Ascending Neural Pathways

Question 1

• Sensory info from somatic segments enters spinal cord through

Answer 1

the dorsal roots of the spinal nerves.
• Then to brain via:
1. The dorsal column–medial lemniscal system OR
2. The anterolateral system.

Question 2

Dorsal column-medial lemniscal fibres

Answer 2

• Large,myelinated fibres
• 30-110m/s
• Discrete types of mechanoreceptive sensation
• High degree of spatial orientation of the nerve fibres with respect to their origin

Question 3

Anterolateral fibres

Answer 3

• Smallerfibres
• Up to 40m/s
• Broad spectrum of sensory modalities
• Less spatial orientation

Question 4

Dorsal column-medial lemniscal controls

Answer 4

• Finetouch
• Vibration
• Proprioception

Question 5

Anterolateral controls

Answer 5

• Pain

* Temperature • CourseTouch

Question 6

Dorsal column-Medial Lemniscal System

Answer 6

• Carries signals up to the medulla in the dorsal columns of spinal cord.
• Synapse in dorsal nuclei (cuneate and gracile)
• Second order neurones cross immediately to opposite side in
medulla.
• Up through brainstem to thalamus via medial lemniscus.

Question 7

Anterolateral System

Answer 7

• Enter spinal cord via dorsal spinal root nerves.
• Synapse in dorsal horns of spinal grey matter.
• Cross to opposite side of cord.
• Ascend through anterior and
lateral white columns of cord.
• Terminate at all levels of lower brain stem and in thalamus.

Question 8

where does the medial lemniscal fibres terminate

Answer 8

In the thalamus, in the thalamic sensory relay area, called the ventrobasal complex.
From the ventrobasal complex, third-order nerve fibres project mainly to the post-central gyrus of the cerebral cortex ie somatic sensory area I.

Question 9

spatial orientation of nerve fibres in dorsal column-medial lemniscal system

Answer 9

• Distinct spatial orientation of nerve fibres from individual parts of body that is maintained throughout
• In thalamus, distinct spatial orientation is maintained, with tail end of body represented by the most lateral portions of the ventrobasal complex and the head/face represented by medial areas of the complex.

Question 10

why does one side of the brain control the opposite side

Answer 10

Because of crossing of the medial lemnisci in the medulla, the left side of the body is represented in the right side of the thalamus and vice versa.

Question 11

where does sensory information in the brain go

Answer 11

Sensory information passes through the thalamus and on to the primary (somatic) sensory cortex which is located in a strip posterior to the post central sulcus of the brain.
It is then projected in a topographical manor to this area, with areas of higher discrimination having a larger proportion of space

Question 12

there are two areas that exist in the parietal lobe

Answer 12

somatosensory areas I and II
I = Much more extensive, somatosensory cortex almost always means area one, high degree of localisation of different body parts
area II = poor localisation

Question 13

The anterolateral pathway: transmission of “less critical” sensory signals

Answer 13

• Signals transmitted do not need
– highly discrete localisation.
– discrimination of fine gradations of intensity.
These are:
Pain, heat, cold, crude tactile, tickle, itch, sexual sensations.

Question 14

Characteristics of transmission in anterolateral pathway

Answer 14

1. the velocities of transmission are only 1⁄3 - 1⁄2 those in the dorsal column–medial lemniscal system ie 8-40 m/s;
2. the degree of spatial localization of signals is poor;
3. the gradations of intensities are far less accurate, with most sensations being recognised in 10-20 gradations of strength, rather than as many as 100 gradations for the dorsal column system;
4. the ability to transmit rapidly changing or rapidly repetitive signals is poor.

Question 15

Fast pain

Answer 15

• Felt within ~ 0.1s after a pain stimulus is applied.
• AKA:
– sharp pain,
– pricking pain,
– acute pain,
– electric pain.
• Fast-sharp pain is NOT felt in most deep tissues of the body.

Question 16

Slow pain

Answer 16

• Begins after 1s or more after pain stimulus applied, then increases slowly over s or min.
• AKA:
– slow burning pain,
– aching pain,
– throbbing pain,
– nauseous pain,
– chronic pain.
• Is usually associated with tissue destruction.
• Can lead to prolonged, almost unbearable suffering.
• Slow pain can occur both in the skin and in almost any deep tissue or organ

Question 17

what are the three types of stimuli that can excite free nerve endings

Answer 17

mechanical (typically fast pain), thermal (typically fast pain), chemical
SLOW PAIN ELICITED BY ALL THREE
No adaption but can increase in sensitivity hyperalgesia

Question 18

what fibres transmit fast and slow pain

Answer 18

fast pain = A theta

slow pain = C fibres

Question 19

Can get “double” pain sensation:

Answer 19

• Sharp pain transmitted by Aδ fibres followed second or so
later by slow pain transmitted by C fibres.
• Fast pain allows for immediate (re)action, slow pain
tends to gets greater over time and stimulates person to keep trying to relieve cause of pain.

Question 20

fast pain pathway

Answer 20

• Signals elicited by mechanical or thermal pain.
• Transmitted via Aδ fibres at ~6-30m/
• Terminate mainly in lamina I (lamina marginalis) of the dorsal horns and there excite 2nd-order neurones of the
neospinothalamic tract.
• The 2nd-order neurones give rise to long fibres that cross immediately to opposite side of cord through anterior
commissure and then turn upward passing to the brain in the anterolateral columns.

Question 21

slow pain pathway

Answer 21

Slow-chronic pain pathway
• Signals mainly elicited by mechanical pain.
• Transmitted via C fibres at ~0.5-2m/s
The peripheral fibres terminate in spinal cord almost entirely in laminae II and III of dorsal horns (ie substantia gelatinosa).
Most signals then pass through 1+ additional short fibre neurons within dorsal horns before entering (mainly) lamina V, in dorsal horn.
Slow-chronic pain pathway
Paleospinothalamic Pathway
a much older system and
• Then give rise to long axons that mostly join fibres from fast pain pathway, passing first through anterior commissure to the opposite side of the cord, then upward to the brain in
the anterolateral pathway.

Question 22

Referred pain

Answer 22

• Pain in a location remote from tissue causing pain.
• eg pain in a visceral organ referred to an area on body
surface.
• Probablemechanism:
– Branches of visceral pain fibres synapse in spinal cord on same 2nd- order neurons that receive signals from skin.
– When visceral pain receptors are stimulated, pain signals from viscera are conducted through some of the same neurons conducting pain signals from skin, so person feels sensations originate in skin.

Question 23

Unconscious proprioception

Answer 23

• Inputfrom:
• Muscle Spindles (Muscle tension)
• Golgiorgans (Proprioception)
• Touch receptors
• 1st order neurone synapses in Dorsal Horn
• 2nd Order neurone Synapses in Ipsilateral Cerebellum
• Ventral tract “double crosses”
• Dorsal tract does not decussate

Question 24

Anterior spinal cord syndrome

Answer 24

• Hyperextension injury

* Infarction of anterior spinal artery

Question 25

Posterior Spinal cord Syndrome

Answer 25

• Penetration injury

* Posterior spinal artery occlusion • Multiple sclerosis

Question 26

Central Spinal cord syndrome

Answer 26

• Hyperextension

* Cord compression

Question 27

Brown-Sequard Syndrome

Answer 27

• Penetrating trauma