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A stimulus is a detectable change in the internal or external environment of an organism that
produces a response in the organism.

The ability to respond to an organism increases the chances of survival for an organism.
For example, to be able to detect and move away from harmful…

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A taxis is a simple response whose direction is determined by the direction of the stimulus.
As a result, a motile organism responds directly to environmental changes by moving its whole body
either towards a favourable stimulus or away from an unfavourable one.

Taxes are classified according to whether…

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A tropism is a growth movement of part of a plant in response to a directional stimulus.
In almost all cases the plant part grows towards or away from the stimulus.

Towards stimulus Positive tropism
Away from stimulus Negative tropism

Plant shoots grow towards light = positive phototropism

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Nervous Organisation

The motor nervous system is subdivided into
Voluntary nervous system Carries nerve impulses to body muscles and is under
voluntary control
Autonomic nervous system Caries nerve impulses to glands, smooth muscle and
cardiac muscle and is not under voluntary control

The peripheral nervous system is divided into

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The Spinal Chord

The spinal chord is a column of nervous tissue that runs along the back and lies inside the vertebral
column for protection.
Emerging at intervals along the spinal chord are pairs of nerves.

Reflex Arc

An involuntary response to a sensory stimulus is called a reflex.

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Importance of Reflex Arcs

Reflexes are involuntary: the actions they control do not need to be considered because there is only
one obvious course of action.

Reflex actions are important for the following reasons:

They are involuntary and therefore do not require the decision-making powers of the brain, thus

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Control of the heart rate is an involuntary reflex.
The part of the nervous system responsible for this type of control is the autonomic nervous system.

The Autonomic Nervous System

The autonomic nervous system controls the involuntary activities of internal muscles and glands.

It has two diversions:

The sympathetic nervous…

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Control by Chemoreceptors

Chemoreceptors are found in the wall of the carotid arteries.
They are sensitive to changes in the pH of the blood that result from changes in carbon dioxide
In solution, carbon dioxide forms an acid and therefore lowers pH.

The process of control works as follows:…

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Stimulus Receptor Neurone and Neurotransmitter Effector Response
High blood Pressure Impulses sent to the medulla which Cardiac Heart rate slows
pressure receptors sends impulses along muscles down to reduce blood
parasympathetic neurones. pressure back to
These secret acetylchlorine which normal
binds to receptors on SAN

Low blood Pressure Impulses sent…

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The central nervous system receives sensory information from its internal and external environment
through a variety of sense cells and organs called receptors.

Receptors are specific ­ they detect one particular stimulus.

Receptor cells are often part of sense organs, such as the ear, eye or skin.

There are many…


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