Middle frontal gyrus  (Petrides, Pandya, HNS2)

Lateral Surface of the Frontal Lobe, Precentral region

The middle frontal gyrus is a wide gyrus that lies between the superior and the inferior frontal sulci, rostral to the precentral gyrus. 
The sulci of the middle frontal gyrus have generated a lot of confusion in the anatomical literature.  In the anterior part of the middle frontal gyrus lies a deep sulcus that has been referred to as the middle frontal sulcus or the intermediate frontal sulcus.  This sulcus was studied in detail by Eberstaller (1890).  It originates in the rostral part of the middle frontal gyrus and runs in a horizontal direction initially (infs-h) and then veers ventrally (infs-v) to reach the medial frontomarginal sulcus (mfms) at the lower edge of the frontal pole (see Figs. 1 and 2).  The horizontal and the vertical branches can be separate is some cases.  The caudal end of the middle frontal sulcus begins close to the rostral end of the superior frontal sulcus.  There is usually a narrow gyral passage that separates them (see * in Figs. 1, 2, 3).  In several cases, this narrow gyral passage may be collapsed and, in these cases, the intermediate frontal sulcus may give the impression of joining the superior frontal sulcus.  The superficial joining of these two sulci may lead to confusing the middle frontal sulcus as a continuation of the superior frontal sulcus when one relies on surface inspection of a brain.  For instance, Ono et al (1990, p. 51) reported that the superior frontal sulcus connects with the frontomarginal sulcus in 16% of the cases in the right hemisphere and 44% of the cases in the left hemisphere.  In our studies, the superior frontal sulcus was never observed to join the frontomarginal sulcus and we could always distinguish the anterior end of the superior frontal sulcus from the posterior end of the intermediate frontal sulcus in coronal or horizontal sections. 

The anterior part of the middle frontal gyrus can be thought of as divided into a dorsal and a ventral middle frontal tier by the horizontal part of the intermediate frontal sulcus.  The caudal part of the middle frontal gyrus is divided into several smaller gyri by various short sulci that have not been the focus of an investigation before.  In our recent examination of the sulcal patterns of the frontal lobe, three short sulci could usually be identified in the posterior middle frontal gyrus.  These posterior middle frontal sulci are quite variable in their directions and may merge with each other creating confusing patterns.  We have referred to these three sulci as the posterior (pmfs-p), the intermediate (pmfs-i), and the anterior (pmfs-a) posterior middle frontal sulci, proceeding in a caudo-rostral direction (Figs. 1, 2, 3).  The posterior and intermediate posterior middle frontal sulci can be more or less vertically oriented and directed towards to the inferior precentral sulcus (Figs. 1, 2, and 3).  It should be noted that a considerable extent of the frontal cortical region lies in the depth of the horizontal extension of the inferior precentral sulcus.  Eberstaller (1890) noted that this is the deepest part of the lateral frontal cortex.  The anterior posterior middle frontal sulcus is a sulcus that varies considerably in its direction and may approach the superior frontal sulcus and merge superficially with it.   

                Another set of short sulci can be identified in the anterior part of the middle frontal gyrus running next to and lateral to the intermediate frontal sulcus.  These sulci we refer to as the paraintermediate frontal sulci (painfs in Figs. 1 and 2).  The dorsal one runs in a more or less vertical direction and lies just behind or below the caudal end of the intermediate frontal sulcus. 

PETRIDES M. AND PANDYA D., HNS2, p. 953-954): It should be noted that a considerable extent of the frontal cortical region lies in the depth of the horizontal extension of the inferior precentral sulcus. Eberstaller (1890) noted that this is the deepest part of the lateral frontal cortex. The anterior posterior middle frontal sulcus is a sulcus that varies considerably in its direction and may approach the superior frontal sulcus and merge superficially with it.

Another set of short sulci can be identified in the anterior part of the middle frontal gyrus running next to and lateral to the intermediate frontal sulcus. These sulci we refer to as the paraintermediate frontal sulci. The dorsal one runs in a more or less vertical direction and lies just behind or below the caudal end of the intermediate frontal sulcus.