5.5.10 Reflex Actions

• Reflex actions are involuntary responses to certain stimuli
• They are very fast and usually have a protective purpose or survival value
• Many different reflex actions are carried out by the human body, including:
  • Yawning
  • Saliva production
  • Swallowing
  • Pulling a part of the body away from a source of pain (sometimes known as the 'withdrawal reflex')
  • Blinking
  • Constricting the muscles of the iris in response to bright light (the pupil reflex)

• Some of these actions, such as blinking, can also be carried out on purpose
  • In these cases, therefore, they are not reflex actions

• Some of these actions, such as the pupil reflex, are always automatic (involuntary)

The sequence of components in a reflex action

• All reflex actions consist of the following sequence:

Stimulus → Receptor → Coordinator → Effector → Response

• Reflex actions in response to a change first require detection
  • Detection involves a stimulus being detected by a receptor cell
• There are different types of receptors
  • Some receptor cells produce electrical activity in nerve cells in response to stimuli
  • Other receptor cells secrete substances in response to stimuli
• The nerve impulses sent by receptor cells travel to a coordinator in the central nervous system
  • In most reflexes the coordinator is the spinal cord
  • Some reflexes may involve unconscious parts of the brain, e.g. the medulla in the brainstem
• From the coordinator the impulse is conducted to the specific effector that will produce the appropriate response
  • The response is the action that is carried out and it is always the same for a given stimulus

The knee-jerk reflex

• The knee-jerk reflex is often used by doctors to assess whether the nervous system of a patient is working properly or not
• Doctors use a specialised hammer to hit a ligament between the knee cap and the tibia, and the leg of the patient will involuntarily straighten in a small kicking motion
• The knee-jerk reflex consists of the following sequence of components:
  • Stimulus - stretching of the quadriceps muscle caused by pressure on the ligament; this pressure is created by the hammer
  • Receptor - stretch receptors in the quadriceps muscle
  • Coordinator - the spinal cord
  • Effector - the quadriceps muscle
  • Response - contraction of the quadriceps muscle, causing the leg to straighten
• In the nervous pathway of the knee-jerk reflex:
  • The stretch receptors send impulses down a sensory neurone which connects directly, via a synapse, with a motor neurone in the spinal cord
    • Note that although relay neurones are normally present in the nerve pathways of reflex actions, there is no relay neurone in the knee-jerk reflex
  • The motor neurone then carries the impulses to the effector (the quadriceps muscle), which contracts
• The knee-jerk pathway shows why reflex actions are so fast and automatic
  • Nerve impulses are delayed by synapses 
  • If these impulses are transmitted via the brain, as occurs in voluntary actions, they have to travel across many synapses
  • In this reflex action the signal only has to cross a single synapse, allowing for a very rapid response
  • Connections from the spinal cord to the brain will still allow information about the stimulus (the pressure caused by the hammer) to be sent to the brain but by the time it receives and processes this information, the response will have already occurred
  • This is why the response happens automatically - the brain has no chance to make a decision

The nervous pathway of the knee-jerk reflex

The blinking reflex

• Blinking is a reflex response caused by something travelling towards the eye at high speed, something contacting the cornea, or by drying of the cornea
• The nervous pathway of this reflex action does, in fact, go via the brain but not via any decision-making areas and the number of synapses is still minimal
• In the nervous pathway of the blinking reflex:
  • Irritation or drying of the cornea sends impulses down the trigeminal sensory nerve to the medulla of the brain, where it connects with other neurones to transmit the signal to the effector muscles
  • Relay neurones are involved in the transmission of impulses to the effectors in the lower eyelid
  • The effectors for the blinking reflex include the superior levator palpebrae muscle, which lowers the upper eyelid, and the orbicularis oculi muscle, which pulls the eyelids inwards and helps to close them

The nervous pathway of the blinking reflex