Lecture 2: Proprioception and Tactile Pathways

Question 1

What are the 4 pathways which transmit different types of somatic sensations (i.e., touch, vibration, proprioception, etc..)?

Answer 1

1. Posterior column-medial lemniscal
2. Trigeminolthalamic pathways
3. Spinocerebellar pathways
4. Anterolateral system

Question 2

What is the primary pathway that transmits discriminative touch, flutter-vibration, and proprioceptive information?

Answer 2

Posterior column-medial lemniscal pathway

Question 3

Size, shape, and texture discrimination, recognition of 3-D shape (stereognosis), motion detection, concious awarness of body positon (proprioception) and limb movement (kinesthesia) involve which system?

Answer 3

Posterior column-medial lemniscal system (PCMLS)

Question 4

What are the characteristic features in terms of afferent fiber conduction, synaptic relays, and somatotopic organization of the PCMLS?

Answer 4

• Afferent fibers with fast conduction velocities and limited number of synaptic relays
• Precise somatotopic organization
• Basis for the accurate localization of touch on different parts of the body, with high fidelity and a high degree of spatial/temporal resolution

Question 5

What is frequency vs. population coding?

Answer 5

Frequency = cell’s firing rate signals stimulus intensity or temporal aspects of the tactile stimulus

Population = distribution in time/space of the number of activated cells signals location of the stimulus as well as its motion/direction

Question 6

The high degree of resolution of the PCMLS results from what; sharpening discrimination between separate points on the skin is critical for?

Answer 6

• Inhibitory mechanisms such as: feed-forward, feedback, and lateral (surround) inhibition
• Critical for two-point discrimination

Question 7

Why does two-point discrimination vary widely over different parts of the body?

Answer 7

Related to the density of peripheral nerve endings

Question 8

If the mechanical pressure applied to peripheral mechanoreceptors is great enough what will occur?

Answer 8

• Mechanical pressure is transduced into an electrical signal by the primary afferent neuron
• If this depolarizes the neuron to threshold, an action potential is produced and relayed to the CNS via PCMLS
• Somatic sensations of touch are evoked

Question 9

What is the tactile receptor density of the digits and perioral region vs. other regions, such as the back?

Answer 9

• Digits and perioral regions have increased density of tactile receptors
• Other regions, like the back have decreased density

*Accuracy with which a tactile stimulus is localized depends on receptor density and receptive field size

Question 10

What is a receptive field and where are there small and large receptive fields; how does this affect sensory discrimination?

Answer 10

• Area of skin innervated by branches of a somatic afferent fiber
• Small receptive fields are found in areas such as fingertips, where receptor density is high = increased discrimination
• Large receptive fields are present in areas with low receptor density (i.e., the back) = decreased discrimination

Question 11

What is the relationship like between the size of the receptive field and representation of that body part in the somatosensory cortex?

Answer 11

• Inverse relationship
• Small receptive fields = larger representation
• Densely innervated body parts are represented by greater number of neurons —> Take up a disproportionally large part of the somatosensory cortex
• Fingertips and lips provide CNS w/ most specific and detailed info about tactile stimulus

Question 12

Primary afferent fibers consist of what 3 parts and what are their locations?

Answer 12

1. A peripheral process extending from the DRG (mechanoreceptor or free nerve ending)
2. A central process extending from DRG into CNS
3. A pseudounipolar cell body in the DRG

Question 13

Large-diameter primary sensory fibers relay what type of information; these fibers enter the spinal cord via?

Answer 13

• Discriminative touch, flutter-vibration and proprioception
• Enter the spinal cord via the medial division of the posterior root and then branch

Question 14

After large-diameter primary sensory fibers enter the spinal cord via the medial division of the posterior root and branch what occurs to these branches; especially the largest set of branches?

