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Posterior column-medial lemniscus pathway
The posterior column-medial lemniscus pathway (called the dorsal column in non-humans) is the sensory pathway responsible for transmitting discriminative sensation from the skin to the thalamus, and on to the cerebral cortex.
Journey of the posterior column-medial lemniscus pathway
Discriminative sensation is well developed in the fingers of humans, and allows us to feel fine textures and determine what an unknown object in our hands is without looking at it.
This fine sensation is detected by Meissner's corpuscles that lie in the dermis of the skin close to the epidermis. When these structures are stimulated by slight pressure, an action potential is started.
The action potential travels up an axon (the cell body of the neuron will be in a dorsal root ganglion). (The neurons are classified as unipolar, so they are regarded as just having one long process, an axon.) So the sensation travels from the skin, along the axon, past the neuronal cell body, and into the dorsal column of the spinal cord.
The axons continues inside the spinal cord, running up the posterior (dorsal) column. Axons from the lower body are most medial (closer to the midline), and run in the gracile tract of the spinal column. Sensory axons from the upper body enter the spinal cord later, so are more lateral and travel up the cuneate tract.
At the brainstem
At the level of the closed medulla oblongata, these axons synapse with neurons in the gracile and cuneate nuclei. The secondary neurons (that start in the nuclei) cross over to the other side of the medulla (as internal arcuate fibres) to form the medial lemniscus.
At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in the posterior columns. For example in the columns, lower limb was medial, upper limb was more lateral. At the medial leminiscus, axons from the leg are more ventral, arm fibres more dorsal. Fibres from the trigeminal nerve (supplying the head) come in dorsal to the arm fibres, and travel up the lemniscus too.
The medial lemniscus rotates 90 degrees at the pons. The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards.
The axons travel up the rest of the brainstem, and synapse at the thalamus (at the ventral posterolateral nucleus). Neurons starting in the thalamus travel up the posterior limb of the internal capsule, and again head and leg swap relative positions. The axons synapse in the primary sensory cortex , with lower body sensation most medial (e.g. the paracentral lobule) and upper body more lateral.
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