Response to Stimuli


Sensory Reception

Stimulus: detectable change in the internal or external environment that produces a response

Stimuli are detected by cells or organs known as receptors. Receptors transform the energy of a stimulus into some form of energy that can be processed by the organisms and leads to a response. The response is carried out by cells, tissues, organs and systems; known as effectors. 

There is communication between the receptors and effectors, this is done by hormones or the nervous system. In the nervous system, each receptor and effector is linked to a central coordinator, which connects the information from stimulus to response.

Stimulus - Receptor - Coordinator - Effector - Response

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Simple Organisms


  • A simple response, whose direction is determined by the direction of the stimulus
  • The organism responds by either moving towards (positive) or away from (negative) the stimuli
  • Directional stimuli, e.g light


  • A form of response where the organism does not move towards or away from a stimulus, but the speed changes. The more unpleasant the stimulus, the more rapidly it moves and changes direction.
  • This results in an increase in random movements, designed to bring the organism into a more favourable condition
  • Less directional stimuli, e.g temperature, humidity


  • A growth movement of part of a plant in response to a directional stimuli
  • It means that the plant grows in a more favourable condition to get resources
  • Directional stimuli, e.g light, water, gravity
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Nervous Control


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Nervous Control

CNS: brain and spinal cord

PNS: pairs of nerves that originate from the brain or spinal cord

Sensory Neurones: carry nerve impulses from receptors towards CNS

Motor Neurones: carry nerve impulses away from CNS to effectors

Voluntary NS: carry nerve impulses to body muscles under voluntary control

Autonomic NS: carry nerve impulses to glands, smooth muscle and cardiac muscle under involuntary control

Sympathetic NS: prepares the body for action (fight or flight mode)

Parasympathetic NS: returns the body to normal bodily functions

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Nervous Control

Reflex Arc:

The pathway of neurones involved in a reflex. They are involuntary and so very fast.

  • Stimulus: heat from the object
  • Receptor: temperature receptors in the skin create a nerve impulse in a sensory neurone
  • Sensory Neurone: passes the nerve impulse to the spinal cord
  • Relay Neurone: links sensory and motos neurone
  • Motor Neurone: carries nerve impulse from spinal cord to muscle in arm
  • Effector: muslce in arm is stimulated to contract
  • Response: contracted muscle causes hand to pull away from hot object

Why are reflex actions important?

  • they are involuntary so do not require decision making of the brain, leaving the brain free to carry out more complex responses
  • they protect the body from harmful stimuli
  • they are very fast as the neurone pathway is short with few synapses
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Control of Heart Rate

Autonomic NS:

  • Sympathetic NS: stimulates effectors to speed up activity: heightens our awareness and prepares us for activity
  • Parasympathetic NS: inhibits effectoes to slow down activity: conserving energy and replenishing the body's reserves

Changes to heart rate are controlled by the medulla oblongata. This has two centres:

1. a centre that increases heart rate linked to SAN by the sympathetic ns

2. a centre that decreases heart rate linked to SAN by the parasympathetic ns

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Control of HR by Chemoreceptors

Chemoreceptors are found in the wall of the carotid arteries and the aorta. They are sensitive to changes in the pH of the blood that result from changes in CO2 concentration.

  • Increased muscular/metablic activity results in more CO2 produced by respiring tissues 
  • This increase in CO2 concentration lowers the blood pH
  • Chemoreceptors in the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the medulla oblongata
  • The centre in the medulla oblongata increases the frequency of implulses to the SAN via the sympathetic NS
  • The SAN increases the heart rate which causes an increased blood flow which removes more CO2 from the lungs so CO2 level returns to normal
  • Blood pH returns to normal and the chemoreceptors detect this and reduce the frequency of nerve impulses to the medulla oblongata
  • The medulla reduces the frequency of impulses to the SAN, which decreases the heart rate to normal
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Control of HR by Baroreceptors

Pressure receptors are within the walls of the carotid arteries and the aorta.

When blood pressure is higher than normal: they transmit a nervous impulse to the centre of the medulla oblongata that decreases heart rate. This centre sends impulses via parasympathetic NS to SAN which decreases the heart rate

When blood pressure is lower than normal: they transmit a nervous impulse to the centre of the medulla oblongata that ncreases heart rate. This centre sends impulses via sympathetic NS to SAN which increases the heart rate

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Pacinian Corpuscle

Pacinian Corpuscles respond to changes in mechanical pressure. 

  • It is specific to a single type of stimulus: only mechanical pressure, e.g pressure
  • It produces a generator potential by acting as a transducer: the transducer converts the information provided by the stimulus into a form that can be understood by the body, namely the nerve impulses. The stimulus is a form of mechanical energy. The pacinian corpuscle transduces the mechanical energy of the stimulus into a generator potenial.

The sensory neurone ending at the centre of the Pacinian Corpuscle has a special type of sodium channel in its plasma membrane. This is a stretch-mediated sodium channel as their permeability to sodium changes when they change shape, e.g by stretching.

  • resting state: the stretch mediated sodium channels of the membrane are too narrow to allow sodium ions to pass along them. It has a resting potential
  • when pressure is applied, the pacinian corpsucle changes shape and the membrane stretches
  • this stretching widens the NA+ channels so sodium ions diffuse into the neurone
  • the influx of NA+ ions changes the potential of the membrane, producing a generator potential that creates an action potential that passes along the neurone
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Receptors in the Eye

The light receptor cells of the mammalian eye are found on the retina. These receptors act as transducers by converting light energy into the electrical energy of a nerve impulse.

Rod Cells

  • rod shaped
  • greater numbers than cone cells
  • more at the periphery of the retina, none at the fovea
  • sensitive to low light intensity
  • give poor visual acuity
  • rhodopsin pigment

Cone Cells

  • cone shaped
  • fewer numbers than rod cells
  • fewer at the periphery of the retina, more concentrated at the fovea
  • not sesnitive to low light intensity
  • give good visual acuity
  • iodopsin pigment
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Cone and Rod Cells


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