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Biomedical subjects

B M Bush

Publications and source records attributed to B M Bush.

At least 19 recordsLinked to original sources
PubMed1992

Parallel reflex and central control of promotor and receptor motoneurons in crayfish.

We describe the reflex and central control of an identified motoneuron (rm 1) to a crayfish muscle receptor, the thoracocoxal muscle receptor organ (TCMRO), and compare it with the in-parallel, 'extrafusal' promotor motorneurons. Rm 1 is spontaneously active in an isolated preparation. This activity is modulated in phase with centrally driven promoter nerve activity, suggesting coactivation of promotor and receptor-motor motoneurons. Rm 1 is autogenetically modulated, in a phase-dependent manner, by stretching the TCMRO: during promotor bursts rm 1 is excited by dynamic stretch, but during remotor bursts it is inhibited by the same stimulus. This effect is mediated by a single, identified TCMRO afferent, the dynamically sensitive T-fibre. At or near maximally stretched lengths of the TCMRO a tonic inhibition of rm 1 is revealed. This effect is mediated by another identified TCMRO afferent, the statistically sensitive S-fibre. The thoracocoxal chordotonal organ is a non-muscular receptor spanning the same joint but signalling the opposite direction of movement. Dynamic movement stimulation of this receptor also excites rm 1, a reflex that could counteract TCMRO slackening. These results demonstrate a complex central and reflex control of the TCMRO, which could regulate reflex gain throughout the step cycle during walking in the intact animal.

Animals↗
PubMed1992

Boxer colitis.

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Animals↗
PubMed1992

Identified proprioceptive afferents and motor rhythm entrainment in the crayfish walking system.

1. In crayfish, Pacifastacus leniusculus, remotion of a walking leg stretches the thoraco-coxal (TC) muscle receptor organ (TCMRO), located at the leg's articulation with the thorax. In vitro, alternate stretch and release of the fourth leg's TCMRO entrained the centrally generated rhythmic motor output to that leg, with the remotor phase of the rhythm entraining to TCMRO stretch, the promoter phase to release. This coordination of motor bursts to afferent input corresponds to that of active, rhythmic movements in vivo. 2. Entrainment was rapid in onset (stable coordination resulting within the first or second stimulus cycle) and was relatively phase-constant (whatever the stimulus frequency, during 1:1 entrainment, remotor bursts began near the onset of stretch and promotor bursts began near the onset of release). Outside the range of 1:1 entrainment, 2:1, 1:2, and 1:3 coordination ratios (rhythm:stimulus) were encountered. Resetting by phasic stimulation of the TCMRO was complete and probabilistic: effective stimuli triggered rapid transitions between the two burst phases. 3. The TCMRO is innervated by two afferents, the nonspiking S and T fibers, which generate graded depolarizing receptor potentials in response to stretch. During proprioceptive entrainment, the more phasic T fiber depolarized and hyperpolarized more rapidly or in advance of the more tonic S fiber. These receptor potentials were modified differently in the two afferents by interaction with central synaptic inputs that were phase-locked to the entrained motor rhythm. 4. Injecting slow sinusoidal current into either afferent alone could entrain motor rhythms: promoter phase bursts were entrained to depolarization of the S fiber or hyperpolarization of the T fiber, whereas the converse response was obtained for remotor phase bursts. 5. During proprioceptive entrainment, tonic hyperpolarization of the S fiber weakened entrained promotor bursts and allowed remotor burst durations to increase. Hyperpolarizing the T fiber weakened entrained remotor bursts and allowed promotor bursts to occur during stretch. These results suggest that the staggered receptor potentials of the two afferents alternately excite opposite burst phases of the rhythm during proprioceptive entrainment. 6. Injecting brief current pulses into either afferent perturbed the timing of entrained bursts in opposite ways, suggesting that, during proprioceptive entrainment, the membrane potential trajectories of the two afferents have reciprocal triggering effects on burst transitions. 7. We infer that entrainment results from 1) complete resetting of burst transitions in a two-phase central oscillator, 2) opposing feedback pathways mediated by a phasic and a tonic afferent, 3) temporally staggered afferent receptor potentials, and 4) the ability of afferent receptor potentials to trigger burst transitions.

Animals↗
PubMed1992

Heterogeneity and central modulation of feedback reflexes in crayfish motor pool.

