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Comprehensive Notes on the Central Nervous System (Transcripts-based)

CNS Overview and Key Concepts

  • CNS evolution and structure: sepulization leads to more neurons in the anterior (head) region forming the brain; rest of CNS attaches to the spinal cord.
  • Surface anatomy of the brain:
    • Gyri: ridges on the brain surface.
    • Sulci: grooves between gyri.
    • Fissure: a particularly deep groove that separates the two cerebral hemispheres.
    • Purpose of folding: increases surface area to fit more neurons inside the skull.
  • Cerebral hemispheres composition:
    • Gray matter: unmyelinated regions, mostly neuronal somas.
    • White matter: myelinated axons forming tracts; white appearance is due to myelin.
    • Brain organization: gray matter in the middle (deep gray matter), white matter surrounding it, and then gray matter on the outside (cortex) again.
  • Lateralization and hemispheric specialization:
    • Cortex has two hemispheres (left and right).
    • Evolutionary dominance: left side of body controlled by right brain and vice versa (contralateral control).
    • Language: ~90\% of people have language centers on the left hemisphere; ~10\% have language centers on the right or split across hemispheres.
    • Right hemisphere specialization: visual-spatial skills.
    • Interhemispheric communication: white matter connects the two hemispheres; cross-hemisphere signaling is common.
  • Cortex architecture and function:
    • Cortex: outer gray matter; executive functions, consciousness, awareness, understanding, communication.
    • Insula (insular cortex): folded region beneath frontal and temporal lobes; associated with self-awareness.
    • Cortex thickness: typically a few millimeters thick (about 2-4\,\text{mm}) yet contains roughly 6\text{ layers} across regions.
    • The term cortex is Greek for "bark".
  • Major brain regions (in brief):
    • Cerebral hemispheres (frontal, parietal, occipital, temporal lobes; insula).
    • Diencephalon (thalamus, hypothalamus, epithalamus).
    • Brain stem (midbrain, pons, medulla).
    • Cerebellum (dorsal, posterior; coordinates movement).
  • Spinal cord organization (overview): gray matter in the center (butterfly shape) surrounded by white matter; white matter consists of myelinated axons forming tracts; gray matter contains interneurons and motor neurons; dorsal and ventral roots.
  • CNS protection overview:
    • Skin, bone (skull and vertebrae).
    • Meninges: dura mater (outer, "hard mother"), arachnoid mater (middle, "spider mother"), pia mater (inner, hugs CNS).
    • Cerebrospinal fluid (CSF) bathes CNS, provides nutrients, removes waste.
    • CSF production: choroid plexus (ependymal cells) within ventricles; CSF circulates through CNS and is renewed (~3 times per day).

Brain Structure: Cerebral Hemispheres and Lobes

  • Four major lobes:
    • Frontal lobe: located at the front; involved in voluntary thought, planning, executive function; contains prefrontal cortex (blue/purple region) which is the last part to develop in humans (usually into the mid-20s).
    • Parietal lobe: superior-dorsal surface; integrates senses.
    • Occipital lobe: visual function (visual processing).
    • Temporal lobe: hearing and memory formation.
  • Insula (insular cortex): folded area located between frontal and temporal lobes; associated with self-awareness; sometimes viewed as underlying cortex beneath other lobes.
  • Frontal-parietal boundary landmark:
    • Central sulcus: separates frontal and parietal lobes; key landmark.
    • Regions around central sulcus:
    • Primary motor cortex (precentral gyrus): just anterior to central sulcus; initiates voluntary motor control; somatotopic map with face area large due to fine motor control for speech and expressions.
    • Primary somatosensory cortex (postcentral gyrus): just posterior to central sulcus; processes tactile and proprioceptive information.
  • Visual areas:
    • Primary visual cortex located in the occipital lobe; processes basic visual input; associated visual areas for higher processing.
  • Language areas (Fernicke’s and Broca’s areas):
    • Fernicke’s area (temporal/parietal association area): language comprehension; speech understanding.
    • Broca’s area (frontal lobe, near the precentral gyrus): speech production; motor aspects of speech.
    • Aphasia types (based on damage):
    • Fernicke’s aphasia: fluent speech with impaired comprehension; speech may be grammatically correct but semantically nonsensical; patients may not understand questions.
    • Broca’s aphasia: non-fluent, labored speech; comprehension relatively preserved; difficulty forming words.
  • Lateralization and language scientists’ notes:
    • Connectivity via white matter allows language and other functions to be distributed and communicate across hemispheres.
  • Prefrontal cortex development and decision-making:
    • Prefrontal cortex is the last adult brain region to mature (often in the mid-20s); implications for impulse control and decision-making in teenagers.

