Overview
This lecture covers the dorsal column medial lemniscus (DCML) pathway, focusing on its role in transmitting discriminative touch, proprioception, pressure, vibration, and stretch sensations from the body to the brain.
Sensations Carried by the DCML Pathway
- The DCML transmits fine/discriminative (two-point) touch, pressure, vibration, stretch, and proprioception.
- Proprioception is awareness of body part position in space even with eyes closed.
Receptors Involved
- Meissner’s corpuscles: detect fine, discriminative touch in the dermal papilla.
- Merkel’s discs: respond to fine touch and superficial pressure in the stratum basale.
- Pacinian corpuscles: respond to deep pressure and vibration, found deeper in the skin and some mucous membranes.
- Peritrichial nerve endings: detect hair movement and some touch.
- Ruffini corpuscles: sense skin stretch and vibration, mainly for stretch.
- Muscle spindles (nuclear chain and nuclear bag fibers): detect muscle stretch—proprioceptive, carried by type 1a fibers.
- Golgi tendon organs: sense tendon stretch—proprioceptive, carried by type 1b fibers.
Nerve Fiber Types
- A fibers are fastest; Aα for muscle spindles and Golgi tendon organs, Aβ for touch/pressure/vibration receptors.
- Myelination determines conduction speed: A (most), B (moderate), C (least/none).
Organization Within the Spinal Cord
- DCML is an ascending (sensory) tract—bundle of axons in the CNS.
- First-order neuron cell bodies are in the dorsal root ganglion (DRG); axons enter dorsal columns.
- Below T6: fibers travel in the fasciculus gracilis (medial, legs/lower trunk).
- Above T6: fibers travel in the fasciculus cuneatus (lateral, arms/upper trunk).
Somatotopic Arrangement
- Medial dorsal column: sacral and lumbar fibers; lateral: thoracic and cervical fibers.
- Fibers remain ipsilateral (same side) in the spinal cord.
Synaptic Relay and Brainstem Pathway
- First-order neuron synapses in the medulla: fasciculus gracilis to nucleus gracilis, fasciculus cuneatus to nucleus cuneatus.
- Second-order neurons cross (decussate) as internal arcuate fibers and ascend as the medial lemniscus.
- Medial lemniscus projects to the ventral posterolateral (VPL) nucleus of the thalamus.
Thalamic and Cortical Targets
- Third-order neurons from VPL project through the posterior limb of the internal capsule to the primary (S1) and secondary (S2) somatosensory cortices in the postcentral gyrus.
- Sensory homunculus: body parts mapped to specific cortical regions (somatotopy).
White Matter Fiber Types
- Projection fibers: internal capsule, corona radiata.
- Commissural fibers: connect hemispheres, e.g., corpus callosum.
- Association fibers: connect regions within the same hemisphere, e.g., arcuate fasciculus.
Key Terms & Definitions
- DCML pathway — dorsal column medial lemniscus, a sensory tract for touch and proprioception.
- Proprioception — conscious awareness of body and limb position.
- First-order neuron — sensory neuron with cell body in DRG.
- Fasciculus gracilis/cuneatus — dorsal spinal cord tracts for lower/upper body input.
- Medial lemniscus — ascending brainstem tract after decussation.
- VPL nucleus — thalamic relay for body somatosensation.
- Somatotopy — spatial body mapping in the cortex.
Action Items / Next Steps
- Review diagrams showing DCML pathway, receptor locations, and sensory homunculus.
- Memorize the three-neuron sequence and main relay points for DCML.
- Study the differences between fasciculus gracilis and cuneatus.