Complex sentence: To study synapses in the peripheral nervous system, Eccles and colleagues used the stretch reflex as a model, which is easily studied because it consists of only two neurons: a sensory neuron (the muscle spindle fibre) and the motor neuron.
Split into simpler sentences: To study synapses in the peripheral nervous system, Eccles and colleagues used the stretch reflex as a model. This reflex is easily studied because it consists of only two neurons: a sensory neuron (the muscle spindle fibre) and the motor neuron.