Gross Anatomy
The CNS consists of the brain (~1400 g) and spinal cord, protected by the skull, three meningeal layers, and CSF. Major divisions: cerebrum, cerebellum, brainstem.
| Structure | Location | Key Function |
|---|---|---|
| Cerebrum | Largest division | Cognition, motor, sensory, language |
| Cerebellum | Posterior fossa | Coordination, balance, fine motor |
| Brainstem | Base of brain | Vital functions, CN nuclei III-XII |
| Spinal cord | Vertebral canal (to L1-L2) | Relay tracts, reflexes |
| Meninges | Brain/cord covering | Protection, CSF circulation |
🩹 Clinical NoteThe falx cerebri separates hemispheres; tentorium cerebelli separates cerebrum from cerebellum — key landmark in herniation syndromes.
Cerebral Lobes and Functions
| Lobe | Primary Function | Key Structures |
|---|---|---|
| Frontal | Motor, executive function, personality | Precentral gyrus (motor), Broca's area |
| Parietal | Sensory integration, spatial awareness | Postcentral gyrus (somatosensory) |
| Temporal | Auditory, memory, language comprehension | Wernicke's area, hippocampus |
| Occipital | Visual processing | Calcarine sulcus (primary visual cortex) |
| Limbic | Emotion, memory, behavior | Amygdala, hippocampus, cingulate gyrus |
🧐 Mnemonic — AphasiaBroca's = Broken speech (expressive, frontal). Wernicke's = Wordy but Wrong (receptive, temporal).
Brainstem and Cerebellum
| Region | Cranial Nerves | Key Centers |
|---|---|---|
| Midbrain | CN III, IV | Pupillary reflex, vertical gaze, RAS |
| Pons | CN V, VI, VII, VIII | Pneumotaxic center, horizontal gaze |
| Medulla | CN IX, X, XI, XII | Cardiac, vasomotor, respiratory centers |
| Cerebellum | — | IPSILATERAL coordination; dysmetria with lesions |
⭐ Boards PearlCrossed findings (contralateral face + ipsilateral body) = brainstem lesion. Wallenberg syndrome: dysphagia, hoarseness, ipsilateral Horner's, ipsilateral facial numbness, contralateral body pain/temp loss.
Spinal Cord Tracts
| Tract | Column | Function | Decussates at |
|---|---|---|---|
| Corticospinal | Lateral | Voluntary motor | Medulla (pyramids) |
| Spinothalamic | Anterolateral | Pain, temperature, crude touch | Within 1-2 spinal levels |
| Dorsal columns (DCML) | Posterior | Vibration, proprioception, fine touch | Medulla (gracile/cuneate nuclei) |
🩹 Brown-Sequard SyndromeHemisection: ipsilateral motor + proprioception loss; contralateral pain/temperature loss below lesion.
⭐ Boards PearlIsolated posterior column loss = B12 deficiency, tabes dorsalis (syphilis), or Friedreich ataxia.
Cranial Nerves
| CN | Name | Type | Function |
|---|---|---|---|
| I | Olfactory | Sensory | Smell |
| II | Optic | Sensory | Vision; afferent pupillary reflex |
| III | Oculomotor | Motor | Eye movement (SR,IR,MR,IO), pupil constriction, eyelid |
| IV | Trochlear | Motor | Superior oblique (down and in) |
| V | Trigeminal | Both | Facial sensation (V1-V3), mastication |
| VI | Abducens | Motor | Lateral rectus (abduction) |
| VII | Facial | Both | Facial expression, taste ant. 2/3 tongue, lacrimation |
| VIII | Vestibulocochlear | Sensory | Hearing and balance |
| IX | Glossopharyngeal | Both | Taste post. 1/3 tongue, gag (afferent), carotid body |
| X | Vagus | Both | Parasympathetics thorax/abdomen, gag (efferent), voice |
| XI | Accessory | Motor | SCM and trapezius |
| XII | Hypoglossal | Motor | Tongue; deviation toward lesion side |
Cerebral Blood Supply
| Artery | Territory | Stroke Deficit |
|---|---|---|
| ACA | Medial frontal/parietal | Contralateral leg more than arm weakness |
| MCA | Lateral cortex | Contralateral face/arm; aphasia (dominant); neglect (non-dominant) |
| PCA | Occipital, medial temporal | Contralateral homonymous hemianopia |
| Basilar | Pons, cerebellum, midbrain | Locked-in syndrome; vertical gaze palsy |
| PICA | Lateral medulla | Wallenberg syndrome |
| Lenticulostriate | Basal ganglia, internal capsule | Lacunar infarcts; pure motor or sensory stroke |
⭐ Boards PearlMCA most commonly occluded in ischemic stroke. tPA window: 3-4.5 hours from symptom onset (if no contraindications).
Microanatomy and CSF
- Astrocytes — BBB support, scar formation (gliosis)
- Oligodendrocytes — CNS myelin (1 cell myelinates MANY axons)
- Schwann cells — PNS myelin (1 cell per 1 axon); can regenerate
- Microglia — CNS macrophages (mesodermal origin)
- Ependymal cells — line ventricles; produce CSF
CSF flow: Choroid plexus → lateral ventricles → foramen of Monro → 3rd ventricle → cerebral aqueduct → 4th ventricle → subarachnoid space → arachnoid granulations → venous sinuses.
Clinical Pearls
⭐ UMN vs LMNUMN: spasticity, hyperreflexia, Babinski positive, minimal atrophy. LMN: flaccidity, hyporeflexia, fasciculations, significant atrophy.
🩹 Meningeal SignsKernig's: cannot extend knee with hip flexed. Brudzinski's: neck flexion causes involuntary knee flexion. Both = meningeal irritation.
🧐 Key DermatomesC5=shoulder, C6=thumb, C7=middle finger, C8=pinky, T4=nipple, T10=umbilicus, L1=inguinal, L4=knee, L5=great toe, S1=heel/small toe, S2-4=saddle.