AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
thebiotutor.com
A2 Biology Unit 5
Responses, Nervous
System & Muscles
1
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Response
Definition
Mechanism
tropism
A growth movement of part of
plant in response to a
directional stimulus
Trevor Chilton
examples
Positive:
Negative:
taxis
A simple response whose
direction is determined by the
direction of the stimulus
Positive:
Negative:
kinesis
Reflex arc
A response by random
movement to an
unpleasant stimulus. The more
unpleasant, the more rapid
An involuntary response to a
See below
Organisms must sense and respond to changes in the external environment in order to
survive.
2
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
sensory
stimulus brought about by
nerves
Annotate the diagram below to explain the following terms: stimulus, receptor,
sensory neurone, intermediate neurone, motor neurone, effector, response
Distinguish between
1. central and peripheral nervous systems
2. voluntary and autonomic nervous systems
3. sympathetic and parasympathetic nervous systems
Control of the heart rate
Annotate the diagram (from Toole and Toole) to explain how a) increased CO2 levels
and b) increased blood pressure result in a lower heart rate.
3
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Receptors
1.
Pacinian corpuscle
It is specific: It responds to only one kind of stimulus, mechanical pressure
4
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
It is a transducer: It converts the energy of the stimulus into a generator potential
Found deep in skin particularly fingers, soles of feet and genitalia, also joints,
ligaments and tendons.
Explain how it works
(use the terms: stretch-mediated sodium channels, resting potential, generator
potential, action potential.)
2. Rods and Cones in the retina.
A generator potential is created by the break down
of the visual pigment rhodopsin (in rods) or
iodopsin (in cones) into two component parts. In
the case of rhodopsin these are opsin (a protein)
and retinal (a derivative of vitamin A). The
pigment reforms in the dark.
Label the diagram here then use the table which
follows to compare the two in terms of visual
acuity and sensitivity to light
feature
Colour
Vision
rods
cones
Sensitivity
to light
Visual
Acuity
Distribution
in the retina
5
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Principles of coordination
Nervous system:
Hormonal system



nerve cells transmit electrical impulses
Stimulate target cells with neurotransmitter substances
Response is short lived




Use chemical messengers (hormones)
Travel in blood system
Stimulate specific target cells
Longer term response
Chemical mediators
Histamine and prostaglandins
 released by specific cells in response to injury
 affect cells in local area
 cause increase in permeability of capillaries
Plant growth factors



diffuse from growing areas to tissues
enable response to light, gravity and water
control seasonal events, e.g. flowering and leaf fall
IAA and phototropism
IAA (indoleacetic acid) causes plant cells to elongate by loosening the bonds between
cellulose microfibrils. It has many effects. Phototropism is only one. It is the growth
of a stem towards a light source (positive photropism)
6
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Nerve impulses
Label the following structures:
cell body, dendrons, axon, Schwann cell, myelin
sheath, node of Ranvier
The nerve impulse is
a self-propagating wave of electrical disturbance that
moves across the surface of the nerve membrane
Resting potential
The inside of the nerve membrane is negatively charged
relative to the outside (about -7mv)
Explain how it is created by gated channels and a sodium potassium exchange pump.
7
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Action potential:
Trevor Chilton
a temporary reversal in charge (depolarisation) across
the membrane (about +30 mv)
Depolarisation results from a stimulus which changes the shape of the gated channels.
They are therefore called voltage gated channels.
Summary of events of action potential
 stimulus causes Na+ voltage gates to open
 Na+ ions stream into axon along their electrochemical gradient
 Depolarisation occurs as axon is more positive inside than out
 Na+ gates close and K+ gates open
 K+ ions flood out causing an ‘overshoot’
 Resting potential is re-established
8
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Movement of action potential
Why does it move? (localised currents)
What’s the difference between myelinated and non-myelinated nerve? (saltatory
conduction)
What other factors might influence the speed of conduction?
Why does it only move in one direction? (refractory period)
What’s meant by ‘all or nothing principle?
The synapse
9
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Summarise the events described in the diagram above (see p179 Toole and Toole)
Explain the following features of synapses:

Unidirectionality

Spacial summation

Temporal summation

Inhibitory synapse
Transmitters and drugs
10
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
The most common transmitter is acetylcholine. This type of synapse is called
cholinergic and the post-synaptic membrane will be fitted with acetylcholine
receptors. The enzyme breaking it down will be cholinesterase.
There are other transmitters, some are excitatory (inducing an action potential), and
some are inhibitory. In all cases, the action depends on the presence of specific
receptor molecules and enzymes. Exam questions are likely to focus on these issues,
possibly taking the example of a drug.
e.g. morphine blocks receptors responsible for the transmission of pain impulses;
some snake venom and natural insecticides contain a competitive inhibitor of
cholinesterase. The effect of constant synaptic transmission is muscle spasm and
paralysis.
The neuromuscular junction
Motor neurones terminate at a neuromuscular junction. The structure is very similar to
the synapse and it works the same way with a transmitter substance in this case
binding to receptor sites on the membranes of muscle fibres, triggering muscle
contraction (see next topic notes).
Muscle structure
11
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
12
AQA A2 Revision notes:
Topic 1. Response, the nervous system and muscles
Trevor Chilton
Sliding filament theory
In a resting muscle, the actin / myosin binding sites are blocked by another protein
called ………………………… When ………………………is released at the
…………………….junction, it binds to ………………… on the muscle fibre
membrane and causes …… ions to be released from the endoplasmic reticulum deep
in the muscle cytoplasm (sarcoplasm). This unblocks the binding sites. The myosin
heads with ADP attached bind with actin sites. The myosin heads change their shape,
ADP is released and the cross bridge contracts to pull one filament over the other,
shortening the muscle fibre.
A molecule of …… now attaches to each myosin head and is ……………… to ADP.
This process is catalysed by …………………. Which is itself activated by …..ions.
The energy released is used to return the …………….. head to its original position.
When nervous stimulation ceases, ions are actively transported back into the
……………
………………………. And the binding sites are blocked again by
…………………….
Energy supply for muscle contraction
ATP is used for the movement of the myosin heads and for the active transport of Ca+
ions.
When oxygen is in short supply (e.g. in energetic exercise) the aerobic supply of ATP
is not sufficient.
Muscle cells store a substance called phosphocreatine In anaerobic conditions it
splits to phosphate and creatine it releasing energy and phosphate for ATP
manufacture. NB it is not directly used as an energy resource.
Fast or slow twitch muscles?
Feature
Fast twitch
Slow twitch
Speed of muscle contraction
Used mainly for
Found in
13