The frontal lobe is the largest lobe of the brain, occupying about one-third of the cerebral hemisphere. As the name implies, the frontal lobe is located in the anterior aspect of the cranial cavity, conforming to the inner surface of the frontal bone.
The frontal lobe is separated from the parietal lobe posteriorly by a groove called the central sulcus, and from the temporal lobe inferolaterally by the lateral sulcus (Sylvian fissure). On its surface, the frontal lobe contains four principal gyri: the precentral, superior frontal, middle frontal, and the inferior frontal gyrus.
The functions of the frontal lobe are numerous. It is associated with higher cognitive functions, such as decision making, motivation, problem-solving, planning and attention. These functions are carried out mainly by the prefrontal cortex of the frontal lobe. It also contains the motor cortex, which is responsible for planning and coordinating voluntary movements. Lastly, the frontal lobe contains the Broca’s area, which is essential for producing the motor component of speech.
This article will discuss the anatomy of the frontal lobe of the brain.
|Location||- Anterior to the parietal lobe (separated by central sulcus)
- Superior and anterior to the temporal lobe (separated by lateral sulcus - Sylvian fissure)
|Principal gyri||Precentral gyrus, superior frontal gyrus, middle frontal gyrus and inferior frontal gyrus|
Prefrontal cortex (superior frontal gyrus, middle frontal gyrus and inferior frontal gyrus)
Motor cortex (precentral gyrus)
Broca’s area (pars opercularis and pars triangularis of inferior frontal gyrus)
Prefrontal cortex: management of higher cognitive functions such as planning, organizing, motivation, discipline, problem-solving, self-control and emotional regulation.
Motor cortex: Control of voluntary movement, spatial orientation
Broca’s area: Speech production
- Location and structure
- Gyri of the frontal lobe
- Blood supply
- Clinical relations
Location and structure
The frontal lobe lies largely in the anterior cranial fossa of the skull, leaning on the orbital plate of the frontal bone. It occupies one-third of the cerebral hemisphere, extending from its most anterior part (frontal pole) posteriorly to the central sulcus, which separates it from the parietal lobe.
Posterior and inferior to the frontal lobe is the temporal lobe, separated from it by the lateral sulcus (Sylvian fissure). The central sulcus demarcates the posterior border of the frontal lobe, while the Sylvian fissure demarcates its inferior border.
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.
Gyri of the frontal lobe
The four principal convolutions on the convexity of the frontal lobe are the precentral gyrus, the superior frontal gyrus, middle frontal gyrus and inferior frontal 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).
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.
The frontal lobe is supplied by the anterior and middle cerebral arteries, which are branches of the internal carotid artery. The anterior cerebral artery mostly supplies the superior and medial surfaces of the frontal lobe, while the middle cerebral artery supplies the inferior and lateral surfaces.
Venous drainage of the frontal lobe is carried out by the the superior cerebral vein, which drains the blood from the superior portion of the lobe into the superior sagittal sinus, while the inferior portion of the frontal lobe is drained by the inferior cerebral veins, which terminate into the superficial middle cerebral veins or the transverse sinus.
Functionally, the entire frontal cortex of the frontal lobe can be deemed as the "active cortex", as opposed to the posterior cortex which can be considered as the “passive (sensory) cortex". It is deemed as such because the frontal cortex initiates an action of some sort, either voluntary limb movement, ocular movement, verbal expression or emotional expression. One way to describe the functional organization of the frontal cortex is to divide it into three areas, each having a specific brain function. These are the prefrontal cortex, the motor cortex and Broca’s area.
The prefrontal cortex corresponds to the superior, middle and inferior frontal gyri of the frontal lobe. It is associated with higher brain functions including decision-making, problem-solving, planning, organizing, motivation, discipline, and emotional regulation. Encompassing all of this, it can be stated that the prefrontal cortex is responsible for internal, purposeful mental action, which involves the ability to predict future consequences that result from current actions and thereby to suppress socially unacceptable responses. Thus, it is thought that the prefrontal cortex is the primary determinant for one’s personality, intelligence and social skills.
The motor cortex corresponds to the precentral gyrus of the frontal lobe. It is responsible for planning, control, and execution of voluntary movements. It involves the primary motor cortex, supplementary motor cortex and premotor cortex.
- The primary motor cortex is mainly responsible for generating neural impulses that travel through the corticospinal tract and control the execution of movement.
- The supplementary motor cortex is thought to be involved in planning of movement, sequencing of movement, and the coordination of the body parts involved in the movement.
- The premotor cortex is thought to manage the preparation for movement, sensory guidance of movement and the spatial guidance of movement.
Finally, Broca’s area encompasses the pars opercularis and pars triangularis of the inferior frontal gyrus. It is mainly responsible for expressive motor speech, which involves verbal fluency, phonological processing, grammar processing and attention during speech. Although Broca’s area is mainly associated with language production, it also plays a significant role in language comprehension. Studies have shown that it’s especially important in providing the ability to use syntactic information to determine the meaning of complex and ambiguous sentences.
Broca’s aphasia, also called non-fluent aphasia, is a type of aphasia characterized by the partial loss of the ability to produce language. It is caused by an injury (e.g. stroke) to the Broca’s area in the inferior frontal gyrus of the dominant cerebral hemisphere. It is sometimes accompanied by right-sided weakness or paralysis of the arm and leg because of the close proximity of the precentral gyrus.
Broca’s aphasia presents with an impaired output of spontaneous speech and a loss of normal grammatical structure. Symptoms include effortful and stilted speech, with most sentences limited to 4 words or less. Specifically, linking words, conjunctions (and, or, and but) and the use of prepositions are lost and speech is reduced to only using nouns and verbs, a pattern known as telegraphic speech. In pure Broca's aphasia, comprehension of speech is intact, and patients are aware of what they want to say, but are unable to express themselves comprehensively to other people.
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