Response to Stimuli

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Sensory Reception

Sensory Reception


A movement away from or towards a stimulus. Direction determined by direction of stimulus.


Unpleasant stimulus causes more rapid movement and changes in direction. Aims to bring the organism back to more favourable conditions.


Growth of a plant towards or away from a stimulus.

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Nervous Control

Nervous Control

Central nervous system (CNS) - made up of brain and spinal cord

Peripheral nervous system (PNS) - made up of pairs of nerves that originate from either the brain or spinal cord

PNS is divided into:

Sensory neurones - from receptors towards CNS

Motor neurones - from CNS towards effectors

Motor nervous system is divided into:

Voluntary nervous system - carries impulses to body muscles and is under conscious control

Autonomic nervous system - carries impulses to glands, smooth muscle and cardiac muscle, and is subconscious

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Reflex Arc

Reflex Arc

Involuntary response to a sensory stimulus = reflex.

Pathway of neurones involved in a reflex = reflex arc.

Spinal reflex:

1) Stimulus                                  5) Motor neurone

2) Receptor                                  6) Effector

3) Sensory neurone                      7) Response

4) Intermediate neurone

Importance of reflex arcs:

Does not require the brain - allows it to make more complex decisions

Effective from birth and do not have to be learned - protection from harmful stimuli

Neurone pathway is short - fast response

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Control of Heart Rate

The Autonomic Nervous System

The two divisions of the autonomic nervous system:

Sympathetic nervous system - stimulates effectors and so speeds up activities, heightening awareness

Parasympathetic nervous system - inhibits effectors and so slows down activity, conserving energy

These two systems are antagonistic.

Control of Heart Rate

Changes in heart rate are controlled by the medulla oblongata.

It has two centres: -increases heart rate by impulse to SAN via sympathetic nervous system

                            -decreases heart rate by impulse to SAN via parasympathetic nervous system

These centres recieve information from two types of receptor, which respond to chemical changes in the blood and pressure changes in the blood.

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Control by Chemoreceptors

Control by Chemoreceptors

Chemoreceptors are found in the wall of the carotid arteries. They are sensitive to changes in pH caused by change in carbon dioxide concentration in the blood.

In solution carbon dioxide forms an acid and so lowers pH.

Drop in pH is detected by chemoreceptors which send more impulses to the centre in the medula oblongata that increases heart rate.

The centre increases impulses to SAN via the sympathetic nervous system, causing heart rate to increase.

Increased blood flow means more carbon dioxide is removed by the lungs, causing pH level to return to normal.

Chemoreceptors decrease frequency of impulses to medula oblongata.

Medula oblongata reduces impulses to SAN, returning heart rate to normal.

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Control by Pressure Receptors

Control by pressure receptors

Presure receptors are found in the carotid arteries.

When blood pressure is higher than normal:

- the receptors transmit a nervous impulse to the centre in the medula oblongata that decreases heart rate

- this centre sends impulses via the parasympathetic nervous system to the SAN

- heart rate decreases

When blood presure is lower than normal:

- the receptors send an impulse to the centre in the medula oblongata that increases heart rate

- this centre sends impulses via the sympathetic nervous system to the SAN

- heart rate increases

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