muscle cells. Nerve terminals, which showed substance P-immunoreaction were observed also close to blood vessels. In substance P- and VIP-immunoreactive nerve terminals the immunoprecipitation was localized in large dense-cored vesicles of about 120 nm in diameter. It is concluded that the intrinsic control of the human heart is most probably regulated by several transmitter candidates. The peptidergic nerves may exert their modulatory interactions in the nerve bundles where they are situated close to each other but a direct effect on the blood vessels and muscle cells cannot be excluded.

Innervation of brain intraparenchymal vessels in subhuman primates: ultrastructural observations

Sympathetic innervation of intraparenchymal blood vessels in the basal ganglia was demonstrated by transmission electron microscopy in arteries, arterioles, and capillaries of the subhuman primate brain. Small arteries (40-120 micron) and some arterioles (12-40 micron) are innervated only at branching sites. However, arterioles occasionally may be innervated at points distal to their origin. Capillary innervation was very infrequently observed.

Innervation of atrial stumps in calves living with the TNS-Brno II artificial heart

The remnant stumps of the atria in six calves surviving with the artificial TNS-Brno II methacrylate heart for up to 5 months were examined by means of the Bodian silver-impregnating technique and by the formalin-induced fluorescence technique visualizing noradrenaline in monoaminergic nerve terminals. The degeneration of the nerve terminal system detected in animals surviving for 2 days and 1 week was followed by regeneration of the vegetative nerve system in four animals surviving for greater than 1 month. Moreover, thick myelinated nerves afferent in character were detected within the atrium of one animal surviving for 5 months.

Histochemical and ultrastructural study on the innervation of human and porcine atrio-ventricular valves

The presence of nerve fibers was investigated in porcine and human atrio-ventricular valves by AChE technique, formaldehyde-induced fluorescence, en bloc silver and gold chloride impregnation and electron microscopy. Elaborate nerve plexuses were observed in every leaflet and in some chordae tendineae of all the samples examined, without significant species differences in the pattern of innervation. The presence of a sensory innervation was inferred from the demonstration, in whole mount samples processed for acetylcholinesterase, of thick myelinated nerve fibers and of endings with dot-like or brush-like appearance. Moreover the results of the combined histochemical and ultrastructural methods showed the existence of both cholinergic and adrenergic efferent nerve fibers. Nerve varicosities with clear or dense-cored vesicles were frequently observed in proximity to blood vessels and to cardiac and smooth muscle bundles, which therefore can be considered as the targets of the efferent nerve supply. The complex pattern of the innervation herein demonstrated suggests the existence of a nervous control of valvular function through the regulation of contractile elements.

Morphology of cardiac nerves in experimental infarction of rat hearts. II. Electron microscopical findings

Alterations of cardiac nerves in myocardial infarction were investigated by electron microscopy after differing intervals in 28 rats. During the first 4 h there are, in non-myelinated nerves within the myocardium, a swelling of the axoplasm with the occurrence of 'pale' axons and swelling of axonal mitochondria and neurosecretory granules. After bursting of the axolemma, these are spilled into the adjacent interstitial space. After 4 h first myelin figures are observed, and in some axons an accumulation of neurofilaments takes place. During the second to seventh day an extensive vesicular disintegration of axonal structures develops. Because of regressive changes, axons cannot be identified with certainty within the necrosis. After two or three weeks nerves with lamellar enfoldings of cytoplasmic processes corresponding to Büngner bands can be seen at the infarction border. These nerves may contain only a few residual axons. Myelinated nerves show a mainly vesicular disintegration. The results are discussed with regard to their functional significance and the special conditions of the animal model, in which ligature of the coronary artery may not only produce ischemia, but may also, by simultaneous ligature of the adjacent cardiac nerves, induce Wallerian degeneration.

Structure of rat aortic baroreceptors and their relationship to connective tissue

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.

Types of nerve terminals in fetal and neonatal rabbit myocardium

With the use of electron microscopy 4 types of axonal profiles were observed in the developing myocardium of rabbits: 1) adrenergic axons which contained mainly small dense-core vesicles and which presumably can store 5-hydroxy-dopamine; 2) cholinergic axons which contained small clear synaptic vesicles and which were acetylcholinesterase-positive; 3) axons which contained large vesicles filled with moderately electron-dense material and which resembled purinergic axons; and 4) profiles filled with mitochondria, vesicles of various sizes, lysosome-like bodies, and microtubules and which resembled sensory terminals.

