PubMed Health⌕ Search

Biomedical subjects

J E Gregory

Publications and source records attributed to J E Gregory.

At least 19 recordsLinked to original sources
PubMed1992

Development of the sensory innervation of muscle spindles in the kitten.

This is a report of the changes observed in the pattern of sensory innervation of muscle spindles in hindlimb muscles of kittens during the first four weeks of life. The structural analysis, made on teased, silver-stained preparations, was complemented by a series of recordings of afferent responses of kitten spindles during ramp-and-hold stretches of the muscle. The primary endings of spindles from newborn animals showed a large degree of variability in their branching pattern and branches formed a network across the intrafusal fibres. In older animals there was less variability and lateral branches of stem axons began to encircle the intrafusal fibres. The process of maturation was characterized by a more uniform shape of the endings and more complete, evenly spaced, annulospiral terminals. Recordings of the responses of primary endings of spindles during muscle stretch showed that several features of the adult response were already present in the newborn, although the overall rate of discharge was very much lower. It was concluded that the changes observed in the structure of the sensory endings of kitten spindles did not have clearly identifiable physiological correlates. It appears that an annulospiral shape of the sensory terminals is not a necessary prerequisite for the generation of stretch responses. The predominant factor which appears to determine the responses of spindles to stretch is the maturity of the intrafusal fibres, in particular, the bag2 fibre.

Aging↗
PubMed1992

Muscle history dependence of responses to stretch of primary and secondary endings of cat soleus muscle spindles.

1. Responses were recorded from both primary and secondary endings of soleus muscle spindles in the anaesthetized cat during slow stretches of the muscle after conditioning contractions at different lengths. 2. After a 5 mm length step and a fusimotor-strength contraction given at the longer length, on return to the initial length the response to a slow test stretch (0.5 mm s-1) showed a change in slope midway through the stretch which was attributed to taking up of slack in intrafusal fibres. 3. The onset of the change in slope during the test stretch depended on the size of the conditioning step. With no conditioning length change, just a fusimotor-strength contraction, there was no slope change and the response consisted of an initial burst followed by a maintained high rate of discharge. 4. Following a conditioning length step, the point of onset of the slope change during the test stretch could be altered by stimulating single identified fusimotor fibres to the spindle. Stimulating some static axons produced large changes in the stretch response while other static axons and dynamic axons had only small effects. 5. Many secondary endings showed a delay in onset of their response to a test stretch, dependent on the size of the preceding conditioning step, signalling the presence of slack in much the same way as the primary endings. Other secondary endings, however, appeared to have stretch responses that were largely independent of muscle conditioning. 6. Muscle history-independent responses of secondary endings were associated with low axonal conduction velocities. It is proposed that secondary endings which remain unaffected by muscle conditioning lie on more distal regions of nuclear chain fibres in the S2-S5 position. Here they are stimulated during both the take-up of slack and the subsequent direct stretch of the intrafusal fibres.

Animals↗
PubMed1992

The central projection of electrosensory information in the platypus.

1. This is the first detailed description of the projection to the cerebral cortex of afferent information coming from electroreceptors in the bill of the platypus, Ornithorhynchus anatinus. 2. In animals anaesthetized with chloralose, with the bill immersed in tap water, applying a potential difference between plate electrodes on either side of the bill produced large amplitude potentials from the surface of a postero-lateral region of cerebral cortex. Response threshold was 300 microV cm-1, somewhat lower than threshold measured for single identified electroreceptors. Electroreceptor threshold was at least three orders of magnitude lower than threshold of mechanoreceptors to electrical stimuli (Gregory, Iggo, McIntyre & Proske, 1989a). 3. Monopolar stimulation of the bill revealed a crossed projection. The map on the cortical surface had the bill oriented dorso-laterally, its base towards the mid-line, the tip on the lateral edge, pointing slightly forwards. The edge of the bill faced backwards. Electrosensory information coming from the edge of the bill appeared to be much more strongly represented than input from the dorsal surface. 4. Weak electrical and mechanical stimuli applied to the bill both evoked large amplitude potentials from the same region of cortex indicating that there was complete overlap between the regions receiving tactile and electrosensory inputs. 5. Inserting microelectrodes into the deeper layers of cortex revealed burst discharges in single cells and groups of cells in response to weak electrical stimulation of the bill. Activity could be recorded over a range of depths from 0.3 to 4 mm, with the majority of responses coming from cells 1-3 mm deep. Histological examination of lesion sites made at 1.1 mm and at 3 mm suggested that cells in the pyramidal and ganglion layers were involved in generating the activity. 6. Some evidence was obtained for interactions at the level of the cerebral cortex between activity generated by tactile and electrosensory inputs. When electrical and mechanical stimuli were both applied to the bill with an interstimulus interval of less than 25 ms, cortical neuronal responses generated by one stimulus were completely suppressed by the other. However no evidence was obtained of a direct convergence at the level of the cortex between the two modalities. 7. Cortical activity could be evoked in response to rapidly changing voltage fields. This observation, together with our earlier finding of a high rate sensitivity of the receptors, emphasizes the high dynamic sensitivity of the system. 8. It is concluded that the electrosensory system of the platypus is closely associated with the sense of touch.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Responses of primary endings of cat muscle spindles to locally applied vibration.

