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PubMed2000

Mammalian spindle orientation and position respond to changes in cell shape in a dynein-dependent fashion.

In animal cells, positioning of the mitotic spindle is crucial for defining the plane of cytokinesis and the size ratio of daughter cells. We have characterized this phenomenon in a rat epithelial cell line using microscopy, micromanipulation, and microinjection. Unmanipulated cells position the mitotic spindle near their geometric center, with the spindle axis lying roughly parallel to the long axis of the cell. Spindles that were initially misoriented underwent directed rotation and caused a delay in anaphase onset. To gain further insight into this process, we gently deformed cells with a blunted glass needle to change the spatial relationship between the cortex and spindle. This manipulation induced spindle movement or rotation in metaphase and/or anaphase, until the spindle reached a proper position relative to the deformed shape. Spindle positioning was inhibited by either treatment with low doses of nocodazole or microinjection of antibodies against dynein, apparently due to the disruption of the organization of dynein and/or astral microtubules. Our results suggest that mitotic cells continuously monitor and maintain the position of the spindle relative to the cortex. This process is likely driven by interactions among astral microtubules, the motor protein dynein, and the cell cortex and may constitute part of a mitotic checkpoint mechanism.

Animals↗

PubMed1998

Dynamics of centromeres during metaphase-anaphase transition in fission yeast: Dis1 is implicated in force balance in metaphase bipolar spindle.

In higher eukaryotic cells, the spindle forms along with chromosome condensation in mitotic prophase. In metaphase, chromosomes are aligned on the spindle with sister kinetochores facing toward the opposite poles. In anaphase A, sister chromatids separate from each other without spindle extension, whereas spindle elongation takes place during anaphase B. We have critically examined whether such mitotic stages also occur in a lower eukaryote, Schizosaccharomyces pombe. Using the green fluorescent protein tagging technique, early mitotic to late anaphase events were observed in living fission yeast cells. S. pombe has three phases in spindle dynamics, spindle formation (phase 1), constant spindle length (phase 2), and spindle extension (phase 3). Sister centromere separation (anaphase A) rapidly occurred at the end of phase 2. The centromere showed dynamic movements throughout phase 2 as it moved back and forth and was transiently split in two before its separation, suggesting that the centromere was positioned in a bioriented manner toward the poles at metaphase. Microtubule-associating Dis1 was required for the occurrence of constant spindle length and centromere movement in phase 2. Normal transition from phase 2 to 3 needed DNA topoisomerase II and Cut1 but not Cut14. The duration of each phase was highly dependent on temperature.

Anaphase↗

PubMed2004

Microtubule plus-end dynamics in Xenopus egg extract spindles.

Microtubule dynamics underlie spindle assembly, yet we do not know how the spindle environment affects these dynamics. We developed methods for measuring two key parameters of microtubule plus-end dynamic instability in Xenopus egg extract spindles. To measure plus-end polymerization rates and localize growing plus ends, we used fluorescence confocal imaging of EB1. This revealed plus-end polymerization throughout the spindle at approximately 11 microm/min, similar to astral microtubules, suggesting polymerization velocity is not regionally regulated by the spindle. The ratio of EB1 to microtubule fluorescence revealed an enrichment of polymerizing ends near the spindle middle, indicating enhanced nucleation or rescue there. We measured depolymerization rates by creating a front of synchronized depolymerization in spindles severed with microneedles. This front could be tracked by polarization and fluorescence microscopy as it advanced from each cut edge toward the associated pole. Both imaging modalities revealed rapid depolymerization ( approximately 30 microm/min) superimposed on a subset of microtubules stable to depolymerization. Larger spindle fragments contained a higher percentage of stable microtubules, which we believe were oriented with their minus ends facing the cut. Depolymerization was blocked by the potent microtubule stabilizing agent hexylene glycol, but was unaffected by alpha-MCAK antibody and AMPPNP, which block catastrophe and kinesin motility, respectively. These measurements move us closer to understanding the complete life history of a spindle microtubule.

Adenylyl Imidodiphosphate↗

PubMed2004

The Caenorhabditis elegans Aurora B kinase AIR-2 phosphorylates and is required for the localization of a BimC kinesin to meiotic and mitotic spindles.

