2.86 Describe the structure and function of the eye as a receptor, 2.87 understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity

Structure:

1. CORNEA
2. EPITHELIUM
3. IRIS
4. PUPIL
5. RETINA
6. SCLERA
7. LENS

Sight is the transmission of light signals through nerve endings to the brain, which interprets the signals and produces visions of what is seen.

- Light enters the eye first through the cornea
- The pupil is the opening that controls how much light passes from the cornea to the lens. The iris, the colored portion of the eye, is actually two sets of muscle fibers that control the size of the pupil.
- Light travels from the lens through a chamber filled with vitreous humor, a semi-solid that helps the eye keep its shape.
- The retina is the back of the inside of the eye and contains light receptors. The receptors are nerve endings with a direct link to the brain via the optic nerve.
- Receptors in the eye consist of rods and cones. Rods detect light regardless of wavelength and help the brain determine light intensity, whether it is bright or dim.
- Cones are only active in bright light and are particular to red, blue, or green. Each type of cone detects a particular light wavelength.

2.84 Responses/ 2.85 Reflexes

Responses
- A reflex is an action that is performed as a response to a stimulus and without conscious thought eg. moving your hand out of the sun.
- The sensory nerve sends an electrical impulse into the nerves through your arm and up to the Central Nervous System.
- The motor nerve sends the signal to the effector and your body reacts to it.

Reflex actions
- The sensory neuron carries the message from the receptor to the motor neuron which carries the message to the effector to make the interneurons work.
- Reflex actions are rapid and done without thought.

- pain receptor detects a stimulus.
- sensory nerve sends the electrical impulse to the CNS.
- the motor nerve sends the signal to the effector.
- the effector produces a response.

2.83 Central Nervous System

- The central nervous system (CNS) is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies.
- This excludes:sponges and radially symmetric animals such as jellyfish.
- It contains the majority of the nervous system and consists of the brain and the spinal cord.
- Together with the peripheral nervous system, it has a fundamental role in the control of behavior.
- The CNS is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity.

2.82 Communication

- Nerve impulse is carried along the inside of the nerve from the cell body to the Synaptic Knob where it connects to the muscle - causing it to extend or retract.
- The endocrine gland produces a chemical known as a hormone.
- The Myelin Sheath increases the nerve conduction between components.
- In mammals the axon is surrounded by a Schwann Cell (contains fat)
- Hormones can be proteins or steroids

2.77b Thermoregulation

- Negative feedback loop - method of control and maintaining constant conditions.
- The average body temperature of a human is 37/38 degrees centigrade.
- In order for it to work we have receptors of body temperature - hypothalamus.
- Responds to a stimulus - temp of body and blood.
- Body temperature feeds into the brain and is compared to the theoretical level of control. (37/38degrees).
- If the body temperature needs to be increased of decreased then it is brought about through the action of the effector.

- Input into hypothalamus, stimulates responses in the skin for cooling, sweating + increased blood flow.
- Blood vessels dilate (widen)

2.77a Thermoregulation

- Homeothermic: Same Temperature.
- Homeostasis: Conditions are kept the same or are constant.
- Mammals remain the same body temperature.
- Homeothermic carries out a process called Thermoregulation.
- no matter the increase or decrease of the surrounding temp. the mouses will stay the same.

2.76 Sensitivity

- Sensitivity is the characteristic of the organisms respond to changes in their environment.
- Changes include: Light, temperature, pressure and chemical.
- Organisms are required to have receptors and effectors such as muscles and glands.
Response is used to survive the changes in the environment.