Central Nervous System
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Central Nervous System — Key Facts for NEET PG
- Reflex arc: Receptor → Sensory neuron → CNS integration center → Motor neuron → Effector
- Spinal cord tracts: Dorsal columns (fine touch, vibration), Spinothalamic (pain, temperature), Corticospinal (voluntary movement)
- Cerebral cortex: Frontal (motor, language), Parietal (somatosensory), Temporal (auditory, memory), Occipital (visual)
- Basal ganglia: caudate + putamen (striatum), globus pallidus, substantia nigra — movement regulation
- ⚡ Exam tip: Dorsal columns decussate in medulla (internal arcuate fibers); Spinothalamic decussates at spinal cord level (2 segments above entry)
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Central Nervous System — NEET PG Study Guide
Neurons and Support Cells
Neurons:
- Cell body (soma): Nissl bodies (RER), neurofibrils, lipofuscin
- Dendrites: Receive signals, have dendritic spines
- Axon hillock: Site of action potential initiation
- Axon: Transmits signals, myelin sheath (oligodendrocytes in CNS, Schwann cells in PNS)
Neuroglial Cells:
| Cell Type | Location | Function |
|---|---|---|
| Astrocytes | CNS | BBB formation, metabolism, K⁺ buffering, water transport |
| Oligodendrocytes | CNS | Myelination (each oligodendrocyte myelinates multiple axons) |
| Microglia | CNS | Macrophages of CNS (CD68+) |
| Ependymal cells | CNS | Line ventricles and central canal, produce CSF |
| Schwann cells | PNS | Myelination (each Schwann cell myelinates one axon segment) |
| Satellite cells | PNS | Support neuronal cell bodies in ganglia |
⚡ Exam tip: Multiple sclerosis = demyelination of CNS axons (oligodendrocyte dysfunction); Guillain-Barré = demyelination of PNS (autoimmune against myelin proteins)
Spinal Cord Organization
Grey Matter (butterfly-shaped):
- Ventral horn: Motor neurons (LMN to skeletal muscles)
- Dorsal horn: Sensory relay (Lamina I–VI)
- Lateral horn: Preganglionic autonomic neurons (T1–L2)
- Central canal: Contains CSF
White Matter (surrounds grey matter):
- Dorsal columns: Fasciculus cuneatus (C2+) and gracilis (lower body) — fine touch, vibration, proprioception
- Lateral columns: Spinothalamic tract (pain, temperature)
- Ventral columns: Corticospinal tract (motor)
Major Ascending Tracts
Dorsal Column-Medial Lemniscal Pathway (conscious proprioception, fine touch, vibration):
| Step | Details |
|---|---|
| 1st neuron | Pseudounipolar neuron in dorsal root ganglion |
| Entry | Enters dorsal horn, ascends 1–2 levels |
| Synapse | Dorsal column nuclei (gracilis, cuneatus) in medulla |
| 2nd neuron | Decussates as internal arcuate fibers → medial lemniscus |
| Synapse | VPL nucleus of thalamus |
| 3rd neuron | Thalamocortical radiations → somatosensory cortex |
Spinothalamic Tract (pain, temperature, crude touch):
| Step | Details |
|---|---|
| 1st neuron | Pseudounipolar neuron in dorsal root ganglion |
| Entry | Enters dorsal horn, synapses (Lamina I, V) |
| Decussation | Crosses at spinal cord level (2 segments above entry) |
| Ascending | Lateral spinothalamic tract (contralateral) |
| Synapse | VPL nucleus of thalamus |
| 3rd neuron | Thalamocortical radiations → sensory cortex |
⚡ Exam tip: Brown-Séquard syndrome = hemisection of spinal cord → ipsilateral motor loss (corticospinal) + ipsilateral proprioception loss (dorsal columns) + contralateral pain/temperature loss (spinothalamic, 2 segments below lesion)
Major Descending Tracts
Corticospinal Tract (voluntary movement):
| Step | Details |
|---|---|
| Origin | Motor cortex (precentral gyrus, Brodmann area 4) |
| Descending | Internal capsule → cerebral peduncle → pons → medulla |
| Decussation | Pyramidal decussation (most fibers cross) |
| Destination | Lateral corticospinal tract → LMN in ventral horn |
| Function | Fine voluntary movement, dexterity |
Other Descending Tracts:
| Tract | Origin | Function |
|---|---|---|
| Rubrospinal | Red nucleus (midbrain) | Flexor muscle tone (replaced by corticospinal in humans) |
| Vestibulospinal | Deiters’ nucleus (pons) | Postural tone, extensor muscles |
| Reticulospinal | Pontine/medullary reticular formation | Autonomic functions, muscle tone |
