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Chapter Fourteen – The Brain: Structure, Protection, and Cranial Nerves

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Brain and Cranial Nerves

The Adult Human Brain

The adult human brain is the central organ of the nervous system, responsible for processing sensory information, controlling motor functions, and enabling higher mental activities.

  • Volume: Ranges from 750 mL to 2100 mL

  • Neural Tissue: Contains almost 97% of the body's neural tissue

  • Average Weight: About 1.4 kg (3 lb)

Six Regions of the Brain

The brain is divided into six major regions, each with distinct functions:

  1. Cerebrum

  2. Cerebellum

  3. Diencephalon

  4. Midbrain

  5. Pons

  6. Medulla oblongata

Cerebrum

Structure and Function

The cerebrum is the largest part of the brain and is responsible for higher mental functions such as reasoning, memory, and voluntary movement.

  • Divided into: Left and right cerebral hemispheres

  • Surface Layer: Gray matter called the neural cortex or cerebral cortex

Neural Cortex

  • Gyri: Elevated ridges that increase surface area

  • Sulci: Shallow depressions

  • Fissures: Deep grooves

Cerebellum

Structure and Function

The cerebellum is the second largest part of the brain, coordinating repetitive body movements and maintaining balance.

  • Hemispheres: Two, covered with cerebellar cortex

Diencephalon

Structure and Divisions

The diencephalon is located beneath the cerebrum and cerebellum, linking the cerebrum with the brain stem.

  • Divisions:

    1. Left thalamus

    2. Right thalamus

    3. Hypothalamus

Thalamus

  • Relays and processes sensory information

Hypothalamus

  • Hormone production

  • Emotion regulation

  • Autonomic function

Pituitary Gland

  • Major endocrine gland connected to hypothalamus via infundibulum (stalk)

  • Interfaces nervous and endocrine systems

Brain Stem

Structure and Function

The brain stem processes information between the spinal cord and higher brain regions.

  • Includes: Midbrain, Pons, Medulla oblongata

Midbrain (Mesencephalon)

  • Processes sight, sound, and associated reflexes

  • Maintains consciousness

Pons

  • Connects cerebellum to brain stem

  • Involved in somatic and visceral motor control

Medulla Oblongata

  • Connects brain to spinal cord

  • Relays information

  • Regulates autonomic functions: heart rate, blood pressure, digestion

Ventricles of the Brain

Origins and Structure

Ventricles are chambers within the brain filled with cerebrospinal fluid (CSF).

  • Neural tube: Encloses neurocoel, which expands to form ventricles lined with ependyma

  • Lateral ventricles: One in each cerebral hemisphere, separated by septum pellucidum

  • Third ventricle: Located in diencephalon, communicates with lateral ventricles via interventricular foramen (foramen of Monro)

  • Fourth ventricle: Extends into medulla oblongata, continuous with central canal of spinal cord, connects with third ventricle via cerebral aqueduct

Brain Protection and Support

Physical Protection

  • Bones of the cranium

  • Cranial meninges

  • Cerebrospinal fluid (CSF)

Biochemical Isolation

  • Blood–brain barrier

The Cranial Meninges

Three layers protect the brain and are continuous with spinal meninges:

  1. Dura mater: Inner (meningeal) and outer (periosteal) fibrous layers, with venous sinuses between

  2. Arachnoid mater: Covers brain, contacts dura mater, subarachnoid space between arachnoid and pia mater

  3. Pia mater: Attached to brain surface by astrocytes

Dural Folds

  • Folded inner layer of dura mater extends into cranial cavity, stabilizing and supporting the brain

  • Contain collecting veins (dural sinuses)

  • Three largest dural folds:

    • Falx cerebri: Projects between cerebral hemispheres, contains superior and inferior sagittal sinuses

    • Tentorium cerebelli: Separates cerebellum and cerebrum, contains transverse sinus

    • Falx cerebelli: Divides cerebellar hemispheres below tentorium cerebelli

Cerebrospinal Fluid (CSF)

  • Surrounds all exposed surfaces of CNS

  • Interchanges with interstitial fluid of brain

  • Functions:

    • Cushions delicate neural structures

    • Supports brain

    • Transports nutrients, chemical messengers, and waste products

  • Choroid plexus: Specialized ependymal cells and capillaries secrete CSF into ventricles, remove waste, adjust composition, produce about 500 mL/day

  • CSF Circulation: From choroid plexus → through ventricles → to central canal of spinal cord → into subarachnoid space

  • Arachnoid villi: Extensions of subarachnoid space through dura mater to superior sagittal sinus

  • Arachnoid granulations: Large clusters of villi that absorb CSF into venous circulation

Blood Supply to the Brain

  • Supplies nutrients and oxygen via internal carotid arteries and vertebral arteries

  • Removed from dural sinuses by internal jugular veins

Brain Cerebrovascular Disease

  • Disorders interfere with blood circulation to brain

  • Stroke or cerebrovascular accident (CVA): Shuts off blood to portion of brain, causing neuron death

  • TIA (Transient Ischemic Attack): Temporary reduction in blood flow, symptoms resolve within 24 hours

Blood–Brain Barrier (BBB)

  • Isolates CNS neural tissue from general circulation

  • Formed by tight junctions between endothelial cells of CNS capillaries

  • Lipid-soluble compounds (O2, CO2), steroids, prostaglandins diffuse into interstitial fluid

  • Astrocytes control BBB by releasing chemicals that regulate permeability

Blood–CSF Barrier

  • Formed by special ependymal cells surrounding capillaries of choroid plexus

  • Limits movement of compounds, allowing chemical composition of blood and CSF to differ

Protection and Support Summary

  • Meninges stabilize brain in cranial cavity

  • CSF protects against sudden movement, provides nutrients, removes wastes

  • Blood–brain and blood–CSF barriers selectively isolate brain from chemicals in blood

Diseases and Syndromes to Know

Disease/Syndrome

Key Features

CTE

Chronic traumatic encephalopathy; neurodegeneration due to repeated head injury

BABINSKI

Babinski sign; abnormal reflex indicating CNS damage

Stroke/TIA

Loss of blood flow to brain; TIA is transient, stroke is permanent

UMNL/LMNL

Upper/Lower Motor Neuron Lesions; affect voluntary movement

Seizure/Epilepsy

Abnormal electrical activity in brain

Concussion

Temporary brain dysfunction due to trauma

CSF – what it should/should not contain

Should be clear, low protein, no blood cells; abnormal contents indicate disease

Huntington's Disease

Genetic neurodegenerative disorder

Meningitis (Viral & Bacterial)

Inflammation of meninges; bacterial is more severe

Alzheimer’s Disease

Progressive memory loss and cognitive decline

Wernicke's

Wernicke's encephalopathy; confusion, ataxia, ophthalmoplegia

Parkinson's Disease

Movement disorder due to loss of dopamine neurons

*Additional info: The notes cover the major anatomical regions, protective structures, and clinical relevance of the brain, suitable for college-level Anatomy & Physiology students. Disease table entries are inferred from standard curriculum topics.*

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