reflex actions

• reflex actions are responses to changes in the environment that do not involve any processing in the brain to coordinate the movement. The nervous pathways are as short as possible so that the reflex is rapid.
• The brain may be informed that the reflex has happened, but it isnt involved in the coordination of the response.
• Reflex actions always have a survival value, a reflex may be used to get out of danger, to avoid damage to part of the body, or it may be used to maintain balance.
• the blinking reflex causes temporary closure of the eyelids to protect the eyes from damage.
• The nervous pathway for the blinking reflex passes through part of the brain, in other words this is a cranial reflex. However, the pathway is a direct pathway that doesnt involve any thought processes in the higher parts of the brain. Since the receptor and effector are in the same place, this is a reflex arch.
• blinking may be stimulated by changes in the environment like:

  a foreign object touching the eye (Corneal reflex0

  sudden bright lights (optical reflex)

  loud sounds

  sudden movements close to the eye.
• the corneal reflex is mediated by a sensory neuron from the cornea, that enters into the pons. A synapse connects the sensory neuron to a relay neuron, that passes the action potential to the motor neurone. The motor neuron passes back out of the brain into the facial muscles which cause the eyelid to blink. this is a very short and direct pathway, so said reflex is very rapid taking about 0.1 seconds. the corneal reflex usually causes both eyes to blink, even if one cornea is affected.
• the sensory neuron involved in the corneal reflex also passes the Action potential (AP) to myelinated neurons in the pons. These myelinated neurons carry the action potential to the sensory region of the cerebral cortex to inform the higher centers of the brain that the stimulus has occurred Allowing the reflex to be overridden by conscious thought.
• The higher parts of the brain (cerebral cortex) can send inhibitory signals to the motor centre of the pons. the myelinated neurons carrying impulses to and from the cerebral cortex transmit action potentials much more rapidly than the non myelinated ones. Thus, the inhibitory action potentials can be the preventing force that inhibit the formation of an action potential in the motor neuron.
• the optical reflex does what?


  The optical reflex protects the light sensitive cells of the retina from damage. The 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.
• the knee jerk reflex is a spinal reflex. The nervous pathway passes through the spinal cord instead of the brain.
• the knee jerk reflex is involved in the coordination of movement and balance. The muscle at the front of the quadriceps contracts to straighten the leg. This muscle is attached to the lower leg via the patella tendon that connects the patella to the lower leg bones at the front of the knee. When the muscles at the front of the thigh are stretched, stretch sensors called muscle spindles detect the increase in length of the muscle. If this stretching is unexpected then a reflex action causes the contraction of that very muscle.
• the knee jerk reflex is part of the mechanism that allows humans to balance on two legs.
• The knee jerk reflex is unusual in that the nervous pathway only consists of two neurons- sensory > motor neuron. As such, because there is one less synapse involved, this response is quicker.
• Why cant the knee jerk reflex not be inhibited?


  There is no relay neuron between the sensory and motor neuron. Inhibition relies on rapid myelinated neurones carrying the inhibitory action potentials to he synapse before the motor neuron is stimulated. In the absence of a relay neuron, the motor neuron is stimulated directly by the sensory neuron.