The Human nervous system

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  • The Human Nervous System
    • Used to respond to changes in the environment and so increase survival
      • Humans have two control systems
        • Nervous system
          • Has 3 parts: detecting stimuli, coordinating and effecting a response
          • Composed of nerve cells, or neurones
            • A neurone has a cell body with extensions leading off it.
              • Several dendrons carry nerve impulses towards the cell body, while a single axon carries the never impulse away from the cell body.
                • Axons and dendrons are only 10 nano m in diameter but can be up to 4m in length in large animals.
                  • A nerve is a discrete bundle of several thousand neurone axons
            • Nerve impulses are passed from the axon of one neurone to the dendron of another at a synapse.
            • Humans have 3 types of neurone: sensory, effector/ motor, relay/ interneurones
          • The Nerve Impulse
            • All animal cell membranes contain a protein pump called a Na+K+ATPase
              • This ues the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in
                • The combination of the Na+K+ATPase pump and the leak channels cause a stable imbalance of NA+ and K+ ions across the membrane.
                  • The imbalance causes a potential difference across all cell membranes called them membrane potential
                    • The membrane potential is always negative inside the cell, and varies in size from -20 to -200mV
                  • The Na+K+ATPase is thought to have evolved as an osmoregulatorto keep the internal waster potential high and so stop water entering animal cells and bursting them.
                • There are also sodium and potassium ion channels in the membrane
                  • These channels are normally closed but when closed they leak
                • In nerve and muscle cells the membranes are electrically excitable
                  • They can change their membrane potential and this is the basis of the nerve impulse
                  • The action potential has 2 phases called depolarisation and repolarisation
                    • 1. Depolarisation
                      • The sodium channels open for 0.5ms, causing sodium ions to diffuse in down their electromagnetic gradient
                        • This makes the inside of the cell more positive
                    • 2. Repolarisation
                      • The potassium channels open for 0.5ms, causing potassium ions to move out down their concentration gradient
                        • This makes the inside more negative again. it restores the original polarity
                    • Since both channels are voltage-gated, they re triggered to open by changes in the membrane potential itself.
                      • The sodium channel opens -30mV and the potassium ion channels open at 0V
                      • How are nerve impulses propagated?
                        • Once an action potential has stated it is propagated along an axon automatically
                          • The local reversal of the membrane potential is detected by the surrounding voyage-gated ion channels, which open when the potential changes enough
                      • The ion channels have 2 other features that help the nerve impulse work effectively:
                        • 1. After an ion channel has opened , it needs a 'rest period' before it can open again
                          • This is called the refractory period, and lasts about 2ms. This means that the upstream ions channels cannot open again
                            • Causing the action potential to move one way along the axon
                          • 2. The ion channels are either open of closed; there is no half-way position
                            • This means that the action potential always reaches +40mV as it moves along an axon
                              • and it is never attenuated (reduced) by a long axon
        • Endocrine (hormonal system)
          • Hormones are secreted by glands into the blood stream.
            • There are 2 different kinds of glands:
              • Exocrine
                • Secrete solutions to the outside, or to the body cavities, usually through ducts
              • Endocrine
                • Do not have ducts but secrete chemicals directly to the tissue fluid,whence they diffuse into the blood stream
            • once a hormone has been secreted by its gland, it diffuses into the blood stream and is carried all round the body to all organs
              • It only effects the target organs, which can respond to it
                • These target organs have specific receptor molecules in their cells to which the hormone binds
                  • These receptor molecules are proteins, and they form specific hormone-receptor complexes
                    • There are 3 different ways in which a hormone can affect cell function
                      • 1. Some hormones affect the permeability of the cell membrane.
                        • They bind to a receptor on the membrane, which then activates a transporter, so substances can enter or leave the cell
                      • 2. Some hormones release a "second messenger" inside the cell. They bind to a receptor on the membrane.
                        • The  receptor then activates and enzyme in the membrane, which catalyses the production of a chemical in the cytoplasm, which affects various aspects of the cell
                      • 3. The steroid hormones are lipid-soluble so can easily pass through membranes by lipid diffusion. They diffuse to the nucleus, where they bind to a receptor, which activate protein synthesis


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