HL 2 Biology

E.5b Brain Function

<<Sympathetic & parasympathetic control>>

Peripheral nervous system: all nerves outside central nervous system

  • two parts: voluntary and autonomic nervous system
  • autonomic: control unconscious processes; this further divides to sympathetic and parasympathetic systems
    • parasympathetic nerves: cause increase in blood flow to gut wall hen guts active
    • sympathetic nerve: cause decrease in blood flow during fasting/when blood needed elsewhere (e.g. muscles when exercising intensely )
    • heart rate –
      • impulses transmitted to heart’s pacemaker by medulla oblongata
      • impulses speed heart rate as carried by sympathetic system
      • impulses slow heart rate when carried by parasympathetic system
    • size of pupil –
      • controlled by muscle fibres in iris
      • impulses carried to radial muscle fibres by neurons from sympathetic system = contract/dilate (bigger) pupil
      • impulses carried to circular muscle fibres by neurons from parasympathetic system = constrict (smaller) pupil
  •  Image

<<The Pupil Reflex>>

  • occurs when bright light shines into eye
    • photoreceptor cells in retina perceive bright light
    • impulses are sent to brain in neurons of optic nerve
    • medulla oblongata (brain stem) process these impulses
    • impulses transmitted via neurons of parasympathetic system to circular muscle fibres in iris
    • constrict pupil
    • amount of light entering eye reduced/ protected from damage
  • sometimes used by doctors to test patient’s brain function 
    • if eye does not constrict when light shone into each eye = maybe damage in medulla oblongata 
    • failure in all repeated brain tests = patient suffered brain death 

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