Answer 14

• One set of branches terminates on second-order neurons in the spinal cord gray matter at, above, and below the level of entry
• Largest set of branches ascends cranially and contributes to the formation of the fasciculus gracilus and fasciculus cuneatus —> collectively termed the posterior columns

Question 15

How are sacral level fibers up to T6 positioned topographically within the posterior columns; what contribution of posterior column do these fibers ultimately form?

Answer 15

• Sacral level fibers are positioned medially
• Fibers from more rostral levels (up to T6) are added laterally
• Forms the fasciculus gracilus

Question 16

How are fibers above T6 and cervical fibers organized topographically in the posterior columns and what do they ultimately form?

Answer 16

• Thoracic level above T6 will be more medial
• Low cervical will be lateral to this and upper cervicals will be most lateral
• Form the laterally placed, fasciculus cuneatus

Question 17

How do SC lesions affect discriminative, positional, and vibratory tactile sensations?

Answer 17

• Ipsilateral reduction or loss
• At and below the segmental level of injury

Question 18

SC lesions resulting in ipsilateral reduction or loss of discriminative, postional, and vibratory tactile senations at or below the segmental level of injury will present how?

Answer 18

• Sensory ataxia, loss of muscle stretch (tendon) reflexes and proprioceptive losses from the extremities due to lack of sensory input
• Patient may also have a wide-based stance and may place the feet to the floor with force, in an effort to create the missing proprioceptive input

Question 19

What are the 2 posterior column nuclei, where are they located, what type of neurons do they contain, what type of input do they receive?

Answer 19

• Nucleus gracilus and nucleus cuneatus are in posterior medulla

- Contain second-order neurons of the PCMLS

• Receive input from first-order neurons (primary afferents) from the ipsilateral DRG

Question 20

How do core “clusters” and outer “shells” of posterior column nuclei (nucleus gracilus and nucleus cuneatus) contribute to the segregation of tactile inputs?

Answer 20

• Core “clusters” receive inputs from rapidly and slowly adapting afferents
• Outer “shells” receive inputs from muscle spindles, joints, and Pacinian corpuscles

Question 21

Explain the route of the axons from the second-order neurons of the posterior column as they ascend, where do they decussate, what do they ascend as and what happens to the fiber?

Answer 21

• The axons of second-order sensory neurons of the posterior column are known as internal arcuate fibers, which loop anteromedially in medulla
• Cross the midline as the sensory decussation, causing the axons of the nuclei to be sent to the contralateral thalamus
• Ascend as the medial lemniscus (ML) on the opposite side
• As ML extends rostrally, it rotates laterally in pons. UE fibers are now medial and LE fibers are now lateral

Question 22

Somatotopic organization of the body is maintained as the medial lemniscus terminates where?

Answer 22

Ventral posterolateral nucleus (VPL) of the thalamus

Question 23

PC lesions at brainstem levels lead to deficits in; on what side?

Answer 23

• Deficits in discriminative touch, vibratory, and positional sensibilities
• Over the CONTRALATERAL side of the body from the lesion

Question 24

What are the 2 ventral posterior nuclei, what separates them, and which is specific for the information from the head and the body?

Answer 24

• Ventral posterolateral nucleus (VPL) receives body proprioceptive info
• Ventral posteromedial nucleus (VPM) receives head prorioceptive information
• Separated by fibers of the arcuate lamina

Question 25

The VPL and VPM are supplied by which arterial branches and what does compromise of this artery lead to?

Answer 25

- **Thalamogeniculate** branches of **Posterior Cerebral artery** - Compromise can result in loss of all tactile sensation over the **contralateral** body and head

Question 26

How do fibers from the contralateral nucleus cuneatus terminate in the VPL versus fibers of the nucleus gracilus?

Answer 26

- Fibers from the contralateral nucleus cuneatus terminate **medial** to those of nucleus gracilis - Remember that the nucleus cuneatus started lateral and the gracilis was medial, but as ML extends rostrally, it rotates laterally in the pons and the fibers switch postions

Question 27

How do rapid/slowly adapting inputs target the VPL vs. Pacinian and joint/muscle inputs?

