Reflex Arc & Movement Control (DP IB Biology)

The Pain Reflex Arc

Reflexes

• Reflex responses are actions of the body that occur without conscious thought

  • Reflexes are automatic and rapid, minimising damage to the body and therefore aiding survival

  • Awareness of a reflex response occurs after it has been carried out; this is because the information takes longer to reach the conscious parts of the brain

• Examples of reflexes include blinking, coughing, and the pupil and knee reflexes

  • Blinking prevents the outer surface of the eye from drying out as well as protecting it from foreign objects

  • Coughing prevents food from entering the airways and removes mucus from the airways during infection or an allergic reaction

  • The pupil reflex prevents damage to the eye from bright light

  • The knee reflex aids balance when standing upright

What is a reflex arc

• A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving conscious regions of the brain

  • A reflex arc therefore brings about a reflex response 

  • Sensory neurones, relay neurones and motor neurones work together in a reflex arc

Order of a reflex arc

• A pain reflex arc is another example of a reflex response

• The skin has receptors for pressure, touch, and pain
  

  • The receptor involved is the pain receptor called a nocireceptor

    • The stimulus may be a sharp pin or hot flame which is detected by the nocireceptor in the skin of the hand

    • An afferent (sensory) action potential is sent along a sensory neurone to the CNS 

    • An electrical impulse is passed to a relay neurone in the spinal cord

      • Relay neurones are found entirely within grey matter of the spinal cord

    • A relay neurone synapses with a motor neurone

      • A synapse is the junction between neurones; nerve impulses cross synapses by diffusion of a chemical called a neurotransmitter

    • A motor neurone carries an impulse to an effector muscle in the hand

    • When stimulated by the motor neurone the muscle will contract and pull the hand away from the sharp object or heat; this is the reflex response

• The reflex arc for a spinal reflex is as follows:

A Reflex Arc Diagram

Spinal reflexes involve relay neurones in the spinal cord as part of a pain reflex

Role of the Cerebellum

• The cerebellum coordinates movement

  • This includes balance; a highly complex function that requires coordination between multiple parts, including the eyes, semicircular canals in the ears, and many muscles

  • Other movements coordinated by the cerebellum are 

    • Posture

    • Walking

    • Hand and finger movements

    • Eye movements

    • Speech

• The cerebellum does not initiate movement, the motor cortex of the cerebrum is responsible for initiating muscle contractions and therefore movement

• Once the movement begins the cerebellum receives feedback signals from the area of the body that is moving and different sense organs, it then sends signals to coordinate and control the movement

• The structure and function of the brain as an organ is covered in more detail here