NB7-1 - Somatosensory System and DLAs

Question 1

List the cutaneous mechanoreceptors and describe their locations, morphologies, and what they detect.

Answer 1

• Meissner’s Corpuscle - encapsulated nerve endings near the surface of the skin which detect fine touch and low frequency (~50 Hz) vibrations
• Merkel Cells - cells associated with nerve endings near the surface of the skin which detect very fine touch
• Pacinian Corpuscle - densely encapuslated nerve endings deeper in the skin which detect deep pressure and higher frequency (~250 Hz) vibrations
• Ruffini Endings - encapsulated and enlarged dendrites of nerve endings deeper in the skin which detect skin stretch.
• Hair Follicles - the roots are surrounded by nerve plexuses which can sense hair movement
• There are also thermoreceptors and nociceptors for temperature and pain sensation, respectively.

Question 2

What are proprioceptors? Where are they located and how do they work?

Answer 2

There are two man types of proprioceptors but all are mechanoreceptors:

Golgi Tendon Organ - located at the origin and insertion of skeletal muscle and detects changes in muscle tension

Muscle Spindle - located within the belly of skeletal muscle and detects changes in muscle length

There are also proprioceptors found within joint angles. Sensory input from all of these receptors informs the brain about the position of a limb in space.

Question 3

How do neurons convey information about stimulus intensity? For example, how would a Pacinian corpuscle signal the difference between deep pressure and really deep pressure being applied to the skin?

Answer 3

Stimulus intensity is signaled via AP frequency. The more intense the stimulus, the more rapidly the receptors trigger AP.

Question 4

List the types of adapting receptors, what they do, and the reason receptor adapatation exists.

Answer 4

Receptor adaptations exists so that neurons don’t continuous signal when a stimulus is unchanging (ie - moderately cold water doesn’t feel cold forever). There are two types of adapting receptors:

• Tonic Receptors - slowly adapting receptors that will steadily decrease the frequency of APs fired in response to an unchanging stimulus
• Phasic Receptor - rapidly adapting receptors that will actually stop firing APs for a time in response to an unchanging stimulus

Question 5

Which of the cutaneous mechanoreceptors are considered to be rapidly or slowly adapting?

Answer 5

Meissner’s and Pacinian Corpuscles are rapidly adapting

Merke’s disk and Ruffini’s endings are slowly adapting

Question 6

What is a spatial receptive field and describe how field differ across the cutaneous mechanoreceptors.

Answer 6

A region of sensory space in which a stimulus will influence sensory neuron firing.

Meissner’s corpuscles and Merkel cells have small receptive fields while Pacinian corpuscles and Ruffini endings have large receptive fields

Question 7

What determine the spatial resolution of a skin area?

Answer 7

1. The types of recptors in that region
2. The density of those receptors in that region
3. The receptive field size of each of those receptors

Question 8

Answer 8

A

Question 9

Name the four different classes of primary afferent fibers, the types of neurons found within each class, what these neurons do, and the level of myelination found within each neuron.

Answer 9

• Aα - include Type Ia (muscle spindle) and Type Ib (golgi tendon) fibers which detect changes in muscle length and tension, respectively. These are the largest diameter and most heavily myelinated primary afferent fibers
• Aβ - include Type II fibers which are non adapting stretch receptors that also innervate the muslce spindle and provide proprioceptive information when the limbs aren’t moving. These are the second largest diameter and second most myelinated primary afferent fibers
• Aδ - include Type III fibers which transmit acute sharp pain and temperature information. These are the third largetst diameter and third most myelinated primary afferent fiber
• C - include Type IV fibers which transmit information about more intense, slow, and deep pain and termperature. These fibers are the smalles diameter primary afferent fiber and are unmyelinated

Question 10

How many pair of spinal nerves are there and from which sections of the spinal cord do they arise?

Answer 10

31 pairs

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

Question 11

Mentally run through the dermatome map with particular focus on the key areas.

Answer 11

Question 12

What are the key nerve distributions we need to know?

Answer 12

Question 13

What is shingles, where does it reside, and how does it function?

Answer 13

Shingles is the varicella-zoster virus (chicken pox) that lies dormant in the DRG after childhood. During times of stress or weakened immunocompetence, the virus can be reactivated. The virus will lower the AP threshold of the sensory fibers in its particular DRG causing constant burning pain or a stabbing sensation and eventually leading to skin inflammation and blistering in the corresponding dermatomal segment

Question 14

Answer 14

G

Question 15

What is the touch pathway? What else is it called? Describe the route of the pathway including the names of the various tracts, nuclei, and neuron classes.

Answer 15

The Touch Pathway, aka Dorsal Column/Medial Lemniscus Pathway, is the major route by which discriminatory touch (touch that can discriminate between two nearby points), vibration, and proprioceptive information travels to the brain. The primary afferent neuron axons, with their cell bodies in the DRG, enter the spinal column via the ipsilateral dorsal roots. Once in the column, the majority of the axons enter the dorsal column and ascend to the medulla via the fasiculi gracilis (lower limbs) & cuneatus (upper limbs). Some axons synapse within the grey matter of the cloumn for spinal reflexes. In the caudal medulla, the primary afferent neurons will synapse onto secondary somatosensory neurons which will decussate and continue on to the thalamus via the medial lemniscus​ where they will synapse onto tertiary somatosensory neurons which will travel to the postcentral gyrus (S1) via the internal capsule.

