Question
Compare the three functional types of neurons — sensory, motor, and interneurons. Describe the direction of impulse transmission, structural features, and location of each type. How do they work together in a reflex arc?
(NEET + CBSE Board pattern)
Solution — Step by Step
| Feature | Sensory (Afferent) | Motor (Efferent) | Interneuron (Association) |
|---|---|---|---|
| Direction | Receptor → CNS | CNS → Effector | Within CNS (connects sensory to motor) |
| Cell body location | Dorsal root ganglion | Ventral horn of spinal cord | Within brain and spinal cord |
| Structure | Usually unipolar or pseudo-unipolar | Multipolar | Multipolar |
| Axon length | Long (from receptor to spinal cord) | Long (from spinal cord to muscle) | Short (local connections) |
| Function | Carry sensory information inward | Carry motor commands outward | Processing, integration, decision-making |
A reflex arc is the simplest neural pathway:
- Receptor detects stimulus (e.g., hand touches hot surface)
- Sensory neuron transmits impulse from receptor to spinal cord
- Interneuron in the spinal cord processes and relays the signal
- Motor neuron transmits impulse from spinal cord to effector
- Effector (muscle) responds — hand pulls away
In the simplest reflexes (like the knee-jerk reflex), there is NO interneuron — the sensory neuron synapses directly with the motor neuron. This is called a monosynaptic reflex.
Neurons are also classified by structure:
- Unipolar — one process extending from cell body (rare in humans, found in embryonic stage)
- Pseudo-unipolar — one process that splits into two branches (most sensory neurons)
- Bipolar — two processes (found in retina, olfactory epithelium)
- Multipolar — many dendrites + one axon (most motor neurons and interneurons)
graph LR
A["Receptor"] --> B["Sensory Neuron"]
B --> C["Interneuron in CNS"]
C --> D["Motor Neuron"]
D --> E["Effector - Muscle"]
B -.-> B1["Afferent: toward CNS"]
D -.-> D1["Efferent: away from CNS"]
C -.-> C1["Integration + Processing"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style C fill:#93c5fd,stroke:#000
style E fill:#86efac,stroke:#000Why This Works
The three neuron types form a logical chain: sense, process, act. Sensory neurons are the input channel, interneurons are the processing unit, and motor neurons are the output channel. This division mirrors how a computer works — input devices, CPU, output devices.
The key biological reason for having interneurons is integration. A simple touch sensation involves millions of sensory neurons, but the decision about how to respond (pull away? grip harder?) requires processing — which interneurons in the spinal cord and brain handle.
Common Mistake
Students often write that “sensory neurons carry impulses from the brain to muscles.” This is backwards. Sensory = afferent = toward CNS. Motor = efferent = away from CNS. Memory trick: Afferent = Arriving at CNS, Efferent = Exiting CNS. Getting this direction wrong causes you to lose marks on reflex arc diagrams.
NEET fact: interneurons make up the vast majority (~99%) of all neurons in the human body. The brain alone has about 100 billion interneurons. Sensory and motor neurons are relatively few — most neural tissue is dedicated to processing, not transmission.