Somatosensory pathways

Broadly speaking, the nervous system can be split into an afferent or sensory division and an efferent or motor division.

The afferent division brings sensory information from the outside world into the brain.

Sensory information may involve special senses - so vision, hearing, taste, and smell - as well as general somatic senses, so the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. These sensations are transduced by sensory receptors, which are present in the cell membrane of highly specialized cells found all over the body.

According to the stimulus they respond to, sensory receptors are classified as mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.

Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body. These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.

Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.

To trigger the release of neurotransmitters, neurons use an electrical signal that races down the axon, known as the action potential.

To help speed up that electrical signal, some axons are intermittently wrapped by a fatty protective sheath called myelin, which comes from glial cells like oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system.

Since the axons in the somatosensory system can be very long, the fact that myelin helps speed up action potentials is super important! But myelin requires energy to generate and takes up space, so not all fibers are myelinated.