Cutaneous electroreceptors in the platypus: a new mammalian receptor.
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Animals↗
Biomedical subjects
J E Gregory
Publications and source records attributed to J E Gregory.
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↗
Aftereffects in the responses of cat muscle spindles and errors of limb position sense in man.
1. This is a report of two series of experiments, one, observations on muscle spindles in the anesthetized cat, the other, based on the animal observations, concerned with limb-position sense in human subjects. 2. When the cat soleus muscle is contracted at a short length and then after a brief period is brought to an intermediate length, the integrated afferent activity recorded in dorsal roots is maintained at a high level. If the contraction is applied while the muscle is held at a long length, the subsequent level of afferent activity back at the intermediate length is much lower. A similar result is obtained if large amplitude vibration of the muscle is used instead of contraction. Recordings from single afferents confirm that these alterations in discharge could be attributed to the primary and secondary endings of muscle spindles. 3. Following the proposal that the resting discharge of muscle spindles contributes to the sense of limb position in man, we have used a sequence of contractions or periods of vibration of biceps and triceps brachii followed by an arm-matching task to look for errors in limb position consistent with the patterns of muscle afferent discharge observed in the animal experiments. 4. Blindfolded subjects were told to contract biceps of one, the test arm, with the arm flexed. After the arm had relaxed, it was placed by the experimenter at an intermediate angle that the subject was asked to match with the other, indicator arm. Then the subject was asked to contract triceps with the arm extended, and the matching task was repeated. 5. The hypothesis was that contracting a muscle while it was short raised the level of afferent discharge coming from it when the arm was moved to an intermediate position so that the muscle would be perceived as more stretched and the arm felt to be displaced further than it really was. Thus, when biceps was contracted with the arm flexed, this would lead the arm in the intermediate position to be perceived as more extended than it really was. When triceps was contracted with the arm extended, this would lead the arm in the intermediate position to be perceived as more flexed than it really was.(ABSTRACT TRUNCATED AT 400 WORDS)
Adult↗
Can lactate-evoked cardiovascular responses be used to identify muscle ergoreceptors?
The increase in blood pressure and heart rate which accompanies muscular exercise is in part a reflex mediated by afferent nerve fibres in the group III and IV (small myelinated and unmyelinated) range. It has been reported that perfusion of lactate ions into hind limb muscles is an effective stimulus for these reflex responses. To investigate this hypothesis further, and to test adequacy of the controls used, a solution containing 15 mM lactic acid was perfused through a hind limb of urethane-anaesthetised rats, the leg's circulation being isolated from the rest of the body. During lactate perfusion, increases were seen in arterial blood pressure and heart rate. Denervation of the entire leg abolished the responses. To locate the receptors involved in the reflex, selective denervations of skin or muscle were performed. Clear responses were never seen when the leg was skinned or denervated by section of cutaneous nerves. On the other hand, responses to lactate perfusion were still seen following section of all nerves supplying the muscles of the leg, leaving the cutaneous innervation largely intact. It is concluded that perfusion of a hindlimb with lactate solutions is not an adequate technique to identify ergoreceptors in muscle.
Animals↗
Responses of muscle receptors in the kitten to succinyl choline.
Responses of soleus muscle afferents were studied in anaesthetised kittens, after intravenous injection of succinyl choline (SCh), a drug which in adult spindles, at low doses, produces a contracture preferentially of bag1 intrafusal muscle fibres and so mimics the effect of dynamic fusimotor stimulation. Following injection of SCh in kittens aged between 1 and 57 days the integrated discharge recorded in large portions of dorsal root during muscle stretch showed a smaller increase than in the adult. Recordings from functionally single afferents of muscle spindles showed that in the youngest animals SCh induced a resting discharge from all spindles, which were normally silent. The response to a ramp-and-hold stretch showed an abrupt rise in firing rate at the start of the stretch followed by a slow decline. The abrupt fall at the end of the ramp, typical of the adult dynamic response, was almost absent in spindles of animals 2-5 days old. In the younger animals SCh allowed primary and secondary spindle endings to be distinguished on the basis of their response to stretch. Conduction velocity of developing Group I nerve fibres was found to increase from 9.5 ms-1 on day 2 to 51.6 ms-1 on day 57, an increase of 0.77 ms-1 per day, while Group II increased from 5.0 ms-1 to 23.2 ms-1 over the same period, an increase of 0.33 ms-1 per day. The two groups appeared to be separate even in the youngest animals. It is concluded that although spindles in the newborn kitten respond vigorously to SCh, the pattern of discharge is quite different from that seen in older animals.
