Ultrastructure and contractures of the pigeon iris striated muscle

A mammal and bird's-eye-view of the pupil during sleep and wakefulness

Besides regulating the amount of light that reaches the retina, fluctuations in pupil size also occur in isoluminant conditions during accommodation, during movement and in relation to cognitive workload, attention and emotion. Recent studies in mammals and birds revealed that the pupils are also highly dynamic in the dark during sleep. However, despite exhibiting similar sleep states (rapid eye movement [REM] and non-REM [NREM] sleep), wake and sleep state-dependent changes in pupil size are opposite between mammals and birds, due in part to differences in the type (striated vs. smooth) and control of the iris muscles. Given the link between pupil dynamics and cognitive processes occurring during wakefulness, sleep-related changes in pupil size might indicate when related processes are occurring during sleep. Moreover, the divergent pupillary behaviour observed between mammals and birds raises the possibility that changes in pupil size in birds are a readout of processes not reflected in the mammalian pupil.

Pupillary behavior during wakefulness, non-REM sleep, and REM sleep in birds is opposite that of mammals

Mammalian pupils respond to light^1,2 and dilate with arousal, attention, cognitive workload, and emotions,³ thus reflecting the state of the brain. Pupil size also varies during sleep, constricting during deep non-REM sleep^4-7 and dilating slightly during REM sleep.^4-6 Anecdotal reports suggest that, unlike mammals, birds constrict their pupils during aroused states, such as courtship and aggression,^8-10 raising the possibility that pupillary behavior also differs between mammals and birds during sleep. Here, we measured pupil size in awake pigeons and used their translucent eyelid to investigate sleep-state-dependent changes in pupil size. Male pigeons constricted their pupils during courtship and other male-female interactions but not while engaging in other waking behaviors. Unlike mouse pupils, the pigeons' pupils were dilated during non-REM sleep, while over 1,000 bursts of constriction and relaxation, which we call rapid iris movements (RIMs), occurred primarily during REM sleep. Consistent with the avian iris being composed largely of striated muscles,^11-15 rather than smooth muscles, as in mammals, pharmacological experiments revealed that RIMs are mediated by nicotinic cholinergic receptors in the iris muscles. Despite receiving input from a parasympathetic nucleus, but consistent with its striated nature, the avian iris sphincter muscle behaves like skeletal muscles controlled by the somatic nervous system, constricting during courtship displays, relaxing during non-REM sleep, and twitching during REM sleep. We speculate that during wakefulness, pupillary constrictions are involved in social communication, whereas RIMs occurring during REM sleep might maintain the efficacy of this motor system and/or reflect the processing of associated memories.

Histochemical, immunohistochemical, and ultrastructural observations on the iris muscles of Gallus gallus

The distribution and typology of fibers in the two muscular systems (sphincter and dilator) of the iris in Gallus gallus were determined histochemically, immunohistochemically, and ultrastructurally. The sphincter muscle in proximity to the ciliary margin was composed predominantly of slow fibers. In the intermediate tract, a large group of fast oxidative fibers were evident and the pupillary margin was exclusively composed of slow fibers. The fast fibers had histochemical and immunohistochemical patterns similar to the alpha fibers in the skeletal control muscle (biventer cervicis). In contrast, the slow fibers were composed of at least three slow types, which were comparable to the isoforms of the different myosins in beta 1 and beta 2 skeletal fibers. In the dilator muscle, the oblique system was uniquely composed of fast oxidative fibers. The radial system was predominantly composed of slow fibers with isoforms of myosins different from the slow fibers of the sphincter and control muscles. Ultrastructural features (width of Z bands, extension of the sarcoplasmic reticulum and SR-T tubule junctions, and number of mitochondria) confirm the histochemical and immunohistochemical assessments of fiber types, even if some peculiar aspects in several fibers were observed. Smooth muscle cells separated from striated fibers were evident at the pupillary margin. The hypothesis of a mesenchymal origin for all irideal striated muscles is discussed.

