A neuron's bushy, branching extensions that receive signals and convey impulses to the cell body. The junction exists between the transmitting neuron's axon tip and the receiving neuron's dendrite or cell body. The synaptic gap, or cleft, is the microscopic gap at this juncture. It contains special substances that can pass information back and forth between neurons. The receiving neuron's dendrite branches out into the synaptic gap, forming a bridge across it. As more dendrites branch off, more branches reach into the gap, creating many small bridges across it. These bridges are where the signal will be transmitted from neuron to neuron. When an impulse travels down an axon, it triggers a series of biochemical events inside the axon that cause it to release neurotransmitters. Neurotransmitters are chemicals that travel from one nerve cell to another through synapses to communicate information such as touch, vision, memory, and thought. They are also called neuromodulators because they can modulate the activity of other neurons. Neurotransmitters bind to receptors on the surface of the receiving cell, causing them to change their activity level. This leads to changes in the behavior of the receiving cell. For example, when acetylcholine binds to receptors on the surface of muscle cells, they become excited and tell the cell body to send electrical signals down the fiber to trigger muscle contraction.