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The Nervous System
The nervous system detects changes and stimuli in the body and surroundings, processes
information, and initiates responses
Central nervous system= brain and spinal cord, processes information and gives response
Peripheral nervous system= pairs of nerves that originate from the brain or spinal cord, consists of
sensory neurones carrying impulses from receptors for the CNS and motor neurones, impulses travel
from the CNS to the effectors
...
They detect one form of energy and convert this into
electrical energy, acting as transducers
...
It is rapid, involuntary, and protective
...
They’re associated with the production of
proteins and neurotransmitters
...

Dendrons- small extensions of the cell body that subdivide into smaller branched fibres (dendrites)
that carry nerve impulses towards the cell body
...
They protect the axon and provide insulation
...
Myelinated neurones transmit impulses faster than unmyelinated neurones
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Nodes of Ranvier- spaces between adjacent Schwann cells, forming gaps 2-3 micrometres long which
occur every 1-3mm in humans

Nerves are bundles of nerve fibres and each nerve fibre (axons/ dendrons) is the long extension of a
single neuron e
...
the axon of a motor neuron
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Nerves may contain entirely motor
fibres or sensory fibres or a mixture of both (mixed nerves)
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Neurons
Sensory neurons transmit nerve impulses from a receptor (sense organs) to a relay or motor neuron
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Relay neurons transmit impulses between neurons e
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from sensory to motor neurons, they have
numerous short processes
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They have a long axon and many short dendrites
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g
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Hydra are aquatic animals found attached to stone or water weed
...
It occasionally contracts and feeds on small animals
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Pacinian corpuscles respond to changes in pressure, converting the energy of the
stimulus into a receptor potential, acting as transducers
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If this potential equals or exceeds a threshold level, an action potential is
produced, which is the same regardless of how much the threshold level is exceeded
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Pacinian corpuscle
Detects changes in the environment and sets up a nerve impulse which is sent to the brain
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More impulses create more pain
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Any action that confers an advantage for survival is said
to have an adaptive value
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The adaptive value of reflex actions includes:
o

o
o

Being involuntary, they don’t need the brain’s decision making powers so it is free to carry
out more complex responses
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Some impulses are sent to the brain, so it’s informed of
what is happening and can override the reflex if necessary
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They’re effective from birth so don’t have to
be learnt
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The spinal cord
A column of nervous tissue running along the back with the vertebral column

Gray matter is denser with neurone cell bodies, synapses
and unmyelinated neurons
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The dorsal root contains sensory neurons and the cell
bodies are contained in the dorsal root ganglion (cell body
of sensory neuron)
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A spinal reflex
This is an involuntary response that follows a sensory stimulus called a reflex
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The simplest forms of reflex arc (e
...
knee jerk reflex)
involve only a sensory and motor neuron
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If the reflex involves the spinal cord but not the brain it is known as a spinal reflex
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If the threshold value is reached a generator
potential is established
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Relay neuron- links the sensory neuron via synapses to the motor neuron within the gray
matter of the spinal cord
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Effector- the biceps muscle of the forearm is stimulated to contract
Response- the hand is raised away from the hot object

The nerve impulse
At resting potential, there are intrinsic proteins in the
membrane with channels that can open and close
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There is a potential difference of -65mV
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The pump pumps out 3Na+ for every 2K- pumped in,
which creates a potential difference with the outside being
more positive
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At intervals of 1-3 micrometres, there are breaks in this myelin where action potentials can
occur
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The action
potentials ‘jump’ from node to node by saltatory conduction
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Speed of nerve impulse transmission
Factors affecting the nerve impulse transmission:
o
o
o
o

