Lecture 13: 2nd to last somatosensation

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18 Terms

1

Which axons have a direct ascending branch?

Axons of Aα and Aß fibers (involved in touch and proprioception)

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2

Which axons have an indirect ascending pathway?

Axons of Aδ and C (involved in Nociception/pain and Thermoreception)

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3

Do the different fibers project to separate regions of the spinal cord?

Yes

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4

Where do the axons fibers of Aα (heavy, myelinated) project to?

Axons fibers project to the central and ventral spinal cord

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5

Where do the axon fibers of Aß (heavy, myelinated) project to?

Axons fibers project to the dorsal horn of the spinal cord, laminae III-V

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6

Where do the axon fibers of Aδ (lightly myelinated) project to?

Axons fibers project to the dorsal horn of the spinal cord, laminae I and IV

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7

Where do the axon fibers of C (unmyelinated) project to?

Axon fibers project to the dorsal horn of the spinal cord, laminae I and V

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8

What is the direct pathway?

The dorsal column-medial lemniscus pathway

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9

What sensory information does the dorsal column- medial lemniscus pathway transmit?

It transmits touch and proprioception information

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10

What’re the steps of the dorsal column pathway?

1⃣ First-order neurons (Aα & Aβ fibers) → Enter dorsal column (ipsilateral)

  • Lower bodyGracile fasciculus

  • Upper bodyCuneate fasciculus

2⃣ Second-order neuronsGracile/Cuneate nucleus (medulla)Decussation (crosses over) to Medial lemniscus

3⃣ Medial lemniscus → Ascends through pons & midbrain

4⃣ Third-order neurons in thalamus (VPN) → Project to Primary Somatosensory Cortex (S1)

🛑 Left S1 = Right body input & vice versa (contralateral processing)

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11

What’re the steps of the Spinothalamic pathway?

1⃣ First-order neurons (Aδ & C fibers) → Enter dorsal horn of spinal cord → Immediately synapse with second-order neurons

2⃣ Second-order neuronsDecussate (cross over) in spinal cord → Ascend via lateral spinothalamic tract (contralateral)

3⃣ Spinothalamic tract → Passes through medulla, pons, midbrain

4⃣ Third-order neurons in thalamus (VPN & other regions) → Project to Primary Somatosensory Cortex (S1)

🛑 Left S1 = Right body pain/temp input (contralateral processing)

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12

What information does the Spinothalamic relay?

Pain and temperature

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13

What’re the areas of the S1?

Area 1, 2, 3a, and 3b

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14

Where is info first relayed to in the S1?

Areas 3a and 3b

  • 3a receives proprioceptive signals carried by Aα fibers

  • 3b receives touch (tactile) signals carried by Aß fibers

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15

What do areas 3a and 3b do?

They relay info to areas 1 and 2 and info is then relayed to higher order regions

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16

What is a somatotopic map?

  • A map in the primary somatosensory cortex (S1) that represents stimulus locations on the body

  • Adjacent neurons in S1 respond to stimuli from adjacent body locations

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17

How is the cortical area in S1 allocated to different body parts?

  • Proportional to sensory receptor density

  • More receptors = larger S1 representation

  • Fine discrimination areas (fingers, lips) occupy more space than areas like the trunk

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18

Why do fingers and lips have a larger representation in S1?

  • High concentration of sensory receptors

  • Crucial for precise tactile perception

  • Require finer discrimination than larger body areas like the trunk

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