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Jai Ganesh

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Motor Neuron

Motor Neuron

Gist

Motor neurons (or motoneurons) are specialized efferent nerve cells originating in the brain or spinal cord that transmit impulses to effector muscles and glands, controlling voluntary and involuntary movements. They are classified into upper motor neurons (brain to spinal cord) and lower motor neurons (spinal cord to muscle).

Motor neurones are cells in the brain and spinal cord that allow us to move, speak, swallow and breathe by sending commands from the brain to the muscles that carry out these functions.

Summary

Motor neurons, also known as efferent neurons, are nerve cells responsible for carrying central nervous system signals towards muscles to cause voluntary or involuntary movement through the innervation of effector muscles and glands. Their nerve fibers are considered to be the longest in the human body. Motor neurons are the most common structure for neurons.

Motor neurons are divided into either upper or lower motor neurons. Each type of motor neuron utilizes different neurotransmitters to relay their signals.

* Upper motor neurons originate primarily in the cerebral cortex (primary motor cortex) and extend towards the brainstem or spinal cord, where they synapse. Upper motor neurons travel down specific pathways, including the pyramidal, extrapyramidal, rubrospinal, tectospinal and reticulospinal tracts. Glutamate is used as a neurotransmitter.
* Lower motor neurons originate in the brainstem (cranial nerve nuclei) and the spinal cord (anterior horn) and project to innervate muscles and glands throughout the body. Acetylcholine is used as a neurotransmitter.

The structure of a motor neuron is characterized by three components: the soma, the axon, and the dendrites. Motor neurons have a large cell body, or soma, and long projections used in transmitting information away from the soma. These projections are referred to as axons and dendrites. Axons send impulses away from the soma and dendrites carry incoming information. Motor neurons are typically considered multipolar in terms of their structure, which translates to having a single axon and multiple dendrites.

Myelin sheath surrounds the axon of the motor neuron. One of the key functions of myelin is to insulate the axon length ensuring that electrical impulses are transmitted quickly and efficiently to the target structure. Each motor neuron divides into many terminal branches which synapse with muscle fibers at neuromuscular junctions (NMJ), also known as myoneural junctions. Electrical impulses are transmitted from the motor neuron to the muscle fiber via axon terminals (also known as neuropodia or axon endfeet), to generate muscle contractions. Axon terminals function to release neurotransmitters which are received by specific parts of the muscle fiber membrane known as motor end plates, signalling muscle contraction.

A motor neuron and its associated muscle fibers make up a motor unit. Fine muscles (e.g. extraocular muscles) have small motor units and therefore can be controlled more precisely in comparison to larger muscles (e.g. biceps brachii).

Details

A motor neuron (or motoneuron), also known as efferent neuron is a neuron that allows for both voluntary and involuntary movements of the body through muscles and glands. Its cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. Innervation takes place at a neuromuscular junction and twitches can become superimposed as a result of summation or a tetanic contraction. Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.

Although the word "motor neuron" suggests that there is a single kind of neuron that controls movement, this is not the case. Indeed, upper and lower motor neurons—which differ greatly in their origins, synapse locations, routes, neurotransmitters, and lesion characteristics—are included in the same classification as "motor neurons." Essentially, motor neurons, also known as motoneurons, are made up of a variety of intricate, finely tuned circuits found throughout the body that innervate effector muscles and glands to enable both voluntary and involuntary motions. Two motor neurons come together to form a two-neuron circuit. While lower motor neurons start in the spinal cord and go to innervate muscles and glands all throughout the body, upper motor neurons originate in the cerebral cortex and travel to the brain stem or spinal cord. It is essential to comprehend the distinctions between upper and lower motor neurons as well as the routes they follow in order to effectively detect these neuronal injuries and localise the lesions.

Disorders causing selective injury to motor neurons are classified as motor neuron diseases.

Additional Information

Motor neurons are nerve cells that carry messages from the brain and spinal cord to your muscles and glands. They control everything from blinking and walking to breathing and digestion.

By acting as messengers between the central nervous system (CNS) and the body, they make movement and essential bodily functions possible.

Motor neurons belong to a broader group called efferent neurons—cells that send signals outward from the CNS. These neurons are crucial for both voluntary actions, like typing, and involuntary ones, like maintaining your posture.

Key Takeaways

* Motor neurons carry messages from your brain and spinal cord to your muscles.
* They’re essential for movement, reflexes, and basic body functions.
* There are two main types: upper and lower motor neurons.
* Damage can cause serious issues, including paralysis and diseases like ALS.
* While some treatments exist, prevention and early detection are key.

(ALS: Amyotrophic lateral sclerosis)

How Do Motor Neurons Work?

Motor neurons act like messengers. They carry electrical signals from your brain or spinal cord to your muscles.

When the message reaches its destination, it triggers the release of a chemical called acetylcholine at a point where the nerve meets the muscle—this spot is called the neuromuscular junction.

Acetylcholine tells your muscle to contract, which produces movement.

Motor neurons have three main parts:

* Soma (cell body): The cell’s headquarters, where energy and proteins are produced.
* Dendrites: Short branches that receive messages from other neurons.
* Axon: A long fiber that carries the outgoing message to a muscle or gland.

Each part works together to ensure your body reacts smoothly and quickly.

What Are the Types of Motor Neurons?

Motor neurons are divided into two main groups, based on where they begin and what they control:

Upper Motor Neurons

These begin in the motor cortex of the brain or the brainstem. They send signals down to the spinal cord to help start and guide movement.

Key pathways include:

* Pyramidal tract: Controls deliberate, voluntary actions like lifting your arm.
* Extrapyramidal tracts: Manage automatic functions like balance and posture. This includes:
* Rubrospinal tract: Helps adjust body balance.
* Tectospinal tract: Affects neck muscle movement.
* Reticulospinal tract: Regulates automatic actions.

Lower Motor Neurons

These are the final link between your nervous system and your muscles. They carry messages directly to muscle fibers.

Lower motor neurons are categorized into:

* Somatic Motor Neurons:
** Alpha neurons: Connect to standard muscle fibers to produce forceful contractions.
** Beta neurons: Interact with both standard and sensory muscle fibers.
** Gamma neurons: Fine-tune muscle tone by connecting to stretch sensors.

* Special Visceral Efferent Neurons: Found in the brainstem, these are part of cranial nerves like the facial nerve and vagus nerve, which control facial expressions and digestion.

* General Visceral Motor Neurons: Part of the autonomic nervous system, they connect to organs like the heart and intestines.

What Do Motor Neurons Do?

Motor neurons allow your brain to control your body. Their responsibilities include:

* Voluntary movements: Walking, writing, smiling
* Involuntary reflexes: Pulling your hand away from something hot
* Autonomic functions: Breathing, heartbeat, digestion

They’re also essential for posture, balance, and muscle coordination—things we often take for granted.

Location

Motor neurons are located in the central nervous system (CNS), specifically in the motor cortex, brainstem, and spinal cord.

While the cell bodies (soma) of motor neurons remain within the CNS, their axons, called efferent fibers, project outward to reach muscles and other peripheral systems such as organs and glands.

These neurons are notable for having some of the longest axons in the body. In fact, the efferent fibers that extend from the base of the spinal cord to the toes represent one of the longest axon pathways in the human body.

With approximately 500,000 motor neurons in total, these cells form an extensive network carrying information from the CNS to peripheral organs, muscles, and glands.

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