The Nervous System

  • Carries out the following:
    -Detects changes or stimuli inside or outside the body
    -Proesses the information
    -Initiates responses
  • Broken up into two main parts:
    -The CNS comprising of the brain and spinal cord
    -The peripheral nervous system, which is made up of pairs of nerves that originate from the brain or the spinal cord, carrying impulses to the CNS.
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Reflex Arc

  • A reflex arc is a rapid, involuntary response resulting from nervous impulses initiated by a stimulus. The action is involuntary and the brain is not involved, generally protective.
  • The withdrawal reflex involves the following:
    -Stimulus - the hot surface.
    -Receptor - temperature and pain receptors in the skin.
    -Sensory neurone sends impulse to the spinal cord.
    -Relay neurone connects sensory neurone to a motor neurone.
    -Motor neurone sends impulse to an effector - a muscle.
    -Response - arm muscles contract and the hand is removed.
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Reflex Arc Diagram (

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Nerve Nets In Hydra

  • Consisting of short extensions joined in different directions, meaning a slow nerve impulse.
  • Not many receptors and effectors, limited number of stimuli.
  • No brain or true muscles.
  • Used to connect to sensory photoreceptors to touch-sensitive nerve cells.
  • Enables Hydra to sense its surroundings and react to stimuli.
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  • Three types of neurones:
    -Sensory - which bring impulses from receptors to CNS
    -Motor - carry impulses from the CNS to the effectors
    -Connector/relay - these recieve impulses from sensory neurones and send them to motor
  • Each cell consists of:
    -A cell body containing a nucleus and granular cytoplasm containing many ribosomes, these clump together to form Nissl granules that produce neurotransmitter.
    -Thin extentions that carry impulses towards the cell body, short extensions are called dendrites and the axon is the long membrane-covered extension, transmitting impulses.
    -At its end, the axon brances off to form synapses with other neurones.
    -Periphral neurones are surrounded by and supported by Schwann cells, in mylenated neuronse these grow around the axons of the nerve cells to form a myelin sheath; acting as an electrical insulator.
    -The myelin sheath has intervals called the Nodes of Ranvier, which are important in transmission of impulses.
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Motor Neurone


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Nerve Impulses

  • Small electrical charges so can only be detected with a pair of microelectrodes and fed into a cathode ray oscilloscope.
  • Resting potentials are typically minus values, indicating that the inside is negative in comparison to the outside:
    -Na+ and K+ are trasnsported across the membrane by active transport.
    -This involves the use of Na+/K+ which maintain the uneven distribution of ions between the membranes; Na+ are pumped out (3) and K+ are pumped in (2).
    -Most K+ channels are open so that K+ can flood out if needed but Na+ channels are closed to stop Na+ from moving in by facillitated diffusion.
    -As a result the outside is more positive compared with the inside.
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Resting Potential

-As well as the last slide, large negative ions such as Cl- make the inside compartively more negative.

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The Action Potential

  • When a stimulus is recieved by a receptor or nerve ending, the reversal of the charges on the axon membrane takes place - as a result the -70mV inside the membrane becomes +40mV.
  • This is the action potential and the membrane becomes depolarised.
  • The sequence occurs as follows:
    -The energy from the stimulus causes the Na+ channels to open, and the sudden influx of Na+ causes the membrane to become depolarised.
    -When +40mV is reached the Na+ channels close to prevent any further impulses.
    -The K+ channels open and K+ flows out again and repolarise the membrane, there is a temperary overshoot of K+ leaving as the K+/Na+ pump restores ionic balance.
    -This is known as the refractory period where no other impulses can occur.
  • The action potential causes a small electrical current across the membrane and as a portion of the membrane becomes depolarised, the next part starts to as well. There is a circuit of localised currents propagated along the axon; and the membrane becomes repolarised post-transmission of current. 
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Action Potential Graph

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Properties Of Nerves And Impulses

  • Occurs as an "all or nothing" principle, unless a threshold is reached then the action potential does not occur; there are no small action potentials.
  • Strength of transmission is dependant on the frequency of action potentials.
  • Two main factors affect the rate of transmission:
    -The diameter of the axon, the greater the diameter the faster the transmission.
    -Myelination speeds up the rate of transmission as it is an electrical insulator; depolarisation does not occur at mylinated parts of the axon; so the current jumps from one Node of Ranvier to the next - called saltatory conduction and it is faster than normal.
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Synapse Structure

  • Neurones are not in direct contact but are seperated by tiny gaps called synapses, these convey impulses one way only. 


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Synaptic Transmission

  • The arrival of an action potential alters permeability so that Ca+ open.
  • This influx of Ca+ ions makes the vesicles containing neurotransmitter travel to the pre-synaptic membrane.
  • Here, it fuses with the membrane and releases the neurotransmitter by exocytosis.
  • The neurotransmitter, acetylcholine, diffuses across the synaptic cleft.
  • At the post-synaptic membrane it binds to receptors on the Na+ channels, the binding causes the channel to change shape.
  • This causes them to open and Na+ to flood in, depolarising it and causing a new action potential in the next neurone.
  • Acetylcholine is broken down by its enzyme and then remanufactured as well as the vesicles that hold it, by the power of ATP, to be reused.
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Function Of Synapses

  • The function of synapses include:
    -Transmit information between neurones.
    -Pass impulses one way only.
    -Acts as junctions.
    -Filter out low stimuli; removing "background noise".
    -To protect the response system from overstimulation. 
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Effects Of Drugs

  • Amplification at a synapse may be due to chemicals mimicking the natural neurotransmitter.
  • A pschoactive drug is one that acts primarily on the central nervous system where it alters brain function, affecting mood and perception; such as heroin or cocaine.
  • Most drugs that affect nerve transmission across synapses are one of two types:
    -Excitory drugs, which stimulate the nervous system into producing more action potentials.
    -Inhibitory drugs, which inhibit the nervous system and so it produces less action potentials.
  • If a drug is taken over a long period of time then the synapse may become altered to compensate and so you become tolerant.
  • Organophosphorous insecticides block the enzyme that breaks down neurotransmitter so it remains in the cleft, causing constant action potentials - if in a muscle junction the uncontrollable muscle contractions may occur. 
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