The Central Nervous System

Central Nervous System (CNS)

  • CNS consists of brain and spinal cord
  • Cephalization:
      - Evolutionary development of rostral (anterior) portion of CNS
      - Increased number of neurons in head
      - Highest level reached in human brain

Embryonic Development

  • Brain and spinal cord begin as neural tube
  • 3 Primary Vesicles form at anterior end:
      1. Prosencephalon (Forebrain)
      2. Mesencephalon (Midbrain)
      3. Rhombencephalon (Hindbrain)
  • Posterior end becomes spinal cord

Secondary Brain Vesicles

  • Primary vesicles → 5 secondary brain vesicles:
      - Forebrain → telencephalon and diencephalon
      - Midbrain remains undivided
      - Hindbrain → metencephalon and myelencephalon
  • Telencephalon ➔ cerebral hemispheres
  • Diencephalon ➔ epithalamus, thalamus, hypothalamus, and retina
  • Mesencephalon ➔ midbrain
  • Metencephalon ➔ pons and cerebellum
  • Myelencephalon ➔ medulla oblongata
  • Central cavity of neural tube → ventricles

Adult Brain Structure

  • Adult brains have four regions:
      1. Cerebral hemispheres
      2. Diencephalon
      3. Brain stem (midbrain, pons, and medulla)
      4. Cerebellum

Gray Matter vs. White Matter

  • Gray Matter:
      - Contains neuron cell bodies and short nonmyelinated neurons
  • White Matter:
      - Mostly myelinated axons (with some nonmyelinated axons)
  • The cerebrum and cerebellum have islands of gray matter (nuclei) within white matter, and an outer cortex of gray matter.

Ventricles of the Brain

  • Ventricles: Fluid-filled chambers that are continuous with each other and with the central canal of the spinal cord.
  • Filled with cerebrospinal fluid (CSF), lined by ependymal cells (neuroglial cells)
  • Paired lateral ventricles:
      - C-shaped chambers located deep in each hemisphere
      - Separated by membranous septum pellucidum
      - Each lateral ventricle connects to the third ventricle via interventricular foramen
  • Third Ventricle: Lies in diencephalon and is connected to the fourth ventricle via cerebral aqueduct
  • Fourth Ventricle: Lies in hindbrain and is continuous with central canal of spinal cord; has three openings to subarachnoid space:
      - Paired lateral apertures in side walls
      - Median aperture in roof

Cerebral Hemispheres

  • Form superior part of brain, account for 83% of brain mass
  • Surface Markings:
      - Gyri: ridges
      - Sulci: shallow grooves
      - Fissures: deep grooves
  • Longitudinal Fissure: Separates two hemispheres
  • Transverse Cerebral Fissure: Separates cerebrum from cerebellum

Lobes of the Cerebral Hemispheres

  • Several sulci divide each hemisphere into five lobes:
      1. Frontal
      2. Parietal
      3. Temporal
      4. Occipital
      5. Insula (buried under portions of temporal, parietal, and frontal lobes)

Major Sulci

  • Central Sulcus: Separates precentral gyrus (frontal lobe) and postcentral gyrus (parietal lobe)
  • Parieto-occipital Sulcus: Separates occipital and parietal lobes
  • Lateral Sulcus: Outlines temporal lobe

Basic Regions of Each Hemisphere

  1. Cerebral Cortex of gray matter (superficially)
  2. White Matter internally
  3. Basal Nuclei deep within white matter

Cerebral Cortex

  • The "executive suite" of the brain; site of conscious mind:
      - Awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding
  • Composed of neuron cell bodies, dendrites, glial cells, and blood vessels, but no axons
  • Composes 40% of mass of brain

Functional Areas of the Cerebral Cortex

  1. Motor Areas: Control voluntary movement
  2. Sensory Areas: Conscious awareness of sensation
  3. Association Areas: Integrate diverse information
       - Each hemisphere is concerned with contralateral side of the body
       - Lateralization of cortical function can occur in only one hemisphere
       - Conscious behavior involves the entire cortex

