The Brain: Structure, Circulation, and Protection (Module 7 – Part I, ANPH 8100)

The Brain: Structure, Circulation, and Protection

Introduction to the Brain

The brain is the central organ of the nervous system, responsible for integrating sensory information, coordinating bodily functions, and enabling cognition, emotion, and consciousness. It weighs approximately 3.5 pounds in adults and contains about 100 billion cells. Despite differences in wiring, males and females have the same amount of brain tissue.

• Brain function determines life and death; without brain activity, life cannot be sustained.

• The brain constitutes 2% of body weight but receives 20% of the body's blood supply.

• All other bodily activities serve to protect and support the brain.

Brain Structure and Appearance

Reasons for Brain Morphology

• Restricted cranial space causes neural tissues to grow back over and cover the diencephalon (hypothalamus, thalamus, epithalamus) and midbrain.

• Continued growth leads to convolutions (gyri and sulci), increasing the surface area and allowing more neurons to fit within the limited space, similar to the villi in the small intestine.

Cerebral Circulation

Major Arteries Supplying the Brain

• The brain is supplied by two internal carotid arteries and two vertebral arteries.

• The internal carotid artery branches into several arteries, including the anterior and middle cerebral arteries, supplying various cortical regions.

• The vertebral arteries join to form the basilar artery, supplying the brainstem, cerebellum, and posterior cerebral regions.

The Circle of Willis

• A circular arterial structure at the base of the brain that provides collateral circulation.

• Common site for cerebral aneurysms, especially at arterial bifurcations.

• If one part is blocked, collateral flow can often compensate, reducing ischemic risk.

Cerebral Blood Flow (CBF)

• Normal adult CBF: 750 mL/min (about 15% of cardiac output).

• Regulated to meet metabolic demands of the brain.

• Too much blood (hyperemia) increases intracranial pressure (ICP), risking tissue damage.

• Too little blood (ischemia) below 18–20 mL/100g/min leads to dysfunction; below 8–10 mL/100g/min causes tissue death.

Factors Affecting Cerebral Blood Flow

• Viscosity of blood

• Degree of vessel dilation/constriction

• Cerebral perfusion pressure (CPP): determined by systemic blood pressure and intracranial pressure

• Autoregulation: cerebral vessels constrict when systemic BP rises and dilate when it falls; they also respond to chemical changes (e.g., CO2 levels)

Meningeal Layers

Dura Mater

• Double-layered outer membrane anchoring the brain to the skull

• Four extensions:

  • Falx cerebri: attaches to crista galli of ethmoid bone

  • Falx cerebelli: runs down the center of the cerebellum

  • Tentorium cerebelli: separates cerebral hemispheres from cerebellum

  • Diaphragma sellae: lines sella turcica, encircling the pituitary gland

Arachnoid Mater

• Middle meningeal layer

• Contains arachnoid villi that protrude into dural sinuses, allowing CSF absorption into venous blood

Pia Mater

• Delicate, innermost layer with fine blood vessels

• Clings tightly to the brain surface

Functions of the Meninges

• Protect CNS organs

• Enclose venous sinuses

• Contain cerebrospinal fluid (CSF)

• Form partitions within the skull

Blood-Brain Barrier (BBB)

Structure and Function

• Protective mechanism maintaining a stable brain environment

• Composed of capillary endothelial cells with tight junctions, basement membranes, neuroglial membranes, and astrocytes

Permitted and Restricted Substances

• Permitted: water, glucose, essential amino acids, some electrolytes

• Restricted: urea, creatinine, proteins, toxins, most antibiotics and drugs

Exceptions and Clinical Relevance

• BBB is absent in the hypothalamus to allow blood sampling for homeostatic regulation

• BBB is immature in newborns and premature infants, increasing CNS vulnerability to toxins

Cerebrospinal Fluid (CSF)

Functions

• Surrounds brain and spinal cord, providing buoyancy and protection

• Prevents brain from crushing under its own weight

• Protects CNS from trauma

• Delivers nutrients and removes wastes

Production and Circulation

• Produced by choroid plexuses (capillaries surrounded by ependymal cells) in the ventricles

• Normal production: 400–600 mL/day

• CSF circulates through ventricles, enters subarachnoid space, and is absorbed into venous blood via arachnoid villi

