What structure forms the junction between a neuron and a target cell?

The synapse is the structure that forms the junction between a neuron and a target cell. This specialized site allows for communication between neurons or between a neuron and another type of cell, such as a muscle cell or a gland cell. During synaptic transmission, neurotransmitters are released from the axon terminal of the neuron into the synaptic cleft, which is the small gap between the two cells. These neurotransmitters then bind to receptors on the target cell, leading to a response.

The other options listed do not serve this specific function. The cell nucleus is the control center of the cell, housing the genetic material and regulating cell activity but does not play a direct role in neuron signaling. The axon is the long, thin structure that conducts electrical impulses away from the neuron's cell body but does not represent the junction itself. The cell body contains the nucleus and other organelles but is not the site of communication between neurons and target cells. Hence, the definition and role of the synapse are critical in understanding neural communication.