Animals↗
Changes in size of the stretch reflex of cat and man attributed to aftereffects in muscle spindles.
1. This is a report of experiments carried out on the cat and on man, which demonstrate that conditioning of a muscle by contraction and movement can lead to changes in amplitude of stretch reflexes elicited in that muscle. 2. In triceps surae of the cat, the reflex response to a brief stretch was recorded after conditioning with a whole-muscle contraction followed by a pause at a length either 5 mm longer or shorter than the length at which the reflex was elicited. Following conditioning at the long length the reflex response was less than half as large as that following conditioning at the short length. 3. The changes in reflex amplitude could be correlated with an altered stretch responsiveness of muscle spindles in the soleus muscle. When the muscle had been held long during conditioning, a subsequent brief stretch applied at an intermediate length elicited fewer impulses in primary endings of spindles than after conditioning at a short length. 4. The same kind of experiment was then carried out on adult human subjects. When a tendon tap was applied to the Achilles tendon after a voluntary contraction and relaxation of triceps surae with the muscle at a long length, (foot dorsiflexed) the reflex was frequently less than half the size it had been after a contraction at a short length (foot plantarflexed). It was concluded that the same kind of spindle aftereffects as observed for cat soleus spindles were responsible for the changes in reflex amplitude. 5. It was found both in the cat and in human subjects that the changes in reflex amplitude after conditioning became progressively less as the test length was made longer. 6. The explanation put forward to account for these observations is that stable cross-bridges form between actin and myosin filaments of passive intrafusal (and extrafusal) fibers. When the muscle is shortened several seconds after a contraction at a long length, the intrafusal fibers, stiffened by the presence of cross-bridges, fall slack. Slack does not develop after a contraction at a short muscle length, as the fiber is stretched to the test length. Since any slack must first be taken up by the test stretch, there is a smaller afferent response and consequently a smaller reflex contraction in response to a tendon tap after conditioning at a long length.(ABSTRACT TRUNCATED AT 400 WORDS)
Adult↗
Vibration-evoked responses from lamellated corpuscles in the legs of kangaroos.
A group of lamellated corpuscles are present in the interosseous region of the legs of macropod marsupials. Structurally, they are similar to, but simpler than the Pacinian corpuscles of eutherian mammals, in having fewer lamellae. Responses of mechanoreceptors with axons coursing in the interosseous nerve were recorded from filaments, containing single functional units, dissected from the sciatic nerve of the wallaby Thylogale billardierii. The receptors were all maximally sensitive to stimuli applied in the interosseous region, where the cluster of lamellated corpuscles is located. Most units had low mechanical thresholds and were sensitive to sinusoidal vibration over a wide range of frequencies. Functional properties generally resembled those of eutherian Pacinian corpuscles, but the marsupial receptors were less rapidly adapting. The afferent nerve fibres conducted at 45 to 60 ms-1, while the diameter of axons in the osmium-stained interosseous nerve ranged between 7.5 and 12 micron. It is suggested that one important function of the receptors might be the detection of ground-borne vibration.
Adaptation, Physiological↗
Toothbrushing and transient bacteremia in patients undergoing orthodontic treatment.
A study was made to determine the extent of bacteremia experienced by patients undergoing orthodontic treatment with fixed appliances during periods of routine oral hygiene--namely, brushing the teeth. Sixteen orthodontic patients made up the population--11 who practiced good oral hygiene and five who demonstrated poor oral hygiene. Blood was drawn aseptically from the median cubital vein of the subjects before and 15 minutes after brushing the teeth. An aliquot of each blood specimen was added to separate blood culture bottles and incubated at 37 degrees C for a period of up to 5 days. Blood was also used to determine the immune status of the subjects. Anaerobic bacteria were recovered from the blood of nine of the 16 patients studied; aerobic bacteria were not recovered. A negative blood culture before brushing and positive blood culture after brushing were expected but did not occur. Some subjects showed bacteremia before brushing and a negative blood culture after brushing. Others showed bacteremia before and after brushing. The unexpected results could be attributed to the patients eating and/or brushing before starting the test. The study showed the capacity of specific anaerobic bacteria to remain in the bloodstream for a 15-minute period. It also demonstrated a presence of bacteria in the bloodstream before the test began.
