The Somatic Sensory System (Prof. Fitzgerald)

Question 1

What are the fcts of the somatic nervous system ?

Answer 1

• perception of events of the skin surface
• pressure and pain in joints and internal organs
• position of limbs in space
• sensory guidance of mvnt (feedback control)

Question 2

What info is provided by the somatosensory system ?

Answer 2

The modality of the stimulus, spatial info, info about intensity and frequency as well as the history of the receptor.

Question 3

What are the different modalities of a stimulus ?

Answer 3

• touch and pressure (static and dynamic)
• thermal (cool and warm)
• itch (mechanical and chemical)
• noxious stimuli (pain) – mechanical, thermal, chemical

Question 4

What imp info (other than modality of the stimulus) does the somatic sensory syst provide ?

Answer 4

• spatial location of the stimulus (where on body surface)
• spatial discrimination of stimulus (how far apart)
• intensity and duration of stimulus (strength + duration)
• nb and freq of stimuli
• history (normal skin, damaged skin, numb skin etc.)

Question 5

How is the somatosensory system anatomicaly organized ?

Answer 5

• primary sensory neurons
• spinal chord (EXCEPT trigeminal system in the brainstem for the head and neck)
• ascending spinal tracts to brainstem + thalamus
• somatosensory cortex
• other brain regions: motor + emotional response
• descending spinal tracts for feedback control

Question 6

Where are the peripheral axons, cell bodies and central axons of primary sensory neurons ?

Answer 6

Peripheral axons in the skin, cell bodies in the dorsal root ganglion, central axons in the spinal chord.

Question 7

What are the four main types of mechanoreceptors in the skin and what are they sensitive to ?

Answer 7

• Merkel receptors in the epidermis are sensitive to pressure (0.3-3Hz, slow pushing)
• Meissner’s corpuscles just below the epidermis are sensitive to flutter (3-40Hz)
• Ruffini endings situated aprox in the middle of the dermis are sentive to stretching (15-400Hz)
• Pacinian corpuscules in the subcutaneous fat below the dermis are sensitive to vibration (10-500Hz, v rapid vibration at the upper range)

Question 8

What do the different type of mechanoreceptors in the skin show ?
What about nociceptors ?

Answer 8

That mechanoreceptors have specialized anatomically recognizable endings in the skin.
Nociceptors, on the other hand, are nerve endings that are depolarized by tissue damaging stimulation: mechanical, thermal or chemical.
Nociceptors DO NOT have anatomically recognizable endings.

Question 9

What kind of channels are expressed in touch receptor endings ?

Answer 9

MS (low threshold)

Question 10

What kind of channels are expressed in pain receptor endings ?

Answer 10

TRPA1, TRPV1, TRPV2, MS (high threshold), ASICs

Question 11

What kind of channels are expressed in hot temp receptor endings ?

Answer 11

TRPV3, TRPV4

Question 12

What kind of channels are expressed in cold temp receptor endings ?

Answer 12

TRPM8

Question 13

Do dorsal root ganglions contain homogenous population of sensory neurons ?

Answer 13

Absolutely not !
Different fctnal populations of neurons occupy the dorsal root ganglion (large A(beta) fibre axons, small unmyelintaed C fiber axons, medium thinly myelinated A(delta) axons).
These cells express different proteins that reflect their different fcts (ions channels, neurotransmitters, receptors, developmental markers).

Question 14

What are the main different axon sizes, levels of myelination, conduction velocities and fcts ?

Answer 14

A(alpha) fibres: thickly myelinated, 13-20μm wide, 80-120m/s
A(beta) fibres: medorately myelinated, 6-12μm wide, 37-73m/s, touch and pressure
A(delta) fibres: thinly myelinated, 1-5μm wide, 5-35m/s
C fibres: unmyelinated, 0.2-1.5μm wide, 0.5-2m/s, pain/itch and temperature

Question 15

How do different primary sensory neurons respond to different stimuli ?

Answer 15

By different patterns of APs (frequencies + durations)

Question 16

“Each dorsal root ganglion has axons in one peripheral nerve”.
Why is this outrageously false ?

Answer 16

Each dorsal root ganglion has axons in several peripheral nerves, so each dorsal root carries information from wide overlapping skin areas

Question 17

What are dermatomes ?

Answer 17

Dermatomes are the areas innervated by each dorsal root ganglion.

Question 18

What are the four spinal segments and which parts of the body do these segments respectively innervate ?

Answer 18

Cervical: upper limbs
Thoracic: trunk
Lumbar: lower limbs
Sacral: pelvic

Question 19

“One dorsal horn neuron will receive inputs inputs from one primary sensory neuron”.
Why is this outrageously false ?