Myelin sheath- action potential jumps from node to node, increasing transmission speed
from 30m/s in an unmyelinated nerve to 90m/s in a myelinated nerve
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Refractory period- once an action potential has been created, there’s a period following
when the inward movement of Na+ is prevented because the sodium voltage gated channels
are closed
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This is the refractory period
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It also
limits the number of action potentials that can pass along a neuron in a given time
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The all or nothing response- the threshold value triggers an impulse
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The action potential is the same regardless of how much the stimulus is above
the threshold value
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Action potential graph
1
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The energy of the stimulus causes
the sodium voltage gated channels
to open and Na+ to diffuse in along
their electro-chemical gradient
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3
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This is
an example of positive feedback
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Once the action potential of +40mV is established, the voltage gates on the sodium channels
close and the voltage gates on the potassium channels begin to open
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With some potassium gated channels now open, the electrical gradient that was preventing
further movement of potassium ions is reversed, causing more potassium channels to open
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6
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The gates on the
potassium channels now close and the activities of the Na+K+ pump causes the resting
potential of -65mV to be re-established- repolarised
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Chemical synapses transmit impulses from one neurone to the
next by neurotransmitters (chemicals)
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The
presynaptic neurone releases the neurotransmitter which has a
swollen portion of axon, the synaptic knob, at its end
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Acetylcholine and noradrenalin are two common neurotransmitters
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g
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Act as junctions- allowing nerve impulses to converge and diverge
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Filter out low level stimuli- e
...
drone of traffic or machinery
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The threshold isn’t reached- all or nothing
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This entails the build up of neurotransmitter in the
synapse by one of two methods; a number of different presynaptic neurones together
release enough neurotransmitter to trigger a new action potential, or where a single
presynaptic neurone releases neurotransmitter many times in a short period
...

Prevent overstimulation and fatigue- where a stimulus is powerful and prolonged, the high
frequency of impulses in the presynaptic neurone leads to the release of considerable
amounts of neurotransmitter
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The synapse is fatigued (can cope with

o

extreme pain)
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Involved in memory and learning- it is thought that synapses have a role in the brain
allowing organisms to recall events and learn to recognise individuals (building up pathways)

Mechanism of transmission across a cholinergic synapse








An action potential arrives at the end of the presynaptic neurone, causing calcium channels
to open and Ca2+ ions to enter the synaptic knob
...

Acetylcholine molecules fuse with receptor sites on two of the five protein subunits that
make up each sodium channel
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The influx of sodium ions generated a new action potential in the postsynaptic neurone
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Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which
diffuse back across the synaptic cleft into the presynaptic neurone (recycling)
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This is stored in synaptic vesicles for future use
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The effects of chemicals on synaptic transmission
Chemicals may mimic the action of natural transmitters causing amplification at the synapse
...
They
may prevent the breakdown of the transmitter e
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inhibiting the enzyme normally doing this
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Most of these drugs were originally developed for therapeutic medicine
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Many are abused, used outside of medical guidance
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E
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tobacco, cannabis,
amphetamines, ecstasy, cocaine, heroin
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Excitory drugs stimulate the nervous system by creating more action potential in post synaptic
membranes
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If drugs are taken over a period of time, the modified synapse adjusts to their new use
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Organophosporus insecticides block the enzyme breaking down the transmitter substance once
attached to the receptor proteins of post synaptic membrane causing a prolonged effect of
neurotransmitters
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If the inhibitor is acting at a neuromuscular
junction, repeated contractions of the muscles occur
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Control in plants
Photoperiodism
The relative length of day and night varies through the year and is the photoperiod
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Plants whose flowering is triggered by
the photoperiod is in two main groups:



Short day plants generally flower in late summer, autumn, or early spring when light periods
are short e
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chrysanthemums and strawberries
Long day plants usually flower in late spring or early summer when the days become longer
and the nights shorter e
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lettuce, clover, many cereals

The flowering of some plants seems to be unaffected by the photoperiod e
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cucumber, tomato
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Photoperiodism= the response of a plant to relative lengths of daylight and darkness
Phytochrome= the photoreceptor responsible for absorbing light, a blue-green pigment found in
minute quantities in plants
...
It exists in two interconvertible forms
Pr, which absorbs red light (wavelength 660nm) and Pfr which absorbs far red light (wavelength
730nm)
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Pr is converted into Pfr when exposed to red
light, rapidly, and Pfr is converted back into Pr when exposed to far red light or in darkness, slowly
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In darkness, Pfr is gradually converted back to Pr
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The balance between
the forms of phytochrome controls flowering in short day and long day plants
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On absorbing light of
a particular wavelength, each form of phytochrome is converted to the other form
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If only one leaf of short day plant or long day
plant is exposed to normal daily light and darkness and the rest of the plant is kept in darkness, the
plant will still flower normally
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Florigen has been suggested to be
responsible for this
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Flowering is triggered by high levels of Pfr, it still flowers if interrupted by short
periods of darkness
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Short day plants- flowering is induced by exposure to dark periods longer than a critical length
(chrysanthemum, tobacco, poinsettia)
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Uninterrupted darkness leads
to flowering
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Red light is the most
effective
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It also means horticulturalists can delay flowering for special occasions (Easter/
Christmas)
Grafting- taking one leaf and attaching it to another plant



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