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. Author manuscript; available in PMC: 2022 Dec 15.
Published in final edited form as: Clin Neurophysiol. 2022 May 18;140:59–97. doi: 10.1016/j.clinph.2022.04.022

Fig. 5. Candidate descending corticospinal pathways activated by transcranial magnetic stimulation (TMS) in the precentral motor hand knob.

Fig. 5.

The insertion in the upper right-hand corner displays a sagittal slice of the motor hand knob with key anatomical landmarks highlighted. The likelihood of direct activation of neurons appears greatest in the lip/rim regions of the motor hand knob. Through synaptic transmission in cortico-cortical projections, activation will spread and activate rostral and caudal parts of M1 potentially contributing to indirect waves (I-waves). The greater preponderance of fast-conducting, monosynaptic cortico-motoneuronal neurons in the caudal (new) M1 (BA4p) compared to the rostral (old) M1 (BA4a) is highlighted. As shown, the exact transition between the rostral parts of the M1 and the caudal of PMd in the lip/rim region of the gyrus is gradual and may vary from subject to subject (highlighted in orange). Please note that this figure focuses on the precentral gyrus and anterior wall of the central sulcus, but additional corticospinal pathways may be activated by TMS via excitation of postcentral primary somatosensory cortex (S1) and its cortico-cortical projections to rostral/caudal M1.