Structure & Function of Peripheral Sensory Receptors Flashcards
What is the role of the somatosensory system?
It allows us to sense: Touch: pressure against skin Temperature of skin Proprioception: position of joints and muscles with respect to body Pain: Tissue-damaging stimuli
How is the somatosensory system different from other sensory systems?
Somatosensory receptors are distributed throughout body.
Other sensory systems (vision, audition, olfaction, taste) are concentrated in small, specialized areas.
What is the basic pathway for all sensory stimuli to be perceived:
Stimulus ⇒ sensory receptors ⇒ spinal cord ⇒ medulla/brainstem ⇒ thalamus ⇒ cortex
Which direction do sensory pathways travel? Which direction do motor pathways travel?
Sensory pathways go up (periphery to cortex)
Motor pathways go down (cortex to muscles)
What is the DRG?
Cell bodies of sensory neurons innervating the neck on down
There is _______ DRG at each level of the spinal cord on each side.
1
DRG possesses __________ cell bodies with ____________ and _____________ process axons
pseudounipolar; peripheral; central
How are sensory receptors generally categorized?
Exteroreceptive
Proprioceptive
Interoceptive
Describe exteroreceptive receptors:
Code information from the external world, mainly via skin
a) Mechanoreceptors: touch
b) Thermoreceptors: warming and cooling
c) Nociceptors: sharp and burning pain
Describe proprioceptive receptors:
Code information about muscle length, muscle tension, joint angles
a) Muscle afferent receptors: Golgi Tendon Organs and Muscle Spindles
b) Joint and tendon afferents
Describe interoceptive receptors:
Code information about changes inside the body
a) Visceral afferent receptors: Localize sensation and pain very poorly
Functions of sensory neuron:
A. Encode stimulus quality, intensity, duration, location
B. Receptive field
C. Stimulus transduction
D. Intensity of the stimulus
E. Speed of action potential transmission
What is the receptive field?
The area in the periphery where application of an adequate stimulus causes response
Describe the example of stimulus transduction: Pacinian corpuscle
⇒ At peripheral terminal stimulus activates receptors and ion channels
⇒ Generates receptor potential
⇒ If receptor potential strong enough, generates action potentials
⇒ Signal (action potentials) are conveyed to spinal cord
Intensity of the stimulus is encoded by:
1) Each neuron: Frequency of action potential firing: rate code
2) Many neurons: Number of neurons firing: spatial summation code
Speed of action potential transmission (conduction velocity) depends on:
⇒ Axon diameter
⇒ Thickness of myelin
–Large myelinated Aα, Aβ fibers conduct APs very fast
–Thin myelinated Aδ fibers conduct APs moderately fast
–Unmyelinated C fibers conduct APs slowly
What is the use of the Compound Action Potential (whole peripheral nerve)?
To determine whether:
axons are missing, damaged or demyelinated
Used to diagnose peripheral neuropathies: Diabetes Herpes zoster infection (shingles) Nerve entrapment Multiple sclerosis Complications from drug treatments Nutritional deficiencies
Environment for receptive terminals:
a) skin
b) muscle
c) joint
Characteristics that set the sensitivity and function of different receptors:
a) Location: superficial vs. deep
b) Type of ending encapsulated or non-encapsulated
c) Slowly adapting vs. rapidly adapting response of the receptor
d) Spatial resolution for stimulus
What is the significance of slowly adapting vs. rapidly adapting responses?
Determines sensitivity to constant vs. changing stimuli:
1) Slowly adapting— Respond best to a sustained, unchanging stimulus. Sense pressure and shape of objects
2) Rapidly adapting— Respond only when stimulus changes (i.e. on and offset). Sense impact and motion of objects on skin
Spatial resolution for a stimulus depends on:
Receptive field size and innervation density
1) Superficial receptors: small receptive field size
Deep receptors: large receptive field size
2) Innervation density: Density is high in very sensitive areas; low in insensitive areas. 2-point discrimination best for fingers, mouth; worst for back, calves.
