Somatosensory System

Question 1

What does the somatosensory system do?

Answer 1

Facilitates the sensation and perception of
– Discriminative touch
– Vibration
– Proprioception
– Temperature
– Pain

Question 2

Where are sensors located?

Answer 2

Sensors are distributed across the whole body in
– skin
– skeletal muscles
– bones and joints
– epithelial tissue
– internal organs
– respiratory and cardiovascular systems

Question 3

What is the function of the somatosensory system?

Answer 3

• Provides information on peri-personal space and the state of well-being of the body
• Constitutes a major source of patient symptom reporting, especially through pain

Question 4

What is numbness?

Answer 4

Numbness

– Term used by patients to describe sense of heaviness, weakness or deadness in part of the body – needs to be clarified

Question 5

What is parasthesia?

Answer 5

– Abnormal spontaneous sensation (burning, tingling, pins & needles

Question 6

What are the modalities of somatosensory system and their receptors?

Answer 6

The somatosensory system comprises five different modalities: touch, vibration, proprioception, pain and temperature. Whereas most of the somatic sensory modalities refer to sensations of the skin, proprioception refers to sensory receptors originating in the skeleto-muscular system.
All receptors of the somatosensory system are pseudo-unipolar neurons.

Their sensory endings can be found in skin, or in (or close to) the muscle. Their fibers run in peripheral nerves and their cell bodies are located in ganglia (dorsal root ganglia when the fibers run in spinal nerves or in cranial nerve ganglia).

Question 6

What are the modalities of somatosensory system and their receptors?

Answer 6

The somatosensory system comprises five different modalities: touch, vibration, proprioception, pain and temperature. Whereas most of the somatic sensory modalities refer to sensations of the skin, proprioception refers to sensory receptors originating in the skeleto-muscular system.
All receptors of the somatosensory system are pseudo-unipolar neurons.

Their sensory endings can be found in skin, or in (or close to) the muscle. Their fibers run in peripheral nerves and their cell bodies are located in ganglia (dorsal root ganglia when the fibers run in spinal nerves or in cranial nerve ganglia).

Question 7

Explain mechanoreceptors as cutaneous receptors

Answer 7

Cutaneous Receptors
There are three classes of cutaneous receptors: 1) mechanoreceptors, 2) thermoreceptors and 3) nociceptors.

Mechanoreceptors respond to indentation of the skin and can be rapidly adapting (RA) or slowly adapting (SA). RA receptors fire as pressure indents the skin and cease responding when constant pressure is applied. SA receptors, in contrast, respond to initial indentation and continue to respond to steady indentation. Thermoreceptors respond to changes in temperature as well as to specific temperatures. Cold fibers respond best around 30°C while warm receptors respond best around 44°C. Nociceptors are polymodal, i.e. they respond to various stimulus modalities – intense pressure, high heat (about 45°C) or noxious chemicals.

Question 8

Explain touch receptors as cutaneous receptors

Answer 8

Touch is transduced by Merkel’s disks (discriminative touch) and Ruffini’s endings (skin stretch)

Question 9

explain the vibration receptors of cutaneous receptors

Answer 9

Vibration is transduced by Meissner’s corpuscles (highest sensitivity / lowest threshold for lower frequencies of about 50 Hz) and Pacinian corpuscles (highest sensitivity / lowest threshold for higher frequencies of about 300 Hz).

Question 10

Explain the pain receptors as cutaneous receptors

Answer 10

Pain (pricking pain by rapidly adapting mechano-sensitive or thermo-sensitive receptors, burning pain by slowly adapting polymodal receptors) and Temperature (cold receptors and warm receptors) are transduced by free nerve endings

Question 11

What are the 5 modalities and their receptors?

Answer 11

• Touch (discriminative touch)
• Vibration
• Proprioception
Above is dorsal column/medial lemniscus

• Temperature sense
• Nociception
Anterolateral system

Question 12

What receptors have fast adaptation (fast motion of objects)?

Answer 12

Small receptive field- meissner corpuscle- abrupt changes in shape (e.g, in corners)

Large receptive field size- pacinian corpuscle - diffuse vibration (e.g. tapping)

Question 13

What are the slow adapting recepptors?

Answer 13

Small receptor field- Merkel- bumps (touch)

Large receptor field- Ruffini’s endings- skin stretch (e.g., gripping a basketball)

Question 13

What are the slow adapting recepptors?

Answer 13

Small receptor field- Merkel- bumps (touch)

Large receptor field- Ruffini’s endings- skin stretch (e.g., gripping a basketball)

Question 14

What is propriocetion ?

Answer 14

Proprioception (position sense), the remaining modality of the somatosensory system, is mediated by proprioceptors. Muscle spindles and Golgi tendon organs are the main proprioceptors of the musculo-skeletal system.

