Nervous & Synaptic Transmission (AQA BIOL5 Notes)

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Nervous & Synaptic Transmission
Neurones
Structure
Myelinated Neurones
Classification
Resting Potential ­ Polarisation
Membrane Structure
Sodium/Potassium Ion Balance
Action Potential ­ Depolarisation
Threshold Value
Summary
Passage of an Action Potential
Non-Myelinated Neurones
Myelinated Neurones (Saltatory Conduction)
The Refractory Period
Factors Affecting the Speed of Transmission
Synaptic Transmission
Cholinergic Synapse
Features of Synapses
Drug Effects of Synapses
Stimulation
Inhibitory
Effects
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Neurones
Structure
A neurone is a specialised cell that carries electrical signals, to do this they have:
Large amounts of rough endoplasmic reticulum for synthesising neurotransmitters
that are proteins.
Dendrons are extensions of the cell membrane and cytoplasm that further divide into
dendrites. These carry nerve impulses from one neurone to the cell body.
The cell elongates into the axon, a long fibre that carries impulses away from the
cell.
Myelinated Neurones
Some neurones have schwaan cells that wrap
around the axon, providing electrical insulation.…read more

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Classification
Nerve cells can be classified into three main types:
Type Structure Function
They transmit impulses from
a receptor (eyes, skin etc.)
Sensory Neurone
to a relay/motor neurone.
They transmit impulses
Relay Neurone between neurones.
They transmit impulses from
a relay/sensory neurone to
an effector (muscle, gland
Motor Neurone
etc.).…read more

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The resting electrical potential across the plasma membrane is roughly 65mV and is
maintained by a balance between Na+ and K+ ions inside and outside the axon.
Membrane Structure
The phospholipid bilayer plays and important part as it is
impermeable to charged particles like Na+ and K+. It contains
three types of intrinsic transport proteins embedded in the
bilayer:
Voltage Gated Channels these allow ions to move in
and out, however they sometimes are shut and thus can regulate when the ions can
move.…read more

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When a stimulus is received there is a reversal of the charges across the axon membrane.
Inside the axon becomes more positive, going from 65mv to +40 mv. This is the axon being
depolarized, and if it is depolarized sufficiently an action potential can be generated.
Voltage gates Na+ channels open ­ these were closed during rest and Na+ diffuses
rapidly in down their electrochemical gradient.…read more

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One of the key points about action potentials is an action potential will only be generated if
the initial stimulus if above a certain threshold value.
If the initial stimulus is not large enough, too few Na+ channels will open and the
axon will not become sufficiently depolarized.
If the stimulus is large enough, then the impulse will be generated at a constant size
and speed.…read more

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NonMyelinated Neurones
Actions potentials are literally propagated along the neurone without loss of size or speed.
The region of the axon that is depolarised is the active zone.
In the active zone, the axon is positively charged and the outside negatively charged. This
difference causes small electrical currents, which flow between the active zone and resting
zones on each side causing the depolarisation of the adjacent region of the axon by opening
gated Na+ channels.…read more

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The area behind the active zone is recovering from depolarisation and is known as the
refractory zone. There are two refractory periods:
Absolute refractory period ­ the neurone cannot respond to any stimulus or conduct
an impulse.
Relative refractory period ­ the neurone can only respond to high intensity stimuli.
This is greatly important as it ensures:
Action potentials only travel in one direction.
Impulses do not merge to become one and thus separate impulses are produced.…read more

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Neurones do not touch and are separated by the synaptic cleft. The neurone with releases
the neurotransmitter is known as the presynaptic neurone and the end of the axon forms
the synaptic knob.
The neurotransmitter, once manufactured is stored in synaptic vesicles, which can fuse to the
presynaptic membrane and release the neurotransmitter into the cleft where it can bind to
receptors on the postsynaptic neurone.…read more

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Cholinergic Synapse
The most common neurotransmitter in vertebrates is acetylcholine (ACh). Synapses that use
ACh as the neurotransmitter as known as cholinergic synapses, they are found in the CNS
and at neuromuscular junctions.
ACh is made from ethanoic (acetyl) acid and choline.
The depolarisation of the presynaptic neurone opens Ca2+ channels and they
diffuse in to the membrane.
The Ca2+ cause synaptic vesicles to release ACh by exocytosis into the
synaptic cleft.…read more

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