1. Somatic Sensory Pathways
Two Pathways:
3 neurons in pathway
1st, 2nd and 3rd order neurons
(from PNS through CNS)
2nd fiber crosses over
(ipsilateral to contralateral)
dorsal column / medial lemniscus system
anterolateral system

2. Somatic Sensory Pathways
Fig. 7- 2
Ganong
Somatic Sensory Pathways
Fig. 13.4

3. Dorsal Column / Medial Lemniscus System
Signals ascend spinal cord via the dorsal (white) columns.
large myelinated fibers (types I and II)
fine touch and proprioception
rapid transmission
high degree of spatial orientation
Fig. 13.5

4. Posterior Column / Medial Lemniscus System
1st order sensory fibers ascend ipsilaterally in dorsal (posterior) columns to the medulla, and synapse in the medulla.
2nd order fibers cross over in the medulla and travel through the brain stem via the medial lemniscus to the thalamus.
3rd order fibers project to the somatosensory areas of the cerebral cortex.
post central gyrus of the parietal lobe

5. Anterolateral System
Signals ascend the spinal cord via the anterior and lateral spinothalamic tracts.
small myelinated and unmyelinated fibers
(types III and IV)
crude touch, pain, temperature
slow transmission
high volume of information
low spatial orientation
Fig. 13.5

6. Anterolateral System
1st order sensory neurons enter spinal cord and synapse in the posterior horn.
2nd order fibers cross over in the spinal cord, and ascend spinal cord to the thalamus via the anterior (ventral) and lateral spinothalamic tracts.
3rd order fibers project to the somatosensory areas of the cerebral cortex.
post central gyrus of the parietal lobe

7. Parietal Lobe Post Central Gyrus
homunculus
visual representation of the human body on the cerebral cortex
Because of crossover, the left side of the body projects to the right side of the cerebrum and vice versa.
Fig

8. Somatic Sensation
Perception depends less on the source of the signal and more on what part of the brain is stimulated.
2 examples from pain perception
phantom pain
referred pain

9. Phantom Pain Amputees feel pain in their (missing) limbs.
filling of cerebral cortical “vacuum” by other afferent signals?
activation of cut nerve endings?

10. Referred Pain Pain from viscera is perceived as coming from the skin.
e.g., heart attack: pain in left arm and shoulder
related to dermatomes
Referred Pain
Fig. 7-7 Ganong
Fig. 16.4

11. Regulation of Pain Sensation
Gate Theory
In the dorsal horn, the synapse between the first and second order neuron is a gateway for pain.
Pain may be blocked by “closing” this gate.
counter irritants
brain’s analgesia system: one of its sites of action
gate
Fig. 16.5
collateral
from touch
receptor
Fig. 7-2
Ganong

12. Sensory Collaterals to the Brain Stem Stimulate the Reticular Activating System
Fig. 16.3

13. Subconscious Proprioception
via spinocerebellar pathways
Fig. 13.4

14. Video Assignment The Brain – Parts 1 and 2
Part 1: The Enlightened Machine
Part 2: Vision and Movement
DVD and VHS versions are available.
Each part lasts about an hour.
See p.13 of Lecture Outlines for checkout information.
As you watch the videos, pay attention to who has what diseases.