WK7: Reading Complexity of Reflex Action

• Six myths revisited


  • Reflex responses are simple
  • Reflex responses are stereotyped and inflexible
  • Reflex responses are repeatedly elicitable
  • Reflexes are spinal
  • Reflex arcs are the neural mechanisms of all reflexes
  • Reflexes are not acquired or modified by learning or experience
• These 'myths' have all been claimed as defining characteristics of all reflexes, but there are no reflexes (at least no skeletomotor reflexes, and probably no autonomic reflexes either) that have all these characteristics and there are many reflexes that have none of them
• Skeletomotor reflex responses


  • They are not stereotyped
  • They are frequently sophisticated and adaptable behaviours
• Frog wiping reflex


  1. Aiming phase: Move foot close to irritated location

  2. Wiping phase: Wipe foot across skin at location of irritation
• Foot position, limb posture and limb used in wiping reflex


  Depend upon location of stimulus
• Path of wiping foot


  Depends upon position of limb with irritant stimulus
• Wiping reflex can work independently of the frog's brain, demonstrating that even spinal reflexes are not simply fixed (stereotyped) patterns of contractions
• Goal-directed behavior


  The ability to achieve the same outcome in different conditions by using different muscles and body segments
• The frog's wiping reflex response can only be understood as a goal-directed action: the goal is to remove an irritating stimulus from the skin
• Eye movement


  A rotation of an eye within its socket
• Extraocular muscles


  • Small skeletal muscles that attach to the eyeball and produce eye movements
  • Continuously active, seldom fatigue, capable of rapid, precise and subtle eye movements
• Eye movements help us see clearly, so clear vision requires precise control of these movements
• Horizontal eye movements


  Movements inwards towards the nose or outwards towards the temple
• Horizontal eye movements


  • Produced by contractions of the lateral and medial recti muscles
  • Medial rectus turns eye towards nose, lateral rectus turns eye towards temple
• Motorneurons innervating the extraocular muscles are located in brainstem nuclei, not the spinal cord
• Reflexes involving the eyes, facial muscles or sensory organs in the head are not spinal, their circuitry is entirely within the brain
• Abducens nuclei


  • Located in the pons
• Oculomotor nuclei


  • Located in the midbrain
• Trochlear nuclei


  • Located in the upper pontine region, close to the border with the midbrain
• Any reflex movement of the eyes must involve motorneurons in one or more of these brainstem nuclei and therefore the reflex circuits cannot be wholly within the spinal cord
• The circuitry of reflex eye movements is entirely within the brain
• The circuitry of the pupillary and eye blink reflexes does not involve the spinal cord at all
• The circuitry of the acoustic startle reflex involves motorneurons in the spinal cord, but since the eliciting acoustic stimulus is detected by receptors in the ear which connect to neurons in the brainstem auditory nuclei, not all neurons in the startle reflex circuits are in the spinal cord
• Any reflex that involves muscles within the head or sensory organs within the head is mediated by circuitry that is at least partly within the brain
• A large number of reflexes are not spinal
• Vestibular-ocular reflex (VOR)


  Reflex eye movements evoked by stimulation of the vestibular organs in the inner ear
• Purpose of the VOR


  • To keep the eyes steady so that the images projected onto the retinas do not jiggle around too much
• In order to see clearly, the images of things being looked at must be kept almost stationary on the retinas
• Even if you try to hold your head as still as you can, there is still sufficient jiggle to make it impossible to see clearly enough to read if you didn't have the VOR
• Counter-rotational eye movements


  Eye movements in the opposite direction to head movements that keep the eyes pointing in the same direction despite head movements
• Types of counter-rotational eye movements


  • Horizontal
  • Vertical
  • Torsional
• How the horizontal VOR works

  1. Head turns left, eyes counter-rotate right

  2. Head turns right, eyes counter-rotate left
• How the vertical VOR works

  As the head tilts forwards, the eyes counter-rotate upwards
• How the torsional VOR works

  As the head rolls, the eyes counter-rotate torsionally
• Orienting reflex


  Reflex that involves a rapid movement of the eyes so that the image that moved or appeared in the corner of the eye (peripheral retina) is located on the foveas (the most sensitive, central part of the retinas where fine details can be resolved)
• Saccade


  Rapid movement of the eyes from one position to another
• Saccades can be both reflexive (elicited by stimuli) or voluntary (initiated by an internal 'act of will')
• The same saccade generating mechanism is involved whether the saccade is reflexive or voluntary
• Neural mechanism for horizontal saccadic eye movements


  1. Activating signal sent to left abducens nucleus

  2. Motorneurons in left abducens nucleus excited, send signals to left lateral rectus muscle

  3. Interneurons in left abducens nucleus activate right oculomotor nucleus

  4. Motorneurons in right oculomotor nucleus send signals to right medial rectus muscle
• Role of nucleus paragigantocellularis dorsalis (NPD)


  • Contains inhibitory neurons that suppress activity in the right abducens nucleus, enabling the saccade to occur