Mechanoreceptors:
mediate tactile/touch sensation
a) Very sensitive to force (low threshold); don’t respond to noxious stimuli
b) Silent without stimulation
c) Have myelinated axons; fast conduction velocities; signal reaches spinal cord quickly
Where would superficial mechanoreceptors be found? What are types of these receptors?
Superficial receptors: Found in border between epidermis and dermis
1) Merkel disks
2) Meissner’s corpuscles
Characterstics of Merkel disks:
- Fine touch, 2-point discrimination; Sharpest resolution of surface texture; bumpy vs. smooth
- Receptive field: Multiple small spots. High density in finger tips, around mouth; Low density on back
- Several Merkel disks innervated by single myelinated axon.
- Slowly Adapting response, encodes amount of force
Characterstics of Meissner’s corpuscles:
- Fine touch, 2-point discrimination; Sense abrupt changes in edges, bumps, corners. Help adjust grip and release when lifting objects; Encode low-frequency vibration
- Receptive field: Single spot.
- Corpuscle encloses a stack of flattened epithelial cells; Sensory terminal entwined between layers; axon is myelinated
- Rapidly Adapting response encodes on/offset of skin indentation
Where would deep mechanoreceptors be found? What are types of these receptors?
Deep receptors: Found in dermis
1) Ruffini Endings
2) Pacinian Corpuscles
Characteristics of Ruffini Endings:
- Sense stretch of skin; Help determine shape of grasped objects
- Receptive field: large and diffuse
- Ruffini ending is encapsulated; myelinated axon surrounds collagen fibrils
- Slowly Adapting response responds to stretching of skin
Characteristics of Pacinian Corpuscles:
- Respond to high frequency vibration; Most sensitive mechanoreceptor, even distribution throughout skin.
- Receptive field: large and diffuse
- Fluid filled capsule wrapped around bare nerve ending; filters out sustained stimuli; myelinated axon
- Rapidly adapting response
Hair Follicle receptors:
- Respond to movement of hairs
- Receptive field: around base of hair follicle
- Bare axon wraps around base of hair follicle; axon is myelinated
- Rapidly adapting response encodes velocity of hair movement
Textures of objects are signaled by ___________
different receptors
What is the spatial summation code?
Overall picture in brain is due to the sum of information provided by the different active and silent fibers
Thermoreceptors:
a) Encode skin temperature (warming, cooling)
b) Discharge continuously (steady rate) at normal skin temperature (32 °C)
What are the types of thermoreceptors?
1) Cooling receptors
2) Warming receptors
Characteristics of Cooling receptors:
- Increase firing rate when skin is cooled. Stop firing when skin is warmed.
- Free nerve endings with myelinated axons
Characteristics of Warming receptors:
- Increase their firing rate when skin is warmed above 32 °C. Stop firing when skin is cooled. Range ≈ 29-43 °C.
- Free nerve endings with unmyelinated axons (C fiber)
- Very small receptive fields
What do thermoreceptors respond best to?
changes in skin temperature
Nociceptors (pain receptors):
Respond to stimuli that damage or threaten to damage tissue
What makes up the majority of sensory neurons in the DRG?
Nociceptors
≈ 70% of all sensory neurons in the dorsal root ganglia
Where is almost all of the innervation of nociceptors provided?
tooth pulp and cornea
Two general types of nociceptors:
1) A-Mechanonociceptors
2) C Polymodal nociceptors
A-Mechanonociceptors:
Axon is myelinated (A∂)
• Respond to intense mechanical force, sometimes intense heat (≥ 52 °C)
• Free nerve endings
• Small receptive fields
• Slowly adapting response
• Mediate fast, initial pain; sharp “pricking” quality; easy to localize
C Polymodal nociceptors:
Axon is unmyelinated (C-fiber)
• Polymodal = many modes: Respond to intense mechanical force, high heat (≥ 45 °C), noxious chemicals (bradykinin, histamine, acid; capsaicin
• Free nerve endings; No sheath– respond to inflammatory chemicals
• Small receptive fields
• Slowly adapting response
• Mediate slow, aching, “burning” quality of pain; difficult to localize
