Descending Somatic Motor Pathways

Introduction

Building on our discussion of sensory pathways, this lecture focuses on the descending somatic motor pathways within the central nervous system. We will explore command centers for motor control, descending pathways in the spinal cord, upper versus lower motor neuron lesions, and lesion localization in sensory and motor pathways. We will also examine the integrating pathways within the cerebellum, with a special emphasis on how these pathways modulate motor activity and maintain balance.

Somatic Motor System Functions

The somatic motor system performs vital functions:

Anti-gravity: Maintains posture against gravity using flexion and extension (contraction and relaxation) of antagonistic muscle groups. This function is crucial for standing, walking, and other weight-bearing activities.
Stable Posture: Maintains an upright position without conscious effort. The system uses continuous feedback loops to adjust muscle tone and maintain balance, ensuring stability during various activities.
Mobility: Controls voluntary movements, including initiation, execution, and cessation. This involves the coordinated activity of multiple muscle groups to produce smooth, purposeful movements.

Spinal Cord Anatomy

Sensory inflow: Via the dorsal root and pseudo-unipolar axon, conveying information from sensory receptors to the central nervous system.
Motor outflow: Cell body in the ventral horn, axon as the lower motor neuron to the target muscle, initiating muscle contraction.
Ventral horn neurons: Myelinated, including:
- Gamma motor neurons: Thin axonal, slow conducting, regulating muscle spindle sensitivity.
- Skeletal motor alpha neurons: Thicker, faster conducting for muscle contraction, providing the primary force for movement.

Pyramidal vs. Extrapyramidal Systems

The initiation of movement involves two primary systems:

Pyramidal System

Evolutionarily newer system, highly developed in primates (including humans).
Responsible for skilled activities like typing, playing musical instruments, writing, etc. These activities require precise, coordinated movements.
Present in mammals; less critical in domestic species, where fine motor skills are less developed.
Experimental destruction of the pyramid in the brainstem (medulla oblongata) revealed that animals could still perform basic motor activities but lost fine motor skills, highlighting the system's role in precise movements.

Extrapyramidal System

More ancient system concerned with automatic muscular activity, postural control, and rhythmic movements. This system operates largely unconsciously, maintaining balance and posture.
Involved in locomotion, feeding, defense, and other semi-automatic survival behaviors. These behaviors are essential for survival and require coordinated muscle activity.
Dominant system in domestic animals and non-mammals, where basic motor functions are more critical than fine motor skills.

Cerebral Cortex - Motor Cortex

The motor cortex is organized similarly to the somatic sensory cortex. Walter Penfield mapped the motor cortex through stimulation during surgeries, identifying specific areas responsible for controlling different body parts.

The primary motor cortex is located in the frontal lobe, just in front of the cruciate sulcus. This area is responsible for initiating voluntary movements.

The somatic sensory and motor cortices are mirrored on both sides of the brain, with each side controlling the opposite side of the body. This contralateral control is a fundamental feature of the nervous system.

Corticospinal Tract

The corticospinal tract originates in the cortex and descends to the spinal cord. This tract is responsible for controlling voluntary movements throughout the body.

The corticonuclear tracts control voluntary movements of the head muscles (mastication, facial expression), governed by cranial nerves (e.g., facial nerve (VII), trigeminal nerve (V)). These tracts are essential for facial expressions, speech, and chewing.

The pyramidal system includes these corticonuclear tracts, following a similar pattern to the corticospinal tract, crossing over to the nuclei in the brainstem. This crossover ensures that each side of the brain controls the opposite side of the head.

Upper vs. Lower Motor Neurons

Upper motor neuron: Located within the central nervous system (brain/spinal cord). These neurons initiate and modulate motor activity.
Lower motor neuron: Peripheral nerve that innervates the target muscle. These neurons directly stimulate muscle contraction.

The pathway consists of a two-neuron relay (or potentially a three-neuron relay with a short interneuron). Interneurons can modulate the signal between the upper and lower motor neurons.

The first neuron (upper motor neuron) has its cell body in the gray matter of the cortex and a long axon that crosses over in the hindbrain (medulla oblongata). This crossover is essential for contralateral control.
It then travels down the spinal cord to synapse with the lower motor neuron in the ventral horn. This synapse transmits the motor signal to the lower motor neuron.
The lower motor neuron's axon innervates the effector muscle, causing it to contract.

