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Receptors and neurones
The nervous system is made up of a complex network of cells called neurones
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– the brain and spinal cord
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 Relay neurones transmit nerve impulses between sensory neurones and motor
neurones
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 When a nerve impulse reaches the end of a neurone, chemicals called
neurotransmitters take the information along to the next neurone, which then
sends a nerve impulse
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Sensory receptors convert stimulus energy into nerve impulses
 different stimuli have different forms of energy, e
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light or chemical
 but our nervous system only sends information in the form of nerve impulses
 sensory receptors convert the energy of a stimulus into electrical energy
 so, sensory receptors are transducers – they convert one form of energy into
another

How receptor cells that communicate via nervous system work
 when a nervous system is in resting state, there’s a difference in charge between the
inside and outside of the cell – this is generated by ion pumps and ion channels
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Voltage is known as potential
difference
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When
a stimulus is detected, the cell membrane is excited and becomes more permeable,
allowing more ions to move in and out of the cell – altering potential difference
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 A bigger stimulus excites the membrane more, causing a bigger movement of ions
and a bigger change in potential difference – so a greater generator potential is
produced
 If the generator potential is big enough it’ll trigger an action potential along a
neurone
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 If the stimulus is too weak the generator potential won’t reach threshold level so
there will be no action potential
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The cell body has extensions that connect to
other neurones – dendrites and dendrons carry nerve impulses towards the cell body, and
axons carry nerve impulses away from the cell body
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Motor neurones
Motor neurones have many short dendrites that carry
nerve impulses from the central nervous system to the
cell body, and one long axon that carries nerve impulses
from the cell body to effector cells
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Neurone cell membranes are polarised at rest
 In a neurones resting state, the outside of the membrane is positively charged
compared to the inside
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 So the membrane is polarised – there’s a difference in charge
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this creates
a sodium ion electrochemical gradient because there are more positive sodium ions
outside the cell than inside
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 This makes the outside of the cell positively charged compared to the inside

Action potentials
Neurone cell membranes become depolarised when they are stimulated
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The membrane becomes more permeable to sodium, so sodium ions
diffuse into the neurone down the sodium ion electrochemical gradient
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Depolarisation – if the potential difference reaches threshold level (around 55mV) voltage gated sodium ion channels open
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This is positive feedback
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The membrane is
more permeable to potassium so potassium ions diffuse out the neurone down the
potassium ion concentration gradient
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This is negative feedback
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The potential
difference becomes more negative than the resting potential (less than -70mV)
Resting potential - the ion channels are reset
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The action potential creates a wave of depolarisation
 When an action potential happens, some of the sodium ions that enter the neurone
diffuse sideways
 This causes sodium ion channels in the next region of the neurone to open up and
sodium ions diffuse into that part
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A bigger stimulus causes more frequent action potentials
 Once the threshold is reached, an action potential will always fire with the same
charge in voltage, no matter how the big the stimulus is
 If the threshold isn’t reached, an action potential wont fire
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 The myelin sheath is an electrical insulator
 In the peripheral nervous system, the myelin sheath is made of a type of cell called
the Schwann cell
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Sodium ion channels are concentrated at the nodes
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 In a non-myelinated neurone, the impulse travels as a wave along the whole length
of the axon membrane
 This is slower than saltatory conduction
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 A synapse is the junction between a neurone and the next neurone, or between a
neurone and an effector cell
 The tiny gap between the cells is called the synaptic cleft
 The presynaptic neurone has a swelling called a synaptic knob
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 When an action potential reaches the end of a neurone it causes neurotransmitters
to be released into the synaptic cleft
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 When neurotransmitters bind to receptors they night trigger an action potential
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g
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 There are many different neurotransmitters, e
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acetylcholine and noradrenaline
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They bind to
receptors called cholinergic receptors, and they’re broken down by
acetylcholinesterase
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
The action potential stimulates voltage gated
calcium ion channels in the presynaptic neurone to
open
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 The influx of calcium ions into the
synaptic knob causes the synaptic
vesicles to move to the presynaptic
membrane
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 the neurotransmitter diffuses across the synaptic cleft and
binds to specific receptors on the postsynaptic membrane
 This causes sodium ion channels in the postsynaptic neurone to
open
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An action potential on the postsynaptic
membrane is generated If the threshold is reached
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This is called synaptic divergence
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This is
called synaptic convergence
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This might not be enough to excite the
postsynaptic membrane to the threshold level and stimulate an action potential
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There are two types:

Spatial summation
 When neurones converge, the small amount of neurotransmitter released from each
neurone can be enough altogether to reach the threshold in the postsynaptic
neurone and trigger an action potential
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Spatial summation allows signals from
multiple stimuli to be coordinated into a single response

Temporal summation
Temporal summation is where two or more nerve impulses arrive in quick succession from
the same presynaptic neurone
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