Q2 LONG ANSWER 10M ★ High-yield

Describe the ascending tracts of the spinal cord. Trace the pathway for pain, temperature and fine touch from the periphery to the cortex.

Physiology · 1st Professional — Pre-Clinical

Introduction

Ascending tracts carry sensory information from receptors to the higher centres. All somatic sensation (except smell) relays in the thalamus before reaching the cortex.

Every pathway has three neurones — first order (dorsal root ganglion), second order (crosses the midline), third order (thalamus → cortex)

Posterior (dorsal) Column–medial Lemniscus Pathway

Feature	Detail
Sensations	Fine (discriminative) touch, vibration, conscious proprioception, two-point discrimination, stereognosis
1st order neurone	Dorsal root ganglion → enters cord → ascends uncrossed in the posterior column
Tracts	Fasciculus gracilis (medial, lower limb) and fasciculus cuneatus (lateral, upper limb, above T6)
2nd order neurone	Nucleus gracilis and cuneatus in the medulla; fibres decussate as internal arcuate fibres → form the medial lemniscus
3rd order neurone	VPL nucleus of thalamus → internal capsule → postcentral gyrus (areas 3, 1, 2)
Site of crossing	Medulla

Spinothalamic (anterolateral) Pathway

Feature	Lateral spinothalamic	Anterior spinothalamic
Sensation	Pain and temperature	Crude touch and pressure
1st order	Aδ and C fibres → dorsal root ganglion	Aβ fibres
2nd order	Substantia gelatinosa (Rexed lamina II) of dorsal horn	Dorsal horn
Crossing	Anterior white commissure, within 1–2 segments	Same, but over several segments
3rd order	VPL thalamus → sensory cortex	Same

CLINICAL PEARL

Note the key difference: the posterior column crosses in the medulla, the spinothalamic tract crosses in the cord, 1–2 segments above entry. This single fact explains the dissociated sensory loss of Brown–Séquard syndrome and of syringomyelia.

Spinocerebellar Tracts

Tract	Carries	Crossing	Destination
Posterior (dorsal) spinocerebellar	Unconscious proprioception from the same side	Uncrossed	Cerebellum via inferior peduncle
Anterior (ventral) spinocerebellar	Unconscious proprioception	Crosses twice — so effectively uncrossed	Cerebellum via superior peduncle

These do not reach consciousness — they inform the cerebellum for coordination

Classification of Sensations

Group	Modalities	Pathway
Superficial (exteroceptive)	Fine touch, crude touch, pain, temperature, pressure	Posterior column + spinothalamic
Deep (proprioceptive)	Joint position, vibration, deep pain, muscle sense	Posterior column + spinocerebellar
Visceral	Hunger, nausea, visceral pain, distension	Autonomic afferents
Cortical (combined)	Stereognosis, two-point discrimination, graphaesthesia, tactile localisation	Require an intact posterior column and cortex

Cortical sensations are lost in a parietal lobe lesion even though the primary modalities are intact — a key localising point

Sensory Receptors

Receptor	Modality	Adaptation
Free nerve endings	Pain, temperature, crude touch	Slow (non-adapting)
Meissner corpuscle	Fine touch (glabrous skin)	Rapid
Pacinian corpuscle	Vibration, deep pressure	Very rapid
Merkel disc	Sustained touch, texture	Slow
Ruffini ending	Stretch, warmth	Slow
Muscle spindle	Length	Slow

Law of specific nerve energies (Müller) — whatever stimulates a receptor, the sensation felt is that of its own modality
Generator (receptor) potential — graded and non-propagated; when it reaches threshold an action potential is fired
Adaptation — explains why we stop feeling our clothes; pain receptors adapt least, which is protective

Sensory Cortex and Homunculus

Primary sensory area — postcentral gyrus, areas 3, 1, 2
Representation is contralateral, inverted (leg at the top near the midline, face at the bottom) and disproportionate
Area is proportional to density of receptors, not size of the part — hence the huge lips, tongue and hand of the sensory homunculus
Secondary sensory area — bilateral representation, less precise

Applied Aspects

Tabes dorsalis (tertiary syphilis) — degeneration of the posterior columns → loss of vibration and joint sense, sensory ataxia, positive Romberg sign, high-stepping stamping gait
Subacute combined degeneration (vitamin B12 deficiency) — posterior column and corticospinal tract affected together
Syringomyelia — a central cavity destroys the decussating fibres → "cape" distribution loss of pain and temperature with preserved touch — the classic dissociated sensory loss
Thalamic syndrome — loss of all sensation followed by spontaneous agonising pain on the opposite side
Parietal lobe lesion — primary sensations preserved but cortical sensations lost: astereognosis, loss of two-point discrimination, sensory inattention
Sensory level — a clear upper border of sensory loss on the trunk localises a spinal cord lesion; remember the cord ends at L1–L2
Useful dermatome landmarks — nipple T4, xiphisternum T7, umbilicus T10, inguinal ligament L1
Peripheral neuropathy → glove and stocking sensory loss, not a dermatomal pattern

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