Contralateral control: The left side of the brain controls the right muscles, and vice versa. This organization allows for coordinated movement and balance.

Paresis: Reduction in voluntary motor activity, indicating a partial loss of motor function.

Paralysis: Complete loss of motor activity, indicating a complete loss of motor function.

If paralysis occurs on one side of the body, with other tests indicating a brain problem, the lesion is likely in the opposite side of the brain. This is due to the contralateral organization of the motor pathways.

A full neurological exam, including cranial nerves and peripheral reflexes, is essential for accurate localization. This exam helps identify the specific location and extent of the lesion.

Summary of Corticospinal Tract

Starts at the cortex, ends at the spine.
If it involves the head, it's the corticonuclear system.
Left brain controls the right body (contralateral).
Two- or three-neuron relay.
Neuron 1: Upper motor neuron.
Neuron 3: Lower motor neuron.

Clinical signs can differ based on whether the lesion affects the upper or lower motor neurons. UMN lesions typically result in spasticity and exaggerated reflexes, while LMN lesions result in flaccidity and diminished reflexes.

Corpus striatum: Important white matter tract; bleeds in this area can cause sensory and motor deficits. The corpus striatum is involved in motor planning and coordination.

Globus Pallidus: Basal nuclei in the deep brain involved in motor control as part of the extrapyramidal system. The globus pallidus helps regulate voluntary movement.

Cerebellum Integration

The cerebellum receives sensory information (ipsilateral) and a copy of the motor plan from the cortex. It then sends a modified plan back to the motor cortex for final execution of motor activity. This feedback loop allows the cerebellum to fine-tune movements and maintain balance.

The cerebellum fine-tunes movement, modifying movement and phase control of movement rather than initiating contraction. It ensures that movements are smooth, accurate, and coordinated.

Cerebellum and Motor Control

The cortex sends a plan to the opposite cerebellum, which has already received sensory information. This allows the cerebellum to compare the intended movement with the actual sensory feedback.
The cerebellum sends a modified copy back to the opposite side of the brain, adjusting the motor plan based on sensory input.
Lesions in the right cerebellum result in deficits on the same side (ipsilateral control). This is because the cerebellum primarily influences motor activity on the same side of the body.

Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) Lesions

Neurological motor pathway diseases are classified as either UMN or LMN diseases, each with different etiologies and clinical signs. This classification is crucial for diagnosing and treating motor disorders.

Upper Motor Neuron Lesions

Typically involve disruption of descending motor pathways (e.g., hemorrhage or trauma to the spine). These lesions affect the communication between the brain and the spinal cord.

Reflexes may still be present because the synaptic reflex arc (sensory input, spinal cord synapse, motor reflex to muscle) remains intact. However, the reflexes are often exaggerated due to the loss of inhibitory control from higher centers.

Reflexes may be exaggerated due to lack of dampening from higher centers. This hyperreflexia is a hallmark of UMN lesions.

Lower Motor Neuron Lesions

Involve damage to the lower motor neuron cell body in the ventral horn or the peripheral nerve. These lesions disrupt the final pathway for motor signals, preventing muscle contraction.

Result in no reflex activity because the impulse cannot reach the neuromuscular junction to initiate contraction. This areflexia is a key characteristic of LMN lesions.

Neurogenic atrophy occurs rapidly due to the lack of acetylcholine release at the neuromuscular junction, which is distinct from disuse atrophy. This rapid muscle wasting is a significant consequence of LMN lesions.

Extrapyramidal System

Includes non-pyramidal descending somatic motor pathways involved in basic automatic movements (feeding, posture, defense). This system is essential for survival and basic motor functions.

Phylogenetically primitive, concerned with automatic muscular activity and postural/rhythmic activity. This system operates largely unconsciously, maintaining balance and posture.

Command centers govern postural and rhythmical activities. These centers include various nuclei in the brainstem and forebrain.

Major Extrapyramidal Tracts

Rubrospinal tract.
Reticulospinal tract (pontine and medullary centers, part of the descending reticular formation).
Vestibulospinal tract.
Tectospinal tract (involved in reflex movements of the head).

