Neuroanat 7

Front 1

Pain can be

Back 1

acute (e.g. pin prick)
– prolonged (adaptive response associated with
inflammation, hyperalgesia and allodynia)
– chronic (permanent and abnormal responses of the CNS
to known or unknown tissue damage)

Front 2

These different aspects of pain are mediated by

Back 2

parallel systems that have distinct structural,
physiological, neurochemical and anatomical
characteristics. These parallel systems arise in the periphery and enter the spinal cord together but ultimately take separate paths (in part) and activate different systems within the CNS.

Front 3

Allodyni is the condition in which

Back 3

non noxious
stimuli becomes noxious after some time.

Front 4

Hyperalgesia is a condition in which

Back 4

the no noxious
stimuli become noxious or noxious stimulus
become evene more noxious.

Front 5

Thermoceptors and Nociceptors are

Back 5

Free nerve endings - no obvious morphologic
specialization

Front 6

Consider the Following Questions

Back 6

Describe the receptors, nerve, and pathways for perceiving pain on the body and face.
Describe the consequences of a lesion in the upper medulla that interrupts the spinal tract of the trigeminal nerve and the spinothalamic tract.
Describe the consequences of a lesion on the left side of the spinal cord at mid-thoracic levels with respect to the perception of touch and acute pain on the legs.
Describe the impact of myelination upon speed of axon conduction, and characterize the thickness of myelination for: 1. slow pain fibers; 2. Golgi tendon organs; 3. Pacinian corpuscles.
Name the thalamic nucleus receiving the medial lemniscus and spinothalamic tracts, and describe the somatotopic organization of the terminals from these tracts.
The secondary axon in an ascending sensory pathway tends to cross to the opposite side. For proprioception, this crossing takes place at what level of CNS? For pain, this crossing takes place at what level of CNS?
Describe the consequences of a lesion in the upper medulla that interrupts the spinal tract of the trigeminal nerve and the spinothalamic tract.
Describe the origins of the ventral trigeminothalamic tract and the synaptic destination in thalamus.

Front 7

Nociceptors (pain receptors) may be

Back 7

high threshold mechanoreceptors, high threshold thermoceptors, chemoceptors (sensitive to chemicals applied to the skin), or polymodal. Others are termed "silent", and do not appear to respond to any stimuli (other than shock which activates everything) until frank damage to tissue has occurred. Similarly, low threshold thermoceptors may be sensitive to warmth or coolness, but generally not both

Front 8

The primary afferents for pain

Back 8

A-delta (type III) fibers
C (type IV) fibers -

Front 9

A-delta (type III) fibers characteristics

Back 9

lightly myelinated
– moderate conduction velocities

Front 10

A-delta fibers terminate primarily in

Back 10

the marginal zone

Front 11

A-delta (type III) fibers primary neurotransmitters are

Back 11

excitatory amino
acids

Front 12

A delta (type III) fibers -

Back 12

best associated with acute, localizable pain and
the neo-spinothalamic tract

Front 13

A-delta (type III) fibers mechanoceptors respond to

Back 13

pinch, squeeze,
physical damage, etc

Front 14

A-delta (type III) fibers thermoceptors respond to

Back 14

noxious (tissue
damaging) thermal (> 52C) stimuli

Front 15

A-delta (type III) fibers not usually responsive to

Back 15

chemical or thermal
stimuli under 52 C

Front 16

C (type IV) fibers characteristics

Back 16

unmyelinated
– slow conduction velocities (2 m/s)

Front 17

C (type IV) fibers terminate primarily in the

Back 17

marginal zone and the
substantia gelatinosa

Front 18

C (type IV) fibers neurotransmitters

Back 18

Substance P, CGRP

Front 19

C (type IV) fibers -best associated with the

Back 19

motivational, affective
aspects of pain and the paleo-spinothalamic
systems

Front 20

C (type IV) fibers -desensitized/destroyed by

Back 20

chemical toxins such as capsaicinC

Front 21

C (type IV) fibers respond to

Back 21

noxious mechanical, chemical and
thermal stimuli (< 15 C – thermal only - or >
45C)

Front 22

Trpv1 channels are

Back 22

capsaicin sensitive. Capsaicin is
homologous to endocannabanoids

Front 23

Heat and cold are coded by subtypes of TRP (transient receptor potential) cation channels that are differentially sensitive to

Back 23

warmth or coolness, sensations that can be elicited by either capsaicin or menthol.

Front 24

Cell bodies of the receptors are in the

Back 24

dorsal root ganglia (DRG)

Front 25

The central processes of the afferents enter the spinal cord via

Back 25

the lateral aspect of the dorsal root.

Front 26

The peripheral (primary) afferent fibers of free nerve endings are all

Back 26

small diameter, and all are either thinly myelinated (A δ , or group III fibers), or unmyelinated (C, or group IV fibers).

Front 27

A δ myelinated, fast pain fibers (A δ - conduction velocity ~ 20 m/s). Conveys

Back 27

acute, sharp, localizable pain.

Front 28

C = unmyelinated, slow pain fibers (C - conduction velocity ~ 2 m/s). Conveys

Back 28

chronic painful stimuli, difficult to localize, slow to adapt.

Front 29

Differences in conduction velocity contribute to the phenomenon of

Back 29

double pain; i.e. first, pain is sharp, and well localized followed by secondary pain sensations that are more burn-like and less well localized.

Front 30

Pain fibers enter

Back 30

Lissauer’s Tract at the dorsal edge of the dorsal horn and bifurcate, running up and down the cord two or three segments before entering the grey matter and synapsing on dorsal horn neurons.

Front 31

A δ fibers terminate monosynaptically primarily on neurons in the

Back 31

the Marginal Zone (lamina I) and lamina V.