1. Movement of the crayfish thoracocoxal leg joint is monitored by a muscle receptor organ (TCMRO) and a chordotonal organ (TCCO). Both receptors span the joint in parallel but signal opposite directions of leg movement. The TCMRO is innervated by afferents responsive to lengthening, which corresponds to leg remotion, whereas TCCO afferents are responsive to shortening of the chordotonal strand, which corresponds to leg promotion. 2. When both receptors are stimulated in parallel, in an otherwise isolated preparation, reflex responses of coxal promoter and remotor motor neurons occur on both stretch and release. By comparison with experiments where one or the other of these receptors is stimulated selectively, we conclude that reflexes evoked by stretch of the two receptors are due to the TCMRO and reflexes evoked by release are due to the TCCO. 3. Reflexes mediated by these receptors are both state dependent and phase dependent. In preparations that produce patterns of reciprocal motor activity in promotor and remotor motor neurons (the active state), the reflex effect depends on the phase of this centrally generated activity. In preparations that are quiescent, or that produce only tonic motor output (the inactive state), the reflex effect is stable, corresponding to a typical resistance (negative feedback) reflex for both directions of receptor movement. 4. In the active state, coxal promotor motor neurons are both excited and inhibited in a phase-dependent manner by stretching the TCMRO. A subgroup of promotor motor neurons is excited by shortening the TCCO. One subgroup of the antagonistic coxal remotor motor neurons receives phase-dependent excitation from stretch of the TCMRO, whereas a second subgroup receives phase-dependent excitation from shortening the TCCO. 5. There are, therefore, at least two ways in which reflex effects can be modulated. At the level of a single motor neuron, the reflex response can vary in gain, and in some cases in sign, in a manner depending on centrally generated motor activity. In addition, at the level of a pool of synergistic motor neurons, the reflex effect is not uniform; instead, different subgroups of motor neurons display different reflex effects, so that the relative levels of excitability of different motor neuron reflex subgroups can also determine the net reflex effect. 6. Excitation of promotor motor neurons by TCCO shortening and of remotor motor neurons by TCMRO lengthening are positive feedback reflexes. The subgroups of motor neurons in which positive feedback reflexes can be evoked in both promotor and remotor pools are termed group 1.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Reflex actions of one proprioceptor on the motoneurones of a muscle receptor and their central modulation in the shore crab.

1. Reflex efferent control of a muscle stretch receptor by a joint proprioceptor of the same limb was studied in an isolated CNS preparation from the shore crab. The influence of 'fictive locomotor' activity on this interjoint reflex was also examined. 2. The thoracic-coxal muscle receptor organ (TCMRO) and the coxo-basal chordotonal organ (CBCO), which monitor movement and position of the first and second joints of the posterior leg, were isolated together with the whole thoracic ganglion complex. The TCMRO, functionally analogous to a mammalian muscle spindle, has two receptor motoneurones. RM1 innervating the receptor muscle alone and RM2 which also supplies the 'extrafusal' promotor muscle. The CBCO is a typical arthropod elastic strand organ, with many sensory neurones but lacking an efferent supply. The TCMRO was fixed at its mid-length, and stretch-hold-release stimuli were applied to the CBCO. Efferent activity was recorded from the cut nerve roots of the four basal limb muscles and intracellularly as excitatory junction potentials (EJPs) from the receptor muscle. 3. A dynamic increase in the frequency of action potentials in RM1 occurred on both stretch and release of the CBCO. During the hold phase the RM1 activity declined from the dynamic response but remained elevated compared to the resting tonic discharge. RM2, identified by EJPs occurring 1:1 with a unit in the promoter nerve, responded in a similar way. 4. One or more promotor motoneurones were usually co-activated with the two receptor efferents in response to input from the CBCO. In a typical example the average spike frequency of RM1 rose from 0 to 27 Hz during the dynamic phases (stretch and release) of the CBCO stimulus, falling to 2.5 Hz during the hold phase, while the corresponding promotor spike frequencies were 25 and 7.5 Hz, respectively. The other three muscle nerves recorded from generally also showed reflex driving by the CBCO. 5. The totally isolated thoracic ganglion could produce a rhythmic, bursting motor output in the absence of any sensory input. During this centrally generated activity the receptor motor innervation was strongly co-activated with the promotor bursts, and the reflex input from the CBCO was overridden or modulated in a phase-dependent manner. 6. The proximally directed interjoint reflex to the receptor muscle probably functions to maintain the tension on the sensory endings of the TCMRO, and so enable them to respond effectively at all times to movements of the basal leg joint.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1989

Primary afferent responses of a crustacean mechanoreceptor are modulated by proctolin, octopamine, and serotonin.

Modulation of sensory responses recorded intracellularly in primary sensory afferents of a crustacean proprioceptor is described. The neuropeptide proctolin enhances the sensory response, whereas the bioamines octopamine and serotonin depress it. The lobster oval organ of the second maxilla, a simple stretch receptor lacking centrifugal control, provides a useful model for studies on nonsynaptic modulation at peripheral sensory loci. Its three large afferents, X, Y, and Z, were prepared for intracellular recording and tested under five experimental conditions: (1) when fully rested, (2) when adapted to maintained stretch and firing tonically, (3) when showing reduced responses after habituation to repetitive stimulation, (4) not stretched but depolarized with current injections, (5) after TTX blockade. The results, taken together, indicate that conductances contributing to the overall amplitude of the receptor potential are major targets for modulators. Thus proctolin increased receptor potential amplitudes with consequent augmentation of spiking, whereas serotonin and octopamine depressed the receptor potentials, often to subthreshold levels with loss of spiking. Octopamine was a less potent agent than serotonin and failed to act upon fibers under TTX blockade. Fibers Y and Z consistently showed sensitivity to the modulators tested. The largest fiber, X, typically was resistant to proctolin, octopamine, and serotonin. Threshold concentrations of 10(-10)-10(-11) M determined in vitro are well below the circulating levels for serotonin and octopamine found in vivo. Proctolin, however, is usually not detectable in the hemolymph, and it is suggested that a significant site of proctolin release may be the oval organ itself.

