A2 Biology Unit 5 Responses, Nervous System & · PDF fileA2 Biology Unit 5 Responses, Nervous System & Muscles . AQA A2 Revision notes: ... 4 Receptors 1

AQA A2 Revision notes: Trevor Chilton

Topic 1. Response, the nervous system and muscles

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A2 Biology Unit 5

Responses, Nervous System & Muscles



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Organisms must sense and respond to changes in the external environment in order to

survive.

Response

Mechanism Definition examples

tropism A growth movement of part of

plant in response to a

directional stimulus

Positive:

Negative:

taxis A simple response whose

direction is determined by the

direction of the stimulus

Positive:

Negative:

kinesis A response by random

movement to an

unpleasant stimulus. The more

unpleasant, the more rapid

Reflex arc An involuntary response to a See below



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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.

sensory

stimulus brought about by

nerves



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Receptors

1. Pacinian corpuscle

It is specific: It responds to only one kind of stimulus, mechanical pressure



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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 rods cones

Colour

Vision

Sensitivity

to light

Visual

Acuity

Distribution

in the retina



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Principles of coordination

Nervous system:

nerve cells transmit electrical impulses

Stimulate target cells with neurotransmitter substances

Response is short lived

Hormonal system

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)



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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.



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Action potential: 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



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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



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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



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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



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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

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A2 Biology Unit 5 Responses, Nervous System & · PDF fileA2 Biology Unit 5 Responses, Nervous System & Muscles . AQA A2 Revision notes: ... 4 Receptors 1