Anatomy of the Nervous System – Comprehensive Study Notes

Embryonic Development

Three germ layers give rise to all tissues
- Endoderm (internal layer)
- Gives rise to: lung alveolar cells, thyroid cells, digestive pancreatic cells
- Mesoderm (middle layer)
- Gives rise to: cardiac muscle, skeletal muscle, kidney tubule cells, red-blood-cell precursors, smooth muscle in gut
- Ectoderm (external layer)
- Gives rise to: skin (epidermis), neurons of brain/spinal cord, pigment cells
Neuro-ectoderm
- Ectoderm thickens → differentiates into neuroectoderm
- Neuroectoderm folds inward → neural groove
- Groove edges converge → neural tube (beneath remaining ectoderm)
- Anterior end ⇒ future brain
- Posterior end ⇒ future spinal cord
- Neural crest (cells at crest of folds) ⇒ peripheral structures (e.g., dorsal root ganglia, autonomic ganglia, melanocytes)

Primary & Secondary Brain Vesicles

Primary vesicles (early encephalon)
- Prosencephalon (forebrain)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain; rhombus-shaped)
Secondary vesicles (5-week embryo)
- Telencephalon → cerebrum (cortex + white matter + basal nuclei)
- Diencephalon → retina, thalamus, hypothalamus, epithalamus
- Mesencephalon (midbrain) → remains undivided; surrounding brain regions grow around it
- Metencephalon → pons (brain-stem) & cerebellum
- Myelencephalon → medulla oblongata
Visual timeline
- $\text{1-month embryo} \rightarrow 3 \text{ primary vesicles}$
- $\text{5-week embryo} \rightarrow 5 \text{ secondary vesicles}$
- Child/adult: mature brain structures (cerebrum, diencephalon, midbrain, pons, medulla, cerebellum, spinal cord)

Spinal Cord Development

Neural tube subdivides into longitudinal plates separated by sulcus limitans
- Alar plate (dorsal) → sensory gray matter (posterior horns)
- Basal plate (ventral) → motor gray matter (anterior & lateral horns)
Week-4: tube + neural canal, neural crest forming peripheral ganglia
Week-6: differentiation of motor neuron cell bodies (ventral) & sensory interneurons (dorsal)
Week-9: distinct gray horns & surrounding white matter; central canal persists

Neuraxis & Cephalic Flexure

Neuraxis = embryonic axis of neural tube (anterior ↔ posterior coordinate)
Cephalic flexure = pronounced bend between brain stem & forebrain that reorients axes; underlies adult right-angle between brainstem and cerebrum

Functional Pathways of the Nervous System

Afferent pathways (Arriving)
- Convey sensory information from receptors → CNS
Efferent pathways (Exiting)
- Carry motor commands from CNS → effectors (muscle, gland)
Organizational chart
- CNS (brain & spinal cord) ↔ PNS (nerves, ganglia)
- PNS divides into afferent (sensory) & efferent (motor)
- Additional sensory & motor neurons interconnect within PNS

Central Nervous System – Macrostructures

Cerebrum = bulk/mass of brain
- Cerebral cortex = outer wrinkled gray matter (gyri & sulci)
- Longitudinal fissure divides right vs left hemispheres
- Corpus callosum = large white-matter commissure enabling inter-hemispheric communication
- Basal nuclei (ganglia) = deep gray matter involved in cognitive/motor processing
- Limbic system = emotion, memory, behavior (includes amygdala, hippocampus, cingulate gyrus, etc.)

Cerebral Cortex: Gross Anatomy

Surface terminology
- Gyrus = ridge of tissue
- Sulcus = groove between gyri
Four major lobes
- Frontal lobe
- Precentral gyrus = primary motor cortex (voluntary movement)
- Prefrontal cortex = personality, executive function, short-term memory, consciousness
- Broca’s area (Areas $44$ & $45$ ) = motor speech production (dominant hemisphere)
- Parietal lobe
- Somatosensation, proprioception, tactile processing, spatial orientation, movement integration
- Temporal lobe
- Primary auditory cortex, auditory association, aspects of memory & language comprehension (part of Wernicke’s area)
- Occipital lobe
- Primary visual cortex + visual association areas

Brodmann’s Areas (1909 cytotectonic map)

