Fig. 4. A schematic diagram of a glutamatergic synapse showing many of the major sources of energy consumption. Movements of ions across the neuronal cell membrane account for a large proportion of the energy consumed. During transmission of the action potential along the axon, Na⁺ and K⁺ ions move through voltage-gated ion channels due to concentration gradients and potential differences across the membrane. When the action potential reaches a synapse, voltage-gated Ca²⁺ channels open, to admit Ca²⁺ ions and trigger the release of vesicles containing glutamate molecules. These glutamate molecules then bind to ligand-gated ion channels, which open admitting Na⁺ molecules that depolarize the post-synaptic neuron. Glutamate in the synaptic cleft is transported into the presynaptic neuron or nearby glial cells by a glutamate co-transporter. Within the pre-synaptic neuron, glutamate molecules are transported into the synaptic vesicles by a glutamate/H⁺ anti-porter. Almost all of these processes require the activity of two pumps, the 3Na⁺/2K⁺ pump and the H⁺ V-ATPase.