Front 32

C fibers terminate monosynaptically on neurons of lamina

Back 32

lamina I (C fibers from the skin may terminate on neurons within the superficial edge of lamina II, the substantia gelatinosa) and polysynaptically on neurons of lamina V.

Front 33

Lamina I neurons comprise several morphologically and neurochemicallly distinct, modality-selective classes that receive input from selective subsets of

Back 33

A δ and C fibers and thus communicate the specific physiological states of the tissues of the body.

Front 34

Lamina V neurons have large dendritic fields that extend across

Back 34

most of the dorsal horn and provide an integrative representation of all primary afferent input from large receptive fields.

Front 35

Small, local circuit neurons within the substantia gelatinosa receive input not only from C fibers but also from

Back 35

neurons of the brainstem and hypothalamus that modulate the activity of projection neurons in lamina V.

Front 36

Significant information processing occurs within the substantia gelatinosa that serves to either

Back 36

“dampen” incoming noxious stimuli or “sensitizes” silent neurons to those noxious inputs.

Front 37

Substance P, CGRP, and other peptides contribute to neurotransmission at the synapse between C fibers and

Back 37

substantia gelatinosa neurons

Front 38

Amino acids, such as glutamate and aspartate and possibly other peptides may mediate

Back 38

responses from A d nociceptors. These neurochemical signatures not only specify the type of information being transmitted from the periphery (fast, sharp pain vs slow, burning pain) but where that information may be transmitted within the CNS.

Front 39

About 90% of the axons arising from lamina I neurons and 50% of the axons arising from lamina V cross the midline via the

Back 39

anterior (ventral) white commissure, just under the central canal area, and ascend in the anterolateral quadrant as the anterolateral system of tracts

Front 40

Crossing occurs over approximately 2-3 segments, which has clinical implications for determining the

Back 40

level of a lesion by loss of pain and temperature sensitivity.

Front 41

Many lamina V neurons involved in the transmission of

Back 41

crude touch and proprioception ascend ipsilaterally in the ventral medial funiculus.

Front 42

The classic neospinothalamic tract, carrying information regarding fast, sharp, well localized pain that was transmitted by the A δ fibers, is comprised of

Back 42

axons arising from cell bodies in lamina I that cross the midline in the anterior white commissure and ascend ventrolaterally in a somatotopic manner directly to the ventral posterolateral thalamus

Front 43

The third order neurons of the neospinothalamic tract system are somatotopically organized in

Back 43

the ventral posterior lateral nucleus of the thalamus

Front 44

The axons of these neurons exit the thalamus via the posterior limb of the internal capsule and terminate in

Back 44

the primary somatosensory cortex at the postcentral gyrus (SI), as well as the adjacent secondary somatosensory cortex (SII) and the retroinsular region. A somatotopic homunculus, similar to that of the medial lemniscal system is found here.

Front 45

The so-called paleospinothalamic tracts which mediate the slower, burning, aching and affect-altering information transmitted by C fiber stimulation is comprised primarily of

Back 45

axons arising from morphologically distinct cell bodies in lamina I. Much of the information carried by these projections may not be somatotopically organized and end in intralaminar thalamic nuclei as opposed to the VPL

Front 46

Other fibers, either direct projections or branches of spinal thalamic afferents, project into

Back 46

omeostatic (parasympathetic) regions of the reticular core of the medulla and pons (spinal reticular projections) as well as into the midbrain periaqueductal grey (PAG) and raphe nuclei (spinal mesencephalic projections).

Front 47

Most of the paleospinothalamic system terminates in the

Back 47

midline and intralaminar thalamic nuclei, although some project directly to limbic cortex (e.g. anterior cingulate and insular cotex, amygdala).

Front 48

Many other areas of cortex also receive input from this system, explaining why ablation of the primary somatosensory cortex does not disrupt pain or suffering, but only

Back 48

only may affect the ability to locate the pain on the body.

Front 49

Portions of the spinoreticular tract also activate descending pain modulatory systems, such as

Back 49

he descending pathways from noradrenergic pontine locus coeruleus and other lateral medullary centers such as the nucleus reticullaris paragigantocellularis. These pathways travel with the raphespinal tract in the dorsolateral funiculi of the spinal cord and terminate in the dorsal horn to inhibit pain information.

Front 50

Painful stimuli from the face are transmitted in a topographically organized fashion via

Back 50

three divisions of the trigeminal (V) nerve (ophtalmic, maxillary and mandibular - information from the ear and pharynx are carried by components of N7, 9 and 10).

Front 51

First order neurons descend ipsilaterally from the pons into the medulla via the

Back 51

spinal trigeminal tract. Especially in the lower medulla caudal to the obex, this tract lies in a prominent position on the lateral surface of the medulla.

Front 52

First order axons from all three divisions will descend to progressively more rostral levels in the medulla as the somatotopic pattern progresses toward

Back 52

the nose and oral invagination.

Front 53

The first order axons terminate in the

Back 53

spinal nucleus of five. Especially in sections through the lower medulla, the appearance of the spinal nucleus is similar to the substantia gelatinosa of the spinal cord

Front 54

The primary afferents terminate in synaptic arrangements similar to that seen in

Back 54

the marginal zone and substantia gelatinosa of the spinal cord

Front 55

Secondary fibers project from the spinal trigemimal nucleus and diffusely cross to the contralateral brainstem to form

Back 55

the ventral trigeminothallamic tract (it will lie just lateral to the dorsal part of the medial lemniscus in medulla and take a position medial and dorsal to the medial lemniscus in the pons).

Front 56

These second order axons terminate in

Back 56

ventral posteromedial thalamic nucleus.

Front 57

Painful stimuli from the face are modulated by

Back 57

reticular and limbic sources in a manner similar to that seen in the spinal cord.