Excitable Epithelium Research Articles

Quentin Bone. 17 August 1931—6 July 2021

Quentin Bone came from a family of well-known artists but, although he shared their talents, he chose to become a scientist because of his interest in natural history. After graduating in zoology from Oxford, he joined the scientific staff of the Marine Biological Association Laboratory at Plymouth, working there until retirement in 1991 and continuing thereafter as an emeritus research fellow. He was interested in all aspects of how fish swim, the structure and innervation of their swimming muscles, their metabolism, flotation and hydrodynamics. He showed that fishes have two distinct, independently innervated locomotory systems, red muscle for cruising and white muscle for rapid swimming bursts, and that the white muscle works on the basis of glycolytic metabolism, the red on oxidative. Later he showed that squids have convergently evolved two sorts of muscle fibre roughly equivalent to those of fishes. Bone took a comparative and evolutionary approach in all his work, looking for changes in design from simple to more complex systems and going right back to amphioxus, a putative ancestor of fishes. He also worked extensively on pelagic marine invertebrates that swim by jet propulsion—such as salps, doliolids and siphonophores—using optical and electron microscopy combined with electrophysiology to establish the neuromuscular basis of locomotion. In one such study he showed how salp chains, though lacking nervous connections between individual zooids, can swim in a coordinated manner using excitable epithelia. In another, using intracellular recordings and whole cell voltage clamp techniques, he was able to link changes in a siphonophore's swimming directly to changes in ion channel kinetics. While Bone's early studies on fish swimming established him as a leader in one important field, his later work on the behavioural physiology of a wide variety of pelagic marine invertebrates adds up to an equally outstanding body of work.

Modelling the effects of short and random proto-neural elongations.

To understand how neurons and nervous systems first evolved, we need an account of the origins of neural elongations: why did neural elongations (axons and dendrites) first originate, such that they could become the central component of both neurons and nervous systems? Two contrasting conceptual accounts provide different answers to this question. Braitenberg's vehicles provide the iconic illustration of the dominant input-output (IO) view. Here, the basic role of neural elongations is to connect sensors to effectors, both situated at different positions within the body. For this function, neural elongations are thought of as comparatively long and specific connections, which require an articulated body involving substantial developmental processes to build. Internal coordination (IC) models stress a different function for early nervous systems. Here, the coordination of activity across extended parts of a multicellular body is held central, in particular, for the contractions of (muscle) tissue. An IC perspective allows the hypothesis that the earliest proto-neural elongations could have been functional even when they were initially simple, short and random connections, as long as they enhanced the patterning of contractile activity across a multicellular surface. The present computational study provides a proof of concept that such short and random neural elongations can play this role. While an excitable epithelium can generate basic forms of patterning for small body configurations, adding elongations allows such patterning to scale up to larger bodies. This result supports a new, more gradual evolutionary route towards the origins of the very first neurons and nervous systems.

Modeling spontaneous activity across an excitable epithelium: support for a coordination scenario of early neural evolution

Internal coordination models hold that early nervous systems evolved in the first place to coordinate internal activity at a multicellular level, most notably the use of multicellular contractility as an effector for motility. A recent example of such a model, the skin brain thesis, suggests that excitable epithelia using chemical signaling are a potential candidate as a nervous system precursor. We developed a computational model and a measure for whole body coordination to investigate the coordinative properties of such excitable epithelia. Using this measure we show that excitable epithelia can spontaneously exhibit body-scale patterns of activation. Relevant factors determining the extent of patterning are the noise level for exocytosis, relative body dimensions, and body size. In smaller bodies whole-body coordination emerges from cellular excitability and bidirectional excitatory transmission alone. Our results show that basic internal coordination as proposed by the skin brain thesis could have arisen in this potential nervous system precursor, supporting that this configuration may have played a role as a proto-neural system and requires further investigation.

Modeling spontaneous activity across an excitable epithelium: Support for a coordination scenario of early neural evolution.

