Cerebrospinal fluid flow in normal beagle dogs analyzed using magnetic resonance imaging.

BACKGROUND: Diseases related to cerebrospinal fluid flow, such as hydrocephalus, syringomyelia, and Chiari malformation, are often found in small dogs. Although studies in human medicine have revealed a correlation with cerebrospinal fluid flow in these diseases by magnetic resonance imaging, there is little information and no standard data for normal dogs. OBJECTIVES: The purpose of this study was to obtain cerebrospinal fluid flow velocity data from the cerebral aqueduct and subarachnoid space at the foramen magnum in healthy beagle dogs. METHODS: Six healthy beagle dogs were used in this experimental study. The dogs underwent phase-contrast and time-spatial labeling inversion pulse magnetic resonance imaging. Flow rate variations in the cerebrospinal fluid were observed using sagittal time-spatial labeling inversion pulse images. The pattern and velocity of cerebrospinal fluid flow were assessed using phase-contrast magnetic resonance imaging within the subarachnoid space at the foramen magnum level and the cerebral aqueduct. RESULTS: In the ventral aspect of the subarachnoid space and cerebral aqueduct, the cerebrospinal fluid was characterized by a bidirectional flow throughout the cardiac cycle. The mean ± SD peak velocities through the ventral and dorsal aspects of the subarachnoid space and the cerebral aqueduct were 1.39 ± 0.13, 0.32 ± 0.12, and 0.76 ± 0.43 cm/s, respectively. CONCLUSIONS: Noninvasive visualization of cerebrospinal fluid flow movement with magnetic resonance imaging was feasible, and a reference dataset of cerebrospinal fluid flow peak velocities was obtained through the cervical subarachnoid space and cerebral aqueduct in healthy dogs.

Locomotion, posture, and the foramen magnum in primates: Reliability of indices and insights into hominin bipedalism.

The position (FMP) and orientation (FMO) of the foramen magnum have been used as proxies for locomotion and posture in extant and extinct primates. Several indices have been designed to quantify FMP and FMO but their application has led to conflicting results. Here, we test six widely used indices and two approaches (univariate and multivariate) for their capability to discriminate between postural and locomotor types in extant primates and fossil hominins. We then look at the locomotion of australopithecines and Homo on the base of these new findings. The following measurements are used: the opisthocranion-prosthion (OP-PR) and the opisthocranion-glabella (OP-GL) indices, the basion-biporion (BA-BP) and basion-bicarotid chords, the foramen magnum angle (FMA), and the basion-sphenoccipital ratio. After exploring the indices variability using principal component analysis, pairwise comparisons are performed to test for the association between each index and the locomotor and postural habits. Cranial size and phylogeny are taken into account. Our analysis indicates that none of the indices or approaches provides complete discrimination across locomotor and postural categories, although some differences are highlighted. FMA and BA-BP distinguish respectively obligate and facultative bipeds from all other groups. For what concerns posture, orthogrades and pronogrades differ with respects to OP-PR, OP-GL, and FMA. Although the multivariate approach seems to have some discrimination power, the results are most likely driven by facial and neurocranial variability embedded in some of the indices. These results demonstrate that indices relying on the anteroposterior positioning of the foramen may not be appropriate proxies for locomotion among primates. The assumptions about locomotor and postural habits in fossil hominins based on foramen magnum indices should be revised in light of these new findings.

Relationship between foramen magnum position and locomotion in extant and extinct hominoids.

From the Miocene Sahelanthropus tchadensis to Pleistocene Homo sapiens, hominins are characterized by a derived anterior position of the foramen magnum relative to basicranial structures. It has been previously suggested that the anterior position of the foramen magnum in hominins is related to bipedal locomotor behavior. Yet, the functional relationship between foramen magnum position and bipedal locomotion remains unclear. Recent studies, using ratios based on cranial linear measurements, have found a link between the anterior position of the foramen magnum and bipedalism in several mammalian clades: marsupials, rodents, and primates. In the present study, we compute these ratios in a sample including a more comprehensive dataset of extant hominoids and fossil hominins. First, we verify if the values of ratios can distinguish extant humans from apes. Then, we test whether extinct hominins can be distinguished from non-bipedal extant hominoids. Finally, we assess if the studied ratios are effective predictors of bipedal behavior by testing if they mainly relate to variation in foramen magnum position rather than changes in other cranial structures. Our results confirm that the ratios discriminate between extant bipeds and non-bipeds. However, the only ratio clearly discriminating between fossil hominins and other extant apes is that which only includes basicranial structures. We show that a large proportion of the interspecific variation in the other ratios relates to changes in facial, rather than basicranial, structures. In this context, we advocate the use of measurements based only on basicranial structures when assessing the relationship between foramen magnum position and bipedalism in future studies.