Responses of muscle spindles in the cat soleus muscle have been studied during vibration applied locally to the belly of the muscle. Bursts of vibration at 170 Hz and with a peak amplitude of 200 microns were applied to a site at which local pressure initiated impulses from the spindle. The response to vibration depended on the conditioning of the muscle immediately beforehand and the placement of the vibrator. The length at which the vibration was applied was called the test length; this was typically 10 mm less than the muscle's maximum length in the body. After a fusimotor strength contraction at a length 2-5 mm longer than the test length, vibration sensitivity, measured on return to the test length, was low. If the muscle was contracted at a length 2-5 mm shorter than the test length, vibration sensitivity was high. The low vibration sensitivity following conditioning at the longer length was attributed to the development of slack in intrafusal fibres. In the presence of slack, stimulation of some static fusimotor fibres was able to restore vibration sensitivity fully. It is suggested that the vibration sensitivity of passive spindles arises largely in bag2 intrafusal fibres.

Animals↗
PubMed1991

Extrafusal and intrafusal motor units in the kitten.

In the lateral flexor digitorum longus muscle (FDL) of anaesthetized kittens aged 3-21 days, alpha (alpha) and gamma (gamma) motoneurone conduction velocities were measured together with motor unit tensions. Conduction velocity lay in the range 8-27 msec-1 for alpha motoneurones and 2-10 msec-1 for gamma motoneurones. Motor unit tetanic tensions were 3-47 mN with the largest units being 2-2.2% of whole muscle tension. The hypothesis was tested that motor axons conducting within the gamma range and presumed to be innervating the intrafusal fibres of muscle spindles, on stimulation, produced measurable tension. Stimulating gamma motoneurones in the adult produces no tension. The hypothesis was based on histological observations that while in the adult, intrafusal fibres were about one third of the size of extrafusal fibres, in kittens the two were nearly equal, both in length and diameter. It was shown by means of signal averaging during stimulation of ventral root filaments that whenever tension was recorded in the muscle an impulse could be detected in the muscle nerve conducting at alpha motoneurone tempo. It is concluded that in the kitten, despite the fact that intrafusal and extrafusal fibres are of similar size, stimulating single gamma motoneurones does not develop measurable tension.

Animals↗
PubMed1991

Where in the muscle spindle is the resting discharge generated?

This is a report of experiments on muscle spindles of the soleus muscle of the anaesthetized cat. Following a step shortening of the muscle, muscle spindles fall silent. At suitable muscle lengths their discharge may restart several seconds later to gradually recover a maintained rate of discharge. These experiments examine the question of where within the spindle the resumption of a resting discharge may originate. It was found that stimulation of some static fusimotor fibres immediately after the shortening led to early recovery of the resting discharge. Stimulation of dynamic and other static gamma motoneurones had much less effect. Since the dynamic gamma axons innervate almost exclusively the bag1 intrafusal fibre, contraction of this fibre appears to have little influence on the mechanisms responsible for restarting the resting discharge. Bag2 and chain fibres do seem to be involved. For primary endings, the bag2 fibre contraction was especially effective since static axons, which did not evoke 'driving' of the afferent response, and which are thought to predominantly innervate bag2 fibres, did restart the resting discharge. For secondary endings, stimulation of nearly all gamma axons led to an early restart of the resting discharge suggesting that here the nuclear chain fibres were responsible.

