A COMPREHENSIVE GUIDE TO MOTOR PATHWAYS

Introduction

Motor pathways are neural pathways that are responsible for controlling voluntary movements in the body. These pathways involve the transmission of signals from the brain to the muscles through a complex network of neurons.

There are two main types of motor pathways in the nervous system: the pyramidal pathway and the extrapyramidal pathway.

The pyramidal pathway, also known as the corticospinal pathway, originates in the cerebral cortex and travels down through the brainstem and spinal cord to reach the muscles. This pathway is responsible for controlling fine, skilled movements, such as writing, typing, and playing an instrument.

The extrapyramidal pathway is a collection of neural pathways that originate in the brainstem and are involved in regulating posture, balance, and muscle tone. This pathway includes several subcortical structures such as the basal ganglia, cerebellum, and reticular formation.

Both of these pathways involve the use of specialized neurons called motor neurons. Motor neurons are located in the spinal cord and brainstem and are responsible for transmitting signals from the nervous system to the muscles. When a signal is received by a motor neuron, it causes the release of a neurotransmitter called acetylcholine, which activates the muscle fibers and causes contraction.

Overall, motor pathways are essential for controlling voluntary movements and maintaining proper posture and balance. Dysfunction of these pathways can lead to a range of motor disorders, such as Parkinson’s disease, Huntington’s disease, and ALS (amyotrophic lateral sclerosis).

 

Upper and Lower motor neurons

Upper motor neurons (UMNs) and lower motor neurons (LMNs) are two types of neurons that are involved in the control of voluntary movements in the body. Here are their definitions with examples:

1) Upper motor neurons (UMNs): UMNs are the neurons that originate in the motor cortex of the brain and descend to the spinal cord to synapse with lower motor neurons. They play a critical role in the control of voluntary movements, such as walking, reaching, and grasping. UMNs are responsible for planning and initiating movements, as well as modulating their strength and direction.

Examples of UMNs include:

• Corticospinal neurons: These are the largest type of UMNs that originate in the primary motor cortex and descend to the spinal cord. They are responsible for controlling fine movements of the hands, fingers, and feet.
• Corticobulbar neurons: These UMNs originate in the motor cortex and terminate in the brainstem. They control the muscles of the face, tongue, and throat, which are involved in speech, chewing, and swallowing.

 

2) Lower motor neurons (LMNs): LMNs are the neurons that originate in the spinal cord and directly innervate skeletal muscles. They are responsible for executing the commands of UMNs and generating muscle contractions.

Examples of LMNs include:

• Alpha motor neurons: These are the primary LMNs that innervate extrafusal muscle fibers, which are responsible for generating force and movement.
• Gamma motor neurons: These LMNs innervate intrafusal muscle fibers, which are involved in the control of muscle tone and sensitivity to stretch.

 

The corticospinal tract, also known as the pyramidal tract, is a pathway that originates in the primary motor cortex of the brain and travels through the brainstem and spinal cord. This tract is responsible for carrying motor commands from the brain to the muscles of the body, allowing us to initiate and control voluntary movements.

The direct motor pathways that make up the corticospinal tract consist of two parts: the lateral corticospinal tract and the ventral corticospinal tract.

The lateral corticospinal tract originates in the primary motor cortex and crosses over to the opposite side of the body in the medulla oblongata of the brainstem. It then travels down the spinal cord on the opposite side of the body, controlling voluntary movements of the limbs and digits.

The ventral corticospinal tract, on the other hand, originates in the primary motor cortex and travels down the spinal cord on the same side of the body. This tract primarily controls voluntary movements of the trunk and proximal limbs.

Together, the corticospinal tract and its direct motor pathways allow for precise and coordinated movement of the body’s muscles. Lesions or damage to these pathways can result in motor deficits such as weakness, spasticity, and paralysis.

 

The indirect motor pathways, also known as extrapyramidal tracts, are responsible for controlling voluntary movements of the trunk and limbs, as well as maintaining posture and balance. These pathways originate in different areas of the cortex, including the primary motor cortex, premotor cortex, and supplementary motor area.

The rubrospinal tract originates in the red nucleus in the midbrain and descends down the spinal cord to control limb movements. It is involved in the control of fine movements and muscle tone.

The reticulospinal tracts originate in the reticular formation in the brainstem and are divided into two pathways: the pontine reticulospinal tract and the medullary reticulospinal tract. The pontine reticulospinal tract facilitates the extensor muscles and is involved in maintaining posture, while the medullary reticulospinal tract inhibits the extensor muscles and facilitates the flexor muscles, allowing for more precise control of movements.

These extrapyramidal tracts receive input from the cortex and other brain regions, such as the basal ganglia and cerebellum, which help to modulate and fine-tune their activity. Dysfunction in these pathways can lead to movement disorders such as Parkinson’s disease, Huntington’s disease, and dystonia.

 

The motor pathways that control the movements of the face muscles involve a complex network of nerves and brain regions. There are two main motor pathways that control the face muscles: the corticobulbar pathway and the facial nerve pathway.

The corticobulbar pathway originates in the primary motor cortex of the brain and descends through the internal capsule of the brain to the brainstem. It then synapses with motor neurons in the facial nucleus, which is located in the pons region of the brainstem. From there, the motor neurons send their axons out to the muscles of the face to control facial expressions.

The facial nerve pathway, on the other hand, originates in the facial nucleus itself. The motor neurons in this pathway directly innervate the muscles of the face. The facial nerve pathway is responsible for controlling the muscles of the forehead, eyebrows, eyelids, cheeks, and lips.

Both of these pathways work together to control the intricate movements of the face muscles. For example, when we smile, the corticobulbar pathway sends signals to the facial nucleus to activate the motor neurons that control the muscles around the mouth, while the facial nerve pathway directly innervates the muscles in the cheeks and around the eyes to create the characteristic expression of a smile.

 

The signs and symptoms of upper and lower motor neuron lesions can vary significantly. Here’s a brief comparison of the two:

a) Upper Motor Neuron Lesion:

• Weakness or paralysis, typically on one side of the body
• Spasticity or increased muscle tone.
• Hyperreflexia or exaggerated reflexes.
• Clonus or rhythmic muscle contractions.
• Babinski reflex or upward extension of the big toe in response to the sole of the foot being stimulated.
• Loss of fine motor control.
• Reduced ability to perform skilled or complex movements.
• Increased muscle tone and stiffness.
• Changes in muscle tone or posture, such as a clenched fist or flexed arm.

 

b) Lower Motor Neuron Lesion:

• Weakness or paralysis, typically in a specific muscle or muscle group.
• Flaccidity or decreased muscle tone.
• Hyporeflexia or reduced reflexes.
• Muscle atrophy or wasting.
• Fasciculations or muscle twitching.
• Fibrillations or spontaneous muscle fiber contractions.
• Reduced or absent deep tendon reflexes.
• Difficulty with voluntary movements, such as walking, grasping, or standing.

It’s important to note that these signs and symptoms can vary depending on the specific location and extent of the lesion. A thorough neurological examination and diagnostic testing, such as imaging or electrophysiological studies, are often needed to determine the underlying cause of the motor neuron lesion and guide appropriate treatment.