Sensory neurons carry incoming information from the sensory receptors of the body toward the central nervous system (brain and spinal cord), whereas motor neurons carry outgoing commands away from the central nervous system to the muscles and glands.
Key Takeaways
- Sensory neurons carry information toward the central nervous system (CNS), while motor neurons send commands away from the CNS to muscles or glands.
- Sensory neurons are afferent and found in dorsal root ganglia, whereas motor neurons are efferent and located in the spinal cord or brain.
- Structurally, sensory neurons are usually unipolar, and motor neurons are typically multipolar.
- Both types work together in reflex arcs, allowing the body to detect and respond to stimuli rapidly.
- Understanding these neurons helps explain how the brain and body communicate to sense the world and produce behavior.
Sensory vs. motor neurons: Differences
Below are the main differences between sensory and motor neurons:
Function
Sensory neurons transmit information from sensory receptors (in the skin, eyes, ears, tongue, etc.) to the CNS, enabling perception of stimuli.
Motor neurons transmit commands from the CNS to effectors (muscles or glands), enabling responses or actions.
In other words, sensory neurons inform the brain/spinal cord of what’s happening, while motor neurons direct the body on how to react.
Direction of Signal (Afferent vs. Efferent)
Sensory input travels toward the central nervous system along afferent pathways, whereas motor impulses travel away from the CNS along efferent pathways.
A helpful shorthand is “afferent arrives, efferent exits” the CNS.
Location of Cell Bodies
The cell bodies of sensory neurons are often located in clusters just outside the spinal cord (called dorsal root ganglia in the peripheral nervous system), since they relay signals from the periphery into the CNS.
In contrast, the cell bodies of motor neurons reside within the central nervous system (for example, in the spinal cord’s ventral horn or in the brain), and their long axons extend out to muscles or glands.
Structure
Structurally, many sensory neurons are unipolar – they have a single long process that splits into two branches, one connecting to the sensory receptor and one entering the CNS.
This allows sensory neurons to quickly transmit signals from the body into the spinal cord.
Motor neurons, on the other hand, are typically multipolar – they have one long axon and many branched dendrites – which is suited to integrating inputs and controlling outputs to multiple muscle fibers.
Mnemonic: A useful memory aid for these terms is “SAME DAVE”, which stands for Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent. This reminds us that sensory neurons are afferent and enter the spinal cord via the dorsal side, while motor neurons are efferent and exit via the ventral side.
What Are Sensory Neurons?
Sensory neurons are specialized nerve cells that carry information from sensory receptors to the central nervous system (CNS).
They respond to external stimuli like touch, temperature, sound, and light, as well as internal signals such as blood pressure or joint position.
You’ll find sensory neurons in receptors located throughout the body — in the skin, eyes, ears, tongue, and internal organs.
Their cell bodies are grouped in clusters called dorsal root ganglia, part of the peripheral nervous system (PNS). From here, their axons enter the CNS, delivering information for processing.
Importantly, not all sensory input reaches conscious awareness. Some sensory neurons stop at the spinal cord, forming part of a reflex arc that allows for quick, automatic responses.
This setup ensures the body can react rapidly to danger — like pulling your hand from a hot surface — even before the brain fully registers the pain.
Structurally, most sensory neurons are unipolar, with a single process that splits to connect the sensory receptor and the spinal cord. This design allows for fast, efficient signal transmission.
In short, sensory neurons are the input system of the nervous system, constantly relaying data from the body and environment to the CNS for perception or reflexive action.
What Are Motor Neurons?
Motor neurons are nerve cells that carry commands from the central nervous system (CNS) to muscles or glands, enabling movement and physiological responses.
Located within the spinal cord or motor areas of the brain, motor neuron cell bodies reside in the CNS, while their axons extend into the peripheral nervous system (PNS) to reach effectors — such as skeletal muscles, smooth muscles, or glands.
When activated, motor neurons release neurotransmitters at the neuromuscular junction, triggering muscle contraction.
Motor neurons play a key role in both voluntary movements (e.g., walking, speaking) and involuntary reflexes.
For example, pulling your hand away from something hot involves motor neurons that receive signals via the spinal cord and immediately activate arm muscles.
Structurally, they are typically multipolar, with many dendrites for receiving input and a single long axon for sending signals to distant targets. This allows motor neurons to integrate complex information before initiating an action.
In essence, motor neurons are the output system of the nervous system — converting decisions made in the CNS into action in the body.
How Sensory and Motor Neurons Work Together (Sensory-Motor Integration)
Sensory and motor neurons do not operate in isolation – in fact, they are two halves of a complete circuit.
The classic example of their cooperation is a reflex arc or any simple stimulus-response pathway.
Typically, a sensory neuron will detect a change or stimulus and send a signal into the CNS.
There, the signal is often passed to an interneuron (also known as a relay neuron), which processes the information and quickly relays it to a motor neuron to produce a response.
Interneurons are the third category of neuron, found entirely within the CNS, that serve as the connectors and processors between input and output.
When you touch something hot, signals travel from the skin to the spinal cord, where interneurons quickly activate motor neurons to pull your hand away—often before you’re even aware of the pain. This quick loop allows the body to respond instantly to danger.
Some reflexes, like the knee-jerk reaction, involve a direct connection between sensory and motor neurons for maximum speed.
More commonly, interneurons serve as intermediaries, helping coordinate appropriate responses.
Regardless of complexity, reflexes follow the same basic pattern:
sensory input → central processing → motor output.
This rapid communication between neurons allows the nervous system to protect the body and maintain functional control without conscious effort.
