The Central Nervous System

The Central Nervous System

Chapter 12

I. Embryonic Tissue: The 3 Germ Layers

  • Ectoderm:

    • Mnemonic: "attract-o-derm"

    • Aspects: Related to attractiveness and perception.

    • Derivatives:

    • Brain

    • Spinal Cord

    • Epidermis (outer skin layer)

    • Lens of the eye

    • Inner ear

    • Connection: How physical appearances, cognition (brain function), and communication (listening) impact attraction.

  • Endoderm:

    • Mnemonic: "intern-organ-oderm"

    • Derivatives:

    • Lining of the digestive tract

    • Lungs

    • Liver

    • Pancreas

  • Mesoderm:

    • Mnemonic: "means-o-derm"

    • Aspects: Related to motion and structures that facilitate movement.

    • Derivatives:

    • Skeleton

    • Muscles

    • Circulatory system

    • Kidneys

    • Gonads (reproductive organs)

II. The Brain

A. Embryonic Development

  • The neural tube begins to form during the 3rd week of embryonic development.

  • The brain develops from the anterior region of the neural tube.

  • Formation of 3 primary brain regions/vesicles:

    • Forebrain: Includes the cerebrum and diencephalon.

    • Midbrain: Central part of the brain.

    • Hindbrain: Includes the pons, medulla oblongata, and cerebellum.

B. Brain Development Structure

  1. Neural Tube Structures:

    • Diagrams and images depicting neural tube, primary and secondary brain vesicles, and adult brain structures were referenced in the transcript.

  2. Basic Pattern of the CNS:

    • Structure:

      • Central cavity surrounded by gray matter.

      • White matter is found outermost.

    • Additional Layer: Cerebrum and cerebellum have an outer layer of gray matter (cortex).

  3. Ventricles (Cavities in the Brain):

    • Lined by ependymal cells that circulate cerebrospinal fluid (CSF).

    • Types of ventricles:

      • Lateral ventricles: 2 in cerebral hemispheres.

      • 3rd ventricle: Located in the diencephalon.

      • 4th ventricle: Sandwiched between the brain stem and cerebellum.

    • Connections:

      • Lateral ventricles connect with the 3rd ventricle via the interventricular foramen.

      • The 3rd ventricle connects to the 4th via the cerebral aqueduct.

      • The 4th ventricle connects to the subarachnoid space and central canal.

  4. Cerebral Hemispheres:

    • Composition: Make up 83% of the brain and enclose the diencephalon and upper brain stem.

    • Structure: Highly convoluted with raised areas called gyri, depressions known as sulci, and deep fissures.

    • Notable Fissures:

      • Longitudinal fissure: Separates cerebral hemispheres.

      • Transverse cerebral fissure: Separates cerebrum and cerebellum.

      • Central sulcus: Separates frontal and parietal lobes.

      • Lateral sulcus: Separates temporal lobe from parietal and frontal lobes.

      • Parieto-occipital sulcus: Separates parietal and occipital lobes.

  5. Cerebral Cortex:

    • Description: Outermost layer of gray matter consisting of interneuron cell bodies, dendrites, and glia.

    • Functionality: Each hemisphere processes info primarily from the opposite side of the body (contralateral).

    • Lateralization: Specialization of function can occur in one hemisphere only.

    • Functional Regions: Contains three interacting areas: motor areas, sensory areas, and association areas.

  6. Motor Areas of the Cerebral Cortex:

    • Primary Motor Cortex:

      • Location: Precentral gyrus of the frontal lobe.

      • Function: Controls voluntary movements of skeletal muscles.

    • Premotor Cortex:

      • Location: Anterior to precentral gyrus.

      • Function: Controls learned repetitious motor skills (e.g., typing, playing an instrument).

    • Broca’s Area:

      • Location: Usually in the left hemisphere.

      • Function: Controls muscles involved in speech.

    • Frontal Eye Field:

      • Location: Anterior to premotor cortex.

      • Function: Controls voluntary eye movements.

  7. Sensory Areas of the Cerebral Cortex:

    • Located in the parietal, insular, temporal, and occipital lobes.

    • Key Sensory Areas:

      • Primary Somatosensory Cortex:

      • Location: Postcentral gyrus of parietal lobe.

      • Function: Receives sensory info from skin and proprioceptors.

      • Somatosensory Association Cortex:

      • Location: Parietal lobe.

      • Function: Integrates sensory input based on past experiences.

      • Primary Visual Cortex:

      • Location: Occipital lobe (buried in calcarine sulcus).

      • Function: Receives info from the retina.

      • Visual Association Area:

      • Location: Occipital lobe.

      • Function: Interprets visual input in light of past experiences.

      • Auditory Cortex:

      • Location: Superior margin of temporal lobe.

      • Function: Receives info from cochlea (hearing).

      • Olfactory Cortex:

      • Location: Uncus of the temporal lobe.

      • Function: Processes info from the nasal cavity (smell).

      • Gustatory Cortex:

      • Location: Insular lobe.

