Neurocranium Overview

Overview

This lecture introduces the "Above the Shoulders Anatomy" module, focusing on the neurocranium—the bones that protect the brain—detailing their features, sutures, landmarks, and foramina.

Frontal Bone

• The frontal bone has a vertical (forehead) and a horizontal (roof of orbits) part.
• The coronal suture separates the frontal bone from the parietal bones; the sagittal suture separates parietals at the midline.
• Bregma is where the coronal and sagittal sutures meet, former site of the anterior fontanelle.
• The vertical part may show a persistent metopic suture due to incomplete midline fusion.
• Internal surface features the sagittal sulcus (origin of superior sagittal sinus) and frontal crest (attachment for falx cerebri).
• Frontal sinuses vary in size and symmetry; located above the superciliary arches and glabella.
• Glabella is a clinical landmark between the eyebrows; abnormal reflexes here can indicate neurological disease.
• The frontal bone articulates with nasal, maxillary, lacrimal, and ethmoid bones via distinct sutures.
• The supraorbital margin may have notches/foramina for the supraorbital and supratrochlear nerves (branches of trigeminal nerve).

Ethmoid Bone

• Sits between the frontal bone’s orbital plates, contributes to the anterior cranial fossa.
• Cribriform plate forms the floor of the olfactory fossa and transmits olfactory nerve fibers.
• Crista galli projects superiorly as an attachment for falx cerebri.
• Ethmoid sinuses lie below the cribriform plate.

Sphenoid Bone

• Sphenoid has a body, lesser wings, greater wings, and pterygoid processes.
• Body includes sella turcica (pituitary fossa) and sphenoidal sinuses.
• Lesser wings form part of anterior cranial fossa and border the superior orbital fissure.
• Greater wings form portions of the orbit, middle cranial fossa, and contain important foramina:
  • Foramen rotundum (maxillary nerve)
  • Foramen ovale (mandibular nerve, accessory meningeal artery, lesser petrosal nerve)
  • Foramen spinosum (middle meningeal artery)

Temporal Bone

• Divided into squamous, petrous, mastoid, and tympanic parts (tympanic covered later).
• Squamous part: lateral wall of middle cranial fossa; connects to zygomatic process.
• Petrous part: houses internal carotid artery and internal acoustic meatus; separates middle/posterior cranial fossae.
• Mastoid part: contains mastoid air cells; muscle attachment site; adjacent to sigmoid sinus.
• Temporal bone forms squamosal and sphenosquamosal sutures.

Parietal Bone

• Paired bones form lateral walls and roof of neurocranium.
• Joined by sagittal suture; meet frontal at coronal suture (bregma), occipital at lambdoid suture (lambda).
• Contains sagittal sulcus for superior sagittal sinus.
• Key landmarks: Pterion (site of potential middle meningeal artery injury) and Asterion (junction of lambdoid, occipitomastoid, and parietomastoid sutures).

Occipital Bone

• Composed of squamous, basilar, and lateral parts.
• Squamous part: posterior skull, contains external occipital protuberance and cruciform eminences.
• Foramen magnum: passage for medulla, vertebral arteries, and spinal arteries.
• Occipital condyles: articulate with atlas (C1) at atlanto-occipital joint.
• Hypoglossal canal transmits hypoglossal nerve.
• Jugular foramen: transmits jugular vein and cranial nerves IX, X, XI.

Key Terms & Definitions

• Neurocranium — Skull bones enclosing the brain.
• Suture — Immovable joint between skull bones.
• Bregma — Intersection of coronal and sagittal sutures.
• Glabella — Smooth area between the eyebrows.
• Falx cerebri — Dural fold separating cerebral hemispheres.
• Foramen rotundum/ovale/spinosum — Openings in sphenoid bone for nerves/arteries.
• Crista galli — Upward projection of the ethmoid bone.
• Pterion — Thinnest area of skull where several bones meet.
• Foramen magnum — Large opening in occipital bone for brainstem.
• Atlanto-occipital joint — Joint between skull and first cervical vertebra.

Action Items / Next Steps

• Review the labeled 3D model images and diagrams linked in the accompanying document.
• Practice identifying sutures, foramina, and landmarks on CT/MRI scans.
• Prepare for next lecture: study the viscerocranium and revisit the sphenoid and ethmoid bones.