Internal coordination models hold that early nervous systems evolved in the first place to coordinate internal activity at a multicellular level, most notably the use of multicellular contractility as an effector for motility. A recent example of such a model, the skin brain thesis, suggests that excitable epithelia using chemical signaling are a potential candidate as a nervous system precursor. We developed a computational model and a measure for whole body coordination to investigate the coordinative properties of such excitable epithelia. Using this measure we show that excitable epithelia can spontaneously exhibit body-scale patterns of activation. Relevant factors determining the extent of patterning are the noise level for exocytosis, relative body dimensions, and body size. In smaller bodies whole-body coordination emerges from cellular excitability and bidirectional excitatory transmission alone. Our results show that basic internal coordination as proposed by the skin brain thesis could have arisen in this potential nervous system precursor, supporting that this configuration may have played a role as a proto-neural system and requires further investigation.

The oral sensory structures of Thaliacea (Tunicata) and consideration of the evolution of hair cells in chordata

We analyzed the mouth of three species, representative of the three orders of the class Thaliacea (Tunicata)--Pyrosoma atlanticum (Pyrosomatida), Doliolum nationalis (Doliolida), and Thalia democratica (Salpida)--to verify the presence of mechanoreceptors, particularly hair cells. In vertebrates, hair cells are well-known mechanoreceptors of the inner ear and lateral line, typically exhibiting an apical hair bundle composed of a cilium and stereovilli but lacking an axon. For a long time, hair cells were thought to be exclusive to vertebrates. However, evidence of a mechanosensory organ (the coronal organ) employing hair cells in the mouth of tunicates, considered the sister group of vertebrates, suggests that tunicate and vertebrate hair cells may share a common origin. This study on thaliaceans, a tunicate group not yet investigated, shows that both P. atlanticum and D. nationalis possess a coronal organ, in addition to sensory structures containing peripheral neurons (i.e., cupular organs and triads of sensory cells). In contrast, in T. democratica, we did not recognize any oral multicellular sensory organ. We hypothesize that in T. democratica, hair cells were secondarily lost, concomitantly with the loss of branchial fissures, the acquisition of a feeding mechanism based on muscle activity, and a mechanosensory apparatus based on excitable epithelia. Our data are consistent with the hypothesis that hair cells were present in the common ancestor of tunicates and vertebrates, from which hair cells progressively evolved.

Nerves in the endodermal canals of hydromedusae and their role in swimming inhibition

N eoturris breviconis (Anthomedusae) has a nerve plexus in the walls of its endodermal canals. The plexus is distinct from the ectodermal nerve plexuses supplying the radial and circular muscles in the ectoderm and no connections have been observed between them. Stimulation of the endodermal plexus evokes electrical events recorded extracellularly as "E" potentials. These propagate through all areas where the plexus has been shown by immunohistology to exist and nowhere else. When Neoturris is ingesting food, trains of "E" potentials propagate down the radial canals to the margin and cause inhibition of swimming. This response is distinct from the inhibition of swimming associated with contractions of the radial muscles but both may play a part in feeding and involve chemoreceptors. Preliminary observations suggest that the "E" system occurs in other medusae including Aglantha digitale (Trachymedusae) where the conduction pathway was previously thought to be an excitable epithelium.

Ionic currents in the myoepithelium of Aglantha digitale

When “fishing” for prey the jellyfish Aglantha digitale undergoes a series of “slow swims” driven by pacemaker neurones at the base of its bell-shaped body wall. To avoid predators Aglantha generates an altogether stronger form of “escape” swimming. During an escape swim, the striated muscle sheet that lines the inside of the body wall gives a strong contraction and water is forced from the bell opening. Neuromuscular synapses are distributed widely within this myoepithelium. An overshooting Na+-dependent action potential in each motor axon sets off a large depolarising post-synaptic potential (psp) with a 1 ms synaptic delay. A spike-like component on its rising phase initiates contraction. During slow swimming, low amplitude impulses in the motor axons set off a more slowly rising psp and spike in the myoepithelium. Although the properties and functions of different excitable epithelia have been studied extensively, the basis of their epithelial impulses is not well understood. Using the loose patch clamp technique to study the muscle spike in Aglantha, we find that the voltage-gated inward (Na+ and Ca2+) and outward (K+) currents that form its ionic basis are inactivated within 10 ms. We suggest that differences in the strength of muscle contraction during swimming arise from differences in the rate of rise of the psp. The more slowly rising psp during slow swimming partially inactivates the voltage-gated currents, producing a reduced Ca2+ influx and a weaker contraction. This flexible response to synaptic input accommodates the two forms of swimming in Aglantha but may be absent in other medusae, which depend on myoid-propagated spikes in their striated muscle sheets rather than the presence of distributed synapses.

