Module 25 - Neurophysiology of TOUCH

Question 1

What is a somatic sensation?

Answer 1

Somatic sensation originates from the activity of afferent nerve fibers whose peripheral processes ramify within the skin

Question 2

Where do the cell’s bodies of the afferent somatic sensation fibers reside?

Answer 2

They reside in ganglia that lie alongside the spinal cord (DRG) and brain stem. They are a part of the PNS.

Question 3

The PCML system transmits information about what type of sensations?

Answer 3

Discriminative touch
Pressure
Vibration
Proprioception

Question 4

What is the somatotopic arrangement of the PCML pathway in the spinal cord?

Answer 4

Look at image.

Question 5

What are the two pathways that supply the brain with information about sensation?

Answer 5

1. Posterior (dorsal) column medial lemniscus pathway
2. Anterolateral system
   
   • Spinothalamic tract
   • Spinoreticular tract
   • Pniomesencephalic tract

Question 6

What is the blood supply to the PCML pathway?

Answer 6

The posterior spinal arteries supply the posterior 1/3 of the spinal cord, which includes the PCML system

Question 7

Describe/draw the route of the PCML pathway.

Answer 7

• The PCML system travels up the spinal cord within the posterior column
• SPINAL CORD = Axons enter the spinal cord from the spinal ganglion and pass directly to the ipsilateral posterior column. Here, rostral fibers (above T6) entre fasciculus cuneatus (lateral) to ascend. Caudal fibers (below T6) enter fasciculus gracilis (medial) to ascend.
• CAUDAL MEDULLA = Primary axons terminate in their specific nuclei located in the caudal medulla: the nucleus gracilis (caudal fibers) or nucleus cuneatus (rostral fibers). From these nuclei, axons of secondary neurons cross the midline as internal arcuate fibers and then form the medial lemniscus.
• ROSTRAL MEDULLA = In the rostral medulla, the fibers travel as the medial lemniscus adjacent to the midline.
• PONS = In the caudal pons, the medial lemniscus flattens in a medial-lateral orientation
• MIDBRAIN = As the medial lemniscus continues to ascend through the rostral pons and midbrain, it moves laterally and extends in an anterior-posterior orientation.
• THALAMUS =The medial lemniscus terminates in the VPL of the thalamus. From the thalamus, fibers project to and terminate in the primary somatosensory cortex.

Question 8

What is sensory transduction?

Answer 8

The process of converting the energy of a stimulus into an electrical signal so that our brain can “understand” it.

Question 9

What are the steps of sensory transduction?

Answer 9

• A stimulus alters the permeability of cation channels in the afferent nerve endings
• This generates a depolarizing current = receptor potential
• If sufficient in magnitude, the receptor potential reaches the threshold for the generation of action potentials in the afferent fiber
• The action potentials fire in a rate that is proportional to the magnitude of the depolarization

Question 10

What are the 4 types of mechanoreceptors for nonpainful stimuli?

Answer 10

• Meissner corpuscle
• Merkel cell - neurite complex
• Ruffini endings
• Pacinian

Question 11

What are the two types of afferent nerves and what distinguishes their “endings”?

Answer 11

• Free nerve endings = pain
• Encapsulated endings = nonpainful sensation
  
  • Surrounded by specialized receptor cells → mechanoreceptors

Question 12

List 3 common properties of mechanoreceptors.

Answer 12

• Force (“mechano”) produces opening of Na+ channels = transduction (therefore, mechanoreceptors are all capable of transduction)
• IF there is an adequate stimulus → depolarization occurs
• NO spontaneous activity
  
  • AP’s only produced when the adequate stimulus is present.

Question 13

In general, how do we feel things?

Answer 13

The process of converting the energy of a stimulus into an electrical signal so that our brain can “understand” it.

• A stimulus alters the permeability of cation channels in the afferent nerve endings = mechanoreceptors
• This generates a depolarizing current = receptor potential
• If sufficient in magnitude, the receptor potential reaches the threshold for the generation of action potentials in the afferent fiber
• The action potentials fire in a rate that is proportional to the magnitude of the depolarization

Question 14

What are the 4 distinct functional properties that categorize how mechanoreceptors respond to stimuli?

Answer 14

• Axon diameter
  
  • Axon diameter determines conduction speed
• Receptive field size
  
  • The area of the skin surface over which stimulation results in a significant rate of action potentials
    
    • It is a functional property which allows you (your brain) to determine the location of the stimulus
    • Every sensory neuron has a “receptive field”: the region of the skin that influences that neuron
• Temporal dynamics of response
  
  • Some afferents fire rapidly; others generate sustained discharge
• Quality
  
  • Respond to mechanical change vs temperature change

Question 15

How are receptor fields of 1st order afferents different?

