Corticospinal tract
Encyclopedia
The corticospinal or pyramidal tract is a collection of axon
s that travel between the cerebral cortex
of the brain
and the spinal cord
.
The corticospinal tract contains mostly motor axon
s. It consists of two separate tracts in the spinal cord: the lateral corticospinal tract
and the anterior corticospinal tract
. An understanding of these tracts leads to an understanding of why for one side of the body is controlled by the opposite side of the brain.
The corticobulbar tract
is also considered to be a pyramidal tract, though it carries signals to motor neurons of the cranial nerve nuclei, rather than the spinal cord.
The neurons of the corticospinal tracts are sometimes referred to as pyramidal tract neurons (PTN), because their axons form part of the pyramidal tracts leading to the spinal cord, which in turn are named such because in cross-section they resemble pyramids as they pass through the medulla
.
Pyramidal tract neurons, however, are not to be confused with pyramidal neurons: a super-class of neurons, found in many parts of the brain including the cerebral cortex and the hippocampus, whose name derives from the pyramid-like shape of the cell body (soma). In fact, corticospinal neurons are both pyramidal tract neurons (because their axons pass through the medullary pyramids) and pyramidal neurons (because their cell body is shaped like a pyramid).
The corticospinal tract is concerned specifically with discrete voluntary skilled movements, especially of the distal parts of the limbs (sometimes called "fractionated" movements).
. About half of its fibres arise from the primary motor cortex
. Other contributions come from the supplementary motor area
, premotor cortex
, somatosensory cortex, parietal lobe
, and cingulate gyrus. The average fiber diameter is in the region of 10μm; around 3% of fibres are extra-large (20μm) and arise from Betz cell
s, mostly in the leg area of the primary motor cortex.
, together with their axons that travel down through the brain stem
and spinal cord
are commonly referred to as upper motor neuron
s. It should be noted, however, that they do not project to muscles, and thus the term 'motor neuron' is somewhat misleading.
Decussation
Some of the neuron
al cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons
(Corticopontine tract), and medulla oblongata
(cortico-bulbar tract), decussating just before they reach their target nuclei. These are called geniculate fibers
. Many more motor cortex neurons, however, extend fibers all the way down to the spinal cord
(corticospinal tract).
Whichever of these two tracts it travels in, a corticospinal axon
will synapse
with another neuron in the ventral horn
. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
, and form part of the posterior limb of the internal capsule
.
The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids
") on the ventral surface of medulla oblongata
- this is where the name pyramidal tract comes from.
After the decussation, the axons travel down the spinal cord
as the lateral corticospinal tract
. Fibers that do not cross over in the medulla oblongata
travel down the separate anterior corticospinal tract
, and most of them cross over to the contralateral side in the spinal cord
, shortly before reaching the lower motor neuron
s.
s) with the lower motor neuron
s, located in the ventral horn of the spinal cord.
In the brain stem
, the lower motor neuron
s are located in the motor cranial nerve nuclei (oculomotor, trochlear, motor nucleus of the trigeminal nerve
, abducens, facial
, accessory
, hypoglossal). The lower motor neuron
axons leave the brain stem
via motor cranial nerves
and the spinal cord
via anterior roots of the spinal nerve
s, respectively, ending up at the neuromuscular plate and providing motor innervation for voluntary muscles.
". A few days after the injury to the upper motor neurons, a pattern of motor signs and symptoms appears, including spasticity
, the decreased vigor (and increased threshold) of superficial reflexes, a loss of the ability to perform fine movements, and an extensor plantar response known as the Babinski sign.
. See extrapyramidal motor system.
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s that travel between 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...
of 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,...
and 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...
.
The corticospinal tract contains mostly motor axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s. It consists of two separate tracts in the spinal cord: the lateral corticospinal tract
Lateral corticospinal tract
The lateral corticospinal tract is the largest part of the corticospinal tract...
and the anterior corticospinal tract
Anterior corticospinal tract
The anterior corticospinal tract is a small bundle of descending fibers that connect the cerebral cortex to the spinal cord...
. An understanding of these tracts leads to an understanding of why for one side of the body is controlled by the opposite side of the brain.
The 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...
is also considered to be a pyramidal tract, though it carries signals to motor neurons of the cranial nerve nuclei, rather than the spinal cord.
The neurons of the corticospinal tracts are sometimes referred to as pyramidal tract neurons (PTN), because their axons form part of the pyramidal tracts leading to the spinal cord, which in turn are named such because in cross-section they resemble pyramids as they pass through the medulla
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...
.
Pyramidal tract neurons, however, are not to be confused with pyramidal neurons: a super-class of neurons, found in many parts of the brain including the cerebral cortex and the hippocampus, whose name derives from the pyramid-like shape of the cell body (soma). In fact, corticospinal neurons are both pyramidal tract neurons (because their axons pass through the medullary pyramids) and pyramidal neurons (because their cell body is shaped like a pyramid).
The corticospinal tract is concerned specifically with discrete voluntary skilled movements, especially of the distal parts of the limbs (sometimes called "fractionated" movements).
The motor pathway
The corticospinal tract originates from pyramidal cells in layer V of the cerebral cortexCerebral 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...
. About half of its fibres arise from 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...
