Chapter 12: The Central Nervous System – Brain Structure and Function

Brain Development and Organization

Embryonic Development of the Brain

The central nervous system (CNS) develops from a hollow embryonic structure called the neural tube. The anterior (rostral) end forms the brain, while the posterior (caudal) end forms the spinal cord. The neural cavity within the tube becomes the ventricular system of the brain and the central canal of the spinal cord.

• Primary brain vesicles (Week 4): Prosencephalon (forebrain), Mesencephalon (midbrain), Rhombencephalon (hindbrain)

• Secondary brain vesicles (Week 5): Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myelencephalon

• Adult derivatives:

  • Telencephalon: Cerebral hemispheres (cerebrum)

  • Diencephalon: Thalamus, hypothalamus, epithalamus, retina

  • Mesencephalon: Midbrain

  • Metencephalon: Pons, cerebellum

  • Myelencephalon: Medulla oblongata

Cephalization refers to the evolutionary concentration of nervous tissue at the anterior end, resulting in complex brains and advanced functions in humans.

Gray Matter vs. White Matter

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; responsible for information processing.

• White matter: Contains myelinated axons; responsible for communication between CNS regions.

• Organization:

  • Spinal cord: Central gray matter ("H" shape), outer white matter

  • Brain stem: Gray matter nuclei scattered within white matter

  • Cerebrum & cerebellum: Outer gray matter (cortex), inner white matter, deep gray nuclei

Ventricular System and CSF Flow

• Ventricles: Interconnected, CSF-filled chambers lined by ependymal cells

• Four ventricles: Two lateral ventricles (in cerebral hemispheres), third ventricle (diencephalon), fourth ventricle (between pons/medulla and cerebellum)

• CSF flow pathway:

  1. Lateral ventricles

  2. Interventricular foramina

  3. Third ventricle

  4. Cerebral aqueduct

  5. Fourth ventricle

  6. Median and lateral apertures

  7. Subarachnoid space

  8. Central canal of spinal cord (minor flow)

Cerebral Hemispheres and Surface Anatomy

Major Features

• Gyri: Raised ridges (e.g., precentral and postcentral gyri)

• Sulci: Shallow grooves (e.g., central sulcus, lateral sulcus, parieto-occipital sulcus)

• Fissures: Deep grooves (e.g., longitudinal fissure, transverse cerebral fissure)

Lobes of the Cerebrum

• Frontal lobe: Voluntary movement, planning, judgment, personality, speech production

• Parietal lobe: Sensory processing (touch, pain, temperature, proprioception)

• Temporal lobe: Hearing, memory, language comprehension, smell

• Occipital lobe: Vision

• Insula: Taste, balance, visceral sensation (hidden beneath lateral sulcus)

Three Basic Regions of Each Hemisphere

• Cerebral cortex: Outer gray matter; site of conscious thought, memory, voluntary movement

• Internal white matter: Myelinated axons connecting cortical areas, hemispheres, and lower CNS

• Basal nuclei: Deep gray matter clusters regulating voluntary movement

Principles of Cortical Function

• Three functional areas: Motor, sensory, association

• Contralateral control: Each hemisphere controls the opposite side of the body

• Lateralization: Some functions are specialized to one hemisphere (e.g., language on the left)

• Complex behaviors require integration across multiple cortical regions

Motor Areas of the Cerebral Cortex

Major Motor Areas (All in Frontal Lobe)

Motor Area	Location	Function
Primary motor cortex	Precentral gyrus	Initiates voluntary skeletal muscle movement
Premotor cortex	Anterior to primary motor cortex	Plans and coordinates learned movements
Broca's area	Inferior frontal lobe (usually left)	Motor speech production
Frontal eye field	Anterior to premotor cortex	Voluntary eye movements

Primary Motor Cortex

• Contains large pyramidal cells whose axons form the corticospinal (pyramidal) tracts

• Controls precise, conscious muscle activity (e.g., writing, speaking)

• Somatotopy: Body regions are mapped to specific cortical areas ("motor homunculus")

• Contralateral control: Left cortex controls right body, and vice versa

Premotor Cortex

• Plans and organizes complex, learned movements (e.g., typing, playing piano)

• Coordinates simultaneous and sequential muscle activity

• Uses sensory feedback to adjust movements

Broca's Area

• Motor speech area: Plans and coordinates muscles for speech

• Usually present in the left hemisphere

Frontal Eye Field

• Controls voluntary eye movements (e.g., reading, tracking objects)

Clinical Correlations

• Damage to primary motor cortex: Contralateral paralysis

• Damage to premotor cortex: Loss of learned skilled movements (muscle strength preserved)

• Neural plasticity: Brain's ability to reorganize and recover function after injury

Sensory Areas of the Cerebral Cortex

Major Sensory Areas

Cortical Area	Location	Function
Primary somatosensory cortex	Postcentral gyrus	Touch, pain, temperature, proprioception
Somatosensory association cortex	Posterior parietal lobe	Recognizes objects by touch
Primary visual cortex	Occipital lobe	Conscious vision
Visual association area	Around visual cortex	Recognizes visual information
Primary auditory cortex	Superior temporal lobe	Hearing
Auditory association area	Temporal lobe	Interprets sounds and speech
Vestibular cortex	Insula/parietal region	Balance and equilibrium
Olfactory cortex	Temporal lobe	Smell
Gustatory cortex	Insula	Taste
Visceral sensory cortex	Insula	Internal organ sensations

• Sensory homunculus: Body map based on sensory receptor density

• Primary areas detect sensations; association areas interpret them

• Damage to association areas can cause agnosias (e.g., astereognosis, visual agnosia)

