Being the most basic units of the human nervous system, neurons play a vital role in sensing and responding to different external as well as internal stimuli. A motor neuron is one of the three types of neurons involved in this process. Read about the structure and function of a motor neuron with reference to a neatly labeled diagram, in this Bodytomy post.
Motor Neuron Disease (MND)
A motor neuron disease affects the normal functioning of motor neurons, resulting in their degeneration and death. This leads to muscle weakness and atrophy to such an extent that basic voluntary muscle activity like speaking, swallowing, and breathing is largely affected.
Let’s begin with the definition of a motor neuron. A motor neuron is basically a nerve cell whose function is to respond to sensory stimulation by producing the required muscular movement. Motor neurons are located in the spinal cord, and their axon protrudes outside to the muscle fibers. The functions of motor neurons are linked to the cerebral cortex of the brain; however, in case of reflexes, it is the spinal cord that ensures quick and responsive motor functioning.
For instance, when one places his/her hand over a flame, the sensory neurons carry the stimulus of pain to the motor neurons via the neural network (interneurons). If this stimulus was to go to the brain and then return with an analyzed response, the person’s hand would keep burning till the motor neuron functioned. Therefore, situations which require an immediate response, like quickly withdrawing the hand as in the previous example, the motor functioning is coordinated by the spinal cord.
As of now, you should have got the basic idea of what exactly a motor neuron is. Let’s dive a bit deeper into the functioning of motor neurons as we refer to a neatly labeled diagram.
Structure, Function, and Location of Motor Neurons
➔ All motor neurons are multipolar neurons. A multipolar neuron has only one axon and densely branched dendrites. The axon, in most cases, is usually long, especially in case of motor neurons as their terminals need to extend to the muscle fibers to function correctly.
➔ As motor neurons are the fundamental units of our body’s response mechanism, they need to be very sensitive to incoming neural messages. This is the reason that these neurons are multipolar; their several extending dendrites sense even the weakest signal transmitted and coordinate muscle movement accordingly.
➔ Their functions classify them into three types, somatic motor neurons, general visceral motor neurons, and special visceral motor neurons. Let’s discuss the respective functions of each of the three.
➔ The somatic neurons stem from the central nervous and are connected to the skeletal muscles, which are responsible for locomotion or movement. Some skeletal muscles include intercostal muscles, thigh and limb muscles, arm muscles, and several others which help in the movement of bones and support the skeleton.
➔ Visceral neurons are specifically designed to stimulate organ-related muscles. The special visceral neurons control the branchiomeric muscles. Branchiomeric muscles are basically muscles of the face and neck. The general visceral neurons stimulate cardiac muscles, smooth visceral muscles, and also certain gland cells.
Now, take a look at the diagrams given below.
Neural Response Mechanism
➔ Above illustrated is the basic functioning of the three different types of neurons to produce a response. When the body receives a stimulation, the receptor/sensory neurons in the affected area are innervated. They transmit the impulse to the motor neurons in order to trigger a response. Sensory and motor neurons are connected by several interneurons; once the impulse reaches the lower motor neurons via the interneurons, the lower motor neurons as per the type of stimulation would either transmit the impulse further to upper motor neurons or directly communicate with the affected muscle fibers. The latter is more evident in case an urgent response is required (reflexes).
➔ When you are pricked by a thorn, your body would, in normal circumstances, follow this process resulting in withdrawal of the affected portion (arm in this case). Likewise, motor neurons also foster several basic voluntary functions like walking, swallowing, speaking, breathing, etc.
➔ Motor neurons, according to location, can be classified into two main types―upper motor neurons and lower motor neurons―both function in unison to carry out motor operations. The upper motor neurons are either located in the cerebral cortex of the brain or the brain stem, whereas the lower motor neurons are located in the spinal cord, and their terminals extend all the way to the muscle fibers and tendons.
➔ When complex motor operations are required, the lower motor neurons consult the upper motor neurons and both work in union to provide a meaningful response. Upper motor neurons are often consulted in case of voluntary motor responses. This is because, voluntary actions are backed by thoughts. For instance, you don’t walk just because you have to, you walk in the direction you wish to go; hence, the motor neurons of the brain have to be consulted in order to acquire coordination between thought and action. In case of reflexes, the nervous system has an automatic response mechanism; hence, the responses in this case are quicker than the former as there is no involvement of upper neurons. Our body is conditioned in such a way that, in case a part of our body is affected or harmed, we tend to pull it or contract it away from the threat. For example, when a boxer hits his opponent in the abdominal area, the affected area is automatically contracted or pulled inward to avoid more harm.
To put the whole thing in a nutshell, we can say that motor neurons located in the cerebral cortex and spinal cord are the output of the stimulus-response mechanism, as they are the ones which initiate muscular movement as per the received sensory stimulus.