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A2 Biology Notes - Grey Matter Chrissie Mayiati




The Nervous System & Nerve Impulses




Neurones are single cells; although there are different types they have the same
basic structures: cell body contains a nucleus, dendrites which conduct impulses
towards cell body and the axon which transmits impulses away from the…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




Motor Neurone




Cell body is always situated within the CNS, the axon conducts impulses from the
CNS to the effectors (muscles or glands). Vey long neurones.

Sensory Neurone




Sensory neurones carry impulses form sensory cells to the CNS

Relay Neurone




Found mostly…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




Reflex Arcs




Reflex arcs are responsible for reflexes ­ rapid, involuntary responses to stimuli
Receptors firstly detect a stimulus (e.g. hot cup) and generate a nerve impulse.
The sensory neurone conducts the nerve impulse to the CNS along the sensory
pathway.
The…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




The Pupil Reflex

The iris controls the size of the pupil: it contains a pair of antaganostic muscles:
radial and circular muscles (controlled by autonomic nervous system)
Radial muscles controlled by a sympathetic reflex (fight vs flight) and circular muscles
are controlled…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




Key Biological Principle: Explaining the potential difference of a neurone

The resting potential of an axon is ­ 70 mV
It is caused by the uneven distribution of ions across the cell surface membrane ­ it
is achieved by the action of…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




Impulse size

The size of a stimulus affects: the frequency of impulses and number of neurones in
a nerve conducting the impulse.




Speed of impulse conduction

Page 7

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A2 Biology Notes - Grey Matter Chrissie Mayiati




The speed of impulse conduction is very fast, allowing fast responses to stimuli. They
are affected by 3 factors.
Temperature ­ the higher temperature, the faster the speed. Homoeothermic
(warm-blooded) animals have faster responses than poikilothermic (cold-blooded)
ones.
Axon Diameter ­ the…

Page 8

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A2 Biology Notes - Grey Matter Chrissie Mayiati




There are three stages leading to the nerve impulse passing along the post synaptic
neurone.
Neurotransmitter Release ­ the presynaptic membrane becomes depolarised due
to the action potential travelling down the membrane. Ca2+ channels open and diffuse
into the membrane, the increased…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




Synapses have two main roles: the control of nerve pathways, allowing flexibility of
response and the integration of information from different neurones allowing a
co-ordinated response.
The posynaptic cell can receive inputs from many synapses at the same time. The
overall effect…

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A2 Biology Notes - Grey Matter Chrissie Mayiati




These synapses make it less likely that an action potential will occur ­ they cause
hyperpolarisation. The neurotransmitter from these synapses opens channels for Cl-
and K+ ions in the postsynaptic membrane. As a result chloride ions will enter the
membrane and…

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