1 - Structure and Function of Peripheral Sensory Receptors

Question 1

What does the somatosensory system allow our body to sense?

Answer 1

Touch: pressure against skin
Temp of skin, heat and cold stimuli
Proprioception: position of joints and muscles
Pain: tissue-damaging stimuli

Question 2

The somatosensory system is distributed throughout what parts of the body?

Answer 2

Skin, muscles, joints, and bone.

Question 3

What is the basic sensory pathway that the stimulus travels to get to the brain?

Answer 3

Stimulus > sensory receptors > spinal cord > medulla/brainstem > thalamus >cortex

Question 4

Cell bodies of sensory neurons of the PNS are located where? What do these innervate?

Answer 4

The dorsal root ganglia, they innervate the neck and below.

Question 5

What is the basic anatomy of the dorsal root ganglia? How many cell bodies does each house?

Answer 5

1 at each spinal level on each side of the spinal cord.

1 DRG has 10,000-20,000 cell bodies in the thoracic level (can be 50,000 at cerv or lumbar).

Length of axons up to 1 meter in leg.

Question 6

What are the three general categories for sensory receptors?

Answer 6

1. Exteroreceptive
2. Proprioceptive
3. Interoceptive

Question 7

Where are exteroreceptive sensory receptors located? What are some receptors that fall into this category?

Answer 7

Receptors that sense the external world on the skin:

• mechanoreceptors sense non-painful touch and vibration
• thermoreceptors sense warning and cooling
• nociceptors sense pain (sharp, dull, pinch, painful heat, -20F wind chill).

Question 8

What is the function of proprioceptive sensory receptors?

Answer 8

Sense muscle length, tension, and joint angle.

• Muscle afferents: muscle spindles sense muscle length and golgi tendon organs sense tension.
• Joint tendon afferents

Question 9

Where are interoceptive sensory receptors located? Give some examples.

Answer 9

In internal organs.

Responsible for visceral afferents (localize sensation and pain poorly)

Ex: baroreceptors, blood pressure, pH

Question 10

What are the characteristics of an encode stimulus?

Answer 10

Quality: brush, pressure, vibration, temp, pain

Intensity: light stroke v. intense pressure

Duration

Location

Question 11

What is the receptive field?

Answer 11

The area in the periphery (ew. skin) where an adequate stimulus causes response from a neuron

Question 12

What are two ways that the intensity of a sensory stimulus is encoded?

Answer 12

Rate code: freq of AP firing per neuron

Spatial summation code: number of neurons firing (info to spinal cord is summated)

Question 13

What two things impact the conduction velocity (speed) of an AP?

Answer 13

Axon diameter: large diameter is faster

Myelination: thicker myelin is faster

Question 14

What are the differences in myelination thickness and their impact on conduction velocity?

Answer 14

1. Large myelin = very fast:
   - Aalpha muscle spindles, golgi tendon organ –> fastest
   - Abeta skin: light tough, vibration, pressure –> a little slower than Aalpha
2. Thin: Adelta = medium:
   - Nociceptors (fast pain)
   - cooling receptors
3. Unmyelinated C fibers: slow
   - nociceptors (slow pain); warm receptors

Question 15

Describe the sensory receptors in the skin?

Answer 15

They have specialized ending that tune sensory neurons to respond to specific physical stimuli

Question 16

What are characteristics that determine the sensitivity and function of the sensory receptor?

Answer 16

1. Location: superficial v. deep
2. Ending: encapsulated v. non-encapsulated
3. Slowly adapting v. rapidly adapting response
4. Spatial resolution

Question 17

What is the difference between a slowly adapting receptor and a rapidly adapting receptor?

Answer 17

Slowly adapting: have sustained, unchanging stimulus.
-encode pressure and shape of objects

Rapidly adapting: changing stimulus

• fire at onset and offset NOT throughout
• encode impact and motion of objects
• important for things moving across skin

Question 18

What does spatial resolution depend on? What is a 2 pt discrimination threshold? What does a sensitive area consist of?

Answer 18

Depends on receptive field size of each neuron and innervation density.

The minimum distance between two detectable stimuli.

High density of receptors and small size of receptive fields.

Question 19

What type of stimuli do mechanoreceptors respond to? What don’t they respond to? How is their info transmitted?

Answer 19

Touch pressure, vibration, and very sensitive to force.

They are silent without stimulation and don’t respond to painful stim

Sent via myelinated axons with fast conduction velocity so signal reaches sp cord fast.

Question 20

What is the function of Merkel disks - superficial (1st type of mechanoreceptor)? Describe their receptive field and location?

Answer 20

Fine touch, 2 pt discrimination, and sense sustained pressure

Receptive field: several small, sensitive spots: touch dome

High density in finger tips, lips, and mouth

Question 21

What are the charactieristics of 2 pt discrimination that the Merkel disks are capable of? What type of response do these receptors have?

Answer 21

Sharpest resolution of texure, surface of objects (bumpy v. smooth), braille.

Slowly adapting response: number of APs fired ~ indentation force

Several disks are innervated by a single axon.

Question 22

What is the function of Meissner’s corpuscles- superficial (2nd type of mechanoreceptors)? Where are they located? What is the receptive field?

Answer 22

Fine touch, 2 pt discrimination, sense abrupt changes in edges, bumps, and corners; adjust grip and release when lifting objects.