The effects of a water-soluble petroleum fraction on the neuroid electrical activity of the hydroid coelenterate Tubularia crocea

Several properties of the excitable epithelium of the hydroid coelenterate, Tubularia crocea, are affected by a water-soluble fraction (WSF) of Platform Holly, Monterey Formation, crude oil (100% WSF= 20 ppm total hydrocarbons). In a flowing seawater system, 0·5 ml pulses of 1 WSF cause a significant increase in the epithelial pulse activity, while a 10·min exposure to 0·1% WSF produces a significant increase in the epithelial pulse frequency. Electrophysiological responses to the volatile and non-volatile subfractions are equal, but significantly lower than those to the whole WSF. Behaviorally, the 100% WSF elicits a more vigorous proximal tentacle closure than either its volatile or non-volatile subfraction. However, the closure response to the volatile subfraction exceeds that for the non-volatile component. Of the two epithelial neuroid pulse systems, the hydranth pulse (HP) system is affected to a greater extent than the neck pulse (NP) system. During 10 minutes' treatment with 100% WSF the amplitude of the neck pulses and hydranth pulses declined by 50% and 80%, respectively. The maximum 3H-toluene uptake from 100% WSF also occurs within this period, and sodium and calcium concentrations fluctuate rapidly. These results suggest that a rapid bioaccumulation of hydrocarbons by T. crocea alters membrane permeability, upsetting critical ion gradients which, in turn, alter the firing frequency of the epithelial conducting systems. This study demonstrates that concentrations of WSF approaching environmentally realistic levels (in areas of chronic pollution) alter the epithelial conducting systems that co-ordinate important elements of behavior in Tubularia. This system provides a sensitive neuroid-behavioral assay for aquatic contaminants.

Lability of conduction velocity during repetitive activation of an excitable epithelium.

Conduction velocity lability was studied in the electrically excitable epithelium of Euphysa japonica by means of intracellular recordings. Three classes of response latency change were identified in response to bursts of stimuli: an initial jump, uniform drift and abrupt jumps in latency. In each case an increase in stimulus frequency produced an increase in latency. The initial jump in latency, which occurred between the first and second response of a series, was related to the afterpotential of the first response. The increased latency of the second response appears to result from the drop in membrane resistance during the hyperpolarizing afterpotential. The uniform drift in latency remains unexplained but may be the result of ion accumulation within the tissue, progressive inactivation of the ionic channels involved in producing the action potential, or junctional phenomena. The abrupt jumps in latency, which often preceded failure to respond, were found to be impulse initiation phenomena.

Cell junctions in the excitable epithelium of bioluminescent scales on a polynoid worm: a freeze-fracture and electrophysiological study.

The bioluminescent scales of the polynoid worm Acholoe are covered by a dorsal and ventral monolayer of epithelium. The luminous activity is intracellular and arises from the ventral epithelial cells, which are modified as photocytes. Photogenic and non-photogenic epithelial cells have been examined with regard to intercellular junctions and electrophysiological properties. Desmosomes, septate and gap junctions are described between all the epithelial cells. Lanthanum impregnation and freeze-fracture reveal that the septate junctions belong to the pleated-type found in molluscs, arthropods and other annelid tissues. Freeze-fractured gap junctions show polygonal arrays of membrane particles on the P face and complementary pits on the E face. Gap junctions are of the P type as reported in vertebrate, mollusc and some annelid tissues. Intracellular current passage also induces propagated non-overshooting action potentials in all the epithelial cells; in photocytes, an increase of injected current elicits another response which is a propagated 2-component overshooting action potential correlated with luminous activity. This study shows the coexistence of septate and gap junctions in a conducting and excitable invertebrate epithelium. The results are discussed in relation to the functional roles of intercellular junctions in invertebrate epithelia. It is concluded that the gap junctions found in this excitable epithelium represent the structural sites of the cell-to-cell propagation of action potentials.

