Sensations result only from those stimuli that reach the cerebral cortex and are consciously perceived. Senses can be defined as special or general.

General Senses

• Receptors for general senses, such as pain, temperature, touch, pressure, and proprioception, are scattered throughout the body.

Pain

• Pain is an unpleasant sensation with a fast component and a slow component.

• Pain can be "gated," referred, or phantom.

Special Senses

• Smell and taste respond to chemical stimulation, vision to light stimulation, and hearing and balance to mechanical stimulation.

Olfaction

• Olfactory neurons have enlarged distal ends with long cilia. The cilia have receptors that respond to dissolved substances in the nasal mucus.

• Axons of the olfactory neurons form the olfactory nerves, which enter the olfactory bulb. Olfactory tracts carry action potentials from the olfactory bulbs to the olfactory cortex of the brain.

• The wide range of detectable odors may result from combinations of receptor responses stimulated by only a few primary odors.

Taste

• Taste buds contain taste cells with hairs that extend into pores. Receptors on the hairs detect dissolved substances.

• There are four basic types of taste: sour, salty, bitter, and sweet.

• The facial nerves carry taste from the anterior two-thirds of the tongue, the glossopharyngeal from the posterior one-third of the tongue, and the vagus from the root of the tongue.

Vision

Accessory Structures

• The eyebrows prevent perspiration from entering the eyes.

• The eyelids protect the eyes from foreign objects.

• The conjunctiva covers the inner eyelids and the anterior surface of the eye.

• Lacrimal glands produce tears that flow across the surface of the eye. Tears lubricate and protect the eye. Excess tears pass into the nasal cavity.

• The extrinsic eye muscles move the eyeball.

Anatomy of the Eye

• The fibrous tunic is the outer layer of the eye. It consists of the sclera and cornea.

• The vascular tunic is the middle layer of the eye. It consists of the choroid, ciliary body, and iris.

• The lens is held in place by the suspensory ligaments, which are attached to the smooth muscles of the ciliary body.

• The retina (nervous tunic) is the inner layer of the eye and contains neurons sensitive to light.

• Rods are responsible for vision in low illumination (night vision).

• Cones are responsible for color vision.

• The fovea centralis has the highest concentration of cones and is the area in which images are detected most clearly.

• The optic disc, or blind spot, is where the optic nerve exits the eye and blood vessels enter.

• The anterior compartment of the eye is filled with aqueous humor, whereas the posterior compartment is filled with vitreous humor. The humors keep the eye inflated, refract light, and provide nutrients to the inner surface of the eye.

Functions of the Complete Eye

• Light passing through a concave surface diverges. Light passing through a convex surface converges.

• Converging light rays cross at the focal point and are said to be focused.

• The cornea, aqueous humor, lens, and vitreous humor all refract light. The cornea is responsible for most of the convergence, whereas the lens can adjust the focus by changing shape (accommodation).

Neuronal Pathways

• Axons pass through the optic nerves to the optic chiasma, where some cross.

• Optic tracts from the chiasma lead to the thalamus.

• Optic radiations extend from the thalamus to the visual cortex in the occipital lobe.