Anatomy of the Brain and Meninges

The Brain

• The brain is protected by cranial bones and cranial meninges
• Three membranous layers surrounding the brain are called meninges
• Meninges protect and support the brain, preventing infections and hold cerebrospinal fluid.

The Meninges

• Cranial meninges are continuous with spinal meninges
• Consist of dura mater, arachnoid mater and pia mater
• Ordered from superficial to deep: dura mater, arachnoid mater, and pia mater
• Dura mater (outermost layer) holds the brain in place
• Arachnoid mater is the middle layer with a spider web-like appearance
• Pia mater is the innermost, very thin and transparent layer, directly on the brain's surface
• Subarachnoid space is between pia mater and arachnoid mater, where cerebrospinal fluid (CSF) circulates

Cranial Bones and Meninges

• Diagram shows cranial meninges layers (dura mater, arachnoid mater, pia mater) and the skull

Blood Flow to the Brain

• Blood flows to the brain through blood vessels branching at the cerebral arterial circle (circle of Willis) at the base of the brain
• Veins return blood from the head to the heart
• The brain uses 20% of the body's oxygen supply
• Interruption of oxygen supply causes weakening, permanent damage, or death of brain cells
• Glucose deficiency can cause mental confusion, dizziness, convulsions, and unconsciousness

Blood Brain Barrier (BBB)

• Protects brain cells from harmful substances and pathogens
• Acts as a selective barrier, preventing many substances from blood to brain
• Nutrients, some amino acids, and electrolytes enter the cerebrospinal fluid (CSF); metabolic waste like urea, creatinine, and toxins cannot.
• Prevents drugs from entering for brain cancer or other central nervous system (CNS) disorders
• Injury, inflammation, or toxins cause BBB breakdown, allowing normally restricted substances into brain tissue.

Cerebrospinal Fluid (CSF)

• Formed by filtration from choroid plexuses, located in ventricle walls
• Choroid plexuses produce CSF
• Ependymal cells form a blood-CSF barrier, controlling substances passing from blood to CSF
• CSF bathes the brain and spinal cord providing mechanical and chemical protection, and circulation of essential nutrients

Principal Parts of the Brain

• Cerebrum, Diencephalon, Brainstem, and Cerebellum (diagram)
  • Diagram shows the location and relative sizes of each of these brain parts

Cerebrum

• Largest part of the brain (83% of the total mass)
• Split into two cerebral hemispheres by the longitudinal fissure
• Each hemisphere is divided into lobes
• Surface is grey matter, deeper is white matter
• Surface is convoluted to increase surface area (more neurons)
• The cerebrum is the conscious brain
• Gyri and sulci (folds and grooves) distinguish between different lobes

Lobes of the Cerebrum

• Frontal Lobe: Largest, at the front, higher-order functioning, memory, voluntary movements
• Parietal Lobe: Separated by the parietal-occipital sulcus, integrated sensations, complex mathematics, reading, writing, and visual skills
• Temporal Lobe: Contains auditory cortex, special senses (hearing, smell), influences personality and emotional response
• Occipital Lobe: Contains the primary visual cortex in the posterior region, visual association area for interpretation

Cerebellum

• Small part of the brain, beneath the occipital lobe, Latin for "little brain"
• Connected to the cerebrum via the brainstem
• Similar anatomy to the cerebrum, with two hemispheres
• Unconscious (subconscious) brain
• Learns its skills
• Functions include muscle coordination, balance, equilibrium, and making motor movements more efficient

Brainstem

• Positioned at the base of the brain, just above the spinal cord
• Consists of midbrain, pons, and medulla oblongata
• Contains vital centers regulating heartbeat, blood pressure, and breathing.
• Also responsible for reflexes (swallowing, coughing, sneezing, blinking).

Diencephalon

• Composed of thalamus, hypothalamus, and epithalamus
• Thalamus: Relays almost all sensory input to cerebral cortex, motor functions, and consciousness
• Hypothalamus: Controls autonomic nervous system, hormone production (releasing/inhibiting, oxytocin, ADH), regulates emotions/behavior, feeding/thirst/temperature (body's thermostat).
• Epithalamus: Consists of the pineal gland (secretes melatonin), habenular nuclei (involved in olfaction).