Neural Pathways: Cortical Areas and Functional Maps

  • Motor and sensory maps around the central sulcus:
    • Primary motor cortex (precentral gyrus) controls motor output; different body parts occupy different cortical areas.
    • Primary somatosensory cortex (postcentral gyrus) processes sensory input; somatosensory map mirrors motor map in many respects.
  • Somatotopy examples:
    • Face/mouth region has a large representation in motor cortex due to fine motor control for speech and eating.
    • Back of knee has a small motor/sensory representation due to less fine motor or tactile requirements.

Basal Nuclei (Basal Ganglia) and Movement

  • Three main nuclei:
    • Caudate nucleus
    • Putamen
    • Globus pallidus
  • Function:
    • Influence on slow or stereotyped movements; cognitive and emotional roles; interpretation and filtering of motor commands.
    • Help to filter out inappropriate responses and refine motor plans (movement selection).
  • Huntington’s disease (genetic):
    • Degeneration of basal nuclei, particularly pathways to the thalamus.
    • Symptoms: Huntington’s chorea—uncontrolled, large ballistic movements of limbs due to loss of normal inhibitory control.

Diencephalon and Limbic System

  • Diencephalon components (three main parts):
    • Thalamus (largest portion): relay station for all sensory input; routes to cortical areas for interpretation (e.g., visual input to occipital cortex via thalamus).
    • Hypothalamus (dark purple): homeostatic regulator; controls temperature, water balance, pupil size, blood pressure; connected to the pituitary gland via the hypothalamic-pituitary portal system; releases various hormones.
    • Epithalamus (lavender): largely pineal gland; secretes melatonin, regulating sleep-wake cycles; melatonin production inhibited by light and stimulated in darkness.
  • Sleep-wake regulation and environmental factors:
    • Light input influences melatonin release via pineal gland; bright light at night can suppress melatonin and affect sleep.

Sleep: Stages, Neurotransmitters, and Importance

  • Sleep is an active brain process with distinct stages assessed by EEG.
  • Brain wave patterns:
    • Awake: low amplitude, high frequency.
    • REM sleep: resembles wakefulness in EEG pattern but with atonia; associated with dreaming.
    • Stage 1: light sleep; short, low amplitude waves.
    • Stage 2: sleep spindles (larger amplitude, slower frequency).
    • Stage 3: more slow-wave activity; deeper sleep.
    • Stage 4: deepest sleep with highest amplitude, lowest frequency (slow waves).
  • Sleep cycles:
    • Night-long cycles progress from wake -> Stage 1 -> Stage 2 -> Stage 3 -> Stage 4 -> back up through Stage 2 to REM; repeat across the night.
    • REM duration tends to increase in later cycles.
  • Importance of REM and sleep in general:
    • REM sleep linked to memory consolidation; deprivation leads to concentration and cognitive problems.
    • Sleep supports immune function; chronic sleep deprivation impairs learning and memory.
  • Neurotransmitters and arousal systems:
    • Awake arousal: acetylcholine, norepinephrine, dopamine support attention and wakefulness.
    • Hypothalamus influence: histamine and orexin promote wakefulness.
    • Dip in wakefulness signals triggers sleep initiation; antihistamines can cause drowsiness by blocking histamine signals.
  • Special note on dolphins:
    • Sleep with one cerebral hemisphere at a time (one hemisphere asleep, the other awake) to surface for breathing and predator awareness; opposite eye closure corresponds to the sleeping hemisphere.