Ventricles of the Brain

• Lined with ependymal cells (supporting cells of the nervous system)

• Choroid plexuses within ventricles produce CSF

Flow of CSF

1. CSF is secreted by choroid plexus in each lateral ventricle

2. Flows through interventricular foramina into the third ventricle

3. Choroid plexus in third ventricle adds more CSF

4. CSF flows down cerebral aqueduct to fourth ventricle

5. Choroid plexus in fourth ventricle adds more CSF

6. CSF flows out two lateral apertures and one median aperture

7. CSF fills subarachnoid space, bathing brain and spinal cord

8. At arachnoid villi, CSF is absorbed into venous blood of dural sinuses

Regions of the Brain

Major Subdivisions

• Cerebrum

• Cerebellum

• Diencephalon (thalamus, hypothalamus, epithalamus)

• Brain stem (midbrain, pons, medulla oblongata)

The Cerebral Hemispheres (Cerebrum)

• Most superior part of the brain; accounts for over 60% of brain weight

• Surface marked by gyri (ridges), sulci (shallow grooves), and fissures (deep grooves)

• Divided into five lobes: frontal, parietal, temporal, occipital, and insula (buried within lateral sulcus)

Functional Areas of the Cerebrum

• Cerebral cortex: site of conscious thought, perception, voluntary movement

• Cerebral white matter: myelinated fibers connecting different brain regions

• Basal nuclei (ganglia): gray matter masses involved in motor control

Types of White Matter Fibers

• Commissural fibers: connect corresponding regions of the two hemispheres (e.g., corpus callosum)

• Association fibers: connect gyri within the same hemisphere

• Projection fibers: connect the cortex with lower brain regions and spinal cord

Diencephalon

• Thalamus: relay station for sensory information; crude awareness of sensation

• Hypothalamus: main homeostatic center; regulates autonomic functions, emotions, temperature, hunger, thirst, sleep-wake cycles, and endocrine activity

• Epithalamus: contains pineal gland; regulates circadian rhythms

Brain Stem

• Midbrain: contains nuclei for cranial nerves III and IV; part of reticular formation

• Pons: regulates breathing rhythm; origin of cranial nerves V–VII

• Medulla oblongata: autonomic reflex center; controls heart rate, blood pressure, and respiration

Cerebellum

• Coordinates voluntary movements, balance, and posture

• Compares intended movement with actual performance and initiates corrective actions

• Divided into anterior, posterior, and flocculonodular lobes; connected by the vermis

Functional Brain Systems

Limbic System

• Encircles upper brainstem and corpus callosum

• Responsible for emotional responses and memory formation

• Hypothalamus acts as the gatekeeper for emotional responses

Reticular Formation

• Network of neurons connecting the cerebral cortex, thalamus, hypothalamus, cerebellum, and spinal cord

• Regulates arousal, consciousness, and filtering of sensory input (Reticular Activating System, RAS)

• Prevents sensory overload by filtering repetitive or weak signals

Summary Table: Major Brain Regions and Functions

Region	Main Structures	Primary Functions
Cerebrum	Frontal, Parietal, Temporal, Occipital, Insula	Conscious thought, voluntary movement, sensory processing
Diencephalon	Thalamus, Hypothalamus, Epithalamus	Sensory relay, homeostasis, circadian rhythms
Brain Stem	Midbrain, Pons, Medulla Oblongata	Autonomic functions, cranial nerves, reflexes
Cerebellum	Anterior, Posterior, Flocculonodular lobes	Coordination, balance, posture

Key Terms and Definitions

• Gyri: Elevated ridges on the cerebral surface

• Sulci: Shallow grooves between gyri

• Fissures: Deep grooves separating major brain regions

• Choroid plexus: Capillary network producing CSF

• Corpus callosum: Largest commissural fiber connecting hemispheres

• Basal nuclei: Subcortical gray matter involved in motor control

• Circle of Willis: Arterial ring at the brain's base providing collateral circulation

• Blood-brain barrier: Selective barrier protecting the brain from harmful substances

Relevant Equations

• Cerebral Perfusion Pressure (CPP):

$CPP = MAP - ICP$

• Where MAP is mean arterial pressure and ICP is intracranial pressure.

Additional info: The notes above are expanded with academic context for clarity and completeness, including definitions, examples, and a summary table for exam preparation.