The Reflex Arc (Edexcel International A Level (IAL) Biology): Revision Note

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

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 $\to$ receptor $\to$ sensory neurone $\to$ relay neurone in spinal cord $\to$ motor neurone $\to$ effector $\to$ 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 $\to$ receptor $\to$ sensory neurone $\to$ relay neurone in brain $\to$ motor neurone $\to$ effector $\to$ response