Having now understood how information is processed at the level of the receptor and the afferent neuron, how is information carried to the brain for us to be conscious of it? This is where the ascending pathways come in (the somatosensory tracts). They are called ascending tracks because they carry information from the periphery up the spinal cord, the brainstem and all the way to the cerebral cortex until it comes to consciousness.
A pathway consists of a tract and the nucleus:
Sensory pathways are called ascending pathways because sensory information is gathered by the sensory receptors in the periphery and it ascends through the spinal cord to the brain.
But remember, the left cerebral hemisphere controls the right side of the body and vice versa. So this means that if the left cerebral hemisphere is to receive information that is coming from the right side of the body, at some point along the ascending pathway, that information is supposed to cross from the right side of the body to the left This process of crossing over is called decussation.
It’s also important to note that a pathway on the left side of the CNS has a matching tract on the right side of the CNS (i.e. paired). And therefore each each side of the body actually sends its own information up to the brain.
In the picture above, there is an afferent neuron which is picking up information from the skin and this information is transduced into action potentials. It travels along the axon of the afferent neuron (pseudo-unipolar neuron). This afferent neuron, carrying information from the periphery to the spinal cord is known as the first order neuron. The cell bodies of the first order neurons are found in the dorsal root ganglion (if it’s in the cranium then the cranial nerve ganglion).
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the cell body is located outside the CNS and a collection of cell bodies outside the CNS is called a ganglion.
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Once the first order neuron reaches the spinal cord it terminates/synapses onto a second order neuron which is usually an interneuron. The second order neurons may be located in the spinal cord or it may be in the brainstem (first order neurons can travel up the spinal cord to the brainstem and synapse with second order neurons there).
The second order neuron then ascends all the way to the thalamus where it terminates on/synapses with third order neurons.
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Remember the function of the thalamus is a filter. It receives all the information, then it sends only the information that is necessary for consciousness to the cerebral cortex —it also helps the cerebral cortex to locate that information.
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The third order neurons carry information from the thalamus to the cerebral cortex. This third order neuron does not decussate (decussation would have already happened at either the first or second order neuron level).