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.

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.

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.

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.

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.

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.

The responses of Golgi tendon organs to stimulation of different combinations of motor units

1. Afferent discharges were recorded from stretch receptors identified as Golgi tendon organs, in the medial gastrocnemius and soleus muscles of the cat. 2. The response of a tendon organ was recorded during stimulation of one or more motor units selected for the intensity of discharge elicited from the receptor during twitch and tetanic contractions. 3. Repetitive stimulation of a single motor unit could evoke in a tendon organ a firing rate of up to 174 impulses/sec. The mean rate for a total of 90 motor units was 65 (+/- 32 S.D.) impulses/sec. No significant difference in effectiveness could be detected between motor units covering a wide range of contraction speeds, tetanic tensions and susceptibility to fatigue. 4. The response of a tendon organ to contraction of several motor units in combination was greater than from stimulating any one motor unit alone but less than predicted from the algebraic sum of individual responses. 5. The relation between firing rate and tension was plotted for combined stimulation of up to ten motor units. The relation was found to be a straight line provided the size of the response elicited by each motor unit in the stimulated bundle was similar or when responses were ranked according to their intensity. When one motor unit evoked a much more powerful response than others it tended to dominate the discharge and disturb the linearity. 6. Evidence is provided that the sites of stimulus transduction for motor units which exert their effect directly on the receptor can be relatively independent of one another. It is argued that on such occasions summation of responses may be attributed to mechanisms operating at the level of impulse generation.

Urethritis attributable to Neisseria meningitidis, group X: a case report

Neisseria meningitidis, Group X, was recovered from the anterior urethra of a 27-year-old male patient with urethritis. This isolate represents the first reported recovery of this organism from this anatomic site in association with this disease. He was treated with probenicid by mouth, followed by 4.8 million units of aqueous procaine penicillin G injected intramuscularly and became asymptomatic in three days. His recovery strongly indicates that the meningococcus was the etiologic agent of the urethritis since the condition promptly cleared with penicillin therapy.

Mycoplasma salivarium in the blood of a child with leukemia

Mycoplasma salivarium was recovered from the blood of a five-year-old girl who had leukemia and subsequently developed pneumonitis. The patient's pneumonitis failed to respond to nafcillin, a cell-wall-active antibiotic, but eventually she recovered from the pneumonia after a regimen of erythromycin. Sputum, oropharyngeal, and nasopharyngeal cultures revealed normal bacterial flora; a blood culture was negative for bacteria. Throat and sputum cultures were negative for mycoplasma; however, M salivarium was recovered from the patient's blood. The patient had a cold hemagglutinin titer of 1:250.

The stiffness of amphibian slow and twitch muscle during high speed stretches

Experiments were carried out to compare the stiffness of cross-bridges in amphibian slow and twitch muscle. An isolated iliofibularis muscle was subjected to rapid, small stretches during contraction of either slow or twitch fibres at a number of different isometric tensions. The method of analysis allowed the compliance of the cross-bridges to be distinguished from other sources of compliance. Provided that the muscle was stretched sufficinetly rapidly to obtain limiting values of stiffness, little difference was found between the mechanical properties of the cross-bridges in slow and twitch muscle. It is concluded that the difference in observed stiffness of the two muscle types is due to a lower turnover rate of cross-bridges and a smaller number of sarcomeres in slow fibres.

A 'late supernormal period' in the recovery of excitability following an action potential in muscle spindle and tendon organ receptors

