 In this video, I will define, contrast, and provide examples of endocrine, paracrine, autocrine, and juxtacrine signaling. Paracrine signaling is a type of intercellular signaling where the chemical message, the paracrine signal, secreted by the signaling cell, travels a short distance through diffusion to bind to receptors on the target cell. Neurotransmission is an example of paracrine signaling, where neurotransmitters are the paracrine intercellular signal that is released by the presynaptic neuron into the synaptic cleft. To diffuse a short distance across the synaptic cleft and bind to receptors on the surface of the postsynaptic cell, these receptors can be ion channels that will open in response to the neurotransmitter, stimulating an electrical signal inside of the postsynaptic cell. Another example of a paracrine signal is a growth factor, like epidermal growth factor. Epidermal growth factor is secreted by signaling cells within the epidermis and binds to receptors on nearby cells, so epidermal growth factor diffuses through the tissue in order to bind to receptors on nearby cells and then stimulate cell division, stimulate mitotic cell division of nearby cells in order to stimulate growth of the epidermal tissue. Endocrine signaling is a form of intercellular signaling where the chemical message is a hormone. Immunizations are intercellular signals released by endocrine glands that travel in the blood. The bloodstream can carry a hormone a long distance from the signaling cell of the adrenal gland to the target cell whose function is being regulated by the hormone. One example of a water soluble hormone is epinephrine. Epinephrine is secreted by the adrenal glands and will travel through the bloodstream all through the body. The adrenal glands produce epinephrine in response to stress in order to help the body respond to stress. Epinephrine will bind to receptors on the surface of cells and then those receptors can activate an intracellular signaling mechanism. One example of a cellular response to stress that's stimulated by epinephrine is the breakdown of glycogen. Glycogen is a carbohydrate, a polysaccharide that functions as a storage form of carbohydrate and epinephrine stimulates the breakdown of glycogen. Glucose is produced and released into the blood and then that glucose can be broken down by cells throughout the body in order to help them cope with the stressful situation. An example of a lipid soluble hormone is testosterone. Testosterone is produced by cells inside of the male gonads. Male gonads also known as the testes contain these cells known as interstitial cells of lydig that will produce testosterone. Then testosterone will travel through the bloodstream and bind to intracellular receptors within target cells. Because testosterone is lipid soluble it's able to cross the plasma membrane and enter the cell in order to bind to an intracellular receptor. Then the intracellular receptor hormone complex functions as the intracellular signal that will regulate gene expression turning on and off the transcription of different genes. This will regulate the functions of the cell. For example in skeletal muscle gene transcription will lead to stimulation of muscle growth leading to increased muscle mass. Neuro hormones are hormones that are secreted by neurons. Neuro hormones are similar to neurotransmitters that they're secreted by neurons but instead of being secreted into a synapse and traveling only a short distance by diffusion, neuro hormones are secreted into the bloodstream and can travel a long distance to reach target cells. Neuro hormones are water soluble hormones that are made from amino acids. And most of the neuro hormones are polypeptides. One example of a neuro hormone is oxytocin. So oxytocin is produced by neurons in the hypothalamus that secrete oxytocin from the posterior pituitary. And then oxytocin travels through the bloodstream to reach target cells in the uterus in the smooth muscle of the uterus. Oxytocin will stimulate contraction. Autocrine signaling is a type of intercellular signaling very similar to paracrine signaling. However, in autocrine signaling the target cell is also the signaling cell. For example, when a signaling cell in the epidermis releases epidermal growth factor, epidermal growth factor can bind to epidermal growth factor receptors on the same cell that secreted the epidermal growth factor. And this can stimulate the cell division of the signaling cell. Juxtocrine signaling is a type of intercellular signaling that requires direct contact between two signaling cells. So these two cells are directly adjacent. One form of juxtocrine signaling is gap junctions that can allow a chemical signal to travel from the cytosol of one signaling cell into the cytosol of the adjacent target cell. Another example of juxtocrine signaling is the interaction between adjacent cells at a desmosome where cell adhesion proteins known as caherins bind two adjacent cells together. This signal then is relayed inside of the cell and activates an intercellular signaling mechanism that can regulate the cell cycle. These cell adhesion proteins can function in a form of contact inhibition to slow down the cell cycle when the tissue is crowded and there's a large number of cells that are contacting one another. This contact inhibition will slow down cell division. Another example of juxtocrine signaling is the communication between leukocytes of our immune system when a leukocyte encounters a pathogen. One way of defending against that pathogen is phagocytosis where the leukocyte will engulf the pathogen. The leukocyte shown in the left in this illustration is a phagocytic leukocyte that has engulfed the pathogen. As that pathogen is digested inside of the leukocyte, fragments from the pathogen can be packaged into MHC proteins that are proteins that function to display the antigens, the fragments from the pathogen on the surface of the phagocytic leukocyte. Phagocytic leukocytes will display antigens to T lymphocytes, also known as T cells, and this form of communication is how the phagocytic leukocytes can communicate to the T lymphocytes what types of infection the body is exposed to so that the adaptive immune response can be coordinated to defend against that infection.