Somatosensation and Pain L2

Question 1

The peripheral nerves consist of bundles of axons grouped by….

Answer 1

The peripheral nerves consist of bundles of axons grouped by fibre diameter.

Question 2

The largest, myelinated axons, are termed ……

the smallest, unmyelinated axons are termed …..

Answer 2

The largest, myelinated axons, are termed Aα and Aβ; the smallest, unmyelinated axons are termed C fibres.

Question 3

describe C fibres

Answer 3

high SA:V ratio

high SA to take up O2

smal lamount of cytoplasm

less susceptibel to anoxia

Question 4

Answer 4

Question 5

Following electrical stimulation of the median nerve (wrist) the action potentials are summed to produce a ….

Answer 5

Following electrical stimulation of the median nerve (wrist) the action potentials are summed to produce a compound action potential.

Question 6

Conduction velocity is related to fibre diameter so Aβ afferents have the _________ delay whereas the unmyelinated C- fibres conduct _____ producing a ______ peak.

Answer 6

Conduction velocity is related to fibre diameter so Aβ afferents have the shortest delay whereas the unmyelinated C- fibres conduct slowly producing a late peak.

Question 7

which axons does anoxia affect first?

Answer 7

Anoxia affects the large Aβ fibres whereas local anaesthetic affects primarily the small ‘C’ and Aδ axons.

Question 8

is Pain evoked when the large Aβ afferents are stimulated alone?

Answer 8

Pain is not evoked when the large Aβ afferents are stimulated alone, even at intensities sufficient to mimic an intense mechanical stimulus.

Question 9

Pain is only evoked by stimulation of which fibres

Answer 9

Pain is only evoked by stimulation of the Aδ and C fibres.

Question 10

n certain conditions e.g. diabetes or multiple sclerosis - what happen s?

Answer 10

n certain conditions e.g. diabetes or multiple sclerosis the myelin sheath of large diameter fibres degenerates resulting in a slowing of nerve conduction or failure of impulse transmission.

Question 11

Answer 11

Question 12

hypoxia affects which fibres first?

Answer 12

largest fibres first

Question 13

Local anaesthetic affects primarily ….

Answer 13

Local anaesthetic affects primarily small C and Aδ fibres

Question 14

Pain is only evoked by stimulation
of _____ fibres
High stimulation of ____ won’t elicit pain

Answer 14

Pain is only evoked by stimulation
of C and Aδ fibres;
High stimulation of Aβ won’t elicit pain

Question 15

describe the cauda equina

Answer 15

The cauda equina, which lies below approximately L2, does not contain the spinal cord but consists of the elongated spinal roots from the lumbrosacral spinal cord; consequently, hollow needles can be inserted into the subarachnoid space to remove CSF for diagnostic purposes (lumbar puncture)

Question 16

in an epidural block - anaesthetic in inserted into ?

Answer 16

the epidural space

Question 17

Answer 17

Question 18

what is a dermatome?

Answer 18

A dermatome is the area of skin innervated by a single dorsal root.

Question 19

do dermatomal boundaries overlap?

Answer 19

Note that dermatomal boundaries overlap by mixing fibres from several dorsal roots in the peripheral nerve.

Question 20

in the spinal cord the cell bodies are at the center?

Answer 20

yes

Question 21

Answer 21

Question 22

The grey matter is divided into functionally distinct laminae (aka _____ laminae).

Answer 22

The grey matter is divided into functionally distinct laminae (aka Rexed’s laminae).

Question 23

T or F

spinal cord:

Afferent and efferent axons run in the white matter

Answer 23

T

Question 24

The white matter can be divided into three regions, defined relative to the grey matter: ….

Answer 24

The white matter can be divided into three regions, defined relative to the grey matter: dorsal, lateral and ventral.

Question 25

describe the DCML system

Answer 25

fien touch and proprioception

• large diameter myelinated axons
• enters spinal cord - bifurcates

those entering:

below mid thoracic: ascend in fasciculus gracilis - terminate in gracile nucleus

above mid thoracic: enter fasciculus cuneatus - terminate in cuneate nucleus

then cross brainstem, ascend to thalamus in Medial lemniscus

Question 26

Answer 26

Question 27

describe the spinothalamic tract?