Question 15

What are muscle spindles?

Answer 15

Muscle spindles are embedded in extrafusal fibers of the muscle. The primary receptor of a muscle spindle is a rapidly adapting receptor with a Ia (read “one a”) afferent fiber. It carries sensory information of muscle stretch. This receptor forms the afferent limb of the myotatic reflex (deep tendon reflex), which will be discussed later during Motor System lectures. The secondary receptor of a muscle spindle is a slowly adapting receptor carrying sensory information of muscle length. It uses a class II (“two”) afferent fiber.

Question 16

What is Golgi tendon organs?

Answer 16

Golgi tendon organs are positioned close to the border between muscle and tendon. Their Ib (read “one b”) afferent fibers form the afferent limb of the reverse myotatic reflex (inverse myotatic reflex).

Note that afferent fibers from these proprioceptors form important pathways to the cerebellum (dorsal and ventral spinocerebellar tracts). Sensory information carried in these tracts contributes to cerebellar functions and does usually not reach consciousness. An overview of mechanoreceptors of skin and muscle and their functions is found in Table 14-2 of Siegel and Sapru, Essential Neuroscience (3rd edition, 2015), page 252. Consider the consequences of slow versus rapid adaptation.

Question 17

What are the types of fibers of somatosensory receptors?

Answer 17

Somatosensory receptors use four different fiber types: Large myelinated fibers, medium myelinated fibers, small myelinated fibers and unmyelinated fibers, which are the thinnest of all fiber types.

Question 18

What is the signigicance of oarge and medium myelinated fibers?

Answer 18

Large and medium myelinated fibers have diameters of up to 20 μm (micrometer) and conduction velocities of up to 120 m/sec (meters per second). They are used by cutaneous mechanoreceptors and proprioceptors and carry sensory information of touch, vibration and proprioception.

Question 19

What is the significance of small myelinated fibers ?

Answer 19

Small myelinated fibers have diameters down to about 1 μm and conduction velocities of down to 5 m/sec. Unmyelinated fibers are between 0.2 and 1.5 μm in diameter. Their conduction velocities range from about 0.5 to about 2 m/sec.

Question 20

What is the difference between small myelinated and unmyelinated fibers?

Answer 20

You should note the significant drop in conduction velocity between small myelinated fibers and unmyelinated fibers. Small myelinated and unmyelinated fibers carry sensory information of pain and temperature

Question 21

Describe A alpha I fiberd

Answer 21

12-20 um diameter

Modality: proprioception, discriminitive touch, vibration

Question 22

Describe A beta II fibers

Answer 22

Diameter- 6-12 um

Modality- proprioception, discriminative touch, vibration

Question 23

Describe A delta III fibers

Answer 23

Diameter- 1-6 um

Modality- pain & temperature

Question 24

Describe C IV fibers

Answer 24

Diameter- 0.2-1.5 um

Modality- pain and temperature

Question 25

How does shingles work?

Answer 25

Herpes zoster (shingles)
– Following an attack of chicken pox (varicella zoster), the virus may become latent in DRG cells (or cranial nerve ganglion cells)

– Reactivation of the virus leads to painful skin irritations in a dermatomal distribution

Question 26

Explain different modalities are carried in different pathways

Answer 26

Touch, vibration and proprioception are carried in a pathway called the dorsal-column / medial lemniscus system. Pain and temperature are carried in a pathway called the anterolateral system. You need to know these two somatosensory pathways at the level of the spinal cord and be able to discuss the clinical relevance at this level. The complete pathways will be shown in subsequent lectures on touch and on pain.

Question 27

There are different columns of white matter in the Spinal Cord…

Answer 27

White matter of the spinal cord may be divided into three columns, or funiculi (pl., sing. funiculus). Their approximate locations are shown in the figure below.

The dorsal column represents a major somatosensory pathway (carrying touch, vibration and proprioception information from the body to higher centres). The ventral column comprises several motor tracts as well as the pathway for pain and temperature information from the body. The lateral column carries, among others, the lateral corticospinal tract.

Question 28

What is the function of dorsal column-medial Lemniscus System?

Answer 28

• Touch (discriminative)
• Vibration
• Proprioception

Question 29

What is the signifucance if Dorsal Column/ Medial Lemniscys System?

Answer 29

fibers of the dorsal root ganglion neurons carrying sensations of touch, vibration and proprioception use the dorsal column / medial lemniscus system. They enter the dorsal column of the spinal cord and ascend within this white matter tract up to the medulla, where they synapse in the nuclei of the dorsal columns (cuneate and gracile nucleus). fibers originating in the upper extremity ascend in the cuneate fasciculus, those originating in the lower extremity in the gracile fasciculus.

