Master PCML (Posterior Column Medial Lemniscus Pathway) Flashcards

Explain the stimulus exposure part (part 1) of the PCML pathway?

One of your fingers is exposed to a stimulus. 

• The energy from the touch or muscle stretch goes from mechanical into electrical energy. This then creates a receptor potential and if it reaches threshold, the action potential propagates down the axon (First order neuron starts).

Explain the stimulus to spinal cord part (part 2) of the PCML pathway?

Action potential starts to travel from the receptors around the stimulus and propagates along the axon, past the neuronal cell body in the dorsal root ganglion. 

• The action potential then reaches the posterior (dorsal) horn of the spinal cord. 

Explain the beginning of the spinal cord journey part (part 3) of the PCML pathway?

Axons continue inside the spinal cord, running up the posterior (dorsal) column.

Explain the lower and upper body tracts part (part 4) of the PCML pathway?

• Axons from the lower body are most medial and run up the gracile tract (fasciculus gracilis) of the spinal column.

• Axons from the upper body enter the spinal cord later and are more lateral and travel up the cuneate tract (fasciculus cuneatus).

Explain the arriving at the medulla oblongata part (part 5) of the PCML pathway?

At the medulla oblongata, these axons synapse with the gracile nucleus and the cuneate nucleus. 

• This marks the end of the first order neuron and the beginning of the second order neuron.

Explain the second order neurons starts and the cross-over begins part (part 6) of the PCML pathway?

The second order neurons cross over to the other side of the medulla (as internal arcuate fibers) to form the medial lemniscus.

Explain the arriving at the thalamus part (part 7) of the PCML pathway?

The axons travel up the rest of the brainstem and synapse at the thalamus (at the ventro-posterior lateral nucleus (VPLN)). This is where the third order neuron starts.

Explain the arriving at the primary somatosensory cortex part (part 8) of the PCML pathway?

Neurons in the thalamus travel up the posterior limb of the internal capsule. 

• The axons synapse in the primary somatosensory cortex (post central gyrus in the parietal lobe) and proprioception is perceived.