Numerical designation based on cytoarchitecture correlates with function
- Area $4$ = primary motor cortex (precentral gyrus)
- Areas $1,2,3$ = primary somatosensory cortex (postcentral gyrus)
- Areas $44,45$ = Broca’s area (speech)
- Areas $39,40$ + $22$ = Wernicke’s area (language comprehension)
- Area $17$ = primary visual cortex; Areas $18,19$ = visual association
- Area $38$ = temporal pole (emotional memory)
- Frontal Areas $9,10$ = higher executive function

Subcortical Structures – Limbic Components

Hippocampus
- Medial temporal lobe; consolidates declarative (long-term) memory; spatial navigation
Amygdala
- Adjacent medial temporal structure; emotional responses, fear conditioning, assigns affective value to stimuli

Basal Ganglia (Basal Nuclei)

Definition: collection of neuronal cell bodies in CNS that modulate motor & cognitive circuits
Major constituents (corpus striatum + pallidum)
- Caudate nucleus (C-shaped, follows lateral ventricle)
- Putamen (lateral portion of striatum)
- Globus pallidus (GP) (internal & external segments) medial to putamen
- Substantia nigra (SN) in midbrain (pars compacta – dopamine source; pars reticulata)
- Subthalamic nucleus (STN) (diencephalic)
- VA/VL thalamic nuclei act as output relays to cortex

Direct vs Indirect Pathways (Motor Modulation)

Abbreviations: $GP<em>{i}$ /SNr = globus pallidus internal segment & substantia nigra pars reticulata (output nuclei); $SN</em>{c}$ = substantia nigra pars compacta; $GP_{e}$ = globus pallidus external; STN = subthalamic nucleus
Direct pathway
- Cortex → striatum (GABA) → inhibits $GP_{i}/SNr$ → disinhibits thalamus (VA/VL) → increased cortical excitation → facilitates movement
Indirect pathway
- Cortex → striatum → inhibits $GP<em>{e}$ → less inhibition on STN → STN excites $GP</em>{i}/SNr$ → stronger inhibition of thalamus → suppresses movement
Dopamine from $SN_{c}$
- D1 receptors on direct pathway (excitatory) enhance movement
- D2 receptors on indirect pathway (inhibitory) reduce suppression → overall pro-movement effect
Clinical significance: imbalance causes disorders (Parkinson’s, Huntington’s, dystonia)

Diencephalon

Acts as gateway between cerebrum & rest of nervous system (except olfaction)
Thalamus
- Bilateral collection of nuclei; relays & processes all sensory modalities except smell; motor, limbic, and cognitive relay
Hypothalamus
- Homeostatic control: autonomic nervous system regulation, endocrine interface (pituitary), temperature, hunger, circadian rhythms

Brain Stem

Midbrain (mesencephalon)
- Integrates visual (superior colliculus), auditory (inferior colliculus), and somatosensory spatial maps; contains cerebral peduncles
Pons (metencephalon)
- Relay between cerebrum ↔ cerebellum; houses nuclei controlling respiration rate, sleep, bladder
Medulla oblongata (myelencephalon)
- Autonomic centers for cardiovascular/respiratory regulation, reflexes (cough, swallow), cranial nerve nuclei

Cerebellum – “Little Brain”

Compares descending motor commands with sensory feedback (proprioception)
Inferior olive (medulla) sends climbing fibers – acts as sensory copy
Fine-tunes movement, posture, balance; error correction (e.g., Is ground rocky? Is gait coordinated?)

Spinal Cord Anatomy

Gray Matter (horns)

Posterior (dorsal) horn: sensory processing (afferent)
Anterior (ventral) horn: somatic motor neuron cell bodies → skeletal muscles
Lateral horn: present in thoracic, upper lumbar, sacral levels; visceral motor neurons of sympathetic division

White Matter (columns/ funiculi)

Posterior columns: ascending sensory tracts
Anterior columns: mixed ascending & descending
Lateral columns: mixed ascending & descending
Ascending tracts (afferent) carry sensory info to brain; descending tracts (efferent) carry motor commands from brain

Cerebral & Spinal Circulation

Circle of Willis (Arterial)

Aorta → heart → common carotid arteries (internal carotids) & vertebral arteries
Carotids + vertebrals form circle at brain base ensuring collateral flow
Orthostatic reflex maintains cerebral perfusion when standing; carotid baroreceptors detect pressure ↓ → heart rate ↑ & blood pressure ↑

Dural Sinuses (Venous)

