Communication via Released Molecules (Chemical Signaling)

Cells communicate over short or long distances by releasing chemical messengers that bind to specific receptors on target cells. The classification depends on the distance the messenger travels.

Mode of Communication

Autocrine Signaling

The signaling molecule acts back on the same cell that released it.

Examples include neurotrophins (like nerve growth factor) released by neurons and neuroglia, which serve as autocrine regulators in the nervous system. Growth factors often function as autocrine regulators.

Paracrine Signaling

A chemical signal acts on a neighboring cell in the same tissue. The diffusion of these signals must be limited (e.g., by rapid endocytosis or destruction by extracellular enzymes).

Neurotransmitters released at the neuromuscular junction (ACh) act on adjacent muscle cells, serving as an excellent example of paracrine signaling. Blood vessels utilize paracrine regulators such as nitric oxide (NO).

Endocrine Signaling

A hormone is produced in one tissue, secreted into the blood (circulation), and carried to distant target organs where it acts. Endocrine glands secrete chemical messengers into the extracellular fluid.

The endocrine system integrates the body's activities. Hormones (e.g., peptides, steroids) must be classified by their chemical nature, which determines their synthesis, transport (e.g., binding to plasma proteins), clearance, and mechanism of action on target cells.

Neurocrine/Synaptic Signaling

A presynaptic neuron releases a neurotransmitter into the narrow synaptic cleft to act on an adjacent postsynaptic neuron or neuroeffector cell. This process is specialized for cell-cell signaling. Chemical synapses afford great specificity and fine control.

Crucial for information transfer in the nervous system. Neurotransmitters activate receptors that are either ionotropic (ligand-gated ion channels, mediating fast responses) or metabotropic (G protein–coupled receptors, mediating slower, biochemically mediated responses). Disorders of neurotransmitter systems are implicated in conditions like schizophrenia and depression, and many psychoactive drugs interact with these systems.