<P> Desmosomes also provide strength and durability to cells and tissues and are located just below adherens junctions . They are sites of adhesion and do not encircle the cell . They are made of two specialized cadherins, desmoglein and desmocollin . These proteins have extracellular domains that interact with each other on adjacent cells . On the cytoplasmic side, plakins form plaques which anchor the desmosomes to intermediate filaments composed of keratin proteins . Desmosomes also play a role in cell - cell signaling . </P> <P> Gap junctions are the main site of cell - cell signaling or communication that allow small molecules to diffuse between adjacent cells . In vertebrates, gap junctions are composed of transmembrane proteins called connexins . They form hexagonal pores or channels through which ions, sugars, and other small molecules can pass . Each pore is made of 12 connexin molecules; 6 form a hemichannel on one cell membrane and interact with a hemichannel on an adjacent cell membrane . The permeability of these junctions is regulated by many factors including pH and Ca2+ concentration . </P> <P> 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 . </P> <P> 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 . </P>

How does an animal cell interact with other cells