The nervous system, reflexes, the eye (pg. 45-47)

• A stimulus is any change in the internal or external environment.
• Receptors detect stimuli. Receptors in the sense organs (the eyes, ears, nose, tongue and skin) are groups of cells that detect external stimuli.
• Effectors are cells that bring out a response to stimuli. They include muscle cells and cells found in glands. Muscle cells contract, whereas glands secrete hormones.
• Receptors communicate with effectors via the nervous system, the hormonal system or sometimes both.
• The Central Nervous System (CNS) coordinates information.

  
  

  1. When receptors in a sense organ detect a stimulus, they send electrical impulses along sensory neurones to the CNS.
  2. The CNS sends electrical impulses along a motor neurone. The effector then responds accordingly.
• The nervous system is made up of all the neurons in your body: sensory neurons, relay neurons and motor neurons.
• The connection between two neurons is called a synapse. The nerve signal is transferred by chemicals called neurotransmitters which diffuse across the gap. These chemicals then set off a new electrical signal in the next neuron.
• The route taken by the information in a reflex is called a reflex arc.
• The reflex arc:

  1. Stimulus is detected by receptors.
  2. Stimulation of the pain receptor.
  3. Impulses travel along the sensory neuron to the CNS.
  4. In the CNS, impulses are passed along a relay neuron, via a synapse.
  5. Impulses travel along a motor neuron, via a synapse.
  6. When impulses reach the effector, it responds to the stimulus.
• Reflex Arc:

  1. Stimulus.
  2. Receptor.
  3. Sensory neuron.
  4. CNS.
  5. Motor neuron.
  6. Effector.
  7. Response.
• Parts of the eye - Diagram

  A) conjunctiva

  B) iris

  C) cornea

  D) pupil

  E) lens

  F) ciliary muscle

  G) suspensory ligaments

  H) retina

  I) fovea

  J) optic nerve

  K) sclera

  
• The conjunctiva lubricates and protects the surface of the eye.
• The sclera is the tough outer layer that protects the eye.
• The cornea refracts light into the eye. The cornea is transparent and has no blood vessels to supply it with oxygen, so oxygen diffuses in from the outer surface.
• The iris controls the diameter of the pupil and therefore how much light enters the eye.
• The lens focuses the light onto the retina.
• The retina is the light-sensitive part that's covered in light receptors called rods and cones. Rods are more sensitive in dim light but can't sense colour. Cones are sensitive to colours but aren't good in dim light. Cones are found all over the retina, but there are loads of them at the fovea.
• The optic nerve carries impulses from the receptors to the brain.
• In bright light, the circular muscles contract and the radial muscles relax, making the pupil smaller so less light enters the eye.
• In dim light, the radial muscles contract and circular muscles relax, making the pupil bigger so more light enters the eye.
• The eye focuses light on the retina by changing the shape of the lens - this is known as accommodation.
• To look at distant objects:

  1. The ciliary muscles relax, which allows the suspensory ligaments to pull tight.
  2. This makes the lens go thin.
  3. So it refracts light by a smaller amount.
• To look at near objects:

  1. The ciliary muscles contract, which relaxes the suspensory ligaments.
  2. The lens becomes fat (more curved).
  3. This increases the amount by which it refracts light.
• Short-sighted people are unable to focus on distant objects. This occurs when the cornea or lens bends the light too much or the eyeball is too long. The images of distant objects are brought into focus in front of the retina.
• Long-sighted people are unable to focus on near objects. This occurs when the cornea or the lens doesn't bend the light enough or the eyeball is too short. The images of near objects are brought into focus behind the retina.
• As you get older, your eye's lens loses flexibility, so it can't easily spring back to a round shape. This means light can't be focused well for near viewing.