BimC kinesins are required for mitotic spindle assembly in a variety of organisms. These proteins are localized to centrosomes, spindle microtubules, and the spindle midzone. We have previously shown that the Caenorhabditis elegans Aurora B kinase AIR-2 is required for the localization of the ZEN-4 kinesin protein to midzone microtubules. To determine whether the association of BimC kinesins with spindle microtubules is also dependent on AIR-2, we examined the expression pattern of BMK-1, a C. elegans BimC kinesin, in wild-type and AIR-2-deficient embryos. BMK-1 is highly expressed in the hermaphrodite gonad and is localized to meiotic spindle microtubules in the newly fertilized embryo. In mitotic embryos, BMK-1 is associated with spindle microtubules from prophase through anaphase and is concentrated at the spindle midzone during anaphase and telophase. In the absence of AIR-2, BMK-1 localization to meiotic and mitotic spindles is greatly reduced. This is not a consequence of loss of ZEN-4 localization because BMK-1 is appropriately localized in ZEN-4-deficient embryos. Furthermore, AIR-2 and BMK-1 directly interact with one another and the C-terminal tail domain of BMK-1 is specifically phosphorylated by AIR-2 in vitro. Together with our previous data, these results suggest that at least one function of the Aurora B kinases is to recruit spindle-associated motor proteins to their sites of action.

Amino Acid Sequence↗

PubMed1995

Analysis of chemically induced spindle aberrations in male mouse germ cells: comparison of differential and immunofluorescent staining procedures.

Differential staining of spindle and chromatin was adapted to meiotic cells of male mice. Immunostaining with antitubulin antibodies was developed. Both methods were used to test acrylamide, diazepam and vinblastine for their potential to cause spindle disturbances in male mouse germ cells. The analyses were performed after in vivo treatment, comparing the classical safranin/brilliant blue spindle staining to an immunofluorescent assay. The two staining methods complemented each other. Differential staining seems to be more sensitive to the detection of misplaced chromatin whereas the immunostaining is superior when scoring for spindle structure aberrations. All three chemicals showed spindle activity. Acrylamide mainly caused multipolar spindles which possibly develop from a separation of mother and daughter centrioles during an unspecific meiotic block. Diazepam elevated the level of monopolar spindles and induced the loss of single chromosomes, suggesting an effect on motor proteins. Vinblastine was the strongest spindle poison by far. It produced high numbers of shortened and monopolar spindles as well as multiple chromosome loss.

Acrylamide↗

PubMed1998

Neurotrophin-3 and trkC in muscle are non-essential for the development of mouse muscle spindles.

Neurotrophin-3 (NT3) or TrkC null mutant mice were examined for the presence of muscle spindles. Muscles of mastication, but not limbs, contained spindles in newborn and adolescent mutants. The intramuscular distribution and morphological properties of spindles in mutant masticatory muscles were indistinguishable from those of wild-type spindles. Intrafusal fibers of NT3- or trkC-deficient spindles-expressed the slow-tonic isoform of myosin heavy chains, characteristic of wild-type spindles. Sensory nerve endings were observed in spindles of mutants by electron microscopy. Thus, NT3 or trkC, which is expressed in wild-type spindles, may serve functions other than those related to spindle assembly. Presumably, proprioceptive neurons innervating jaw muscles are dependent on factors other than NT3 for survival and maintenance.

Animals↗

PubMed2001

Stimulation of chemosensitive afferents from multifidus muscle does not sensitize multifidus muscle spindles to vertebral loads in the lumbar spine of the cat.

