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Hormonal system Nervous system
Communication by hormones Communication by nerve impulses
Transmission is by blood system Transmission by neurones
Transmission and response slow Transmission and response rapid
Hormones travel across body but only target Nerve impulses travel to specific parts of body=
organs respond = response widespread response localised…

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Made by cells located throughout the plant
Some plant growth factors affect tissues that release them, not a distant target organ

Indoleacetic acid (IAA) causes plant cells to ELONGATE.

1. Cells in tip of shoot produce IAA, transported downwards
2. IAA initially transported to all sides of shoot
3. Light…

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Structure of neurons

CELL BODY ­ contains nucleus. Has lots of RER which produces proteins and
DENSDRONS ­ small extensions of body cells which subdivide into dendrites that carry nerve
impulses TOWARDS cell body.
AXON ­ single long fibre that carries nerve impulses AWAY from cell body

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o SENSORY NEURONES ­ transmit nerve impulses from receptor to intermediate or motor
neurone. Contain of dendron and axon
o INTERMEDIATE NEURONES ­ transmit impulses between neurones.
o MOTOR NEURONES - transmit nerve impulses from intermediate or sensory neurones to
and effector. Have long axon and many short dendrites.


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6. Most Na+ channels closed, most K+ channels open = axon membrane more permeable to K+
which diffuse out faster = increases p.d between ­ive inside and +ive outside.
7. Electrical gradient established. Further outward K+ movement more difficult because they
are attracted to overall negative state inside axon =…

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Passage of action potential

Moves rapidly down axon by the reversal of electrical charge reproduced at different points along
axon. Size constant.
As one region produces action potential, it acts as stimulus for the depolarisation of next region, and
it's previous region undergoes repolarisation.
Action potentials are regenerated along each…

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Sheath of myelin acts as electrical insulator, preventing action potentials forming. Action potentials
occur at nodes of Ranvier = localised circuits between nodes. Action potentials jump from node to

Action potentials pass down myelinated axon faster.

Factors that affect speed at which action potential travels:


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o Ensures that action potential is propagated in
one direction only
o It produces discrete impulses ­ action potentials not merged together

How can an organism perceive the size of the stimulus?

o Number of impulses passing
in a given time. The larger the stimulus, the more impulses
o Different…

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Functions of synapses

Transmtiing impulses from one neurone to another, and in doing so act as junctions, allowing:

o Single impulse along one neurone to be transmitted to lots of different neurones at a
synapse = simultaneous responses
o Number of impulses combined at synapse to produce single response.


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Inhibition: on POSTSYNAPTIC membrane of some synapses, protein channels with Cl- can be made to
open which makes inside postsynaptic membrane more negative at resting potential
hyperpolarisation) = less likely new action potential will be created. = INHIBITORY SYNAPSES

Effects of drugs on synapses

They stimulate the nervous system by…


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