Neurofacile 1 : La Voie Pyramidale.

Neurocampus

3 Feb 201807:46

Summary

TLDRThis video explains the voluntary motor pathways in the human nervous system, focusing on the two-neuron structure of motor transmission. It details the function of the first motor neuron (N1), which connects the brain to the spinal cord, forming the pyramidal tract. The video also covers the topographic organization of motor functions in the brain, the effects of cortical lesions, and clinical implications for motor recovery. Additionally, it touches on the phenomenon of motor plasticity and the role of the corticospinal tract in controlling muscle movements. Future videos will delve into related topics like spasticity and motor rehabilitation.

Takeaways

🧠 Human voluntary motor control is transmitted through a fast neural pathway involving two neurons: N1 (central pathway) and N2 (peripheral pathway).
🦴 N1, or upper motor neurons, transmit signals from the brain to the spinal cord, forming part of the pyramidal tract, while N2 transmits from the spinal cord to muscles.
🔺 The pyramidal tract gets its name from the triangular shape of the cell bodies located in the motor cortex.
🌍 N1 axons descend through the white matter, crossing at the base of the medulla, and synapse in the anterior horn of the spinal cord with N2.
🖐️ The motor cortex is organized topographically, with each area corresponding to specific body parts, forming the motor homunculus.
💡 Topographical organization helps understand vascular hemiplegias, with specific arteries (middle cerebral and anterior cerebral) supplying different regions of the motor cortex.
🩸 A stroke in the middle cerebral artery affects the upper body and face, while one in the anterior cerebral artery affects the lower body.
♻️ Cortical plasticity allows adjacent areas to compensate for motor function loss after damage, aiding in post-injury motor recovery.
🔀 90% of pyramidal tract fibers cross at the medulla (decussation), controlling the contralateral side of the body, while 10% remain uncrossed, innervating the ipsilateral side.
🤸 The direct corticospinal tract ensures rapid recovery of axial muscle control due to double innervation from both hemispheres.

Q & A

What are the two types of motoneurons involved in voluntary motor activity?
-The two types of motoneurons involved are upper motoneurons (N1), which transmit signals from the brain to the spinal cord, and lower motoneurons (N2), which transmit signals from the spinal cord to the muscles.
Where are the cell bodies of upper motoneurons located?
-The cell bodies of upper motoneurons (N1) are located in the grey matter of the brain, specifically in the motor cortex.
What is the pyramidal tract and how is it named?
-The pyramidal tract is a collection of axons that transmit motor signals from the motor cortex to the spinal cord. It is named after the triangular shape of the cell bodies that form it.
What is the role of the internal capsule in the transmission of motor signals?
-The internal capsule is a region in the brain where the axons of the upper motoneurons (N1) gather and concentrate before descending to the spinal cord. It plays a crucial role in transmitting motor signals to the body.
How does the somatotopic organization of the motor cortex influence motor control?
-The motor cortex is organized somatotopically, meaning that specific areas of the cortex control specific parts of the body. This organization creates a topographic map, where larger areas control more finely controlled parts like the hands and face.
What happens when there is an interruption of blood flow to the middle cerebral artery?
-An interruption of blood flow to the middle cerebral artery can cause hemiplegia, particularly affecting the upper limbs and face, as this artery supplies the lateral part of the motor cortex responsible for these areas.
What is the difference between proportional and non-proportional hemiplegia?
-Proportional hemiplegia affects the entire half of the body equally and typically results from lesions in the subcortical regions, whereas non-proportional hemiplegia varies in intensity across different body parts and indicates a cortical lesion.
How does brain plasticity aid in motor recovery after a lesion?
-Brain plasticity allows adjacent areas of the cortex to take over the function of damaged regions, such as when a hand's motor function is partially restored by nearby areas following a lesion.
What is the decussation of the pyramids, and why is it important?
-The decussation of the pyramids refers to the crossing of the motor fibers at the base of the brainstem, which ensures that each cerebral hemisphere controls the opposite side of the body. It is crucial for understanding the location and effects of motor impairments.
How does the corticospinal tract ensure balance control after hemisphere lesions?
-Balance control is maintained after lesions in one hemisphere because the axial muscles receive dual innervation from both the crossed and uncrossed corticospinal fibers, allowing rapid recovery of postural control.

Outlines

00:00

🧠 Human Voluntary Motor Control Pathways

The human voluntary motor system is transmitted via a fast, two-neuron pathway. The first motor neuron, called the upper motor neuron (N1), sends signals from the brain to the spinal cord through the central pathway, while the second motor neuron, called the peripheral motor neuron (N2), transmits signals from the spinal cord to the skeletal muscles. The N1 neurons form the pyramidal tract, originating from the motor cortex and extending through the spinal cord. Each region of the motor cortex corresponds to specific body parts, creating a somatotopic map known as the motor homunculus. This map explains how different areas of the body are represented in the brain, with larger representations for parts like the hands. Understanding this mapping aids in diagnosing conditions like hemiplegia, where a stroke affecting the middle cerebral artery impacts motor function of the face and upper limbs, while anterior cerebral artery involvement leads to leg-related issues.

05:01

🔄 Motor Control Pathway Decussation and Clinical Implications

The pyramidal tract's crossing, or decussation, at the medulla allows each hemisphere to control the opposite side of the body (contralateral innervation). This crossing is significant for diagnosing central paralysis. Lesions above the decussation cause contralateral paralysis, while lesions below lead to ipsilateral paralysis. Although 90% of pyramidal fibers cross, 10% form an uncrossed corticospinal tract for ipsilateral innervation. These uncrossed fibers mainly control muscles that maintain posture and balance, explaining why axial muscles recover quickly after brain damage. Additionally, the termination of N1 fibers in the spinal cord involves synapses that both excite target muscles and inhibit antagonist muscles, preventing counterproductive contractions. Disruptions in this inhibitory control can lead to spasticity.

