Lesson 4: Speed, myelin, and reflexes
Why some responses beat your awareness to the finish line.
Start here
Step on something sharp and your foot lifts before you consciously decide to move it. Nothing skipped your nervous system — the route was just shorter.
In this lesson you will
- Explain how myelin affects signal speed.
- Describe how a spinal reflex is coordinated.
- Compare a reflex pathway with a pathway that involves the brain.
Not every message takes the same route or moves at the same speed. Two features matter most: whether the axon is wrapped in myelin, and whether the response is coordinated in the spinal cord or requires processing in the brain.
Myelin: insulation that speeds signals
Many axons are wrapped in a fatty covering called myelin, with small gaps along the way. Signals effectively jump between the gaps, which makes conduction much faster than in unmyelinated axons of similar size.
Reflexes: coordinated close to home
In a withdrawal reflex, sensory neurons carry information into the spinal cord, which can coordinate a motor response directly. Information is also sent onward to the brain — that is why you feel the pain a moment after you have already moved.
When the brain is in the loop
Deciding whether to catch a falling glass, and how, requires processing in the brain: sensory information, prediction, and motor planning. That route takes longer but allows far more flexible behavior.
Diagram
Bare axon vs. myelinated axon
Notice that the myelin covering leaves small gaps — the signal is renewed at those gaps instead of at every point along the membrane.
Myelin is a fatty covering wrapped around many axons, with small gaps along its length. Because the signal is renewed only at those gaps, a myelinated axon conducts much faster than an unmyelinated axon of similar size.
Compare: reflex pathway vs. brain-based response
Open both states, then read the summary that appears underneath.
Coordinated in the spinal cord
A protective withdrawal can be organized by circuits in the spinal cord while information also travels up to the brain.
- Few synapses, so the response is very fast
- Happens before conscious awareness of the sensation
- Limited flexibility: a protective response, not a plan
View both states to unlock the comparison summary.
Predict: which signal arrives first?
Predict, then reveal the explanation.
Two axons are the same diameter. One is myelinated, one is not. Which carries its signal faster?
Sort it: reflex or brain-based?
Decide how each response is most likely coordinated, then read the explanation.
Item 1 of 4
Jerking your hand back from a hot pan
Real-world connection: myelin, injury, and rehabilitation
Conditions that affect myelin, such as multiple sclerosis, can slow or disrupt signal conduction and change movement, sensation, or vision. Research supported by organizations such as the National Institute of Neurological Disorders and Stroke studies how myelin is maintained and repaired, and how rehabilitation helps people regain function after nervous-system injury.
Three Key Takeaways
- 1Myelin speeds conduction by letting signals jump along gaps in the covering.
- 2Some rapid reflexes are coordinated in the spinal cord while the brain is still informed.
- 3Speed and flexibility are a trade-off built into the routes signals take.