Essentials of Anatomy and Physiology

Purpose of Sensations—to detect changes in
the external or internal environment to
enable the body to respond appropriately
to maintain homeostasis

Sensory Pathway—pathway of impulses for a
sensation

1. Receptors—detect a change (usually very specific)
   and generate impulses.


2. Sensory neurons—transmit impulses from recep-
   tors to the CNS.


3. Sensory tracts—white matter in the CNS.


4. Sensory area—most are in the cerebral cortex; feels
   and interprets the sensation.

Characteristics of Sensations

1. Projection—the sensation seems to come from the
   area where the receptors were stimulated, even
   though it is the brain that truly feels the sensation.


2. Intensity—the degree to which a sensation is felt; a
   strong stimulus affects more receptors, more
   impulses are sent to the brain and are interpreted as
   a more intense sensation.


3. Contrast—the effect of a previous or simultaneous
   sensation on a current sensation as the brain com-
   pares them.


4. Adaptation—becoming unaware of a continuing
   stimulus; if the stimulus remains constant, there is
   no change for receptors to detect.


5. After-image—the sensation remains in the con-
   sciousness after the stimulus has stopped.

Cutaneous Senses—provide information
about the external environment and the
skin itself

1. The dermis has free nerve endings that are recep-
   tors for pain, heat, and cold, and encapsulated nerve
   endings that are receptors for touch and pressure
   (see Fig. 9–1).


2. Sensory areas are in parietal lobes.


3. Referred pain is visceral pain that is felt as cuta-
   neous pain. Common pathways in the CNS carry
   both cutaneous and visceral impulses; the brain
   usually projects sensation to the cutaneous area.

Muscle Sense—knowing where our muscles
are without looking at them

1. Stretch receptors in muscles detect stretching.


2. Sensory areas for conscious muscle sense are in
   parietal lobes.
   3. Cerebellum uses unconscious muscle sense to
   coordinate voluntary movement.

Sense of Taste (see Fig. 9–2)

1. Chemoreceptors are in taste buds on the tongue;
   detect chemicals (foods) in solution (saliva) in the
   mouth.


2. Five basic tastes: sweet, sour, salty, bitter, and
   savory; foods stimulate combinations of receptors.


3. Pathway: facial and glossopharyngeal nerves to
   taste areas in parietal-temporal lobes.

Sense of Smell (see Fig. 9–2)

1. Chemoreceptors are in upper nasal cavities; several
   hundred different ones; detect vaporized chemicals
   (many combinations possible).


2. Pathway: olfactory nerves to olfactory bulbs to
   olfactory areas in the temporal lobes.


3. Smell contributes greatly to what we call taste.

Hunger and Thirst—visceral (internal)

sensations

1. Receptors for hunger: in hypothalamus, detect
   changes in GI hormones and nutrient levels in the
   blood; hunger is projected to the stomach; adapta-
   tion does occur.


2. Receptors for thirst: in hypothalamus, osmorecep-
   tors detect changes in body water (water–salt pro-
   portions); thirst is projected to the mouth and
   pharynx; adaptation does not occur.

The Eye (see Figs. 9–3 through 9–6)

1. Eyelids and eyelashes keep dust out of eyes; con-
   junctivae line the eyelids and cover white of eye.


2. Lacrimal glands produce tears, which flow across
   the eyeball to two lacrimal ducts, to lacrimal sac to
   nasolacrimal duct to nasal cavity. Tears wash the
   anterior eyeball and contain lysozyme to inhibit
   bacterial growth.


3. The eyeball is protected by the bony orbit (socket).


4. The six extrinsic muscles move the eyeball; inner-
   vated by the 3rd, 4th, and 6th cranial nerves.


5. Sclera—outermost layer of the eyeball, made of
   fibrous connective tissue; anterior portion is the
   transparent cornea, the first light-refracting struc-
   ture.


6. Choroid layer—middle layer of eyeball; dark blue
   pigment absorbs light to prevent glare within the
   eyeball.

The Senses 217

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