STUDY DESIGN: Electrophysiologic recordings from muscle spindle afferents innervating the lumbar multifidus muscle of the cat while loading the L6 vertebra at its spinous process and while exposing the segmentally adjacent lumbar multifidus muscles to algesic and inflammatory mediators. OBJECTIVES: The purpose of this study was to investigate a possible mechanism underlying muscle spasm and pain in the lumbar spine. The hypothesis was tested that stimulation of chemosensitive afferents with receptive endings in the paraspinal muscle increases the discharge of paraspinal muscle spindle afferents during loading of a lumbar vertebra. The presence of such a phenomenon would provide a mechanism by which pain or inflammation could alter segmental lumbar biomechanics and contribute to lumbar spine dysfunction. SUMMARY OF BACKGROUND DATA: Muscle pain, tenderness, and altered muscle tone are often associated with musculoskeletal disorders. The literature suggests that stimulation of Group III and IV muscle afferents sensitive to algesic or inflammatory metabolites increases the stretch sensitivity of muscle spindles via a reflex pathway involving gamma-motoneurons. The reflex increase in muscle spindle activity, in turn, reflexly increases the excitability of alpha-motoneurons leading to enhanced muscle tone and the further accumulation of muscle metabolites and subsequent pain. Studies in the cervical spine support this hypothesis. It has not been investigated in the lumbar spine. METHODS: Single unit activity from muscle spindles in the L6 multifidus muscle were recorded from the cut peripheral end of the L6 dorsal root in alpha-chloralose-anesthetized cats and in decerebrate unanesthetized cats. The L6 vertebra was loaded at its spinous process using a force-feedback motor. Ramp and hold loads were delivered at 25%, 50%, 75%, and 100% body weight. Chemosensitive afferents in the L5 and L7 multifidus muscle were stimulated by bathing (subfascial injection) or infiltrating (intramuscular injection) the L5 and L7 multifidus muscles with bradykinin or capsaicin. RESULTS: Loading the L6 vertebra stimulated muscle spindles in the L6 multifidus muscle. Neither the saline volume control nor bradykinin nor capsaicin injected subfascially or intramuscularly affected the response of L6 multifidus muscle spindles to ramp and hold vertebral loads in the alpha-chloralose-anesthetized cat. In addition, neither saline nor bradykinin nor capsaicin injected intramuscularly affected the activity of L6 multifidus muscle spindles to ramp and hold vertebral loads in the unanesthetized decerebrate cat. CONCLUSIONS: These results indicate that stimulation of small diameter muscle afferents in a deep muscle of the lumbar spine does not sensitize muscle spindles to vertebral loads. These data do not support the hypothesis that fusimotor reflexes evoked by chemosensitive muscle afferents contribute to muscle spasm or to changes in muscle tone in the lumbar spine. In addition, the present results do not provide evidence for the pain-spasm-pain cycle in the lumbar spine.

Action Potentials↗

PubMed1997

Discharge of human muscle spindle afferents innervating ankle dorsiflexors during target isometric contractions.

1. There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the 'acceleration threshold' and the factors contributing to variability of 'acceleration threshold'. 2. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed 'ramp-and-hold' template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required 'ramp' rate and 'hold' force was adjusted between trials (range, 0.5-5% MVCs-1 and 0.5-20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any 'after-effects' on spindle afferent responses in subsequent contractions. 3. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the 'acceleration threshold'. Of twenty-six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01-11.9% MVC). 4. Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. 5. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after-effects are controlled, much of the residual variability in 'acceleration threshold' for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

Acceleration↗

PubMed1971

Muscle spindle response at the onset of isometric voluntary contractions in man. Time difference between fusimotor and skeletomotor effects.

1. Impulses in single muscle afferents were recorded from the median nerves of waking human subjects with percutaneously inserted tungsten needle electrodes. During isometric voluntary contractions, unitary discharges were analysed from muscle spindle endings in the wrist and finger flexor muscles and the electromyographic activity from these muscles was recorded simultaneously.2. When the subject activated the muscle portion in which a spindle was located, the afferent discharge increased in spite of the mechanical unloading effects of the skeletomotor contraction indicating a concomitant fusimotor activation. This was valid for slowly rising contractions as well as small fast rising twitches.3. The time of onset of spindle acceleration was determined in relation to the time of onset of the electromyographic activity for thirty-one units studied altogether in more than seven hundred contractions. It was found that spindle acceleration regularly occurred after the onset of the electromyographic activity.4. There was a considerable variation from one test to the other, for the individual units, with regard to the exact time of onset of spindle acceleration, although spindle acceleration occurred mostly within 0.5 sec after the onset of the electromyographic activity in sustained contractions and within 0.1 sec in small fast rising twitches. It was not possible to assess to what extent this variation was accounted for by variations in the mechanical unloading effects of the skeletomotor contraction or variations in the timing of the fusimotor outflow.5. For many units, spindle acceleration did not occur until 10-50 msec after the onset of the skeletomotor contraction. This time is of the same order of magnitude as the time difference in latency from the spinal cord to the recording points in the two systems, as estimated from reasonable assumptions.6. It was concluded that the fusimotor system does not participate in the initiation of voluntary contractions in man, but that the skeletomotor activity is initiated by descending impulses from supraspinal structures and their effects on the neuronal organization within the spinal cord.7. The fact that fusimotor activation occurs also in very small and short lasting twitches, when spindle acceleration must have a negligible influence on the skeletomotor outflow, suggests that the fusimotor and the skeletomotor systems are rigidly co-activated in voluntary contractions.8. The finding that spindle acceleration does not occur until 10-50 msec after the onset of the electromyographic activity suggests that there is an approximately simultaneous onset of the fusimotor and the skeletomotor outflows from the spinal cord.

Adult↗

PubMed1977

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.

Action Potentials↗

PubMed1978

Muscle spindle activity in man during shortening and lengthening contractions.

1. The responses of forty-one muscle spindle endings, mostly in tibialis anterior, were studied in human subjects during voluntary movements of the ankle joint performed at various speeds against different external loads. 2. During slow shortening contractions, the discharge rates of spindle endings in the contracting muscle accelerated after the appearance of the first e.m.g. potentials but before sufficient force had been generated to move the limb. With some endings, the discharge rate decreased during the shortening movement while the e.m.g. activity was increasing, but it always remained higher than before the onset of contraction. If the speed of the movement was increased fewer spindle discharges were seen during muscle shortening. If the shortening contraction was opposed by an external load, so that greater effort was required to perform the same movement, more discharges were seen and the discharge pattern became less modulated by the change in muscle length. 3. These findings indicate that during shortening contractions the fusimotor system is activated together with the skeletomotor system. However, the fusimotor drive is generally insufficient to maintain a significant spindle discharge unless movement is slow or the muscle is shortening against an external load. 4. During lengthening contractions the spindle responses were greater than to passive stretch of similar amplitude and velocity, suggesting heightened fusimotor outflow. 5. During shortening and lengthening contractions small iregularities in the speed of movement occurred commonly. Unintended acceleration of a shortening movement caused a pause in spindle firing, and unintended acceleration of a lengthening movement caused an increased discharge from spindle endings. These spindle responses were associated with corresponding alterations in the discharge pattern of the voluntarily activated motor units at latencies consistent with the operation of spinal reflex mechanisms. 6. It is suggested that a functional role for the fusimotor activation during slow shortening contractions is to provide spindle endings with a background discharge so that they can detect irregularities in the movement and initiate the appropriate reflex correction.

Action Potentials↗

PubMed1991

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions.

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force. 2. Once the afferent had been identified, subjects were instructed to dorsiflex the foot slowly to recruit the spindle ending, to continue the ramp contraction until a predetermined target force was reached, and then to hold that force until requested to relax. 3. Five muscle spindle afferents maintained a constant discharge frequency during the hold phase of the isometric contraction. Following relaxation of the contraction two spindle afferents from tibialis anterior, exhibited a post-contraction discharge despite the absence of detectable electromyographic activity (EMG). 4. The discharge frequency of most of the spindle afferents (72%) declined progressively during the isometric contraction. The mean firing rates had declined to two-thirds of those at the onset of the contraction by 30 s, and to half after 1 min. The decline in spindle firing rate commenced during the ramp phase of the contraction and was statistically significant by 10 s, when force was held constant. The extent of the decline was greater for those units with the higher initial firing rates and for those units studied after many preceding contractions. 5. In the same contractions a progressive increase in EMG was required to maintain force and consequently the change in EMG was inversely related to the change in spindle discharge. While many mechanisms may contribute to the decline in spindle discharge during a sustained isometric contraction, it is argued that the result will be a progressive disfacilitation of alpha-motoneurones, which may contribute to the decline in motor unit firing rates during a sustained contraction.

Adult↗

PubMed1995

Increased resting discharge of human spindle afferents following voluntary contractions.

