Sensory Pathways and Dermatomes

Overview

This lecture covers the organization and clinical relevance of sensory pathways and dermatomes in the peripheral nervous system, as well as major ascending sensory tracts in the spinal cord.

Dermatomes and Spinal Cord Organization

• Dermatomes are body regions served by specific sensory spinal nerves.
• Sensory nerves carry information from receptors into the spinal cord via spinal nerves.
• Spinal nerves have both sensory (dorsal root) and motor (ventral root) components.
• Neurological exams may test dermatomes to assess the location and severity of spinal cord injury.
• The ASIA (American Spinal Injury Association) evaluation tests dermatomal sensory and motor function bilaterally.
• Each spinal cord region (cervical, thoracic, lumbar, sacral) has its own set of numbered spinal nerves.
• Loss of sensation between adjacent dermatomes may suggest injury at that spinal cord segment.

Somatosensory Cortex and Sensory Perception

• The primary somatosensory cortex maps sensory input from specific body regions (somatotopy).
• The more sensory receptors a body part has, the more area it occupies in the cortex diagram ("homunculus").
• This area handles conscious sensation (e.g., touch, temperature), not subconscious monitoring (e.g., blood pH).
• Brodmann areas 1, 2, 3 are primary for somatosensory input; 5 and 7 are associative areas.
• Astereognosis is the inability to identify objects by touch alone.

Major Ascending Sensory Pathways

• Lateral Spinothalamic Tract (LSTT): Transmits pain and temperature via nociceptors; ascends laterally in the spinal cord.
• Ventral Spinothalamic Tract (VSTT): Transmits crude touch and pressure; ascends ventrally, provides redundancy by traveling across multiple segments.
• Dorsal Column-Medial Lemniscus (DCML): Handles fine touch, vibration, and proprioception; ascends via dorsal columns, synapses in the medulla, then thalamus.
• Decussation is when sensory signals cross to the opposite side of the spinal cord or brainstem.
• Medullary nuclei (gracile and cuneate) process signals from lower and upper body, respectively.

Cranial Nerve Sensory Pathways

• Some sensory information (e.g., from the face via trigeminal nerve, cranial nerve V) bypasses spinal nerves and enters the brainstem directly.
• The trigeminal ganglion relays facial sensation to the pons, brainstem, thalamus, and internal capsule.

Key Terms & Definitions

• Dermatome — Skin region innervated by a single spinal nerve.
• ASIA Evaluation — Clinical test for spinal cord injury using sensory and motor assessment.
• Somatotopy — Organization of the cortex according to body region input.
• Homunculus — Diagram showing body part representation in the somatosensory cortex.
• Astereognosis — Inability to recognize objects by touch.
• Decussation — Crossing of nerve fibers from one side of the CNS to the other.
• Lateral Spinothalamic Tract — Ascending pathway for pain and temperature.
• Ventral Spinothalamic Tract — Ascending pathway for crude touch and pressure.
• Dorsal Column-Medial Lemniscus — Ascending pathway for fine touch, vibration, proprioception.

Action Items / Next Steps

• Review diagrams of dermatomes, somatosensory cortex, and ascending spinal tracts.
• Prepare for upcoming section on cranial nerve pathways in the next lecture video.