The Nervous System
- Carries out the following:
-Detects changes or stimuli inside or outside the body
-Proesses the information
- 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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- 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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- 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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-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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- Neurones are not in direct contact but are seperated by tiny gaps called synapses, these convey impulses one way only.
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- 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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