Edexcel International A Level Biology

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8.3 The Reflex Arc

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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

8-3-the-reflex-arc

The spinal cord contains both grey matter and white matter

  • 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 rightwards arrow receptor rightwards arrow sensory neurone rightwards arrow relay neurone in spinal cord rightwards arrow motor neurone rightwards arrow effector rightwards arrow response

A reflex

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 rightwards arrow receptor rightwards arrow sensory neurone rightwards arrow relay neurone in brain rightwards arrow motor neurone rightwards arrow effector rightwards arrow response

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Naomi H

Author: Naomi H

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.