Diameters and bone thickness at the margin of the foramen magnum in dry skulls from pediatric population: a cross-sectional anatomical study.

PURPOSE: The purpose of the study was to estimate the size and bone thickness at the margin of the foramen magnum in a pediatric population. METHODS: Sixty occipital bone specimens from the collection of macerated skulls at the Department of Anatomy, University of Zagreb, were examined and measured using a vernier scale/caliper. For the purpose of analysis, specimens were divided into two age groups: 1-6 years and 7-18 years of age (before and after the fusion of ossification centers in the occipital bone). We measured the following: antero-posterior and transverse diameters of the foramen magnum, bone thicknesses at the basion, opisthion, two paramedial points on the anterior and posterior margins, and at the occipito-squamous junction. RESULTS: Data presented in this study show that diameters of the foramen magnum increase with age, whereas bone thickness shows variable behavior depending on the measured area. CONCLUSIONS: Increases in diameters in specimens from the younger age group and their absence in specimens from older subjects reflect the growth pattern of the basilar part of occipital bone. Variability of bone thickness at the margin of the foramen magnum and lack of its association with age of the subjects may be attributed to various factors and may potentially affect the clinical presentation of compression syndromes at the level of foramen magnum.

Morphologic Evaluation of Lumbosacral Nerve Roots in the Vertebral Foramen: Measurement of Local Pressure of the Intervertebral Foramen.

STUDY DESIGN: The prospective cohort study. OBJECTIVE OF THE STUDY: The objective was to evaluate the relationships between local pressure changes of the intervertebral foramen during lumbar spine extension and lumbar foraminal morphology. SUMMARY OF BACKGROUND DATA: The physiological states of lumbosacral nerve roots in the vertebral foramen remain controversial. METHODS: We evaluated 56 lumbosacral vertebral foramens in 21 patients with L4-degenerative spondylolisthesis. All patients underwent L4-5 posterolateral fusion (PLF). The local pressure of the intervertebral foramen was measured intraoperatively, and measurement was performed before and after L4-5 PLF. We defined the changes in the ratio of local pressure between lumbar flexion to extension as percent pressure. The sagittal angular motion, distance between the inferior cortex of the cranial pedicle and superior cortex of the caudal pedicle, posterosuperior margin of the superior vertebral body and superior articular facet, posteroinferior edge of the superior vertebral body and inferior articular facet, and the intervertebral disc height were measured using preoperative functional plain radiographs and CT images. RESULTS: The average local pressure of the intervertebral foramen significantly increased during lumbar extension. However, the L4-5 vertebral foraminal pressure after PLF were nearly identical. There was no significant correlation between percent pressure and lumbar range of motion. Furthermore, there were no significant correlations between percent pressure and each morphologic parameter of the lumbar foramen. CONCLUSIONS: There were no significant relationships between the lumbar foraminal morphology and intervertebral foraminal pressure changes during lumbar extension, and L4-5 vertebral foraminal pressure was not affected by the lumbar posture after L4-5 posterior fusion. On the basis of the results, the external dynamic stresses on the nerve roots in the vertebral foramen might be improved by lumbar posterior fusion using instrumentation without direct decompression of the vertebral foramen.

Angiomatosis leptomeníngea temporo-occipital de diagnóstico en edad adulta / Adult diagnosis of temporo-occipital leptomeningeal angiomatosis

No disponible

Physiology-based MR imaging assessment of CSF flow at the foramen magnum with a valsalva maneuver.

