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1. Ascending Tracts

Introduction

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A. Dorsal Column - Medial Lemniscal (DCML) Pathway

Sensations Carried

• Fine/discriminative touch
• Vibration sense
• Conscious proprioception (joint position sense)
• Two-point discrimination
• Stereognosis

Three-Neuron Arc

Fasciculus Gracilis vs. Cuneatus

Flowchart - DCML Pathway

Peripheral receptor (skin, muscle, joint)
        ↓
1st order neuron (DRG cell body)
        ↓
Enters dorsal horn → ascends IPSILATERALLY
 in fasciculus gracilis (below T6)
 or fasciculus cuneatus (above T6)
        ↓
Nucleus gracilis / cuneatus (medulla)
        ↓
Internal arcuate fibers → CROSS (sensory decussation)
        ↓
Medial lemniscus (contralateral ascent through brainstem)
        ↓
VPL nucleus of THALAMUS
        ↓
Thalamocortical radiation (posterior limb of internal capsule)
        ↓
PRIMARY SOMATOSENSORY CORTEX (postcentral gyrus)

Labelled Diagram - DCML Pathway

Clinical Note

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B. Anterolateral (Spinothalamic) Pathway

Sensations Carried

• Pain
• Temperature (crude)
• Crude touch and pressure

Flowchart - Spinothalamic Tract

Peripheral nociceptors / thermoreceptors
        ↓
1st order neuron (DRG cell body)
        ↓
Enters dorsal horn → synapses in REXED LAMINAE I, II, V
        ↓
2nd order neuron CROSSES immediately in anterior commissure
        ↓
Ascends CONTRALATERALLY as spinothalamic tract
(in anterolateral white matter)
        ↓
VPL nucleus of THALAMUS
        ↓
Thalamocortical fibers
        ↓
PRIMARY SOMATOSENSORY CORTEX

Labelled Diagram - Spinothalamic (DCML also shown)

Key Difference between the Two Pathways

Clinical Relevance: Brown-Séquard Syndrome (Hemisection of cord)

• Ipsilateral loss of fine touch, vibration, proprioception (DCML)
• Contralateral loss of pain and temperature (spinothalamic), 1-2 levels below lesion

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2. Corticospinal Tract

Introduction

Origin

• 30% - Primary motor cortex (precentral gyrus, Brodmann area 4)
• 30% - Premotor and supplementary motor areas (area 6)
• 40% - Somatosensory cortex (parietal lobe, areas 3, 1, 2, 5, 7)

Course - Flowchart

PRIMARY MOTOR CORTEX (precentral gyrus, area 4)
        ↓
Corona radiata (upper cerebral white matter)
        ↓
POSTERIOR LIMB OF INTERNAL CAPSULE
 (face fibers: anterior; arm: middle; leg: posterior)
        ↓
BASIS PEDUNCULI (cerebral peduncles, midbrain)
 (middle 1/3: CST; medial 1/3: face → arm; lateral 1/3: leg)
        ↓
PONS (dispersed fascicles, ventral pons)
        ↓
MEDULLARY PYRAMIDS (ventral medulla)
        ↓
PYRAMIDAL DECUSSATION (cervicomedullary junction)
 ~85% cross → LATERAL CORTICOSPINAL TRACT (contralateral spinal cord)
 ~15% do NOT cross → VENTRAL CORTICOSPINAL TRACT (ipsilateral)
        ↓
Synapse on INTERNEURONS (mainly) or LMNs in anterior horn
        ↓
SKELETAL MUSCLE (voluntary movement)

Labelled Diagrams

Key Facts

UMN vs. LMN Lesion

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3. Functions and Connections of the Thalamus

Introduction

Structure

• Divided by an internal Y-shaped white matter sheet (internal medullary lamina) into multiple nuclear groups
• Surrounded laterally by the reticular nucleus (a thin shell of inhibitory GABAergic neurons)

