Main Difference – Sensory Neurons vs Motor Neurons
Sensory, relay and motor neurons are the three types of neurons that build the nervous system of animals. They carry information as action potentials which occur on the membrane of neurons. These action potentials are carried over a long distance, from sensory organs to the central nervous system and from the central nervous system to the effector organs like muscles and glands. The central nervous system consists of the brain and the spinal cord. Movements and the responses of the body are controlled by the central nervous system, with the help of both sensory and motor neurons. The main difference between sensory and motor neurons is that sensory neurons carry signals from receptors to the spinal cord and brain whereas motor neurons carry signals from the central nervous system to effector organs.
Key Areas Covered
1. What are Sensory Neurons
– Definition, Structure, Characteristics, Function
2. What are Motor Neurons
– Definition, Structure, Characteristics, Function
3. What is the difference between Sensory and Motor Neurons
– Comparison of Key Differences
Key Terms: Sensory neurons, Five Senses, Motor neurons, Lower motor neurons, Upper motor neurons, Somatic motor neurons, autonomic motor neurons
What are Sensory Neurons
Sensory neurons are the afferent neurons which are responsible for converting external stimuli into internal electrical impulse. The nerve impulse travels along the afferent nerve fibers to the brain via the spinal cord. The cell body of the sensory neuron is located in the dorsal ganglia of the spinal cord. Sensory neurons are composed of five primary senses: sight, smell, taste, touch, and hearing.
Five Senses
Rod cells and cone cells in the retina are activated by light. This activates specialized nerves called retinal ganglia. The nerve impulses generated in the retinal ganglia are transferred to the brain via the optical nerve, sensing the sight. In sensing smell, the odor of a molecule dissolves in the mucus and attaches to microvilli. Dendrites of the sensory neurons are present in the microvilli. The contact of odor molecules with the dendrites stimulates the sensory neurons to send the impulses to the brain, sensing the smell. Tastes buds are also sensory neurons which are present on the tongue. The sensory neurons in the tongue work cooperatively with the olfactory neurons in the nose. Free nerve endings and corpuscles are the two types of neurons found on the skin. Free nerve endings are embedded in the dermis. They detect mechanical stimuli like touch, pressure, and stretch. They also detect temperature and danger (nociception) as well. The inner hair cells in the ear stimulate the afferent audio nerve and signals send to the brain, allowing an organism to sense different sounds.
Figure 1: Sensory neuron
What are Motor Neurons
Motor neurons are the efferent neurons which carry signals from the spinal cord to the effector organs, facilitating muscle contraction and secretion of substances from glands. Two types of motor neurons are found: upper motor neurons and lower motor neurons. Lower motor neurons originate in the spinal cord and synapse with muscle fibers while upper motor neurons are cortico-spinal interneurons. Upper motor neurons arise from the motor cortex and descend to the spinal cord. They activate lower motor neurons. Two categories of lower motor neurons occur in the peripheral nervous system: somatic motor neurons and autonomic motor neurons. The somatic motor neurons act on voluntary muscles in the neck, arms, and legs. The cell body of the somatic motor neurons is located in the central nervous system, either in the brain or spinal cord. The autonomic motor neurons innervate heart muscles, smooth muscles which surround the gastrointestinal tract, and glands. Sympathetic and parasympathetic neurons are the two classes of the autonomic motor neurons.
Figure 2: Lower motor neuron
Difference Between Sensory and Motor Neurons
Definition
Sensory Neurons: Sensory neurons are nerve cells that are responsible for converting external stimuli into internal electrical impulses.
Motor Neurons: Motor neuron is a nerve cell whose cell body is located in the spinal cord and axon fiber projects outside of the spinal cord. It directly or indirectly controls effector organs like muscles and glands.
Axon
Sensory Neurons: Sensory neurons consist of a short axon.
Motor Neurons: Motor neurons consist of a long axon.
Receptor
Sensory Neurons: Sensory neurons consist of a receptor.
Motor Neurons: Motor neurons do not consist of a receptor.
Cell Body
Sensory Neurons: Cell body of the sensory neuron is situated in the dorsal root ganglion of the spinal cord, and no dendrites are found in it.
Motor Neurons: Cell body of the motor neuron situated in the ventral root ganglion of the spinal cord and consists of dendrites.
Dendrons
Sensory Neurons: Sensory neuron consists of one long dendron.
Motor Neurons: Motor neuron consists of many short dendrons.
Function
Sensory Neurons: Sensory neurons carry signals from the outer part of the body into the central nervous system.
Motor Neurons: Motor neurons carry signals from the central nervous system to the outer parts of the body.
Afferent/Efferent Pathways
Sensory Neurons: Sensory neurons follow the afferent pathway.
Motor Neurons: Motor neurons follow the efferent pathway.
Number
Sensory Neurons: An adult has around 10 million sensory neurons in the body.
Motor Neurons: Around half million of motor neurons are found in the body.
Multipolar/Unipolar
Sensory Neurons: Sensory neurons are unipolar.
Motor Neurons: Motor neurons are multipolar.
Found in
Sensory Neurons: Sensory neurons are found in skin, eyes, ears, tongue, and nose.
Motor Neurons: Motor neurons are mainly found in muscles and glands.
Conclusion
Sensory and motor neurons are two of types of neurons found in the central nervous system of animals. Sensory neurons carry impulses from sensory organs like skin, nose, eye, ear, and tongue to the central nervous system. Motor neurons carry impulses from the central nervous system to the effector organs like muscles and glands. The main difference between sensory and motor neurons is in their structure and function in animals. The central nervous system is capable of coordinating the functions in the body by the use of both sensory and motor neurons.
Reference:
1.” Introducing the Neuron.” Boundless. N.p., 20 Oct. 2016. Web. Available here. [Viewed 31 May 2017].
2. Lodish, Harvey. “Overview of Neuron Structure and Function.” Molecular Cell Biology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. Available here. [Viewed 31 May 2017].
3. Cheprasov, Artem. “Types of Neurons: Sensory, Afferent, Motor, Efferent & More.” Study.com. N.p., n.d. Web. Available here. [Accessed 31 May 2017].
Image Courtesy:
1. “1401 Receptor Types”By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Blausen 0657 MultipolarNeuron” By BruceBlaus – Own work (CC BY 3.0) via Commons Wikimedia
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