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October, 2006
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Serotonin: A biochemical messenger and regulator, synthesized from the essential amino acid L-Tryptophan. In humans it is found primarily in the central nervous system, gastrointestinal tract, and blood platelets. Serotonin mediates several important physiological functions including neurotransmission, gastrointestinal motility, hemostasis, and cardiovascular integrity. Multiple receptor families (Receptors, Serotonin) explain the broad physiological actions and distribution of this biochemical mediator.

The Synapse

JoVE 10997

Neurons communicate with one another by passing on their electrical signals to other neurons. A synapse is the location where two neurons meet to exchange signals. At the synapse, the neuron that sends the signal is called the presynaptic cell, while the neuron that receives the message is called the postsynaptic cell. Note that most neurons can be both presynaptic and postsynaptic, as they both transmit and receive information. An electrical synapse is one type of synapse in which the pre- and postsynaptic cells are physically coupled by proteins called gap junctions. This allows electrical signals to be directly transmitted to the postsynaptic cell. One feature of these synapses is that they can transmit electrical signals extremely quickly—sometimes at a fraction of a millisecond—and do not require any energy input. This is often useful in circuits that are part of escape behaviors, such as that found in the crayfish that couples the sensation of a predator with the activation of the motor response. In contrast, transmission at chemical synapses is a stepwise process. When an action potential reaches the end of the axonal terminal, voltage-gated calcium channels open and allows calcium ions to enter. These ions trigger fusion of neurotransmitter-containing vesicles with the cellular membrane, releasing neurotransmitters into the small space

 Core: Biology

G-protein Coupled Receptors

JoVE 10718

G-protein coupled receptors are ligand binding receptors that indirectly affect changes in the cell. The actual receptor is a single polypeptide that transverses the cell membrane seven times creating intracellular and extracellular loops. The extracellular loops create a ligand specific pocket which binds to neurotransmitters or hormones. The intracellular loops holds onto the G-protein.

The G-protein or guanine nucleotide-binding protein, is a large heterotrimeric complex. Its three subunits are labeled alpha (α), beta (β), and gamma (γ). When the receptor is unbound or resting, the α-subunit binds a guanosine diphosphate molecule or GDP, and all three subunits are attached to the receptor. When a ligand binds the receptor, the α-subunit releases the GDP and binds a molecule of guanosine triphosphate (GTP). This action releases the α-GTP complex and the β-γ complex from the receptor. The α-GTP can move along the membrane to activate second messenger pathways such as cAMP. However there are different types of α-subunits and some are inhibitory, turning off cAMP. The β-γ complex may interact with potassium ion channels which release potassium (K+) into the extracellular space resulting in hyperpolarization of the cell membrane. This type of ligand-gated ion channel is called a G-prot

 Core: Biology


JoVE 10143

Source: Jaideep S. Talwalkar, MD, Internal Medicine and Pediatrics, Yale School of Medicine, New Haven, CT

The physical examination requires the use of all of the provider's senses to gain information about the patient. The sense of touch is utilized to obtain diagnostic information through palpation.

 Physical Examinations I
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