Reflex Mythology and the Complexity of Reflex action

• Reflex Mythology


  Common misconceptions regarding reflexes
• 6 Common Misconceptions


  • Simple responses: responses are things like blinks, muscle twitches and knee jerks
  • Stereotyped responses: performance always involves the same movements and/or muscle contractions
  • Mediated by spinal circuits (circuits located in the spinal cord)
  • Mediated by reflex arcs: mechanisms are basically chains of neurons (pathways) between sensory receptors and muscles
  • Not acquired or modified by learning and experience (because they are innate)
  • Repeatable: the same stimulus always evokes the same response (not influenced by internal states)
• Some skeletomotor reflex responses – such as blinks – are somewhat stereotyped
• Most reflex responses are not stereotyped because they are adapted to the conditions of execution
• Wiping reflex in frogs and toads


  • Eliciting stimulus: irritation at a location on the skin
  • Response: movement involving extension of rear leg wiping the 'toes' over the stimulus
• The frog's wiping reflex


  1. Frame 1

  2. Frame 2

  3. Frame 3
• Adaptability of the wiping reflex


  • Foot positioning depends upon stimulus location
  • Foot used depends upon stimulus location
  • How the leg actually moves when wiping is different for different starting postures
  • The position to which the foot should be moved depends upon the position of the forelimb
• The frog's spinal cord "knows" where the limbs are as well as where the stimulus is
• The reflex response is not stereotyped
• The reflex response is able to achieve the same outcome (removal of the source of irritation) under different circumstances (location of stimulus, body posture, body condition)
• This is a basic kind of goal-directedness
• Other skeletomotor reflexes have this same character, e.g., scratching reflexes (the mammalian equivalent of wiping reflexes) and withdrawal reflexes
• Of course not: any reflexes that involve a sensory organ in the head (ears, tongue, nose, eyes) involve neurons in the brain
• Eye-movement


  A movement of the eye in its socket
• Extraocular Muscles


  • Responsible for eye movements
  • Horizontal (left/right) movements of an eye are produced by contracting the medial and lateral recti

  
• Motor nuclei


  • Medial recti motorneurons located within the oculomotor nuclei at midbrain level
  • Lateral recti motorneurons located within the abducens nuclei at the medullary level
  • All the extra-ocular muscles are driven by motorneurons located in nuclei within the brainstem
• Connectivity


  1. Signals from the left abducens nucleus and right oculomotor nucleus will move the eyes to the left

  2. Signals from the left oculomotor nucleus and right abducens nucleus will move the eyes to the right
• Brainstem connects to spinal cord and controls vital functions like breathing, heart rate, blood pressure, and digestion.
• Cerebral cortex is responsible for higher functions such as thinking, memory, language, and consciousness.
• The brain is divided into three main parts - cerebrum, cerebellum, and medulla oblongata.
• Cerebellum is involved in coordinating voluntary muscle activity and maintaining posture and balance.
• The basal ganglia play a role in controlling movement and learning new skills.
• Thalamus relays information between different parts of the nervous system and plays a role in processing sensory input.
• The cerebellum coordinates voluntary muscle activity and maintains posture and balance.
• Hypothalamus regulates various bodily functions such as body temperature, hunger, thirst, sleep, and emotions.
• The limbic system includes structures involved in emotion, motivation, and memory.
• The cerebrum is further subdivided into two hemispheres connected by the corpus callosum.
• Each hemisphere has four lobes - frontal lobe, parietal lobe, temporal lobe, and occipital lobe.
• Medulla oblongata regulates automatic processes such as breathing, swallowing, and heartbeat.
• The thalamus acts as a relay station between different areas of the brain and helps process sensory information.
• Synaptic transmission involves neurotransmitters released by presynaptic neuron binding to receptors on postsynaptic neuron.
• Hypothalamus regulates body temperature, hunger, thirst, sleep-wake cycles, and emotions.