Superior sagittal sinus in longitudinal fissure absorbs CSF
Drains → confluence of sinuses (joins occipital & straight sinuses) → transverse sinuses → sigmoid sinuses → internal jugular veins → heart/lungs

Meninges – Protective Membranes

Dura mater (“tough mother”): outer fibrous covering brain & spinal cord
Arachnoid mater (“spider mother”): thin fibrous membrane; forms loose sac
- Arachnoid trabeculae: filamentous mesh resembling spider web
Subarachnoid space: CSF-filled, between arachnoid & pia
Pia mater (“tender mother”): thin membrane following gyri & sulci; closest to neural tissue
Lumbar puncture: needle through dura & arachnoid into subarachnoid space (lumbar cistern) to withdraw CSF safely (cord ends $\approx L1/L2$ )
Meningitis: inflammation of meninges (bacterial/viral/fungal)

Ventricles & CSF Circulation

Choroid plexus (specialized ependymal cells + capillaries) produces CSF (~ $500\,\text{mL/day}$ )
Ventricular system
- Lateral ventricles (1 & 2): right & left within cerebral hemispheres
- Third ventricle: midline diencephalon
- Fourth ventricle: between pons/medulla & cerebellum
- Cerebral aqueduct connects 3rd → 4th
- Median aperture (foramen of Magendie) & lateral apertures (foramina of Luschka) allow CSF → subarachnoid space
CSF circulation path illustrated (ventricles ↔ subarachnoid ↔ dural sinuses)

Stroke Awareness – F.A.S.T.

Face drooping
Arm weakness
Speech difficulty
Time to call emergency services
Stroke = disruption of blood supply; may be ischemic or hemorrhagic; rapid intervention critical

Ganglia vs Nuclei

Ganglia = collections of neuron cell bodies in PNS
- Dorsal root ganglion (DRG): sensory cell bodies entering spinal cord via dorsal root
- Cranial nerve ganglia: analogous sensory ganglia associated with cranial nerves
- Autonomic ganglia (sympathetic chain, parasympathetic ganglia)
Nuclei = collections of neuron cell bodies in CNS (gray matter)
- Examples: caudate, putamen, dentate, pallidum, substantia nigra, subthalamic nucleus
Functional difference: DRG mediates primary sensory input; basal nuclei modulate motor & cognitive circuits

Nerves – Peripheral Bundles of Axons

Composition: axons + connective tissue + blood vessels (vascular supply)
Connective tissue layers
- Epineurium: outer fibrous sheath of whole nerve
- Perineurium: sheath around each fascicle (bundle)
- Endoneurium: delicate layer around individual axons (myelinated or unmyelinated)
Analogy: parallels skeletal muscle (epimysium, perimysium, endomysium)

Spinal Nerves & Plexuses

Sensory (afferent) fibers enter via dorsal root
Motor (efferent) fibers exit via ventral root (somatic & autonomic)
DRG present on dorsal root (sensory cell bodies)
Total of $31$ spinal nerves:
- $8$ cervical, $12$ thoracic, $5$ lumbar, $5$ sacral, $1$ coccygeal
Plexus = network of intersecting nerves (cervical, brachial, lumbar, sacral) containing mixed fibers; no cell bodies (purely axons)

Cranial Nerves (Overview – detailed in PPT 13B)

Twelve pairs (CN I–XII); arise from brain/brainstem; may carry sensory, motor, or mixed components; some contain cranial nerve ganglia (e.g., trigeminal, geniculate)

Discussion Prompts & Learning Objectives (Self-Check)

Model embryonic stages (play-doh activity)
Draw & label spinal cord cross-section (showing gray horns, white columns, roots, DRG)
List brain structures & functions (cortex lobes, basal nuclei pathways, limbic, diencephalon, brain stem, cerebellum)
List spinal cord structures & functions (horns, columns, tracts)
Define ganglia & distinguish from nuclei
Explain direct vs indirect basal nuclei pathways

Summary – “Can You…?”

Relate embryonic development to adult nervous structures
Identify major adult regions (cerebrum, diencephalon, brainstem, cerebellum, spinal cord)
Locate cortical regions via universal landmarks (lobes, gyri, sulci, Brodmann areas)
Describe spinal cord regions in cross-section (horns & columns)
Explain basal nuclei direct/indirect motor pathways & significance
Compare/contrast basal nuclei (CNS nuclei) with peripheral root ganglia (DRG)