Action Potentials↗
PubMed1983

U100 insulin.

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Animals↗
PubMed1983

Subcellular biochemical studies of a naturally occurring enteropathy in the dog resembling chronic tropical sprue in human beings.

Subcellular biochemical features of a naturally occurring enteropathy in the dog resembling chronic tropical sprue in human beings were examined. Affected dogs had reduced xylose absorption and low concentrations of serum folate, RBC folate, and serum vitamin B12. Histologic examination of peroral jejunal biopsy specimens revealed villous atrophy and a variable infiltration of lymphocytes and plasma cells in the lamina propria. Biochemical assessment of the principal subcellular organelles was achieved by the assay of specific marker enzymes in homogenized jejunal biopsy specimens and in the gradient fractions after isopyknic centrifugation on continuous sucrose-density gradients. Activities of brush-border marker enzymes were reduced. However, there was no change in the equilibrium density of this organelle, indicating that these quantitative brush-border changes were not accompanied by an alteration in the composition of the microvillous membrane. Activities of lysosomal enzymes were increased, and there was evidence of enhanced lysosomal fragility and a proliferation of the endoplasmic reticulum. These findings were consistent with an impairment of the functional capacity of the small intestinal mucosa and implicated lysosomes and the endoplasmic reticulum in the disease process.

Animals↗
PubMed1982

Impulse-coded and analog signaling in single mechanoreceptor neurons.

Although most sensory neurons convey temporally coded impulses to the central nervous system, certain nonspiking receptors use only graded afferent signals. Each of three large nerve fibers from the lobster oval organ, a mechanoreceptor subserving ventilation, carry both impulses and graded potentials. Thus, both impulse frequency and receptor potential amplitude are available for information transfer.

Action Potentials↗
PubMed1982

Some aspects of the auditory pathway and audition in the European mole, Talpa europaea.

The structure of the lower auditory pathway and certain responses to sounds were studied in the European mole (Talpa europaea). While the animal possesses a structurally normal cochlear nuclear complex and auditory midbrain, the medial superior olive, although small, is prominent and the lateral superior olive is lacking. This characteristic has been observed in other species with essentially low frequency hearing. Behavioural and electrophysiological responses to sound were evoked between 0.1 and 15 kHz, and moles were apparently at least sensitive to tones of 5-8 kHz as they were to low tones. In general, evidence is presented for the mole having essentially normal mammalian auditory characteristics.

Acoustic Stimulation↗
PubMed1981

Positive feedback to a muscle receptor stabilized by concurrent self-inhibition.

The two non-spiking afferent neurones (T and S fibres) of the crab's thoracic-coxal muscle receptor organ mediate antagonistic reflex controls upon the specific receptor motoneurone (Rm1). Depolarization of the 'in-series' T fibre, by receptor muscle stretch or intracellular current injection, reflexly excites Rm1, whereas length- or current-induced S fibre depolarization inhibits Rm1. The latter, autogenetic negative feedback stabilizes the concurrent positive feedback over the physiological range of receptor muscle lengths.

Afferent Pathways↗
PubMed1979

Biochemical changes in the jejunal mucosa of dogs with naturally occurring exocrine pancreatic insufficiency.

The roles of extracellular and intracellular mechanisms in the degradation of brush border proteins have been investigated by studying the small intestinal mucosa of dogs with naturally occurring exocrine pancreatic insufficiency. Peroral jejunal biopsies were homogenised and the organelles separated by isopycnic centrifugation on continuous sucrose density gradients. The distributions of marker enzymes for the principal subcellular organelles were determined in the gradients and related to the specific activities in the homogenates. There were increased activities of the brush border carbohydrases zinc-resistant alpha-glucosidase, maltase and sucrase in the pancreatic insufficient animals, but no change in lactase activity. The activity of gamma-glutamyl transferase was also higher in the affected group; the activities of two other brush border enzymes, alkaline phosphatase and leucyl-beta-naphthylamidase, however, were unaltered. These findings with an increase in the modal density of the brush border from 1.20 to 1.22 are consistent with an enhanced glycoprotein content of the microvillus membrane. There were also rises in the activities of lysosomal enzymes. N-Acetyl-beta-glucosaminidase activity was increased in the soluble fractions and the percentage latent enzyme activity was reduced, findings indicative of an increased fragility of the lysosomal membrane. There were no marked alterations in the activities or density gradient distributions of marker enzymes for the other organelles, stressing the specificity of the changes in the brush borders and lysosomes. These findings are compatible with the degradation of certain exposed brush border proteins by pancreatic proteases and suggest that when this is defective, intracellular degradative mechanisms may be stimulated.

Acetylglucosaminidase↗