The labeled parts and the direction of the flow of electrical signals are as follows:
(a):
Sensory neuron
(b):
Spinal cord (CNS)
(c):
Motor neuron
(d):
Effector (muscle in arm)
The flow of electrical signals: Receptors in hand → Sensory neuron → Spinal cord (CNS) → Motor neuron → Effector (muscle in arm)