Neurones, impulses and co-ordination

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What are responses from the nervous system like?
Short lived and rapid.
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What are responses from the hormonal system like?
Slower, less specific, long lasting and widespread.
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What are chemical mediators?
Chemicals released from some mammalian cells that have an effect on cells in their immediate surroundings.
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Name 2 chemical mediators
Histamine and Prostaglandin
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Describe how IAA controls the tropisms of a plant
IAA is in the tip of the plant shoot. It is initially transported to all sides then it moves from the light side to the shaded side of the shoot as a result of light. There is a higher concentration on the shaded side so the plant bends towards light
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Describe the effect of a high concentration of IAA on the growth of root and stem cells.
Decreases growth of root cells, increases growth of stem cells
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What are dendrons?
Small extensions of the cell body dividing into smaller branches called dendrites.
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What is an axon?
Single long fibre that carries nerve impulses away from the cell body
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What do Schwann cells do?
Surround the axon and protect it, provide electrical insulation and carry out phagocytosis.
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What is the Myelin Sheath?
It is a covering around the axon made up of the membranes of the Schwann cells. Membranes rich in lipid called myelin.
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What are the Nodes of Ranvier?
Gaps between adjacent Schwann cells where there is no myelin sheath.
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Name the 3 different types of neurones
Sensory Neurones, Motor Neurones and Intermediate Neurones
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What is a nerve impulse?
Self-propagating wave of electrical disturbance that travels along the surface of the axon membrane
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Describe the movement of ions when the membrane is at resting potential
Potassium ions actively transported in, naturally diffuse out. Sodium ions actively transported out, naturally diffuse back in.
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What is the ratio of the active transport of sodium ions out to the active transport of potassium ions in?
3:2 (3 sodiums to 2 potassiums)
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Why is the axon membrane 100x more permeable to potassium ions when the axon is polarised?
Most of the gates that allow potassium ions to move through are open; most gates that allow sodium ions to move through are closed.
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Describe what happens during depolarisation
Sodium voltage gated channels open due to stimulus so more sodium ions diffuse into the axon so there is a reversal in p.d. At +40mV, sodium channels close + potassium channels open.
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Describe what happens during repolarisation
Electrical gradient preventing movement of K+ ions reversed so K+ ions diffuse out of the axon through open voltage gated channels. More K+ ions diffuse out so more channels open.
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Describe what happens during hyperpolarisation
Outward diffusion of K+ ions causes inside of axon to be more negative than resting potential. K+ channels close and the sodium-potassium pump resumes its job.
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Which transmits an nerve impulse faster: myelinated or unmyelinated axon?
Myelinated axon
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How is the transmission of a nerve impulse different in a myelinated and unmyelinated axon?
Myelinated: action potential jumps from node to node (saltatory conduction); Unmyelinated: action potential travels down the whole axon
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Name 3 factors that affect the speed of a nerve impulse
Myelin Sheath, Diameter of axon (the greater the faster), Temperature
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What are 3 roles of the refractory period?
1) Ensure action potential is propagated in one direction only. 2) To produce discrete impulses. 3) To limit the no. of action potentials
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What is the 'all or nothing principle'?
Stimulus has to be at or above a threshold frequency, otherwise no action potential can be triggered.
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Name two ways in which an organism can perceive the size of a stimulus
1) Number of impulses in a given time. 2) Having different neurones with different threshold values.
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What is a synapse?
point where the axon of one neurone connects with the dendrite of another or an effector
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What is a neurotransmitter?
A chemical that transmits impulses from one neurone to another.
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What is the synaptic cleft?
Small gap that separates neurones (20-30nm wide)
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What is the presynaptic neurone?
The neurone that releases the neurotransmitter
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What is the synaptic knob?
Swollen portion of axon at the end of the presynaptic neurone.
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What do synaptic vesicles do?
Store neurotransmitters.
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What does the synaptic knob contain?
Many mitochondria and endoplasmic reticulum.
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How does a neurotransmitter set up a new action potential in the postsynaptic neurone?
Diffuses across synapse and binds with a receptor on the postsynaptic neurone.
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What is Spatial summation?
No. of different presynaptic neurones together release enough neurotransmitter to exceed threshold frequency and create a new action potential.
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What is Temporal summation?
Single presynaptic neurone releases neurotransmitters many times over a short space of time, exceeding threshold frequency and creating an action potential.
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How can chloride ions inhibit action potentials?
Protein channels carrying Cl- open so there is an inward diffusion of Cl- ions making the inside of the postsynaptic membrane more negative.
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What does the influx of calcium ions do?
It causes the synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
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What happens when acetylcholine fuses with receptor sites on the postsynaptic membrane?
Sodium ion channels open
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What does acetylcholinesterase do?
Hydrolyses acetylcholine into choline and acetyl
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What happens once acetylcholinesterase carries out its job?
Choline and acetyl diffuse back into the presynaptic membrane.
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What does the breakdown of acetylcholine prevent?
Another action potential being generated
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What is the role of mitochondria in the presynaptic neurone?
Release ATP to recombine choline + acetyl into acetylcholine
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Card 2


What are responses from the hormonal system like?


Slower, less specific, long lasting and widespread.

Card 3


What are chemical mediators?


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Card 4


Name 2 chemical mediators


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Card 5


Describe how IAA controls the tropisms of a plant


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