Difference between Upper and Lower Motor Neurons: Definition & Similarities

Ans. Motor neurons, also known as efferent neurons, are the nerve cells that are responsible for transmitting signals away from the CNS towards the muscles in order to cause involuntary movement. Motor neurons release neurotransmitters in order to trigger the responses to lead to the movement of muscles. These neurons allow us to talk, move, eat, breathe, and swallow. In the absence of these cells, we won't be able to perform basic life functions.

The motor neurons are responsible for integrating signals from the brain to the glands, organs, and muscles. The afferent neurons, that is sensory neurons, on the other hand, carry the information from the tissues and sensory organs to the Central Nervous System (CNS).

These neurons form many complex and controlled circuits all throughout the body. Motor neurons are classified into two types based on their function which are:

• Lower motor neurons = These neurons travel from the spinal cord to the rest of the muscles of the body.
• Upper motor neurons = These neurons travel from the brain to the lower motor neurons in the spinal cord.

Ans. Motor nerves are responsible for transmitting the signals from the Central Nervous System to the sensory organs. Therefore, they help to initiate actions.

• Motor neurons lack receptors.
• These neurons are present at the spinal cord's ventral root ganglion.
• They consist of multiple dendrites and a long axon.
• Motor neurons are also referred to as multipolar neurons.

The structure of a motor neuron mainly comprises three components: the axon, the soma, and the dendrites.

• Axon refers to the long extension structure that stems from the soma. It works to communicate information that it receives down its body.
• Soma refers to the cell body where the nucleus is present. It controls the cells. Proteins are produced in the soma in order to maintain the function of a neuron.
• Dendrites refer to the branch-like structures that are found at the end tips of the neuron. These are used to send and receive information to and from other neurons.

Ans. Upper motor neurons (UMNs) originate in the brain's gray matter. They carry the impulses in order to initiate movement. These are the first-order neurons. It is a term introduced in 1886 by William Gowers. They are found in the brainstem and cerebral cortex. These neurons carry information down in order to activate lower motor neurons and interneurons. This in turn directly signals the muscles in the body to relax or to contract. They are mainly responsible for controlling voluntary movement throughout the body.

The transmission of nerve impulses from the upper motor neurons to the lower motor neurons takes place through a neurotransmitter that is known as glutamate via glutamatergic receptors. Some of the pathways or tracts through which upper motor neurons can travel serve many functions. These tracts are pyramid, extrapyramidal, tectospinal, rubrospinal, and reticulospinal tracts. The upper neurons travel through these tracts where they then synapse or form the connections with the neurons that are located within the spinal cord, known as the lower motor neurons.

Ans. Lower motor neurons are present in the spinal cord. These neurons are mainly responsible for transmitting signals directly to the glands and organs present in the body along with skeletal muscles. The lower motor neurons receive information from the upper motor neurons either via interneurons or directly. Then they stimulate the activity of these neurons by extending their fibers all throughout the body. The few sub-classifications of lower motor neurons are somatic, general visceral motor neurons, and special visceral effects.

Ans. Somatic, general visceral motor neurons, and special visceral efferent are some subtypes of lower motor neurons.

Somatic motor neurons are originally in the central nervous system. Specifically, they are located in the brain stem from where they project their axons to the skeletal muscles. These somatic motor neurons can be further divided into alpha, beta, and gamma motor neurons.

• Alpha motor neurons are larger relatively. They generate movement by causing muscle contractions.
• Beta motor neurons also innervate intrafusal fibers as well as extrafusal muscle fibers.
• Gamma motor neurons are also referred to as muscle spindles. They do not directly cause any motor function.

Special visceral efferent neurons, also referred to as branchial motor neurons mainly innervate the muscles of the neck and head. They are present in the brain stem.

General visceral motor neurons contribute to both the parasympathetic and sympathetic functions of the Autonomic Nervous System (ANS).

Ans. The few pathways or tracts through which the upper motor neurons can travel and that serve various functions are:

• In the pyramidal pathway, the upper motor neurons are responsible for controlling the conscious movements of the body. It starts in the motor strip present in the frontal lobe. The nerve impulses travel through it to reach the spinal cord.
• The extrapyramidal tract is any tract or pathway that is located outside the pyramid tract. Here, the neurons are involved in the subconscious motor functions such as balance and posture.
• Rubrospinal pathway is an extrapyramidal tract that is involved in the involuntary movements of the body. These movements are used to improve or maintain the balance of the body.
• Tectospinal pathway or tract is also an extrapyramidal tract. It is responsible for the muscle movements within the neck.
• At last, the reticulospinal tract is vital for controlling the autonomous actions of the body.

Ans. Neurons are the fundamental units of our nervous system that are specialized to transmit or communicate information to various parts of the body. All the neurons are composed of three different parts which are the cell body, dendrites, and axon. There are mainly three types of neurons which are:

• Sensory Neurons = The sensory neurons are responsible for converting the signals from the external environment into internal stimuli. They are activated by the sensory inputs and carry the information to the spinal cord and the brain.
• Motor Neurons = These neurons are present in the central nervous system. They are multipolar and they extend their axons outside the CNS or the Central Nervous System. They are the most common types of neurons present in the body that communicate information from the brain to the body muscles.
• Interneurons = The axons of interneurons only connect to the nearby motor and sensory neurons. Their main role is to pass the signal between the sensory and motor neurons.

The main functions of a neuron are:

• Electrical Synapse = When any two neurons are connected to each other through a gap junction, an electric synapse is a result. The gaps have ion channels that directly help in the transmission of an electric signal. They are relatively faster than chemical synapses.
• Chemical Synapse = The action potential in chemical synapses affects the other neurons through a gap that is present between both the neurons, which is known as a synapse. This action potential is then carried along the axon to a postsynaptic ending. This initiates the release of neurotransmitters, a kind of chemical messengers. These neurotransmitters then excite the postsynaptic neurons. This generates an action potential.