A sensory neuron that delivers sensations directly to the CNS is called a first-order neuron. 

A sensory neuron that delivers sensations directly to the CNS is called a first-order neuron.

The cell body of a first-order general sensory neuron is located in a spinal ganglion or cranial nerve ganglion.

 In the CNS, the axon of that sensory neuron synapses (passes signals to) an interneuron known as a second-order neuron, which may be located in the spinal cord or brainstem.

For us to become aware of the sensation, the second-order neuron in turn must pass on the signal to a third-order neuron in the thalamus.

Somewhere along its length, the axon of the second-order neuron crosses over to the opposite side of the CNS in a process called decussation. As a result, the right side of the thalamus receives sensory information from the left side of the body, and the left side of the thalamus receives sensory information from the right side.

 The axon of the third-order neuron then ascends (without crossing back over) and synapses on neurons of the primary somatosensory cortex. Therefore, the right cerebral hemisphere receives sensory information from the left side of the body, and the left cerebral hemisphere receives sensations from the right side.