BACKGROUND AND PURPOSE: MR imaging is currently not used to evaluate CSF flow changes due to short-lasting physiological maneuvers. The purpose of this study was to evaluate the ability of MR imaging to assess the CSF flow response to a Valsalva maneuver in healthy participants. MATERIALS AND METHODS: A cardiac-gated fast cine-PC sequence with ≤15-second acquisition time was used to assess CSF flow in 8 healthy participants at the foramen magnum at rest, during, and immediately after a controlled Valsalva maneuver. CSF mean displacement volume VCSF during the cardiac cycle and CSF flow waveform App were determined. A work-in-progress real-time pencil-beam imaging method with temporal resolution ≤56 ms was used to scan 2 participants for 90 seconds during which resting, Valsalva, and post-Valsalva CSF flow, respiration, and HR were continuously recorded. Results were qualitatively compared with invasive craniospinal differential pressure measurements from the literature. RESULTS: Both methods showed 1) a decrease from baseline in VCSF and App during Valsalva and 2) an increase in VCSF and App immediately after Valsalva compared with values measured both at rest and during Valsalva. Whereas fast cine-PC produced a single CSF flow waveform that is an average over many cardiac cycles, pencil-beam imaging depicted waveforms for each heartbeat and was able to capture many dynamic features of CSF flow, including transients synchronized with the Valsalva maneuver. CONCLUSIONS: Both fast cine-PC and pencil-beam imaging demonstrated expected changes in CSF flow with Valsalva maneuver in healthy participants. The real-time capability of pencil-beam imaging may be necessary to detect Valsalva-related transient CSF flow obstruction in patients with pathologic conditions such as Chiari I malformation.

Magnetic-motor-root stimulation: review.

Magnetic stimulation can activate the human central and peripheral nervous systems non-invasively and virtually painlessly. Magnetic stimulation over the spinal enlargements can activate spinal nerves at the neuroforamina (magnetic-neuroforamina stimulation). This stimulation method provides us with information related to the latency of compound-muscle action potential (CMAP), which is usually interpreted as peripheral motor-conduction time (PMCT). However, this stimulation method has faced several problems in clinical applications. One is that supramaximal CMAPs were unobtainable. Another is that magnetic stimulation did not usually activate the spinal nerves in the spinal canal, i.e., the cauda equina, which prevented an evaluation of its conduction. For these reasons, magnetic-neuroforamina stimulation was rarely used to evaluate the conduction of peripheral nerves. It was mainly used to evaluate the conduction of the corticospinal tract using the parameter of central motor-conduction time (CMCT), which was calculated by subtracting PMCT from the latency of motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the primary motor cortex. Recently, supramaximal stimulation has been achieved in magnetic-neuroforamina stimulation, and this has contributed to the measurement of both CMAP size and latency. The achievement of supramaximal stimulation is ascribed to the increase in magnetic-stimulator output and a novel coil, the magnetic augmented translumbosacral stimulation (MATS) coil. The most proximal part of the cauda equina can be reliably activated using the MATS coil (magnetic-conus stimulation), thus contributing to the measurement of cauda equina conduction time (CECT) and cortico-conus motor-conduction time (CCCT). These recent developments in magnetic-motor-root stimulation enable us to more precisely evaluate the conduction of the proximal part of peripheral nerves and that of the corticospinal tract for lower-limb muscles. In this review article, we summarise the basic mechanisms, recent topics, clinical applications, comparison to electrical stimulation, pitfalls, safety and additional issues in magnetic-motor-root stimulation.

Characterization of cyclic CSF flow in the foramen magnum and upper cervical spinal canal with MR flow imaging and computational fluid dynamics.