Mindmap

Keywords

💡Motor Neuron

A motor neuron is a type of nerve cell responsible for transmitting signals from the brain or spinal cord to muscles, enabling voluntary movement. In the video, two types of motor neurons are discussed: upper motor neurons (N1) that transmit information from the brain to the spinal cord, and lower motor neurons (N2) that carry signals from the spinal cord to muscles. These neurons are central to understanding how the nervous system controls movement.

💡Pyramidal Tract

The pyramidal tract refers to a major neural pathway that carries motor signals from the brain's cortex to the spinal cord. The video explains that the upper motor neurons (N1) travel along this tract, and it is so named because the nerve cells in the brain's motor cortex have a triangular shape. The pyramidal tract plays a critical role in voluntary motor control, especially for fine movements.

💡Cortex Motor

The motor cortex is the region of the brain responsible for planning, controlling, and executing voluntary movements. It is located in the frontal lobe and is where motor neurons (N1) originate. In the video, the motor cortex's organization is shown to be topographical, meaning different regions correspond to different parts of the body, like the face or arms.

💡Somatotopy

Somatotopy is the organization of the brain's motor cortex based on body regions. It describes how specific areas of the motor cortex are dedicated to controlling movement in particular body parts. The video introduces this concept when discussing how the motor cortex projects signals to different muscles, leading to a 'motor homunculus,' a distorted representation of the body where areas like the hands are oversized due to their greater motor control.

💡Hemiplegia

Hemiplegia refers to paralysis on one side of the body, often caused by damage to the brain or spinal cord. The video explains that different types of hemiplegia occur depending on which part of the brain's vascular system is affected. For example, damage to the middle cerebral artery may lead to paralysis primarily affecting the arm and face, while damage to the anterior cerebral artery may cause leg paralysis.

💡Decussation of the Pyramids

Decussation of the pyramids is the crossing of nerve fibers in the medulla oblongata, where most fibers in the pyramidal tract switch sides. This means that each hemisphere of the brain controls the opposite side of the body. The video highlights the importance of this crossing in diagnosing central paralysis, as lesions above or below the decussation lead to contralateral or ipsilateral paralysis, respectively.

💡Corticospinal Tract

The corticospinal tract is the primary pathway for motor signals traveling from the cerebral cortex to the spinal cord. The video explains that 90% of the fibers in this tract cross over in the medulla (forming the decussation), while 10% remain uncrossed, allowing both sides of the body to receive motor innervation. This tract is essential for fine motor control of the limbs.

💡Capsule Internal

The internal capsule is a region of white matter in the brain through which motor signals pass from the cortex to the spinal cord. The video emphasizes the importance of the internal capsule, as lesions here can lead to significant motor deficits, including massive hemiplegia. It is a key site where motor fibers are densely packed, making it vulnerable to strokes.

💡Synapse

A synapse is the junction between two neurons where nerve signals are transmitted. The video specifically discusses the synapse between the upper motor neuron (N1) and the lower motor neuron (N2) in the spinal cord, called the medullary synapse. This synapse allows for the transmission of signals that cause muscle contraction, playing a crucial role in movement control.

💡Cerebral Plasticity

Cerebral plasticity refers to the brain's ability to reorganize itself by forming new neural connections. The video touches on this concept when discussing motor recovery after a lesion, explaining how adjacent brain areas can take over functions lost due to damage, allowing for partial recovery of motor function. This plasticity is vital for rehabilitation after strokes or other brain injuries.

Highlights

The human voluntary motor control is transmitted through the nervous system by a rapid pathway consisting of only two neurons: the upper motor neuron (N1) and the lower motor neuron (N2).

The upper motor neuron (N1) transmits information from the brain to the spinal cord, while the lower motor neuron (N2) transmits it from the spinal cord to the striated muscles.

N1 neurons form the pyramidal tract, with their cell bodies located in the motor cortex of the brain, specifically in the frontal cortex.

The term 'pyramidal tract' comes from the triangular shape of the cell bodies of N1 neurons located in the motor cortex.

The motor cortex is organized topographically, meaning different regions control specific parts of the body. This organization is known as the motor homunculus.

The motor homunculus illustrates that areas such as the hands and face have a larger representation in the motor cortex due to their higher complexity of movement.

Damage to different arteries supplying the motor cortex can result in distinct types of hemiplegia (paralysis), depending on which artery is affected.

An interruption of blood flow to the middle cerebral artery causes hemiplegia with a predominance of facial and upper limb paralysis.

Damage to the anterior cerebral artery results in hemiplegia with more prominent paralysis in the lower limbs.

In cases of cortical lesions, the brain's plasticity allows adjacent areas to take over some of the motor functions, aiding in recovery.

Pyramidal fibers cross at the base of the medulla oblongata, ensuring that each hemisphere of the brain controls the opposite side of the body.

Lesions occurring above the pyramidal decussation (crossing) result in contralateral paralysis, while lesions below the decussation cause ipsilateral paralysis.

Approximately 90% of pyramidal fibers cross over, while the remaining 10% continue uncrossed and control muscles bilaterally.

The synapse between N1 and N2 occurs in the anterior horn of the spinal cord, where excitatory signals cause muscle contraction.

Inhibitory signals are also sent to the antagonist muscle to prevent opposition during movement, a key mechanism for coordinated motion.