Frontal lobe

Surfaces

The frontal lobe exhibits three cortical surfaces: lateral, medial and inferior surfaces. The convexity of the lateral surface presents four important convolutions (gyri), which are the precentral gyrus, superior frontal gyrus, middle frontal gyrus and inferior frontal gyrus.

On the medial (interhemispheric) surface, the frontal lobe extends down to the cingulate sulcus. From posteriorly to anteriorly, it consists mainly of the paracentral lobule (an extension of the precentral and postcentral gyri), and the medial extension of the superior frontal gyrus.

The inferior surface of the frontal lobe (or frontobasal, or orbital) is the smallest cortical surface, lying on the floor of the anterior cranial fossa. The medial part of the inferior surface is traversed by the deep olfactory sulcus, which contains the olfactory tract and olfactory bulb. The olfactory sulcus separates the straight gyrus (a long and narrow gyrus) and the orbital gyri. There are four orbital gyri, divided by the H-shaped orbital sulcus into the anterior and posterior orbital gyri superiorly, and the medial and lateral orbital gyri inferiorly.

Precentral gyrus

The precentral gyrus is the most posterior part of the frontal lobe, situated between the central sulcus posteriorly and the precentral sulcus anteriorly. Inferiorly, it is bordered by the lateral sulcus (Sylvian fissure).

The precentral gyrus contains the primary motor cortex (Brodmann’s area 4), which is responsible for integrating signals from different brain regions to modulate motor function: each primary motor cortex sends instructions for voluntary movement to the contralateral (opposite) side of the body.

The primary motor cortex contains pyramidal cells, which are types of multipolar neurons found in several other areas of the brain. A portion of the primary motor cortex contains characteristic giant pyramidal cells called Betz cells, whose axons form the corticospinal and corticobulbar tracts. The Betz cells and their axons are referred to as the upper motor neurons in these tracts. The pyramidal neurons and the Betz cells are arranged somatotopically, which means that, depending on where in the precentral gyrus they originate, they will supply different regions of the body.

The ankle and foot, for example, receive input from neurons originating in the most medial part of the precentral gyrus (near the paracentral lobule), whereas the head receives input from neurons originating from the most lateral part of the precentral gyrus.

The body parts which are controlled by each part of the motor cortex are typically represented by the motor homunculus, a cartoon human whose body parts are drawn at sizes proportional to the input they receive from their corresponding areas within the motor cortex (sensory information received from the body by the somatosensory cortex is often depicted similarly by the sensory homunculus).

Thus, it comes as no surprise that the motor areas for the hand and face occupy the largest part of the precentral gyrus, since they exhibit the most complex and finely controlled set of movements.

If you want to learn more about the motor and sensory homunculi, be sure to check out study unit below!

Anterior to the primary motor cortex in the precentral gyrus is the premotor area, or premotor cortex (Brodmann’s area 6), and the supplemental motor cortex. These regions of the cortex occupy the anterior part of the precentral gyrus and the posterior parts of the superior, middle, and inferior frontal gyri. These regions receive input from the sensory cortex, thalamus, and basal ganglia, and thus play an important role in the initiation and sequencing of movements.

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Superior frontal gyrus

The superior frontal gyrus makes up about one third of the frontal lobe. It extends from the rectus gyrus anteriorly to the precentral sulcus posteriorly, which separates it from the precentral gyrus. The superior frontal gyrus is also extended onto the medial surface of the frontal lobe up until the cingulate sulcus, which separates it from the anterior cingulate gyrus. Laterally, it is bounded by the superior frontal sulcus, which also separates it from the middle frontal gyrus.

The supplementary motor area is located along the most medial portion of the superior frontal gyrus, immediately facing the precentral gyrus.

Middle frontal gyrus

The middle frontal gyrus is usually the largest of the three frontal gyri and lies between the superior and the inferior frontal sulci. The superior frontal sulcus separates the middle frontal gyrus from the superior frontal gyrus, while the inferior frontal sulcus separates it from the inferior frontal gyrus.

The anterior part of the middle frontal gyrus is occupied by a deep sulcus known as the middle frontal sulcus or the intermediate frontal sulcus, which divides this part of the gyrus into dorsal and ventral middle frontal tiers.

Inferior frontal gyrus

The inferior frontal gyrus constitutes a large part of the anterolateral prefrontal cortex. It can be divided into three parts:

• Pars opercularis (or opercular part): refers to the most dorsal, vertically oriented part of the gyrus, that covers the insula (hence its name “opercularis” that means “covering”).
• Pars triangularis (or triangular part): middle triangularly-shaped part of the gyrus.
• Pars orbitalis (or orbital part): most ventral part of the gyrus, extending up until the lateral orbital sulcus.

In the dominant hemisphere (which is usually the left hemisphere in right handed people), the pars opercularis and pars triangularis are referred to as the Broca’s speech area, which is vital for producing the motor component of speech.

Encompassing parts of the middle and inferior frontal gyri, just rostral to the premotor region, is an area called the frontal eye fields (Brodmann's area 6,8,9), which is responsible for voluntary control of conjugate (horizontal) eye movements.