Electrical properties of an excitable epithelium.

The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is composed of a single layer of broad (70 micrometers), thin (1--2 micrometers) cells which are joined by gap junctions and simple appositions. Although the epithelium lacks nerves, it is excitable; electrically stimulating the epithelium initiates a propagated action potential. The average resting potential of the epithelial cells is -46 mV. The action potential, recorded with an intracellular electrode, is an all-or-nothing, positive, overshooting spike. The epithelial cells are electrically coupled. The passive electrical properties of the epithelium were determined from the decrement in membrane hyperpolarization with distance from an intracellular, positive current source. The two-dimensional space constant of the epithelium is 1.3 mm, the internal longitudinal resistivity of the cytoplasm and intercellular junctions is 196 omega cm, and the resistivity of both apical and basal cell membranes is greater than 23 k omega cm2. Although the membrane resistivity is high, the transverse resistivity of the epithelium is quite low (7.5 omega cm2), indicating that the epithelium is leaky with a large, transverse, paracellular shunt.

BIOLUMINESCENCE AND OTHER RESPONSES SPREAD BY EPITHELIAL CONDUCTION IN THE SIPHONOPHOREHIPPOPODIUS

1. Four responses are spread by through-conducting excitable epithelia in the nectophores: luminescence, blanching, muscular involution, and secretion. Swimming, which is independently controlled by the nervous system, is inhibited by epithelial impulses.2. Luminescent flashes are correlated one for one with epithelial impulses. At least three impulses must be propagated before the first flash is recorded. Flashes sum and facilitate. Pacemaker-like after-discharges may continue after stimulation has ceased. Not all regions of the epithelium luminesce equally, and the active area can shift during a single luminescent episode, although the excitatory impulses pass across all regions equally. No steady luminescent glow has been observed. Comparisons are drawn with other luminescent systems.3. Luminescence is generated intracellularly within the exumbrellar epithelium, but blanching (opacity) is associated with formation of granules in the adjacent mesogloea. The response builds up to saturation level within ...

Locomotion and propagated skin impulses in salps (Tunicata: Thaliacea).

1. Various observations by M. Fedele on the mechanism of forward and reverse locomotion, on the neurogenic origin of the locomotor rhythm and on the coordinated behavior of salp chains are confirmed or extended. Salpa fusiformis was the species chiefly studied. 2. The striated muscle fibers of the body wall exhibit nonpropagative, graded responsivity. The fibers are multiply-innervated. Adjacent fibers are not electrically coupled. 3. Intracellular recordings are reported from a pacemaker and presumed motor neurons in the brain. The locomotor rhythm is exhibited by deafferented and isolated brains. In the intact animal, sensory input can modify the rhythm and alter the firing sequence of the muscles. The rhythm is accelerated by reduction, and inhibited by elevation of the ambient light intensity. 4. The outer skin is a conducting epithelium. The cells conduct action potentials at ca. 17 cm/sec and are connected by gap junctions. Three other independently conducting inner epithelial territories are described. Propagated impulses in the excitable epithelia are believed to enter the nervous system via neurosensory processes in the skin, extending the effective fields of these receptors. 5. Salp chains show coordinated responses but, except in their earliest developmental stages, impulses are probably not through-conducted along the chain, but are relayed from one zooid to the next by an unknown mechanism. 6. Comparisons are drawn between salps and other pelagic tunicates where conducting epithelia have previously been reported.

Similarities between sodium channels in excitable membranes and in epithelia.

The inhibitory effects of the pyrazine derivative, amiloride, on sodium transport in an amphibian epithelium has been studied as a function of pH. It is concluded that the charged (guanidinium) group interacts with a negatively charged acid grouping in the membrane. Similarities between sodium channels in excitable membranes and epithelia are highlighted.