Introduction
A neuron or nerve cell, which is electrically excitable, communicates with other cells via synapses, which are specialized connections. In all species, the neuron is the main component of nervous tissue, except for sponges and placozoa. Fungi and plants are both devoid of nerve cells. The roles of three different types of neurons can generally be identified. Sense neurons, which impact the cells of the sensory organs, send signals to the spinal cord or brain in response to stimuli like touch, sound, or light. Motor neurons get guidance from the brain and spinal cord to regulate anything from glandular secretion to muscle contractions. Interneurons connect neurons in the same area of the spinal cord or brain. Signals are sent and received by neurons, also known as nerve cells, from your brain. While neurons differ from other types of cells in appearance and function, they share many characteristics. Neurons have unique extensions called axons that enable them to communicate with other cells via chemical and electrical impulses. Dendrite, which resembles roots, are root-like extensions that allow neurons to receive these impulses. At birth, the human brain is thought to have 100 billion neurons. Unlike other cells, neurons do not divide or regenerate. They cannot be replaced, even if they are dead.
A neural circuit is created when numerous neurons are linked together. A normal neuron is made up of an axon, dendrites, and the cell body (soma). The axon and dendrites are filaments extending from the soma, which is a compact structure. Typically, dendrites branch out widely and reach a few hundred micrometres from the soma. In humans, the axon can extend as far as 1 meter from the soma, whereas, in other species, it can extend more. This swelling is known as the axon hillock. Although its branches, it often keeps its diameter unchanged. Axon terminals, where the neuron can send a signal across the synapse to another cell, are located at the distal point of the axon's branches. Dendrites and axons in neurons can be absent. Before firing impulses down the axon, the majority of neurons first receive signals from the soma and dendrites. Most synapses allow impulses to move from the axon to the dendrite of one neuron to another. However, synapses can link a dendritic to another dendrite or an axon to another axon. The signalling technique combines chemical and electrical methods. Voltage gradients are maintained across the membranes of neurons, allowing for their electrical activity. A neuron will produce an action potential, an all-or-nothing electrochemical pulse if the voltage varies significantly and quickly enough. The synaptic synapses are activated as this potential rapidly ascends the axon.
Neurons are the basic building blocks of the nervous system, and glial cells support them structurally and metabolically. The nervous system is made up of the central nervous system, which is composed of the brain and spinal cord, and the peripheral nervous system, which includes the autonomic and somatic nervous systems. The bulk of neurons in vertebrates are situated in the central nervous system, although many sensory neurons can be found in sense organs like the retina and ear. However, peripheral ganglia also contain some neurons. Axons in the peripheral nervous system's nerves can produce fascicles (like strands of wire make-up cables). The central nervous system contains axon clusters known as tracts.
Neuronal Types
Neurosensory Cell
We can sense and discover our surroundings with the aid of sensory neurons. Major sensations like touch and pain can facilitate safe movement across the environment. For our body to receive information about our surroundings, sensory neurons are essential. They can communicate temperature information and instruct us on when to stay away from hot objects. Additionally, sophisticated motions like picking up objects are supported by sensory neurons.
Neuromotor Cells
The body's motion is managed by motor neurons. Our muscles are controlled by these neurons, which also make sure that our arms and legs move in unison. Lower motor neurons and higher motor neurons are types of motor neurons that are found in the brain and spinal cord, respectively. The degree of differentiation between upper and lower motor neurons involves control each exerts over functions of the body.
Interneurons
The body's interneurons are the neurons that are most prevalent. They serve as the body's signal controllers, transmitting crucial data from one end of the nervous system to the other. In between other neurons, such as motor or sensory neurons, are the interneurons. They are in charge of transmitting electrical signals. The signals from neurons can also be regulated by
interneurons. They have control over what is and is not sent along. They have a multipolar structure that enables them to take in several impulses before sending a single order to a different neuron.
Basic Neuronal Functions
- Neuron can simultaneously take in and process stimuli (impulses) from the body as well as from external sources. They can translate stimuli into changes in membrane potential. Neurons can quickly communicate this change in membrane potential over extensive distances by using the conduction mechanism.
- Sending, receiving, and processing are all functions that neurons carry out to keep the nervous system's communication flowing. In actuality, there are about 100 million neurons surrounding our nervous system, and each one is capable of receiving information from tens of thousands of distinct cells.