Association Areas, Lateralization, White Matter, and Basal Nuclei

Major Association Areas

• Prefrontal cortex: Executive functions (planning, judgment, personality, working memory)

• Wernicke's area: Language comprehension (usually left hemisphere)

• Posterior association area: Integrates sensory information, spatial awareness, recognition

Hemispheric Lateralization

• Left hemisphere: Language, logic, mathematics

• Right hemisphere: Creativity, spatial awareness, facial recognition

• Corpus callosum: Largest commissural tract connecting hemispheres

Cerebral White Matter

Fiber Type	Connects
Association fibers	Regions within the same hemisphere
Commissural fibers	Left and right hemispheres
Projection fibers	Cerebrum with brain stem and spinal cord

Basal Nuclei

• Major components: Caudate nucleus, putamen, globus pallidus

• Functions: Initiate and regulate voluntary movement, suppress unwanted movement, regulate muscle tone

• Clinical disorders: Parkinson disease (reduced movement), Huntington disease (excessive involuntary movement)

The Diencephalon

Major Regions and Functions

Structure	Major Functions
Thalamus	Sensory relay, motor relay, consciousness, sensory filtering
Hypothalamus	Homeostasis, ANS control, endocrine regulation, hunger, thirst, temperature, circadian rhythms, emotions
Epithalamus	Contains pineal gland; regulates melatonin and sleep cycles

• Thalamus: Gateway to the cerebral cortex; relays and filters sensory information (except olfaction)

• Hypothalamus: Master regulator of homeostasis; controls ANS, pituitary gland, temperature, hunger, thirst, circadian rhythms, emotions

• Epithalamus: Contains pineal gland (secretes melatonin for sleep-wake regulation)

The Brain Stem

Regions and Functions

Brain Stem Region	Major Functions
Midbrain	Visual and auditory reflexes, motor pathways, eye movement
Pons	Bridge between cerebrum and cerebellum, breathing regulation, cranial nerves V–VIII
Medulla oblongata	Cardiovascular control, respiration, protective reflexes, cranial nerves VIII–XII
Reticular formation	Consciousness, posture, muscle tone, autonomic regulation
Reticular activating system	Alertness, wakefulness, attention

• Midbrain: Contains cerebral peduncles (motor tracts), superior colliculi (visual reflexes), inferior colliculi (auditory reflexes), nuclei for CN III and IV

• Pons: Relays information, regulates breathing (pneumotaxic and apneustic centers), nuclei for CN V–VIII

• Medulla oblongata: Contains pyramids (corticospinal tracts), site of pyramidal decussation (contralateral control), vital centers for heart rate, blood pressure, respiration, nuclei for CN VIII–XII

• Reticular formation: Regulates muscle tone, posture, pain, consciousness, sleep-wake cycles

• Reticular activating system (RAS): Maintains alertness and filters sensory input; damage can cause coma

The Cerebellum

Anatomy and Function

• Located inferior to occipital lobes, posterior to pons and medulla

• Two hemispheres connected by the vermis

• Cerebellar cortex: Outer gray matter for processing movement information

• Arbor vitae: Internal white matter ("tree of life") carrying signals

• Deep cerebellar nuclei: Output centers

• Functions: Coordination of voluntary movement, balance, posture, muscle tone, motor learning, error correction

Clinical Correlations

• Ataxia: Loss of coordinated movement

• Intention tremor: Tremor during voluntary movement

• Dysmetria: Inability to judge distance

• Dysdiadochokinesia: Difficulty with rapid alternating movements

• Nystagmus: Abnormal eye movements

High-Yield Exam Facts and Summary Tables

Structure/Area	Key Function/Fact
Gyri	Ridges of cortex
Sulci	Shallow grooves
Fissures	Deep grooves
Central sulcus	Separates frontal and parietal lobes
Precentral gyrus	Primary motor cortex
Postcentral gyrus	Primary somatosensory cortex
Basal nuclei	Regulate movement
Thalamus	Sensory relay station
Hypothalamus	Homeostatic control
Pineal gland	Secretes melatonin
Superior colliculi	Visual reflexes
Inferior colliculi	Auditory reflexes
Pons	Bridge, breathing regulation
Medulla	Vital autonomic centers
Reticular activating system	Maintains consciousness
Arbor vitae	Cerebellar white matter
Cerebellum	Coordinates movement and balance

Additional Key Concepts

• Dural septa: Falx cerebri (separates cerebral hemispheres), tentorium cerebelli (separates cerebrum from cerebellum), falx cerebelli (separates cerebellar hemispheres)

• Blood-brain barrier (BBB): Protects brain from toxins; allows oxygen, glucose, some drugs; blocks many wastes

• Brain waves: Beta (alert), alpha (relaxed), theta (children/light sleep), delta (deep sleep)

• Memory centers: Hippocampus (converts short-term to long-term memory), prefrontal cortex (working memory, planning)

• Spinocerebellar tracts: Carry proprioceptive information to cerebellum for coordination

• Lateral spinothalamic tract: Carries pain and temperature sensations

Ultimate Exam Memorization List

• Gyri = ridges; Sulci = shallow grooves; Fissures = deep grooves

• Central sulcus: Frontal vs. parietal lobes

• Precentral gyrus: Primary motor cortex; Postcentral gyrus: Primary somatosensory cortex

• Basal nuclei: Smooth movement; Thalamus: Sensory relay; Hypothalamus: Homeostasis

• Pineal gland: Melatonin; Superior/inferior colliculi: Visual/auditory reflexes

• Pons: Bridge; Medulla: Vital centers; RAS: Consciousness

• Arbor vitae: Cerebellar white matter; Cerebellum: Coordination and balance