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to Stimuli
A2 Biology Unit 5

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Sensory Reception
Stimulus and Response
Stimulus: a detectable change in the internal or external environment
of an organism that produces a response in the organism.

The ability to respond to stimuli increases the chances of survival for an
organism, e.g. the ability to detect and move away from harmful stimuli,…

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e.g. Earthworms move away from light, negative phototaxis. This increases
chances of survival because it takes them into the soil where they are better able
to conserve water, find food and avoid predators.

Kineseis: a form of response in which the organism does not move
towards or away from…

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Nervous Control
Nervous Organisation
The nervous system has two major divisions:
1. Central Nervous System (CNS) ­ made up of the brain and spinal cord
2. Peripheral Nervous System (PNS) ­ made up of pairs of nerves that
originate from the brain or spinal cord

The peripheral nervous system is…

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Reflex Arc
An involuntary response, to a stimulus, which you do not have to think about, is
called a reflex, e.g. withdrawal of hand from a hot or sharp object. The pathway
of neurones involved in a reflex is known as a reflex arc.

Reflex arcs involve just three neurones:…

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Control of Heart Rate
The Autonomic Nervous System
Autonomic means `self-governing'. The autonomic nervous system controls the
involuntary (subconscious) activities of internal muscles and glands. It has two

1. Sympathetic Nervous System
In general, stimulates effectors and speeds up any activity. Acts like an
emergency controller and stimulates receptors…

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2. A centre which decreases heart rate, which is linked to the sinoatrial
node by the parasympathetic nervous system

Which of these centres is stimulated depends on the information they receive
from two types of receptor, which respond to one of the following:
Chemical changes in blood
Pressure changes in…

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Control by Pressure Receptors
Pressure receptors occur within the walls of the carotid arteries and the
aorta. They operate as follows:

When blood pressure is higher than normal
They transmit a nervous impulse to the centre in the medulla oblongata
that decreases heart rate. This centre sends impulses via the…

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Role of Receptors ­ Pacinian
Pacinian Corpuscles
Pacinian corpuscles respond to changes in mechanical pressure. As with all
sensory receptors, a Pacinian corpuscle:

Is specific to a single type of stimulus ­ responds only to mechanical
Produces a generator potential by acting as a transducer

All stimuli are…




Just to let you know in the kineses paragraph you have got the changing direction the wrong way round, it  should be 

unfavourable conditions= move faster change direction often(you put change direction less)

favourable conditions= move slower and change direction fewer times(you put change direction more)

Her Dudelmann


@Ace, AQA are annoyingly unclear or this point, often directly contradicting themselves. Logically, the kineses paragraph in the notes makes sense, and some exam questions back this up, but the textbook disagrees.

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