      • Function: Receives info from taste buds.

  8. Lateralization of Brain Function:

    • In 90% of individuals:

      • Left hemisphere: Dominates in language, mathematics, and logic (tendency towards right-handedness).

      • Right hemisphere: Involved in intuition, emotion, artistry, and musical skills.

    • In 10% of individuals:

      • Right hemisphere dominance or neither hemisphere dominance leading to left-handedness or ambidextrous traits.

  9. Cerebral White Matter:

    • Found deep to the cortex; consists of myelinated interneurons.

    • Types of tracts:

      • Commissural Tract Fibers: Connect right and left hemispheres (e.g., corpus callosum).

      • Association Tract Fibers: Connect different parts within the same hemisphere.

      • Projection Tract Fibers: Connect cerebral hemispheres to lower brain and spinal cord.

III. Diencephalon

  • Composition: Consists of thalamus, hypothalamus, and epithalamus.

A. Thalamus

  • Characteristics:

    • Contains multiple nuclei and encloses the 3rd ventricle.

    • Functions: Acts as a "gateway" to the cerebrum; all information going to/from the cerebral cortex passes through it.

    • Involved in memory processing.

B. Hypothalamus

  • Characteristics:

    • Contains several nuclei and is the chief integration center of the autonomic nervous system.

    • Functions:

    • Regulation of body temperature

    • Regulation of food intake (hunger)

    • Water balance (via ADH) and thirst

    • Biological rhythms and drives

    • Regulation of hormones produced by the anterior pituitary

    • Production of oxytocin.

C. Epithalamus (including Pineal Gland)

  • Functions: Produces melatonin (a hormone) that induces sleep and acts as an antioxidant.

  • Interaction: Works with hypothalamus to regulate sleep-wake cycles.

IV. Brain Stem

  • Composition: Comprises midbrain, pons, and medulla oblongata.

A. Midbrain

  • Functions:

    • Serves as a conduction pathway between higher and lower brain centers.

    • Contains the cerebral aqueduct connecting the 3rd and 4th ventricles.

    • Houses visual and auditory reflex centers.

B. Pons

  • Function: Conduction pathway between higher and lower brain centers and contains a respiratory center regulating rate and depth of breathing.

C. Medulla Oblongata

  • Functionality: Pathway between the cerebrum and spinal cord.

    • Site of decussation (crossing over of nerve fibers).

    • Contains nuclei regulating heart rate, blood vessel diameter, respiratory rate, vomiting, coughing, swallowing, hiccupping, and sneezing.

V. Cerebellum

  • Function: Processes information from proprioceptors, visual, and equilibrium pathways.

  • Responsible for balance, posture, and smooth, coordinated muscular movements.

  • Structure: Composed of the arbor vitae.

VI. Functional Brain Systems

  • Comprised of networks of neurons working together.

A. Limbic System

  • Location: Within the cerebrum and diencephalon.

  • Function: Acts as the center for emotions and plays a role in memory.

B. Reticular Activating System (RAS)

  • Location: In the brain stem.

  • Function: Maintains the alert state of the cerebral cortex; filters unimportant sensory input.

VII. Higher Mental Functions

A. Brain Wave Patterns

  • Description: Brain waves indicate patterns of neuronal electrical activity and are recorded using an EEG (electroencephalogram).

  • Types of brain waves:

    • Alpha Waves (8-13 Hz): Indicative of a calm, relaxed state of wakefulness, representing an “idling” brain.

    • Beta Waves (14-30 Hz): Rhythmic patterns observed when mentally alert or concentrating on tasks.

    • Theta Waves (4-7 Hz): Irregular waves typical in children; abnormal in awake adults.

    • Delta Waves (4 Hz or less): Occur during deep sleep or when the RAS is suppressed (e.g., during anesthesia), indicates brain damage if seen in awake adults.

B. Sleep/Wake Cycles

  • Definition: Sleep is a state of partial unconsciousness from which a person can be aroused.

  • Types of Sleep:

    • NREM (non-rapid eye movement) Sleep: Occurs in 4 stages typically. Nightmares occur during stages 3 and 4.

    • REM (rapid eye movement) Sleep: Most skeletal muscles are inhibited during this stage; oxygen usage is high, and dreaming predominantly occurs.

  • Duration of Sleep Stages:

    • Stage 1: ~30-45 min (easy to awaken);

    • Stage 2: EEG becomes more irregular, with sleep spindles appearing, normal vital signs;

    • Stages 3-4: Sleep deepens with theta and delta waves predominating; vital signs decline gradually; dreams and deep sleep characteristics observed.

  • Sleep Cycle: Regulated by the hypothalamus; alternates between REM and NREM sleep with partial arousals occurring. Longest dreams happen just before waking.

C. Language

  • Broca’s Area: Damage results in the ability to understand language, but difficulty in speech production occurs.

  • Wernicke’s Area: Damage leads to an inability to comprehend spoken or written language while still allowing for nonsensical speech output.

