The process of sensory-motor integration eventually gives rise to the patterns of human behavior. This makes it important to distinguish between sensory and motor neurons, and we will examine the matter in the following article.
Sensory neurons belong to the sensory components of the functional nervous system. Essentially, they take information from the internal and the external environment via the sensory receptors situated at the surface of our bodies. Within the body, sensory neurons process the information into sensory nerves (which are essentially bundles of axons) along the spinal cord up to the level of the cerebral cortex inside the brain. From this point, this information becomes synthesized and integrated in circuitry that is distributed across the lobes in the cerebral hemispheres and runs loops into the subcortical structures, such as the basal ganglia, thalamus, amygdala (emotional centers), and memory structures like hippocampus, among others.
Motor neurons are essential components of motor systems in humans. There are two parallel pathways that allow motor systems inside the human organism to produce our behavior:
- Visceral motor system with its sympathetic, parasympathetic, and enteric divisions, autonomic ganglia and nerves.
- Somatic motor system with its motor nerves.
Both of these run information down to the effectors, which are mainly comprised of smooth muscles, cardiac muscles, skeletal muscles, and glands.
Essentially, sensory neurons are responsible for delivering information about smells, taste, sounds, etc. to the brain. Motor neurons, on the other hand, are responsible for responding to the commands made by the brain upon delivering sensory information.
In neurobiology, one of the important principles on which the human sensory and motor systems are based is known as the sensory pathways principle. Let’s consider one example.
The skin is innervated by nerve fibers, which are grown by cells. The axons, of which the fibers consist, reach the neurons on the skin surface. These are known as peripheral neurons, or 1st order neurons. The other branch reaches to the central nervous system, where 1st order neurons connect via synapses with 2nd order neurons, which, in their turn, connect further to 3rd order neurons. The latter connect to the vast network of interconnected cells and some processing station within the brain, for example, the cerebral cortex. Such chain of processing allows for the signal to be delivered from the output in the skin to, eventually, the cerebral cortex.
This is the case of a sensory pathway, where information flows in an ascending direction, and describes the functioning of sensory neurons.
Motor neurons, on the other hand, follow the pathway of the opposite direction:
The cortical network also has a motor component. The sensory signal is integrated in some kind of network, which coordinates and produces an output, which, eventually, reaches motor neurons, which ultimately supply some effector, for example, skeletal muscle.
This output pathway is known as a motor pathway (also known as descending pathway) from the higher levels to the lower. (For example, from the cerebral cortex down to the spinal cord, which may lead, for example, to the contraction of the skeleton muscle).
|Sensory neurons||Motor neurons|
|Transmit sensual information (light, sounds, smell, touch, pressure, limb position) to the brain||Carry signals from the effectors like muscles and glands|
|Dendrites from the cell bodies are located outside of the CNS; the axons propagate to the spinal cord or brainstem to convey information to the brain.||Are situated in the spinal cord|
In this video you can see a brief lecture that will help you understand the differences between sensory and motor neurons: