Ultrastructure of the female pedal gonad in Phoxichilidium femoratum (Chelicerata, Pycnogonida)

Phoxichilidium femoratum is a common species of sea spiders - a small and unique group of chelicerates with unusual adult anatomy. In particular, substantial parts of the reproductive system in pycnogonids (unlike euchelicerates) are located in the appendages. Existing studies of pycnogonid gonads are often limited to light-microscopic level, cover a small range of species, and focus on the contents of the gonad diverticula. Ultrastructural data are rare and contradictory, and the organisation of the gonad wall and the gonoducts is unknown. Here we present a detailed light and transmission electron microscopy-based examination of the pedal portion of the adult female reproductive system in Phoxichilidium femoratum Rathke, 1799. We describe its gross anatomy and the ultrastructure of the gonad diverticulum, oviduct and gonopore, as well as development of the oocytes. Each gonad diverticulum is enclosed in the extracellular matrix of the horizontal septum and bears some internal cellular lining. However, neither the gonad lining, nor the septum sheath cells, ever form a continuous epithelial layer. Oocytes, which undergo maturation in the diverticulum, remain, until very late in the process, attached to the gonad wall though specialised stalk cells. Interestingly, stalk cells do not participate in egg envelope or yolk formation: both are synthesized endogenously in the oocytes. The oviduct is supplied with musculature, which assists in egg transport to the gonopore, whereas the gonopore itself is surrounded by specialised glands.

Early lecithotrophic stages of Nymphon grossipes, and the role of larval appendages and glands in different larval types of pycnogonids

Nymphon grossipes is a common subtidal species belonging to a small and unique group of chelicerates, that is, the sea spiders. These animals have an anamorphic phase during post-embryonic development and often hatch as small, oligomeric and exotrophic larvae (protonymphons) with four postocular segments, cheliphores, and two pairs of larval legs. A common alternative to protonymphons is a large lecithotrophic larval type, where animals hatch at more advanced stages and have a foreshortened anamorphic development. Based on external morphology, N. grossipes was believed to be an intriguing intermediate between these two conditions and its hatchlings were called "lecithotrophic protonymphons." Here, we examine the anatomy and ultrastructure of instars I and II and review the variety of roles of larval appendages and associated glands in other sea spiders in order to correctly place the larva of this species among pycnogonid larval types. Compared to "typical protonymphons," N. grossipes young hatch with an advanced segmental and appendage composition: six postocular segments instead of four, buds of walking legs 1 and hidden buds of walking legs 2. This state corresponds to the instars II/III (rather than larvae) of Nymphon brevirostre and Pycnogonum litorale. Modifications of the larval appendages, chelar, and spinning glands are aligned with ecological needs of different larval types along a few typical dimensions: locomotion and feeding, dispersal, and attachment to the parent. Although the main challenge for N. grossipes young is secure attachment to the egg package while they growth, there are some discrepancies in their anatomy: N. grossipes retains an oyster basket, but an otherwise nonfunctional digestive system, and a strong silken thread for attachment, but no corresponding reduction of the larval legs. Thus, it is likely that the switch to lecithotrophy happened in the recent evolutionary history of this species.

Anatomical changes in postembryonic development of Pycnogonum litorale

Sea spiders (Pycnogonida) are a small group of arthropods, sister to other chelicerates. They have an unusual adult bauplan, oligosegmented larvae, and a protracted postembryonic development. Pycnogonum litorale (Strøm, 1762) is an uncommonly long-lived sea spider with a distinctive protonymphon and adult anatomy. Although it was described ~250 years ago, little is known about its internal organization and development. We examined the anamorphic and early epimorphic development of this species using histology, light microscopy, and SEM, and provide the first comprehensive anatomical study of its many instars. Postembryonic development of P. litorale includes transformations typical of pycnogonids: reorganization of the larval organs (digestive, nervous, secretory), formation of the abdomen, trunk segments (+ appendages), primary body cavity and reproductive system. Specific traits include the accelerated articulation of the walking legs, formation of the subesophageal and posterior synganglia, and the system of twin midgut diverticula. In addition, P. litorale simultaneously lose the spinning apparatus and all larval appendages. We found that developmental changes occur in synchrony with changes in ecology and food sources. The transition from the anamorphic to the epimorphic period in particular is marked by considerable anatomical and lifestyle shifts. HIGHLIGHTS: Postembryonic development of P. litorale includes numerous anamorphic and epimorphic stages. The instars acquire abdomen, trunk segments, body cavity, and gonads, while losing all larval appendages. Developmental changes are synchronized with changes in lifestyle and food sources.

Postembryonic development of pycnogonids: A deeper look inside

Sea spiders form a small, enigmatic group of recent chelicerates, with an unusual bodyplan, oligosegmented larvae and a postembryonic development that is punctuated by many moults. To date, only a few papers examined the anatomical and ultrastructural modifications of the larvae and various instars. Here we traced both internal and external events of the whole postembryonic development in Nymphon brevirostre HODGE 1863 using histology, SEM, TEM and confocal microscopy. During postembryonic development, larvae of this species undergo massive reorganization: spinning apparatus and chelar glands disappear; larval legs redifferentiate; three new segments and the abdomen are formed with their corresponding internal organs and appendages; circulatory and reproductive systems develop anew and the digestive and the nervous systems change dramatically. The body cavity remains schizocoelic throughout development, and no traces of even transitory coeloms were found in any instar. In Nymphon brevirostre, just like in Artemia salina LINNAEUS 1758 the heart arises through differentiation of the already existing schizocoel, and thus the circulatory systems of arthropods and annelids are not homologous. We found that classical chelicerate tagmata, prosoma and opisthosoma, are inapplicable to adult pycnogonids, with the most striking difference being the fate and structure of the seventh appendage-bearing segment.

Controlling high-frequency collective electron dynamics via single-particle complexity

We demonstrate, through experiment and theory, enhanced high-frequency current oscillations due to magnetically-induced conduction resonances in superlattices. Strong increase in the ac power originates from complex single-electron dynamics, characterized by abrupt resonant transitions between unbound and localized trajectories, which trigger and shape propagating charge domains. Our data demonstrate that external fields can tune the collective behavior of quantum particles by imprinting configurable patterns in the single-particle classical phase space.

Relationship between EMG and muscle force after spinal cord injury

OBJECTIVE

The purpose of this study was to compare the electromyography (EMG) score during contraction of a given muscle to the independently measured manual muscle test (MMT) score for that same muscle (or muscle group), to determine whether EMG measures could serve as a reasonable approximation of muscle contraction force in persons with acute spinal cord injury (SCI).

METHODS

We examined the strength of relationship between surface-recorded EMG and estimated muscle strength using the MMT in a population of 45 subjects with acute (<1 week) traumatic SCI. Eight different muscle groups were compared in each individual; measures were repeated on these subjects approximately 2 months later. A 6-point numeric index was used for assignment of EMG scores, all of which were done in a blinded fashion by 1 investigator from tape-recorded evaluations.

RESULTS

Nearly all of the individual muscle comparisons led to positive and significant (P < .01) correlations between EMG and MMT scores, at both the acute and subacute time points following injury.

CONCLUSIONS

These findings support the use of EMG scoring as an indicator of recovery of volitional strength following SCI in a given subject. However, caution must be used when attempting to extrapolate EMG scores to absolute forces or when comparing EMG scores among different subjects.

Distribution and latency of muscle responses to transcranial magnetic stimulation of motor cortex after spinal cord injury in humans

Noninvasive transcranial magnetic stimulation (TMS) of the motor cortex was used to evoke electromyographic (EMG) responses in persons with spinal cord injury (n = 97) and able-bodied subjects (n = 20, for comparative data). Our goal was to evaluate, for different levels and severity of spinal cord injury, potential differences in the distribution and latency of motor responses in a large sample of muscles affected by the injury. The spinal cord injury (SCI) population was divided into subgroups based upon injury location (cervical, thoracic, and thoracolumbar) and clinical status (motor-complete versus motor-incomplete). Cortical stimuli were delivered while subjects attempted to contract individual muscles, in order to both maximize the probability of a response to TMS and minimize the response latency. Subjects with motor-incomplete injuries to the cervical or thoracic spinal cord were more likely to demonstrate volitional and TMS-evoked contractions in muscles controlling their foot and ankle (i.e., distal lower limb muscles) compared to muscles of the thigh (i.e., proximal lower limb muscles). When TMS did evoke responses in muscles innervated at levels caudal to the spinal cord lesion, response latencies of muscles in the lower limbs were delayed equally for persons with injury to the cervical or thoracic spinal cord, suggesting normal central motor conduction velocity in motor axons caudal to the lesion. In fact, motor response distribution and latencies were essentially indistinguishable for injuries to the cervical or thoracic (at or rostral to T10) levels of the spine. In contrast, motor-incomplete SCI subjects with injuries at the thoracolumbar level showed a higher probability of preserved volitional movements and TMS-evoked contractions in proximal muscles of the lower limb, and absent responses in distal muscles. When responses to TMS were seen in this group, the latencies were not significantly longer than those of able-bodied (AB) subjects, strongly suggestive of "root sparing" as a basis for motor function in subjects with injury at or caudal to the T11 vertebral body. Both the distribution and latency of TMS-evoked responses are consistent with highly focal lesions to the spinal cord in the subjects examined. The pattern of preserved responsiveness predominating in the distal leg muscles is consistent with a greater role of corticospinal tract innervation of these muscles compared to more proximal muscles of the thigh and hip.

Latency of changes in spinal motoneuron excitability evoked by transcranial magnetic brain stimulation in spinal cord injured individuals

OBJECTIVES

To examine the basis for delay in the excitatory effects of transcranial magnetic stimulation (TMS) of motor cortex on motoneuron pools of muscles left partially-paralyzed by traumatic spinal cord injury (SCI).

METHODS

The effect of subthreshold transcranial magnetic stimulation (TMS) on just-suprathreshold H-reflex amplitude was examined in subjects (n = 10) with incomplete cervical SCI, and in able-bodied (AB) subjects (n = 20) for comparison. EMG activity was recorded from the soleus and the abductor hallucis muscles, and H-reflex was elicited by stimulation of the tibial nerve behind the knee. Comparison of the peak-to-peak amplitude of the TMS-conditioned H-reflex to that of the H-reflex alone (i.e. unconditioned H-reflex) was made for different conditioning-test intervals with multivariate analysis of variance and (when called for) t testing.

RESULTS

The absolute latencies of motor responses to suprathreshold TMS delivered during a weak voluntary contraction of the soleus and abductor hallucis were significantly prolonged in the SCI group relative to AB subjects. For the TMS-conditioned H-reflex, the time-course effect of TMS on the H-reflex amplitude in different AB subjects included an early effect (typically facilitation, but occasionally inhibition) seen between -5 and 0 ms, followed by a later period (i.e. >5 ms) of H-reflex facilitation. In contrast, the earliest indication of a TMS effect on H-reflex excitability in SCI subjects was between 5 and 10 ms after TMS. This difference between SCI and AB subjects of approximately 10 ms was similar to the prolongation of TMS-evoked response latencies in the soleus and the abductor hallucis muscles of the SCI subjects.

CONCLUSIONS

The results suggest that motor conduction slowing after traumatic SCI most likely occurs across the population of the descending tract axons mediating the TMS-evoked motor responses.

"Threshold-level" multipulse transcranial electrical stimulation of motor cortex for intraoperative monitoring of spinal motor tracts: description of method and comparison to somatosensory evoked potential monitoring

Numerous methods have been pursued to evaluate function in central motor pathways during surgery in the anesthetized patient. At this time, no standard has emerged, possibly because each of the methods described to date requires some degree of compromise and/or lacks sensitivity.

