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Major Pathways of General Sensation

Dorsal Column-Medial Lemniscus Pathway A. Function: The dorsal column-medial lemniscus pathway

mediates tactile discrimination, vibration sensation, form recognition, and joint & muscles sensation (conscious proprioception).

B. Receptors include Pacini's & Meissner's tactile corpuscles, joint receptors, muscle spindles, and Golgi tendon organs.

C. First-order neurons are located in the dorsal root ganglia at all levels. They project axons to the spinal cord through the medial root entry zone. First-order neurons give rise to:

1. The gracile fasciculus from the lower extremity. 2. The cuneate fasciculus from the upper extremity. 3. The collaterals for spinal reflexes, e.g. myotatic

reflex (deep tendon reflex). 4. The axons that ascend in the dorsal columns and

terminate in the gracile & cuneate nuclei of the caudal medulla.

D. Second-order neurons are located in the gracile & cuneate nuclei of the caudal medulla. They give rise to axons & internal arcuate fibers that decussate and form a compact fiber bundle, i.e. medial lemniscus. The medial lemniscus ascends through the contralateral brain stem and terminates in ventral posterolateral (VPL) nucleus of the thalamus.

E. Third-order neurons are located in the VPL nucleus of the thalamus. They project through the posterior limb of the internal capsule to the postcentral gyrus, which is the primary somatosensory cortex (Brodmann's areas 3, 1, & 2).

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Lateral Spinothalamic tract A. Function: The lateral spinothalmic tract mediates pain &

temperature sensation. B. Receptors are free nerve endings. The lateral

spinothalamic tract receives input from fast- & slow-conducting pain fibers, i.e. A-δ & C respectively.

C. First-order neurons are found in the dorsal root ganglia at all levels. They project axons to the spinal cord through the dorsolateral tract of Lissauer (lateral root entry zone) to second-order neurons.

D. Second-order neurons are found in the dorsal horn. They give rise to axons that decussate in the ventral white commissure and ascend in the contralateral lateral funiculus. Their axons terminate in the VPS nucleus of the thalamus.

E. Third-order neurons are found in the VPL nucleus of the thalamus. They project through the posterior limb of the internal capsule to the primary somatosensory cortex (Brodmann's areas 3, 1, & 2).

Summary

A. Pain & Temperature: • Receptors in dermis and epidermis, • Dorsal root ganglion to dorsal horn of spinal cord, • Cross to lateral spinothalamic tract & ascend to VPL

nucleus of the thalamus, • Project to the postcentral gyrus. • Reflex arc present at the spinal cord level.

B. Pressure & Crude Touch: • Receptors in dermis, • Dorsal root ganglion to dorsal horn of spinal cord, • Cross to ventral spinothalamic tract & ascend to VPL

nucleus of the thalamus, • Project to the postcentral gyrus.

C. Proprioception, Fine Touch, & Vibration: • Receptors in muscles, joints and tendons.

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D. Sensation From Face: • Input via the trigeminal nerve (CN: V). • Projections to trigeminal ganglion (pons) and nucleus

(medulla). • Fibers cross and ascend to the VPM nucleus in

thalamus.

Oral Physiology Assignment By Noor Al-Deen M. Al-Khanati

Reference: High-Yield Neuroanatomy, 2nd Edition, James D. Fix, Ph.D.