The pyramidal tracts include both the

corticobulbar tract The corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the Medullary pyramids (brainstem), medullary pyramids, which are part of the brainstem's medulla oblonga ...

and the

corticospinal tract The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neu ...

. These are aggregations of

efferent nerve fiber Efferent nerve fibers are axons (nerve fibers) of efferent neurons that exit a particular region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS). The efferen ...

s from the

upper motor neuron Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles ...

s that travel from the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

and terminate either in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

(''corticobulbar'') or

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

(''corticospinal'') and are involved in the control of motor functions of the body. The corticobulbar tract conducts impulses from the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

to the

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

s. These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other motor functions. The

conducts impulses from the brain to the spinal cord. It is made up of a

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

and anterior tract. The corticospinal tract is involved in voluntary movement. The majority of fibres of the corticospinal tract cross over in the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

, resulting in muscles being controlled by the opposite side of the brain. The corticospinal tract contains the

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s of the pyramidal cells, the largest of which are the

Betz cell Betz cells (also known as pyramidal cells of Betz) are giant pyramidal cells (neurons) located within the fifth layer of the grey matter in the primary motor cortex. These neurons are the largest in the central nervous system, sometimes reaching 1 ...

s, located in the

primary motor cortex The primary motor cortex ( Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor c ...

. The pyramidal tracts are named because they pass through the

pyramids A pyramid () is a Nonbuilding structure, structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a Pyramid (geometry), pyramid in the geometric sense. The base of a pyramid ca ...

of the

. The corticospinal fibers converge to a point when descending from the

internal capsule The internal capsule is a paired white matter structure, as a two-way nerve tract, tract, carrying afferent nerve fiber, ascending and efferent nerve fiber, descending axon, fibers, to and from the cerebral cortex. The internal capsule is situate ...

to the brain stem from multiple directions, giving the impression of an inverted pyramid. Involvement of the pyramidal tract at any level leads to

pyramidal signs Pyramidal signs indicate that the pyramidal tract is affected at some point in its course. Pyramidal tract dysfunction can lead to various clinical presentations such as spasticity, weakness, slowing of rapid alternating movements, hyperreflexi ...

. The

myelin Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

ation of the pyramidal fibres is incomplete at birth and gradually progresses in cranio-caudal direction and thereby progressively gaining functionality. Most of the myelination is complete by two years of age and thereafter it progresses very slowly in cranio-caudal direction up to twelve years of age.

Structure

The term ''pyramidal tracts'' refers to

s that originate in the

and terminate in the

(''corticospinal'') or

(''corticobulbar''). Nerves emerge in the

, pass down and may cross sides in the

, and travel as part of the

until they

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

with

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

s in the

grey column The grey columns are three regions of the somewhat ridge-shaped mass of grey matter in the spinal cord. These regions present as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which ...

of the spinal cord. There is some variation in terminology. The ''pyramidal tracts'' definitively encompass the

s, and many authors also include the

Corticospinal tract

Nerve fibres in the corticospinal tract originate from

pyramidal cells Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal cells are the primary excitation units of the mammalian prefrontal cort ...

in layer V of the

. Fibres arise from the

(about 30%),

supplementary motor area The supplementary motor area (SMA) is a part of the motor cortex of primates that contributes to the control of movement. It is located on the midline surface of the hemisphere just in front of (anterior to) the primary motor cortex leg representa ...

and the

premotor cortex The premotor cortex is an area of the motor cortex lying within the frontal lobe of the brain just anterior to the primary motor cortex. It occupies part of Brodmann's area 6. It has been studied mainly in primates, including monkeys and human ...

(together also about 30%), and the

somatosensory cortex The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bod ...

parietal lobe The parietal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus. The parietal lobe integra ...

