Surface Anatomy Of The Forebrain - The Frontal Lobe

Name the 4 gyri that make up the lateral surface of the frontal lobe, in order from most posterior to most anterior.

1. Precentral gyrus → right in front of the central sulcus (most posterior)

2. Superior frontal gyrus → highest and most anterior

3. Middle frontal gyrus → sits between superior and inferior

4. Inferior frontal gyrus → lowest, next to the lateral sulcus

How many major functional divisions does the frontal lobe have, and what are they?

The frontal lobe has 4 major functional divisions:

1. Primary motor cortex

2. Premotor + supplementary motor areas

3. Frontal eye field

4. Prefrontal cortex

• Broca’s area is also an important frontal lobe region but its not counted as the “big four”

Describe the anatomical boundaries of the precentral gyrus.

• Posterior boundary: Central sulcus

• Anterior boundary: Precentral sulcus

• Its the most posterior gyrus of the frontal lobe, running parallel to the central sulcus on the superolateral surface

Describe the anatomical boundaries of the superior frontal gyrus, and what makes it distinctive compared to the other frontal gyri?

• The superior frontal gyrus is the topmost frontal gyrus

• Inferior boundary: the superior frontal sulcus (separates it from the middle frontal gyrus)

• It extends all the way forward to the frontal pole

• What makes it distinctive:

• Its the highest and longest of the frontal gyri

• It occupies the entire superior part of the frontal lobe

Describe the anatomical boundaries of the middle frontal gyrus.

• The middle frontal gyrus sits between the superior and inferior frontal gyri

• Superior boundary: Superior frontal sulcus

• Inferior boundary: Inferior frontal sulcus

• It occupies the middle horizontal strip of the frontal lobe’s lateral surface

Describe the anatomical boundaries of the inferior frontal gyrus.

• The inferior frontal gyrus sits below the middle frontal gyrus

• Superior boundary: Inferior frontal sulcus

• Inferior boundary: Lateral sulcus (Sylvian fissure)

• Its the lowest gyrus on the lateral frontal lobe, sitting just above the temporal lobe

Name all the sulci of the frontal lobe and state what each one separates.

• Central sulcus → separates precentral gyrus (frontal) from postcentral gyrus (parietal)

• Precentral sulcus → separates precentral gyrus from the rest of the frontal lobe

• Superior frontal sulcus → separates superior from middle frontal gyrus

• Inferior frontal sulcus → separates middle from inferior frontal gyrus

• Lateral sulcus (Sylvian fissure) → forms the inferior boundary of the inferior frontal gyrus, separating it from the temporal lobe

The inferior frontal gyrus is divided into 3 parts. Name them in order from most anterior to most posterior.

1. Orbital part → most anterior

2. Triangular part → middle, wedge-shaped

3. Opercular part → most posterior

Which 2 parts of the inferior frontal gyrus together form Broca’s area, and what are the corresponding Brodmann area numbers?

• Broca’s area = two parts of the inferior frontal gyrus:

• Opercular part → Brodmann area 44

• Triangular part → Brodmann area 45

• Together, BA 44 + BA 45 form Broca’s area, essential for the motor production of speech and language

Why is the inferior frontal gyrus clinically the most important of the 4 frontal gyri?

• The inferior frontal gyrus is the most clinically important frontal gyrus because it contains Broca’s area

• Broca’s area = opercular (BA 44) + triangular (BA 45) parts

• It controls the motor planning and production of spoken and written language

• Damage here causes Broca’s aphasia → non-fluent speech but preserved language comprehension

Where is the primary motor cortex located within the frontal lobe?

• The primary motor cortex sits on the precentral gyrus

• The precentral gyrus lies just in front of the central sulcus and just behind the precentral sulcus

• The primary motor cortex takes up most of this gyrus with the remaining small portion used by premotor/supplementary motor areas

Where are the representations of the leg and foot found in the primary motor cortex - on the lateral or medial surface?

• The leg and foot areas of the primary motor cortex are on the medial surface of the hemisphere

• They sit in the paracentral lobule where the precentral gyrus curves over the medial wall

• This region is supplied by the anterior cerebral artery which is why ACA strokes cause contralateral leg weakness

Where are the premotor and supplementary motor areas located within the frontal lobe?

• These areas sit just in front of the primary motor cortex

• They occupy the remaining parts of the precentral gyrus

• They also extend into the posterior portions of the superior, middle and inferior frontal gyri

Distinguish between the premotor cortex and the supplementary motor area (SMA) in terms of their location and general function.

Premotor Cortex

• Location: Lateral surface of the precentral region

• Function: Plans externally guided movements

Supplementary Motor Area

• Location: Medial surface of the hemisphere

• Function: Plans internally generated movements

Shared role

• Both are higher-order motor areas that prepare movements before the primary motor cortex executes them

What is the frontal eye field, where is it located, and what is its function?

• The frontal eye field is a diffuse region of frontal cortex located near Brodmann area 8

• Its job is to generate voluntary saccades - rapid eye jumps that shift gaze from one point to another

What collective term describes the complex processes mediated by the prefrontal cortex?

The complex mental processes handled by the prefrontal cortex is executive function.

Executive function includes:

• Planning

• Working memory

• Decision-making

• Goal-directed behaviour

A patient has a lesion causing personality change, impaired planning, and impulsivity, but intact motor and sensory function. Which area is most likely affected?

Prefrontal cortex