D. Memory

  • Definition: The storage and retrieval of information, which tends to decline with age.

  • Stages of Memory:

    • Short-term Memory (STM): Temporary storage for approximately 7-8 pieces of information.

    • Long-term Memory (LTM): Storage capacity appears limitless. Actual transfer from STM to LTM can be influenced by:

    • Emotional state during learning (best retained when alert, motivated, aroused).

    • Rehearsal (repetition enhances memory retention).

    • Association with already stored information (linking new info to old).

E. Protection of the Brain

  • Structures:

    • Skull, meninges, cerebrospinal fluid (CSF), and the blood-brain barrier.

  1. Meninges:

    • Dura Mater (Outermost Meninge): Leathery, two-layered membrane (fused except at the superior sagittal sinus).

    • Arachnoid Mater (Middle Meninge): Contains subarachnoid space with CSF and blood vessels; arachnoid villi protrude into the superior sagittal sinus to return CSF to the bloodstream.

    • Pia Mater (Innermost Meninge): Delicate adherent layer to the brain that is well-vascularized.

  2. Cerebrospinal Fluid (CSF):

    • Functions: Protection, nourishment, buoyancy for brain and spinal cord.

    • Producing and circulation characteristics: Produced by choroid plexuses within each ventricle, circulating in the subarachnoid space and central canal.

    • Maintained at a constant rate for production and drainage.

  3. Blood-Brain Barrier (BBB):

    • Functions: Restricts metabolic wastes, toxins, and most drugs from entering brain tissue, while allowing fat-soluble substances (O2, CO2, alcohol) to pass.

    • Notable Areas: Absent near the vomiting center and hypothalamus, which monitor the toxic composition of blood.

  4. Homeostatic Imbalances:

    • Traumatic Brain Injury: Includes concussion, contusion, hemorrhages, cerebral edema, leading cause of death in the US.

    • Cerebrovascular Accidents (Strokes): Result from insufficient blood supply leading to brain area death.

    • Degenerative Brain Disorders: Includes Alzheimer’s disease, Parkinson’s disease, Huntington’s disease.

VIII. Spinal Cord

  • Description: Functions as a major reflex center (e.g., knee-jerk, withdrawal reflex).

  • Extends from the medulla to the L1/L2 vertebrae.

  • Features:

    • Cervical and lumbar enlargements where nerves serving arms and legs arise.

    • Conus Medullaris: Tapered inferior region.

    • Cauda Equina: Nerve roots originating from the conus medullaris.

    • Filum Terminale: Anchors spinal cord to the coccyx.

    • Protects by vertebrae, meninges, and CSF.

A. Cross-Sectional Anatomy

  • Features a division by anterior median fissure and posterior median sulcus.

  • Gray Matter:

    • Gray commissure containing the central canal.

    • Dorsal (posterior) horns: Composed mainly of interneurons.

    • Ventral (anterior) horns: Some interneurons, predominately somatic motor neuron cell bodies.

    • Lateral horns: Houses autonomic (sympathetic) motor neurons.

  • White Matter:

    • Consists of myelinated and unmyelinated neurons traversing in three directions:

    • Ascending Tracts: Transport sensory information upward in the spinal cord.

    • Descending Tracts: Carry motor information downward in the spinal cord.

    • Commissural Fibers: Transport information laterally across the spinal cord.

IX. Spinal Cord Trauma & Disorders

  1. Spinal Cord Trauma:

    • Damage to the ventral root or horn causes flaccid paralysis.

    • Damage to the primary motor cortex results in spastic paralysis.

    • Paraplegia: Transection of the spinal cord between T1 and L1.

    • Quadriplegia: Transection in the cervical region.

  2. Poliomyelitis:

    • Caused by the poliovirus destroying motor neurons in the ventral horns, leading to flaccid paralysis.

    • Can lead to death if the diaphragm becomes paralyzed.

    • Prevention: Via vaccination.

  3. Amyotrophic Lateral Sclerosis (ALS):

    • Characterized by progressive destruction of motor neurons in the ventral horns and descending tracts.

    • Cause is often unknown; typically results in death within 5 years.

X. Congenital Disorders

  • Deafness: May result from maternal rubella infection.

  • Brain Damage: Possible due to maternal exposure to radiation or drugs, including nicotine.

  • Cerebral Palsy: Impairs voluntary muscle control, often caused by temporary oxygen deficiency during birth.

  • Anencephaly: Failure of the cerebrum and part of the brain stem to develop.

  • Spina Bifida: Caused by inadequate vertebral enclosure, predominantly linked to folic acid deficiency in the maternal diet, resulting in various form variations:

    • Meningocele: Protrusion of meninges only.

    • Myelomeningocele: Protrusion of both meninges and spinal cord.

  • Associated Syndromes:

    • Chiari Type II: Associated with lumbar meningomyelocele.

    • Dandy-Walker Malformation: Linked with spina bifida abnormalities.

  • Tethered Cord Syndrome: Can occur as a result of spina bifida occulta, leading to issues such as bladder incontinence.