Animals↗
PubMed1991

Two kinds of resting discharge in cat muscle spindles.

1. The behavior of primary endings of cat soleus muscle spindles was studied during shortening steps carried out at different muscle lengths. 2. Spindles were of two kinds: one, silent spindles, whose afferents fell silent after the shortening, at least over part of the range of lengths tested. The second, spontaneous spindles, resumed firing at all lengths. 3. For silent spindles, the duration of the silent period, measured at lengths where they did recover a resting rate, depended directly on muscle length and became shorter at longer lengths. This is what would be expected if the slack introduced in the spindle by the shortening step was removed more rapidly at longer lengths by the higher passive tension. For spontaneous spindles, on the other hand, the duration of the silent period after the shortening was largely independent of muscle length and depended on the spindle's rate of firing immediately before the shortening. 4. At intermediate lengths the discharge of slack spontaneous spindles remained unaffected by an isometric muscle contraction. It was therefore not possible to produce a pause in the discharge, behavior normally taken as typical of spindles. The discharge could be interrupted by the contraction if this was combined with a large shortening movement. 5. It is proposed that when intrafusal fibers are slackened by a shortening step, the resting discharge in spontaneous spindles is generated by a maintained depolarization of the annulospiral ending resulting from extension of the terminal coils by forces from within the receptor. A shortening contraction compresses the spirals to interrupt the discharge. The sensory endings of silent spindles remain below threshold until the spirals have been opened out sufficiently by external stretch.

Afferent Pathways↗
PubMed1991

Effect of muscle length on phasic stretch reflexes in humans and cats.

1. This is a report of the effects at different muscle lengths of the muscle's immediate history on the tendon jerk and Hoffman (H)-reflex in triceps surae of human subjects and cats. 2. In adult human subjects the size of the tendon jerk was measured as electromyogram (EMG) and torque in response to a tendon tap. Before each test tap the muscle was conditioned by a maximum voluntary contraction carried out with the foot either plantarflexed or dorsiflexed by 30 degrees from the test position. After a contraction with the foot dorsiflexed, the subsequent reflex response was smaller than after a contraction with the foot plantarflexed. 3. The same conditioning procedure was carried out with the H-reflex. The reflex was elicited by transcutaneous electrical stimulation of the tibial nerve in the popliteal fossa. Here the reflex after a contraction with the foot dorsiflexed was larger than after plantarflexion. In other words, the effects of conditioning were the opposite for the tendon jerk and H-reflex. 4. The effects of muscle conditioning were tested over a range of muscle lengths. As the test length was made progressively longer, that is, the foot more dorsiflexed, the difference in size of the tendon jerk following the two forms of conditioning became less, whereas for the H-reflex it remained the same. 5. These findings were confirmed in cats anesthetized with alpha-chloralose. The tendon jerk was elicited by a quick stretch applied to the triceps surae muscle group, and the H-reflex represented by the monosynaptic reflex recorded from the central, cut end of the ventral root in response to electrical stimulation of the triceps nerve. Muscle conditioning consisted of a 1-s period of stimulation at 20 pulses/s, at fusimotor strength, of the peripheral end of the cut ventral root at a muscle length 5 mm longer or shorter than the test length. In the cat, as in human subjects, the effect of conditioning on the tendon jerk reversed at long muscle lengths, whereas the monosynaptic reflex showed no reversal. 6. It had been proposed previously that the effects of conditioning on stretch reflexes could be explained by development of slack in the intrafusal fibers of muscle spindles after a contraction at a longer-than-test length. The presence of slack lowers the resting discharge of spindles and reduces the afferent response to a tendon tap.(ABSTRACT TRUNCATED AT 400 WORDS)

Achilles Tendon↗
PubMed1990

Errors in force estimation can be explained by tendon organ desensitization.