Major Nuclear Groups

Connections - Diagram

Functions Flowchart

THALAMUS - FUNCTIONS
       |
       ├─ SENSORY RELAY
       │    ├─ VPL → Somatic sensation (body) → Postcentral gyrus
       │    ├─ VPM → Somatic sensation (face) + Taste → Postcentral gyrus
       │    ├─ LGN → Vision → Occipital cortex (area 17)
       │    └─ MGN → Hearing → Temporal cortex (Heschl's gyrus)
       │
       ├─ MOTOR RELAY
       │    └─ VA/VL ← Cerebellum (dentate nucleus) + Basal ganglia
       │            → Motor cortex (area 4 + area 6)
       │
       ├─ LIMBIC / MEMORY
       │    └─ Anterior nucleus ↔ Cingulate gyrus, hippocampus (Papez circuit)
       │
       ├─ AROUSAL / CONSCIOUSNESS
       │    └─ Intralaminar nuclei ← Reticular formation → Widespread cortex
       │
       └─ CORTICAL MODULATION
            └─ Reticular nucleus - gates thalamic output (inhibitory, GABAergic)

Key Points

• All sensory systems (except olfaction) relay in the thalamus before reaching cortex
• Reciprocal connections: every cortical area that receives thalamic input also sends fibers back to the same thalamic nucleus (layer VI of cortex → thalamus)
• Thalamic stroke (VPL infarct): contralateral sensory loss + thalamic pain (Dejerine-Roussy syndrome)

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4. Structure, Connections and Functions of the Cerebral Cortex

Introduction

Gross Structure

• Thrown into folds: gyri (ridges) and sulci (grooves)
• Major sulci divide each hemisphere into 4 lobes: frontal, parietal, temporal, occipital (+ insular cortex)
• Surface area: ~2,500 cm² (most hidden in sulci)

Histological Structure - Layers of Neocortex (6 layers)

Special Cortical Types

• Granular cortex (koniocortex): thick layer IV, prominent granule cells - sensory areas (postcentral gyrus, primary visual cortex)
• Agranular cortex: thick layer V, no layer IV - motor areas (precentral gyrus)
• Allocortex: 3-layered older cortex (hippocampus, olfactory cortex)

Functional Areas (Brodmann's Areas)

Connections

Cortical Connections Flowchart

CEREBRAL CORTEX CONNECTIONS
        |
        ├─ ASSOCIATION FIBERS (within same hemisphere)
        │    ├─ Short (U-fibers): adjacent gyri
        │    └─ Long fasciculi:
        │         ├─ Arcuate fasciculus: frontal ↔ temporal (language)
        │         ├─ Uncinate fasciculus: frontal ↔ temporal (memory/emotion)
        │         ├─ Superior longitudinal fasciculus
        │         └─ Cingulum: cingulate gyrus (limbic)
        │
        ├─ COMMISSURAL FIBERS (between hemispheres)
        │    ├─ Corpus callosum (largest) - connects all lobes
        │    ├─ Anterior commissure - connects temporal lobes
        │    └─ Posterior commissure
        │
        └─ PROJECTION FIBERS (cortex ↔ subcortical structures)
             ├─ Thalamocortical / Corticothalamic (internal capsule)
             ├─ Corticospinal (descending motor)
             ├─ Corticobulbar (to cranial nerve nuclei)
             └─ Corticopontine (to cerebellum via pons)

Key Functions

• Frontal lobe: voluntary motor control, executive function, personality, Broca's speech area (dominant), inhibition of behavior
• Parietal lobe: somatic sensation, spatial awareness, body image; angular gyrus (reading, calculation)
• Temporal lobe: hearing, memory (hippocampus), Wernicke's speech area (dominant), olfaction
• Occipital lobe: vision; dorsal stream (where/how) via parietal, ventral stream (what) via temporal
• Association cortex (70% of neurons): integrates information across modalities; heteromodal association cortex in angular gyrus and prefrontal cortex is responsible for highest cognitive functions

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5. Visual Pathway with Lesions

Introduction

Anatomy of the Visual Pathway

Retinal Organization

• Nasal retina receives input from the temporal (lateral) visual field
• Temporal retina receives input from the nasal (medial) visual field
• Macula (central vision): represented disproportionately in the cortex