CSF flow has been shown to exhibit complex patterns in MR images in both healthy subjects and in patients with Chiari I. Abnormal CSF flow oscillations, according to prevailing opinion, cause syringomyelia and other clinical manifestations that affect some patients with the Chiari I malformation. For this article, we reviewed the literature on PC MR of CSF flow, collected the published CFD studies relevant to CSF flow, and performed flow simulations. PC MR creates cine and still images of CSF flow and measurements of flow velocities. CFD, a technique used to compute flow and pressure in liquid systems, simulates the CSF flow patterns that occur in a specific geometry or anatomy of the SAS and a specific volume of flow. Published PC MR studies show greater peak CSF velocities and more complex flow patterns in patients with Chiari I than in healthy subjects, with synchronous bidirectional flow one of the characteristic markers of pathologic flow. In mathematic models of the SAS created from high-resolution MR images, CFD displays complex CSF flow patterns similar to those shown in PC MR in patients. CFD shows that the pressure and flow patterns vary from level to level in the upper spinal canal and differ between patients with Chiari and healthy volunteers. In models in which elasticity and motion are incorporated, CFD displays CSF pressure waves in the SAS. PC MR and CFD studies to date demonstrate significant alterations of CSF flow and pressure patterns in patients with Chiari I. CSF flow has nonlaminar complex spatial and temporal variations and associated pressure waves and pressure gradients. Additional simulations of CSF flow supplemented by PC MR will lead to better measures for distinguishing pathologic flow abnormalities that cause syringomyelia, headaches, and other clinical manifestations in Chiari I malformations.

Accuracy and reproducibility of phase-contrast MR imaging measurements for CSF flow.

BACKGROUND AND PURPOSE: PCMR, widely used for the evaluation of blood flow, has been adopted for the assessment of cerebrospinal fluid flow in a variety of disorders. The purpose of this study was to evaluate the accuracy and reproducibility of 2 fast PCMR techniques for measuring CSF flow. MATERIALS AND METHODS: Velocities were calculated from RPC and CPC images of fluid flowing in a tube at a constant velocity. Error and the COV were computed for average and peak velocities. Additionally, measurements of sinusoidally fluctuating flow and of CSF flow in 5 healthy volunteers were acquired with the RPC and CPC acquisitions. RESULTS: For constant velocity experiments, error for the RPC and CPC acquisitions averaged +1.15% and +8.91% and COVs averaged 1.29% and 3.01%, respectively. For peak velocities of >or=12.6 cm/s, error with RPC or CPC ranged from -33.3% to -36.9% and COVs were 0%-4% for RPC and 1%-7% for CPC. For peak velocities of 250%. For fluctuating flow, both acquisitions showed similar flow patterns. In volunteer studies, peak systolic and diastolic velocities were not significantly different. CONCLUSIONS: The RPC and CPC sequences measure velocities on the order of CSF flow with an average error of >or=9%. The 2 techniques significantly overestimate peak velocities <6.4 cm/s, with maximum errors of 209% and 276% and maximum COVs of 100% and 73% for the RPC and CPC sequences, respectively. Measurements of CSF velocities in human volunteers and of sinusoidally fluctuating phantom velocities did not differ significantly between the 2 techniques.

Feasible and accurate occipitoatlantal transarticular fixation: an anatomic study.

BACKGROUND: Defining the anatomic zones for the placement of occiput-C1 transarticular screws is essential for patient safety. OBJECTIVE: The feasibility and accuracy of occiput-C1 transarticular screw placement were evaluated in this anatomical study of normal cadaveric specimens. MATERIAL AND METHODS: Sixteen measurements were determined for screw entry points, trajectories, and lengths for placement of transarticular screws, as applied in the technique described by Grob, on the craniovertebral junction segments (occiput-C2) of 16 fresh human cadaveric cervical spines and 41 computed tomographic reconstructions of the craniovertebral junction. Acceptable angles for screw positioning were measured on digital x-rays. RESULTS: All 32 screws were placed accurately. As determined by dissection of the specimens, none of the screws penetrated the spinal canal. Screw insertion caused no fractures, and the integrity of the hypoglossal canal was maintained in all the disarticulated specimens. CONCLUSION: Viable transarticular occiput-C1 screw placement is possible, despite variability of the anatomy of the occipital condyle.

The transverse occipital ligament: anatomy and potential functional significance.

