Somatosensory neurons are most sensitive to mechanical force, temperature change, and tissue damage. Nociception is the processing of pain signals, which could result from any of these stimuli.

While somatosensory neurons are found within specialized touch receptors, the touch receptor organ for the tactile system is the skin. These sensors will detect stimuli for pressure, temperature change, and pain. Though these sensations can be detected in some internal regions of the body as well, the sensation of touch must be triggered by an external stimulus. As such, the receptors must have contact with the external environment (via the skin).

Sensors for proprioception can be found within joint tissues, muscles and tendons. These sensors detect spatial orientation and motion, stretch and compression, and internal pain. Stimuli for proprioception must be triggered by the body's internal environment.

The only entirely true statement is that proprioceptive stimuli are internal and generated by body position and movement, while tactile stimuli are external. Proprioception is the awareness of the body in space, which allows animals to have the sensation of the placement of their bodies without needing to see what they are doing or feel their way. Tactile perception is the body's interaction with an external stimulus, such as pain, temperature, or pressure.

While most tactile stimuli are transmitted through the spinal trigeminal pathway, certain sensations use the main sensory trigeminal pathway.

Taste buds are bulb-shaped and open to the environment through a small pore. Chemical compounds bind to small cellular hairs of taste receptor cells to stimulate an action potential, relaying the taste to the brain. Basal cells are located at the bottom of the taste bud. As taste receptors die, basal cells will differentiate into new receptors to maintain the taste bud.

Options must be considered carefully here, as all answer choices relate to the gustatory system. The only similarity is that salty (sodium ion) and sour (hydrogen ion) tastes both involve the passing of these ions through sodium channels. (Hydrogen can also pass through proton transport channels.) While sour tastes can block the efflux of potassium ions, this is not true for salts, making this an incorrect answer choice.

Hydrophilic (polar) molecules are able to diffuse across the olfactory mucosa, while hydrophobic (nonpolar) molecules are first bound to transport proteins. We are looking for the answer choice that contains one polar and one nonpolar molecule. Ethanol is polar and methane is nonpolar, making this the correct answer choice.

Methane, ethane, and benzene are all nonpolar. Ethanol and ammonia are both polar.

Olfactory neurons are the sole nerve cells that receive sensory information from the external environment. Basal cells will differentiate into neurons while columnar cells secrete mucosa. The true statement is that cilia density on the ends of the olfactory neurons determines odor sensitivity; the cilia contain many odor-specific receptor proteins.

After taste receptor cells contact solutes in the saliva, action potentials cause a calcium influx through ion channels, which leads to the release of the neurotransmitter that induces an action potential in the afferent nerve. This action potential is carried to the brain for integration and interpretation.

Chloride ions do not enter the receptor during this process.

The incus, along with the malleus and stapes, are three bones found in the middle ear, which act as a lever system to transmit sound waves to the oval window.

The pinna is simply the skin and cartilage component of the outer ear. The semicircular canals are found in the inner ear, and are responsible for adapting the head and body to positional changes, thus maintaining balance. The perilymph is the fluid found within the inner ear. 

The malleus, incus, and stapes are the three bones of the ear. Sound enters the external ear by using air as a medium. In the middle ear, vibrations from the air transition to vibrations through bone. The inner ear converts these bone vibrations to fluid vibrations in the cochlea, which converts the fluid vibrations to electrical stimuli for the nervous system.