Responding to Changes in the Environment

All organisms respond to chnges in their environment and this section explores some of the ways in which they do this. The human eye is studied as an example of a receptor organ.

Stimuli: Changes in external or internal environment which cause a response

Receptor: Cells in the body which detect stimuli

Effector: Part of the body which responds to stimuli

Central Nervous System

Brain
Spinal Chord

Nerves: a bundle of nerve cells. A nerve cell is called a neurone.

Senesory Neurone: Carries impulses from the receptor to the central nervous system
Motor Neurone: Carries impulses from teh central nervous system to the effectors
Realy neurone: connects the sensory neurone to the motor neurone.

Motor Neurone

Myelin Sheath: Is fatty and insulates electricity inside the axon.

Dendrites: Recieve nerve impulses from sensory cells.

Reflex Action, Involuntary/Voluntary

Reflex arc is the path taken by an impulse in a reflect arc
Stimulus->Receptor->Sensory neurone->Relay neurone->Motor Neurone-> Effector->Response

Spinal Chord (Reflex arc only concerned with stimulus from the skin)

Stimulus (sharp object) -> Sensory neurone (through dorsal route in white matter) -> (synaps in grey matter) -> realy neurone -> (synaps) -> motor neurone (in white matter) -> muscle effector (move hand)

The Eye

Know the structure of the eye. Will put image here someday.

How we see

Light rays from objects are refracted by the cornea and focussed by the lengs. This forms an upside down image on the retina. This upside down image is corrected by our brain so we see the correct way up.

Eye

Focussing on Distant objects

ciliary muscles relax
suspensory ligaments become tight
lends get pulled thin and flat. (less convex)
It is the first of the above images

Focussing on near Objects

ciliary muscles contract
suspensory ligaments relax
lens becomes fatter and rounder (more convex)
second of above images

Rods and Cones: Contain light sensative pigments which are bleached in light and generate an impulse

Rods	Cones
120 million present	6 million present - mainly on fovea
senstive to low light concentrations	sensitive to bright light concentrations
black and white image	colour image
fuzzy image	sharp and clear image

Plant Response

Phototrophic response: growth in response to directional stimulus, light

Top of the plant shoot is the receptor of light for growth

Auxn is a chemical growth regulating substance, which is produced at the tip of shoots when exposed to light.

Auxin diffuses down the shoot, on the side which is shaded
- it causes cells in zone of cell elongation to grow faster
- therefore one side is long/bigger than the other, so the plant bends towards the light.

Responding to Changes in the Environment

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