📚 Lecture Overview
This lecture covers the detailed anatomy of the medial and inferior surfaces of the cerebral hemispheres, focusing on their defining sulci, gyri, and functional areas. Understanding these structures is crucial for mapping cortical functions—such as vision, olfaction, motor control, and behavior—to their precise anatomical locations and clinical presentations.
🎯 Key Concepts & Definitions
- Insula: The cerebral cortex region hidden deep within the lateral sulcus.
- Paracentral Lobule: A U-shaped gyrus on the medial surface surrounding the end of the central sulcus, responsible for motor and sensory control of the contralateral lower limb and perineum.
- Cuneus: A wedge-shaped (triangular) gyrus on the medial occipital lobe located between the parieto-occipital and postcalcarine sulci, essential for vision.
- Uncus: An anterior hook-like projection on the inferior surface that forms part of the olfactory (piriform) cortex.
- Gyrus Rectus: A straight gyrus on the medial-most aspect of the orbital surface, separated from the rest of the orbital surface by the olfactory sulcus.
📖 Main Content
1. Medial Surface of the Cerebrum
The medial surface contains key sulci and gyri that regulate limbic, motor, sensory, and visual functions.
Main Sulci
- Cingulate sulcus: Follows the curvature of the corpus callosum through the frontal and parietal lobes. It terminates by passing upwards behind the central sulcus to continue on the lateral surface as the postcentral sulcus.
- Callosal sulcus: Lies directly above the corpus callosum, separating it from the cingulate gyrus.
- Central sulcus: Its terminal end extends onto the medial surface, located anterior to the ascending terminal branch of the cingulate sulcus.
- Parieto-occipital sulcus: Runs vertically from the superomedial border to meet the calcarine sulcus.
- Calcarine sulcus: Located at the caudal end of the medial surface. It joins the parieto-occipital sulcus, which divides it into precalcarine and postcalcarine segments.
Main Gyri & Functional Areas
- Cingulate gyrus: Located between the callosal and cingulate sulci; functions as part of the limbic association cortex.
- Paracentral lobule: Surrounds the termination of the central sulcus. It is continuous anteriorly with the precentral gyrus (motor) and posteriorly with the postcentral gyrus (sensory). It controls the contralateral leg, foot, and perineum.
- Medial frontal gyrus: Frontal lobe portion anterior to the cingulate sulcus; functions as part of the prefrontal area (responsible for behavior, attention, and concentration).
- Precuneus: Lies anterior to the parieto-occipital sulcus.
- Cuneus and Lingual gyrus: Form the visual cortex. The centre for vision lies below the precalcarine sulcus and on both sides of the postcalcarine sulcus.
- Premotor area: Located in the frontal lobe, directly in front of the motor portion of the paracentral lobule.
2. Inferior Surface of the Cerebrum
The inferior surface is divided by the stem of the lateral sulcus into two main regions:
A. Orbital Part (Anterior)
Bounded by the medial orbital border, superciliary border, and the stem of the lateral sulcus.
- Olfactory sulcus: Lies lateral to the medial orbital border, separated from it by the gyrus rectus. It is concerned with the olfactory system.
- H-shaped orbital sulcus: Divides the lateral orbital surface into anterior, posterior, medial, and lateral orbital gyri. These form part of the prefrontal area.
B. Tentorial Part (Posterior)
Bounded by the hippocampal border, medial occipital border, inferolateral border, and the stem of the lateral sulcus.
- Rhinal sulcus: A small anterior sulcus near the temporal pole that forms the lateral boundary of the uncus.
- Collateral sulcus: Runs posteriorly as a continuation of the rhinal sulcus, ending near the occipital pole.
- Parahippocampal gyrus: Located lateral to the anterior part of the collateral sulcus; constitutes part of the olfactory cortex.
- Lingual gyrus: Located between the posterior part of the collateral sulcus and the calcarine sulcus (associated with vision).
- Occipito-temporal sulcus: Runs parallel and lateral to the collateral sulcus, dividing the lateral tentorial surface into medial and lateral occipito-temporal gyri.
3. Clinical Correlations
- Primary Motor Cortex Lesions: Damage to the primary motor cortex in one hemisphere results in paralysis of the contralateral (opposite side) extremities.
- Calcarine Sulcus Lesions: Damage involving the calcarine sulcus results in a loss of vision in the contralateral (opposite) visual field.
📊 Visual Learning
Diagram 1: Visual Cortex Subdivisions (Medial Surface)
Diagram 2: Divisions of the Inferior Surface
Diagram 3: Functional Mapping of the Paracentral Lobule
💡 Important Points to Remember
- The Insula is the hidden lobe located deep within the lateral sulcus.
- The paracentral lobule controls both motor and sensory functions of the opposite leg, foot, and perineum.
- The cingulate gyrus belongs to the limbic association cortex.
- The medial frontal gyrus regulates behavior, attention, and concentration (prefrontal area).
- The visual cortex centers are located in the cuneus and lingual gyrus, specifically around the calcarine sulcus.
- A lesion in the calcarine sulcus leads to vision loss in the opposite visual field (contralateral hemianopia).
- The rhinal sulcus acts as the lateral boundary for the uncus.
- Olfactory structures on the inferior surface include the uncus, parahippocampal gyrus, and olfactory gyrus.
- The collateral sulcus is the posterior continuation of the rhinal sulcus.
- The H-shaped orbital sulcus divides the orbital surface into four distinct orbital gyri (anterior, posterior, medial, and lateral).
⚠️ Common Exam Questions
Common MCQ & Short-Answer Focus Areas
- Paracentral Lobule Representation: Questions frequently ask which body parts are represented here (leg, foot, perineum) and emphasize that it controls the contralateral side of the body.
- Olfactory Structures: Examiners often ask you to identify which gyri on the inferior surface are responsible for olfaction. Remember: uncus, parahippocampal gyrus, and olfactory gyrus.
- Boundaries of the Cuneus: Be prepared to identify it as the triangular gyrus situated between the parieto-occipital and postcalcarine sulci.
Common Exam Traps
- The "Ipsilateral vs. Contralateral" Trap: Questions regarding lesions in the primary motor cortex or calcarine sulcus will offer "ipsilateral paralysis" or "same-side vision loss" as distractors. Always remember these pathways cross, resulting in contralateral deficits.
- The "Paracentral Lobule Representation" Trap: MCQs might ask which body part is not represented in the paracentral lobule, listing options like "upper limb," "face," or "hand." These are represented on the lateral surface, not the medial surface's paracentral lobule.
- Sulcus Misidentification: Confusing the collateral sulcus with the calcarine sulcus. Remember: Calcarine is on the medial surface (visual), while Collateral is on the inferior tentorial surface (separating parahippocampal/lingual gyri from occipito-temporal gyri).
📝 Quick Review Checklist
I can locate and describe the boundaries of the paracentral lobule.
I understand that the paracentral lobule controls motor and sensory functions of the contralateral leg, foot, and perineum.
I can identify the visual cortex structures (cuneus, lingual gyrus, and calcarine sulcus) and predict the clinical outcome of a calcarine sulcus lesion.
I can identify which structure is hidden deep to the lateral sulcus (Insula).
I know the components of the olfactory cortex on the inferior surface (uncus, parahippocampal gyrus, and olfactory gyrus).
I can differentiate between the orbital and tentorial parts of the inferior surface of the brain.
I can trace the path of the cingulate sulcus and its relationship to the cingulate gyrus (limbic lobe).