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 involves special senses - like vision, hearing, taste, and smell - as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. These sensations are felt by sensory neurons all over the body.
These sensory neurons have receptors that are classified according to the stimulus they respond to - there are 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.
The somatosensory pathways are made up of a relay of four neurons.
The first neuron is called the first order neuron or sensory neuron, which has the sensory receptors and converts stimuli from the outside world into an impulse that can be passed through a synapse to the next neuron in series.
Next is the second order neuron, and it may have its cell body in the spinal cord or up in the brainstem.
The second order neuron then takes the impulse to the third order neuron, which has its cell body in the thalamus.
Finally, the third order neuron takes the impulse to the fourth order neuron, which has its cell body further up in the sensory cortex of the brain.
Now let’s zoom into first order or sensory neurons. First order neurons are called pseudounipolar neurons, which means that they don’t have separate dendrites and axons; instead, there’s only one axon that extends out from the cell body, and it has two branches: a peripheral branch and a central branch.