"BIOLOGICAL BASEBALL" PAGE 2
A few reactions are fast because they skip the brain altogether--the knee-jerk reflex, for example, requiring only a few nerve-cell-to-nerve-cell interconnections, and thus happening in "the blink of an eye." The neural pathway makes a short loop from sensory cells through interneurons to motor nerve cells, which control the appropriate muscle groups. A message is sent to the brain at the same time it is sent to the muscles, so we actually become aware of the knee jerk as it happens.
Unfortunately for the batter, swinging the bat requires more than a simple reflex pathway. Judgment, decision making, and total body movement add up to hundreds of thousands of nerve cells working simultaneously. And conduction velocity along the nerve fiber isn't the only factor affecting reaction time.
When one nerve cell signals another, it does so by releasing a chemical substance which crosses a tiny gap between the cells, called the "synapse." It takes about half a thousandth of a second for the chemical transmitter to flow across the gap, a period called "synaptic delay."
The longest delays probably involve the nerve cells that make the decision to swing. These decision-making cells receive their input from the eye by way of the brain's visual cortex. It takes at least 43 thousandths of a second for information about the velocity and trajectory of the baseball to be sent from the retina to the higher visual cortex. What happens during the actual "decision" is a neurological mystery - -but once the decision is made, a signal is sent to the cerebellum initiating a series of pre-programmed, reflex-like actions (for a practiced batter).
The last fraction of the ball's flight across the plate is consumed by commands to the bat-swinging muscles. Amazingly, even after the batter has started his swing he still has some ability to reverse his decision and check his swing.
Moving a single group of muscles may require the interaction of numerous nerve cells and involve multiple synaptic delays, as the body receives sensory information (sees the ball), processes it (makes a decision), and coordinates muscle action (swings the bat). Practice eliminates wasted time by speeding up the decision making -- somehow the obvious mistakes and fruitless actions a novice spends time thinking about are simply ignored by the practiced player, and his brain saves the time needed to consider them. But basic reaction time due to nerve conduction and synaptic delay remains an irreducible constant of the game.