Question 16

At what nucleus in the thalamus do the secondary neurons of the touch pathway synapse?

Answer 16

The secondary somatosensory neurons from the neck, trunk, and extremities will synapse at the ventral posterolateral nucleus (VPL) while the neurons from the head will synapse at the ventral posteromedial nucleus (VPM).

Question 17

Why do synapses occur along the touch pathway? Why don’t the neurons just travel directly to the post-central gyrus?

Answer 17

Each synapse is an opportunity for the nervous system to alter the signal. It will do this by having other neurons also synapse there. This helps to improve the contrast of the signal.

Question 18

What is the pain pathway? What else is it called? Describe the route of the pathway including the names of the various tracts, nuclei, and neuron classes.

Answer 18

The pain pathway, aka antero-lateral or spinothalamic pathway, is the major route by which pain, temperature, and crude touch information travels to the brain. The primary afferent neurons, with their somas in the DRG, enter the spinal column via the ipsilateral dorsal root and synapse onto secondary pain neurons within the substantia gelatinosa in the dorsal horn. The secondary neurons then decussate over to one of the contralateral spinothalamic tracts which are either anterior to or lateral to the ventral horn (hence antero-lateral pathway) where they ascend to the ventral posteriolateral (VPL) thalamic nucleus. In the midbrain, they give off collateral axons to the reticular formation which are involved with emotive pain and the analgesic pathways. At the VPL nucleus of the thalamus, the seconday fibers synapse onto tertiary pain neurons which will travel to the post-central gyrus (S1) via the internal capsule.

Question 19

Answer 19

E

Question 20

How do the touch and pain pathways for the face differ from the rest of the body?

Answer 20

All touch and pain info from the face comes from the trigeminal nerve (CNV). Both touch and pain fibers enter the brainstem at the potine level (not the medullary level). The touch fibers will immediately decussate while the pain fibers will descend through the spinal trigeminal tract into the medulla and synapse at the spinal trigeminal nucleus. The secondary fibers will then decussate. Both types of fibers will then ascend to the thalamus via the trigeminal lemniscus (not the medial lemniscus or spinothalamic tract). These fibers will synapse onto tertiary neurons at the ventral posteromedial (VPM) nucleus of the thalamus (not the VPL) which will then travel to the post-central gyrus via the internal capsule.

Question 21

What is Lissauer’s tract and why is it significant?

Answer 21

Lissauer’s Tract is essentially an alternative pathway for pain information to get to the brain. The primary pain fibers, upon entering the dorsal horn, will give off collateral axons that will ascend or descend one to two spinal segments and then synapse onto secondary pain fibers. This is clinically significant because a lesion within the spinal column that would normally block pain sensation from a region can be avoided via this pathway.

Question 22

What is Brown Sequard Syndrome? What usually causes it? Describe how it affects the sensory pathways.

Answer 22

Brown Sequard Syndrome is a lesion to one half of the spinal cord (hemisection), usually at only one or two spinal levels and typically caused by trauma. It will present with ipsilateral touch, vibration, and proprioception (touch pathway) sensory loss and with ipsilateral pain, and temperature (pain pathway) loss at the level of the lesion. However, contralateral sensations will be intact (pain/temp won’t be affected because of lissauer’s tract). Below the level of the lesion, the touch pathway will be lost but the pain pathway will remain intact (due to decussation). However, contralateral sensation below the level of the lesion, will remain intact for the touch pathway but will be lost for the pain pathway beginning about 2 levels below the lesion. Refer to image

Question 23

What is Syringomyelia? What usually causes it? Describe how it presents and why it presents in that way.

Answer 23

Syringomyelia is a fluid filled cavity (aka cyst or syrinx) within the central canal, most commonly in the cervical cord. It progressively expands and often compresses the anterior white commisure and ventral horns. Typically caused by congenital malformation, infections, or trauma. The touch pathway is bilaterally unaffected by syringomyelia, at all levels, because it does not decussate in the spinal cord. At the level of the lesion, excluding the two top and bottom levels (due to Lissauer’s tract) the pain pathway is disrupted. Below the level of the lesion, the pain pathway is unaffected because the fibers have already decussated by the time they ascend to the lesioned levels. Typically presents with bilateral paralysis of upper extremities (ventral horn compression) and a cape-like (along the dorsum) distribution of sensory loss.

Question 24

What causes phantom limb syndrome? What are the larger implications of this cause?

Answer 24

Cortical plasticity - when a particular region of the sensory homunculus stops receiving input (due to an amputation), nearby neurons may begin synapsing on that region. This is what triggers phantom sensations (including pain). If this is true, neuronal rewiring can occur much later in life than previously thought.