Alteration of motor cortex excitability in response to intermittent photic stimulation.

OBJECTIVE: To measure the duration of cortical silent period (C-SP) in response to different frequencies of intermittent photic stimulation (IPS). METHODS: Ten normal volunteers without a history of seizures were studied. First, using a magnetic stimulator we determined the resting motor threshold (RMT). The C-SP produced by a single magnetic shock at 120% of RMT was measured in the maximally contracted abductor pollicis brevis. C-SP was then measured while subjects were exposed to IPS at 5, 10, 15, 20, 30, 50, and 60 Hz with closed eyes. RESULTS: The average duration of C-SP was 135+/-33. 2 ms with no photic stimulation. This did not change significantly when subjects were exposed to IPS at 5, 10, 15, 20, and 30 Hz. The duration of C-SP, however, decreased significantly at higher frequencies. At 50 Hz it was 116.2+/-43 ms (P=0.003) and at 60 Hz it was 112.7+/-44.6 ms (P<0.001). CONCLUSION: Our findings suggest that motor cortex inhibition is altered in response to IPS at 50 and 60 Hz.

Changes in excitability of motor cortical circuitry in primary restless legs syndrome.

OBJECTIVE: To investigate the excitability of segmental and suprasegmental systems in patients with primary restless legs syndrome (pRLS) by measuring the cortical silent period (C-SP) and the peripheral silent period (P-SP). BACKGROUND: There is some evidence that RLS may be the motor manifestation of normal CNS periodicity that becomes disinhibited under certain conditions. The mechanism of this disinhibition is unclear. DESIGN/METHODS: Ten patients with pRLS and 10 normal age-matched subjects were studied. The mixed nerve P-SP produced by electrical stimulation of the median and common peroneal nerves was recorded during maximal contraction of the abductor pollicis brevis (APB) and tibialis anterior (TA) muscles. The C-SP produced by a single magnetic shock to motor cortex at 150% of resting threshold was also measured during maximal contraction of the APB and TA muscles. The average of 5 to 10 trials at each site was obtained and compared using Student's t-test. RESULTS: Resting central motor threshold was not significantly different between pRLS patients and the control group. The average duration of the C-SP was shorter in the APB (74.5+/-37.7 versus 129.56+/-35.95 msec, p<0.05) and TA (66.81+/-25.63 versus 136.1+/-40.35 msec, p<0.05) in patients with pRLS. The P-SP duration, however, was not significantly different between two groups in either limb. CONCLUSION: The supraspinal inhibitory system is impaired in pRLS.

Cortical projections to spinal motoneurons: changes with aging and amyotrophic lateral sclerosis.

Peristimulus time histograms (PSTHs) of discharging single motor units, recorded from the extensor digitorum communis (EDC) during randomly applied cortical magnetic stimulation, were obtained in 42 normal subjects aged 24 to 83 years and 42 patients with amyotrophic lateral sclerosis (ALS) aged 37 to 84 years. Normal subjects had an early period of increased firing probability occurring at about 20 msec poststimulus, reflecting an underlying compound excitatory postsynaptic potential (EPSP) induced by fast-conducting, descending volleys of the corticomotoneuronal core facilitating the single spinal motoneuron. There was an age-dependent, linear decline in the amplitude of the EPSP (r = 0.673). We estimated that by age 50 years about 35% of corticomotoneurons are lost or nonfunctioning in normal controls. Compared with age-matched controls, the EPSP in most patients with ALS was reduced, and it was unmeasurable in six. We postulate this reflects a loss of corticomotoneurons. Seven (16.7%) patients phenotypically the same as the others had EPSPs that were larger than age-predicted values. This may reflect glutamate-induced excitotoxicity in a subset of ALS. In a single patient with chronic spinal muscular atrophy the EPSP was normal.