1. 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)

2. 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

3. 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

4. Figure 21.4 Testing the patellar reflex.

5. Figure 21.5 Testing the calcaneal tendon reflex.

6. Figure 21.6 Testing the plantar reflex.