Limbic System, Emotion, Memory, and Psychosomatic Concepts

  • Core limbic structures:
    • Amygdala: processes emotions (fear, anger, happiness, etc.); drives emotional responses.
    • Hippocampus: central to learning and memory consolidation; site of long-term potentiation (LTP).
    • Rhinencephalon: olfactory structures; links odors to memories and emotions.
    • Thalamus and hypothalamus: integrate limbic signals with cortical processing and autonomic responses.
  • Limbic system and emotion-memory integration:
    • Olfactory cues can trigger memories due to Rhinencephalon–hippocampus–amygdala connections.
    • Emotions modulate memory encoding via amygdala–hippocampus interactions; emotionally charged events are often remembered better.
  • Psychosomatic illnesses:
    • Emotional stress can manifest as physical symptoms; pain and other sensations may be interpreted in the cortex via the thalamus under limbic influence.
    • Important reminder to clinicians: pain can be real even when physical pathology is not evident; biopsychosocial aspects matter.
  • Learning and memory concepts:
    • Learning: acquisition of new information.
    • Memory: retention and recall of information; improved by repetition and emotional salience.
    • Neural plasticity: physical changes in synapses during learning; hippocampus is a key site for plasticity.
    • Synaptic changes: repeated firing strengthens synapses via increased neurotransmitter release, receptor density, and dendritic spine growth — a process known as long-term potentiation (LTP).
  • Memory processes and brain regions:
    • Procedural (skill-based) memory involves cerebellum and basal nuclei; often automatic and not requiring conscious effort.
    • Declarative (fact-based) memory involves hippocampus and associated cortical areas; requires conscious recall.

Memory, Learning, and Synaptic Plasticity: Mechanics of Change

  • Synapse dynamics (illustrative):
    • When an action potential arrives at a presynaptic terminal, neurotransmitter release may cause the postsynaptic neuron to reach threshold and fire.
    • Repeated activation at a synapse strengthens the connection: more neurotransmitter release, more postsynaptic receptors, and larger dendritic spines (spine growth).
    • This strengthening across multiple sessions underlies long-term potentiation (LTP), a cellular substrate for memory.
  • Memory consolidation and interconnections:
    • Hippocampus links with amygdala and cortex to consolidate and retrieve memories.
    • Emotional associations strengthen memories; repetition across senses (reading, listening, note-taking) reinforces memory traces.
  • Practical study tips derived from memory science:
    • Use multiple senses to study (read, listen, take notes) to create multiple memory traces.
    • Create connections between new material and existing knowledge (e.g., mnemonics like "banana in the ocean").
    • Emphasize repetition to promote stronger synaptic connections.

The Brain Stem and Cerebellum: Foundations of Life-Support and Movement

  • Brain stem sections and key roles:
    • Midbrain (top):,
    • Superior colliculi: track and follow moving objects; visual tracking and eye movements.
    • Inferior colliculi: startle reflex and sound localization.
    • Substantia nigra: dopaminergic neurons projecting to basal nuclei; initiates motor plans; degeneration linked to Parkinson’s disease (difficulty initiating movement).
    • Pons: relays motor signals to the cerebellum; involved in regulating respiration via signals to the diaphragm.
    • Medulla oblongata: vital autonomic centers (cardiovascular, respiratory); vomiting center; swallowing and vomiting control.
  • Cerebellum:
    • Coordinates voluntary motor movements; integrates multiple muscle groups for smooth, accurate action.
    • Damage leads to ataxia (uncoordinated, slowed movements, swaying, staggering).