Answer 27

- Rapidly and slow adapting inputs target the VPL core - Pacinian and joint/muscle inputs target VPL shell

Question 28

The VPL for the trunk and extremities contains 2 populations of identified neurons, what are their functions?

Answer 28

1. **Third-order neurons:** large-diameter axons that traverse posterior limb of the **internal capsule** and terminate in the **primary (S1) and secondary (SII) somatosensory cortices** 2. **Local circuit interneurons** (inhibitory): receive excitatory corticothalamic inputs and influence the firing rates of third-order neurons

Question 29

Axons from third-order neurons (thalamus) terminate in primary somatosensory (S1) cortex, which comprises which gyri and sulci?

Answer 29

- Postcentral gyrus and posterior paracentral gyrus - Bordered by central sulcus (anteriorly) and postcentral sulcus (posteriorly)

Question 30

Which 2 arteries provide blood supply to the SI cortical areas; which artery if compromised produces loss over upper body/face and which for lower limb?

Answer 30

- The ACA and MCA - MCA lesions = tactile loss over **contralateral** upper body and face - ACA lesions = tactile loss over **contralateral** lower limb

Question 31

What are the functional subdivisions of the primary somatosensory cortex (S1) from anterior to posterior?

Answer 31

- **Broadmann areas: 3a, 3b, 1 and 2** - **Area 3a:** located in depths of the central sulcus, abuts area 4 (primary motor cortex) - **Areas 3b and 1:** extend up the bank of the sulcus onto the shoulder of postcentral gyrus - **Area 2:** lies on the gyral surface and abuts area 5 (somatosensory association cortex)

Question 32

Lesions in subdivision area 1, area 2, and area 3b of the somatosensory cortex would produce what kind of deficits; which is the most profound and what does this suggest?

Answer 32

- Lesions in **area 1** produce a deficit in **texture discrimination** - Lesions in **area 2** result in a loss of **size and shape discrimination (astereognosis)** - Lesions in **area 3b** have a more **profound effect** ---\> deficits in **both** **texture and size/shape discrimination** - Suggests that there is **hieracrchial processing** of tactile information in SI cortex and that area 3b performs initial processing and distributes information to areas 1 and 2

Question 33

Typically, SI lesions include larger areas, encompassing mutliple subdivision and result in what kind of deficits?

Answer 33

Loss of proprioception, position sense, vibratory sense, and pain/thermal sensations on the **contralateral** side of the body

Question 34

How is it possible to have cross sensory syndrome/findings with brainstem lesions?

Answer 34

- Sensory deficits of trunk/extremities contralateral to lesion, but sensory deficits of face/CN ipsilateral to lesion - The **right face** and **left arm/leg** lack proprioceptive info - All dependent on where the fibers cross and damage occuring at one level will likely damage multiple tracts.

Question 35

Where is the Secondary Somatosensory (SII) cortex, what does it contain, and receives inputs from where?

Answer 35

- Lies **deep** in the **inner face** of the **upper bank** of **lateral sulcus** - Contains somatotopically representation of body surface - Inputs arise from **ipsilateral SI cortex and Ventral Posterior Inferior (VPI) of the thalamus**

Question 36

What inputs does the Parietal cortical somatosensory region receive, where is it located?

Answer 36

- Posterior to area 2, includes **area 5 and area 7 (7b)** - Receives some medial lemniscal input and inputs from SI (tactile inputs)

Question 37

Lesions in the parietal association area can produce what deficits?

Answer 37

- **Agnosia** - Contralateral body parts are lost from the personal body map - **Sensation** is **not radically altered**, but the **limb is not recognized** as part of the patient's own body

Question 38

The spinocerebellar pathways transmit what kind of information?

Answer 38

- Proprioceptive and limited cutaneous info to the cerebellum - Includes information about limb position, joint angles, and muscle tension/length