Innervation of myocardial microcirculation; terminal autonomic axons associated with capillaries and postcapillary venules in mouse heart

Efferent terminal axons are associated with numerous capillaries and postcapillary venules in both the atria and ventricles of mouse heart. These axons possess ultrastructural characteristics which are typical of peripheral autonomic fibers in other tissues. Many are found near pericytes, in a relationship closely resembling that between terminal axons and smooth muscle cells of larger vessels. To demonstrate adrenergic terminals, mice were pretreated with 5- or 6-hydroxydopamine; examination of these animals' hearts revealed that both adrenergic and cholinergic axons terminate near pericytes and endothelial cells. The results of this study are consistent with the view that there may be a functional innervation of capillaries and postcapillary venules of the mouse heart.

The structure and innervation of the sinu-atrial node of the mole heart

The sinu-atrial node in the heart of the mole, Mogera wogera, contains myocytes which are devoid of atrial specific granules and which may be classified into two types: electron-lucent (majority) and dark (minority). Numerous unmyelinated axon terminals, containing synaptic vesicles, face the nodal myocyte surface with interspaces of less than 300 nm. There are about 6 terminals per myocyte profile and about 10 per profile at nuclear levels. Of a total of 2717 of these terminals 85.2% are 50-200 nm from the nodal myocytes, and 9.6% form closer neuromuscular junctions, with less than 50 nm interspace and some membrane specialization. Such specilizations are almost exclusively found in relation to the dark, minority-type of nodal myocyte. Myelinated nerve fibers are also numerous within the mole SA node, and may terminate almost directly on a nodal myocyte. A few nerve fibers contain many mitochondria and may represent afferent endings. Nodal capillaries are smaller and fewer than capillaries in the juxtanodal myocardium, probably indicating a lesser blood supply to the SA node than to the ordinary myocardium.

Fluorescence histochemical and electron-microscopical observations on the innervation of the atrial myocardium of the adult human heart

The existence of both adrenergic and cholinergic innervation of the atrial myocardium of the adult human heart was demonstrated by means of fluorescence induced by formaldehyde or glyoxylic acid and by electron microscopy. The adrenergic fluorescing axons (1) followed the course of blood vessels as typical perivascular nerve plexuses, and (2) formed a three-dimensional fairly dense nerve net obviously not related to the blood vessels. The varicosities frequently came into close apposition on myocardial cells. Several types of nerve terminals were differentiated at electron microscopy: (1) an "adrenergic" type containing small (diameter 450-700 A) dense-cored vesicles and usually (in various proportions) small "empty" and/or large (900-1500 A) dense-cored vesicles, (2) a "cholinergic" type containing small (ca. 500 A) "empty" vesicles and occasionally also some large (mean diameter ca. 1200 A) dense-cored vesicles, (3) a "pale" type containing only a few or no vesicles, (4) a "disintegrated" type containing degenerated mitochondria, autophagic vacuoles, and occasional normal-looking mitochondria, (5) nerve terminals containing a large number of mitochondria in addition to varying vesicle populations, and (6) a (possibly baroreceptive type of) nerve terminal containing myelinlike lamellated structures. The "disintegrated" and the "pale" types of nerve terminals possibly represent different stages of axonal degeneration, or may correspond to diminution in the transmitter substance concentration under certain pathophysiologic conditions, respectively. Nerve terminals crowded with mitochondria may be sensory and involved in mechano-or chemoreceptive functions. In preliminary experiments convincing evidence was obtained that the glyoxylic acid-induced fluorescence histochemical method will be suitable for comparative studies on (human) clinical specimens, e.g., for analyzing the degree of the functional activity of the intrinsic adrenergic innervation of the myocardium under various pathophysiologic conditions. The modification which appeared most appropriate for such studies is described in detail, and is proposed for use as a standard method in other similar or related studies on human clinical series. The essential criteria for analyzing the specimens at fluorescence microscopy are suggested as well.