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CNS Physiology - 1st Year MBBS (Up to Tracts)

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1. Organization of the Nervous System

• Central Nervous System (CNS): Brain + Spinal cord
• Peripheral Nervous System (PNS): Cranial nerves, spinal nerves, and their ganglia

• Somatic nervous system: Voluntary control of skeletal muscle; conscious sensory input
• Autonomic nervous system (ANS): Involuntary control of visceral organs (sympathetic and parasympathetic)

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2. The Neuron - Basic Unit of the CNS

Structure

1. Dendrites - receive incoming signals
2. Cell body (soma) - contains nucleus, metabolic center
3. Axon (nerve fiber) - conducts impulses away from the soma
4. Axon terminals (synaptic boutons) - transmit signals to the next cell

Types of Neurons (by function)

Glia (Supporting Cells)

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3. Resting Membrane Potential (RMP)

• Value: -70 mV (inside negative relative to outside)
• Maintained by:
  • High intracellular K+ (maintained by Na+/K+ ATPase pump - 3 Na+ out, 2 K+ in)
  • High extracellular Na+
  • Negatively charged intracellular proteins
  • Selective permeability (resting membrane more permeable to K+ than Na+)

• K+: -90 mV (favors hyperpolarization)
• Na+: +60 mV (favors depolarization)

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4. Action Potential (AP)

Phases

1. Resting (-70 mV): Voltage-gated Na+ and K+ channels are closed
2. Depolarization: Stimulus reaches threshold (~-55 mV), Na+ channels open rapidly → Na+ rushes in → membrane potential rises to ~+30 mV
3. Repolarization: Na+ channels inactivate; K+ channels open → K+ flows out → membrane returns toward -70 mV
4. Hyperpolarization (after-potential): K+ channels briefly over-correct → membrane dips below -70 mV
5. Return to resting: Na+/K+ pump restores ionic balance

Key Concepts

• Threshold: ~-55 mV; below this, AP will not fire
• All-or-none law: Either fires fully or not at all
• Absolute refractory period: No AP possible (Na+ channels inactivated)
• Relative refractory period: AP possible with suprathreshold stimulus
• Saltatory conduction: In myelinated fibers, AP "jumps" between Nodes of Ranvier → faster conduction

Conduction velocity depends on:

• Myelination (increases speed greatly)
• Axon diameter (larger = faster)

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5. Synapse and Synaptic Transmission

Structure

• Presynaptic terminal: Contains synaptic vesicles filled with neurotransmitters
• Synaptic cleft: ~20-40 nm gap
• Postsynaptic membrane: Contains receptors

Mechanism (Chemical Synapse)

1. AP arrives at presynaptic terminal
2. Voltage-gated Ca2+ channels open → Ca2+ flows in
3. Synaptic vesicles fuse with membrane (exocytosis) → neurotransmitter released
4. NT binds to postsynaptic receptors → ion channels open
5. Generates either EPSP or IPSP
6. NT removed by reuptake, enzymatic degradation, or diffusion

EPSP vs IPSP

Summation

• Temporal summation: Repeated stimuli from same neuron add up over time
• Spatial summation: Multiple synapses fire simultaneously → effects add up

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6. Neurotransmitters (Key ones for 1st year)

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7. The Spinal Cord - Structure

Cross-section anatomy

• Gray matter (H-shaped): Contains neuron cell bodies
  • Anterior horn: Motor neurons (lower motor neurons - LMN)
  • Posterior horn: Sensory relay neurons
  • Lateral horn (T1-L2): Preganglionic sympathetic neurons; (S2-S4): Parasympathetic
• White matter: Contains myelinated axons organized into funiculi (columns)
  • Posterior funiculus (dorsal column): Ascending sensory tracts
  • Lateral funiculus: Mix of ascending and descending tracts
  • Anterior funiculus: Mainly descending motor tracts

Rexed's Laminae

• Laminae I, II (substantia gelatinosa): Pain and temperature processing
• Laminae III, IV: Touch, pressure
• Lamina V: Wide dynamic range neurons
• Lamina IX: Alpha and gamma motor neurons
• Lamina X: Surrounds the central canal

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8. ASCENDING TRACTS (Sensory Pathways)

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A. Anterolateral System (Spinothalamic Tract)

1. 1st order neuron: Cell body in dorsal root ganglion (DRG) → enters spinal cord through posterior root → travels in Lissauer's tract (posterolateral fasciculus) for 1-2 segments → synapses in posterior horn (laminae I and V)
2. 2nd order neuron: Axon crosses obliquely via anterior white commissure over 2-3 spinal segments to the contralateral side → ascends as the anterolateral (spinothalamic) tract → synapses in VPL nucleus of thalamus
3. 3rd order neuron: From thalamus → through posterior limb of internal capsule → primary somatosensory cortex (postcentral gyrus)

