Table 1 Summary of TMS parameters indicative of cortical excitability change in ALS patients

Motor evoked potentials (MEP)

Electrical signals recorded from descending motor pathways or from muscles after stimulation of motor pathways within the brain

Integrity of the descending corticospinal tract

Reduced

[45]

Motor Threshold (MT)

Minimal intensity of motor cortex stimulation required to elicit a reliable MEP of minimal amplitude in the target muscle

Cortical motor neuronal membrane excitability

Reduced (early) Increased (late)

[47]

Central motor conduction time (CMCT)

Time required for neural impulses to travel through the central nervous system on their way to the target muscles

Conduction between the primary motor cortex and spinal cord

Increased

[48]

Cortical silent period (CSP)

Temporary interruption of electromyographic signal from a muscle following a MEP triggered by TMS over the primary motor cortex (M1)

M1 function. The CSP is typically shortened if M1 is affected, but often grossly prolonged if areas outside M1

Normal or shortened

[49]

Short interval intracortical inhibition (SICI)

Inhibition of MEPs upon a subthreshold conditioning stimulus applied at predetermined short time intervals (7-10 ms) prior to a test stimulus

Balance between inhibition and facilitation. Reduction or absence of SICI is a biomarker of cortical inhibitory GABAergic neuron dysfunction

Reduced

[50]

Intracortical facilitation (ICF)

Increased excitability can be elicited with a similar protocol as SICI but at longer interstimulus interval of 6–30 ms

Intracortical synaptic excitability

Increased

[51]