I have not necessarily read these papers yet, so do not take these as recommendations, but simply papers I am curious to read. Most of them are not recent.

This week it has come to my attention that a number of unicellular organisms (such as Paramecium) produce action potentials, which are triggered by sensory events and produce behavioral responses.

• Pineda and Ribera (2009) - Evolution of the Action Potential. Actually about development rather than evolution.
• Davis et al. A Model of Drosophila Larva Chemotaxis.
• Casale et al. Cortical Interneuron Subtypes Vary in Their Axonal Action Potential Properties.
• Hinrichen (1988). Paramecium - a model system for the study of excitable cells. Paramecium is a pretty cool model system I got interested in this week (see below).
• Saimi and Kung (1988). Ion channels in Paramecium, yeast and Escherichia coli. Comparative physiology of unicellular organisms.
• Mechaly et al (2005) - Molecular diversity of voltage-gated sodium channel alpha subunits expressed in neuronal and non-neuronal excitable cells.
• Naitoh and Eckert (1968) - Electrical properties of Paramecium caudatum- all-or-none electrogenesis. All-or-none spikes in unicellular Paramecium.
• Kung and Saimi (1982). The Physiological Basis of Taxes in Paramecium.
• Goldsworthy (1983). The evolution of plant action potentials. A theory that action potentials originate from a membrane repair mechanism.
• Eckert and Brehm (1979). Ionic Mechanisms of Excitation in Paramecium. Basis of action potentials in paramecium.
• Machemer and Ogura (1978) - Ionic conductances of membranes in ciliated and deciliated Paramecium. Spatial segregation of ionic channels in paramecium.
• Harz and Hegemann (1991). Rhodopsin-regulated calcium currents in Chlamydomonas. Fig. 3A presumably indicating Ca2+ action potential in Chlamydomona (unicellular alga).
• Taylor (2009). A Fast Na+ /Ca2+ -Based Action Potential in a Marine Diatom. That is another unicellular organism.