ABSTRACT
APs are, under normal circumstances, propagated from the cell body of a neuron to the axon terminal, as befits the role of neurons in passing on 'information' to other neurons and to effector cells. However, if a neuron is artificially excited (depolarized) somewhere along the axon, an AP can be propagated in both directions - towards the central nervous system (CNS) and out towards the periphery. This can be demonstrated by tapping the ulnar nerve at the elbow (sometimes if we knock this nerve, we say that we've knocked our 'funny bone'); involuntary movement of the muscles of the hand and a sensation of tingling can be experienced. The movement of the muscles of the hand is caused by APs being propagated from the elbow to the muscles of the hand and causing contraction, and the tingling sensation is caused by propagation of APs from the elbow to the CNS, where the brain assumes that the parts of the hand normally innervated by those neurons have in some way been stimulated. Thus, AP propagation in axons has the potential to be bidirectional, but is normally unidirectional. Sensory neurons are excited by stimuli in the periphery, and this information is conveyed to the CNS. Motor neurons are activated by descending input from other parts of the CNS, and this information is conveyed to the periphery. Conduction of an AP in the 'proper' direction is known as orthodromic conduction, and conduction of an AP in the 'incorrect' direction is known as antidromic conduction.