Protection of the CNS: Meninges, CSF, and Circulation

  • Meninges layers (superficial to deep):
    • Dura mater: hard, protective outer layer.
    • Arachnoid mater: web-like middle layer; subarachnoid space beneath contains CSF.
    • Pia mater: delicate inner layer tightly hugging brain/spinal cord.
  • Cerebrospinal fluid (CSF):
    • Produced by choroid plexus (ependymal cells) located in ventricles.
    • CSF circulates through ventricles and subarachnoid space; bathes CNS; removes wastes; provides nutrients.
    • Ventricular system: two lateral ventricles, third ventricle (near diencephalon), fourth ventricle (near hindbrain).
    • CSF turnover: about
      3\times per day in adults.
  • Blood supply and metabolism in CNS:
    • CNS weight vs blood supply: about 2\% of body weight but receives about 15\% of blood supply.
    • Neurons have high metabolic demand: about 20\% of resting oxygen consumption and 50\% of resting glucose utilization.
    • Circle of Willis: arterial circle at the base of the brain that provides collateral flow; protects tissue from ischemia due to blockage elsewhere.
    • Stroke and watershed damage: blockages or ruptures can reduce blood flow downstream, causing tissue death.

Blood-Brain Barrier and Two Special Regions

  • Blood-brain barrier (BBB):
    • Tight endothelial junctions formed with astrocyte endfeet prevent most substances from passing between blood and brain tissue.
    • Purpose: protect CNS from pathogens while allowing selective nutrient transport.
  • Areas without a BBB (two exceptions):
    • Vomiting center in the medulla: requires sensing of toxins in blood.
    • Hypothalamus: regulates osmolarity and water balance; needs dynamic exposure to blood contents.

Spinal Cord: Structure, Nerves, and Protective Features

  • Location and general function:
    • Extends from foramen magnum to approximately between L1-L2 in adults; enables brain–body communication (ascending sensory, descending motor) and cross-communication.
  • Meninges continue around the spinal cord.
  • Gray matter and white matter arrangement:
    • Gray matter in the center (butterfly shape) with dorsal (sensory) and ventral (motor) horns; lateral horns present in thoracic and upper lumbar regions for sympathetic neurons.
    • White matter surrounds gray matter with ascending, descending, and commissural tracts.
  • Dorsal root and ventral root:
    • Dorsal root: afferent (sensory) fibers entering the spinal cord; contains dorsal root ganglia where cell bodies of sensory neurons reside.
    • Ventral root: efferent (motor) fibers leaving the spinal cord.
    • A spinal nerve is formed where dorsal and ventral roots mix and exit the spine.
  • Spinal nerves: 31 pairs total; mixed sensory and motor.
  • Cauda equina: below L1–L2, a bundle of nerve roots resembles a horse’s tail; safe site for lumbar puncture and epidural injections.
    • Lumbar puncture (CSF sample or injection) typically performed below L3 to avoid the spinal cord.
  • Clinical protective features:
    • Vertebral column protects spinal cord.
    • Spinal nerves can be moved aside to avoid injury during procedures (e.g., lumbar puncture).

Reflexes: Pathways and Examples

  • Reflex basics: an involuntary, automatic response to a stimulus; spinal cord often handles the reflex, sometimes subject to brain processing later.
  • Reflex arc components:
    • Receptor senses the stimulus.
    • Afferent neuron transmits signal to CNS.
    • CNS integrates information.
    • Efferent neuron transmits a response to an effector (muscle or gland).
    • Resulting behavior or action.
  • Stretch (myotatic) reflex example:
    • Tap on patellar tendon stretches muscle spindles in quadriceps.
    • Afferent signal travels to spinal cord; motor neurons in quadriceps trigger contraction.
    • Antagonist (hamstring) contraction is inhibited (reciprocal inhibition) to allow leg extension.
    • Monosynaptic reflex: involves a single synapse between the afferent and efferent neuron; minimal interneuronal involvement.
  • Withdrawal and cross-extensor reflex (protective reflex):
    • Step on a nail example: pain stimulus travels via afferent fibers to dorsal horn; interneurons coordinate response to withdraw the affected leg.
    • If weight-bearing must be maintained, the contralateral leg extends to support body weight (cross-extensor reflex).
    • This reflex involves multiple synapses and interneurons across both sides of the spinal cord.

