Pyramidal and extrapyramidal tracts

Definitions & Key takeaways

Pyramidal tracts are long nerve pathways in the brain that transmit signals from the cerebral cortex to the spinal cord, through the medullary pyramids. They control the voluntary motor function of the body and face. Neurons in the pyramidal tract are composed of upper motor neurons that directly innervate lower motor neurons in the anterior horn of the spinal cord. In contrast to the pyramidal tracts, the extrapyramidal tracts do not start in the cerebral cortex. Instead, their upper motor neurons are located within nuclei in the brain stem and they send their axons down the spinal cord and are involved in the control of movement and coordination.

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Introduction0:00–0:51

In order for you to flex your bicep in the mirror, your brain and brainstem has to send a motor signal through the spinal cord to the muscles in the body.
These motor signals are carried through two tracts, the pyramidal and extrapyramidal tracts. Neurons in the pyramidal tract are composed of upper motor neurons that directly innervate lower motor neurons in the anterior horn of the spinal cord.
Neurons in the extrapyramidal tract do not directly innervate lower motor neurons, but instead help coordinate muscle movement by indirectly activating or inhibiting groups of lower motor neurons through interneurons.
These groups of lower motor neurons usually innervate multiple muscles that share the same function, usually either flexors or extensors.
The pyramidal pathway is the primary pathway that carries out motor commands for voluntary movement. And it can be broken down into two main tracts, the corticospinal tract and the corticobulbar tract.

Pyramidal pathways0:51–1:05

Corticospinal tract1:05–2:10

The corticospinal tract originates in the motor cortex where the cell bodies of the upper motor neurons are located. The axons of these neurons travel together as fibers through the internal capsule to reach the brainstem where they form the medullary pyramids on the ventral surface of the brainstem.
At the level of the medulla, these fibers divide, and 90% of them form the lateral corticospinal tract which cross over to the opposite side of the medulla at the pyramidal decussation, while the remaining 10% of them form the anterior corticospinal tract which does not cross over just yet, and both tracts then travel through the spinal cord.
Neurons from the lateral corticospinal tract synapse on lower motor neurons in the anterior horn, while the neurons in the anterior corticospinal tract cross over in the spinal cord first before they synapse on the lower motor neurons in the anterior horn.
The upper motor neurons activate the lower motor neurons which leave the spinal cord and innervate the different skeletal muscles.
The lateral corticospinal tract controls the muscles of the limbs while the anterior corticospinal tract controls those in the trunk.
The corticobulbar tract also starts in the motor cortex where the upper motor neurons are located. The axons of these upper motor neurons form the cortical bulbar tract which travels lateral to the corticospinal tract to reach the brainstem.

Corticobulbar tract2:10–3:28

These axons will depart the tract and synapse directly with the contralateral lower motor neurons for Cranial Nerves V, VII, XI, and XII at their corresponding levels of the Pons and medulla.
Some of the upper motor neurons branch into two fibers which synapse with both the ipsilateral and contralateral motor nuclei.
These include cranial nerves V, which controls the muscles for chewing, XI, which controls the muscles of the neck, and the part of VII that innervates the muscles in the upper half of the face.
This means the muscles innervated by these nerves receive motor signals from the motor cortex from both hemispheres of the brain.
The upper motor neurons of cranial nerve VII that control the lower half of the face, and cranial nerve XII, which control tongue movement, crossover in the brainstem without branching and only synapse with the contralateral nuclei.
So the muscles innervated by these cranial nerves only receive motor information from the contralateral cerebral cortex.
In contrast to the pyramidal tracts, the extrapyramidal tracts do not start in the cerebral cortex. Instead, their upper motor neurons are located within nuclei in the brain stem and they send their axons down the spinal cord.

Extrapyramidal pathways3:28–3:52

These tracts includes the pontine reticulospinal tract, medullary reticulospinal tract, the lateral vestibulospinal tract, and the rubrospinal tract.

Pontine & medullary reticulospinal tracts3:52–5:10

The pontine or medial reticulospinal tract and the medullary or lateral reticulospinal tract help coordinate movement and posture.
The reticular formation is a network of nuclei that are found throughout the brain stem. Reticular nuclei in the pons give rise to the pontine reticulospinal tract which travels ipsilaterally through the ventral medial portion of the spinal cord where they act on interneurons to indirectly activate the lower motor neurons in the anterior horn that innervates the extensor muscles in the trunk and proximal limbs to help maintain posture.
The medullary reticulospinal tract starts in the medullary reticular nuclei which gives off an ipsilateral and a contralateral branch.
Both branches travel through the ventromedial spinal cord where they either directly or indirectly inhibit lower motor neurons in the anterior horn that innervates the extensor muscles in the limbs.
They also synapse with interneurons that synapse on the corticospinal tract, forming a connection between the extrapyramidal and pyramidal pathways which work together to coordinate an activity like walking.
Finally, the medullary reticulospinal tract regulates lower motor neurons that control breathing muscles like the intercostal muscles between the ribs.
The lateral vestibulospinal tract assists in balance and posture. Changes in the body’s posture are detected by vestibular sensory receptor cells in the inner ear and these travel to the large vestibular nucleus in the pons and medulla.

Lateral vestibulospinal tract5:10–5:55

This information is then sent to the lateral vestibular nucleus located in the pons, which sends out motor neurons down the ventromedial spinal cord to synapse with interneurons in the spinal cord on the ipsilateral side of the body.
Most of these interneurons activate lower motor neurons that control the extensor muscles in the legs and butt, but some also inhibit the lower motor neurons to the flexor muscles.
So when you’re falling over to one side, the leg muscles on that side will extend and keep you from falling over. The rubrospinal tract originates in the red nucleus in the midbrain and it assists with speed in movements.

Rubrospinal tract5:55–6:30

It immediately crosses over to the contralateral side at the ventral tegmental decussation before traveling down through the ventral lateral spinal cord in the neck region where they synapse with interneurons that activate the flexor muscles and inhibit extensor muscles in the upper limbs It is thought to play a role in controlling movement velocity, and that’s because damage to the tract causes a person’s movements to slow down.
Like moving in slow motion. Alright, as a quick recap, pyramidal tracts include the corticospinal and corticobulbar tracts which consist of upper motor neurons from the cerebral cortex which activate lower motor neurons in the spinal cord and brainstem which innervate skeletal muscles.

Review6:30–7:01

Extra-pyramidal tracts include the rubrospinal tract, the pontine reticulospinal tract, medullary reticulospinal tract, and lateral vestibulospinal tract, and these all indirectly modulate movement through interneurons which either activate or inhibit lower motor neurons.