The afferent fibers of the posterior root, which originate from the nerve cells of the spinal ganglion, transmit sensory signals to the posterior horn cells of the spinal cord,and these pass them on to the brain (C). The relay may also take place in the medulla ob-longata. However, the afferent fibers may also run to the anterior horn cells and trans-mit the signal directly to these cells. The re-sulting muscle reaction is called reflex, the underlying neuronal circuit is called reflexarc (D). In general, the afferent fibers do notrun directly to the motor neuron (mono-synaptic reflex arc) but via interneurons thatare interposed (multisynaptic reflex arc) (E).
The monosynaptic intrinsic reflex (stretchreflex) and the multisynaptic extrinsic reflex(withdrawal reflex) are of clinical impor-tance. In the stretch reflex (F), the muscle is briefly stretched by a tap on its tendon. Stimulation of the muscle receptors results in a momentary contraction of the muscle as a counter reaction. The reflex in-volves only a few neurons at any level of the spinal cord. In the withdrawal reflex (G), skin receptors are stimulated (pain); the with-drawal movement is brought about by the coordinated action of several muscle groups. The signal spreads through several levels of the spinal cord and involves many interneurons.