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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