, and

cingulate gyrus The cingulate cortex is a part of the brain situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cin ...

supplies the rest. The cells have their bodies in the cerebral cortex, and the

s form the bulk of the pyramidal tracts. The nerve axons travel from the cortex through the posterior limb of internal capsule, through the

cerebral peduncle The cerebral peduncles (In Latin, ''ped-'' means 'foot'.) are the two stalks that attach the cerebrum to the brainstem. They are structures at the front of the midbrain which arise from the ventral pons and contain the large ascending (sensor ...

and into the

and anterior

. Here they form two prominences called the medulla oblongatary pyramids. Below the prominences, the majority of axons cross over to the opposite side from which they originated, known as

decussation Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term Chiasm (anatomy), chi ...

. The axons that cross over move to the outer part of the medulla oblongata and form the

lateral corticospinal tract Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

, whereas the fibres that remain form the

anterior corticospinal tract The anterior corticospinal tract (also called the ventral corticospinal tract, medial corticospinal tract, direct pyramidal tract, or anterior cerebrospinal fasciculus) is a small bundle of Descending neuron, descending fibers that connect the cer ...

. About 80% of axons cross over and form the lateral corticospinal tract; 10% do not cross over and join the tract, and 10% of fibres travel in the anterior corticospinal tract. The nerve axons traveling down the tract are the

s of the

s. These axons travel down the tracts in the

white matter White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, also called Nerve tract, tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distr ...

of the spinal cord until they reach the

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

l level of the muscle that they will innervate. At this point, the axons

with

lower motor neuron Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve lower ...

s. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an

that then synapses with a lower motor neuron. This generally occurs in the

anterior grey column The grey columns are three regions of the somewhat ridge-shaped mass of grey matter in the spinal cord. These regions present as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which are ...

. Nerve axons of the lateral corticospinal tract that did not cross over in the medulla oblongata do so at the level of the spinal cord they terminate in. These tracts contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal tracts myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from

s, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord. Lower pons horizontal KB

Corticobulbar tract

Fibres from the

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

motor cortex The motor cortex is the region of the cerebral cortex involved in the planning, motor control, control, and execution of voluntary movements. The motor cortex is an area of the frontal lobe located in the posterior precentral gyrus immediately ...

travel with the corticospinal tract through the internal capsule, but terminate in a number of locations in the

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

( cortico-mesencephalic tract),

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

( Corticopontine tract), and

( cortico-bulbar tract). The

s of the corticobulbar tract synapse with interneurons or directly with the lower motor neurons located in the motor

cranial nerve nuclei A cranial nerve nucleus is a collection of neuron cell bodies (gray matter) in the brain stem that is associated with one or more of the cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nucl ...

, namely

oculomotor The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...

, trochlear, motor nucleus of the

trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...

abducens The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocula ...

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

and accessory and in the

nucleus ambiguus The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary part of the reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innerva ...

to the hypoglossal,

vagus The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in the autonomic nervous system, which is responsible for regulating involuntary functions within the human body. This nerve carries both sensory and motor fiber ...

and

accessory nerve The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerv ...

s. These nuclei are supplied by nerves from both sides of the brain, with the exception of the parts of the facial nerve that control muscles of the lower face. These muscles are only innervated by nerves from the contralateral (opposite) side of the cortex.

Function

The nerves within the corticospinal tract are involved in movement of

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s of the body. Because of the crossing-over of fibres, muscles are supplied by the side of the brain opposite to that of the muscle. The nerves within the corticobulbar tract are involved in movement in muscles of the head. They are involved in swallowing,

phonation The term phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, ''phonation'' is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the defi ...

, and movements of the tongue. By virtue of involvement with the

, the corticobulbar tract is also responsible for transmitting

facial expression Facial expression is the motion and positioning of the muscles beneath the skin of the face. These movements convey the emotional state of an individual to observers and are a form of nonverbal communication. They are a primary means of conveying ...

. With the exception of lower muscles of facial expression, all functions of the corticobulbar tract involve inputs from both sides of the brain. The extrapyramidal system refers to tracts within the spinal cord involved in involuntary movement but not part of the pyramidal tracts. Their functions include the control of posture and

muscle tone In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state.O’Sullivan, S. B. (2007) ...

Clinical significance

# Damage to the fibres of the corticospinal tracts, anywhere along their course from the cerebral cortex to the lower end of the spinal cord, can cause an

upper motor neuron syndrome Upper motor neuron syndrome (UMNS) is the motor control changes that can occur in skeletal muscle after an upper motor neuron lesion. Following upper motor neuron lesions, affected muscles potentially have many features of altered performance inc ...