Course - Flowchart

RETINAL GANGLION CELLS
        ↓
OPTIC NERVE (CN II)
(ipsilateral eye only; carries all visual info from that eye)
        ↓
OPTIC CHIASM (at sella turcica)
 ┌─────────────────────────────────────────┐
 │  Nasal (medial) retinal fibers → CROSS  │
 │  Temporal (lateral) retinal fibers →    │
 │  remain IPSILATERAL                     │
 └─────────────────────────────────────────┘
        ↓
OPTIC TRACT
(carries: ipsilateral temporal + contralateral nasal fibers)
(= all visual info from CONTRALATERAL visual field)
        ↓
LATERAL GENICULATE NUCLEUS (LGN) of thalamus
        ↓
OPTIC RADIATIONS (geniculocalcarine tract)
  ┌────────────────────────────────────────────┐
  │ UPPER fibers → parietal lobe               │
  │   (carry inferior visual field)            │
  │ LOWER fibers → loop through temporal lobe  │
  │   (Meyer's loop - carry superior visual    │
  │   field)                                   │
  └────────────────────────────────────────────┘
        ↓
PRIMARY VISUAL CORTEX (area 17)
Calcarine sulcus, occipital lobe
  Upper bank = inferior visual field
  Lower bank = superior visual field
  Posterior pole = macula (largest representation)

Lesions and Visual Field Defects

Lesion Table

Key Rule: Pre-chiasmal vs. Post-chiasmal

• Pre-chiasmal lesion (optic nerve): monocular defect in one eye only
• At chiasm: bitemporal heteronymous defect
• Post-chiasmal lesion (optic tract → cortex): contralateral homonymous defect

Congruity

• Optic tract lesions: incongruous (asymmetric defects in both eyes)
• Optic radiation / cortical lesions: congruous (symmetric, identical defects)

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6. Organ of Corti

Introduction

Structure of the Cochlea (Context)

• Perilymph: high Na⁺, low K⁺ (like ECF)
• Endolymph: high K⁺, low Na⁺ (like ICF) - maintained by stria vascularis in the lateral wall of scala media

Structure of the Organ of Corti

Components

Labelled Diagram

Mechanism of Auditory Transduction - Flowchart

SOUND WAVES enter external auditory canal
        ↓
TYMPANIC MEMBRANE vibrates
        ↓
OSSICLES (malleus → incus → stapes) transmit vibration
(amplified by lever action + area difference of TM vs. oval window)
        ↓
STAPES pushed into OVAL WINDOW
        ↓
Fluid waves in SCALA VESTIBULI (perilymph)
        ↓
BASILAR MEMBRANE vibrates at specific location
(base = high frequency; apex = low frequency - tonotopy)
        ↓
Organ of Corti moves UP and DOWN
        ↓
SHEARING FORCE between reticular lamina and tectorial membrane
        ↓
STEREOCILIA of hair cells bend
        ↓
TIP LINKS open K⁺/Ca²⁺ mechanosensitive channels
        ↓
K⁺ influx into hair cell (from endolymph) → DEPOLARIZATION
        ↓
Voltage-gated Ca²⁺ channels open at base of hair cell
        ↓
Neurotransmitter (glutamate) released onto cochlear nerve fibers
        ↓
Action potential in SPIRAL GANGLION NEURONS (1st order)
        ↓
Axons form COCHLEAR NERVE (CN VIII) →
        ↓
Cochlear nuclei (dorsal + ventral) in brainstem
        ↓
Superior olivary nucleus (bilateral), inferior colliculus, MGB (medial geniculate body, thalamus)
        ↓
PRIMARY AUDITORY CORTEX (Heschl's gyrus, areas 41, 42 - temporal lobe)

Key Concepts

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Sources

• Neuroanatomy through Clinical Cases, 3rd Edition (Blumenfeld)
• Guyton and Hall Textbook of Medical Physiology (Hall)
• Costanzo Physiology, 7th Edition
• Bradley and Daroff's Neurology in Clinical Practice
• TeachMeAnatomy.info (DCML pathway diagram)
• Medbullets (Visual field defects diagram)