BACKGROUND: Knowledge of the anatomy of ligaments that bind the craniocervical junction is important for treating patients with lesions of this region. Although the anatomy and function of these ligaments have been well described, those of the transverse occipital ligament (TOL) have remained enigmatic. OBJECTIVE: To describe the anatomy and functions of the transverse occipital ligament. METHODS: Via a posterior approach, 9 cadaveric specimens underwent dissection of the craniocervical junction with special attention to the presence and anatomy of the TOL. RESULTS: The TOL was identified in 77.8% of the specimens. The ligament was found to be rectangular with fibers running horizontally between the lateral aspects of the foramen magnum. The attachment of each ligament near the occipital condyle was consistent, and each ligament was found superior to the transverse portion of the cruciform ligament and inserted just posterior to the lateral attachment sites of the alar ligaments. The average width, length, and thickness of the TOL was 0.34, 1.94, and 0.13 cm, respectively. The TOL in some specimens also had connections to the alar and transverse ligaments. CONCLUSION: The TOL was found in the majority of our specimens. The possible functions of this ligament when attached to the alar ligaments include providing additional support to these structures in stabilizing lateral bending, flexion, and axial rotation of the head. Knowledge of this ligament may aid in further understanding craniocervical stability and help in differentiating normal from pathology via imaging modalities.

Variability of human foramen magnum size.

The foramen magnum is an important landmark of the skull base and is of particular interest for anthropology, anatomy, forensic medicine, and other medical fields. Despite its importance, few osteometric studies of the foramen magnum have been published so far. A total of 110 transverse and 111 sagittal diameters from Central European male and female dry specimens dating from the Pleistocene to modern times were measured, and related to sex, age, stature, ethnicity, and a possible secular trend. Only a moderate positive correlation between the transverse and the sagittal diameter of the foramen magnum was found. Surprisingly, neither sexual dimorphism, individual age-dependency, nor a secular trend was found for either diameter. Furthermore, the relationship between the individual stature and foramen magnum diameters was weak: thus foramen magnum size cannot be used as reliable indicator for stature estimation. Further consideration of possible factors influencing the variability of human foramen magnum size shall be explored in larger and geographically more diverse samples, thus serving forensic, clinical, anatomical, and anthropological interests in this body part.

Magnetic resonance measurement of blood and CSF flow rates with phase contrast--normal values, repeatability and CO2 reactivity.

BACKGROUND: Similarity in flow pulsatility has been proposed as a basis for semi-automated segmentation of vessel lumens for MR-based flow measurement, but re-examinations of salient aspects of the methodology have not been widely reported. METHODS: 12 normal control subjects underwent repeated (3*Baseline+1*5%CO2) phase contrast measurements of CSF flow through the cerebral aqueduct and foramen magnum, and CBF through the 6 large cranial vessels at the level of the 1st vertebra. Average flows were calculated for regions temporally correlated (0.3 < or = Rthreshold < or = 0.95) to user defined seed points and their 3 x 3 neighbours. RESULTS: Arterial CBF averaged 710ml/min, with low variability (+/- 4%/17%, intra-individual/group CV respectively) and was the only flow to respond significantly to 5%/mmHg CO2. Venous outflow was much smaller (298ml/min +/- 10%/ 72%), possibly due to the weak venous pulse and variable venous anatomy. Average CSF flows exceeded the classical 0.4ml/min CSF production rate and were highly variable--aqueduct: 0.6ml/min (+/- 50%/93%), foramen magnum: -2.7ml/min (+/- 158%/226%). CONCLUSIONS: This preliminary analysis identified procedural steps that can improve the accuracy and repeatability of MR flow measurements, but the process remains user-dependent for the weakly pulsatile foramen magnum CSF and venous flows where variability remains a significant confound even to relatively large perturbations such as CO2 administration.

Posture-induced changes in intracranial pressure: a comparative study in patients with and without a cerebrospinal fluid block at the craniovertebral junction.

OBJECTIVE: The aim of this study was to determine posture-induced changes in intracranial pressure (ICP) when patients with hydrocephalus or idiopathic intracranial hypertension remained supine for 1 hour and then sat up and remained sitting for 3 hours. METHODS: Continuous ICP was monitored using a fiberoptic extradural sensor in: 1) 259 patients with hydrocephalus or idiopathic intracranial hypertension with free cerebrospinal fluid (CSF) flow through the craniovertebral junction and Sylvian aqueduct, 2) 20 patients with hydrocephalus secondary to aqueductal stenosis with free CSF flow through the craniovertebral junction, and 3) 97 patients with hydrocephalus associated with Chiari malformation. The maximum ICP difference (DeltaICP) was calculated as the difference between mean ICP in the supine position and minimum ICP value after changing body position. The mean ICP difference (DeltaICPmean) was calculated as the difference between the mean ICP in the supine position and the mean ICP while the patient was in a sitting position. RESULTS: In the complete sample, the median of DeltaICP was 13 mm Hg (interquartile range 10-17). The median of DeltaICPmean was 8 mm Hg (interquartile range 5-11). Both DeltaICP and DeltaICPmean were significantly greater in patients without obstruction in the craniospinal junction than in those with Chiari malformation (P = 0.005 and P = 0.014, respectively). No differences were found in DeltaICP or DeltaICPmean between patients with Sylvian aqueduct stenosis and those without (P = 0.777 and P = 0.346, respectively). CONCLUSION: ICP reduction after a change in body position is significantly greater in patients with free CSF flow through the craniospinal junction than in those with Chiari malformation, indicating the difficulty or impossibility of CSF displacement into the spinal canal in the latter.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section.

