Somatosensation and Principles of Sensory Transduction

What are the four attributes of sensation

1) Modality-type of stimulus
→ the type of the stimulus such as touch, temperature, proprioception, etc

2) Duration-timing on/off
→ how long the stimulus is encountered for

3) Intensity-amount of stimulus
→ the amount of stimulus that you receive

4) location on body or in space
→ location of the stimulus on the body or in the surrounding space

What is the organization of somatosensory neurons and what are their modalities?

Dorsal Root Ganglion
→ nerves will have their cell bodies located in the dorsal root ganglion and will immediately bifurcate into two axonal branches
→ peripheral branch goes out into the periphery
→ central axon goes into the dorsal horn of the spinal cord

Modalities:
→ Touch (crude and two-point discrimination)
→ Proprioception
→ Vibration
→ Pain
_> Temperature

What are the six different somatosensory receptors found in the skin?

Somatosensory Receptors in the skin can be subdivided into encapsulated and nonencapsulated
Encapsulated - nerve endings are surrounded in connective tissue, encapsulation may help to open the mechanosensitive ion channels
→ Pacinian Corpuscle - vibration and deep pressure
→ Meissner Corpuscle - light touch
→ Ruffini Ending - stretch and pressure

Nonencapsulated - nerve endings are bare
→ Merkel Endings - light touch/texture
→ Free Nerve Endings - pain, temperature, itch, etc - have dedicated cells depending on what the modality is
→ Endings around Hair

How do mechanoreceptors transduct and create action potentials?

Mechanoreceptors will create action potentials that will send signals down the peripheral axons of the somatosensory nerves

1) These cells contain mechanosensitive ion channels that open in response to pressure or touch
→ stimulus opens these channels leading to sodium entering into the cell causing depolarization known as the receptor potential
→ this action potential will travel to the axon and lead to activation of voltage gated sodium and potassium channels - sending the signal down to the dorsal root ganglion
→ eventually leads to glutamate release from the central axon branch

What are the three kinds of free nerve endings and how do they function

Thermoreceptors - responsible for detecting innocuous or painless temperatures such as warm and cool temps
→ express thermosensitive ion channels at their endings and are activated by changes in temperature

Pruriceptors - detect itches
→ immune cells will secrete molecules such as histamine activating chemo sensitive ion channels

Nociceptors - pain receptors that detect burning heat, freezing cold, painful chemicals, and mechanical pain
→ Nociceptors have a different cell type based on the individual four modalities

What is adaptation in our sensory systems?

All sensory systems except for pain undergo adaptation, where a sensory receptor becomes less responsive to constant, unchanging stimulus over time

1) Patients will have a rapid burst of action potentials during a constant stimulus with an eventual decrease

2) Allows for increase in dynamic range or the range of stimulus intensities a given receptor can respond to
→ also allows us to quickly adapt when there is an increase in stimulus intensity

3) Action potentials determine our tactile perception (sense of touch)

What are the rapid adapting vs slowly adapting receptors?

Rapid - Meissner and Pacinian

Slow - Merkel and Ruffini Endings

What is the receptive field and density of somatosensory receptors?

Receptive field for somatosensory receptors is the dermatome where stimulation produces a response in that receptor
→ neurons that are located deeper in the skin have a larger receptive fields than those closer to the epidermal surface

Density refers to how many sensory receptors are packed into an area of skin
→ the density is determined by two point discrimination with areas with high density having a much higher two point discrimination than areas with a lower density

What are proprioceptors?

Proprioceptors are mechanoreceptor cells of the muscles, tendons and joints and are involved in proprioception or our orientation in space
→ also respond to stretch and tension in muscles and tendons
→ contribute to spinal reflexes

There are two major proprioceptors the muscle spindle fibers and golgi tendon organs

What are gamma motor neurons?

Gamma motor neurons are a specialized lower motor neuron that help to control the sensitivity of muscle spindles

1) When extrafusal muscle fibers contract and shorten the whole muscle will decrease in length
→ without gamma neurons, these intrafusal fibers would also contract at their ends and become slack and unresponsive
→ the gamma neurons help to keep the muscle spindle fibers taut in order to keep them sensitive to stretch

What is the Labeled Line Concept?

Labeled Line Concept states that each sensory nerve fiber is labeled for one specific type of information from the periphery to the brain

1) no matter where that nerve fiber is stimulated or by whatever stimulus it will always interpret that stimulus as the single information that nerve fiber carries
→ for example, a nerve fiber for the visual system will always interpret activity as light even if its stimulated by pressure

What are the large myelinated, small myelinated, and unmyelinated axons?

Large Myelinated
→ Ia/Ib - proprioceptive types
→ A-beta and A-alpha - skin touch

Small Myelinated
→ A-delta fibers - temperature and pain

Unmyelinated
→ C - pain

What are the two sensory pathways to the thalamus?

1) Dorsal Column/Medial Lemniscus
→ conveys fine touch, proprioception and vibration
→ enters at the dorsal column, travels up through the fasciculus gracilis (medial, lower body) or the fasciculus cuneatus (lateral, upper body) before synapsing at the nucleus gracilus and cuneatus
→ Decussate and form the medial lemniscus before synapsing at the ventral posterolateral nucleus
→ project to the posterior limb of the internal capsule and then to the somatosensory cortex

2) Spinothalamic Pathway
→ conveys pain and temperature
→ enters at the dorsal horn where they synapse and then axons decussate at the anterior white commissure in the spinal cord
→ ascend to the VPL where they synapse and then project to the somatosensory cortex

How does the trigeminal nerve relay sensory information from the face?

Trigeminal nerve will transmit sensory to the trigeminal ganglion before splitting sensory info between three nuclei before relaying to the ventral posterior medial of the thalamus

1) Mesencephalic Nuclei (midbrain)
→ proprioception of the face such as jaw position

2) Principal or Chief Sensory Nuclei
→ fine touch and vibration of the face

3) Spinal Trigeminal Nucleus
→ pain and temperature from the face

What is the transmission of sensory information from the primary somatosensory cortex to the higher-order somatosensory areas

1) Primary Somatosensory Cortex (SI)
→ receives information from the 3rd order neurons of the thalamus via the cortical column layer IV. Subdivided into Brodmann areas that carry different modalities of sensation
→ Area 3b - touch, pain and temp
→ Area 3a - conscious proprioception
→ Area 1 - Texture
→ Area 2 - size and shape

2) Info is then sent to the Secondary Somatosensory Cortex
→ integrates information from both sides of the body and helps us undergo object recognition

3) Info is then projected to the Posterior Parietal Cortex
→ integrates visual and vestibular input along with our somatosensation to create spatial awareness

What are the Cortical Columns in the SI?

Cortical columns are vertical organization of neurons in the cerebral cortex and help to organize the SI or Primary Somatosensory Cortex

→ Layer IV receives input from the thalamus before sending information to layers II and III
→ Layer II/III processes info before sending info to V and VI
→ V and VI are the major outputs to the thalamus and other regions of the brain - create a feedback loop that modulates sensory input

What is the Somatotopic Map of the Primary Somatosensory Cortex?

legs medial, face lateral, arms in between

What is Synthesesia?

Exposure to one type of sensory info leads to activation of another unrelated sensory pathway

1) common type is the graphene color synthesia
→ seeing numbers causes people to see them in color