Week 2 - B - Pharmacology 2a - Somatosensory system (axon types and muscle fibres)

Question 1

The somatosensory system has different types of receptors distributed throughout the body Touch, pressure and vibration Joint and muscle position sense Temperature Pain Itch What are each of these sensations known as?

Answer 1

Touch pressure and vibration - mechanosensation Joint and muscle position sense - proprioception Temperature - thermosensation Pain - Noiciception Itch - Pruriception

Question 2

Mechanoreceptos are sensitive to stretch of the membrane The channels are opened by physical stimulus instead of chemical What is the sensory structure in skeletal muscle activated by stretch called?

Answer 2

This is the muscle spindle

Question 3

What type of fibres make up the muscle spindle? - these fibres have more of a sensory than contractile function What type of motor neuron innervates these muscle fibres? What are the muscl fibres that play a role in the actual contractual force of the muscle? What type of motor neuron innervates them?

Answer 3

Intrafusal muscle fibres make up the muscle spindle - detect stretch Gamma motor neurones innervate these muscle fibres The muscle fibres that play a role in the actual contracile force of the muscle are the extrafusal muscle fibres - innervated by alpha motor neurones

Question 4

What is a motor unit?

Answer 4

This is a single motor neurone and all the skeletal fibres it innervates

Question 5

When the intrafusal fibres are stretched, this generates a graded action potential which travels along the axon into the dorsal root of the the spinal cord and into the dorsal horn What type of neurons are passing through the dorsal root ganglion?

Answer 5

These are bipolar pseudo-unipolar type neurons From skin to dorsal root ganglion and to the dorsal horn of the spinal cord

Question 6

Primary sensory afferents differ greatly in their diameter, extent of myelination, conduction velocity and associated sensory receptor. What does a smaller diameter do to the speed of conduction? What does an increase in myelination do to the speed of conduction

Answer 6

Smaller diameter decreases the speed of conduction of electrical signals Increase in myelination increases the speed of conduction of electrical signals

Question 7

What are the four different axons coming from the skin known as? Axons from muscle and tendons have slighlty different names, what are they?

Answer 7

Axons from skin - Aa (Alpha), Ab(Abeta), As(Adelta) and C Axons from muscles and tendons - Group I, II, III, IV

Question 8

Which type of sensory receptor has the thickest diameter and thickest myelin hence the fast conduction speed?

Answer 8

This would be the proprioceptors of skeletal muscle - Type Aa

Question 9

Which sensory receptors have no myelin and are very thin hence a low conduction speed?

Answer 9

This would be the pain and itch receptors - Type C (group IV)

Question 10

• What are the two different classifications for pain fibres?
• What is the classiifcation for proprioception of skeletal muscle and mechanocreceptors of skin?
• What is the classification for temperature and itch?

Answer 10

• Pain fibres can be classified as Adelta or C (group III or IV)
• Proprioception is Aa and mechanorecptors is Ab
• Temperature is Adelta
• Itch is C

Question 11

The RF (receptive field) of an afferent neurone is the region that when stimulated with an adequate stimulus causes a response in that neurone What is the ability to discern that two nearby objects touching the skin are truly two distinct onbects?

Which mechanoreceptors in the skin sense this?

Answer 11

This is two point discrimination Pacinian(dermis/pressure) are deeper than Meissners (epidermis/ vibration) and therefore the receptive field is larger for Pacinin afferent nerve endings - two point discimination is therefore also greater in Pacinian corpuscles

Question 12

The area needed for discrimination decreases as the function of the area is more specific Give an example of this?

Answer 12

Finger tips have a smaller discrimination in the forearm as the function of the finger tip is more specific Due to a more specific function, we are able to feel two separate points more clearly

Question 13

Skin is richly innervated and contains different sensory receptors Which receptors are for pain and heat? Which are for pressure? Which are for vibration?

Answer 13

Pain and heat are free nerve endings Pressure is Pacinian Corpuscles Vibration is Meissner’s Corpuscles

Question 14

Type C Axons are often depicted as only pain sensing but have the other modalities as seen Name some of these other modalities? Does Type C contain any myelin?

Answer 14

Itch, stroke, sensual touch

No myelin sheath

Question 15

Is the receptive field of Pacinian corpuscles or Meissner’s corpuscles greater? Remember the sensations of which they control

Answer 15

Pacinian corpuscles - pressure has a far greater receptive field than of Meissner’s corpuscles which are receptors for vibration Pacinian corpsucles are depper - bigger receptive field required (dermis) Meisnners are in the epidemris

Question 16

Area of skin innervated by the left and right dorsal roots of a single segment is a dermatome What do area of skin do each of these different dermatomes innervates: C4,5,6,7,8, T1, T2, T4, T8, T10, S2, S3, S4 L2,3,4,5, S1?

Answer 16

C4 - shoulder C5 - Lateral arm C6 - Lateral forearm and thumb C7 - index and middle finger(posteior), just middle anterior C8 - pinky T1 - medial forearm, T2 medial arm and sternum T4- nipple T8 - xiphoid process T10 - Umbilcus S2 - Back of knee S3 - Buttocks S4 - Perineum L2 - Medial thigh L3 - anterior knee L4 - Medial shin and ankle L5 - lateral shin and dorsum of foot S1 - lateral foot and heel

Question 17

What infection typically causes a dermatomal rash due to the virus reactivating in a single nerve ganglion?

Answer 17

This would be shingles

Question 18

Which dermatome is this liekly to be?

Answer 18

T6 dermatome - below nipple and above xiphoid process

Question 19

What fibre class are pain and temperature fibres? What ascending tract is for pain and temperature sensation

Answer 19

Pain and temperate fibre class - Adelta and C Spinothalamic tract is for pain and temperature sensation

Question 20

When the first order neurones enter the dorsal horn of grey matter before decussating, what tract carries the fibres from dorsal column to dorsal horn where the substantia gelatinosa is located (where the fibres synapse)?

Answer 20

It is the Lissauers tract (lissauers fascicularis) is where the fibres are carried to the dorsal horn to synapse in the substantia gelatinosa before decussating