Upper motor neurons are motor neuron

Motor neuron

In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...

s that originate in the motor region

Motor cortex

Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...

 of the cerebral cortex

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 or the brain stem

Brain stem

In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

 and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

. The main effector neurons for voluntary movement lie within layer V of the primary motor cortex

Primary motor cortex

The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

 and are called Betz cell

Betz cell

Betz cells are pyramidal cell neurons located within the fifth layer of the grey matter in the primary motor cortex, M1. They are named after Vladimir Alekseyevich Betz, who described them in his work published in 1874. These neurons are the largest in the central nervous system, sometimes reaching...

s. The cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 bodies of these neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s are some of the largest in the brain, approaching nearly 100μm in diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

.

One of the most important areas in the frontal lobe is our primary motor control area which is the precentral gyrus. The precentral gyrus is the most posterior gyrus of the frontal lobe and it lies anterior to the central sulcus. The pyramidal cells of the precentral gyrus are also called upper motor neurons. The fibers of the upper motor neurons project out of the precentral gyrus ending in the brainstem where they will decussate (i.e., cross) within the lower medulla oblongata to form the lateral corticospinal tract on each side of the spinal cord. The fibers that do not decussate will pass through the medulla and continue on to form the anterior corticospinal tracts. At this point the upper motor neuron will synapse with the lower motor neuron whose axons will innervate skeletal muscle.

These neurons connect the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 to the appropriate level in the spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

, from which point nerve signals

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 continue to the muscles by means of the lower motor neuron

Lower motor neuron

Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...

s. The neurotransmitter glutamate transmits the nerve impulses from upper to lower motor neurons where it is detected by glutamatergic receptors.

Pathways

Upper motor neurons travel in several pathways through the CNS:

Tract

Pathway

Function >- | corticospinal tract Corticospinal tract The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

from the motor cortex Motor cortex Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...  to lower motor neurons in the ventral horn of the spinal cord

>- | corticobulbar tract Corticobulbar tract The corticobulbar tract is a white matter pathway connecting the cerebral cortex to the brainstem. The 'bulb' is an archaic term for the medulla oblongata; in modern clinical usage, it sometimes includes the pons as well...

from the motor cortex Motor cortex Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...  to several nuclei in the pons Pons The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...  and medulla oblongata Medulla oblongata The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

>- | Colliculospinal tract Tectospinal tract In humans, the tectospinal tract is a nerve pathway which coordinates head and eye movements. It is part of the indirect extrapyramidal tract...

from the superior colliculus Superior colliculus The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...  to lower motor neurons

>- | rubrospinal tract Rubrospinal tract The rubrospinal tract is a part of the nervous system. It is a part of the lateral indirect extra-pyramidal tract.-Function:In humans, the rubrospinal tract is one of several major motor control pathways. It is smaller and has fewer axons than the corticospinal tract, suggesting that it is less...

from red nucleus Red nucleus The red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...  to lower motor neurons

>- | vestibulospinal tract Vestibulospinal tract The vestibulospinal tract is a component of the extrapyramidal system and also can be classified as a component of the medial pathway. The vestibulospinal fibers relay information from nuclei to motor neurons, like other descending motor pathways. Specifically, the vestibular nuclei receive...

from vestibular nuclei, which processes stimuli from semicircular canals

>- | reticulospinal tract Reticulospinal tract The reticulospinal tract is an extrapyramidal motor tract which travels from the reticular formation.-Functions:...

from reticular formation Reticular formation The reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...

Regulates various involuntary motor activities and assists in balance.

Lesions

Damage to the descending motor pathways anywhere along the trajectory from the cerebral cortex to the lower end of the spinal cord gives rise to a set of symptoms called the "upper motor neuron syndrome". Signs may appear over days or weeks following brain or spinal cord damage, including muscle weakness

Weakness

Weakness is a symptom represented, medically, by a number of different conditions, including: lack of muscle strength, malaise, dizziness, or fatigue. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a...

, decreased motor control

Motor control

Motor control are information processing related activities carried out by the central nervous system that organize the musculoskeletal system to create coordinated movements and skilled actions...

 including a loss of the ability to perform fine movements, increased vigor (and decreased threshold) of spinal reflexes including spasticity

Spasticity

Spasticity is a feature of altered skeletal muscle performance in muscle tone involving hypertonia, which is also referred to as an unusual "tightness" of muscles...

, and an extensor plantar response known as the Babinski sign.