Physiology · MU Sofia

83. Somatosensory system: afferent pathways and central processing of information

Covers the two main somatosensory afferent routes, the dorsal column-medial lemniscus pathway for discriminative touch, vibration and proprioception and the anterolateral spinothalamic pathway for pain, temperature and crude touch, tracing the three neurons of each. It then explains central processing in the thalamus, the primary somatosensory cortex with its somatotopic homunculus and four Brodmann fields, and the association cortex S2, closing with recap questions.

Inside: 4 sections · 5 labelled pictures · 10 flashcards · 5 min

Topic 82 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 4 sections, about 682 words (5 min of reading), with 10 flashcards.

In this note

  1. Somatosensory system: afferent pathways and central processing of information.
  2. Afferent Pathways
  3. Central Processing
  4. Recap Questions

Afferent Pathways

The somatosensory system uses two main pathways to transmit information from the sensory receptors to the brain:

  • Dorsal Column-Medial Lemniscus Pathway:
  • Function: This pathway is responsible for transmitting information about discriminative touch, pressure, vibration, two-point discrimination, and proprioception. It primarily uses large, myelinated, fast-conducting nerve fibers (Group I and II).
  • Pathway:
  1. 1. First-order neurons: Their cell bodies reside in the dorsal root ganglia or cranial nerve ganglia. Their axons ascend ipsilaterally in the dorsal columns of the spinal cord to the nucleus gracilis (lower body) and nucleus cuneatus (upper body) in the medulla.
  2. 2. Second-order neurons: These neurons decussate (cross the midline) in the medulla and ascend in the medial lemniscus to the thalamus.
  3. 3. Third-order neurons: Located in the ventral posterolateral nucleus (VPL) of the thalamus, these neurons project to the somatosensory cortex (S1).
  • Anterolateral (Spinothalamic) Pathway:
  • Function: This pathway carries information about pain, temperature, and crude touch. It utilizes smaller, slower-conducting nerve fibers (Group III and IV).
  • Pathway:
  1. 1. First-order neurons: Their cell bodies are in the dorsal root ganglia. Their axons enter the spinal cord and synapse in the dorsal horn.
  2. 2. Second-order neurons: These neurons decussate in the spinal cord and ascend in the anterolateral tract to the thalamus.
  3. 3. Third-order neurons: Located in the VPL of the thalamus, these neurons project to the S1 cortex.

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