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4.1.2 Nerves


(a) Outline the roles of sensory receptors in mammals in converting different forms of energy
into the nerve impulses

Specialised cells that can detect changes in our surroundings
They are energy transducers that convert one form of energy to another
Each transducer is adapted to detect changes in…

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Length of dendrites or dendron Long (often single dendron)- Many short dendrites
runs from sensory receptor to
the cell body just outside the
CNS; has dendrites at the end
of axon

Length of Axon Short- runs from just outside Long- runs from inside CNS to
CNS into the CNS effector…

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Sensory Neurone

Carry impulses via a dendron from sense organs to the brain or
spinal cord
Cell bodies are inside structures called dorsal root ganglia, just
outside spinal cord




Relay Neurone



Relay neurones have their cell bodies and cytoplasmic processes
inseide the brain or spinal cord
Adapted to carry impulses…

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Term Function Structure
Axon Nerve fibre; a single process
extending from the cell body of
a neurone and carrying nerve
impulses away from it
Dendrite Carries nerve impulses from One of the shorter branching
adjacent neurons into the cell processes of the cell body of a
body neurone. All dendrites…

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Myelin Sheath Protection of the nerve A complex material formed of
fibre protein and phospholipid (fat)
Insulation of the nerve that is laid down as a sheath
fibre around the axons of certain
neurons
Increases the rate of
transmission of nerve
impulses

Nerve Bundle containing 100s
and 1000s of axons…

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Sensory Neurone Transmit impulses inwards
from sense organs to CNS

Synapse Reaching a synapse, Minute gap across which nerve
impulses causes the impulses pass from one
release of a neurone to the next, at the end
neurotransmitter, which of a nerve fibre
diffuses across the gap and
triggers an electrical…

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When a neurone is transmitting an action potential it is said to be at rest.
It is actively transporting ions across its cell surface membrane
Sodium potassium pumps use ATP to pump 3 sodium ions out of the cell for every 2 potassium
ions that are pumped in
Plasma membrane…

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At rest the sodium ion channels are kept closed
Sodium potassium pump uses ATP to actively transport Na+ out for every 2 K+ brought into the
axon
A few of these potassium ions diffuse back out as some potassium channels are open
If some of the sodium channels are opened…

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This causes a large influx of sodium ions and the depolarisation reaches +40mV, which is an action
potential
Once this value is reached the neurone will transmit an action potential because many
voltage-gated sodium ion channels open
Action potential is self-perpetuating- once it starts at one point in the neurone,…

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After an action potential the sodium and potassium ions are in the wrong places. The concentrations
of these ions inside and outside the cell must be restored by the action of sodium and potassium
pumps.

For a short time after each action potential it is impossible to stimulate the cell…

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