Figure 21.1 The five basic components of reflex arcs. Stimulus Skin Interneuron 1 Receptor 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section)

Figure 21.2 Monosynaptic and polysynaptic reflex arcs. Spinal cord Interneurons Sensory (afferent) neuron Sensory receptor (muscle spindle) Sensory receptor endings in fingertip Nail Sensory (afferent) neuron Motor (efferent) neuron Motor (efferent) neuron Effector (biceps brachii muscle) Effector (quadriceps femoris muscle) Monosynaptic reflex Polysynaptic reflex 2

Figure 21.3 The patellar (knee-jerk) reflex—a specific example of a stretch reflex. + Quadriceps (extensors) 3a 3b 3b + 1 Patella Spinal cord (L2 L4) Muscle spindle Tapping the patellar ligament excites muscle spindles in the quadriceps. 1 Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 The motor neurons (red) send activating impulses to the quadriceps, causing it to contract, which extend the knee. 3a The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b  Excitatory synapse  Inhibitory synapse 3

Figure 21.4 Testing the patellar reflex.

Figure 21.5 Testing the calcaneal tendon reflex.

Figure 21.6 Testing the plantar reflex.