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Lecture 1 Intro to Physiology

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Michelle French

Lecture 1 Text Notes Cell-to-Cell Communication - electrical signals – changes in a cell’s membrane potential; one of two basic physiological signals - chemical signals – molecules secreted by cells into the extracellular fluid; responsible for most communication within the body - target cells/targets – cells that receive electrical/chemical signals - there are four basic methods of cell-to-cell communication: gap junctions, contact-dependent signals, local communication, long-distance communication - gap junction • allows direct cytoplasmic transfer of signals between adjacent cells • simplest form of communication • protein channels that create cytoplasmic bridges between adjacent cells • forms from the union of connexins on two adjacent cells • connexin – membrane-spanning protein • connexon – protein channel formed by united connexins, that can open and close • when channel is open, the cells act as one cell with multiple nuclei • when open – ions, small molecules (amino acids, ATP, CAMP) diffuse directly between cytoplasm in each cell • larger molecules cannot pass through gap junction • only means by which electrical signals can pass directly between cells - contact-dependent signals • occurs when surface molecules on one cell membrane bind to surface molecules of another cell’s membrane • occurs in the immune system during growth and development • example – when nerve cells send out long extensions that grow from the central axis of the body to the distal ends of the limbs • cell adhesion molecules (CAMs) – role in cell-to-cell adhesion; act as receptors in cell-to-cell signalling and transfer signals in both directions across membranes - local communication • when chemicals diffuse through the extracellular fluid • accomplice by paracrine and autocrine signalling • paracrine signal – chemical signal that acts on cells in the immediate vicinity of the cell that secreted it • autocrine signal – chemical signal that acts on the cell that secreted it • some molecules can act as both autocrine and paracrine signals • signal molecules reach target by diffusing through the interstitial fluid • distance as limiting factor in diffusion, therefore limited to adjacent cells • several important classes of enzymes act as local signals – cytokines (regulatory peptides), eicosanoids (lipid-derived paracrine and autocrine signal molecules) - long-distance communication • combination of electrical signals carried by nerve cells and ch • emical signals transported through the blood • responsibility of nervous and endocrine systems • endocrine system uses hormones • nervous system uses neurocrines • hormone – chemical signal that is secreted into the blood and distributed over the body by circulation; comes in contact with most cells of the body • hormone target cells – cells that have receptors for the hormone • nervous system uses a combination of chemical signals – first along the neuron until reaching the end of the cell, then translated into a neurocrine • neurocrine – chemical signal secreted by the neuron; diffuses from the neuron across a narrow extracellular space to a target cell • neurotransmitter – a neurocrine that diffuses across neurons and has a rapid effect • neuromodulator – neurocrine that diffuses and acts more slowly as an autocrine or paracrine signal • neurohormone – neurocrine that is release by a neuron and diffuses into the blood • neurohormones and classic hormones are similar and blur the distinction between the nervous and endocrine systems – a continuum rather than two distinct systems - molecules can function as signals by more than one method - cytokines • can act as both a local and long-distance signal • initially referred to proteins that modulate immune responses but now includes regulatory peptides • all nucleated cells synthesize and secrete cytokines in response to stimuli • control cell development, cell differentiation, immune response • development and differentiation – cytokines function as autocrine or paracrine signals • stress and inflammation – cytokines act on more distant targets and are transported through circulation similar to hormones • differ from hormones however – act on a broader spectrum of target cells • not produced by specialized cells the way hormones are • made on demand unlike most protein and peptide hormones • are different from hormone signal pathways Signal Pathways - chemical signals (paracrine, autocrine, hormones) released from cells into the extracellular compartment – not very specific for finding targets - cells respond to only some chemical signals due to target-cell receptor proteins - receptor proteins – place chemical signals bind; a cell cannot respond to a chemical signal if it lacks the appropriate receptor proteins for that signal - receptor protein responses initiate when binding occurs - common features of signal pathways: • signal molecule is a ligand that binds to receptor; brings information to the target cell • ligand-receptor binding activates the receptor • receptor in turn activates one or more intracellular molecules • last signal molecule in the pathway initiates synthesis of target proteins or modifies the existing ones to create a response Control Pathways: Response and Feedback Loops - homeostasis – the ability of the body to maintain a relatively stable environment; uses a physiological control system - regulated variables – key functions controlled by homeostasis - basic parts to a physiological control system: • input signal – consists of a regulated variable and a specialized sensor • sensor – if the variable moved out of its desired range then the sensor is activated and sends a signal to the controller • controller – programmed to respond to certain input signals, and acts as an integrating center • integrating center – evaluates information coming from the sensor; initiates a response that brings the regulated variable back in range (often a neuron or endocrine cell) • effector – muscles and tissues controlled by integrating centers; effect a change Cannon’s Postulates on Regulated Variables and Control Systems 1. The nervous system has a role in preserving the “fitness” of the internal environment. • fitness – conditions compatible with normal function • nervous system coordinates and integrates blood volume, blood osmolarity, blood pressure, body temperature, etc. 2. Some systems of the body are under tonic control. • tonic control – can be regulated to increase or decrease input • example – neural regulation diameter in blood vessels • increased input from nervous system decreases diameter • decreased input from nervous system increases diameter 3. Some systems of the body are under antagonistic control. • systems not under tonic control • antagonistic control usually by hormones or the nervous system • examples – insulin, glucagon • insulin decreases the concentration of glucose in the blood • glucagon increases the concentration of glucose in the blood • in pathways controlled by the nervous system – sympathetic and parasympathetic divisions often have the opposite effect • example – chemical signals from a sympathetic increase heart rate but signals from a parasympathetic neuron decreases heart rate 4. One chemical signal can have different effects in different tissues. • chemical signals can have different effects based of the receptor and signal pathway of the target cell • example – epinephrine constricts or dilates based on whether the vessel has α- or β-adrenergic receptors Homeostasis Pathways - local control • isolated change around a cell or tissue evokes a response • response can be paracrine or autocrine • resp
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