Here we report observations on the sense of muscle tension in human subjects and compare them with responses of tendon organs in cat hindlimb muscles. Human subjects learned under visual guidance to estimate a 4% maximum voluntary contraction (m.v.c.) of elbow flexors of one arm. When they were able to reproduce this force reliably without visual feedback, they repeated the estimation immediately after a 5 second m.v.c. or a 5 second period of relaxation. In a second experiment the 4% m.v.c. was generated under visual control with one arm, and matched with the other, test arm, without visual feedback. The matching task was then repeated after test arm conditioning. In both experiments subjects reported an accurate match using significantly more than the reference force ("overmatched") after an m.v.c. The overmatching was greatest during the first 5 second period following the conditioning contraction, and during the subsequent 20 seconds it gradually declined to near reference levels. The size of the matching error was directly proportional to the duration of the conditioning contraction. In the first experiment extension of the arm immediately following conditioning increased the error, in the second it slightly decreased it, although tension continued to be overmatched. In a series of experiments on the soleus muscle of anaesthetised cats responses of tendon organs to 10% of maximum contraction were seen to drop sharply when preceded by a conditioning maximum contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Relations between identified tendon organs and motor units in the medial gastrocnemius muscle of the cat.

In one medial gastrocnemius muscle of each of several cats, the response was recorded of a single tendon organ to the contraction of a single motor unit which strongly excited the receptor. The motor unit was depleted of its glycogen and the depleted muscle fibres identified in PAS-stained transverse sections. The site of maximum tendon organ sensitivity was marked and the tendon organ identified in the same sections. Five pairs of tendon organs and motor units were studied completely. Each tendon organ was found to have one or two (mean 1.6) depleted muscle fibres attached to it, included in the bundle of fibres attached to the end (mean no. 14.4) and side (mean no. 5.6) of the tendon organ. A correlation was found between tendon organ discharge rate and the tension calculated from cross-sectional area measurements of the depleted muscle fibres attached to the tendon organ, with variation between individual pairs of tendon organs and motor units. One estimate of the average sensitivity of the sample was 28 imp/s/mN. A nearly linear discharge rate vs. tension relation was found for single tendon organ and motor unit pairs when tension was graded during a series of fatiguing contractions. Under these conditions the sensitivity, measured as the slope of the relation between discharge rate and motor unit tension recorded at the common tendon, varied between 0.11 and 0.30 imp/s/mN for 6 pairs.

Animals↗
PubMed1990

Effects of muscle history on the stretch reflex in cat and man.

1. This is a report of experiments on cat and man which demonstrate effects of a muscle's previous history of contraction and length changes on the size of the stretch reflex. 2. In adult human subjects the size of the tendon jerk was measured in ankle extensor muscles by tapping the Achilles tendon. Muscle conditioning consisted of a maximum voluntary contraction with the foot dorsiflexed or plantarflexed by 30 deg from the test position, after which the subject was asked to relax while the foot was held still for several seconds before being returned to the test position and a tendon tap given. After a contraction of the lengthened muscle the tendon jerk was smaller than after a contraction of the shortened muscle. 3. The experiment was then repeated, but instead of a tendon jerk an H (Hoffmann) reflex was elicited by transcutaneous electrical stimulation of the tibial nerve in the popliteal fossa. The reflex after a conditioning contraction of the lengthened muscle was larger than after a contraction of the shortened muscle. In other words muscle conditioning produced opposite effects on the tendon jerk and H reflex. 4. These findings were confirmed in cats anaesthetized with chloralose. After a conditioning contraction of triceps surae at a length 5 mm longer than the test length (hold-long) a quick tendon stretch produced a smaller reflex response than following a conditioning contraction with the muscle 5 mm shorter than the test length (hold-short). The reverse trend was seen with a reflex elicited by direct electrical stimulation of the muscle nerve, which stimulates the H reflex. 5. One consequence of a conditioning contraction is that it leads to an alteration of the level of resting discharge of muscle spindles. We propose that the larger tendon jerk after a contraction of the shortened muscle is the result of changes in stretch sensitivity of muscle spindles. The reverse effect on the H reflex we attribute to a rise in the level of resting discharge of muscle spindles, which, we propose, leads to reflex inhibition of motoneurones. 6. We support this conclusion with evidence from an experiment in which the size of the conditioning step was systematically altered. Even quite small hold-short conditioning steps led to depression of the H reflex in man and the monosynaptic reflex in cats. Recordings from single afferents showed that such small steps were also accompanied by a detectable rise in spindle resting discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1989

Tendon organ afferents in the knee joint nerve of the cat.