The distribution of acetylcholine receptor clusters and sites of transmitter release along chick ciliary ganglion neurite-myotube contacts in culture

Acetylcholine receptors accumulate along the length of cholinergic neuron-skeletal muscle contacts in vitro. The main purpose of this study was to describe, in a quantitative way, the distribution of acetylcholine receptor clusters induced by ciliary ganglion neurons over a period of time extending from hours to weeks after contacts are established. Neurites were filled with Lucifer Yellow and receptor clusters were identified with rhodamine-bungarotoxin. A cluster located within 5 micron of a nerve process or 10 micron of the base of a growth cone was considered to be a neurite-associated receptor patch (NARP). The first synaptic potentials were evoked 20 min after growth cone-myotube contact, and, after 24 h of co-culture, greater than 60% of the nerve-muscle pairs tested were functionally connected. NARPs appear rapidly; the first clusters were detected approximately 6 h after the neurons were plated. They were composed of several small subclusters or speckles of rhodamine-bungarotoxin fluorescence. The initial accumulation of receptors may occur at the advancing tips of nerve processes because NARPs were found at greater than 80% of the growth cone-muscle contacts examined between 12 and 24 h of co-culture. Over the 3-wk period examined, the mean incidence of NARPs ranged between 1.0 and 2.6 per 100 micron of neurite-myotube contact, with the peak observed on the second day of co-culture. During the first 3 d in culture, when the neurons were multipolar, nearly all of the primary processes induced one or more clusters. With time, as the neurons become unipolar (Role and Fischbach, 1987) NARPs persisted along the remaining dominant process. Measurements made during the third day of co-culture suggest that NARPs disappear along shorter neurites before they retract. Synaptic currents were detected by focal extracellular recording at 55% of the NARPs. The fact that spontaneous or evoked responses were not recorded at 45% suggests that contacts with clusters exhibit two functional states. Two types of presynaptic specialization at identified NARPs observed by scanning electron microscopy appear to be correlated with the functional state.

Effects of melatonin implants on structures and behaviors of the house finch (Carpodacus mexicanus) eye

This study aimed to determine the extraretinal effects of melatonin upon the eyes of an avian species, the House Finch (Carpodacus mexicanus). Twelve birds (full-grown, second-year males) each received a Silastic tubing intraperitoneal implant, six containing melatonin (average release = 24 micrograms/d/bird; = M birds) and six being empty (= C birds). Microscopic study of pupillary and palpebral behaviors during the final week demonstrated lesser pupillary diameters and interpalpebral distances in M birds under all test conditions. These effects could have diminished mean light levels reaching parts of the retina. Characteristics of the relative miosis and ptosis of M birds resemble signs in some CNS disorders, such as altered inhibition of the Edinger-Westphal nucleus, and especially lesions in, or lowered activity of, higher sympathetic centers (a subtype of Horner's syndrome). Weights of eyes and their parts were the same in M and C birds, contrasting with previously reported results from male Golden Hamsters, possibly due to species differences and/or preexperimental attainment of full growth in the finches. Effects of melatonin on pupillary and palpebral behaviors, demonstrated here for the first time, foster caveats for simplistic experimental designs and interpretations with melatonin when sensory-neural-behavioral interactions are affected. Quantitative changes in pupillary and palpebral behaviors may, nevertheless, provide a window for monitoring central actions of melatonin in living test subjects in chronic studies.

The visual fields of the tawny owl, Strix aluco L

The uniocular retinal field of Strix aluco is highly asymmetrical. The maximum width of 124 degrees is less than that recorded in any other vertebrate. Maximum retinal binocular field width equals 48 degrees and the optic axes diverge by 55 degrees. Maximum binocularity occurs above the bill whose tip lies outside of the visual field. The cyclopean retinal field has a maximum width of 201 degrees. Limited data on the visual fields of the pigeon are also presented. All of these data are compared with visual field widths in other species and the significance of the owl eye's tubular shape, its nasad asymmetry, and the possible factors influencing binocular field width are discussed.

Embryonic development of the smooth and striated musculatures of the chicken iris

Both smooth muscle and striated muscle are present in the iris of the chick embryo. The two types of musculature form mixed clusters which include undifferentiated cells and many nerve fibres, but they are structurally quite distinct and have different origins. The smooth musculature originates around the 10th day from a laminar invagination (iridial lamella) of the posterior epithelium, and is therefore an ectodermal derivative. The striated musculature appears slightly later than the smooth musculature and originates from undifferentiated cells which are regarded as mesenchymal. After the 15th day in ovo the smooth musculature stops growing; its cells become confined to an area very near the pupillary margin and many develop pigment granules in the sarcoplasm. Many smooth muscle cells seem to undergo regressive changes; however, cells with the typical appearance of visceral muscle cells are still present in the iris of 3-month-old chickens. High density of innervation and vascularization, wide range of striated muscle fibre diameters, presence of lipid vacuoles and of large clusters of mitochondria in the striated fibres, occurrence of peripheral couplings of the sarcoplasmic reticulum, and presence of numerous fibroblast processes in the interstices between fibres, characterize the sphincter pupillae of the mature iris.

