Response to Stimuli

Chapter 9

Seonsory Reception

Nervous Control

Control of Heart Rate

Role of Receptors

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Sensory Reception

A stimulus is a dectable change in the internal ot external enviroment of an orgaanism that produces a repsonse. The ability to respond to a stimulus increases an organisms chance of survival. Receptors transfer the energy of a stimulus into a form that can be processed by the organism and leads to a response. They are tranducers. The response is carried out by effectors which can include cells, tissues, organs and systems

Taxis- a simple respose where direction of movement is determined by the direction of stimulus. An organism can respond directly to change in the enviroment by moving its body either towards the stimulus (positive respnse) or away from stimulus (negative response)

Kinesis- results in an increase of rapid movements, Organism doesn't move towards/away from stimulus. The more intense the stimulus, the more rapid the movements, Important when stimulus is less directional (eg, heat/humidity)

Tropoism-a growth movement of part of a plant in response to a directional stimulus. Positive Phototropism, shoots and leaves responding by growing towards the light. Positive Geotropism, roots responding by growing down in soil.

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Nervous Control

CNS- central nervous system- brain and spinal cord

PNS- peripheral nervous system- pairs of nerves that originate from brain or spinal cord


It is divided into:

Sensory neurones- carry impulses away from recpetors to CNS

Motor neurones- carry nervous impulses from CNS to effectors

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Nervous Control: Reflex Arc

Reflex- involentary response to a stimulus

The pathway of neuroones involved in a reflex is called a reflex arc. They contain 3 neurones: sensory, intermediate, motor.


1) Stimulus

2) Receptor

3) Sensory Neurone

4) Coordinator (intermediate neurone)

5) Motor neurone

6) Effector

7) Response

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Nervous Control: Importance of Reflex Arc

Involentary- doesn't require descions from the brain

Brain can override responses if necessary

Protects body from harmful stimuli

Innate- doesn't need to be learnt

Short pathway- few synapses

Synapses- slow since diffusion across cleft is passive

Neurones- fast sine node of Ranvier allow impulse to jump along

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Control of Heart Rate- ANS

Autonomic Nevous System

Controls 2 subconsious activities of muscles and glands.

Have 2 main divisions:

Sympathetic- speeds up activities allowing us to cope with stressful situations (flight or fight)

Parasympathetic- Inhibits and slows down activities allowing energy to be conserved.

These are antagonist

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Control of Heart Rate

Changes to heart rate are controlled by regions of the brain called the medulla oblongata.

It has 2 main divisions:

One is connected the the sino-atrial node through the SNS

Other is connected to the sino-atrial node through the PNS

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Control of Heart Rate- Chemoreceptors

Found in the wall if the carotid arteries and dectect changes in pH as a result of CO2 concentration

When CO2 conc in blood is too low, chemoreceptors dectect drop in pH and send impulses to section of medulla oblongata responsible for increasing heart rate. This section then increases the number of impulses sent to the SA node via the SNS,

This results in an increase in heart rate which causes blood pH to return back to normal

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Control of Heart Rate- Pressure receptors

Occur in wall of carotid arteries and in the aorta.

When blood pressure is too high, impulses are sent to the medulla oblongata which sends impulses to the SA node via the PSN, decreasing heart rate.

When blood pressure is too low, impulses are sent to the medulla oblongata which then sends impulses ti the SA node via the SNS, increasing heart rate

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Role of Receptors- Sensory Reception

A sensory receptor will:

Only respond to a specific type of stimulus (eg/ light, pressure)

Produce an action potential by acting as a transducer. This means that it can convert the information to a form that the human body can interpret

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Role of Receptors- Pacinian Corpuscle

Responds to mechanical pressure

Occurs in ligaments and joints so that it's possible to tell which direction a joint is changing.

The neurone of a pacinian corpuscle is in the centre of layers of tissue, each seperated by gel

The sensory neurone of pacinian corpuscle has stretch mediated sodium channels in its plasma membrane:

  • During resting state, strech-mediated sodium channels are too narrow to allow sodium through. Therefore, it has resting potential.
  • When pressure is applied, membrane of neurone is stretched causing sodium channels to widen, allowing sodium to diffuse into neurone
  • Influx of sodium ions cause a change in the polarity of the neuron, creating a generator potential
  • If enough ions move in, this creates an action potential which moves along the neuron
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Role of Receptors- Eye

Light receptors found in retina, rod and cone cells. Both types of cells convert light energy into a nervous impulse, so act as transducers.

Rod Cells

Breakdown the pigment rhodopsin to generate an action potential. Easily broken down in low light intensity. Many rod cells connected to same neuron so can fuction in low light intensity. Threshold must be reached in bi-polar cells that they are attached to. Many to one rod cell is connected to same neuron, only one impulse is generated. Impossible for brain to determine which rod cell was stimulated to begin with, whch results in rod cells having poor visual acuity.

Cone Cells

3 types of cone cells that all respond to different wavelengths. Colour interpreted depends of the proportion of each type that is stimulated. Breakdown pigment iodopsin to create an action potential. Only broken down in high light intensities. Connected to one bi-polar cell, so can't combine each threshold. When 2 adjacent cone cells stimulated, two seperate impluses are sent to brain. Therefore have higher visual acuity.

Light contracted by lens to centre of eye called forea. Has many cone cells as high lught intensity.

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