Brain, Cranial Nerves, and Meninges

• These notes cover the brain, cranial nerves, and meninges.

Directional Terms for the Brain

• Ventral (toward the belly): above the midbrain is inferior, and below is anterior.
• Dorsal (toward the back): above the midbrain is superior, and below is posterior.
• Rostral (toward the nose): above the midbrain is anterior, and below is superior.
• Caudal (toward the tail): above the midbrain is posterior, and below is inferior.

Cerebrum

• Composed of gray matter (neuron cell bodies) and white matter (myelinated axons).
• Arranged in gyri (ridges) and sulci (valleys).
• Lateral sulcus separates the frontal and parietal lobe from the temporal lobe.
• Central sulcus separates the frontal lobe from the parietal lobe.
• Longitudinal Fissure separates the right and left hemispheres, connected by the corpus collosum, and contains the falx cerebri.
• The lobes include frontal, parietal, temporal, occipital, and insular.

Diencephalon

• Diencephalon: the central core of the brain.
• Includes the epithalamus, thalamus, and hypothalamus.

Brainstem

• The brainstem consists of the midbrain, pons, and medulla oblongata.

Cerebellum

• Cerebellum: has 2 hemispheres united by the vermis.

Ventricular System

• Choroid plexuses within the ventricles secrete CSF.
• Includes two lateral ventricles, the 3rd ventricle, and the 4th ventricle.

Meninges

• Meninges: connective tissue covers of the brain and spinal cord.
• Dura Mater (tough mother): tough, thick external fibrous layer and thickest covering.
  • Consists of 2 layers, the periosteal (outer) and meningeal (inner).
  • Periosteal layer formed by periosteum of the skull, adheres to the inner surface of the cranium but is not continuous with the spinal cord.
  • Meningeal layer in contact with the Arachnoid Mater, continuous with dura of spinal cord, and continuous with one another except for at the dural venous sinuses and dural septae.
• Arachnoid Mater (spider mother): thin, intermediate layer.
  • Separated by Pia Mater by subarachnoid space, which contains CSF.
• Pia Mater (tender mother): delicate internal vascular layer most closely associated with external surface of brain.

Meningeal Spaces

• Epidural (extradural) space: between periosteal dura and bone of the cranium.
  • It is a potential space.
  • Epidural hematomas can form when torn meningeal vessels bleed into the space between the dura and the cranium.
• Subdural space: between dura and arachnoid mater.
  • It is a potential space.
  • Subdural hematomas can form when torn cerebral bridge veins push the dura away from the arachnoid mater.
• Subarachnoid space: between the arachnoid and pia mater.
  • It is a true space that contains CSF and cerebral blood vessels.
  • Subarachnoid hemorrhages can form when cerebral arteries rupture and bleed into the subarachnoid space.

Divisions of the Cranial Cavity

• Dural septae formed by meningeal layer of dura mater.
• Falx Cerebri runs from Crista galli anteriorly to the Tentorium cerebelli posteriorly, located between R/L cerebral hemispheres.
• Tentorium Cerebelli runs between occipital bone, temporal bone and sphenoid bone and separates the cerebellum from occipital lobes.
• Falx Cerebelli runs from tentorium cerebelli to occipital bone and separates cerebellar hemispheres.
• Diaphragma Sellae forms partial roof over hypophysial fossa and covers the hypophyseal fossa.

Vasculature of the Dura

• Meningeal Arteries run in periosteal dura layer.
  • Anterior Meningeal A. supplies the anterior cranial fossa.
  • Middle & Accessory Meningeal As. supplies the middle cranial fossa; the Middle Meningeal A. associated with Pterion
  • Posterior Meningeal A. supplies the posterior cranial fossa.
• Meningeal veins run with meningeal arteries.

