Receptor proteins on the cell surface have the ability to bind specific signaling molecules secreted by other cells.  Cell signaling allows cells to communicate with adjacent cells, nearby cells (paracrine) and even distant cells (endocrine). This binding induces a conformational change in the receptor which, in turn, elicits a response in the corresponding cell.  These responses include changes in gene expression and alterations in cytoskeleton structure.  The extracellular face of the plasma membrane has a variety of proteins, carbohydrates, and lipids which project outward and act as signals.  Direct contact between cells allows the receptors on one cell to bind the small molecules attached to the plasma membrane of different cell. In eukaryotes, many of the cells during early development communicate through direct contact.
Synaptic signaling, an integral part of nervous system activity, occurs between neurons and target cells.  These target cells can also be neurons or other cell types (i.e. muscle or gland cells).  Protocadherins, a member of the cadherin family, mediate the adhesion of neurons to their target cells at synapses otherwise known as synaptic junctions.  In order to for communication to occur between a neuron and its target cell, a wave of depolarization travels the length of the neuron and causes neurotransmitters to be released into the synaptic junction.  These neurotransmitters bind and activate receptors on the post-synaptic neuron thereby transmitting the signal to the target cell.  Thus, a post-synaptic membrane belongs to the membrane receiving the signal, while a pre-synaptic membrane is the source of the neurotransmitter.  In a neuromuscular junction, a synapse is formed between a motor neuron and muscle fibers.  In vertebrates, acetylcholine released from the motor neuron acts as a neurotransmitter which depolarizes the muscle fiber and causes muscle contraction.  A neuron’s ability to receive and integrate simultaneous signals from the environment and other neurons allows for complex animal behavior.