been properly defined yet. Synapses A synapse typically involves a junction between an axon terminal of one neuron, known as the presynaptic neuron, and the dendrites or cell body of a second neuron, known as the postsynaptic neuron. Less frequently, axon-to-axon or dendrite-to-dendrite connections occur. Some neurons within the CNS have been estimated to receive as many as 100 000 synaptic inputs! What does a synapse look like? The axon terminal of the presynaptic neuron conducts electrical signals called action potentials towards the synapse. The end of the axon terminal has a slight swelling known as the synaptic knob. This is where chemical messengers called neurotransmitters are made and strored. The synaptic knob of the presynaptic neuron is located near the postsynaptic neuron. The space between the two neurons is called the synaptic cleft, and is too wide to allow current to pass directly from one cell to another, preventing the transference of action potentials between neurons. What happens at a synapse? A electrical signal (an action potential) is initiated and transmitted to the axon terminal of the presynaptic neuron. This stimulates voltage-regulated calcium ion channels in the synaptic knob to open. The concentration of calcium ions becomes much higher outside the neuron compared to inside, so calcium ions flow into the synaptic knob through the open calcium channels. The increased calcium ion concentration inside the neuron causes the release of neurotransmitter from the synaptic cleft. The neurotransmitter moves across the synaptic cleft, and binds to receptors on the postsynaptic