Reflex কী?

Example 35.3 (What the reflex is for)

Standing, you sway; each sway stretches the muscles of one side of the ankle, whose spindles fire and whose reflex contraction pulls you back before you notice. Carrying a tray, a sudden extra weight stretches the arm muscles and the reflex stiffens them within $30\,\mathrm{ms}$, before the tray tips. The stretch reflex is a servomechanism: it opposes any change of a muscle’s length, and the brain adjusts its sensitivity to set the body’s tone — higher when you brace, lower when you sleep.

Example 35.6 (Speed)

Action potentials travel at $1\,\mathrm{m}/\mathrm{s}$ in thin unmyelinated fibres and up to $100\,\mathrm{m}/\mathrm{s}$ in the thick myelinated fibres of the stretch reflex. From the knee to the spinal cord and back is about $1.5\,\mathrm{m}$: some $15\,\mathrm{ms}$ of travel, to which the spindle’s response, one synapse and the muscle’s activation add another $15\,\mathrm{ms}$ — the $30\,\mathrm{ms}$ of the knee jerk. A message from a toe to the brain, $1.8\,\mathrm{m}$ away, takes about $20\,\mathrm{ms}$; the decision to move it, and its execution, some $150\,\mathrm{ms}$ more.