1. Discharge patterns have been recorded from five types of stretch receptor; frog muscle spindles, lizard tendon organs, cat soleus tendon organs and primary and secondary endings of cat soleus muscle spindles.2. The fully adapted discharge of each type of receptor is irregular, especially for frog spindles and primary endings of cat spindles as compared with the other three types (the ;regularly firing' receptors). Frog spindles and some cat spindle primary endings would maintain a discharge at very low mean rates (1/sec or less) while the remaining receptors would stop suddenly, as soon as their rate of discharge fell below a critical value characteristic for each individual ending.3. This pattern of discharge suggests that there is a peak in the excitability of ;regularly firing' receptors at a time following a preceding impulse, which corresponds to the intervals between impulses at each particular receptor's slowest rate of maintained firing, and that the excitability subsequently falls again. Primary endings of cat muscle spindles also showed some evidence of such a ;late supernormal period', but frog spindles did not.4. Direct evidence for the ;late supernormal period' was obtained from experiments in which a maintained discharge was restarted by an antidromic action potential in a receptor which had stopped firing, and to which had been applied a stretch just too small to restart the discharge.5. It is shown in an Appendix that a model receptor in which the recovery of excitability following an impulse has a hyperbolic time course, and in which Gaussian distributed noise is superimposed on the generator potential, can have a discharge pattern very closely resembling that of a frog spindle (cf. Buller, 1965).6. After addition of a late supernormal period to the model, its discharge pattern could mimic closely that of a lizard or cat tendon organ, or of a secondary ending of a cat spindle.

Responses of tendon organs in a lizard

1. In the lizard Tiliqua the tendons of the caudo-femoralis muscle are supplied by a nerve which runs separately from the muscle nerve. 2. Recordings of afferent discharges in the tendon nerve revealed the presence in the tendon of stretch-sensitive mechanoreceptors which responded to both passive changes in limb position and to muscle contraction. 3. A preparation of the tendon and its nerve were dissected free of surrounding tissue and studied in isolation while recording the activity of single functional units. The minimum tension in the tendon necessary for a maintained response from a receptor lay in the range 5-35 g (mean 16 g) and the firing rates at these tensions were in the range 5-14 impulses/sec (mean 9 impulses/sec). 4. Receptors showed a steep increase in firing rate with increase in tension up to about 120 g. The firing rate 30 sec after the onset of a tension change did not exceed 40 impulses/sec. 5. During the tension change the receptor responded with a burst of impulses whose frequency depended on the velocity of stretch. With large, rapidly rising tension steps peak firing rates of up to 300 impulses/sec were observed. 6. Tension and length changes recorded during rapid tendon-stretches were very similar, with little sag in tension at the new length. The response of all units however continued to fall throughout the stretch. Some of the possible causes of this adaptation have been discussed.

An electrophysiological investigation of the receptor apparatus of the duck's bill

1. The properties of receptors in the duck's bill have been studied by recording from units isolated by dissecting fine filaments from the maxillary and ophthalmic nerves.2. The units studied were divisible into three groups, phasic mechanoreceptors responsive to vibration, thermoreceptive units, and high threshold mechanoreceptors.3. Vibration-sensitive mechanoreceptors (113 units) had small receptive fields, showed a rapidly adapting discharge to mechanical stimulation of the bill, were sensitive to vibratory but not to thermal stimuli and showed no background discharge.4. Temperature receptors (twenty-one units) were insensitive to mechanical stimulation and showed a temperature-dependent background discharge. Sudden cooling produced a transient increase in discharge frequency.5. High threshold mechanosensitive units (eight units) gave a slowly adapting discharge to strong mechanical stimulation and were insensitive to vibratory and thermal stimulation.6. It is concluded that the low-threshold, vibration-sensitive responses come from Herbst corpuscles. No specific function can yet be assigned to the Grandry corpuscles.

Mycoplasma recovery from the male genitourinary tract: voided urine versus the urethral swab

A study of Mycoplasma recovery from the genitourinary tract was made on a group of 50 males attending a venereal disease clinic. The purpose of the investigation was to compare recovery rates of mycoplasma from two types of clinical specimens-the urethral swab and voided urine. The total number of positive cultures did not differ significantly when either the swab or urine was used; Mycoplasma hominis type 1 was the only taxonomic species isolated, either alone or mixed with T-strain mycoplasma. Recovery rates of the two types from both the swab and the urine did not differ significantly. Age did not relate to the presence of mycoplasma in the group studied.