Gross Anatomy of the Brain

Gross Anatomy of the Brain: Surface Features and Landmarks

• Gyrus: Defined as a ridge on the surface of the brain.
• Sulcus: Defined as a valley or groove on the surface of the brain.
• Fissure: A deep valley or significantly deep groove in the brain architecture.
• Central Sulcus: The prominent groove separating the frontal lobe from the parietal lobe.
  • Precentral Gyrus: Located in the frontal lobe; serves as the primary motor cortex.
  • Postcentral Gyrus: Located in the parietal lobe; serves as the primary somatosensory cortex.
• Functional Lobes and Specific Areas:
  • Frontal Lobe: Associated with motor function and personality.
  • Parietal Lobe: Associated with somatosensory processing.
  • Temporal Lobe: Associated with auditory processing and olfaction (smell).
    • STG: Superior Temporal Gyrus.
    • MTG: Middle Temporal Gyrus.
    • ITG: Inferior Temporal Gyrus (located near the temporal pole).
  • Occipital Lobe: Associated with vision and vision association.
  • Parieto-occipital Sulcus: The boundary between the parietal and occipital lobes.
  • Lateral Fissure: The deep groove separating the temporal lobe from the frontal and parietal lobes.
• Cortical Mapping (Homunculus):
  • Both the motor (precentral) and sensory (postcentral) gyri follow a topographical map: Trunk (superior/medial), Arm (lateral), and Head (inferior/lateral near the lateral fissure).
• Brainstem and Lower Structures:
  • Pons: Part of the brainstem.
  • Medulla: The lower part of the brainstem.
  • Cerebellum: Located posterior to the brainstem.

Medial View and Internal Structures

• Paracentral Lobule: A U-shaped structure on the medial surface encompassing the medial continuations of the precentral and postcentral gyri; involved in motor and sensory functions.
• Cingulate Gyrus: Part of the limbic system, involved in emotions and behavior.
• Corpus Callosum: The primary gateway for communication between the two cerebral hemispheres. It is divided into four main parts:
  1. Rostrum: The anterior-most, thin portion tapering inferiorly.
  2. Genu: The "knee" or anterior bend.
  3. Body: The long central part.
  4. Splenium: The thickened posterior end.
• Diencephalon and Related Structures:
  • Thalamus: A major sensory gateway; features the interthalamic adhesion (mass intermedia).
  • Hypothalamus: Located inferior to the thalamus; includes the mamillary body.
  • Pineal Gland: Located in the epithalamus; responsible for circadian rhythms.
  • Pituitary Gland: Attached to the hypothalamus via the infundibulum.
• Other Internal Landmarks:
  • Fornix: A white matter bundle associated with the limbic system.
  • Cerebral Aqueduct: Connects the third and fourth ventricles.
  • Fourth Ventricle: Located between the pons/medulla and the cerebellum.
  • Calcarine Fissure: Located in the occipital lobe; divides the visual fields (superior and inferior retinal fields).

The Meninges and Protective Barriers

• Protection Layers: The brain is protected by the skull, the meninges, and Cerebrospinal Fluid (CSF).
• Three Layers of Meninges:
  1. Dura Mater: "Tough mother"; the outermost, thickest layer. It forms partitions like the falx cerebri and contains dural sinuses (e.g., superior sagittal sinus, internal sagittal sinus).
  2. Arachnoid Mater: Middle layer made up of collagen fibers. It has no sensory innervation.
  3. Pia Mater: "Delicate mother"; the innermost layer closely adhering to the brain surface. It includes the epipial layer and the intima.
• Meningeal Spaces:
  • Epidural Space: The space between the dura mater and the skull.
  • Subarachnoid Space: The space between the arachnoid and pia mater; it contains Cerebrospinal Fluid (CSF) and major blood vessels (e.g., veins, penetrating vessels like those in the Virchow-Robin space).

Neurological Innervation of the Dura

• Nerve Supply:
  • Cranial Nerve V (Trigeminal Nerve): Provides sensory and stretch receptors above the tentorium cerebelli.
    • Ophthalmic Division ($CN V_1$): Includes anterior meningeal branches of the anterior ethmoidal nerve, posterior ethmoidal nerve, and the tentorial nerve (recurrent meningeal branch).
    • Maxillary Division ($CN V_2$): Meningeal branch.
    • Mandibular Division ($CN V_3$): Meningeal branches, including the nervus spinosus.
  • Cranial Nerve X (Vagus Nerve): Provides vasomotor control and some pain fibers to the posterior fossa (distributed via $C2$ fibers).
  • Cervical Nerves ($C1 – C3$): Provide sensory and stretch receptors below the tentorium cerebelli.
    • Fibers from $C2$ and $C3$ enter through the hypoglossal canal and are distributed to the floor of the posterior cranial fossa via $CN XII$.
  • Cervical Sympathetic Chain: Provides vasomotor control.

Ventricular System and Cerebrospinal Fluid (CSF)

• Structure of the Ventricles:
  • Lateral Ventricles: Located in each hemisphere. Parts include the anterior (frontal) horn, body (central part), atrium (collateral trigone), posterior (occipital) horn, and inferior (temporal) horn.
  • Interventricular Foramen (Foramen of Monro): Connects lateral ventricles to the third ventricle.
  • Third Ventricle: Located in the diencephalon. Recesses include the supraoptic, infundibular, suprapineal, and pineal recesses.
  • Cerebral Aqueduct (Aqueduct of Sylvius): Connects the third and fourth ventricles.
  • Fourth Ventricle: Features apertures for CSF to enter the subarachnoid space:
    • Median Aperture (Foramen of Magendie).
    • Lateral Apertures (Foramina of Luschka).
  • Central Canal: Continuation of the ventricular system into the spinal cord.
• CSF Dynamics:
  • Production: Produced in the choroid plexus.
  • Production Rate: Approximately $20\,ml/hr$, totaling up to $500\,ml/day$.
  • Volume Stats:
    • Total volume: Between $120 – 140\,ml$ (or $125 – 150\,ml$ depending on measurement).
    • Location Distribution: $20\%$ in the ventricles ($25\,ml$) and $80\%$ in the subarachnoid space around the brain and spine.
  • Brain Buoyancy: The brain's weight is significantly reduced when suspended in CSF.
    • Weight in air: $\sim 1400\,g$.
    • Weight in CSF: $\sim 25\,g$.
• CSF Cisterns: Areas where the subarachnoid space is enlarged.

Hydrocephalus

• Condition defined as an accumulation of CSF, which can result from:
  • Overproduction of CSF.
  • Blockage of flow (e.g., at the interventricular foramen or cerebral aqueduct).
  • Under-reabsorption of CSF by the arachnoid granulations.
• Treatment: Commonly involves the surgical placement of a CSF shunt.
  • The shunt includes a valve to regulate pressure.
  • It typically drains fluid from the lateral ventricle to another part of the body.