| Tectospinal | Superior colliculus | Head and neck reflexes |
⚡ Exam tip: Upper motor neuron signs (corticospinal lesion): Spasticity, hyperreflexia, clonus, Babinski sign; Lower motor neuron signs (ventral horn lesion): Flaccid paralysis, hyporeflexia, fasciculations, atrophy
Brainstem
Midbrain:
- Superior colliculus: Visual reflexes
- Inferior colliculus: Auditory processing
- Cerebral peduncle: Corticospinal, corticopontine fibers
- Red nucleus: Rubrospinal tract origin
- Substantia nigra: Dopaminergic neurons (Parkinson disease)
Pons:
- Respiratory centers: Apneustic, pneumotaxic
- Pontine nuclei: Cerebellar connections
- Superior olivary nucleus: Sound localization
- Trigeminal nuclei: Face sensation
Medulla:
- Respiratory centers: Medullary respiratory center (inspiratory, expiratory)
- Cardiovascular center: Vasomotor center
- Dorsal motor nucleus of X: Parasympathetic output
- Nucleus tractus solitarius: Taste, visceral afferents
- Olive: Cerebellar connections
⚡ Exam tip: Locked-in syndrome = ventral pontine lesion → quadriplegia, anarthria, consciousness preserved; Patient communicates via vertical eye movements (intact cortical function)
Cerebellum
Anatomy:
- Archicerebellum (vestibulocerebellum): Flocculonodular lobe — vestibular function, balance
- Paleocerebellum (spinocerebellum): Vermis — trunk and proximal muscle coordination
- Neocerebellum ( cerebrocerebellum): Lateral hemispheres — planning and initiation of movement
Connections:
- Inputs: climbing fibers (from inferior olive), mossy fibers (from many sources)
- Outputs: Deep nuclei → thalamus → motor cortex (dentate, emboliform, globose, fastigial)
Cerebellar Lesions (ipsilateral):
| Sign | Description |
|---|---|
| Ataxia | Wide-based gait, incoordination |
| Dysmetria | Past-pointing (finger-nose test) |
| Intention tremor | Tremor worse at end of movement |
| Dysdiadochokinesia | Cannot perform rapid alternating movements |
| Hypotonia | Decreased muscle tone |
| Nystagmus | Oscillating eye movements |
⚡ Exam tip: Midline cerebellar lesions (vermis) → truncal ataxia, no appendicular signs; Lateral hemispheric lesions → appendicular ataxia, dysmetria, intention tremor
Basal Ganglia
Components:
- Striatum: Caudate nucleus + putamen (receives cortical inputs)
- Globus pallidus (GP): Internal (GPi) and External (GPe) segments
- Substantia nigra: Pars compacta (dopamine) + Pars reticulata (output)
- Subthalamic nucleus: excitatory input to GPi
Direct and Indirect Pathways:
| Pathway | From Striatum | Effect on Thalamus | Movement |
|---|---|---|---|
| Direct | Inhibits GPi | Disinhibition | Facilitates |
| Indirect | Inhibits GPe | Less inhibition of STN → more excitation of GPi | Inhibits |
Parkinson Disease (basal ganglia disorder):
- Pathology: Loss of dopaminergic neurons in substantia nigra pars compacta
- Features: Resting tremor, rigidity, bradykinesia, shuffling gait, masked facies, micrographia
- Treatment: Levodopa (DOPA decarboxylase → dopamine), MAO-B inhibitors, COMT inhibitors, anticholinergics
⚡ Exam tip: Huntington disease = autosomal dominant CAG repeat in HTT gene → chorea, psychiatric symptoms, dementia; Caudate atrophy; Hemiballismus = lesion of subthalamic nucleus → wild flinging movements
Diencephalon
Thalamus:
- VPL: Somatosensory (body)
- VPM: Somatosensory (face)
- MGN/LGN: Auditory (medial) and visual (lateral) geniculate bodies
- Anterior nucleus: Papez circuit (memory)
- Pulvinar: Visual association
- Intralaminar nuclei: Reticular activating system
Hypothalamus:
- Anterior (parasympathetic): Preoptic area — thermoregulation (cooling), osmoreceptors
- Posterior (sympathetic): Mammillary bodies — thermoregulation (heating)
- Lateral: Feeding center — lesions → anorexia
- Ventromedial: Satiety center — lesions → hyperphagia/obesity
- Arcuate: Release-inhibiting hormones, dopamine (prolactin-inhibiting)
Hypothalamic Hormones:
| Hormone | Target | Function |
|---|---|---|
| TRH | Anterior pituitary | ↑ TSH |
| CRH | Anterior pituitary | ↑ ACTH |
| GnRH | Anterior pituitary | ↑ FSH, LH |
| GHRH | Anterior pituitary | ↑ GH |
| Somatostatin | Anterior pituitary | ↓ GH |