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B. Posterior Column - Medial Lemniscal Pathway (PCML)

1. 1st order neuron: Cell body in DRG → enters spinal cord → axon ascends IPSILATERALLY in the posterior funiculus (dorsal columns)
   • Fibers from lower limb/trunk: Fasciculus gracilis (medial; T7 and below)
   • Fibers from upper limb/neck: Fasciculus cuneatus (lateral; C2-T6)
   • Synapses at nucleus gracilis / nucleus cuneatus in the caudal medulla
2. 2nd order neuron: Axons cross as internal arcuate fibers → form the medial lemniscus → ascend to VPL nucleus of thalamus
3. 3rd order neuron: VPL → through posterior limb of internal capsule → primary somatosensory cortex

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Comparison: Spinothalamic vs Posterior Column

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9. DESCENDING TRACTS (Motor Pathways)

• Upper Motor Neuron (UMN): Cell body in cortex or brainstem
• Lower Motor Neuron (LMN): Cell body in anterior horn of spinal cord

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A. Lateral Motor System

1. Lateral Corticospinal Tract (Most important)

1. UMN: Cell body in primary motor cortex (precentral gyrus)
2. Axons converge in corona radiata → descend through posterior limb of internal capsule
3. Continue through crus cerebri (midbrain) → descend as small fascicles through pons (separated by transverse pontocerebellar fibers)
4. Fibers regroup in the medulla → form the pyramid (hence also called the pyramidal tract)
5. At the caudal medulla: ~85% of fibers decussate at the pyramidal decussation (decussation of pyramids)
6. Cross to contralateral side → descend as lateral corticospinal tract in the lateral funiculus
7. Synapse on LMN cell bodies in the lateral anterior horn
8. LMN axons exit through the anterior root → reach target muscles

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2. Rubrospinal Tract

• UMN: Red nucleus (midbrain tegmentum)
• Axons cross immediately as the ventral tegmental decussation
• Descend as rubrospinal tract in the lateral column
• Descend only to cervical spinal cord levels
• Synapse on interneurons → influence flexor muscles of the upper limb

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B. Medial Motor System

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10. UMN vs LMN Lesion (Clinical Application)

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11. Reflex Arc

1. Receptor - detects stimulus
2. Afferent (sensory) neuron - carries signal to spinal cord
3. Integration center - spinal cord (interneurons)
4. Efferent (motor) neuron - carries response signal
5. Effector - muscle or gland

Monosynaptic Reflex (Stretch/Myotatic Reflex)

• Example: Knee jerk (patellar reflex - L3/L4)
• Tapping the tendon stretches the muscle → activates muscle spindles (Ia afferents) → directly synapse on alpha motor neurons → muscle contracts
• Only ONE synapse involved (monosynaptic)

Polysynaptic Reflex

• Example: Withdrawal (flexor) reflex
• Involves interneurons → can cross midline (crossed extensor reflex)

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12. Quick Summary Table - Spinal Cord Tracts

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Exam Tips for 1st Year MBBS

1. The spinothalamic tract crosses in the spinal cord (2-3 segments above entry); the PCML crosses in the medulla - this explains different deficits in hemisection (Brown-Sequard syndrome)
2. Lateral corticospinal tract carries ~85% of corticospinal fibers; it decussates at the pyramidal decussation in caudal medulla
3. UMN = spastic paralysis; LMN = flaccid paralysis - remember BUSHY (Babinski, UMN features: hyperreflexia, spasticity; LMN: hyporeflexia, atrophy, fasciculations)
4. All three conscious somatosensory and motor tracts end at the VPL thalamus (sensory) or VPL/VL thalamus (motor relay) before reaching the cortex
5. The posterior column carries information ipsilaterally all the way to the medulla before crossing

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Neurological Syndromes - Complete Comparison Table

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PART 1: Spinal Cord Syndromes

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Key: Brown-Séquard Deficits at a Glance

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PART 2: Brainstem Syndromes (Crossed Paralysis Syndromes)

Hallmark: Ipsilateral CN palsy + contralateral hemiparesis = lesion is in the brainstem

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MIDBRAIN Syndromes

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PONTINE Syndromes

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MEDULLARY Syndromes

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PART 3: Other Named CN and Cranial Base Syndromes

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PART 4: Cerebrovascular / Hemispheric Syndromes

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PART 5: Memory Mnemonics

• Same side: motor loss + posterior column loss
• Opposite side: pain/temperature loss

• All have: ipsilateral CN + contralateral body

• Dysphagia, Yaw nystagmus (vertigo), Ataxia, Dissociated sensory loss (face ipsi / body contra), Sympathetic (Horner)

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