Influence of activity-induced axonal hypoexcitability on transmission of descending and segmental signals.

In this experiment, the changes in excitability of motor axons produced after natural activity were measured in nine healthy subjects using 1 min of maximal voluntary contractions (MVC) of the abductor digiti minimi (ADM) by studying the relationship between stimulus intensity applied to the ulnar nerve and the size of the ADM compound muscle action potential (CMAP). On cessation of the contraction, there was a prominent right-shift of the input-output curve: the intensity required to produce a control CMAP approximately 60% of maximum, generated a post-contraction response approximately 25% of maximum. Similar changes occurred in the input-output curves obtained by recording the ulnar nerve volley evoked by same test stimulus for CMAP. Motor-evoked potential (MEP) and F-waves (and H-reflex in one subject) were recorded from ADM before and after 1 min of MVC. On cessation of contraction, the MEP input-output curves exhibited a significant right-shift: the stimulus required to evoke a pre-contraction maximum MEP ( approximately 60% of maximum CMAP) generated a post-contraction response approximately 65% of initial values. One minute of MVC produced similar decreases of F ( approximately 35%)- and H ( approximately 30%)-ADM responses. All responses recovered their control value in 15-20 min after the end of contraction. The almost identical depressive effect produced by 1 min of MVC on peripherally and centrally generated muscle responses suggests a common conditioning factor. These findings are discussed within the context of activity-induced motor axonal hyperpolarizion.

Ulnar nerve compression neuropathy at Guyon's canal caused by crutch walking: case report with ultrasonographic nerve imaging.

This report is the first account of Guyon's syndrome after the bilateral use of forearm crutches. Crutch palsy is usually neuropraxic in nature and associated with clinical and electrophysiologic recovery of nerve function, especially if patients are instructed to not bear excessive weight on the wrist. The present case history follows this pattern. In establishing the diagnosis of a focal compression neuropathy, a combination of clinical assessment and neurophysiologic studies are typically used. This report describes the additional application of ultrasound imaging to verify the diagnosis and to track changes in the appearance of the nerve during follow-up.

Prulifloxacin as a trigger of myasthenia gravis.

Fluoroquinolones has been rarely associated with exacerbation of myasthenia gravis (MG). We present a case of MG following a treatment with prulifloxacin, a new broad-spectrum oral fluoroquinolone. Fluoroquinolones of any generation may interfere with neuromuscular transmission and should be avoided in patients with MG.

Ulnar sensory nerve impairment at the wrist in carpal tunnel syndrome.

In previous studies, changes in impulse transmission of ulnar motor axons have been documented in patients with carpal tunnel syndrome (CTS). We examined ulnar sensory conduction in 144 CTS hands. In particular, conduction parameters of the dorsal ulnar cutaneous branch (DUC) running outside Guyon's canal were compared with those of the superficial sensory branches (U4 and U5) passing through the canal. U4 and U5 response amplitudes and U5 conduction velocity were significantly lower than in controls. Conduction parameters of the DUC were similar in both groups. Patients with more severely impaired median conduction had smaller ulnar sensory action potentials. We propose that the ulnar nerve may be subject to compression in Guyon's canal as a consequence of high pressure in the carpal tunnel of CTS patients. This may provide insights into the mechanisms underlying extra-median spread of sensory symptoms in CTS patients.

Changes in corticomotor excitability of forearm muscles in relation to static shoulder positions.

We examined whether the recruitment properties of the corticospinal pathway to forearm muscles are influenced by variations of the shoulder joint angle. Flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles motor evoked potentials (MEPs) in response to transcranial magnetic stimulation were examined during different static positions of shoulder joint in the horizontal plane: from 30 degrees abduction to 30 degrees adduction. We found that at 30 degrees shoulder adduction, maximum slope and plateau phase of the ECR and FCR input-output relationship (i.e., relation between MEP size and stimulus intensity) were significantly higher and lower than at 30 degrees abduction of the shoulder joint, respectively. Intracortical inhibition (ICI) and intracortical facilitation (ICF) of the FCR were assessed using a paired-magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30 degrees abduction to 30 degrees adduction. On the contrary, no variation in the amount of ICI occurred in relation to the same changes in shoulder position. FCR H-reflex to electrical stimulation of median nerve at elbow did not differ significantly between the two shoulder positions. We conclude that shoulder position influences the recruitment efficiency (gain) of the corticospinal volleys to motoneurones of forearm muscles. It is proposed that activity of peripheral receptors signaling static shoulder position influences corticomotor excitability of forearm muscles mainly at cortical level, although C3-C4 propriospinal system could be also involved. It is proposed that the above changes in corticomotoneuronal excitability to forearm muscles as function of shoulder joint position are part of a global proximal-distal synergy operating throughout reaching movements.