Wednesday, 8 April 2009

iGCSE Biology - Coordination


  • Stimulus -> Receptor -> Coordination -> Effector -> Response.
  • Information transferred through small electrical charges called impulses.
  • Receptors are transducers of energy.
  • Central Nervous System made up of brain and spinal cord.
  • Sensory Neurone -> CNS -> Motor Neurone.
  • The cell body of a motor neurone is in the CNS, and it has extensions called dendrons, which in turn have extensions called dendrites.
  • Dendrons and dendrites form junctions between neurones, called synapses.
  • The fibre which carries the nerve impulses is the axon.
  • Sensory neurones have their cell body branching out from their side.
  • The axon is covered with a myelin sheath which stops short circuits.
  • The outer coat of the eye is the sclera.
  • The transparent part at the front of the eye is the cornea.
  • Behind the cornea is a coloured ring called the iris.
  • In the middle of the iris is the pupil which lets light into the eye.
  • Underneath the sclera is the choroid, which is dark and thus stops light reflecting inside the eye.
  • At the back of the eye is the retina where light is transduced by rods and cones.
  • The impulses are then brought to brain through the optic nerve.
  • Rods work in low light but cannot distinguish colours.
  • Cones work only in bright light and can distinguish colours.
  • Cones are concentrated in the middle of the retina in the fovea.
  • To form an image on the retina, light needs to be refracted.
  • Light is refracted both in the cornea and in the lens.
  • The iris regulates how much light enters the eye by adjusting the size of the pupil.
  • There are two types of muscles in the iris: circular muscles and radial muscles.
  • To constrict the pupil, the radial muscles relax, and the circular muscles contract.
  • To relax the pupil, the radial muscles contract, and the circular muscles relax.
  • Changes which allow us to see at different distances are called accommodation:

                     Lens is connected to sclera by suspensory ligaments.

                     Suspensory ligaments are connected to ciliary muscles.

                     When ciliary muscles relax, suspensory ligaments are pulled              tight and lens is flattened.

                     When ciliary muscles contract, suspensory ligaments are          slackened and lens is more rounded.

  • Reflex actions bypass the brain.
  • Nerve pathway of a reflex action is called a reflex arc:

               Impulses enter CNS through dorsal root.

                     Sensory neurones create synapses with relay neurones which   in turn create synapses with motor neurones.

                     Motor neurones emerge from CNS through the ventral root.

  • Middle of spinal cord consists of grey matter.
  • Outer part of spinal cord consists of white matter.
  • Synapses are actually gaps between neurones:

               Impulses cause the end of a neurone’s axon to secrete a              chemical called a neurotransmitter.

                     The neurotransmitter diffuses across the gap and causes and    impulse in the second neurone.

                     The neurotransmitter is then broken down by an enzyme.

                     Each neurone can form thousands of synapses with other           neurones.

  • The largest part of the brain is the cerebrum.
  • The cerebrum:

                     Contains sensory areas that receive and process information     from our sense organs.

                     Contains motor areas where all of our conscious actions   originate.

  • Behind and under the cerebrum is the cerebellum.
  • Directly underneath the cerebrum is the brain stem, or medulla.
  • The pituitary gland and hypothalamus are just above the medulla. The pituitary gland secretes hormones.