OBJECT

The goal of this study was to develop and evaluate a protocol for intraoperative monitoring of spinal motor conduction that: 1) is safe; 2) is sensitive and specific to motor pathways; 3) provides immediate feedback; 4) is compatible with anesthesia requirements; 5) allows monitoring of spontaneous and/or nerve root stimulus-evoked electromyography; 6) requires little or no involvement of the surgical team; and 7) requires limited equipment beyond that routinely used for somatosensory evoked potential (SSEP) monitoring. Using a multipulse electrical stimulator designed for transcranial applications, the authors have developed a protocol that they term "threshold-level" multipulse transcranial electrical stimulation (TES).

METHODS

Patients considered at high risk for postoperative deficit were studied. After anesthesia had been induced and the patient positioned, but prior to incision, "baseline" measures of SSEPs were obtained as well as the minimum (that is, threshold-level) TES voltage needed to evoke a motor response from each of the muscles being monitored. A brief, high-frequency pulse train (three pulses; 2-msec interpulse interval) was used for TES in all cases. Data (latency and amplitude for SSEP; threshold voltage for TES) were collected at different times throughout the surgical procedure. Postoperative neurological status, as judged by evaluation of sensory and motor status, was compared with intraoperative SSEP and TES findings for determination of the sensitivity and specificity of each electrophysiological monitoring technique. Of the 34 patients enrolled, 32 demonstrated TES-evoked responses in muscles innervated at levels caudal to the lesion when examined after anesthesia induction and positioning but prior to incision (that is, baseline). In contrast, baseline SSEPs could be resolved in only 25 of the 34 patients. During surgery, significant changes in SSEP waveforms were noted in 12 of these 25 patients, and 10 patients demonstrated changes in TES thresholds. Fifteen patients experienced varying degrees and durations of postoperative neurological deficit. Intraoperative changes in TES thresholds accurately predicted each instance of postoperative motor weakness without error, but failed to predict four instances of postoperative sensory deficit. Intraoperative SSEP monitoring was not 100% accurate in predicting postoperative sensory status and failed to predict five instances of postoperative motor deficit. As a result of intraoperative TES findings, the surgical plan was altered or otherwise influenced in six patients (roughly 15% of the sample population), possibly limiting the extent of postoperative motor deficit experienced by these patients.

CONCLUSIONS

This novel method for intraoperative monitoring of spinal motor conduction appears to meet all of the goals outlined above. Although the risk of postoperative motor deficit is relatively low for the majority of spine surgeries (for example, a simple disc), high-risk procedures, such as tumor resection, correction of vascular abnormalities, and correction of major deformities, should benefit from the virtually immediate and accurate knowledge of spinal motor conduction provided by this new monitoring approach.

Central cord syndrome of cervical spinal cord injury: widespread changes in muscle recruitment studied by voluntary contractions and transcranial magnetic stimulation

Muscle recruitment after central cord syndrome (CCS), a cervical spinal cord injury leading to a weaker motor function in the upper limbs versus the lower limbs, was examined in 14 individuals by means of voluntary muscle contractions and transcranial magnetic stimulation (TMS). Previously obtained data from able-bodied (AB) and non-CCS spinal cord injured subjects were used for comparison. Surface EMG was recorded from as many as six pairs of affected muscles. Individual muscle EMG activity was scored from 0 to 5. Cortical stimulation was applied while subjects maintained a weak contraction in each muscle. When CCS subjects attempted to produce a maximal voluntary contraction of an isolated muscle, this frequently resulted in cocontraction of nonsynergists in the same limb or/and in other limbs. Although the EMG scores in both upper and lower extremity muscles improved within postinjury time, in general, the lower extremity muscles, particularly the distal ones, demonstrated better recovery than the upper extremity muscles. CCS and AB subjects showed a similar high probability of "well-defined" responses to TMS (amplitude >150 microV) in all studied muscles. In contrast, latencies to TMS-evoked motor responses were prolonged by significant amounts after CCS. The delays in muscle responses were not significantly different from those observed in subjects with more severe cervical injury. Despite improvement in EMG scores, repeated measurements of TMS-evoked muscle response latencies in the same CCS subjects did not reveal significant shortening in central conduction latency. This argues against remyelination as an important contributor to the recovery process.