1. The aim of this study was to assess the incidence of lasting alterations in discharge rate of muscle spindle afferents innervating human ankle and toe dorsiflexor muscles following isometric contractions. 2. The subjects performed controlled isometric ankle dorsiflexions maintained for approximately 5 s. During the contraction the discharge of all but one spindle afferent increased above the precontraction level. After complete relaxation, there was prolonged enhancement of the discharge rate of nineteen of fifty-five muscle spindle afferents and none of three Golgi tendon organ afferents. Ten of the nineteen spindle afferents had been silent prior to the contraction. For the population of fifty-five spindle afferents, the mean 'postcontraction' discharge rate was 65% higher than the mean precontraction discharge rate, with the mean rate increasing from 2.3 to 3.9 Hz (P < 0.001). The mean duration of the enhanced postcontraction discharge was 52 s (range, 8-240 s). 3. Stretch applied to the tendon of the receptor-bearing muscle in twelve of fourteen spindle afferents with an enhanced postcontraction discharge rate eliminated or reduced the enhanced discharge rate. 4. The high incidence of an enhanced spindle discharge after voluntary contraction (35% of spindle afferents) suggests that muscle 'history' should be taken into account when interpreting changes in spindle discharge rates. The enhanced discharge rates following contraction probably reflect a long-lasting effect of the contraction-associated increase in fusimotor drive on intrafusal stiffness, rather than the persistence of fusimotor drive following relaxation.

Adult↗

PubMed2003

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive.

There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system. We examined the validity of this idea in human subjects by recording from muscle spindles in the relaxed ankle and toe extensor muscles during a strong and sustained physiological activation of muscle sympathetic outflow. Unitary recordings were made from 20 primary and 17 secondary muscle spindle afferents via a tungsten microelectrode inserted percutaneously into the peroneal nerve in 10 awake, healthy subjects seated with the legs supported in the extended position. ECG, blood pressure, respiration and calf circumference were also recorded. The majority of the muscle spindles were spontaneously active at rest; a background discharge was induced in four silent spindles by vibrating the tendon. A sustained increase in muscle vasoconstrictor activity, an increase in calf volume and a fall in pulse pressure were produced by subjects performing a 30-40 s maximal inspiratory breath-hold. Despite this strong increase in muscle sympathetic outflow no significant changes occurred in the discharge of either primary or secondary muscle spindle afferents, measured as a change in mean frequency and variability over sequential 5 s epochs and compared with the preceding period of rest. Strong chemoreceptor-driven sympathetic bursts during sustained expiratory breath-holds also failed to modulate the firing of 14 spindle endings. We conclude that a sustained, physiological increase in muscle sympathetic activity causes no detectable change in muscle spindle firing, lending no support to the concept that the sympathetic nervous system can influence the sensitivity of human muscle spindles directly.

Adolescent↗

PubMed1998

Bub1p kinase activates the Saccharomyces cerevisiae spindle assembly checkpoint.

Saccharomyces cerevisiae BUB1 encodes a protein kinase required for spindle assembly checkpoint function. In the presence of spindle damage, BUB1 is required to prevent cell cycle progression into anaphase. We have identified a dominantly acting BUB1 allele that appears to activate the spindle assembly checkpoint pathway in cells with undamaged spindles. High-level expression of BUB1-5 did not cause detectable spindle damage, yet it delayed yeast cells in mitosis at a stage following bipolar spindle assembly but prior to anaphase spindle elongation. Delayed cells possessed a G2 DNA content and elevated Clb2p mitotic cyclin levels. Unlike cells delayed in mitosis by spindle damage or MPS1 kinase overexpression, hyperphosphorylated forms of the Mad1p checkpoint protein did not accumulate. Similar to cells overexpressing MPS1, the BUB1-5 delay was dependent upon the functions of the other checkpoint genes, including BUB2 and BUB3 and MAD1, MAD2, and MAD3. We found that the mitotic delay caused by BUB1-5 or MPS1 overexpression was interdependent upon the function of the other. This suggests that the Bub1p and Mps1p kinases act together at an early step in generating the spindle damage signal.

Alleles↗

PubMed2000

Clathrin in mitotic spindles.

