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.
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.
How Do Motor Neurons Compare to Other Neuron Types?
| Neuron Type | Function | Direction of Signal | Location | Target |
|---|---|---|---|---|
| Motor Neurons | Carry signals from the central nervous system to muscles and glands | CNS → Body (efferent) | Cell bodies in spinal cord/brainstem; axons extend to muscles or organs | Muscles (skeletal, cardiac, smooth), glands |
| Sensory Neurons | Transmit information from sensory receptors to the central nervous system | Body → CNS (afferent) | Cell bodies in dorsal root ganglia; dendrites in skin, eyes, ears, etc. | Brain and spinal cord |
| Interneurons | Relay messages between sensory and motor neurons | Within CNS (CNS ↔ CNS) | Entirely within the brain and spinal cord | Other neurons |
What Happens When Motor Neurons Are Damaged?
Damage can occur in either upper or lower motor neurons. The symptoms and causes differ based on the location.
Upper Motor Neuron Damage
Common causes:
- Multiple sclerosis (MS)
- Stroke
- Cerebral palsy
- Brain or spinal cord injuries
Symptoms include:
- Muscle stiffness
- Weakness
- Poor motor control
- Exaggerated reflexes
Lower Motor Neuron Damage
Causes may include:
- Infections (like Lyme disease)
- Peripheral nerve trauma
- Certain viruses
Symptoms:
- Muscle paralysis or weakness
- Loss of reflexes
- Muscle shrinkage (atrophy)
Motor Neuron Disease (MND)
The most well-known MND is amyotrophic lateral sclerosis (ALS). It affects both upper and lower motor neurons and progressively weakens muscles over time.
Symptoms of ALS:
- Muscle stiffness and twitching
- Difficulty moving or speaking
- Progressive loss of strength
Although ALS has no cure, therapies like occupational therapy, physical therapy, and speech therapy can help people manage symptoms and maintain quality of life.
Why Are Motor Neurons So Important?
Motor neurons are vital because they connect your brain’s instructions to your body’s actions. Without them, you couldn’t move, breathe, or respond to your environment.
Whether you’re giving a presentation, going for a run, or simply breathing while you sleep, motor neurons are behind the scenes, making it possible.
FAQs
Can motor neurons regenerate?
In general, motor neurons have a limited ability to heal after injury. This is why damage can be so serious.
Do motor neurons control digestion?
Yes. Through the autonomic nervous system, they help regulate the muscles of your digestive system.
Are motor neurons in the brain or body?
Their cell bodies are in the brain and spinal cord, but their axons extend throughout the body to reach muscles and glands.
References
Liden, D. (n.d.). What is a Motor Neuron? (with pictures). Info Bloom. Retrieved June 16, 2021, from https://www.infobloom.com/what-is-a-motor-neuron.htm
McLaughlin, K. (2020, May 16). Motor Neuron. Biology Dictionary. https://biologydictionary.net/motor-neuron/
Moini, J. & Piran, P. (2020) Histophysiology. Functional and Clinical Neuroanatomy. Academic Press. https://doi.org/10.1016/B978-0-12-817424-1.00001-X
Physiopedia. (n.d.). Motor Neurone. Retrieved June 16, 2021, from https://www.physio-pedia.com/Motor_Neurone
Slater, C. R. (2009). Neuromuscular junction (NMJ): mammalian development. DEVELOPMENTAL NEUROBIOLOGY, 529.
Zayia, L. C., & Tadi, P. (2020). Neuroanatomy, Motor Neuron. StatPearls [Internet].
Further Reading
- Nicholls, J. G., Martin, A. R., Wallace, B. G., & Fuchs, P. A. (2001). From neuron to brain (Vol. 271). Sunderland, MA: Sinauer Associates.
- Pereda, A. E. (2014). Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience, 15(4), 250-263.
