Nerves and Muscles Part 1

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  • Created on: 23-05-14 15:09
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  • Nerves and Muscles
    • Nerves and Homones
      • Nervous communications consists of neurones passing electrical impulses along their length.
        • Neurones pass directly to target cells.
      • Hormonal communication  consists of the endocrine system secreting hormones directly into the bloodstream.
        • Hormones pass to all parts of the body but stimulate target cells in a lock and key fashion- the hormone receptors have specific hormone receptor proteins on their surface.
          • Hormones= chemical signalling messengers that are released directly into the bloodstream.
    • Chemical Mediators
      • Histamine
        • Largely responsible for inflammation.
        • Made by White cells called mast cells in response to cell damage or bacterial infection.
        • Affects cells locally-makes blood capillaries more permeable so white blood cells and fluid accumulates in the affected area.
      • Prostaglandins
        • Group of substances with a variety of localised effects including inflammation.
        • First found in prostate gland but secrete to most organs in the body.
        • Effects= stimulating contraction of smooth muscle (uterus) and controlling platelets during blood clotting,
    • Motor Neurones
      • A motor neurone is a cell that carries nerve impulses away from the CNS to an effector such as a gland or muscle.
      • Myelinated means that the axon is protected by a fatty myelin sheath made from several layers of cell membrane.
        • Myelin sheath= non-conducting fatty layer around the axon.
          • Role of myelin sheath= Insulate the axon, protect the axon and spped up the transmission of the nerve impulses.
        • Sections are made by individual schwann cells that wrap around the axon many times.
          • In between the section of myelin there are nodes of Ranvier where the axon membrane is exposed.
            • Role of myelin sheath= Insulate the axon, protect the axon and spped up the transmission of the nerve impulses.
      • Key Features
        • A cell body that contains the nucleus and other organelles.
        • An elongated axon that carries impulses away from the cell body.
        • One or more dendrites that take impulses towards the cell body.
    • Resting Potential
      • Resting potential is the potential difference across the axon membrane of a nerve cell when the cell is at rest.
      • Once resting potential is established, the neurone is ready to transmit an impulse.
        • Resting potential is as a result of two processes occurring together.
          • Active Transport-sodium-potassium pump is a protein responsible for the active transport of positive ions.
          • Unequal facilitated diffusion- there are sodium channels and potassium channels.
            • Both are gated so that by changing their shape they can open up and allow the ions to diffuse freely.
              • Difference between the two is that the potassium channels are more 'leaky' and so potassium diffuses out faster than sodium diffuses in.
                • High conc of positive ions outside axon so the area has an overall positive charge compared to inside.
                  • There are negative ions inside and outside the axon membrane mostly cl- and negatively charged proteins.
                    • It's the movement of positive ions that establishes a resting potential and brings about an action potential.
    • Generation of an action potential
      • Action potential is a nerve impulse that rapidly reverses a resting potential and spreads rapidly along an axon.
      • For about a millisecond, the resting potential is reversed in one area of the axon so that the inside becomes positively charged. Reversal spreads along the axon while the original area recovers and establishes a resting potential again.
        • Depolatisation- Gated Na+ channels open, allowing Na+ to diffuse in and reverse the resting potential.
        • Repolarisation- gates Na+ channels close preventing further diffusion inwards. Gated K+ chanels open allowing + charged potassium ions to diffuse out rapidly.
        • Hyperpolarisation- an 'undershoot' results from the gated K+ channels still being open when the active transport mechanism begins to re-establish a resting potential by pumping sodium channels out.
        • The gate K+ channels return to their normal permeability and a resting potential is re-established.
      • Self-propogating- means that depolarisation in one area of the axon will automatically cause depolarisation of the next region.
        • Both sodium and potassium channels are voltage gated which means that their shape and therefore permeabilty depends on the voltage (charge) across the membrane.


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