Lecture 13: 2nd to last somatosensation

Which axons have a direct ascending branch?

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

Which axons have an indirect ascending pathway?

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

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

Yes

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

Axons fibers project to the central and ventral spinal cord

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

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

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

What is the direct pathway?

The dorsal column-medial lemniscus pathway

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

It transmits touch and proprioception information

What’re the steps of the dorsal column pathway?

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

• Lower body → Gracile fasciculus

• Upper body → Cuneate fasciculus

2⃣ Second-order neurons → Gracile/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)

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 neurons → Decussate (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)

What information does the Spinothalamic relay?

Pain and temperature

What’re the areas of the S1?

Area 1, 2, 3a, and 3b

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

What do areas 3a and 3b do?

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

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

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

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