The Brain and Cranial Nerves: Anatomy & Physiology Study Notes

Brain Anatomy

Overview of the Adult Human Brain

The adult human brain is the central organ of the nervous system, responsible for processing sensory information, coordinating movement, and regulating vital functions.

• Neural Tissue: The brain contains almost 97 percent of the body's neural tissue.

• Typical Brain:

  • Average weight: 1.4 kg (3 lb.)

  • Average volume: 1200 mL

• Size Variation: Brain size varies among individuals. On average, male brains are about 10 percent larger than female brains, primarily due to differences in body size.

• Intelligence: There is no correlation between brain size and intelligence.

Brain Development

Formation and Early Structure

The brain develops from a hollow neural tube, a process that begins early in embryonic development and is visible by the fourth week.

• Neurocoel: The central cavity within the neural tube is called the neurocoel.

• Primary Brain Vesicles: Rapid expansion of the neurocoel in the cephalic (head) region creates three primary brain vesicles:

  • Prosencephalon (forebrain)

  • Mesencephalon (midbrain)

  • Rhombencephalon (hindbrain)

• These vesicles are named according to their positions along the neural tube.

Primary Brain Vesicles at 4 Weeks of Development

At four weeks, the brain consists of three distinct regions:

• Prosencephalon: Forms the forebrain.

• Mesencephalon: Forms the midbrain.

• Rhombencephalon: Forms the hindbrain.

• Spinal Cord: Extends caudally from the rhombencephalon.

• Neurocoel: The central cavity running through these regions.

Secondary Brain Vesicles and Adult Brain Structures

As development continues, the primary vesicles subdivide into secondary vesicles, which give rise to major adult brain regions.

Primary Vesicle	Secondary Vesicle	Adult Brain Structure	Associated Ventricles
Prosencephalon	Telencephalon	Cerebrum (cerebral hemispheres, cortex, basal nuclei)	Lateral ventricles
Prosencephalon	Diencephalon	Diencephalon (thalamus, hypothalamus, epithalamus)	Third ventricle
Mesencephalon	Mesencephalon	Midbrain	Cerebral aqueduct
Rhombencephalon	Metencephalon	Pons, cerebellum	Fourth ventricle
Rhombencephalon	Myelencephalon	Medulla oblongata	Central canal

Additional info: The subdivision of brain vesicles is a critical process in neurodevelopment, as it determines the organization and specialization of the adult brain's major regions.