A2 Biology Notes - Grey Matter

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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 cell body.
There is an insulating layer called the myelin sheath around the axon which is made
up of Schwann cells wrapped around the axon ­ the sheath affects how quickly
never impulses travel through the axon.
Not all organisms have myelinated axons.

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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 in the CNS ­ they have a large number of connections with other nerve
cells.

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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 sensory neurones enter the spinal cord through the dorsal route.
Sensory neurones from a synapse with a relay neurone.

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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 by parasympathetic reflex (rest & digest)
When the pupil constricts the radial muscles relax while the circular muscles contract
(high light)
When the pupil dilates the radial muscles contract while the circular muscles relax
(low

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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 sodium-potassium pumps in cell surface membrane.

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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.

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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 wider the diameter, the faster the impulse travels. Marine
invertebrates who live at low temperatures have developed thick axons to speed up
their responses.

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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 calcium concentration cause the synaptic vesicles
which contain neurotransmitters to fuse with the presynaptic membrane and release
their contents into the synaptic cleft by exocytosis.

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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 of the synapses determines whether an action potential will be
generated, affected by: type of synapse & the number of impulses received.
Excitatory synapses
Excitatory synapses make postynaptic membrane more permeable to Na+ ions.

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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 potassium ions will leave the membrane so there will be a greater
potential difference across the membrane as the inside becomes more negative than
usual.

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