Somatic Sensory Pathways

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
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

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Parietal LobePost 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. 14.21
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

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
• brains 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 AssignmentThe 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.