Adolescent↗
Aftereffects in the responses of cat muscle spindles.
Responses have been recorded from primary endings of muscle spindles in the cat soleus muscle. Changes in spindle responsiveness were measured following a period of conditioning that consisted of a series of rapid stretches or of tetanic ventral root stimulation. In the testing procedure the response of a single spindle afferent was recorded to stimulation of a dynamic fusimotor axon during a slow stretch. Changes in gross afferent discharge coming from the muscle were measured by integrating the activity recorded in dorsal roots. If, after conditioning stretches, the muscle was immediately returned to its initial length, the spindle responded to the test fusimotor stimulation with a high-frequency burst of afferent impulses. If the muscle was held stretched for 3 s after conditioning the response to the brief test tetanus was small or "depressed." It has been suggested that conditioning stretches result in detachment of stable crossbridges in intrafusal fibers and that these bridges then reform over the next few seconds at whatever length the muscle happens to have at the time. When it is long, shortening the muscle back to the initial length leads to the development of slack in intrafusal fibers because of the passive stiffness they have acquired from the presence of the stable bridges. Under these conditions a brief test fusimotor tetanus will lead to a depressed response because the slack must first be taken up before a full response can be generated. It was possible to reverse the depression by interposing an extrafusal contraction during the period between the conditioning and test sequences. It is suggested that lateral compression from the contracting extrafusal fibers and the stretch they impose as they relax reduces any intrafusal slack and thereby reduces the depression. A more quantitative measure of intrafusal slack than the test for depression is to determine the delay in onset of the afferent response to a longer fusimotor tetanus. The delay was short a long initial muscle lengths where, if the muscle was left undisturbed, it soon disappeared completely and spontaneously. It is suggested that at long lengths passive tension in the muscle tends to remove any slack in intrafusal fibers and therefore removes any after effects. The rise in resting discharge of muscle afferents after a conditioning tetanus applied to the ventral root ("postcontraction sensory discharge") can be accounted for by the same hypothesis.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Fusimotor axons in the kitten.
In kittens 1- to 23-days old growth of axons in the soleus nerve has been studied using the structural parameters nerve length, internodal length, and axonal diameter. In addition, single functional fusimotor axons were isolated in lumbosacral ventral roots, and the responses of muscle spindles in soleus were studied during fusimotor stimulation. While nerve length over the soleus nerve to lumbar spinal root increased from 41 to 76 mm during the 22 days, mean internodal length increased from 250 to 410 microns. Mean axon diameter increased from 2.1 to 4.1 microns. In the youngest animals values for both internodal length and axon diameter were distributed uniformly about the mean. From day 11 onward the distributions became bimodal, including a growing number of new axons in the small-myelinated range. Filaments of ventral root were isolated that on repetitive stimulation had a specific excitatory effect on the discharge of muscle spindles. The responses could be attributed to axons that were not associated with measurable tension and were therefore likely to be fusimotor fibers. Measurements of the conduction velocity of skeletomotor and fusimotor axons showed that conduction speed increased progressively with age for both groups, but the rate of increase was more than three times faster in the most rapidly conducting skeletomotor axons compared with the fusimotor axons. The distribution of conduction velocities for fusimotor fibers showed two peaks, one in the range typical for conduction in unmyelinated fibers, 0.5-1.0 m/s, the second at 3-4 m/s. The small number of values in the range of 1-2 m/s was attributed to the process of myelination. It is suggested that conduction speed increases discontinuously over this part of the range as impulse conduction changes from continuous propagation to saltatory transmission. Eighteen fusimotor axons could be classified as having either a static or a dynamic action on spindle discharge. Repetitive stimulation of fusimotor fibers during a ramp-and-hold stretch of the muscle produced a characteristic response. Fibers classified as dynamic had little effect on the response of the spindle when the muscle was held at a particular length but greatly increased the response during a length change. Static fusimotor fibers, on stimulation, increased the response of the spindle at constant length but did not evoke a selective increase during the length change. Both kinds of fusimotor effects were characterized by overall low firing rates.
