Physiology of Somatosensation Flashcards
What type of info does the somatosensory system give about the skin?
Pain
Temp
Pressure
Vibration
Texture
Shape
What type of info does the somatosensory system give about the muscles and joints?
Pain
Length
Degree of stretch
Tension
Contraction
Position
What are cell body locations for the primary afferent neurons involved in somatosensation? What is conveyed at each location?
- Dorsal root ganglion- Information about the neck and below
- Mesencephalic nucleus- Proprioceptive information about the face
- Trigeminal ganglion- Everything else about the face
Which fiber types are myelinated?
Aα- I; Diameter: 13-20; Conduction Velocity: 75-120
Aβ - II; Diameter: 6-12; Conduction Velocity: 35-75
Aδ- III; Diameter: 1-5; Conduction Velocity: 5-35
List the fiber speeds from fastest to slowest
I> II>III>IV
Which fiber types respond to temperature?
Aδ and C fibers
Where are cold receptors located? What about heat receptors?
Cold receptors: Upper dermis
Heat receptors: Upper dermis and deeper dermis
NOTE: Thermoreceptors have a small receptive field
What is the transduction of thermoreceptors?
- Conformation of non-selective cation channels are temperature- dependent
- These cation channels change conformation based on temperature
Cold receptors (<30° C)
Heat receptors (>32° C)
What are two types of cold receptors and which fiber types can each be found on?
- CMR1 (TRPM8)- On C fiber endings
- TRPA1- On A delta fibers
What are two types of heat receptors and which fiber types can each be found on?
- TRPV1- On C fiber endings
- TRPV2- On A delta fiber endings
What is the adaptation for thermoreceptors?
Low and slow
*This makes sense because thermoreceptors allow us to survive
Thermoreceptors are considered polymodal. Why?
Because they respond to more than one thing, chemicals and temp.
NOTE: Nociceptors are naked nerve endings
Where in the body are nociceptors abundant?
In epithelia and connective tissue
Nociceptors respond to…
- Chemicals from damaged tissue (chemosensitive)
- Intense or deep touch (pinching, poking) (mechanosensitive)
- Extreme temperature (thermosensitive)
NOTE: Nociceptors are polymodal and are naked nerve endings.
Which fibers types are nociceptors found on?
-
C fibers- Dull Pain (Difficult to distinguish where the pain is coming from)
- Small diameter
- Unmyelinated
-
A delta- Sudden sharp pain
- Medium diameter
- Lightly myelinated
NOTE: Nociceptors have large receptive fields
Types of skin mechanoreceptors
-
Hair follicle receptor
- Light touch
- Moving touch
-
Pacinian corpuscles
- Deep pressure
- Vibration
-
Meissner’s corpuscles
- Fine touch
- Close to the dermis
-
Merkel receptors
- Pressure
- Texture
-
Ruffini endings
- Skin stretch
- Deep pressure
- Deep in skin
Do accessory organs around the receptor transform energy?
No.
Accessory organs direct, lessen, amplify the stimulus but they don’t transform energy
Ex: Hair
Hair follicle receptor are _______ (tonic/phasic) receptors.
Phasic
*They adapt quickly; frequency of APs slow down dramatically
What fiber types are hair follicle receptors found on?
A beta
A delta
What is the fiber for Pacinian Corpuscle?
A beta
What allows the rapid adaptation seen in pacinian corpuscle?
Because of its fluid filled layers, the fluid disperses and evens itself out so that receptor in the middle is no longer being distorted. The receptor is no longer feeling pressure and channels close and action potentials are gone, even though the stimulus is still there.
What is the transduction mechanism for Merkel’s cells?
- Neurotransmitter released by Merkel’s cell
- Neurotransmitter bind to ligand gated channels on primary afferent neuron
- AP propogated
Which receptor is the only complex receptor in the somatosensory system?
Merkel’s cell
*Complex receptors separate the cell from the primary afferent neuron
Muscle spindle
Acessory organ
Fiber type
Adaptation
Acessory organ: Intrafusal fibers
Fiber type: A alpha and A beta
Adaptation: low; slow
Golgi Tendon
Fiber type
Adaptation
Fiber type: A alpha
Adaptation: very low
*Prevents too much contraction
Where are the cell bodies of first order, second order, and third neurons located?
First order: Dorsal root ganglion, trigeminal ganglion, mesencephalic nucleus
Second order: Ipsilateral, dorsal horn of spinal cord, ipsilateral side of medulla, or ipsilateral pons
*Cross the midline to the contralateral side
Third order: In contralateral thalamic nuclei
- Ventral posterior lateral nucleus
- Ventral posterior medial nucleus
*Projects to primary somatosensory cortex
What are the main sensory fiber types and what does each detect?
A alpha (I)- Proprioception
- Large diameter, myelinated fibers
A beta (II)- Fine Touch
- Large diameter, myelinated fibers
A delta (III)- Pain and temperature
- Small diameter, lightly myelinated fibers
C (IV)- Pain and temperature
- Small diameter, unmyelinated fibers
What are the 3 main types of ascending sensory systems? What type of info does each type carry?
- Dorsal Column- Medial Leminscus
- Discriminative touch, vibration, proprioception
-
Spinthalamic/ Spinoreticular/ Spinomesencephalic
- Crude touch, pain and temperature
-
Spinocerebellar
- Proprioception
*Proprioception is somewhat conscious but some is also unconscious
What are the three major divisions of the somatosensory cortex?
-
Primary somatosensory (SI)
- In the postcentral gyrus
- You are able to feel touch but won’t know the significance of touch
-
Seconday Somatosensory (SII)
- Superior bank of the lateral fissure
- Gives the significance of touch
-
Posterior Parietal Association
- Broadmann’s area 5 and 7
Explain the sensory homunculus theory.
- Primary somatosensory cortex is “topographically” organized
- The amount of cortex devoted to each body part is proportional to its level of sensory acuity
What 5 components are necessary for conscious awareness?
- Stimulus has occurred
- Stimulus quality
- Stimulus location
- Stimulus intensity
- Stimulus duration
What Brodmann’s area contributes to the primary somatosensory cortex (SI)?
-
Area 3a
- Receives input from proprioceptors in muscles
-
Area 3b
- Mostly receives cutaneous input from slowly and rapidly adapting receptors
-
Area 1
- Receives input from rapidly adapting cutaneous mechano-receptors
-
Area 2
- Mostly receives input from joints and deep tissues
Brodmann’s area 3a receives input from_________
Proprioceptors in muscles
Brodmann’s area 3b receives input from_________
Cutaneous input from slowly and rapidly adapting receptors
Brodmann’s area 1 receives input from_________
Rapidly adapting cutaneous mechano-receptors (fine touch)
Brodmann’s area 2 receives input from_________
Joints and deep tissues (plays a role in proprioception)
Secondary somatosensory cortex (SII) is innervatd by _________.
Neurons from SI
The seconday somatosensory cortex projects to ________
Insula cortex
The secondary somatosensory cortex is believed to involved in…
- Self-awareness
- Emotion and psychopathology
- Perception and Cognition
The insula innervates regions of the ______ lobe believed to be important for tactile memory.
temporal
Which division of the somatosensory cortex is critical for higher order processing?
Posterior Parietal Cortex
*Brodmann’s areas 5 and 7
The circuitry necessary for discimative somesthesis must be sent to the _______________ for the meaning of the stimulus to be appreciated.
Posterior parietal association cortex
Stereognosis
The ability to visualize and know what object is by tactile information only
*Requires an intact posterior parietal cortex
Graphesthesia
Know what someone is drawing on skin without looking at it
*Requires an intact posterior parietal cortex
