Reflexes
Reflexes are automatic and un ordered responses that your body does to minimise damage to your body. It can do it through CNS or Hormones. The Path the message takes is the REFLEX ARC.
It goes:
The Reflex Arc goes through the Central Nervous System
A stimuli makes a receptor active, then it sends an electrical signal through sensory neurones to the spinal cord. Then it is passed onto another neurone called the relay neurone. Between the neurones there are gaps called synapse. The message is chemically passed between these. From the relay neurone the impulse gets relayed to the motor neurone. The impulse travels from the motor neurone to the effector muscle and the reflex takes place.
The Eye
The CONJUCTIVA lubricates and protects the surface of the eye.
The CORNEA refracts light into eyes. It is transparent, it has no blood vessels near it so if diffuses oxygen from the air.
The IRIS controls the diameter of the pupil and therefore how much light enters.
The LENS focuses the light into the Retina which contains light sensitive cells i.e. (Rod and Cone cells).
Rod Cells are more sensitive in dim light but don’t sense colour.
Cone cells are sensitive in colour but aren’t good in dim light. Cone cells are found all in the retina but there are loads in the fovea
The optic nerve is where the message is sent to the brain.
Iris Reflex
It bright light the circular muscle contract so more of the retina is covered, in dim light the circular muscles relax so less of the retina is covered so the pupil is bigger and you can see more clearly.
Focusing on near and far things
To focus on things far we make the lens thin by contracting ciliary muscles, which to look at close things we relax the ciliary muscles so the lens is fat.
Long sighted people are unable to focus on near things so it makes it blurry, this can be corrected by putting a thin glass in front.
Short sighted people are unable to focus on distant objects. This is fixed by putting some thick and fat glass in front.
Hormones
Hormones are chemical messages sent in the blood:
Hormones are chemically released in the blood and are carried by blood plasma, they travel around the whole body but only effect target cells. They are produced in glands and are pretty long lasting but travel slowly.
Comparison of Hormones and Nerves
Homeostasis
Usually we only control the water content and Body Temperature.
Water is lost from the body by: skin as sweat, via the lungs as breath and out of the penis as urine.
Body Temperature is kept at around 37oC as the enzymes in the body work best at that temperature. Your brain is a thermostat and if it is too cold or hot your CNS will send messages to the effectors.
The Skin plays an important role in maintaining body temperature:
Smaller Organisms cool down quicker
- Smaller organism have bigger surface area to volume ratio
- Lose heat much more quicker and gain heat much more quickly.
Response in Plants
Plants have stimuli as well. They can sense the direction of light through the use of auxin. When a shoot is exposed to light it accumulates on the other side and auxin makes the plant grow so it moves the plant towards the light.
The plant shoots are negatively geotropic so they grow opposite gravity. The roots are positively geotropism so they grow towards the ground.
