ABSTRACT
Sensation and movement, comprising the second section of the book, provide important context for the cognitive processes bridging them. The story begins in Chapter 6 with the mechanical senses of touch, balance, and hearing. The chemical senses include smell and taste.
The earliest single-cell organisms mechanically sensed internal pressure. High pressure caused pores to open, expelling cellular contents. Later, cells evolved membrane channels that admitted or expelled particular ions. Most of today's ion transport channels appeared by 600 million years ago.
The advent of multicelled animals called for new mechanisms of mechanical sensing. Hair cells detected disturbances through the deflection of an embedded hair. Such cells were subsequently adapted for touch, hearing, and balance. Early cnidarians probably had them in the form of stinging cells, which some believe gave rise to neurons. Others, however, think that neurons evolved along with muscles, building into diffuse neural nets controlling movement.
Chemical sensing likewise originated with membrane channels. In multicellular animals, the sense of smell began with a small cluster of specialized cells connected to a nervous system, sometime during the transition from chordates to vertebrates.
It is surely significant that the senses of balance, smell, and taste all emerged 520-530 million years ago. A likely explanation is that a new type of sensory cell had evolved, a short one with a broad synaptic end that allowed immediate connections with neurons outside the central nervous system.