| Dopamine | Anterior pituitary | ↓ Prolactin |
| Oxytocin | Posterior pituitary | Uterine contraction, milk let-down |
| ADH | Posterior pituitary | Water retention |
⚡ Exam tip: Korsakoff syndrome = thiamine (B1) deficiency → mammillary body damage → anterograde amnesia, confabulation; Wernicke-Korsakoff from chronic alcoholism
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Central Nervous System — Comprehensive NEET PG Notes
Meninges and CSF
Meninges:
| Layer | Features |
|---|---|
| Dura mater | Outer layer; epidural space (potential), venous sinuses |
| Arachnoid mater | Spider-web projections; subarachnoid space (contains CSF and vessels) |
| Pia mater | Delicate, closely adherent to brain surface |
Subarachnoid cisterna:
- Lumbar cistern: Below L2 — site for lumbar puncture
- Cisterna magna: Below cerebellum — cerebellar infarcts can be seen here
CSF Circulation:
- Produced by choroid plexus in ventricles (80%) and ependymal cells
- Lateral ventricles → foramen of Monro → 3rd ventricle → cerebral aqueduct → 4th ventricle
- 4th ventricle → foramina of Magendie (median) and Luschka (lateral) → subarachnoid space
- Arachnoid granulations → venous sinuses (superior sagittal) → venous circulation
CSF Properties:
- Volume: ~150 mL
- Pressure: 70–180 mmH₂O (lying), 200–250 mmH₂O (sitting)
- Composition: Low protein, glucose 60–70% of plasma, few cells
⚡ Exam tip: Lumbar puncture — needle enters at L3–L4 (below conus medullaris at L1–L2); Contraindications: ↑ ICP (risk of herniation), coagulopathy, infection at puncture site; Queckenstedt maneuver — manual compression of jugular veins → ↑ ICP → ↑ CSF pressure (blocked if spinal block)
Blood-Brain Barrier
Structure:
- Tight junctions between endothelial cells
- Basement membrane (pericytes embedded)
- Astrocyte end-feet (perivascular)
Functions:
- Protects CNS from blood-borne toxins
- Maintains stable environment for neuronal function
- Selectively allows necessary substances
Areas Without BBB:
- Circumventricular organs: Subfornical organ, area postrema (detects blood toxins, triggers vomiting)
- Choroid plexus: Selective secretion
⚡ Exam tip: Multiple sclerosis — inflammatory demyelination; Progressive multifocal leukoencephalopathy (PML) — JC virus reactivation in immunocompromised; Autoimmune encephalitis — antibodies against neuronal surface antigens (NMDA-R, VGKC)
Limbic System
Components:
- Hippocampus (Ammon’s horn): Declarative memory, spatial navigation
- Amygdala: Emotional processing, fear conditioning
- Fornix: Hippocampal connections
- Parahippocampal gyrus: Memory encoding
- Mammillary bodies: Part of Papez circuit
- Anterior thalamic nucleus: Part of Papez circuit
Papez Circuit (memory): Hippocampus → Fornix → Mammillary bodies → Anterior thalamic nucleus → Cingulate gyrus → Parahippocampal gyrus → Hippocampus
⚡ Exam tip: Klüver-Bucy syndrome = bilateral amygdala lesions → hyperorality, hypersexuality, visual agnosia, placidity; Bilateral hippocampal damage → anterograde amnesia (cannot form new memories)
Cerebral Cortex Functions
Lobes and Functions:
| Lobe | Area | Function |
|---|---|---|
| Frontal | Broca area (BA 44, 45) | Speech production |
| Frontal | Prefrontal cortex | Executive function, planning, personality |
| Frontal | Primary motor (BA 4) | Voluntary movement |
| Frontal | Premotor, supplementary motor | Movement planning |
| Parietal | Primary somatosensory (BA 1, 2, 3) | Somatic sensation |
| Parietal | Somatosensory association (BA 5, 7) | Spatial orientation |
| Temporal | Wernicke area (BA 22) | Speech comprehension |
| Temporal | Primary auditory (BA 41, 42) | Sound processing |
| Temporal | Hippocampus | Memory |
| Occipital | Primary visual (BA 17) | Visual perception |
| Occipital | Visual association (BA 18, 19) | Visual integration |
Dominant Hemisphere (usually left):
- Language (Broca and Wernicke areas)
- Logical reasoning
- Sequential processing
Non-dominant Hemisphere (usually right):
- Spatial abilities
- Music and art appreciation
- Holistic processing
⚡ Exam tip: Broca aphasia = non-fluent speech, good comprehension, naming difficulty; Lesion: inferior frontal gyrus; Wernicke aphasia = fluent but meaningless speech, poor comprehension; Lesion: superior temporal gyrus
