Docosahexaenoic acid (DHA) is a 22-carbon omega 3 PUFA highly enriched in the neuronal cell membranes and rod outer segment membranes. When DHA is depleted from these cell membranes it is replaced nearly quantitatively by a 22-carbon omega 6 PUFA, docosapentaenoic acid, which has similar, but less potent, biophysical and physiological properties to DHA. It is speculated that omega 6-docosapentaenoic acid is a buffer to prevent the possible catastrophic effects of DHA depletion on brain and visual function. The primary insult from the loss of DHA from cell membrane glycerophospholipids, and replacement by omega 6-docosapentaenoic acid, is on the flexibility/compression of the membrane lipids which affects the optimal function of integral membrane proteins (receptors, voltage-gated ion channels and enzymes). This leads to effects on second messenger systems, and subsequently affects neurotransmitter concentrations due to 'weakened' signals from the initiating receptors. Remembering there are more than 80 billion neurones and many times more synaptic connections between neurons, a very small loss of "efficiency" in signal due to altered properties of membrane proteins would likely result in meaningful changes in brain and visual function. Additionally, impairment of neurotransmission could be due, in part, to sub-optimal brain energy metabolism (glucose entry into the brain), which is significantly reduced in omega 3 deficiency. Many studies report that dietary omega 3 deficiency results in changes in learning, coping with stress, behavioural changes, and responses in visual function. It is thus concluded that DHA is an essential fatty acid for optimal neuronal function.