Blockade of ganglionic transmission during synaptogenesis decreases alpha-bungarotoxin binding in the chick ciliary ganglion and iris

The role of normal synaptic activity in the biochemical development of the nervous system has been examined in the chick embryo. Chlorisondamine, a ganglionic blocking drug, was administered in ovo during the period of synaptogenesis in the parasympathetic ciliary ganglion. Following treatment with chlorisondamine, nicotinic binding sites (as measured with [125I] alpha-bungarotoxin) were significantly reduced in both the ganglion and its end organ, the striated iris muscle. While the number of [125I] alpha-bungarotoxin binding sites eventually approached control levels in the iris, binding in the ciliary ganglion remained below normal values through hatching.

Functional maturation of motor nerve terminals in the avian iris: ultrastructure, transmitter metabolism and synaptic reliability

1. The transformation of easily fatigued embryonic neuromuscular junctions into highly reliable mature terminals was examined by studying functional and morphological changes during development of the avian iris. The mature ability to follow repetitive electrical nerve stimulation was correlated with the rate of acetylcholine (ACh) synthesis and choline uptake, and with the fine structure of the nerve terminals and the post-synaptic elements.2. The terminals of the ciliary nerve of the chick initially form functional synaptic contacts with the iris muscle at embryonic St. 34-40. At the onset of this period, no Na(+)-dependent high affinity choline uptake can be demonstrated, and the low level of ACh synthesis present is sensitive to Na(+) removal. At St. 36 [(3)H]ACh synthesis begins to increase, the increment being Na(+)-dependent.3. ACh synthesis in the embryonic iris was insensitive to a conditioning [K(+)](o) depolarization even as late as St. 43. Just before hatching, depolarization elicits some augmentation in synthesis, but by 2 days ex ovo this release-induced response has increased by an order of magnitude.4. Concurrently with the acquisition of the ability to respond to depolarization with accelerated synthesis, neuromuscular transmission in the iris becomes reliable and secure during stimulation at 20 Hz. Embryonic junctions rapidly block during such stimulation, and the failure is shown to be presynaptic in origin, resulting most probably from failure to sustain adequate levels of transmitter release.5. Ultrastructural examination of the developing ciliary terminals revealed few synaptic vesicles at early stages, and a dearth of other specializations. The sequence of development from these small structurally undistinguished endings to large en plaque junctions completely filled with vesicles was reconstructed and compared to other neuromuscular junctions. Morphological maturation appears progressive with little evidence of discontinuity signalling functional status, but it is only after the terminals enlarge and become closely packed with vesicles that mature synaptic reliability is found.6. The temporal correlation between responsiveness of transmitter synthesis to depolarization and reliable neuromuscular transmission suggests that modulation of neurotransmitter metabolism in response to demand signals the achievement of junctional maturity.

Acetylcholine receptors in the ciliary ganglion and in the iris muscle of the chick: specific binding and effect on the synaptic transmission of the neurotoxin from Naja naja siamensis

1 A specific binding of Naja naja siamensis neurotoxin was found both in the iris and in the ciliary ganglion of the chick. 2 Naja-toxin (125 nM) caused a complete block of the iris muscle contraction induced by carbamylcholine. 3 Naja-toxin had a different effect on the two neuronal populations present in the ganglion: it blocked the synaptically evoked response of the ciliary cells, while the response of the choroid ones was only slightly reduced. The effects were the same in a wide range of concentrations (125 to 2500 nM). 4 The results obtained in the iris show the existence of an acetylcholine receptor population similar to the nicotinic receptor of the skeletal muscle. 5 In the ciliary ganglion the results confirm the existence of different acetylcholine receptors on the two cell types.

Mechanisms controlling choline transport and acetylcholine synthesis in motor nerve terminals during electrical stimulation

Electrical stimulation of the chick ciliary nerve leads to a frequency-dependent increase in the Na+-dependent high affinity uptake of [3H]choline (SDHACU) and its conversion to acetylcholine (ACh) in the nerve terminals innervating the iris muscle. The forces that drive this choline (Ch) uptake across the presynaptic membrane were evaluated. Depolarization with increased [K+] out or veratridine decreases Ch accumulation. In addition to the electrical driving force, energy is provided by the Na+ gradient. Inhibition of the Na,K-ATPase decreased the Ch taken up. Thus, changes in the rate of Ch transport are dependent on the electrochemical gradients for both Ch and Na+. Ch uptake and ACh synthesis were increased after a conditioning preincubation with high [K+] out or veratridine. As is the case for electrical stimulation, this acceleration of Ch uptake and ACh synthesis was strongly dependent on the presence of Ca++ in the incubation medium. Na+ influx through a TTX-sensitive channel also contributed to this acceleration. Inasmuch as membrane depolarization reduces the initial velocity of Ch uptake and ACh synthesis, their increases during electrical stimulation therefore cannot be the direct effect of the depolarization phase of the action potential. Instead they are the result of the ionic fluxes accompanying the presynaptic spike. It is concluded that stimulation of Ch uptake and ACh synthesis by nerve activity depends first, on the ACh release elicited by Ca++ influx after depolarization and second, on the activation of the Na,K-ATPase due to Na+ entry. Furthermore, it is suggested that the release of ACh after stimulation drives translocation of cytoplasmic ACh into a protected compartment (probably vesicular). This recompartmentation of intraterminal ACh stimulates ACh synthesis by mass action, allowing further accumulation of Ch.

