Unit 2. Organization and brain coverings, Universidad Europea Presentation

Index

• Bone protection
• Meninges
• Ventricular system
• Venous drainage of the brain
• Blood supply
• Questions

Bone Protection

Introduction to Bone Protection

The central nervous system (CNS) is sustained and protected by bones and membranous meninges. In fact, the brain is localized inside the cranial cavity and the spinal cord in the vertebral canal of the vertebral column. Inside their bony coverings, the brain and the spinal cord are surrounded by three protective membranes: the meninges.

The skull supports the structures of the face like nerves, sensory organs, cavities of the digestive and respiratory systems and mimic muscles and muscles of mastication. The skull is composed of two parts: the neurocranium and the viscerocranium or facial skeleton.

Neurocranium Bones

The neurocranium is composed of eight bones: four are single bones: frontal, occipital, sphenoid and ethmoid bones. Two are paired bones: parietal and temporal bones.

Single Bones of the Neurocranium

• Frontal
• Occipital
• Sphenoid
• Ethmoid

Paired Bones of the Neurocranium

• Parietal
• Temporal

Neurocranium - Frontal Bone

The frontal bone is located in the anterior part of the neurocranium. Two main parts are distinguished: the superior portion articulates with the two parietal bones by the coronal suture and the orbital portion that curves downward to make the upper margins of the orbits.

Neurocranium - Ethmoid Bone

The ethmoid bone is located in the anterior part of the base of the cranium, where the small cribriform plate has apertures through which olfactory nerve fasciculi enter the cranium. The crista galli is a sharp upward projection for the attachment of the falx cerebri.

Neurocranium - Sphenoid Bone

The sphenoid bone has a bat shape having a centrally placed body with greater and lesser wings on its sides. It is located in the middle of the cranial floor and has a deep depression, the sella turcica which lodges the hypophysis.

Neurocranium - Parietal Bones

The two parietal bones form the sides and roof of the cranium and articulate with each other in the midline at the sagittal suture. Posteriorly, they articulate with the occipital bone at the lambdoid suture.

Neurocranium - Temporal Bones

The temporal bones are at the lateral side of the cranium. Several parts can be distinguished: the squamous, the petrous and the tympanic parts and the mastoid, the styloid and the zygomatic processes.

Neurocranium - Lateral View

1. Zygomatic process
2. Mastoid process
3. Styloid process

Neurocranium - Cranial Fossa

The brain sits over the base of the skull, that is divided into three cranial fossae. The anterior cranial fossa is formed by the frontal, the ethmoid and the sphenoid bones and contains the two frontal lobes of the cerebrum. The middle cranial fossa is formed by the temporal and the sphenoid bones and hosts the temporal lobes. The posterior cranial fossa is formed by the occipital and the petrous parts of the temporal bones. It hosts the cerebellum and contains the foramen magnum where the brainstem lodges.

Vertebral Column

Functions of the Vertebral Column

The vertebral column is the central bony pillar of the body and has several functions.

• Support for the skull
• Support for the upper limbs
• Support for the thoracic cage
• Transmission of body weight to the lower limbs
• Protection of the spinal cord

The vertebral column laterally has four curves (kyphosis or lordosis): the cervical, thoracic, lumbar and sacral curves. When viewed from the back, the vertebral column runs straight.

Vertebral Column Composition

The vertebral column is composed of 33 vertebrae being the 5 sacral fused to form the sacrum and the lower 3 coccygeal fused too. The vertebrae are joint by fibrocartilage intervertebral discs, making it a flexible structure.

• 7 x cervical vertebrae
• 12 x thoracic vertebrae
• 5 x lumbar vertebrae
• 5 x sacral vertebrae
• 4 x coccygeal vertebrae

Cervical Vertebrae C1 and C2

The two first cervical vertebrae are slightly different, because they have to sustain the skull but, at the same time, they have to permit its rotation. C1 vertebra is also known as atlas and lacks the body. C2 vertebra is also known as axis, is very robust and has the odontoid process or dens, that is articulated with the vertebral arch of atlas allowing the skull rotation.

Meninges

Meninges of the Brain

The brain, inside the cranium, is surrounded by three protective membranes that are called meninges. The external one is the dura mater and is a hard fibrous membrane. The arachnoid mater is the middle layer and is a spider-like membrane composed of processes called arachnoid trabeculae. The dura mater and the arachnoid mater are separated by the subdural space. The internal meninge is the pia mater, a fine and delicate membrane intimately attached to the brain. The arachnoid and the pia mater are separated by the subarachnoid space, which is filled with cerebrospinal fluid (CSF).

The dura mater is a hard and fibrous membrane, while the arachnoid mater is delicate and semitransparent.

Meninges of the Brain - Dura Mater

The dura mater is composed of two layers, periosteal layer, that covers the internal layer of the cranium bones and the meningeal layer or the dura mater proper. In the cavity of the neurocranium, the meningeal layer has extensions inserted in brain fissures that separate different parts of the brain. For example, the falx cerebri is the dura mater extension, inserted in the longitudinal fissure that separates both cerebral hemispheres.

Meninges of the Brain - Hemorrhages and Hematomas

Location of epidural hematoma Location of subdural hematoma Position of subarachnoid hemorrhage