Forms of Intracellular Signaling

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Forms of Intracellular Signaling

Cell signaling is part of any communication process that governs the basic activities of cells and coordinates cell actions.
It is the ability of cells to perceive and correctly respond to their microenvironment that is the basis of development, tissue repair, immunity, and homeostasis.
Communication between cells is common in nature.
The cells of multicellular organisms use a variety of molecules as signals, such as peptides, proteins, amino acids, nucleotides, steroids, and lipids.

Cell signaling can be classified as mechanical and biochemical based on the type of the signal.
Mechanical signals are the forces exerted on the cell and produced by the cell.
Biochemical signals are the biochemical molecules such as proteins, lipids, ions, and gases.
These signals can be classified based on the distance between signaling and responder cells.
Signaling between and amongst cells is divided into the following:

Gap junctions in animals are connections between the plasma membranes of neighboring cells.
These water-filled channels allow small signaling molecules, called intracellular mediators, to diffuse between the two cells.
Small molecules, such as calcium ions (Ca²⁺), are able to move between cells, but large molecules, like proteins and DNA, cannot fit through the channels.
The specificity of the channels ensures that the cells remain independent, but can quickly and easily transmit signals.
The transfer of signaling molecules communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received.

Signals that act locally between cells that are close together are called paracrine signals.
Paracrine signals move by diffusion through the extracellular matrix.
These types of signals usually elicit quick responses and last for a short duration.
Paracrine ligand molecules are quickly degraded by enzymes or removed by neighboring cells.
Paracrine signaling plays an important role in early development, coordinating the
activities of neighboring cells
One example of paracrine signaling is the transfer of signals across synapses between nerve cells.

The cells of the nervous system provide rapid communication with distant cells.
Their signal molecules, neurotransmitters, do not travel to the distant cells through the circulatory system like hormones.
The long, fiber-like extensions of nerve cells release neurotransmitters from their tips very close to the target cells.
The narrow gap between the two cells is called the chemical synapse.

Signals from distant cells are called endocrine signals; they originate from endocrine cells.
In the human body, many endocrine cells are located in endocrine glands such as the thyroid gland, hypothalamus, and pituitary gland.
These types of signals usually produce a slower response but have a long-lasting effect.
The ligands released in endocrine signaling are called hormones, signaling molecules that are produced in one part of the body but affect other body regions some distance away.
Hormones travel large distances between endocrine cells and their target cells via the bloodstream.