Answer 15

• The “size” of the receptive field depends on how widespread the branching of its terminals are
• A small receptive field: more precise information about the location
• Large receptive field: can’t give accurate information about where the stimulus is within this field.

Question 16

What is spatial acuity? What does it depend on?

Answer 16

Ability to distinguish different points on the skin
Dependent on
Innervation density = how many receptors in the area
The receptive field size of the receptor

Question 17

What is spatial acuity? What does it depend on?

Answer 17

• Ability to distinguish different points on the skin
• Dependent on
  
  • Innervation density = how many receptors in the area
  • The receptive field size of the receptor

Question 18

What is two-point discrimination?

Answer 18

• A test that we often use to look and see how well the nerves are working within your skin.
• It is also defined as at what point can we tell that there are two stimuli versus one.
• Some places the two-point discrimination is smaller in some areas, others it is larger
• If you take a look at the image of the hand, we know that the two-point discrimination in the fingers is in a small distance, that distance gets bigger in the forearm.
• Fingers = two-point discrimination is LOW
• Upper arm and leg = two-point discrimination is LARGER

Question 19

Explain what it means that receptors differ in their ability to sustain discharge.

Answer 19

• Slower adapting receptors = Merkel Ruffini = STATIC→ they respond throughout the stimulus. This means that if you were to hit yourself with the hammer and leave the hammer on the skin, these Merkel Ruffini receptors would keep firing and they would let you know that the hammer is still sitting there.
• Rapidly adapting receptors = Meissner Pacinian = DYNAMIC → they quickly fire when you get hit by the hammer and they quickly fire again when the hammer comes off from the skin. They respond at the onset and offset of the stimulus.
• Both of these receptors allows for information to be obtained about static (slowly adapting) and dynamic (rapidly adapting) aspects of a stimulus.

Question 20

What are some of the features that characterize the receptive fields of the Meissner’s corpuscle?

Answer 20

• Rapid adapting!!!
• Dense innervation
• 40% of mechanosensory innervation in the hand
• Lie in tips of dermal papillae close to the surface of the skin
• Small receptive field

Question 21

What are some of the features that characterize the receptive fields of the Merkel receptors?

Answer 21

• Slow adapting!!!
• 25 % of mechanosensory innervation in the hand
• Increase number in fingertips
• Lie in the “fingerprint” ridges on the fingers surface
• Small receptive field - highest spatial resolution
• Sensitive to points, curves, and edges (form and texture)

Question 22

What are some of the features that characterize the receptive fields of the Pacinian corpuscles?

Answer 22

• Rapid adapting!!!
• 10-15% of mechanosensory innervation in the hand
• Located deep in the dermis or subcutaneous layer
• Large receptive field - boundaries difficult to define
• Well suited to detect vibration transmitted through objects (relation to tool use)

Question 23

What are some of the features that characterize the receptive fields of the Ruffini corpuscles?

Answer 23

• Slow adapting!!!
• 20% of mechanosensory innervation in the hand
• Located deep in the skin and in ligaments and tendons
• Sensitive to stretching of the skin produced by digit/limb movement
• Aids in localization of finger position (and conformation of the hand)

Question 24

TAKE HOME MESSAGE ABOUT TRANSDUCTION

Answer 24

• Different receptor types respond best to different kinds of stimuli
• Different receptor types have different targets in the CNS so they can contribute to different aspects of feature extraction
• Make up different “channels” of information

Question 25

What is the hierarchy of the sensory system? (Read the answer to this question from bottom to top)

Answer 25

• Convergence from many input neurons on the neurons at each of these levels
  
  • Excitatory and inhibitory interneurons modulating activity at each step
• Receptive fields at the 1st order neuron are easy: AP means stimulus and no AP means no stimulus
• Receptive fields of higher-order neurons depend on incoming signals (APs) and local processing
• A given neuron will preferentially respond to some input features (like size or shape) but not others (like vibration)

Question 26

How does CNS “figure out” a stimulus is moving? Use reading brail as an example to explain.

Answer 26

• The row of receptors on a finger moving across a row of raised braille letters will detect the different stimulus
  
  • Merkel - details
  • Meissner - coarser representation
  • Pacinian & Ruffini - no detail but rather tracking of finger movement and position
• For movement, some neurons will detect orientation like left or right, others will be the base → tip of where you are touching.
• As you go up the hierarchy chain, the more possibilities there are, the more neurons are needed
• The features that get analyzed increased in complexity as you go up the hierarchy (more and more neurons are being recruited because the features are being analyzed at such a high level of complexity that all the neurons need to work together)

Question 27

What are the two keys messages for the implication of the sensory cortex organization and its importance for the ability of detection in movements?