The figure on the following slide shows the topographical organization in the spinal cord, indicating where fibers originating from leg, trunk, or arm are located

Question 30

What is the function of Anterolateral system?

Answer 30

• Pain (nociception)

* Temperature

Question 31

Explain in detail the anterolateral (pain) System

Answer 31

For the subsequent discussion of the ascending fiber tract within the spinal cord carrying sensory information of pain and temperature we are using the terminology consistent with your Haines Atlas. Your Siegel and Sapru textbook refers to the major ascending pathway conveying pain signals and running in the lateral funiculus of the spinal cord as “neospinothalamic tract”.

Fibers of the anterolateral system enter the spinal cord segments also via the dorsal roots of the spinal nerves. The central axons of dorsal root ganglion neurons carrying sensations of pain and temperature synapse within the dorsal horn (part of the grey matter) of the spinal cord. The axons of the second order neurons cross to the contralateral side through the anterior white commissure and ascend in the ALS fiber tract of the spinal cord (white matter). Another term used for the anterolateral system fiber tract is spinothalamic tract, indicating the origin (spinal cord) and the destination (thalamus).

The topographical organization of fibers of different origin is shown on the diagram on the following slide, indicating the localization of fibers originating in the arm, trunk and leg

Question 32

How is Lissauer’s Tract medically relevant?

Answer 32

Lissauer’s Tract can be clinically relevant in the cases of spinal injury.

When we draw sensory (or motor) pathways, we usually focus on the major fiber connections. For the Anterolateral System, however, we have to include some additional details, which will help us to better understand one of the spinal cord lesions (Brown-Sequard Syndrome) discussed in the clinical correlations below.

Not all fibers follow the simple pathway as outlined in the preceding diagrams, but collaterals of the primary afferent fibers may ascend (or descend) one or two segments in the dorsolateral fasciculus (or dorsolateral tract, or zone of Lissauer) of the spinal cord

Question 33

What is Brown-Séquard Syndrome?

Answer 33

Hemisection of the spinal cord as a result of a slow-growing mass, or a traumatic lesion interrupting ascending and descending fibers on one side of the cord.

A more detailed description of the disease can be found in the figure legend of Figure 6-8 of Haines Atlas, 9th edition.

The key sensory finding in this disease process is a dissociated sensory loss pattern: touch, vibration and proprioception are lost on one side of the body, whereas pain and temperature are lost on the other side of the body. We will focus in lecture on the sensory losses of the disease process, but you have to be aware that there are also motor losses,

Question 34

What are the somatosensory findings of Brown-Séquard Syndrome?

Answer 34

• At the level of the lesion on the ipsilateral side (which is the side of the lesion), we find loss of sensations for all somatosensory modalities: touch, vibration, proprioception, as well as pain and temperature.
• Below the level of the lesion on the ipsilateral side (the same side as the lesion) the sensory loss of touch, vibration and proprioception (which started at the level of the lesion) continues through all dermatomes below.
• Below the level of the lesion on the contralateral side (opposite to the side of the lesion), pain and temperature sensations are lost.

Question 35

Explain the caveat of Lissauers tract

Answer 35

CAVEAT: There are exceptions. When we take Lissauer’s tract into consideration, we find that one or two segments on the contralateral side may be spared from sensory losses. You should be able to explain the role of Lissauer’s tract in preserving pain and temperature sensations in one or two segments on the contralateral side.

The concept of sparing of segments is often challenging. We recommend regular, spaced practicing with ‘lesions’ at various levels using the schematic that follows to cement your understanding of segmental sparing (e.g., spinal cord hemisection at T6 on the left – draw the lesion and relevant fibers on the schematic and answer the following questions

Question 36

What is the clinical correlation of Brown-Sequard Syndrome at level of lesion

Answer 36

Loss of touch, vibration, proprioception, pain & temp. On the contralateral side because of Lissauers side

Question 37

What are the clinical correlations of Brown-Sequard syndrome below level of lesion?

Answer 37

• loss of touch, vibration, proprioception contralaterally but not ipsilaterally
• pain & temperatureloss ipsilaterally but not contralaterally

Beginning approx. 2 levels below lesion

Question 38

What is the clinical correlation to Syringomyelia?

Answer 38

A pathologic enlargement of the central canal of the spinal cord. As the cavity expands, it interrupts fibers that cross through the anterior white commissure.

A more detailed description of the disease can be found in the figure legend of Figure 6-8 of Haines Atlas, 9th edition.

You should also consider the impact of Lissauer’s tract on the distribution of sensory losses, compared to the actual size of a lesion (spinal cord segments affected).

The same schematic as was recommended for reviewing Brown-Séquard syndrome is recommended here for Syringomyelia. Choose the level(s) of the lesion, draw it on the schematic and then add relevant fibers to ascertain associated the sensory losses (if any).