Answer 19

One dorsal horn neuron will receive inputs from many primary sensory neurons. This a direct consequence of the dorsal root ganglion containing axons in several peripheral nerves.

Question 20

How are sensory neurons organized in the spinal chord ?

Answer 20

Each type of sensory neuron has a different pattern of terminals and synapses in the spinal cord.
Good luck learning those… because the somatosensory ‘circuits’ formed by primary sensory neuron synapses and dorsal horn neurons are complex.

Question 21

What is a receptive field (RF) ?

Answer 21

The receptive field of an individual sensory neuron is the particular region of the sensory space (e.g., the body surface, or the visual field) in which a stimulus will modify the firing of that neuron.

Question 22

What are RFs for dorsal horn neurons ?

Answer 22

These are the areas of skin that evoke AP for particular neurons when stimulated.

Question 23

How do RFs vary ?

Answer 23

Receptive fields may be a single modality e.g. touch,
or several modalities e.g. touch, pinch, heat.
Receptive fields may very small (e.g. 1mm2) or very large (e.g. the sole of the foot).

Question 24

How are the RFs of neighbouring neurons organized ?

Answer 24

Each neuron has a receptive field that is adjacent to its neighbouring neuron, this allows the elaboration of a somatotopic map in the somatosensory cortex.

Question 25

How are the ascending pathways to the brain different for pain and touch ?

Answer 25

Proprioreceptors and mechanoreceptors travel through the dorsal column-medial lemniscal pathway: a 1st order afferent synapses w/ a second order neuron in the dorsal column nuclei that decussates in the medial lemniscus in the medulla and then synapses a 3rd thalamo-crotical neuron in the thalamus.
Nociceptors and thermoreceptors travel through the spinothalamic tract: a 1st order afferent synapses a second order neuron in the dorsal horn that decussates immediately and travels though the anterolateral quadrant to synapse a 3rd order thalamo-cortical neuron in the thalamus.

Question 26

What does the motor homunculus show ?

Answer 26

That the sensory pathways maintain their anatomical organization.

Question 27

Why is the rodent whisker system a good example of somatotopic mapping ?

Answer 27

After a v precise mapping of the whisker cortex, by recording cortical activity in response to whisker stimulation in awake rodents, it has been observed that the arrangement of neurons in the cortex innervating the whiskers conserve the organization of the whiskers themselves.

Question 28

What other factors, other than perception of events on the skin surface in the somatosensory cortex, must be considered ?

Answer 28

• Motor systems: sensory feedback is essential for accurate movements
• Emotions: experiences of pleasure and pain
• Attention: focussing upon physical events in the environment
• Learning: many learned behaviours require touch
• Constant feedback with the environment: maps are not fixed and static but constantly changing and dynamic.

Question 29

Give an example of where the somatosensory syst can go wrong ?

Answer 29

Herpes zoster.

Question 30

How does herpes zoster (shingles) work ?

Answer 30

Shingles occurs when the virus that causes chickenpox starts up again in the body. After children (of sometimes adults) get better from chickenpox, the virus remains, dormant, in the dorsal root ganglions. In some people, it stays dormant forever. In others, the virus “wakes up” when disease, stress, or aging weakens the immune system.

Question 31

What are the symptoms of the shingles infection ?

Answer 31

• Pain or bruised feeling - usually on one side of the face or body - often along with a fever, chills, headache or upset stomach. People will often feel unwell for several days before the rash appears
• Tingling, itching or prickling skin and an inflamed, red skin rash several days later
• A group or long strip of small, fluid-filled blisters
• Deep burning, searing, aching or stabbing pain, which may occur once in a while or last a long time

Question 32

What is post-herpetic neuralgia (PHN) ?

Answer 32

PHN is a nerve pain (neuralgia) that persists after a shingles rash has cleared. If the pain goes, but then returns at a later date, this too is called PHN.
Common signs and symptoms of postherpetic neuralgia include:
- severe pain that continues for more than one to three months in the same place that the shingles occurred, even after the rash goes away
- burning sensation on the skin, even from the slightest pressure
- sensitivity to touch or temperature changes

Question 33

What are some of the effects of nerve damage on the skin ?

Answer 33

• nerve damage causes changes in intact nearby sensory axons

- increased sodium channels and TRPV1 change the properties of these sensory nerves

Question 34

What are some of the effects of nerve damage on the spinal chord ?

Answer 34

• altered A and C fibre input changes the properties of dorsal horn neurons and so alters messages to the brain
• altered patterns of A and C fibre input activates glial cells, which in turn affect dorsal horn neuron excitability