- The sensory (afferent) neurons serve as the nervous system's front-desk personnel for incoming stimuli. They take in and process impulses from the environment as well as the body. These neurons can concurrently send the impulses to the central nervous system (CNS) for processing through the conduction mechanism.
- The term "communication neurons" refers to interneurons or internuncial (association) neurons. In addition to creating connections between sensory and motor neurons, they also seek to create connections within themselves. The transmission of the CNS's processed impulses to the motor neurons is another important job performed by these neurons.
- The effector organs receive the nerve impulses from the motor (efferent) neurons (e.g., muscle tissues or glands). They regulate the movement of the body's muscles, including the heart, kidney, liver, diaphragm, sexual organs, and glands, both directly and indirectly.
Upper Motor Neuron vs. Lower Motor Neuron
The fundamental components of the human body are neurons. Another name for UML is upper motor neuron lesions. They're frequently referred to as higher motor neurons. Damaged neurons known as UML may be found in the brain and spinal cord. LMN is another term for lower motor neuron lesions. Upper motor neurons, or UMN, are lesions of the neurons in the spinal cord and brain, which is the fundamental distinction between them and lower motor neurons. While lower motor neurons, which directly innervate the muscles, are injured in the cranium and spinal cord. There will be hypertonia, hyperreflexia, and spasticity in the upper motor neuron. Because LMN, or lower motor neurons, are controlled by UMN, or upper motor neurons, there will be a loss of control of LMN if UMN is damaged. Increased reflexes, spasticity, and tonicity are the results.
Difference Between Upper Motor Neuron and Lower Motor Neuron in Tabular Form
| Parameters of Comparison | Upper Motor Neuron | Lower Motor Neuron |
| Location | It is entirely contained by the central nervous system. | It is either contained in the grey matter of the spinal cord or the cranial nerve nuclei in the brain stem. |
| Transmits | Transmits motor instructions from the brain to the synapses of lower motor neurons. | Collects the motor impulses that the body's muscles receive from higher motor neurons. |
| Cell Bodies | The cell bodies of upper motor neurons are bigger than those of lower motor neurons. | In comparison, cell bodies are minuscule. |
| Classification | Listed in groups based on the paths they travel. | Classification is based on the type of muscle fibres they innervate. |
| Muscle tone | It can either be stiffness or spasticity, depending on the muscular tone. | Hypotonia in LMN denotes unusually low levels of muscular tone. |
What is Upper Motor Neuron?
William Gowers coined the term "upper motor neurons" (UMNs) in 1886. They are present in the cerebral cortex and brainstem and transmit information to lower motor neurons and interneurons, which in turn activate muscles directly. The primary source of voluntary movement comes from UMNs in the cerebral cortex. The bigger pyramidal cells in the cerebral cortex are these. In layer V of the primary motor cortex, which is located just below the cerebral cortex's surface, are large pyramidal cells known as Betz cells. With a diameter of over 0.1 mm, Betz cell neurons have the largest cell bodies in the brain. These neurons link the brain to the proper level of the spinal cord, from which point lower motor neurons carry on the nerve signals to the muscles. The nerve impulses are passed from upper to lower motor neurons by the neurotransmitter glutamate, where they are picked up by glutamatergic receptors.
There will be hypertonia, hyperreflexia, and spasticity in the upper motor neuron. Because LMN, or lower motor neurons, are controlled by UMN, or upper motor neurons, there will be a loss of control of LMN if UMN is damaged. Increased reflexes, spasticity, and tonicity are the results. The upper motor neuron was initially described as a bodily component by William Gowers in 1886. The cerebral cortex and brain stem both include the upper motor neuron or UML. The cerebral cortex contains the upper motor neurons, or UML, which are the main component of voluntary movement. Inter motor neurons and lower motor neurons receive input from UMN.
The top motor neurons control the lower motor neurons. The cerebral cortex contains pyramidal cells, including a sizable type referred to as Betz cells. Betz cells are the largest cell bodies in the brain, measuring 0.1 mm in diameter. Upper motor neurons are the pyramidal cells of the precentral gyrus. The top motor neuron travels down to the root of the spinal cord. in the neural canal that is situated above the spinal cord anterior horn. Pyramidal insufficiency is the medical term for any UMN damage. Numerous neural pathways allow upper motor neurons to exit the central nervous system. Exaggerated and spastic reflexes may be the result of damage to the upper motor neuron (UMN). A group of muscles are affected by an upper motor neuron lesion, which is different from muscular atrophy. Deep reflex hyperactivity and the Babinski sign turn negative.