In glabrous skin only (thick skin on sole of feet or palms). High density in finger tips, lips, and mouth.

Receptive field is a single spot.

Question 23

What are the histological characteristics of Meissner’s corpuscles? Describe their response to stimuli?

Answer 23

They enclose a stack of schwann cells; Axon is myelinated.

Rapidly adapting response based on onset and offset of indentation.

APs fired ~ # times the skin is indented, low frequency.

Question 24

What is the function of Ruffini corpuscles (3rd type of mechanoreceptor)? Where are they located and what is their receptive field?

Answer 24

Sense the stretch of skin and gravity force against skin.

Located in the dermis of the skin.

Large and diffuse receptive field.

Question 25

Describe the histological characteristics of Ruffini corpuscles? What is their response?

Answer 25

Axon surrounds collagen fibrils, axon is myelinated, and they have a slowly adapting response to skin stretch.

They determine the shape of grasped object as skin stretches around it.

Question 26

What is the function of Pacinian Corpuscles (4th type of mechanoreceptor)? What is their receptive field? Where are they located.

Answer 26

Detect tiny, high-frequency vibrations such as those of a tuning fork and violin strings.

Extremely sensitive.

Very large and diffuse receptive field.

Located deep in the skin.

Question 27

What are the histological characteristics of pacinian corpuscles? What type of response do they have?

Answer 27

Large fluid filled capsule wrapped around a bare nerve ending: capsule filters out sustains stimuli.

Axon myelinated.

Rapidly adapting response.

Question 28

What is the function of hair follicle receptors (5th type of mechanoreceptor)? What is their response and receptive field?

Answer 28

Movement of hairs.

Receptive field: myelinated axon wraps around the base of follicle to encode velocity of hair movement.

Rapidly adapting response.

Question 29

Explain how the texture of objects are signaled using the example of braille dots moving across the skin?

Answer 29

AP firing pattern from 3 receptor types.

Merkel and Meissner’s encode the spacing/texture of the dots well (Pacinian corpuscles don’t distinguish spacing well)

Question 30

What is the spatial summation code?

Answer 30

The overall picture in the brain due to the sum of the information from the pattern of activation of different fibers.

Question 31

What is the function and action of thermoreceptors?

Answer 31

Encode skin temp such as warming and cooling.

Fire APs continuously; slow rate at a normal skin temp (32 degrees C)

Question 32

When do cooling receptors increase their firing rate? What is their structure? What is their receptive field?

Answer 32

Increase firing rate when skin is cooled and stop when skin is warmed.

Free nerve ending that’s thinly myelinated (Adelta)

Very small receptive field with infrequent distribution.

Question 33

When do warming receptors increase their firing rate? What is their structure and receptive field?

Answer 33

Increase firing rate when skin is warmed and stop when skin is cooled.

Free nerve ending that’s unmyelinated (C fiber)

Very small receptive field, best range in 33-42 degrees C.

Question 34

Cooling and warming receptors respond best to what?

Answer 34

CHANGES in skin temps, but adapt to constant temps.

They respond in opposite ways to cooling and warming skin.

Question 35

How do Pacinian Corpuscles sense vibration?

Answer 35

1. Vibration impinges on the PC: its onion like structure is wrapped around a bare nerve ending with schwann cell fluid between each layer.
2. Sensory neuron terminal activated and activates mechanoreceptor ion channels in neuron membrane
3. Generator potential
4. AP
5. AP propagates to spinal cord.

Question 36

How is compound action potential measurement used as a clinical diagnostic tool?

Answer 36

It records the activity from a whole peripheral nerve to determine what type of axon is missing, damaged, or demyelinated.

Question 37

What is the sequence of firing using compound action potential diagnostic tools?

Answer 37

Aalpha and Abeta nerves fire first, which are the largest component on the graph.

Next is Adelta.

Then milliseconds later (long) C fibers are the small APs added to make a small amplitude.

Question 38

What are nociceptors and what is their function? How common are they in the body?

Answer 38

Receptors that respond to stimuli that damage or threaten to damage tissue.

70% of all sensory neurons are nociceptors.

All are slowly adapting and have free nerve endings.

Question 39

What are the two general types of nociceptors?

Answer 39

1. A-mechanonociceptors

2. C Polymodal nociceptors

Question 40

What do A-mechanonocicaptors respond to? What is their structure? What type of response do they have?

Answer 40

Respond to intense force and somtimes intense heat.

Free nerve ending with myelinated axon (Adelta).

Small receptive fields and slow adapting response.

Question 41

What are some examples of pain that would cause a response from A-mechanonociceptors?

Answer 41

Fast pain that’s sharp, shooting, electrical “pricking” pain.

Easy to localize pain.

Immediate pain when you stub your toe, hit your thumb, or have your hand under very hot water.

Question 42

What do C polymodal nociceptors respond to? What is their structure and what type of response do they have?

Answer 42

Respond to intence force, high heat over > or equal to 45 degrees C, and chemicals such as prostaglandins, acid, and histamines.

Unmyelinated axons and free nerve endings for access to inflammatory chemicals.

Slow adapting response.

Question 43

What type of pain would cause a response from C polymodal nociceptors?

Answer 43

Slow pain, long-lasting, burning, aching, dull pain.

Pain that’s difficult to localize.