. Other contributions come from the supplementary motor area
Supplementary motor area
The supplementary motor area is a part of the sensorimotor cerebral cortex . It was included, on purely cytoarchitectonic arguments, in area 6 of Brodmann and the Vogts...
, premotor cortex
Premotor cortex
The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...
, somatosensory cortex, parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
, and cingulate gyrus. The average fiber diameter is in the region of 10μm; around 3% of fibres are extra-large (20μm) and arise from 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, mostly in the leg area of the primary motor cortex.
Upper motor neurons
The neuronal cell bodies in the motor cortexMotor 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:...
, together with their axons that travel down through 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 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...
are commonly referred to as upper motor neuron
Upper motor neuron
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem 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...
s. It should be noted, however, that they do not project to muscles, and thus the term 'motor neuron' is somewhat misleading.
DecussationDecussationDecussation is used in biological contexts to describe a crossing.Examples include:* In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin...
and synapses
Some of the neuronNeuron
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...
al cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), 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...
(Corticopontine tract), 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...
(cortico-bulbar tract), decussating just before they reach their target nuclei. These are called geniculate fibers
Geniculate fibers
The geniculate fibers are the fibers in the region of the genu of the internal capsule; they originate in the motor part of the cerebral cortex, and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of...
. Many more motor cortex neurons, however, extend fibers all the way down to 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...
(corticospinal tract).
- Most of the corticospinal fibers (about 80%) cross over to the contralateral side in the medulla oblongataMedulla oblongataThe 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...
(pyramidal decussation). Those that cross in the medulla oblongata travel in the lateral corticospinal tractLateral corticospinal tractThe lateral corticospinal tract is the largest part of the corticospinal tract...
. - 10% Enter the lateral corticospinal tract on the same side.
- The remainder of them (10%) cross over at the level that they exit the spinal cordSpinal cordThe 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...
, and these travel in the anterior corticospinal tractAnterior corticospinal tractThe anterior corticospinal tract is a small bundle of descending fibers that connect the cerebral cortex to the spinal cord...
.
Whichever of these two tracts it travels in, a corticospinal axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
will synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
with another neuron in the ventral horn
Anterior horn (spinal cord)
The anterior horn of the spinal cord is the ventral grey matter section of the spinal cord. The anterior horn contains motor neurons that affect the axial muscles while the posterior horn receives information regarding touch and sensation...
. This ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
From cerebral to motor neurons
The motor axons move closer together as they travel down through the cerebral white matterWhite matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
, and form part of the posterior limb of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
.
The motor fibers continue down into the brainstem. The bundle of corticospinal axons is visible as two column-like structures ("pyramids
Pyramid of medulla oblongata
The anterior or ventral portion of the medulla oblongata is named the pyramid and lies between the anterior median fissure and the antero-lateral sulcus....
") on the ventral surface of 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...
- this is where the name pyramidal tract comes from.
After the decussation, the axons travel down 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...
as the lateral corticospinal tract
Lateral corticospinal tract
The lateral corticospinal tract is the largest part of the corticospinal tract...
. Fibers that do not cross over in the 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...
travel down the separate anterior corticospinal tract
Anterior corticospinal tract
The anterior corticospinal tract is a small bundle of descending fibers that connect the cerebral cortex to the spinal cord...
, and most of them cross over to the contralateral side 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...
, shortly before reaching 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.
Lower motor neurons
In the spinal cord, the axons of the upper motor neuron connect (most of them via interneurons, but to a lesser extent also via direct synapseSynapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s) with 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, located in the ventral horn of the spinal cord.
In 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...
, 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 are located in the motor cranial nerve nuclei (oculomotor, trochlear, motor nucleus of the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...
, abducens, facial
Facial
A facial is a procedure involving a variety of skin treatments, including: steam, exfoliation, extraction, creams, lotions, facial masks, peels, and massage...
, accessory
Accessory nerve
In anatomy, the accessory nerve is a nerve that controls specific muscles of the shoulder and neck. As part of it was formerly believed to originate in the brain, it is considered a cranial nerve...
, hypoglossal). 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...
axons leave 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...
via motor cranial nerves
Cranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
and 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...
via anterior roots of the spinal nerve
Spinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...
s, respectively, ending up at the neuromuscular plate and providing motor innervation for voluntary muscles.
Sensory pathways
- Spinothalamic tractSpinothalamic tractThe spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch...
- Spinocerebellar tractSpinocerebellar tractThe spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position ....
- Visual pathway
- Olfactory systemOlfactory systemThe olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...
- Posterior column pathway
Corticospinal tract damage
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 SyndromeUpper Motor Neuron Syndrome
Upper Motor Neuron Syndrome is a term used to describe the motor control changes that can occur in skeletal muscle after an upper motor neuron lesion....
". A few days after the injury to the upper motor neurons, a pattern of motor signs and symptoms appears, 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...
, the decreased vigor (and increased threshold) of superficial reflexes, a loss of the ability to perform fine movements, and an extensor plantar response known as the Babinski sign.
Extrapyramidal motor pathways
These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to support voluntary movement and help control posture and muscle toneMuscle tone
In physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. It helps maintain posture, and it declines during REM sleep.-Purpose:Unconscious nerve impulses maintain the...
. See extrapyramidal motor system.