What is Lower Motor Neuron?
There will be hypotonia, hyporeflexia, and flaccidity in the lower motor neurons. Atrophy will occur in this instance as well, but it will be caused by the motor nerve muscles' denervation (loss of nerve supply). In addition, fasciculation—the spontaneous contracting of tiny muscle fibres caused by activation of a small number of single muscle fibres—occurs in Lower motor neuron injuries. In the lower motor neurons, there will be hypotonia, hyporeflexia, and flaccidity. In this case, atrophy will also happen, but it will be brought on by denervation of the motor nerve muscles (loss of nerve supply). Additionally, Lower motor neuron damage results in fasciculation, which is the spontaneous contraction of tiny muscle fibres brought on by the stimulation of a small number of single muscle fibres.
In the direction of the spinal cord’s bottom mean, lower motor neurons begin to fire. Lower motor neurons deliver the signals that cause muscles to contract. Lower motor neurons are those fibres that originate from cranial nerves. Muscles are acted upon by the lower motor neurons. Since the lower motor neuron is coupled to the upper motor neuron, it is hypothesized that if the upper motor neuron is eliminated, the lower motor neuron will be free to target muscles and exert more force. However, if the lower motor neuron is destroyed, the muscles of the upper motor neuron become paralyzed and flaccid. Therefore, if lower motor neurons die, we will ultimately lose a great deal. Only one muscle is serviced by the lower motor neuron. Loss of lower motor neuron reflexes and a positive Babinski sign are both present.
Lower motor neurons (LMNs) are motor neurons having a motor function that are found in the brainstem's anterior grey column, anterior nerve roots (spinal lower motor neurons), or cranial nerve nuclei (cranial nerve lower motor neurons). Spinal lower motor neurons, which innervate skeletal muscle fibres and serve as a conduit between upper motor neurons and muscles, are the basis for all voluntary movement. Lower motor neurons of the cranial nerve regulate facial, tongue, and eye movements as well as chewing, swallowing, and vocalization. Atrophy of the muscles and flaccid paralysis can result from damage to the lower motor neurons.
Difference between Upper Motor Neuron and Lower Motor Neuron In Points
- Upper motor neurons, or UMNs, are located in the cerebral cortex of the brain, whereas lower motor neurons, or LMNs, are found in the spinal cord.
- While the Lower Motor Neuron enables it to receive inputs from the higher component of the neuron system, the Upper Motor Neuron carries information about the various portions of the body.
- The Lower Motor Neuron received the signal and transmitted it to the other regions of the body, whereas the Upper Motor Neuron is utilized to transmit signals to the various muscles.
- While the Lower Motor Neuron is based on two separate types of neurons, the Upper Motor Neuron is based on just one type.
- While the LMN is located in the cranial nerves and anterior horn cells, the Upper Motor Neuron is dispersed across the pyramidal system.
Conclusion
The top motor neurons receive nerve impulses from the central nervous system, while the lower motor neurons pass those signals on to the muscles. The somatic nervous system, which regulates voluntary muscular movements, is made up of both upper motor neurons and lower motor neurons. The upper motor neuron transfers impulses from the brain to the synapses of the lower motor neurons, whereas the lower motor neuron is the section of the central nervous system that links with the muscles. Upper and lower motor neurons of the somatic nervous system are divided into motor units. They control the induction of voluntary muscular contractions.
Simply put, we are familiar with the terms UMN, LMN, and lesions. There is no unique definition for the specific term or word used in some lines because they are all biological terminology. However, I made an effort to outline the differences between top motor neurons and lower motor neurons. Additionally, after reading this, you may have gained some insight into how to avoid illnesses that affect both upper and lower motor neurons. Here, we merely clarify and delve deeper into the neuron component so that, if you read this essay as its whole, you will learn as much as possible about these two crucial words.
References
- https://n.neurology.org/content/68/19/1571.short
- https://www.sciencedirect.com/science/article/pii/S1388245706001143
