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Nervous & Synaptic Transmission

Myelinated Neurones
Resting Potential ­ Polarisation
Membrane Structure
Sodium/Potassium Ion Balance
Action Potential ­ Depolarisation
Threshold Value
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

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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…

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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

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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

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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…

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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…

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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…

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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.…

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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…

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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+…


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