Answer 27

• Map (medial are the legs to lateral where it represents the arms and continues for the face) shows where analysis of a particular body part occurs.
• The amount of brain devoted to a body part is related to tactile acuity
  
  • The lower two-point discrimination, or the smaller amount of skin that we can sense that it is two-point, those areas actually take up more place in the brain. (for example = digits compared to the leg)

Question 28

What are the different functional properties of the Brodmann Areas 1,2, 3a, and 3b?

Answer 28

• Similarities:
• They are all concerned with sensation
• Each area has a medial to the lateral representation of the body
• Functional properties of neurons in each area are distinct (this means that each of these neurons responds to a similar type of sensation)
• Cytoarchitecture is also distinct (these neurons physiologically look the same)
• Areas 3b and 1
  Respond to cutaneous stimuli
  Areas 3a
  Respond to proprioceptors
  Area 2
  Respond to tactile & proprioception

Question 29

Where does the thalamus, in particular, VPL and VPM project its synapses to?

Answer 29

• Diagram = The thalamus (recall that the PCML synapses on the thalamus) will synapse to projections to the Broadman areas –> 3b and areas 1 and 2. (purple box in the image)
• All of these areas are interconnected with one and another.
• From the primary somatosensory cortex –> the information will then go to the secondary somatosensory cortex
• The information is processed at higher levels of complexity as we go up the hierarchy.
• *Amygdala and hippocampus = emotional component of touch (later modules)

Question 30

What happens when there is any type of deficits/lesions in area 3b?

Answer 30

• Extensive deficits in ALL forms of tactile sensation
• Info from cutaneous receptors not passed to areas 1 &2

Question 31

What happens when there is any type of deficits/lesions in area 1 but area 3b is intact?

Answer 31

• Can’t perceive the texture of objects
• Information from 3b intact, so you can perceive touch

Question 32

What happens when there is any type of deficits/lesions in area 2 but area 3b and area 1 are intact?

Answer 32

• Can’t perceive the size and shape of objects
• Information from 3b and 1 intact, so you can perceive touch and texture

Question 33

Where do the Broadman areas 3a,3b, 1, and 2 lay their projections? What are the functional implications of these areas?

Answer 33

• Secondary somatosensory cortex → which then goes to the Amygdala and hippocampus (known as limbic structures) → tactile learning and memory
• Parietal areas 5 and 7 → which then goes to motor and premotor cortical areas → integration of sensory and motor information

Question 34

What are instances you as a clinician can suspect spinal cord injury?

Answer 34

• Unconscious patients
• Head injury and facial fractures
• Falls from heights
• Near drowning
• Pain, deformity or stab near the spinal cord
• Neurological deficit
• Unexplained shock
• Unexplained respiratory difficulty
• Unexplained urinary retention and priapism

Question 35

Why do we use the AIS - Asia impairment scale?

Answer 35

• Used to classify spinal cord injuries which is based on painful and nonpainful sensation.
• It determines the severity and level of injury to the spinal cord
• Can be used as a predictor for recovery = to monitor rehabilitation → often done according to the following guideline
  
  • ER
  • 72 hours post-injury
  • Weekly
• IT GUIDES CARE !!!

Question 36

What are the three components to the AIS exam and the associated tracts to these components?

Answer 36

• Strength testing
• Light touch sensation → PCML
• Pinprick sensation → anterolateral system
  
  • Both the left and right side of the body are tested

Question 37

What are the three components to testing in the AIS exam and how is each component tested?

Answer 37

• Sensory exam
  
  • All 28 dermatomes are tested
  • Light touch & pinprick
• Motor exam
  
  • Muscle power in 10 key myotomes using the MRC grading
    
    • Upper limb = C5 - T1
    • Lower limb = L2 - S1
• Anal rectal exam
  
  • The bulbocavernosus reflex indicates the absence or presence of spinal shock. Spinal shock usually occurs between 24 and 72 hours after spinal injury. Spinal shock is manifested by the absence of bulbocavernosus reflex, hypotension, bradycardia, and complete loss of motor, sensation, and reflexes.

Question 38

What is the difference between complete or incomplete spinal cord injury?

Answer 38

• Complete
  
  • Absent voluntary anal contraction
  • Absent deep anal pressure
  • S4-5 sensory score 0
• Incomplete
  
  • There is some preserved sensation and/or muscle contraction in the lower sacral segments

Question 39

What is the final AIS scale?

Answer 39

Question 40

How is a spinal cord injury classified using the AIS scale?

Answer 40

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