Brain Organization, Protection, and Blood Supply

Central Nervous System: Development

The brain and spinal cord develop from the ectodermal neural tube

Brain: Primary Brain Vesicles

The anterior region of the neural tube would then expand to form three regions: prosencephalon, mesencephalon, and rhombencephalon

Brain: Secondary Brain Vesicles

Both the prosencephalon and rhombencephalon subdivide further; the prosencephalon (forebrain) forms the telencephalon and diencephalon, and the rhombencephalon (hindbrain) forms the metencephalon and myelencephalon

Brain: Structural Development: Telencephalon

The telencephalon develops into the cerebrum from the secondary brain vesicle

Brain: Structural Development: Diencephalon

The diencephalon forms the thalamus, hypothalamus, epithalamus, and third ventricle from the secondary brain vesicle

Brain: Structural Development: Mesencephalon

The mesencephalon gives rise to the midbrain from the primary brain vesicle

Brain: Structural Development: Metencephalon

The metencephalon becomes the pons, cerebellum, and upper part of the fourth ventricle from the secondary brain vesicle

Brain: Structural Development: Myelencephalon

The myelencephalon forms the medulla oblongata and the lower part of the fourth ventricle from the secondary brain vesicle

Brain Development: Nervous Tissue

The walls of the developing brain regions form nervous tissue.

Brain Development: Ventricles

The hollow interior of the developing neural tube becomes the brain's ventricles, which are fluid-filled spaces.

Brain Development: Neural Crest Tissue

The neural crest tissue expands and becomes especially important during development, as most skull bones and meningeal membranes develop from it

Brain: Four Major Parts

The brain consists of a brainstem, cerebellum, diencephalon, and cerebrum

Brain: Four Major Parts: Brain Stem

The brainstem is continuous with the spinal cord that consists of the medulla oblongata, pons, and midbrain

Brain: Four Major Parts: Cerebellum

The cerebellum is posterior to the brainstem

Brain: Four Major Parts: Diencephalon

The diencephalon is superior to the brainstem that consists of the thalamus, hypothalamus, and epithalamus

Brain: Four Major Parts: Cerebrum

The cerebrum is the largest part of the brain that is supported by the diencephalon and brainstem

Cranial Meninges

Three protective membranes that surround and support the brain inside the skull that are continuous with the spinal meninges

Cranial Meninges: Layers

From superficial to deep, the cranial meninges are dura mater, arachnoid mater, and pia mater, similar to the spinal meninges

Cranial Meninges: Dura Mater

However, the cranial dura mater is the outermost meningeal layer and has two layers (periosteal and meningeal), unlike the spinal dura mater, which has one layer

Cranial Meninges: Dura Mater: Periosteal Layer

The periosteal layer is the external layer of the cranial dura mater and is attached to the inner surface of the skull.

Cranial Meninges: Dura Mater: Meningeal Layer

The meningeal layer is the internal layer of the cranial dura mater and lies deep to the periosteal layer.

Cranial Meninges: Dura Mater: Function

The two layers are fused together, except when they separate to enclose the dural venous sinuses to drain venous blood from the brain into the jugular veins

Cranial Meninges: Dura Mater: Surface

Brain blood vessels travel along the surface of the brain before entering the brain tissue that penetrate through a loose-fitting sleeve of pia mater

Cranial Meninges: Dura Mater: Extensions

Three extensions of the dura mater separate parts of the brain:

• The falx cerebri separates the two hemispheres of the cerebrum

• The falx cerebelli separates the two hemispheres of the cerebellum

• The tentorium cerebelli separates the cerebrum from the cerebellum

Brain Blood Supply and Drainage

The brain receives most of its blood through the internal carotid and vertebral arteries; it drains via dural venous sinuses into the internal jugular veins, which return it to the heart.

Brain: Oxygen and Glucose Use

The brain uses about 20% of the body's oxygen and glucose even while at rest.

Brain: Neuron Activity

When neuron and neuroglia activity increases in a specific brain region, blood flow to that region also increases.

Brain Blood Flow: Brief Reduction

A brief reduction in brain blood flow can cause disorientation or loss of consciousness; 1-2 minutes can impair neuronal function, and 4 minutes can cause permanent brain injury.

Brain: Glucose Storage

The brain stores very little glucose, so it requires a continuous supply of glucose from the blood.

Brain Glucose: Decrease in Blood Glucose

Low blood glucose entering the brain can cause mental confusion, dizziness, convulsions, and loss of consciousness, and a rapid drop in blood glucose can cause diabetic shock, which may involve seizures, coma, and possibly death.

Blood-Brain Barrier

A selective barrier that controls which substances can pass from the blood into brain tissue, composed of tight junctions between endothelial cells of brain capillaries and a thick basement membrane surrounding the capillaries.

Blood-Brain Barrier: Function

The BBB allows certain substances to enter brain tissue while restricting or preventing others.

Blood-Brain Barrier: Lipid-Soluble Substances and Water

Lipid-soluble substances, such as O₂, CO₂, steroid hormones, alcohol, nicotine, and caffeine, as well as water, can easily cross the BBB by diffusing through endothelial cell membranes.

Blood-Brain Barrier: Glucose

Glucose, a water-soluble substance, crosses the BBB rapidly through facilitated transport.

Blood-Brain Barrier: Proteins and Drugs

Proteins and most antibiotic drugs normally cannot pass from the blood into brain tissue through the BBB.

Blood-Brain Barrier: Breakdown

Trauma, certain toxins, and inflammation can damage or break down the BBB.

Blood-Brain Barrier: Selective Permeability

Lipid-soluble substances cross easily, glucose crosses by facilitated transport, most ions cross slowly, and proteins and most antibiotics do not cross.