Long-Distance Signaling

In order to communicate signals across the organism, some cells must be able to send information long distances. Long-distance signaling is a type of communication whereby a signaling molecule acts on a target cell far from the signaling cell. Long-distance signaling is also called endocrine signaling. A hormone is a chemical secreted by an endocrine gland that targets a particular cell to produce a response. Hormones are distributed by the bloodstream throughout the whole body. An endocrine cell, which is a cell that secretes a hormone, is the signaling cell in long-distance signaling. Hormones are able to find their target cells because of their high affinity, or tendency to bind, to their receptors. The target cell, which is a cell that responds to a specific signal molecule, for a given hormone has a receptor, a protein that binds to a specific extracellular signal molecule that then begins a cell response specific to that hormone.

When a hormone binds to a receptor, it forms a receptor-ligand complex. A ligand is any molecule that binds tightly and specifically to a receptor protein. In this case, the ligand is the signal molecule. The formation of the receptor-ligand complex causes a conformational change in the receptor, which in turn triggers a series of reactions that ends with the desired cell response. For example, epinephrine is a hormone produced in the adrenal glands, located superior to the kidneys. When an animal is under stress, the adrenal glands release epinephrine into the bloodstream. Muscle cells are target cells for epinephrine. When epinephrine binds to the receptors on muscle cells, the cells respond by increasing the breakdown of glycogen into glucose 1-phosphate, which provides energy to the muscle cells in the form of ATP, adenosine triphosphate. Another way the body can transmit signals over distances is with specialized cells. Neurons are specialized cells in the body that are made of a cell body from which protrude dendrites and one long axon that can be up to 1 meter long. The ends of a neuron contain chemicals called neurotransmitters; a neurotransmitter is a small signaling molecule secreted by a presynaptic neuron. The junction at which a neuron meets a target cell is called a synapse. Nerve-cell signaling is called neuronal signaling, or transmission of signals by nerve cells at a synapse, also called synaptic signaling. Some neuronal signaling cascades involve chains of many neurons signaling each other, but some neurotransmitters are able to travel to locations far from the cell body of the neuron because the axons of a neuron can be very long, such as the distance from the spinal cord to a toe.

Note that this type of signaling is in contrast to long-distance, or endocrine, signaling, because neurotransmitters are not carried by the bloodstream. Instead, the bulk of the signaling mechanism occurs by means of electrical impulses along the nerve cell. It is not until the neurotransmitter is released from the end of the neuron that it must diffuse between the end of the nerve cell and the membrane of the target cell, a distance of 20 to 40 nanometers. In synaptic signaling, neurons, which are long cells, release neurotransmitters close to the target cell.