Cerebrospinal Fluid Disorders
Hydrocephalus:
| Type | Cause | Features |
|---|---|---|
| Obstructive (non-communicating) | Block in ventricular system | Acute ↑ ICP, papilledema |
| Communicating | Block in subarachnoid space | Gradual onset, dementia |
| Normal pressure | Impaired CSF absorption | Dementia, gait disturbance, incontinence |
| Ex vacuo | Brain atrophy (elderly) | Ventricular enlargement without ↑ ICP |
Intracranial Pressure:
- Normal: 10–20 mmHg
- Compensation: CSF displacement, venous compression
- Cushing triad: Hypertension, bradycardia, irregular respirations (late sign of ↑ ICP)
- Herniation types: Uncal (CN III, ipsilateral pupil), central (diencephalon), tonsillar (brainstem)
Spinal Cord Lesions
| Level | Syndrome | Features |
|---|---|---|
| C2–C3 | High cervical | Quadriplegia, respiratory failure, sensory loss below clavicle |
| C5–C6 | Radiculopathy | Deltoid/biceps weakness, sensory loss in arm |
| T4 | Thoracic | Paraplegia, sensory loss below T4, bladder dysfunction |
| L1–L2 | Cauda equina | Lower motor neuron signs, saddle anesthesia, bowel/bladder dysfunction |
| Conus medullaris | Conus | Early bladder symptoms, erectile dysfunction |
⚡ Exam tip: Anterior cord syndrome = anterior spinal artery occlusion → loss of motor (corticospinal) and pain/temperature (spinothalamic), preserved dorsal column function; Brown-Séquard = hemisection → ipsilateral motor + proprioception loss, contralateral pain/temperature loss
Autonomic Nervous System
Parasympathetic (craniosacral):
- Cholinergic throughout (ACh at all synapses)
- Receptors: Nicotinic (ganglia), Muscarinic (end organs)
- Functions: Rest and digest,瞳孔缩小,促进分泌,心率减慢
Sympathetic (thoracolumbar):
- Pre-ganglionic: ACh (nicotinic receptors)
- Post-ganglionic: Mostly norepinephrine (α, β receptors), sweat glands (ACh, muscarinic)
- Functions: Fight or flight,瞳孔开大,心率加快,血压升高
Autonomic Receptors:
| Receptor | Location | Effect | Blockers |
|---|---|---|---|
| α1 | Vascular smooth muscle | Vasoconstriction | Prazosin |
| α2 | Presynaptic nerve terminals | ↓ NE release | Yohimbine |
| β1 | Heart | ↑ HR, ↑ contractility | Metoprolol |
| β2 | Bronchial, vascular smooth muscle | Bronchodilation, vasodilation | Propranolol |
| M1–M5 | Parasympathetic end organs | Variable | Atropine |
⚡ Exam tip: Autonomic dysreflexia = lesion above T6 → triggers below lesion → massive sympathetic discharge → hypertension, bradycardia; Triggered by bladder distension, bowel impaction; Shy-Drager syndrome = multiple system atrophy with autonomic failure
Sleep
Wakefulness:
- Reticular activating system (RAS): Midbrain reticular formation projects to thalamus → cortex
- Neurotransmitters: Acetylcholine, dopamine, norepinephrine, serotonin
NREM Sleep (stages 1–3):
- Stage 1: Light sleep (theta waves), sleep spindles and K-complexes appear
- Stage 2: True sleep onset, sleep spindles (12–14 Hz), K-complexes
- Stage 3 (slow-wave): Delta waves (0.5–2 Hz), restoration, GH and prolactin release
REM Sleep:
- Desynchronized EEG (like wakefulness)
- PGO waves (ponto-geniculo-occipital)
- Dreaming occurs here
- Muscle atonia (via descending pathways)
- Penile/clitoral erection (parasympathetic)
Sleep Cycles:
- 4–5 cycles per night
- Early: More N3
- Late: More REM
⚡ Exam tip: Narcolepsy = loss of hypocretin/orexin neurons → daytime somnolence, cataplexy, hypnagogic hallucinations, sleep paralysis; ** REM sleep behavior disorder** = loss of muscle atonia → acting out dreams (often precedes Parkinson disease)
Practice Questions for NEET PG
- Trace the dorsal column-medial lemniscal pathway from periphery to cortex.
- Compare upper motor neuron and lower motor neuron lesions.
- A patient has resting tremor, rigidity, and bradykinesia. Where is the lesion?
- Describe the CSF circulation pathway.
- What is the role of the limbic system in memory?
- A patient has loss of pain and temperature sensation but preserved proprioception. Which tract is affected and why?
- Compare the direct and indirect pathways of the basal ganglia.
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