Question

What is the function of unmyelinated neurons? (hint :pain)

Answer 1

Unmyelinated neurons are slow conducting neurons that lack a myelin sheath around their axons. These are also known as type C fibers unmyelinated fibers are known to be of two types: peptidergic and non-peptidergic fibers. Peptidergic fibers detect signals for heat and cold. Non-peptidergic neurons detect signals for response to mechanical stimuli. The Schwann cells in these neurons rather than producing myelin instead form Remak fibers in peripheral nerves. These Schwann cells form a groove around these fibers. These unmyelinated neurons form fibers that lack nodes and internodes. Lack of myelin will lead to lack of insulation, promotion of continuous impulse to be passed between axons. These neurons have voltage dependent sodium channels present along the entire axonal length allowing the impulse to be conducted across the axons.

These unmyelinated fibers transmit signals (cutaneous and visceral) to the CNS from the skin in response to thermal, mechanical and chemical signals. Their basic function is in sensation, touch/heat and pain. They are a part of somatosensory system acting as nociceptors or thermoreceptors and send the signals to the CNS. Gentle human touch and cold pain (response to cold temperature) also stimulates these fibers.

In response to pain, mTORC1 signaling is activated in some C fiber sensory fibers in skin, due to inhibition of Tsc2, a negative regulator. Due to Tsc2 inhibition, the cell body and axon diameter of these unmyelinated fibers increases. This will result in transmission of sensation of pain. Signals from the nociceptors are processed in the somatosensory and multimodal cortical areas of the CNS.

The C type thermonoceptors respond to slow as well as quick sensation of pain due to thermal stimulation. The slow fibers respond slowly after there is onset of the sustained heat stimulus. The fast receptor neurons respond very rapidly. Further, they adapt to the heat stimulus which is sustained over time