Motor Areas of the Cerebral Cortex

  • Primary Motor Cortex: Located in precentral gyrus, controls voluntary movement
  • Premotor Cortex: Anterior to precentral gyrus; helps plan movements, coordinates actions
  • Broca’s Area: Anterior to inferior premotor area; involved in speech production
  • Frontal Eye Field: Controls voluntary eye movements, located within and anterior to premotor cortex; superior to Broca’s area

Primary Motor Cortex Details

  • Contains large pyramidal cells that allow conscious control of precise, skilled, skeletal muscle movements
  • Pyramidal (Corticospinal) Tracts: Formed from long axons that project down spinal cord
Pyramidal Cells
  • Large neurons in precentral gyrus
  • Allow conscious control of precise, skilled muscle movements
Motor Homunculi
  • Represent contralateral motor innervation; body is represented upside-down in the motor cortex
  • Areas controlling muscles with precise control (face, tongue, hands) are disproportionately large
  • Example of somatotopy: neurons controlling foot movements are located together, those controlling hand movements are distinct.

Cerebral Cortex Sensory Areas

  • Areas responsible for conscious awareness of sensation, located in various lobes:
      - Primary Somatosensory Cortex:
        - Located in postcentral gyrus of parietal lobe
        - Receives general sensory info from skin and proprioceptors
        - Capable of spatial discrimination: identification of body region being stimulated
      - Somatosensory Association Cortex: Integrates sensory input from primary somatosensory cortex to understand objects
      - Visual Areas: Primarily located in occipital lobe (primary visual cortex)
      - Auditory Areas: Located in temporal lobes (primary auditory cortex)
      - Vestibular Cortex: For conscious awareness of balance
      - Olfactory Cortex: For awareness of odors
      - Gustatory Cortex: Involved in perception of taste
      - Visceral Sensory Area: Perception of visceral sensations (e.g., stomach, bladder)

Damage Implications

  • Damage to primary sensory areas leads to loss of conscious awareness
  • Damage to visual cortex results in functional blindness; damage to visual association area results in inability to comprehend visuals despite seeing.

Multimodal Association Areas

  • General Function: Receive and integrate inputs from multiple sensory areas; critical for memory and decision making
  • Divided into:
      - Anterior Association Area (Prefrontal Cortex)
        - Complex region involved in intellect, cognition, personality
      - Posterior Association Area
        - Spans across temporal, parietal, occipital lobes; involved in language comprehension and identifying patterns and faces
      - Limbic Association Area
        - Connects emotional responses and memory formation in limbic system

Clinical Considerations

  • Damage to anterior association area can result in personality changes, lack of inhibitions
  • Lesions in posterior association areas can cause disorientation and lack of awareness of body parts (e.g., denial of body part belonging to oneself)

Lateralization of Cortical Function

  • Lateralization: Division of labor between hemispheres
  • Cerebral Dominance: Refers to the hemisphere dominant for language (90% left-sided dominance) leading to right-handedness
      - Left Hemisphere: Language, math, logic
      - Right Hemisphere: Visual-spatial tasks, intuition, emotion, music

White Matter of Cerebral Hemispheres

  • Composed of myelinated fibers bundled into tracts:
      - Association Fibers: Connect different parts of the same hemisphere
      - Commissural Fibers: Connect gray matter of both hemispheres
      - Projection Fibers: Connect hemispheres with lower brain or spinal cord
        - Internal Capsule: Compact band of projection fibers between thalamus and basal nuclei
        - Corona Radiata: Projection fibers radiate through white matter to cortex

Basal Nuclei

  • Involve functions related to:
      - Muscle movement/motor control
      - Cognition and emotion
      - Regulation of intensity of slow or stereotyped movements
      - Inhibition of incorrect responses
      - Disorders include Parkinson’s and Huntington’s diseases

