Action potential A2 Biology

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D
Action potentials
1.3 Converting energy into nerve impulses, resting potentials and action potentials
Sensory receptors
A receptor converts an external or internal stimulus into an electrical signal. Sensory receptors detect changes in their
surroundings. The table below outlines the main stimuli and the receptors which detect them:
Stimulus Receptor Location A thermoreceptor detects changes in
auditory receptors (the cilia in temperature. These are found all over the
sound inner ear skin. Similarly, chemoreceptors in the nose
the cochlea)
skin and and mouth detect taste and smell (in the
temperature thermoreceptors nose, these are in olfactory cells lining the
hypothalamus
inner surface in the nasal cavity). All muscles
light rods and cones retina
have propioreceptors which are also known
taste chemoreceptors taste buds as `stretch receptors' ­ these detect the
chemoreceptors (olfactory change in muscle fibre length. This is how we
smell nose
cells) know the position or stretch of our body (for
example, knowing when our arm is stretched
pressure Pacinian corpuscles skin
out without seeing it). Also in the skin are
propioreceptors (stretch Pacinian corpuscles which are your pressure
position/stretch muscles
receptors)
receptors detecting pressure on the skin.
Neurones have very specialised cell surface membranes filled with many channel proteins. These are specialised to
sodium and potassium channel proteins. One type of channel protein, the sodium-potassium pump actively transports
+ +
sodium ions (Na ) out of the cell, and potassium ions (K ) into the cell. There are also voltage-gated channels which are
channel proteins which have a gate which when open allows a certain ion through. Gated channels are specific to ions, so
one gated channel will only allow through sodium or potassium ions.
gated sodium gated channel opens, sodium-potassium
+
channel (closed) Na diffuses into cell pump
The inside of the membrane is more negatively-charged than the outside, although they both have a negative charge. The
membrane is said to be polarised, meaning it has a voltage (potential difference) across it. There is a high concentration
of sodium ions outside of the cell, and a high concentration of potassium ions inside the cell.
Sensory receptors are known as energy transducers. This means that they convert energy from one form to another.
They detect stimuli (energy changes) ­ whether it be a thermoreceptor detecting a change in thermal energy, or a
Pacinian corpuscle detecting a pressure change ­ and convert this energy into a form of electrical energy, called a nerve
impulse. It is this stimulus, or energy, which causes the sodium voltage-gated channels to open along the membrane.
When the sodium gated channels open, this causes an influx of sodium ions, as the permeability of the membrane to
sodium ions increases. This movement of sodium ions across the membrane (down the concentration gradient, so into
the cell) causes a change in charge, so the inside of the cell becomes less negative. This is called depolarisation. The
bigger the energy of the stimulus, the more sodium gated channels open, and so the more permeable the membrane to
sodium ions and therefore the more sodium ions enter the cell ­ resulting in a larger depolarisation.
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Triggering an action potential
Because the number of sodium ions which can enter the cell during the process varies so greatly, sometimes there is only
a small depolarisation, where the result is only a slight increase in voltage across the membrane. This small depolarisation
is known as a generator potential.…read more

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Events of an action potential
Now that you understand the behaviour of a neural membrane at its resting state and you know what causes an action
potential to be generated, you should be able to understand the events of a single action potential. Action potentials are
triggered by ionic movements over the membrane which bring about changes in electronegativity, and all of the events
which make up one action potential are also due to ionic movements.…read more

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Transmission of an action potential
The resting neurone has a stable balance of sodium and potassium ions, which is created by the sodium-potassium pump.
The ions are sorted so that there are mainly sodium ions outside the cell and mainly potassium ions inside the cell, also so
that the inside of the cell is more negatively charged than the outside (the cell interior usually has a stable charge of
-60mV).…read more

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The myelinated sheath won't allow sodium or potassium ions to diffuse through it, and so the transmission of an action
potential along the axon happens in a `jumping action'. The areas between the myelination, called nodes of Ranvier, will
receive the influx of sodium ions as per usual, but the local current is elongated so that the ions diffuse from one node of
Ranvier along to the next, inducing another action potential at the following node.…read more

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