chp 16

Overview of the Cranial Meninges

• Buoyancy: Keeps the brain from crushing under its own weight and limits movement within the skull via encasement in a protective structure.

• Cranial Meninges: Three membranes that protect the brain; they are the dura mater, arachnoid mater, and pia mater.

Dura Mater

• Definition: Translates to "tough mother"; composed of dense regular connective tissue consisting of two layers:

  • Endosteal Layer: Also known as the periosteum, it is the outer layer surrounding the bone.

  • Meningeal Layer: The inner periosteal layer that anchors the brain.

• Function: Provides structural support and protection, creating folds to anchor the brain within the skull.

Arachnoid Mater

• Position: Located beneath the dura mater.

• Subarachnoid Space: This space is filled with cerebrospinal fluid (CSF) and is crucial for cushioning the brain.

• Arachnoid Trabeculae: Collagen fibers that maintain the structure of the arachnoid mater and prevent collapse.

Pia Mater

• Definition: Directly attached to the brain, follows its contours including gyri and sulci.

• Function: Provides support and nourishment to the brain; heavily vascularized with astrocytes (form blood-brain barrier).

Dural Sinuses

• Formation: Created by the separation of the meningeal layer from the endosteal layer in areas of major fissures, these sinuses are crucial for draining CSF and maintaining blood volume.

• Components: Includes the superior sagittal sinus, inferior sagittal sinus, transverse sinus, and occipital sinus.

Cerebrospinal Fluid (CSF) Flow

• Production: Generated at the choroid plexus in the ventricles of the brain.

• Pathway:

  • Lateral ventricle → Interventricular foramen → Third ventricle → Cerebral aqueduct → Fourth ventricle.

  • CSF can exit through the medial and lateral apertures into the subarachnoid space, which surrounds the brain and spinal cord.

• Reabsorption: CSF enters the dural sinuses through arachnoid granulations, where it is returned to the bloodstream via the internal jugular vein.

Meningeal Folds and Their Sinuses

• Falx Cerebri: A sickle-shaped fold located in the longitudinal fissure separating the two hemispheres of the cerebrum, containing superior and inferior sagittal sinuses.

• Tentorium Cerebelli: A fold that separates the cerebellum from the cerebrum at the transverse fissure, containing the transverse sinus.

• Falx Cerebelli: Located in the vermis of the cerebellum, associated with the occipital sinus.

• Diaphragma sellae: A layer of dura mater surrounding the pituitary gland, providing protection.

Central Nervous System (CNS) Structure

• Gray Matter: Comprised of neuron cell bodies where information processing occurs; referred to as nuclei in the CNS.

• White Matter: Composed of axons that transmit signals, arranged into tracts in the CNS; referred to as nerves in the peripheral nervous system (PNS).

CNS Terminology

• Nuclei vs. Ganglia: Nuclei are collections of cell bodies in the CNS; ganglia are collections of cell bodies in the PNS.

• Tracts: Bundles of axons in the CNS that transmit information; can be further classified into:

  • Arcuate Fibers: Connect gyri within the same hemisphere.

  • Longitudinal Tracts: Run from lobe to lobe within the same hemisphere.

  • Commissural Tracts: Connect corresponding areas of both hemispheres, the main tract being the corpus callosum.

Projection Tracts

• Definition: Connect the brain to the brainstem, crucial for motor coordination and somatic movements.

• Examples: Includes important pathways for sensory input and motor output.

Cranial Nerves

• Identification: Each cranial nerve (CN) is designated by "CN" or "N" followed by a Roman numeral.

• Functionality Overview: Nerves may be sensory (e.g., olfactory nerve for smell), motor (e.g., controlling muscle movement), or mixed.

Recap of Functions and Connections

• Sensory input is processed in the brain (mainly in the parietal lobe for somatosensory).

• Motor commands originate from the frontal lobe and are executed through the body.

• Coordination between cerebral hemispheres and various brain structures is crucial for complex functions like movement and sensation.

• Understanding these intricate connections is essential for advanced comprehension of neurobiology and anatomy in the CNS.