How does an action potential come from a pain sensor in your small toe and travel up the spine?

Great question! Your body has so many neurons specialised in the detection of various signals, such as light or sound. So how does your brain know where a signal came from?

Neurons are the specialised cells of the central nervous system. Inputs arrive at the dendrites of a neuron, then traveling down to their cell body. Provided that the signal that arrived at the dendrites is strong enough, an action potential is then generated at the axon level, which travels down to the dendritic tree of a different neuron. The specialised neurons all over your body detect various different types of signals, which are then sent out to the central nervous system (the spinal cord and brain). In a way, because signals are coming from a particular neuron or a group of them, that may be sufficient for your brain to resolve where the signals are coming from. However, because your brain is integrating information from all the sensors all over your body, resolving the source of information may get tricky if there is too much information coming in at once. To get around that, the way your nervous system develops before and after birth is critical.

There is a field of biology that deals specifically with how all of our systems emerge from a single cell, and that is Development Biology. Thanks to the contribution of various scientists, we know that the development of Nervous System, like that of any other system in our bodies, requires a series of orchestrated events in both time and place, which culminate with a fully developed being. Throughout the course of development, individual axons find their way from the periphery into the spinal cord (or in the case of your face, directly to a brain area called the hindbrain). This is achieved due to combination of signals in both the axons of the neurones and in the dendrites of the neurones at the destination of the axons. This ensures that neurones from your right foot find the right foot-specific neurones in the brain. This ensures that unique pathways from areas all over your body are correctly connected to the central nervous system, and then also within the central nervous system as well. 

Information that arrives at the spinal cord or hindbrain, is then propagated to other brain areas. At the very top of this hierarchy, is the neocortex – which is wrinkly part of the brain that is divided into two hemispheres. The neocortex, which is proportionally larger in humans than other vertebrates, has dedicated areas that receive and encode different types of information, e.g., the part of the brain on the back of your head/top of the neck receives and processes visual information. Information arriving from the base of this hierarchy is already spatially segregated, and this allows the neocortex to map information according to its source. This type of arrangement is designated topographic mapping (you can read more about it here https://en.wikipedia.org/wiki/Topographic_map_(neuroanatomy)). As a result, you have dedicated sub-areas in your brain that respond only to information coming from your right foot, and within that area, you have smaller groups of neurones that only respond to information coming from your small toe on the right foot. In the case of the mapping related to pain reception or touch, this type of mapping in the brain is called somatotopy, and you can read more about it here https://en.wikipedia.org/wiki/Somatotopic_arrangement. This type of mapping is set during embryonic development, but it is refined during childhood all the way into adulthood. This happens because neurones are incredibly plastic and can quickly change the way they connect to each other. In this way, at the various different levels of information processing in the spinal cord and then brain, the spatial information of the source of the signal (pain, or touch) is never lost, and the brain can therefore easily pinpoint the precise location of where the signal originated.