Somatosensory Cortex and Sensory Testing

Somatosensory Cortex and Pathways

• The postcentral gyrus in the brain is the primary somatosensory cortex.
• The thalamus relays sensory information from the face (VPM) and body (VPL) to the cortex.
• The posterior funiculus in the spinal cord carries discriminative senses.
• The posterior horn of the spinal cord houses most sensory nuclei.
• The anterolateral system in the spinal cord carries light touch, pain, and temperature sensations.

Sensory Testing

• Sensory exams assess conscious awareness and sensory perception.
• Unconscious sensory systems are indirectly evaluated based on their contribution to movement.
• Screening questions include inquiring about changes in sensation or asymmetry.
• It's important to consider dermatome overlap and nerve distribution.
• The trigeminal nerve supplies sensation to the face and cheek mucosa.
• Sensory testing should follow a systematic sequence, often from head to toe, covering all dermatomes.

Surface Landmarks and Dermatomes

• C2: posterior half of the skull.
• C3: medial end of clavicle.
• C4: medial acromion and below clavicle.
• C5: lateral elbow and lateral acromion.
• C6: 1st digit (and 2nd digit).
• C7: 3rd digit.
• C8: 5th digit (and 4th digit).
• T1: medial elbow.
• T2: anterior axilla.
• T4: nipple line.
• T6 or T7: xiphoid process.
• T10: umbilicus.
• T12: anterior iliac crest/pubic symphysis.
• L1: inguinal region (upper medial thigh).
• L2: medial thigh-mid-distance.
• L3: medial knee.
• L4: medial malleolus.
• L5: base of great toe (and lateral aspect leg/plantar aspect to heel).
• S1: lateral heel (base of 5th digit, fibula head, lateral malleolus, little toe).
• S2: posterior knee.
• S3: ischial tuberosity.

Sensory Testing Sequence

• The testing sequence for discriminative and deep sensations is based on joints and body regions, not dermatomes,.
• Peripheral neuropathy often affects sensory distribution.
• Sporadic distribution is asymmetrical and affects different regions on each side of the body.

Semmes-Weinstein Monofilament Test

• The Semmes-Weinstein Monofilament (SWM) test objectively evaluates light touch and deep pressure.
• SWM filaments are color-coded and standardized for force application.
• Lower filament numbers indicate less force required to bend.
• SWM testing is useful for monitoring changes in sensation, especially in nerve damage, reattachments, or peripheral neuropathies.

Light Touch Sensation

• Light touch can be assessed grossly using a cotton wisp, facial tissue, or camel hair brush.
• Variability in testing can be achieved by altering the time interval between applications or asking the patient if they feel anything.
• Hair follicles are exquisitely sensitive to light touch.

Pressure Touch Sensation

• Pressure touch can be assessed grossly using a pencil eraser or finger pressure.
• Objective methods include using graded weights or a SWM aesthesiometer.
• The SWM test is widely researched and used clinically for specific measurement of sensibility.

Temperature Sensation

• Temperature sensation is evaluated using hot and cold stimuli, such as water-filled test tubes.
• Dermatomal order and general tactile procedures are followed.
• Patient responds with "hot," "cold," or "can't tell" for each contact.

This quiz covers the somatosensory cortex, sensory pathways, and the methods of sensory testing. It highlights crucial concepts such as the function of the thalamus, spinal cord pathways, and the importance of assessing sensory perception through systematic exams. Understanding these elements is vital for evaluating sensory function in clinical settings.