Unit 4

Cutaneous Senses Part 1

(Touch, Temperature, Pain, Tickle, and Itch)

1. Skin - the receptive surface

The body’s largest organ

Protects against injury, heat, and light radiation, helps regulate body temperature

Helps eliminate harmful substances

Helps eliminate harmful substances

Acts as a barrier

Acts as an external sensory organ

2. Types of Skin

Hairy Skin - covers most of the body

Glabrous Skin - hairless skin. Thicker than hairy skin (palms, soles of feet)

3. Skin components and structure

1. Varies in thickness from ~ .5mm to 5mm

1. Within one square inch of skin 

• 650 swear glands

• 65 hair follicles

• 19 yds of capillaries

• 78 yds. of nerves

• Thousands of sensory cells/nerve endings

B. Layers of Skin 

1. Epidermis - outer most layer

• Composed of mostly dead skin cells

2. Dermis 

• Thick connective membrane

3. Hypodermis 

• Connective tissue

IV. Types of receptors - collectively referred to as somatosensory receptors.

There are 3 major receptors

1. Mechanoreceptors - respond to sensation to the skin

1. RA (Rapidly adapting receptors) 

Fire the second when the stimulus is applied or removed, but not in between

Responds to fast vibration, rough texture

Ex: If you run your fingers across sandpaper, RA receptors fire because your skin is feeling fast changes in texture. 

2. SA (Slowly adapting receptors)

Continue to fire as long as the stimulus is applied

Responds to skin stretching, fine details, smooth texture

Ex: If you just press your finger down and hold it still on the sandpaper or if the texture never changes and is at a constant pressure, the RA receptors mostly stop responding 

B. Thermorceptors - these receptors respond to temperatures

1. Warm Fibers fire when the temperature increases (86F - 122F)

2. Cold Fibers fire when the temperature decreases (68F - 113F)

C. Nociceptors - they respond to anything that can cause harm, from intense pressure, high heat, or burning chemicals. 

Ex. You touch a hot stove—nociceptors in your skin immediately fire, signaling burning pain to your brain, so you pull away fast.

V. Relationship between receptor type and perception

1. Specificity theory suggests that specific perceptions are associated with specific nerve endings. However, tests failed to find any correlation between receptor type and sensory experience

2. Pattern theory- The pattern of firing among the receptor types better predicts skin perception. 

• Each receptor responds to many different types of stimulation. Thats why if you just activate a nociceptor it won't cause anything to happen.

• Ex: If light touch and pain signals fire together in a certain way, your brain might interpret that as a sharp or itchy feeling. 

VI. Relationship between physiology and skin acuity

1. Two-point threshold - a measure of skin acuity (detail)

1. Refers to the smallest separation between two points on the skin that is perceived as two points

• Back and legs have very low acuity

• Fingers, face, lips, have very high acuity

B. Representation on the cortex

1. Penfield and Ramussen stimulated different parts of the somatosensory cortex. The top of the cortex is focused on the bottom part of body (legm feet), while going down the cortex it focues up of the body (Lips, face)

2. Homonculus - discovered that the somatosensory cortex was systematically laid out.

3. The amount of cortex dedicated to a region

of the body is highly correlated with its acuity.

• High acuity = large somatosensory cortex devoted to processing in that area (e.g., lips, face, fingers, etc.)

• Low acuity = smaller somatosensory cortex devoted to processing in that area (e.g., legs, arms, trunk, etc.)

Perception of Pain

1. Functional significance of pain

Pain perception keeps you from harming your body: 

DANGER! DANGER! Do something!

• People born without the ability to feel pain (CIP; congenital insensitivity to pain) tend to die young (accumulation of skin/bone damage, chewing off ethe nds of fingers)

2. The Puzzle of Pain (5)

1. Phantom Limb pain - 70% of phantom limb perception is accompanied by pain

2. Meaning of the situation Context often affects one’s perception of pain

• Reported 25% of men seriously wounded in battle requested morphine but over 80% of civilians about to undergo surgery requested pain medication. This is because Men in war, injury means getting home. People going to surgery do not want to be in pain during surgery.