Additional Notable Concepts and Highlights

  • Neuronal regeneration:
    • CNS neurons regenerate poorly; regeneration is limited to two areas: olfactory bulb and hippocampus.
  • Tau tangles and neurodegeneration:
    • Tau neurofibrillary tangles observed in traumatic brain injury and neurodegenerative diseases (e.g., CTE, Alzheimer’s disease); debate on whether tangles cause neuron death or result from it.
  • Memory consolidation and hippocampus anatomy:
    • Hippocampus has a distinctive curled structure important for forming and consolidating memories; site of plastic changes during learning.
  • Practical clinical notes:
    • Understanding of the CNS sections helps interpret deficits (language, movement, memory, emotion).
    • Recognition of structural landmarks (central sulcus, hippocampus, basal nuclei, circle of Willis) guides assessment and intervention.

Key Terms and Landmarks Summary (quick reference)

  • Gyri and Sulci: brain folds and grooves; increase surface area.

  • Fissure: deep groove separating hemispheres.

  • Gray matter vs White matter: somas vs myelinated axons.

  • Cortex: outer gray matter; 2-4 mm thick; 6 cortical layers.

  • Central sulcus: divides frontal (motor) and parietal (sensory) cortices; near precentral and postcentral gyri.

  • Wernicke’s (Fernicke’s) area: language comprehension (temporal/parietal association cortex).

  • Broca’s area: language production (frontal near precentral gyrus).

  • Prefrontal cortex: executive function; late-maturing.

  • Thalamus: sensory relay to cortex.

  • Hypothalamus: homeostasis; endocrine regulation via pituitary.

  • Epithalamus: pineal gland; melatonin.

  • Circle of Willis: collateral cerebral circulation.

  • Basal nuclei: caudate, putamen, globus pallidus; motor filtering and cognitive/emotional roles.

  • Huntington’s disease: caudate/putamen degeneration; chorea.

  • Limbic system: emotion, memory, smell integration (amygdala, hippocampus, rhinencephalon).

  • Hippocampus: learning and memory; plasticity center.

  • Cerebellum: coordination of movement; ataxia with damage.

  • Brainstem: midbrain, pons, medulla; vital autonomic control; reflexes.

  • BBB: tight endothelial junctions; astrocyte endfeet; two BBB-exempt areas (medulla vomiting center and hypothalamus).

  • CSF and choroid plexus: CSF production and circulation; ventricles.

  • Spinal cord anatomy: gray matter inside, white matter outside; dorsal root ganglia; dorsal/ventral roots; cauda equina.

  • Reflexes: stretch reflex (monosynaptic); withdrawal and cross-extensor reflex.

  • Neurotransmitters and arousal: acetylcholine, norepinephrine, dopamine; histamine and orexin in hypothalamus.

  • Numerical and factual references (for quick recall):

    • Language lateralization: \approx 90\%\, left-hemisphere language dominance; \approx 10\%\, right or bilateral.
    • Cortex thickness: 2-4\,\text{mm}.
    • Cortex mass contribution: ~40\% of total brain mass.
    • Brain energy use: \approx 20\% of resting oxygen; \approx 50\% of resting glucose.
    • Spinal nerves: 31\text{ pairs}.
    • CSF turnover: ~3\times per day.
    • Parkinson’s disease: degeneration of dopaminergic neurons in the substantia nigra.
    • CTE occurrence after concussions: about 17\% of individuals with concussions.
    • Spinal cord ends around the vertebral level L1-L2; lumbar puncture typically below L3.