Vasculature of the Brain

• Arterial Supply to the Brain:
  • Vertebral A arises from Subclavian A. and enters the skull through Foramen Magnum.
    • They come together to form the Basilar A., which supplies the cerebellum and brainstem.
    • Basilar A. branches to form the Posterior Cerebral Arteries, which supply the inferior surface and occipital pole
  • Internal Carotid A. - arises from Common Carotid A. and enters the skull through the Carotid Canal.
    • Terminal branches are Anterior and Middle Cerebral Arteries.
      • Anterior Cerebral Artery supplies medial and superior surfaces and frontal pole.
      • Middle Cerebral Artery supplies lateral surface and temporal pole.

Dural Venous Sinuses

• Cerebral veins drain the brain into dural venous sinuses.
• Spaces found between the periosteal and meningeal dura, mostly along dural infolds.
• Sinuses: Superior sagittal, Inferior sagittal, Straight, Transverse, Sigmoid, Confluence of Sinuses.
• Confluence of Sinuses – meeting of superior sagittal, straight, occipital, and transverse sinuses
• Cavernous sinus Located on either side of sella turcica.
  • Cavernous sinus contains the ICA, CN III, CN IV, CN V1, CNV2, CNVI
  • Receives blood from cerebral veins, ophthalmic veins, and emissary veins, providing a pathway for infection into cranial cavity.
  • Surrounds the pituitary gland and sphenoid sinuses, which can compress structures in the cavernous sinus and cause paralysis of the extraocular muscles and sensory loss of the forehead and maxillary region.
• Receives blood from veins draining the scalp and brain as well as CSF from arachnoid granulations and drains into the Internal Jugular V.

Cranial Floor & Fossae

• Anterior Cranial Fossa comprised of the Frontal, ethmoid, and sphenoid bone that contains the frontal lobe of the brain.
  • Important landmarks are: Cribriform plate – passage of CN 1 (olfactory), Crista galli – projection from cribriform plate for attachment of falx cerebri
• Middle Cranial Fossa contains the Sphenoid & Temporal bones that contains the Temporal lobes of brain, hypothalamus and pituitary gland.
  • Important landmarks are: Sella turcica – portion of the sphenoid bone with hypophyseal fossa for pituitary gland, Optic canal for passage of CN 2 (optic), Carotid canal for passage of the Internal Carotid A. (ICA) and sympathetic nerves, Foramen Spinosum – Middle Meningeal A., Foramen Rotundum – Maxillary Division of Trigeminal N. (CN V2), Superior Orbital Fissure – Ophthalmic Division of Trigeminal N. (CN V1), Oculomotor N. (CN III), Trochlear N. (CN IV), Abducent N. (CN VI), Foramen Ovale – Maxillary Division of Trigeminal N. (CN V3)
• Posterior Cranial Fossa comprised of the Temporal, Occipital, and some sphenoid and parietal that contains the Cerebellum, Medulla and pons (occipital lobe sits on top of cerebellum).
  • Important Landmarks: Foramen magnum – medulla/spinal cord and vertebral arteries, Accessory N. (CN XI), Internal acoustic meatus passage of Facial N. (CN VII) and Vestibulocochlear N. (CN VIII), Jugular foramen passage of CN IX (Glossopharyngeal N.), CN X (Vagus N.), CN XI (Accessory), and internal jugular veins, Hypoglossal canal – CN XII (hypoglossal)

Cranial Nerves

• 12 pairs numbered I-XII from rostral to caudal according to their attachment to the brain
  • Are motor, sensory, or mixed.
    • Mnemonic: Some Say Marry Money, But My Brother Says Big Brains Matter More
• CN I: Olfactory - sensory
• CN II: Optic - sensory
• CN III: Oculomotor - motor
• CN IV: Trochlear - motor
• CN V: Trigeminal: - both
  • Ophthalmic division (CN V₁) – sensation from cornea, skin of forehead, scalp, eyelids, nose
  • Maxillary division (CN V2) – sensation of skin of face over maxilla
  • Mandibular division (CN V3) – sensation of skin over mandible, motor to muscles of mastication
• CN VI: Abducent - motor
• CN VII: Facial - both
• CN VIII: Vestibulocochlear - sensory
• CN IX: Glossopharyngeal - both
• CN X: Vagus - both
• CN XI: Spinal accessory - motor
• CN XII: Hypoglossal - motor