Aging↗
The discharge of cat tendon organs during unloading contractions.
Tendon organs respond preferentially to contractions of a select set of motor units. If a tendon organ is given a resting discharge by raising the passive tension in the muscle, other motor units can be identified which on contraction interrupt the discharge, presumably by unloading the passive tension. These experiments investigate the possibility that stimulation of motor units with unloading effects can reduce the response of a tendon organ to a loading contraction. We find that over most of the range of muscle lengths the unloading contraction produces only a transient lowering of the firing rate. Only at short muscle lengths where further shortening is accompanied by a steep fall in tension is the unloading contraction able to induce a sustained reduction of afferent discharge.
Action Potentials↗
Responses of muscle receptors in the kitten.
Responses have been recorded in single functional afferents supplying muscle receptors of the soleus muscle in kittens aged between 1 and 23 days. Recordings of nerve volleys in whole dorsal or ventral roots in response to muscle nerve stimulation showed that conduction velocity for afferent and motor fibres was similar and increased from 9 m s-1 in a 2-day-old animal to 33 m s-1 in a 23-day-old animal. Of a total of 215 single functional axons isolated from 28 animals, 82 showed responses typical of tendon organs and 103 resembled muscle spindles, leaving 30 unidentified units. Both muscle spindles and tendon organs were characterized by having an over-all low firing rate compared with responses of adult receptors. The most immature receptors also lacked a maintained response to the hold phase of a ramp-and-hold stretch. Spindles in animals over the whole range of ages, including the youngest animals could be shown to be supplied with a fusimotor innervation. On a few occasions it was possible to isolate single fusimotor axons to a spindle; such axons were sometimes found to conduct impulses at a speed characteristic of non-myelinated fibres. Evidence was obtained for the existence of a beta-(skeletofusimotor) innervation of spindles. On four occasions it was possible to isolate single functional beta-axons in the ventral root. The incidence of beta-innervation appeared to be higher than in the adult.
Action Potentials↗
Site of impulse initiation in tendon organs of cat soleus muscle.
A continuing controversy surrounds the question of whether Golgi tendon organs are examples of receptors in which impulses may be generated at more than one site. This paper reports a systematic examination of a number of models incorporating single or multiple impulse generators and of the compatibility of their predictions with experimental observations. Two phenomena, in particular, that must be accounted for are nonlinear summation and cross-adaptation. When two motor units each with a direct effect on the tendon organ are stimulated together, the rate of discharge is greater than either individual rate but is less than their sum. In cross-adaptation a conditioning response elicited by one motor unit contraction produces adaptation of the discharge associated with stimulation of a second motor unit. A model with a central impulse generator can be modified to account for nonlinear summation by postulating a nonlinear transformation in the generator current-to-impulse rate conversion. Experiments measuring summation of responses to stimulation of three inputs produced results that did not support this model. Another variation of the model, which had a nonlinearity in the tension-to-current step and cross-connections (mechanical or neural) between tendon strands stressed by contracting muscle fibers, was able to account for the observations. A second model that provided the right predictions was a multiple impulse generator with cross-connections. Which of the two models best fits the experimental observations can be decided by comparing the calculated summation coefficients and cross-adaptation coefficients. A central impulse generator predicts a negative correlation, the multiple impulse generator a positive correlation. All of the observations were made using tendon organs of cat soleus muscle. Responses were recorded to stimulation of filaments of ventral root. In a comparison between 20 pairs of responses from six tendon organs the correlation between summation and cross-adaptation coefficients was found to be significantly positive. We conclude that the tendon organ model that accurately predicts all of the experimental observations incorporates multiple generators.
Adaptation, Physiological↗
Opsonization of pneumococci by whole serum from sickle cell disease patients.