3. Effect of Culture - Some evidence suggests that one’s cultural background affects the perception of pain

4. Impact of Experience on Pain

5. Placebo Effects: If a patient believes that a substance will reduce pain, it often does, even though the substance is chemically inert (Doesn’t have effect on the body)

6. Acupuncture - needles are inserted into particular regions of the body

• Has been shown to produce an analgesic effect

III. Gate Control Theory

Developed by Melzack and Wall in 1965.

A. The gate control system involves cells in the spinal cord called the substantia gelatinosa

B. Cells in the substantia gelatinosa receive input from three sources.

1. Large diameter fibers –highly myelinated fibers, causing rapid signals to be sent to the spine. May mediate initial response to damage, but is generally overridden by inhibitory signals. Tends to close the gate, especially (reduces pain) by rubbing, heat, and cold packs.

2. Small diameter fibers – these are less myelinated and thus are slower in sending pain signals to the spine. Associated with nociceptors (signals to spine concerning damage or potential damage to skin). Tends to open the gate (increases pain).

3. Central control system (Top down) – Efferent nerves from the brain to the spine. Related to cognitive functions associated with expectation, attention, distraction, etc. Tends to close the gate (decreases pain).

C. Transmission cells (T-cells) – these cells receive inhibitory or excitatory information from these fibers; T-cell activity ultimately determines how much pain signals are sent to the brain.

Smell & Taste

1. Taste - works in conjunction with smell, but is an independent sense. 

Flavor is a combination of smell and taste.

1. Taste primaries - receptors on tongue respond to only 4 basic tases

1. Sweet

2. Sour

3. Salty

4. Bitter

B. Basic taste receptor structures called taste buds

1. Taste buds are located in pits & grooves of the mouth, throat, pharynx, inside cheeks, and soft palate

2. Humans possess 9000 - 20,000 taste bids

3. Taste buds in clusters lying within lying within small elevations on the tongue called papillae.

4. Several different types of papillae

1. Circumvallate - Does have taste buds

2. Foliate - Does have taste buds

3. Fungiform - Does have taste buds

4. Filiform - Does NOT have taste buds

C. Structure of taste buds

1.  Each taste bud contains 50 - 150 microvilli (tufts of cilia), which extend into taste pore.

2. Chemicals absorbed in the saliva attach to receptors along microvilli

D. Two types of neural coding from taste bud activation

1.  Specificity coding (also called Direct line label) - there is some evidence to suggest that a particular taste is experienced because certain taste buds fire to certain substances.

2.  Across - Fiber Patterning - other evidence suggests that a particular taste is experienced because of the particular pattern of firing across taste buds

3.  Both mechanisms appear to code for taste perception

1. Smell

1.  Microsmatic - poor sense of smell (humans are microsmatic)

2.  Macrosmatic - good sense of smell (dogs are macrosmatic; 100 X better than humans)

3.  Most mammals (including humans) have a separate organ (vomeronasal) that responds to pheromones (implicated in sexual behavior of most mammals).

1.  Menstrual synchrony - refers to the phenomenon where women who live together often find that their monthly menstrual cycle becomes synchronized. Synchronization appears to be due to pheromone excretion.

II.  Sketch of the olfactory process (see figure below):

1. Molecules of odorant are absorbed into the olfactory mucosa, contains the olfactory cili

2. Molecules attach to receptors on the cilia and a signal is sent to the olfactory bu

3.  From the olfactory bulbs, information then sent along the olfactory tract.

III. Neural code for odor

1.  Pattern of neural activity in olfactory bulbs appears to determine smell perception.

2.  Two different types of patterns (see figures below):

1.  The Regional Sensitivity Effect - different areas on the mucosa are sensitive to some odorants and are not as sensitive to other odorants.

2. The Chromatographic Effect - as molecules travel across the mucosa, they are absorbed gradually; heavy molecules absorbed first by the lighter molecules

Mercaptan, a sulfur derivative, is added to natural gas to give it a distinctive rotten egg smell for safety