PY10.1-20 | Central Nervous System Physiology — Summary & Reflection

REFLECT

Before reading the summary, test yourself on these integrative questions:

1. A patient has ipsilateral loss of proprioception and contralateral loss of pain and temperature below T6. Draw the cross-section of the spinal cord and shade the damaged area. Name the syndrome and explain why each modality is lost on a different side.

1. Compare resting tremor and intention tremor. For each, name the structure involved, the neurotransmitter deficiency (if any), and one associated clinical feature besides the tremor.

1. A patient speaks fluently but makes no sense, and cannot understand what you say to them. Where is the lesion? Which artery is most likely occluded? What would the EEG show if this patient also had seizures?

1. Explain the gate control theory of pain. How does rubbing a painful area reduce pain? How does TENS work? What descending system provides additional pain modulation, and what neurotransmitters does it use?

1. A medical student is studying for an anatomy exam. They read a chapter on the brachial plexus (encoding), sleep for 8 hours, and remember it the next morning (retrieval). Which brain structure is essential for the consolidation step between encoding and retrieval? Which type of sleep is most important for this type of declarative memory? What cellular mechanism underlies synaptic strengthening during consolidation?

1. Perform a mental neurological examination: list, in order, the components you would examine to differentiate a cerebellar lesion from a basal ganglia lesion at the bedside (PY10.19).

KEY TAKEAWAYS

Key Takeaways — Central Nervous System Physiology

Foundations (PY10.1-PY10.6):
- CNS = brain + spinal cord; PNS = somatic + autonomic (sympathetic + parasympathetic + enteric)
- Major neurotransmitters: ACh (NMJ, autonomic), glutamate (excitatory), GABA (inhibitory in brain), glycine (inhibitory in spinal cord), catecholamines (dopamine, noradrenaline, adrenaline), serotonin
- Synapses: chemical (unidirectional, modifiable) vs electrical (gap junctions, no delay)
- Reflex arc: receptor, afferent, integration centre, efferent, effector. Stretch reflex = monosynaptic; withdrawal = polysynaptic
- Receptors encode stimulus by frequency coding and population coding; classified by modality, location, and adaptation rate

Sensory Pathways (PY10.7, PY10.8):
- DCML (fine touch, proprioception, vibration): DRG to ipsilateral dorsal columns to nucleus gracilis/cuneatus to contralateral medial lemniscus to VPL thalamus to S1 cortex. Crosses in medulla
- Anterolateral/Spinothalamic (pain, temperature, crude touch): DRG to dorsal horn to contralateral anterolateral column (crosses within 1-2 segments) to VPL thalamus to S1 cortex
- Pain: fast (A-delta) vs slow (C fibres); gate control theory (A-beta touch closes gate); descending modulation via PAG to raphe magnus to dorsal horn (serotonin, enkephalins)
- Referred pain: convergence of visceral and somatic afferents on the same dorsal horn neuron

Motor Pathways (PY10.9, PY10.10):
- Pyramidal: cortex to posterior limb of internal capsule to crus cerebri to pyramid to decussation (85-90% cross) to lateral corticospinal tract to alpha motor neuron
- UMN lesion: spastic paralysis, hyperreflexia, Babinski positive, clonus, no atrophy
- LMN lesion: flaccid paralysis, areflexia, Babinski negative, atrophy, fasciculations
- Brown-Sequard (hemisection): ipsilateral DCML loss + ipsilateral UMN signs + contralateral pain/temperature loss
- Syringomyelia (central cord): bilateral cape-like loss of pain and temperature, preserved DCML

Cerebellum (PY10.11):
- Vestibulocerebellum (flocculonodular): balance, eye movements. Lesion: truncal ataxia, nystagmus
- Spinocerebellum (vermis): axial coordination. Lesion: gait ataxia
- Cerebrocerebellum (lateral hemispheres): planning, motor learning. Lesion: intention tremor, dysmetria, dysdiadochokinesia
- Cerebellar signs are ipsilateral (double-crosser rule)

Basal Ganglia (PY10.12, PY10.13):
- Direct pathway (D1, facilitates movement): cortex to striatum to GPi (inhibition) = thalamic disinhibition
- Indirect pathway (D2, inhibits movement): cortex to striatum to GPe to STN to GPi = thalamic inhibition
- Parkinson's (SNc dopamine loss): TRAP — resting Tremor, Rigidity, Akinesia, Postural instability
- Huntington's (striatal degeneration): chorea. Hemiballismus (STN lesion): violent flinging
- Decerebrate rigidity: intercollicular transection. Decorticate: above red nucleus

Thalamus (PY10.14):
- VPL: body sensation. VPM: face sensation. LGN: vision. MGN: hearing. VL/VA: motor (cerebellar + basal ganglia output). Anterior: limbic (Papez circuit)
- Dejerine-Roussy syndrome: thalamic stroke to VPL causing contralateral thalamic pain

Hypothalamus and Limbic System (PY10.15):
- Hypothalamus: temperature (anterior = cooling, posterior = heating), appetite (lateral = feeding, ventromedial = satiety), ADH/oxytocin (supraoptic/paraventricular), circadian rhythm (SCN), pituitary control (arcuate)
- Hippocampus: declarative memory consolidation. Amygdala: fear and emotion. Mammillary bodies: Papez circuit, Wernicke's encephalopathy

Cerebral Cortex (PY10.16):
- Motor cortex (area 4): precentral gyrus, motor homunculus
- Broca's (areas 44-45): non-fluent aphasia. Wernicke's (area 22): fluent aphasia
- Prefrontal: executive functions, personality (Phineas Gage)
- Left hemisphere dominant for language in 95% of right-handed people

Sleep and EEG (PY10.17):
- NREM: N1 (theta), N2 (spindles, K-complexes), N3 (delta, growth hormone, restorative)
- REM: paradoxical sleep, dreaming, muscle atonia, EEG like waking
- Flip-flop switch: VLPO (sleep) vs ARAS (wake), stabilised by orexin. Narcolepsy = orexin loss
- EEG: beta (active), alpha (relaxed, eyes closed), theta (drowsy), delta (deep sleep)

Learning, Memory, Speech (PY10.18):
- Declarative memory: hippocampus-dependent, consolidated during SWS
- Procedural memory: basal ganglia + cerebellum, hippocampus-independent
- LTP: NMDA receptors, Ca2+ influx, AMPA receptor insertion, CREB for long-term
- Aphasias: Broca's (non-fluent), Wernicke's (fluent, no comprehension), conduction (can't repeat), global (both)

Clinical Examination (PY10.19, PY10.20):
- Systematic: higher functions to motor to sensory to reflexes to cranial nerves
- Key UMN vs LMN signs at the bedside differentiate cortical from spinal from peripheral lesions