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

Magnetic stimulation of the descending and ascending tracts at the foramen magnum level.

To test the possibility that stimulation over the foramen magnum activates ascending tracts as well as descending tracts, we studied 4 patients with myoclonic epilepsy all of whom had enhanced cortical long loop reflexes (LLRs) and 10 normal subjects, using our previously reported method (Ugawa et al., Ann. Neurol., 1994, 36: 618-624). For latency comparisons, peripheral nerve stimulation at the elbow and spinal motor root were also performed. In all patients, magnetic stimulation at the foramen magnum consistently elicited long loop reflexes as well as direct responses caused by stimulation of the descending tracts. In contrast, no LLRs were ever seen in any normal subjects. The latencies of both types of response were the same whether stimulation used upward or downward current in the brain, although the former was always more effective. This indicates that stimulation at the level of the foramen magnum activates ascending tracts as well as descending tracts at a fixed position. The threshold for LLRs was lower than that for activation of the descending tracts. This threshold difference is compatible with the hypothesis that large diameter fibers from muscle afferent conduct the central afferent volley for LLRs (Marsden et al., Brain, 1977, 100: 185-200).

Clinical utility of magnetic corticospinal tract stimulation at the foramen magnum level.

We applied magnetic stimulation of the corticospinal tract at the foramen magnum level to 19 patients with various neurological disorders. Results were consistent with our previous speculation that activation occurs at the foramen magnum level. This method was clinically useful for the following conditions. (1) Detection of subclinical lesion: one patient who had transient ischemic attack that caused no clinical symptoms at examination was shown to have dysfunction of the corticospinal tract. (2) Multiple lesions: our method disclosed at least one lesion above and below the foramen magnum in two patients with multiple sclerosis. (3) Unmasking of dysfunction of the corticospinal tract masked by peripheral neuropathy: magnetic stimulation showed conduction delay in the corticospinal tract in two patients in whom no pyramidal signs were evident because of muscular atrophy due to neuropathy. One patient had multiple sclerosis and chronic inflammatory demyelinating polyradiculoneuropathy, the other had degenerative ataxia and neuropathy. (4) Association of disorders: conduction delay rostral to the foramen magnum, which should not occur in patients with only cervical myelopathy, was shown in a patient with cervical myeloradiculopathy and amyotrophic lateral sclerosis. We conclude that this magnetic stimulation method which is less painful than electrical stimulation has extensive clinical usefulness.

Magnetic stimulation of corticospinal pathways at the foramen magnum level in humans.

Magnetic stimulation done with a double cone coil placed over the back of the head activated descending motor pathways and produced electromyographic responses in muscles of the arms and legs. The latencies of these responses were the same as those of responses to electrical brainstem stimulation. The threshold was lowest when the coil was placed over the inion or below it on the median line. Placement of the coil on the side ipsilateral to the muscle was more effective than placement on the contralateral side. These results indicate that activation occurs at the foramen magnum level (just below the pyramidal decussation). Collision experiments that used cortical and magnetic brainstem stimulation indicated that the major part of the responses to the latter stimulation were conducted via the large diameter component of the corticospinal tract. Collision experiments done with the peripheral nerve and magnetic brainstem stimulation showed that this stimulation produced a single descending volley in the descending tract. We conclude that magnetic brainstem stimulation produces a single descending volley in the corticospinal tract at the foramen magnum level with less discomfort.