This article is concerned with the question of whether some of the slowly adapting responses recorded in the mid-range of joint position from the posterior articular nerve (PAN) of the cat hindlimb and which are therefore candidates for joint position sensors, are in fact coming from muscle receptors of the popliteus muscle. Evidence is provided that in addition to afferents of primary endings of spindles, secondary endings of spindles and tendon organs are represented in PAN as well. Removal of the fleshy part of the popliteus muscle does not reliably remove muscle afferent contamination in PAN.

Afferent Pathways↗
PubMed1989

Responses of electroreceptors in the platypus bill to steady and alternating potentials.

1. This is a report of further observations on the response characteristics of electroreceptors in the bill of the platypus, Ornithorhynchus anatinus, first described by Gregory, Iggo, McIntyre & Proske (1987). 2. The main finding is that, with the bill immersed in water, applying a potential difference between large plate electrodes on either side of the bill, produced detectable responses in a population of electroreceptors to field strengths as low as 4 mV cm-1. Threshold for individual receptors lay between 4 and 25 mV cm-1. 3. An electric dipole placed in the water close to the receptive field could also elicit responses, threshold being lowest when the cathode was near the centre of the field. On several occasions the most sensitive spot was seen, under the microscope, to correspond to the mouth of a mucous sensory gland (Andres & Von Düring, 1984). Response intensity fell when the dipole was moved further away, the drop being less steep in a direction over the top of the bill towards the mid-line. 4. For individual receptors the latency of the first impulse initiated by supramaximal voltage pulses was 1.1-1.8 ms. Latencies tended to be shorter when the site of the receptor lay closer to the recording electrodes. Plotting each latency against conduction path length for eleven receptors gave an approximately linear relation from which was calculated an average axonal conduction velocity of 56 m s-1. The plot yielded an estimate of impulse initiation time of 0.8 ms. It is argued that this is too short to include a synaptic delay. A peripheral synapse is found in all non-mammalian electroreceptors. 5. Electroreceptors responded to both steady and rapidly changing potential gradients. For ramp-shaped gradients of 1-50 V s-1 peak firing rate was approximately proportional to log stimulus velocity. In response to sinusoidal potential changes a 1:1 relation between each afferent impulse and the peak of the stimulating waveform could be obtained over the range 12-300 Hz. Threshold was at its lowest at 50-100 Hz. Tuning curves measured with the bill immersed in water were little different from those obtained by focal stimulation with the bill in air. 6. It is concluded that platypus electroreceptors, supplied by the trigeminal nerve, and which are therefore not part of the acoustico-lateralis system as in non-mammalian electroreceptors, are also unique in not having a peripheral synapse. Furthermore, they are able to respond to both steady and rapidly changing voltage gradients.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1989

Responses of electroreceptors in the snout of the echidna.

1. This is a report of experiments which provide evidence in support of the existence of an electric sense in the echidna, or spiny anteater Tachyglossus aculeatus. It is the first known example of electroreception in a terrestrial animal. 2. In each of four animals anaesthetized with alpha-chloralose, afferent responses were recorded in dissected filaments of the infraorbital branch of the trigeminal nerve which supplies skin of the upper jaw. Recordings were obtained from a total of forty-seven units identified as electroreceptors, by their responses to weak voltage pulses using focal stimulation of the moist skin surface. 3. In the absence of a stimulus, some receptors had an irregular resting discharge; others were silent. The receptive field for each receptor consisted of a discrete spot. Receptive fields were restricted to the tip of the snout. Cathodal stimulation over the receptive spot was excitatory for the duration of an applied voltage pulse. Reversal of stimulus polarity silenced any on-going activity and was followed by a post-anodal rebound excitation. 4. Receptor threshold was best measured not in air but with the snout immersed in tap water. An electric field was applied between a pair of large plate electrodes on either side of the snout. Threshold for thirty receptors lay in the range 1.8-73 mV cm-1. Measurements of response latency and of conduction path length gave estimates of axonal conduction velocities for the afferent fibres of 10-18 m Receptors responded to sinusoidally changing voltage gradients over the range 0.5-200 Hz with a maximum sensitivity at 20 Hz. 5. In one experiment a receptor site was marked with fine pins. Serial sections of the piece of underlying skin revealed a large mucus-secreting gland at the marked spot. Similar glands in skin of the platypus have previously been shown to be the sites of electroreceptors. 6. In a behavioural experiment an echidna was trained to choose between two identical tap water-filled troughs, one of which had a weak electric field across it. The animal learned to detect field strengths down to 1.8 mV cm-1 which corresponded to threshold for the most sensitive receptor measured in a subsequent electro-physiological experiment. It is concluded that the echidna, like the other Australian representative of the monotremes, the platypus, has an electric sense. It remains to be determined what kinds of sources of electric fields the animal encounters in its normal habitat.