Characteristics of Extrapyramidal Tracts

Some tracts are predominantly inhibitory, while others are facilitatory. The balance between these inhibitory and facilitatory influences is crucial for coordinated movement.

Several nuclei are involved, including those in the forebrain (Globus Pallidus, basal nuclei), midbrain (red nuclei) and hindbrain (vestibular nuclei). These nuclei work together to regulate motor activity.

Most of the extrapyramidal systems decussate across the midline, contributing to contralateral control.

Red nucleus/rubrospinal tract: This is the extrapyramidal system most like the pyramidal system, being somatotopically arranged, and facilitating semi-skilled activity. The rubrospinal tract plays a role in motor coordination and control.

Vestibular nuclei: Strongly facilitatory, important for posture, and closely connected to the cerebellum. The vestibular nuclei help maintain balance and spatial orientation.

Reticulospinal tract: Has both inhibitory and facilitatory components, but the net output is inhibitory. The reticulospinal tract modulates muscle tone and reflexes.

The motor centers of the brainstem reside within this brain region. These centers are essential for basic motor functions and survival.

Summary of Major Pathways

Corticospinal tract: Cortex to the spine, involved in skilled motor activity; passes through the pyramid of the midbrain, pontine region, and ventral medulla oblongata.
Rubrospinal tract: Midbrain region, semi-skilled motor activity.
Reticulospinal tract: Mesh-like structure, involved in various functions.
Vestibulospinal tract: Pons region, maintains posture and balance.
Tectospinal tract: Tectum of the midbrain; mediates reflex activity (e.g., pupillary light reflex, auditory reflex).

The inhibitory signals from upper motor neurons originate higher up in the brain (rostral to the midbrain). Lesions affecting these inhibitory signals can lead to extensor rigidity due to the unopposed facilitatory activity. This rigidity is a common sign of UMN lesions.

Final Points on the Extrapyramidal System

Facilitated and inhibitory pathways predominate, but the majority are inhibitory overall.
Necessary for flexion, extension, and muscle relaxation. This system ensures smooth, coordinated movements.
The final common pathway is via the lower motor neuron to the muscle. The LMN is the final link in the motor pathway.
Lesions can affect both upper and lower motor neurons, resulting in a variety of motor deficits.

Example: C8-T2 Lesion

A lesion at C8-T2 (cervical to thoracic 2) will affect the nerves going to the forelimb, leading to muscle atrophy.

In the hindlimb, upper motor neuron signs may be seen due to disruption of inhibitory effects, leading to exaggerated reflexes.

Spinal Cord Lesions and Loss of Function

The most surface tracts are affected first in crushing lesions. This is because these tracts are located on the outer surface of the spinal cord.

Loss of conscious proprioception may be an early sign of a spinal cord lesion. Proprioception is the sense of body position and movement, and its loss can indicate damage to the sensory pathways in the spinal cord.

Complete loss of sensory and motor activity indicates a more serious crushing lesion. This indicates severe damage to the spinal cord, affecting both sensory and motor pathways.

It can be challenging to differentiate between sensory and motor deficits at times. A thorough neurological exam is essential for accurate diagnosis.

Cerebellum: Evolution and Function

Archicerebellum (flocculonodular lobe): Most ancient, involved in balance and posture.
Paleocerebellum: Evolved for tone and posture, associated with muscle spindles and Golgi tendon organs.
Neocerebellum: Coordinates skilled output with higher centers.

Deep gray nuclei are present within the cerebellum, playing a crucial role in cerebellar function.

The cerebellum does not initiate contraction but modulates and controls the range, amplitude, and phase of movements, modifying movement but not initiating contraction. It ensures that movements are smooth, accurate, and coordinated.

Cerebellar disease involves incoordination rather than paralysis. This incoordination is a hallmark of cerebellar disorders.

Cerebellar Lesions and Clinical Signs

Intention tremor: Jerky movements as one attempts to control and pick something up. This tremor is characteristic of cerebellar lesions.

Clinical Localization

Lesions above the site of decussation: Clinical signs on the opposite side. This is due to the contralateral organization of the motor pathways.
Lesions below the site of decussation: Clinical signs on the same side. This is because the motor pathways have already crossed over at this point.

Neurological exams involve assessing cranial nerves and mentation to gather a comprehensive clinical picture. This comprehensive assessment is essential for accurate diagnosis and treatment planning.