8.3 The Reflex Arc

• The central nervous system (CNS) consists of the brain and spinal cord
  • The brain processes information about external and internal stimuli and co-ordinates the body's responses
  • The spinal cord connects the brain to the rest of the nervous system as well as coordinating some reflex responses
• 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
• 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

Spinal reflexes

• The pathway of a spinal reflex involves relay neurones located in the spinal cord
  • The spinal cord is made up of types of tissue known as grey matter and white matter
    • Grey matter contains the cell bodies of motor neurones along with relay neurones
    • White matter contains long myelinated axons that carry information through the spinal cord

• Pulling a foot away from a sharp object is an example of a spinal reflex 
  • The stimulus of a sharp pin is detected by a receptor cell in the skin of the foot
    • The skin has receptors for pressure, touch, and pain
  • A sensory neurone sends electrical impulses to the CNS 
  • An electrical impulse is passed to a relay neurone in 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 leg 
  • When stimulated by the motor neurone the muscle will contract and pull the foot up and away from the sharp object; this is the reflex response
• The reflex arc for a spinal reflex is as follows

stimulus  receptor  sensory neurone  relay neurone in spinal cord  motor neurone  effector  response

Spinal reflexes involve relay neurones in the spinal cord

Cranial reflexes

• The pathway of a cranial reflex involves relay neurones located in the brain
• The pupil reflex is an example of a cranial reflex
  • Changing pupil diameter enables the eye to control the amount of light hitting the retina 
  • The diameter of the pupil in the eye is determined by two sets of muscles
    • The circular muscles contract to constrict the pupil
    • The radial muscles contract to dilate the pupil
  • The two sets of muscles work antagonistically, meaning that when one set of muscles contracts the other relaxes, and vice versa
• When bright light falls on the eye, the following events occur
  • The light level is detected by photoreceptors in the retina
  • A sensory neurone sends electrical impulses to the CNS 
  • An electrical impulse is passed to a relay neurone in the brain
  • A relay neurone synapses with a motor neurone
  • A motor neurone carries an impulse to the effector muscle; in this case the circular muscle in the iris 
  • When stimulated by the motor neurone the muscle will contract and constrict the pupil; this is the reflex response
• When in dim light the same process occurs, but the motor neurone stimulates the radial muscles in the iris, causing them to contract and dilate the pupil
• The reflex arc for a cranial reflex is as follows

stimulus  receptor  sensory neurone  relay neurone in brain  motor neurone  effector  response