Babinski Reflex for the MCAT: Understanding Plantar Responses

When you stroke the bottom of someone’s foot, what their toes do can tell you a lot about their nervous system. This is the basis of the Babinski reflex—a clinical test with major MCAT significance.

Is the response normal or pathological? Is it expected in a newborn or an adult? Today’s blog uses King of the Curve’s clear visual to break it down once and for all.

What Is the Babinski Reflex?

The Babinski reflex (or plantar reflex) tests upper motor neuron integrity via a simple foot stimulus.

• Method: Stroke the sole of the foot, heel to toe.

• Expected Response (in adults): Toes flex downward.

• Abnormal Response (Babinski sign): Toes fan outward and big toe extends upward.

Visual Breakdown

🦶 Normal Plantar Response (Left Panel)

• Toes flex downward

• Seen in healthy adults

• Indicates intact corticospinal tract

🦶 Babinski Sign (Right Panel)

• Toes fan and dorsiflex

• Normal in infants <1 year

• In adults, indicates upper motor neuron lesion (UMNL)

When Is the Babinski Reflex Normal?

• ✅ Infants (up to ~12 months): Because the corticospinal tract is not fully myelinated yet.

• 🚫 Adults: Suggests neurological damage if present (e.g., stroke, spinal cord lesion, ALS).

MCAT-Style Question Example

Question:
A 40-year-old man has a stroke. When the sole of his foot is stroked, his toes fan outward and the great toe dorsiflexes. What does this indicate?

A. Lower motor neuron lesion
B. Intact corticospinal tract
C. Upper motor neuron lesion
D. Sensory nerve dysfunction

✅ Correct Answer: C — The Babinski sign in adults signals upper motor neuron involvement.

Test Tips for the MCAT

🧠 Know how to differentiate:

• Babinski = UMNL

• Normal flexion = LMNL or intact CNS

🔍 Also review:

• Other primitive reflexes (e.g., Moro, grasp)

• Neurological development milestones in infants

Final Thoughts

The Babinski reflex isn’t just for neurologists—it’s MCAT gold. It connects neurodevelopment, clinical signs, and motor pathway integrity.

Learn it once. Visualize it forever.

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