Lecture on Brain Structure and Function

Components of the Brain

  • The brain consists of two main components: gray matter and white matter.

    • Gray Matter

    • Makes up approximately 40% of the brain.

    • Forms a surface layer covering the cerebrum and cerebellum, known as the cortex, as well as nuclei deep within the brain.

    • Contains:

      • Cell bodies of neurons.

      • Dendrites.

      • Synapses connecting neurons to each other.

    • Appears darker due to relatively low presence of myelin compared to white matter.

    • White Matter

    • Constitutes roughly 60% of the brain.

    • Located in the deeper regions, subcortical areas, beneath the cortex.

    • Characteristic white color due to high lipid fat content (myelin).

    • Contains:

      • Axons or nerve fibers that bundle together to form tracts.

      • Facilitates communication between different brain regions.

      • Example: The corpus callosum, a bundle of axons connecting the left and right cerebral hemispheres, enabling transfer of information between them.

  • Comparison of Gray and White Matter in the Brain and Spinal Cord:

    • Brain:

    • Gray matter primarily located on the outer surface (except certain deeper nuclei).

    • White matter situated internally.

    • Spinal Cord:

    • Arrangement is reversed; gray matter is encased within white matter.

Terminology Distinctions in the Central and Peripheral Nervous Systems

  • Groups of neuron cell bodies:

    • Central Nervous System (CNS): referred to as nuclei.

    • Peripheral Nervous System (PNS): referred to as ganglia.

  • Bundles of axons:

    • CNS: termed tracts.

    • PNS: form peripheral nerves.

The Meninges

  • Composed of three membranes enclosing the brain and spinal cord:

    • Functions:

    • Protection.

    • Structural framework for blood vessels.

    • Facilitation of cerebrospinal fluid (CSF) circulation.

  • Layers of Meninges:

    • Dura Mater: Outermost layer, beneath cranial bone.

    • Comprises two layers:

      • Periosteal layer: akin to the periosteum covering bone surfaces.

      • Meningeal layer.

    • Contains the superior sagittal sinus

    • Subdural Space: Space between the dura mater and the arachnoid mater.

    • Arachnoid Mater: Middle layer, encasing the brain.

    • Contains the subarachnoid space, filled with CSF and houses all cerebral arteries and veins.

    • Pia Mater: Innermost layer, adheres closely to the brain surface.

    • Composed of delicate connective tissue and contains numerous tiny blood vessels.

    • Translates to "tender matter".

Cerebrospinal Fluid (CSF)

  • Description: Transparent, colorless liquid serving as a protective cushion for the brain and spinal cord; provides nourishment.

  • Location: Found within brain ventricles.

    • Total of four ventricles in the brain:

    • Two lateral ventricles.

    • One third ventricle.

    • One fourth ventricle.

  • Production of CSF:

    • Generated from blood plasma within choroid plexuses, located in each ventricle.

    • Choroid plexus comprises a network of blood vessels and ependymal cells, responsible for producing and secreting CSF.

  • Circulation of CSF:

    • Starts journey in the lateral ventricles.

    • Moves through the foramina of Monro (interventricular foramen) to reach the third ventricle.

    • Continues through the aqueduct of Sylvius (cerebral aqueduct) to the fourth ventricle.

    • Connects to the spinal canal via the fourth ventricle with openings called lateral and median apertures.

  • Reabsorption of CSF:

    • Occurs through protrusions known as arachnoid granulations, primarily found within major venous sinuses like the superior sagittal sinus.

    • Arachnoid granulations facilitate removal of excess CSF from subarachnoid space into bloodstream, maintaining appropriate pressure and composition of CSF.

Dural Sinuses (Venous Sinuses)

  • Comprise large, thin-walled veins located within the dura mater.

  • Role: Drain deoxygenated blood and CSF from the brain, returning it to systemic circulation.

  • Arachnoid granulations possess villi penetrating the dura mater and extending into venous sinuses.

Blood-Brain Barrier (BBB)

  • Encloses inner lining of microvessels (capillaries) throughout the brain.

  • Exhibits selective permeability, controlling substance passage from the bloodstream to extracellular fluid of the brain.

  • Only lipid soluble, low molecular weight molecules, and positively charged particles can cross the BBB without assistance.

  • Certain areas (circumventricular organs) lack the BBB, permitting direct blood access to neurons.

  • Components of BBB:

    • Endothelial cells: Form non-fenestrated capillary wall.

    • Astrocyte end feet: Wrap around capillaries.

    • Pericytes: Surround blood capillaries.

Blood-Cerebrospinal Fluid (CSF) Barrier

  • Exists at the choroid plexus and composed of ependymal cells with tight junctions facing the ventricles.

  • Vasculature of choroid plexus consists of fenestrated capillaries.

  • Functions similarly to BBB but less selective due to fenestrated endothelium.

  • Aids in the exchange and removal of metabolites while preventing blood-borne substances from entering the brain.

Embryonic Brain Development

  • Neural tube enlargements, referred to as vesicles, form the basis of the brain's complexity.

    • Primary Vesicles:

    • Prosencephalon: Forebrain.

    • Mesencephalon: Midbrain.

    • Rhombencephalon: Hindbrain.

Brain Anatomy: Brainstem

  • Components: Midbrain, pons, medulla oblongata.

  • Functions: Regulation of essential life-sustaining functions.

  • Midbrain:

    • Houses the cerebral aqueduct linking third and fourth ventricles.

    • Features:

    • Superior and inferior colliculi on posterior surface - visual and auditory reflex centers.

    • Cerebral peduncles on anterior surface containing descending pyramidal tracts (corticospinal tract).

    • Called corpora quadrigemina (Latin for quadruplet bodies).

  • Pons:

    • Located above medulla and below midbrain.

    • Function: Transmission of signals between forebrain and cerebellum; regulates respiration rate and depth with medulla.

  • Medulla Oblongata:

    • Lowest segment of brainstem; relays signals between spinal cord and brain.

    • Controls autonomic functions: respiration, heart rate, coughing, vomiting, and digestion.

    • Contains pyramidal tracts, including corticospinal tract; has a crucial crossing point known as decussation of the pyramids.

Reticular Formation and Reticular Activating System (RAS)

  • Comprises a net-like structure of brainstem nuclei extending throughout the brainstem.

  • Functions:

    • Regulation of muscle activity.

    • Transmission of sensory information to the cerebral cortex.

    • Filters repetitive stimuli through habituation.

  • RAS: Critical for maintaining alertness and consciousness; coordinates sleep-wake cycles. Injury affects consciousness potentially causing coma.

Cerebellum Anatomy and Function

  • Cortex consists of gray matter with folds called folia; white matter resembles tree-like structure known as arbor vitae (Latin for tree of life).

  • Layers of Cerebellar Cortex:

    • Molecular layer.

    • Purkinje cell layer.

    • Granular layer.

  • Cerebellar Vermis: Central portion bridging right and left hemispheres.

  • Cerebellar Peduncles: Connect cerebellum to brainstem.

  • Function: Processes sensory information regarding body position and movement; ensures motor coordination and balance, related to unconscious proprioception through spinocerebellar tract.