reflex actions

• blinking reflex


  • blinking reflex causes temporary closure of the eyelids to protect the eyes from damage
  • nervous pathway for blinking reflex passes through part of brain - the reflex is a cranial reflex
  • however the pathway is a direct pathway that does not involve any thought processes in the higher parts of the brain. since the receptor and effector are in the same place - this is called a reflex arc
• blinking may be stimulated by sudden changes in the environment such as:

  • foreign object touching the eye (corneal reflex)
  • sudden bright light (optical reflex)
  • loud sounds
  • sudden movements close to the eye
• optical reflex


  • protects light sensitive cells of the retina from damage.
  • stimulus is detected by the retina and the reflex is mediated by the optical centre in the cerebral cortex. The optical reflex is a little slower than the corneal reflex
• corneal reflex 1


  • reflex is mediated by a sensory neurone from the cornea, which enters the pons. A synapse connects the sensory neurone to relay neurone, which passes the action potential to the motor neurone.
  • the motor neurone passes back out of the brain to facial muscles, causing the eyelid to blink
  • corneal reflex is rapid
  • causes both eyes to blink
• corneal reflex 2


  • sensory neurone in corneal reflex passes the action potential to myelinated neurones in the pons.
  • myelinated neurones carry action potential to sensory region in cerebral cortex, to inform higher centres of brain that stimulus occurred.
  • allows reflex to be overridden by conscious control.
• corneal reflex 3

  • higher parts of brain send inhibitory signals to motor centre in the pons.
  • myelinated neurones carrying impulses to and from cerebral cortex transmit action potentials more rapidly than non-myelinated relay neurones in pons. inhibitory action potentials prevent formation of an action potential in motor neurone
• knee jerk reflex 1


  • spinal reflex - nervous pathway passes through the spinal chord rather than through the brain
  • knee jerk reflex involved in coordinated movement and balance.
  • muscle at front of quadriceps contracts to straighten leg.
  • muscle is attached to lower leg via the patella tendon that connects patella to lower leg bones at front of the knee
  • when muscles at front of thigh are stretched, specialised stretch receptors called muscle spindles detect increase in length of muscle. if stretching unexpected, reflex action causes contraction of same muscle
• knee jerk reflex 2


  • when standing still, muscle in front of thigh will stretch if knee is bending or body is leaning backwards.
  • contraction of muscles straightens the knee or brings body back above the legs.
  • such a response must be rapid, so that body can remain balanced
  • consists of two neurons: sensory and motor
  • there is one less synapse involved and response is quicke
• knee jerk reflex 3 


  • because there's no relay neurone, brain cannot inhibit the reflex. inhibition relies on rapid myelinated neurones carrying inhibitory action potentials to synapse before motor neurone is stimulated.
  • in absence of relay neurone, motor neurone stimulated directly by sensory neurone and there is delay to enable inhibition
• knee jerk reflex 4


  • while we are walking or running, knee must bend and will stimulate muscle spindles
  • however, complex pattern of nervous impulses coming from cerebellum able to inhibit reflex contractions.
  • as action potentials sent to muscles behind thigh (hamstring), stimulating it to contract, inhibitory action potentials sent to synapse in reflex arc to prevent reflex contraction of opposing muscle