Answer 27

nociseptive pathway

• axons in layers 1 5 6 7 of dorsal horn.
• he axons ascend in the contralateral, anterolateral, white matter and this pathway is often referred to as the anterolateral system.
• cross early, synapse early

Question 28

describe the The spinoreticular tract

Answer 28

The spinoreticular tract projects from laminae VII and VIII and terminates in the reticular formation and thalamus.

Some axons in this pathway do not cross the mid-line – that is they travel ipsilaterally.

Question 29

describe the spinomesencephalic tract

Answer 29

spinomesencephalic tract projects from laminae I and V, via the anterolateral quadrant of the spinal cord to the mesencephalic reticular formation and the periaqueductal gray.

Question 30

describe Brown-Sequard syndrome

Answer 30

The Brown-Sequard syndrome describes the consequence of a hemi-section through the spinal column.

It affects both the spinothalamic tract and DC-ML on the same side of the cord resulting in:

a loss of pain and temperature sensation below and contralateral to the site of the lesion

and loss of fine touch and proprioception below and ipsilateral to the site of the lesion.

Question 31

D: Syringomyelia

Answer 31

Syringomyelia is caused by a fluid-filled cavity within the spinal (usually cervical) cord. The cavitation usually disrupts the decussating fibres of the anterolateral system but not the ascending fibres of the DC- ML system resulting in the characteristic “cape-like” distribution of loss of pain and temperature sensation in the upper limbs and trunk but preservation of touch and pressure sensation.

Question 32

Answer 32

Question 33

diagram of syringomylia

Answer 33

Question 34

Posterior column syndrome picture

Answer 34

Question 35

Picture for complete transection

Answer 35

Question 36

describe trigeminal neuralgia

Answer 36

Trigeminal neuralgia (aka Tic douloureux) is a syndrome whereby gentle stroking of the face or mouth provokes a massive stabbing pain. This is a classic example of allodynia.

The ophthalmic and maxilliary branches are purely sensory while the mandibular is a mixed sensory and motor branch.

Question 37

Distinct modalities of sensory information are processed separately in the trigeminal system but where?

Answer 37

Distinct modalities of sensory information are processed separately:

• the principal sensory nucleus encodes tactile information,
• the trigeminal nucleus encodes pain and temperature.

Question 38

how does lateral inhibition modify input to the brain?

Answer 38

allows the amplification of differences in the output of neighbouring neurons and effectively enhances contrast.

Question 39

describe whast happening here:

hint - lateral inhibition

Answer 39

(A) shows the increase in receptive field size as one goes from the fingers to the lower forearm.

B1 shows a schematic excitatory receptive field with an inhibitory surround.

B2, upper trace, shows a stimulus being applied to the excitatory area of a cell located in the dorsal column nuclei, while the lower trace shows a reduction in excitation when the inhibitory surround, of the same cell, is also stimulated.

B3 shows a simplified circuit that may underlie the responses shown.

Question 40

Stimulation of a single point of skin produces an excitatory response surrounded by …

Answer 40

Stimulation of a single point of skin produces an excitatory response surrounded by a small inhibitory response

Question 41

Stimulation of a single point of skin produces an excitatory response surrounded by a small inhibitory response.

why is this good?

Answer 41

When stimulating two adjacent points of skin (further right) lateral inhibition suppresses excitation of the neurons between the points thus sharpening the focus or spatial clarity of the two points. In this way it is easy to see how inhibitory interactions could be important for fine tactile discriminations such as reading Braille.

Sharpens the repsonse

Question 42

what is this used for?

posterior part of the ventral medial nucleus (VMPO)

part of the thalamus

Answer 42

pain and temperature sensing

Note the steady increase in discharge rate with increases in temperature. The receptive field was located on the contralateral ulnar hand.

Question 43

mechanism of 2 point stimulation of the skim

Answer 43

just FYI

Question 44

A cell’s receptive field is not always obvious.

describe what is meant by this

Answer 44

the failure of one input unmasks the presence of inputs which are ordinarily suppressed by ‘inhibitory’ mechanisms

Question 45

where is S1?

Answer 45

posterior to cetnral sulcus

Question 46

fat

Answer 46

mamba