Neurology 102

A somatic reflex arc is one in which there is the simplest possible arrangement of elements to permit a response to stimuli, and in which the final element in the chain is skeletal muscle. In the crude sketch given here, you see the basic elements of this system. 1 is some sensory transducer in the periphery, for example, a Pacinian corpuscle or other tactile sensor in the skin. Shown here in blue is 2, the pseudo-unipolar sensory neuron in the circuit. Its soma is physically located in a craniospinal ganglion (pictured here as a dorsal root ganglion, but it could also be on a cranial nerve). Drawn in black is 3, an interconnector neuron, whose soma is found in the CNS. Drawn in red, 4 is a motor neuron whose soma is in the ventral horn of the gray H of the spinal cord. The last element involved is 5, the effector organ, which in the case of this type of arc, will always be skeletal muscle.

Here's how the system works: something impinges on the transducer, which causes the afferent fiber of the pseudo-unipolar sensory neuron to fire. That signal is transmitted via its efferent fiber into the CNS, specifically into a synapse with an interconnector neuron in the dorsal horn of the gray H. That neuron then sends a signal to a synapse with the motor neuron in the ventral horn. The afferent motor fiber (axon) of the motor neuron—which may actually be several meters in length—leaves the CNS and terminates at a motor end plate on some myofiber. When it fires it initiates contraction of 5.

Notice that this loop is completely independent; it's not necessary to have CNS involvement beyond the "relay" at the interconnector neuron. Let's say you inadvertently put your hand on a hot stove burner. You will of course immediately remove it, and in doing so you are making use of this type of arc, bypassing conscious thought. In fact, the sensation of uncomfortable heat makes it to the CNS after the motor response to withdraw your hand is initiated. In other words, you move your hand away before you "know why" you're doing it. Of course, it's possible to override this loop with direct CNS input. As is always true, the Brain is The Boss. If you really, really want to, it's possible to hold your hand on that hot burner, ignoring the somatic reflex. Some people have done things like this, but most of us haven't got that level of willpower.

The doctor is testing your somatic reflex arc when he taps your knee with a rubber mallet and studies the knee-jerk reaction. The impulse of the hammer tap travels to the spinal cord and back to the effector muscle, in a reactive arc. Diagnostics can be derived from observing the speed and tone of the reaction.