. # A few days after the injury to the upper motor neurons, a pattern of motor signs and symptoms appears, including

spasticity Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles. ...

, hyperactive reflexes, a loss of the ability to perform fine movements, and an extensor

plantar Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

response known as the Babinski sign. # Symptoms generally occur alongside other sensory problems. # Causes may include disorders such as

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

cerebral palsy Cerebral palsy (CP) is a group of movement disorders that appear in early childhood. Signs and symptoms vary among people and over time, but include poor coordination, spasticity, stiff muscles, Paresis, weak muscles, and tremors. There may b ...

subdural hemorrhage A subdural hematoma (SDH) is a type of bleeding in which a collection of blood—usually but not always associated with a traumatic brain injury—gathers between the inner layer of the dura mater and the arachnoid mater of the meninges surroun ...

abscess An abscess is a collection of pus that has built up within the tissue of the body, usually caused by bacterial infection. Signs and symptoms of abscesses include redness, pain, warmth, and swelling. The swelling may feel fluid-filled when pre ...

es and

tumour A neoplasm () is a type of abnormal and excessive growth of tissue (biology), tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tiss ...

s, neurodegenerative diseases such as

multiple system atrophy Multiple system atrophy (MSA) is a rare neurodegenerative disorder characterized by tremors, slow movement, muscle rigidity, postural instability (collectively known as parkinsonism), autonomic dysfunction and ataxia. This is caused by progr ...

, inflammation such as

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasion ...

and

multiple sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

, and trauma to the spinal cord, including from slipped discs. # If the

is damaged on only one side, then only the lower face will be affected, however if there is involvement of both the left and right tracts, then the result is

pseudobulbar palsy Pseudobulbar palsy is a medical condition characterized by the inability to control facial movements (such as chewing and speaking) and caused by a variety of neurological disorders. Patients experience difficulty chewing and swallowing, have i ...

. This causes problems with swallowing, speaking, and

emotional lability In medicine and psychology, emotional lability is a Medical sign, sign or symptom typified by exaggerated changes in mood or affect (psychology), affect in quick succession. Sometimes the emotions expressed outwardly are very different from how th ...

. # Severe disabling involuntary movements such as hemiballismus or severe

chorea Chorea, or (rarely) choreia, () is an abnormal involuntary movement disorder, characterized by quick movements of the hands or feet. It is one of a group of neurological disorders called dyskinesias. The term ''chorea'' is derived , as the move ...

might exhaust the patient and become a life-threatening situation. # In the past, this condition was treated by partial section of the pyramidal tract either at the

or at the

cerebral crus The cerebral crus (crus cerebri. ''crus'' means ‘leg’ in Latin.) is the anterior portion of the cerebral peduncle which contains the motor tracts, traveling from the cerebral cortex to the pons and spine. The plural of which is cerebral crura ...

(pedunculotomy).

Additional images

File:Gray683.png, Dissection of brain-stem. Lateral view. File:Gray689.png, Superficial dissection of brain-stem. Ventral view. File:Gray764.png, The motor tract. File:Medulla spinalis - tracts - English.svg File:Gray672.png, Diagram of the principal fasciculi of the spinal cord, from

Gray's anatomy ''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter and first published in London in 1858. It has had multiple revised editions, and the current edition, the 42nd (October 2020 ...

In popular culture

In ''

National Lampoon's European Vacation ''National Lampoon's European Vacation'' is a 1985 American comedy film directed by Amy Heckerling and written by Robert Klane based on characters created by John Hughes. The second film in National Lampoon's ''Vacation'' film series, it stars ...

'', the Griswold family wins a vacation on a game show called ''Pig in a Poke'' when their opponents fail to correctly answer a question about the pyramidal tracts, despite Clark Griswold (played by Chevy Chase) mistakenly answering that they are a housing development outside Cairo.

References

External links

*
McGill
* {{Authority control Central nervous system pathways Motor system