An in vitro opsonic activity test was developed to measure the ability of whole serum from sickle cell disease patients to enhance the phagocytosis of Streptococcus pneumoniae by normal white blood cells. At the 30- and 60-minute incubation time periods, there was a 2- to 3-log reduction in the number of colonyforming units of S pneumoniae with both normal and sickle cell serum indicating opsonization of the organisms. However, the reduction in colony-forming units was significantly greater with normal than with sickle cell serum at both time periods (P <.0005 and P <.025 for the 30- and 60-minute time intervals, respectively). The sickle cell sera used in this assay were obtained from pediatric patients (age range, 1 to 15 years) and from adults (age range, 16 to 30 years). Based on this assay, sera from the adult sickle cell patients had a lower mean opsonic activity than that of the pediatric group. The significance of the lower opsonic activity in the adults with sickle cell disease is unknown and requires additional investigation.
Adolescent↗
Muscle receptors in the cross-reinnervated soleus muscle of the cat.
1. Discharges have been recorded from afferents of the soleus muscle following reinnervation by the nerve of a fast twitch muscle, extensor digitorum longus. Recordings were made 227-449 d post-operatively.2. The gross afferent discharge from the cross-reinnervated soleus suggested the presence of fewer mechanosensitive receptors than in normal muscles, as judged by discharges seen during a maximal muscle twitch.3. A comparison of receptors in the cross-reinnervated muscle with afferents from a self-reinnervated muscle showed that many of the responses in the self-reinnervated muscle were also abnormal. It was concluded that much of the disruption resulted from the surgical interference and that rather less could be attributed to the foreign nerve.4. A detailed analysis of response characteristics of receptors in cross-reinnervated soleus muscles of five cats showed that afferent conduction velocities of identified spindles and tendon organs were generally lower than normal and responses to muscle stretch or vibration were often atypical. A large number of afferents which could not be classified as muscle spindles or tendon organs included a group called contraction receptors. These responded generally only during maximal muscle contractions and with a rather feeble discharge. A second group consisted of afferents in which impulses could be elicited by electrical stimulation of the nerve but not by any mechanical activity in the muscle.5. In a further five animals a detailed study was made of the motor supply of muscle spindles. A fusimotor innervation was common, but invariably stimulation of the gamma fibre had a static action on the spindle. No purely dynamic fusimotor fibres were encountered. There were many static beta fibres (skeletofusimotor) no dynamic betas and three axons conducting in the alpha range, which developed no tension, yet produced specific intrafusal effects. Two of these had a mixed static-dynamic action while the third was purely static.6. It was concluded that in the cross-reinnervated soleus muscle the majority of afferents were abnormal in one or other respect. The central action of such abnormal receptors would have to be taken into account when seeking explanations of the transformation of a muscle's mechanical properties following reinnervation by a foreign nerve.
Action Potentials↗
Kava and prohibition in Tanna, Vanuatu.
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Cultural Characteristics↗
Motor unit contractions initiating impulses in a tendon organ in the cat.
1. Responses, identified as coming from Golgi tendon organs, were recorded in afferent nerve fibres supplying the cat soleus muscle. 2. Receptor discharge was recorded during stimulation of single, selected motor units. The degree of summation of responses to combined stimulation of pairs of motor units was compared with the effect of stimulating each motor unit separately. Whenever individual response frequencies differed by a large amount there was little summation on combined stimulation. 3. The response of tendon organ to the second of a pair of closely spaced tetanic contractions of a motor unit showed adaptation of the discharge. Adaptation could also be induced by a conditioning contraction from another motor unit. The amount of 'cross-adaptation' could be correlated with the difference in individual response frequencies of the two motor units and with the degree of summation on combined stimulation. 4. A mechanical model is proposed to account for these observations. The muscle fibre from each motor unit is envisaged to pull on a collagen strand which supports one of the receptor terminals. The non-linear summation of responses on combined stimulation and the phenomenon of cross-adaptation are accounted for by mechanical cross-links between collagen strands such that one muscle fibre pulls on more than one receptor terminal.
Action Potentials↗
The discharge rate: tension relation of Golgi tendon organs.
Tendon organs are preferentially sensitive to activity in a small selected number of motor units. We have isolated single tendon organ afferents and the motor units exerting effects on them in the soleus muscle of the cat. Using the method of distributed stimulation it has been possible to grade motor unit tension over a wide range and record the corresponding firing rates of the receptor. The plot of firing rate against tension was found to be highly non-linear and did not conform to the simple power function previously attributed to the relation.
Action Potentials↗