Distinguish between:
$(1)$ Receptor proteins and Receptor cells.
$(2)$ Aqueous humor and Vitreous humor.

(N/A) $(1)$ Receptor proteins: When an impulse arrives at the axon terminal,synaptic vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft. The released neurotransmitter binds to specific proteins on the postsynaptic membrane called receptor proteins.
Receptor cells: These are specialized cells that detect stimuli. For example,the organ of Corti is located on the basilar membrane and contains hair cells,which act as auditory receptor cells.
$(2)$ Aqueous humor: This is a thin,watery fluid present in the space between the cornea and the lens.
Vitreous humor: This is a transparent,jelly-like substance present in the space between the lens and the retina.