Effects of long term denervation on smooth muscle of the chicken expansor secundariorum

Denervation of the expansor secundariorum muscle of the adult and 2 week chicken, by sectioning the brachial plexus, resulted in an approximate twofold increase in dry weight over 8 weeks. Unlike skeletal muscle, no ultrastructural changes were exhibited by the smooth muscle cells for a period of up to 5 months post denervation. No evidence of hypertrophy of the individual muscle cells was observed, but following colchicine treatment a definite increase in the number of mitotic figures was noted within muscle bundles indicating that the increase in dry weight of the expansor muscle is due to hyperplasia of the smooth muscle cells. The results are discussed in relation to in vitro studies of the interaction of sympathetic nerves with smooth muscle.

Synaptic transmission and cell death during normal ganglionic development

1. During normal embryonic development of the chick ciliary ganglion, cell death over a 4-day period (Stages 35-39) reduces the number of ganglion cells by half, from 6500 to 3200. Both ciliary and choroid populations are affected by approximately the same amount.2. Previous to cell death, preganglionic fibres form functional synapses on all ganglion cells, indicating that synapses form on cells which are destined to die.3. Shortly before the period of cell death, there is a failure of transmission in approximately half the cells. Some evidence suggests that transmission failure in at least some of the cells is of preganglionic origin.4. Cell death is nearly synchronous with the establishment of peripheral connexions by ganglion cells, at least with respect to the ciliary population which forms functional synapses with iris muscle. This implies that those cells which die do so because they have failed to form adequate peripheral connexions.5. It is suggested that many of the cells in which transmission has failed die, bringing transmission through the ganglion back to 100%. However, transmission failure appears to be a transitory phenomenon in other cells which survive and probably results from death of their preganglionic elements. Restoration of transmission would then be brought about by the formation of new or more effective synapses by surviving preganglionic fibres.

The onset and development of transmission in the chick ciliary ganglion

1. The onset and development of transmission has been studied electro-physiologically in the isolated chick ciliary ganglion from Stage 25 (Hamburger & Hamilton, 1951) until 28 days after hatching. Ultrastructure of the synapses was concomitantly investigated.2. Synaptic transmission began at Stage 26(1/2) and was 100% in both cell groups, ciliary and choroid, by Stage 33. It was initially chemical until Stage 41 when effective electrical coupling first appeared in the ciliary population. The proportion of electrically transmitting synapses increased to 80% by 1-2 days post-hatching.3. Few morphological synapses were present at Stage 33(1/2) when all ganglion cells were transmitting. A scarcity of synaptic vesicles persisted until late in embryonic development when all ciliary cells possessed calyces. At hatching the calyces were filled with synaptic vesicles.4. Initial synaptic contacts were by fine terminal branches often on the intricate processes of early ganglion cells. Calyces formed from Stage 36(1/2) and there was a concomitant retraction of ganglion cell processes, so that by Stage 40 all ciliary cells had simple calyces. The calyx was a transitory structure, which from the first week post-hatching began to break up into a cluster of boutons.5. Chemical post-synaptic potentials (PSPs) were at Stage 40 long (30 x the membrane time constant) and further prolonged by eserine. By Stage 43, PSPs had become markedly shortened and were unaffected by eserine. No simple explanation can be offered for the changes in PSP time course and sensitivity to anticholinesterases during development.6. Intracellular records from Stage 40 ciliary cells, which all possess calyces, showed 1-2 mV amplitude, diphasic, fast decaying electrical coupling potentials (CPs). Later in development the CPs became 20-40 mV amplitude, more slowly decaying and monophasic. This seemed to be correlated with faster presynaptic conduction velocities and myelination of the cell soma. Such changes in CPs may reflect a shift from capacitative to more resistive coupling and point to several factors contributing in varying degrees to the electrical transmission.7. Presynaptic fibres innervating ciliary cells were from the start of lower threshold and faster conduction velocity than those innervating ciliary cells, as occurred in the adult. It is concluded that these preganglionic fibres were probably specified by the time transmission starts and that they selectively innervated the proper post-synaptic cells.