Central Nervous System (CNS)
1. Nervous System Divisions
- Central nervous system (CNS)
→ Brain + Spinal cord - Peripheral nervous system (PNS)
→ Nerves outside CNS
→ Divided into somatic and autonomic systems
2. Brain — Main Parts
Three major divisions:
- Cerebrum
- Cerebral hemispheres
- Diencephalon (thalamus, hypothalamus)
- Brainstem
- Cerebellum
Brainstem (Very High Yield)
- Located at the base of the brain
- Connects the spinal cord to the cerebrum
- Divided into:
- Midbrain
- Pons
- Medulla
- Cerebellum lies posterior to the brainstem
3. Spinal Cord
Location and extent
- Lies within the vertebral canal
- Continuation of the medulla oblongata
- Ends as conus medullaris
|
Age |
Level of conus medullaris |
|
Birth |
L3 |
|
Adult |
L1–L2 |
- Below the conus → Cauda equina
- Nerve roots from L2 to sacrum
- Adult spinal cord occupies 2/3 of vertebral canal
4. Internal Structure of Spinal Cord
Cross-section appearance
- Central grey matter → “butterfly” shape
- Surrounding white matter
White matter
- Contains ascending (sensory) and descending (motor) tracts
Grey matter horns (Exam gold)
|
Horn |
Function |
|
Anterior (ventral) |
Motor neurons → skeletal muscle |
|
Posterior (dorsal) |
Sensory input |
|
Lateral |
Autonomic (visceral) function |
|
Intermediate |
Visceral and pelvic organ control |
5. Spinal Nerve Roots
Formation
- Anterior (ventral) root → motor fibres
- Posterior (dorsal) root → sensory fibres
- Roots unite → mixed spinal nerve
After exiting intervertebral foramen:
- Posterior ramus
- Intrinsic back muscles
- Skin of the back
- Anterior ramus
- Trunk and limb muscles
- Skin of trunk and limbs
- Visceral organs
6. Blood Supply of the Spinal Cord (High Yield)
Main supply: Vertebral arteries
Anterior spinal artery
- Single vessel formed from both vertebral arteries
- Supplies anterior 2/3 of spinal cord
- Anterior horn
- Lateral horn
- Anterior & lateral columns
- Damage → motor loss ± pain/temperature loss
Posterior spinal arteries (paired)
- Arise from:
- Posterior inferior cerebellar artery (PICA) or
- Vertebral artery
- Supply posterior 1/3 of spinal cord
- Posterior columns
- Damage → loss of proprioception & vibration
Exam Pearls
- Anterior horn = motor
- Posterior horn = sensory
- Conus medullaris in adults = L1–L2
- Below L1 = cauda equina
- Anterior spinal artery = motor pathways
Anterior vs Posterior Spinal Artery Syndrome (High-Yield)
|
Feature |
Anterior Spinal Artery Syndrome |
Posterior Spinal Artery Syndrome |
|
Artery involved |
Anterior spinal artery |
Posterior spinal arteries |
|
Cord territory supplied |
Anterior 2/3 of spinal cord |
Posterior 1/3 of spinal cord |
|
Tracts affected |
– Corticospinal tracts – Spinothalamic tracts – Anterior horn cells |
– Dorsal columns |
|
Motor function |
❌ Bilateral motor paralysis below lesion |
Motor function preserved |
|
Pain & temperature |
❌ Lost below lesion |
Preserved |
|
Vibration & proprioception |
Preserved |
❌ Lost |
|
Autonomic function |
❌ Bladder & bowel dysfunction common |
Usually preserved |
|
Reflexes (initially) |
Reduced → later UMN signs |
Usually normal |
|
Common causes |
– Aortic pathology (AAA repair) – Severe hypotension – Vertebral artery pathology |
– Atherosclerosis (rare) – Trauma (hyperextension) |
|
Clinical clue |
Paralysis + loss of pain/temp with intact vibration |
Sensory ataxia with preserved strength |
|
Prognosis |
Poor |
Better |
CNS Lesion Localisation – High-Yield Table
|
Lesion Site |
Key Clinical Features |
Important Clues |
|
Cerebral Cortex |
• Contralateral weakness & sensory loss • Cortical signs (aphasia, neglect, seizures) |
• Face/arm > leg (MCA) • Leg > arm (ACA) |
|
Internal Capsule |
• Pure motor hemiplegia • Face, arm & leg equally affected |
• No cortical signs |
|
Brainstem |
• Cranial nerve palsy on lesion side • Contralateral limb weakness/sensory loss |
“Crossed signs” = brainstem |
|
Cerebellum |
• Ataxia • Dysmetria • Intention tremor |
• Ipsilateral signs |
|
Spinal Cord (Complete lesion) |
• Bilateral motor & sensory loss below level |
• Clear sensory level |
|
Anterior Spinal Cord |
• Motor paralysis • Loss of pain & temperature • Proprioception preserved |
• Anterior spinal artery syndrome |
|
Posterior Spinal Cord |
• Loss of vibration & position sense • Sensory ataxia |
• Positive Romberg |
|
Hemisection (Brown-Sequard) |
• Ipsilateral motor weakness • Ipsilateral loss of vibration • Contralateral loss of pain |
• Penetrating trauma |
|
Cauda Equina |
• LMN weakness • Saddle anaesthesia • Early bladder dysfunction |
• Asymmetric, radicular pain |
|
Conus Medullaris |
• Early bladder & bowel dysfunction • Symmetrical saddle anaesthesia |
• Minimal leg weakness |
|
Peripheral Nerve |
• LMN weakness • Sensory loss in nerve distribution |
• Reduced reflexes |
|
Neuromuscular Junction |
• Fatigable weakness • Normal sensation |
• Myasthenia gravis |
|
Muscle (Myopathy) |
• Proximal weakness • Normal sensation & reflexes |
• Difficulty climbing stairs |
High-Yield
- Cortical signs present → Cortex
- Pure motor, no cortical signs → Internal capsule
- Crossed signs → Brainstem
- Ipsilateral ataxia → Cerebellum
- Clear sensory level → Spinal cord
- Early bladder involvement → Conus / Cauda equina
- LMN signs → Peripheral nerve