3b.control and co-ordination

• The brain of man is covered and protected by three cranial meninges:

  • Duramater: outer thick double-layered membrane
  • Arachnoid membrane: thin net-like membrane
  • Piamater: thin membrane closely attached to the brain
• Corpus callosum:

  • Transverse internal connection between two cerebral hemispheres beneath the cerebral cortex
  • Made up of a wide, flat bundle of myelinated fibers
  • Coordinates the functions of the two cerebral hemispheres
• Arbor vitae:

  • Characteristic arrangement of white matter in the form of a tree in the cerebellum
  • Surrounded by grey matter
• Sympathetic division is called thoraco-lumbar division because:

  • Preganglionic nerves arise from thoracic and lumbar regions of the spinal cord
• Parasympathetic division is called cranio-sacral division because:

  • Preganglionic nerves arise from the brain and sacral region of the spinal cord
• Absolute refractory period:

  • Time in which even a very strong threshold stimulus cannot initiate a second action potential
• Relative refractory period:

  • Time during which a second action potential can be initiated by a larger than normal stimulus
  • Coincides with depolarization and repolarization
• All or none principle:

  • The action potential occurs in response to a threshold or super threshold stimulus but not to a subthreshold stimulus
  • When an action potential is generated, it cannot be stopped in the middle and travels completely to its destination
  • The nerve impulse is either conducted totally or not conducted at all, hence it is called the 'all or none' principle
• Rods and cones of the human eye:

  • Rods contain a purplish-red protein called Rhodopsin or visual purple, which requires Vitamin A and opsin for its formation. Rods are necessary for vision in dim light (scotopic vision)
  • Cones contain iodopsin, important for daylight vision and color vision. There are three types of cones for red, blue, and green colors. Equal stimuli produce white colors, while different levels of stimuli produce different colors
• Blind spot:

  • The site on the retina where the optic nerve exits the eyeball is called the blind spot
  • It does not contain photoreceptor cells, so no image is formed at that point
• Yellow spot:

  • The posterior portion of the retina is called the yellow spot
  • This region contains only cones, providing sharp central vision
• Organ of Corti:

  • The sensory ridge formed by the cochlear epithelium on the basilar membrane is called the 'Organ of Corti'
  • It contains hair cells that act as auditory receptors