Air↗
PubMed1988

The responses of muscle spindles in the kitten to stretch and vibration.

Discharges of muscle spindle afferents from the soleus muscle were studied in kittens aged 1-21 days and in adult cats. Vibration applied longitudinally to the tendon elicited one impulse for each cycle of vibration over the range 1-200 Hz for the kittens and up to 450 Hz for the adult. Threshold amplitudes were generally higher in the kitten than in the adult. In response to large ramp and hold stretches applied at long muscle lengths kitten spindles showed rate saturation during the length change. Dynamic index, that is the peak rate during the length change minus the rate at the final length became progressively smaller at longer muscle lengths. No sign of saturation was seen at comparable muscle lengths in the adult. It is suggested that in the newborn the bag1 intrafusal fibre is not functional and that the dynamic response is produced only by the afferent terminals on the bag2 fibre. Another difference between kitten and adult was the length sensitivity measured under dynamic conditions. This increased much more steeply with stretch rate in the kitten. One possible explanation for the higher dynamic length sensitivity is a lack of elastic fibres surrounding intrafusal fibres of immature spindles.

Aging↗
PubMed1988

Receptors in the bill of the platypus.

1. Afferent responses were recorded from filaments of the trigeminal nerve in each of two platypuses (Ornithorhynchus anatinus) anaesthetized with alpha-chloralose. All receptive fields were located along the lateral border of the upper bill. Discrete receptive fields could be identified as belonging to two distinct classes of sensory receptor. 2. The most prominent response was an irregular resting discharge which could be increased or decreased by weak electric pulses. These receptors were insensitive to moderately strong mechanical stimulation, and it was concluded that they were electroreceptors. 3. Each electroreceptor had a single spot of maximum sensitivity on the bill surface. When the stimulating electrode over this spot was the cathode it excited the receptor for the duration of the stimulating pulse, using stimulus strengths as low as 20 mV. When it was the anode, it inhibited the discharge. Cathodal excitation was followed by rebound inhibition and anodal inhibition by rebound excitation. 4. Receptors responded to cathodal steps with an initial high-frequency burst of impulses, followed by a lower maintained rate of discharge. Rapidly changing pulses were similarly effective in exciting receptors, adding support to the claim that platypuses are able to detect moving prey by the electrical activity associated with muscle contraction. 5. The centres of the receptive fields of two electroreceptors were marked by the insertion of fine entomological pins. Histological examination established the presence of a large mucus-secreting gland at the marked spot. The epidermal duct of the gland contained an elaborate myelinated innervation, with morphologically distinct axon terminals that we identify as the electroreceptors. 6. As well as electroreceptors, the skin of the bill contained three kinds of mechanoreceptors: slow-adapting receptors, rapidly adapting, vibration-sensitive receptors and receptors with an intermediate adaptation rate. The slowly adapting receptors were characterized by their low threshold to mechanical stimuli, irregular discharge and significant dynamic sensitivity. Vibration receptors showed maintained responses to sinusoidal vibration of the skin up to 600 Hz. 7. These experiments confirm an earlier report that the platypus bill is an electrodetector organ. The presence of electroreceptors of a unique structure and supplied by the trigeminal nerve indicates that electroreception has evolved independently in monotremes. This in turn emphasizes that monotremes are a highly evolved group which split off from the main mammalian stem a long time ago.

Action Potentials↗