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Transcrannial magnetic stimulation was developed because transcrannial electric stimulation was too painfull (because the flowing electricity on the scalp may cause skin burns). A brief, high current pulse is produced in a coil of wire, called the magnetic coil. A magnetic field is produced with lines of flux passing perpendicular to te plane of the coil, which ir placed tangential to the scalp. It can reach up to 2T and lasts about 100micro seconds. An electric field is induced perpendicular to the magnetic field.The voltage may excite neurons, but more important are the induced currents. A magnetic field if changed rapidly enough will induce an electric field sufficient to stimulate neurons (new action potentials) or change the resting membrane potentials in the underlying cortex. TMS induces disorder.

The current loops become weaker near the center of the current and there is no current in the center. If the field is parallel to the neuronal element, then the field will be most effective where intensity changes as a function of distance.

With TMS there is an early D wave that reflects direct activation of descending axons. Later comes a series of I (indirect) waves that reflect synaptic activation of the corticospinal neurons (are generated by long posynaptic network). When current in the brain is directed in the pasterior-anterior direction, first you see the I waves.

Spatial resolution - TMS is not spacially focal. Temporal resolution

Limitations - 1. Effects of stimulation are limited to superficial cortical regions and cannot be used to investigate gate functions of medial cortex or subcortical structures.

Cortico conduction time - Time from motor cortex to the motor neuron pool in the spinal cord or brainstem. Is calculated by taking the latency of the MEP and subtracting the peripheral conduction time.

For assessments of cirtical excitability we use different parameters.

The recruitment curve - the growth of the MEP suze as a function of stimulus intensity and background contraction.

Short intracortical inhibition - used with pair pulses initial conditioning stimulus is elicited, but small enough so that no descending influence on the spinal cord can be detected and then we use a suprathershold level stimulation. at very short intervals, less than 5ms, we see inhibition, and in longer (8 ms to 30ms) there is facilitation.

Long intracortical inhibition - is assessed with paired suprathreshold pulses at intevals from 50ms to 200ms.

Plasticity

rTMS between 0.2 Hz and 1 Hz will cause a decrease in brain excitability. more than 5Hz will cause an increase in brain ecitability.

With TMS you can find out if intra-cortical connections have been reduced in patients suffering from strokes and so forth.

Can be used in conjuction with other non-invasive techniques. With EEG you could see how activation of premotor cortex is transmitted to the rest of the brain.

Central motor conduction time delays could be and indication of demyenelating injury. It can also be used to see cortical changes that can be seen in the behavior by assesing cortical excitability. In parkinsons disease there is no change in the motor threshold, but there is an increase in the motor recruitment curve.

TMS can also have therapeutical applications.

Transcrannial electrical stimulation

tDCS can be used to manipulate the membrane potential of neurons and modulate spontaneous firing rates, but is insufficient on it won to discharege resting neurons or axons. TES - anodal stimulation will facilitate the motor cortex (evoking larger motor evoked potentials) and cathodal stimulation will inhibit it. Anodal stimulation ( from 9 to 13min) lasts for 1-2 hours, but after 26 minutes of excitation it resulted in inhibition. While cathodal lassted for 1hour and nine minutes.

Electrode size matters. Larger electrodes can act as reference electrodes and smaller electrodes may allow a selective stimulation of muscles. The position of the reference electrode is critical. It is best to use that with a contralateral reference electrode.

tDCS can help with learning, improve cognitive rehabilitation, boosts memory.

Advantages of tDCS:

Easier to blind participants (sham conditions)

Robustness agains brain-derived neurotropic factor polymorphisms.

tRNS - random noise stimulation.

tACS - sinusoidal applied current stimulation. Allows manipulation of intrinsic cortical oscillations with externally applied frequencies.

The after-effects of tES dominantly depend on the stimulation parameters, including the stimulation duration, the current density, the electrode size, the current density, the type of current, the timing of the stimulation, and the electrode montage. Also wakefullness of the participants, the state of the participants, individual differences in the neuroanatomy of the brain, genetic polimorphismus, handedness and experimental paradigm.