Subconfluent cultures of Madin-Darby canine kidney (MDCK) and CV-1 cells were immunostained with two monoclonal antibodies (MAbs), MAb X-22 and MAb 23, against clathrin heavy chain and with polyclonal antiserum against a conserved region of all mammalian clathrin light chains. In interphase MDCK and CV-1 cells, staining by all three antibodies resulted in the characteristic intracellular punctate vesicular and perinuclear staining pattern. In mitotic cells, all three anti-clathrin antibodies strongly stained the mitotic spindle. Staining of clathrin in the mitotic spindle was colocalized with anti-tubulin staining of microtubular arrays in the spindle. Staining of the mitotic spindle was evident in mitotic cells from prometaphase to telophase and in spindles in mitotic cells released from a thymidine-nocodazole block. In CV-1 cells, staining of clathrin in the mitotic spindle was not affected by brefeldin A. On Western blots, clathrin was detected, but not enriched, in isolated spindles. The immunodetection of clathrin in the mitotic spindle may suggest a novel role for clathrin in mitosis. Alternatively, the recruitment of clathrin to the spindle may suggest a novel regulatory mechanism for localization of clathrin in mitotic cells.

Animals↗

PubMed1979

Physiological properties of muscle spindles in dorsal neck muscles of the cat.

1. Single-fiber recording was used to examine the properties of 107 spindle endings in cat biventer cervicis (BC) and complexus (CM) muscles. Responses of receptors were examined following muscle contraction and ramp and hold stretch. Twenty-two endings in splenius (SP) were also examined, but their responses could not be quantitated because the anatomy of SP prevented the application of appropriate stretches. 2. Conduction velocitites of spindle afferents ranged from 13 to 90 m/s. Endings with primary response patterns usually had faster conduction velocities than secondary endings, but there was overlap in the conduction velocity ranges of the two subgroups. 3. Most neck spindle afferents could be classified as either primary or secondary by a constellation of physiological criteria including dynamic response pattern, dynamic index, and variability of resting discharge frequency. However, 22 of 107 endings from BC and CM had responses with characteristics intermediate between primary and secondary responses. The possible sources of these characteristics are discussed. 4. Despite the similarity in properties between spindles of different neck muscles, the length sensitivities of CM spindles were high compared to those of BC spindles. CM spindles showed length-related modulation of firing frequency over a more restricted range of initial muscle lengths than did BC spindles. 5. Eight Golgi tendon organs (GTO) were identified by their characteristics responses. Conduction velocities obtained for five GTO afferent nerves ranged from 50 to 67 m/s. Recordings were also made from receptros in deep muscles surrounding the vertebrae. These receptors had properties characteristic of muscle spindles.

Adaptation, Physiological↗

PubMed1980

Glycogen-depletion method of intrafusal distribution of gamma-axons that increase sensitivity of spindle secondary endings.

1. The glycogen-depletion method was used in cat tenuissimus muscles to investigate whether the action of static gamma-axons that increase the sensitivity of spindle secondary endings to muscle length is associated with a specific pattern of distribution of these axons to intrafusal muscle fibers. 2. In each experiment, a single gamma-axon acting on a secondary ending of a precisely located spindle was repetitively stimulated, and subsequently the intrafusal muscle fibers of that spindle were examined for glycogen depletion. 3. The gamma-axons whose repetitive stimulation increased the length sensitivity of secondary endings depleted all the chain fibers in one or both poles of the spindle, with the bag fibers being inconstantly involved. The gamma-axons whose stimulation did not exert this effect produced much more restricted patterns of glycogen depletion. 4. The length sensitivities of two secondary endings belonging to the same spindle were similarly affected by a single gamma-axon, whereas a gamma-axon could increase the sensitivity of a secondary ending without altering that of the primary ending of the same spindle. 5. The action exerted by single gamma-axons on secondary endings appears related to their intrafusal distribution since enhancement of the secondary endings sensitivity was observed only when all the chain fibers of at least one spindle pole were activated. 6. If several static gamma-axons supplying a given spindle are firing together, an increase in the length sensitivity of the secondary endings of that spindle can be expected since probably all chain fibers are activated.

Animals↗