Diencephalon

  • Composed of:
      1. Thalamus:
         - Major relay station for sensory information; projects to cerebral cortex
         - Involved in memory and sensory integration
      2. Hypothalamus:
         - Controls autonomic nervous system functions: blood pressure, heart rate, temperature regulation
         - Links nervous system to endocrine system
      3. Epithalamus:
         - Contains pineal gland, regulates sleep-wake cycles via melatonin secretion

Brain Stem

  • Composed of: midbrain, pons, medulla oblongata
  • Controls automatic behaviors necessary for survival and connects higher and lower neural centers
  • Associated with cranial nerves
Midbrain
  • Contains:
      - Cerebral peduncles (motor tracts)
      - Cerebral aqueduct (connects third and fourth ventricles)
      - Periaqueductal gray matter (pain suppression)
      - Corpora quadrigemina (visual and auditory reflex centers)
Pons
  • Connects midbrain and medulla
  • Contains nuclei for cranial nerves and regulates breath rhythm
Medulla Oblongata
  • Blends into spinal cord, regulates heart rate, blood pressure, and respiration
  • Contains pyramids (motor tracts) with decussation (crossing over)

Cerebellum

  • Located dorsal to pons and medulla; involved in motor coordination and balance
  • Composed of left and right hemispheres connected by vermis; has distinctive foliation
  • Functions: Fine-tunes motor activity, processes input from body position and momentum, and cognitive functions.

Functional Brain Systems

Limbic System

  • Emotional brain with structures involving memory and emotional responses
  • Includes: amygdala, cingulate gyrus, and hippocampus
  • Responsible for emotional impact and regulating psychosomatic illnesses

Reticular Formation

  • Governs brain arousal and consciousness
  • Filters stimuli; regulated by sleep centers, alcohol, and drugs

Higher Mental Functions

Language

  • Broca’s Area: Speech production (damaged in individuals may understand but not speak)
  • Wernicke’s Area: Understanding of spoken and written language (damaged individuals can speak, but words lack meaning)

Amnesia

  • Loss of memory; can be retrograde (loss of past memories) or anterograde (inability to form new memories)

Sleep and Consciousness

  • Sleep can be measured through EEG, reflecting electrical activity of the brain
  • Two types of sleep: NREM (Non-Rapid Eye Movement) and REM (Rapid Eye Movement)
  • Disorders include insomnia and narcolepsy

Protection of Brain

Meninges

  • Three layers:
      1. Dura Mater: Tough outer layer, forms partitions in skull
      2. Arachnoid Mater: Middle layer with spider-web like extensions
      3. Pia Mater: Delicate inner layer tightly adherent to brain
  • Meningitis: Inflammation of the meninges, can affect brain leading to encephalitis

Cerebrospinal Fluid (CSF)

  • Forms a cushion around the brain, enables buoyancy, reduces weight by 97%
  • Produced by choroid plexus, functions in nourishment and protection

Hydrocephalus

  • Abnormal buildup of CSF leading to increased pressure in ventricles, can be treated by shunt

Blood Brain Barrier

  • Selectively allows substances to pass from blood to brain; protective mechanism against harmful substances
  • Absent in areas like the vomit center and hypothalamus to monitor blood composition

Brain Injuries & Disorders

  • Traumatic injuries including concussion, contusion, hemorrhage, and brain edema
  • CVA (Stroke): Ischemia can lead to paralysis and sensory deficits
  • Degenerative Disorders: Alzheimer's, Parkinson's, and Huntington’s disease

Spinal Cord

  • Central region is gray matter surrounded by white matter
  • Terminates at the conus medullaris; CSF-filled central canal
Trauma and Disorders
  • Paresthesias: sensory loss due to dorsal root damage
  • Paralysis: motor loss due to ventral root damage; types include flaccid or spastic paralysis
Clinical Imbalances
  • Cerebral Palsy: due to brain damage, results in affected motor and speech control
  • Spina Bifida: incomplete vertebral formation, degree of severity varies

Developmental Aspects

  • Gender-specific development; maternal influences on CNS development; aging effects on cognitive function.