lec 12-13 Organization of the CNS handout (2024)

Organization of the CNS

Goals

• Learn the general organization of the CNS.

• Identify the three meningeal membranes.

• Describe the formation and roles of cerebrospinal fluid (CSF).

• Understand the formation and roles of the blood-brain barrier (BBB).

• Analyze the anatomy of a section of the spinal cord.

Overview of Content

Development of the CNS

Hollow Tube Development

• The CNS develops from a hollow tube, starting at the third week of embryonic development.

Brain Development Timeline

• By the 4th week, the human brain specializes in three primary regions:

  • Forebrain

  • Midbrain

  • Hindbrain

Further Specialization at 4-6 Weeks

• The brain continues to specialize:

  • Cerebrum develops prominently from the forebrain.

  • Diencephalon falls under the forebrain.

  • Formation of various structures including the midbrain and hindbrain.

Ventricles and Cerebrospinal Fluid

Formation and Anatomy

• Ventricles: Fluid-filled cavities (remnants of the hollow tube) within the brain.

• Central Canal: Continuation of the ventricles within the spinal cord; lined by ependymal cells.

Role of CSF

• Formed in ventricles; circulates within the subarachnoid space.

• Provides cushioning for the CNS, maintains solute concentrations, and removes waste products.

Structural Overview of CNS

Grey Matter vs White Matter

• Grey Matter: Composed of unmyelinated cell bodies, axon terminals, and dendrites; organized into nuclei.

• White Matter: Composed of myelinated axons; characterized by its whitish appearance due to myelination.

Support and Protection

• The CNS is protected by:

  • Bony structures: Skull and vertebral column.

  • Meninges: Three protective membranes (Dura mater, Arachnoid, Pia mater).

  • Cerebrospinal fluid (CSF).

  • Blood-brain barrier (BBB): Regulation of substances entering the CNS.

Meninges

Layers of Meninges

• Dura Mater: Tough outer layer; protects CNS.

• Arachnoid Mater: Middle layer with trabeculae and villi that absorb CSF.

• Pia Mater: Inner layer closely associated with brain tissue.

Clinical Relevance

• Meningitis: Infection causing swelling and pressure on the brain.

• Potential for bleeding (epidural, subdural, subarachnoid) from head injuries.

Blood-Brain Barrier

Structure and Function

• The BBB is formed by endothelial cells with tight junctions, preventing unwanted substances from entering the CNS while allowing essential nutrients to pass.

• Lipophilic substances can diffuse easily; polar substances require specialized transport mechanisms.

Regional Neuroanatomy and Function

Major Structures of the CNS

• Spinal Cord: Contains ascending and descending tracts responsible for sensory and motor information.

• Brainstem: Includes midbrain, pons, and medulla; coordinates vital functions and cranial nerves.

• Cerebellum: Involved in balance and coordinated movement.

• Diencephalon: Divided into thalamus (relay center) and hypothalamus (regulates internal environment).

• Cerebrum: The largest brain structure, involved in higher-order functions, sensory perception, and motor coordination.

Localization of Function

Functional Areas of the Brain

• Various functions are localized to specific areas in the architectural layout of the cerebral cortex, including processing of senses and coordination of movement.

• Understanding of lateralization: left hemisphere typically corresponds with language and analytical tasks, while the right hemisphere correlates with spatial abilities and emotional processing.

Broca's and Wernicke's Areas

• Critical for language processing; damage to these areas can lead to various forms of aphasia, impairing speech comprehension and production.

Cerebrospinal Fluid (CSF) Creation

• Formation Location: CSF is primarily produced in the brain's ventricles by specialized cells known as ependymal cells.

• Choroid Plexus: The majority of CSF is formed in the choroid plexus, a network of blood vessels and ependymal cells located in each of the brain's four ventricles.

• Filtration Process: Blood plasma is filtered through the ependymal cells, and ions and nutrients are actively transported to form the CSF composition.

• Composition: CSF is largely composed of water, electrolytes (like sodium and potassium), glucose, and very few proteins and cells, making it distinctly different from blood plasma.

• Circulation: Once formed, CSF circulates through the ventricles, flows into the subarachnoid space surrounding the brain and spinal cord, and is eventually absorbed into the bloodstream through the arachnoid villi.