The diagram below shows a reflex arc.

Label I and II. 

I. ..................................................................

II. ..................................................................

Outline how stimuli can be detected by human sensory receptors.

Explain how sound is perceived by the ear.

I: sensory neuron
II: motor neuron
(both needed)

sensory receptors transfer stimulus energy into electrochemical energy;
mechanoreceptors respond to touch/pressure/movement/sound waves;
thermoreceptors respond to temperature changes;
chemoreceptors detect chemicals/molecules;
photoreceptors respond to electromagnetic stimulation/light;

sound waves are (funneled through the ear canal) causing ear drum to vibrate;
vibrations of ear drum cause the bones of the middle ear/ossicles/malleus, incus and stapes/hammer, anvil and stirrup to move;
lever system of middle ear bones increases pressure on the oval window;
vibrations are transmitted from oval window through (fluid-filled) cochlea;
stimulation of hair cells/mechanoreceptors in cochlea;
vibrations are transformed into nerve impulses/action potentials;
impulse sent to brain along auditory nerve;

Most candidates were able to identify the sensory and motor neurones indicated.

This question proved difficult for many candidates. Many incorrectly described a reflex arc rather than outlining how sensory receptors detect stimuli.  

This section on sound perception was very well done by the majority of candidates with many receiving full marks.