Comprehensive Study Guide to the Motor System and Movement Disorders (copy)

Hierarchical Organization of the Motor System

• The motor system is a hierarchical system as described by Hughlings Jackson. It becomes less sophisticated as it descends and more sophisticated as it ascends the hierarchy.

• The hierarchy is divided into direct pathways (pyramidal), indirect pathways (extrapyramidal), and control circuits (basal ganglia and cerebellum).

Cortical Motor Areas and Functions

• Premotor Area (PMA):

  • Features bilateral activation.

  • Projects to the brainstem where the reticulospinal tracts originate.

  • Primarily responsible for postural fixation, such as fixating the shoulders and stabilizing the hips.

  • Receives input from the prefrontal cortex and the parietal lobe (specifically tactile and visual information).

• Supplementary Motor Area (SMA):

  • Responds to internal cues.

  • Involved in motor planning and movement sequences stored in motor memory.

  • Projects to the primary motor cortex and the pyramidal tract.

Anatomy of Descending Pathways and Neurons

• Tract Definition: A tract is composed of nerve fibers that share a common origin, destination, and set of functions.

• Two-Neuron Chain: Typical descending pathways consist of a series of two motor neurons: Upper Motor Neurons ($UMNs$) and Lower Motor Neurons ($LMNs$).

• Upper Motor Neurons ($UMNs$):

  • Located entirely within the Central Nervous System ($CNS$).

  • Cell bodies originate in the cerebral cortex, cerebellum, and brainstem.

  • They form the descending tracts.

  • This is considered a direct activation pathway because of its direct connection to the lower motor neurons.

• Lower Motor Neurons ($LMNs$):

  • Cell bodies are located in the spinal ventral horns (anterior horns) or the corresponding cranial motor nuclei in the brainstem.

  • They make up the spinal and cranial nerves.

  • Composed of alpha motor neurons ($A-\alpha$).

  • Axons proceed through peripheral somatic nerves to innervate skeletal muscles.

  • Classified as the "Final Common Pathway."

Classification of Descending Motor Systems

• Pyramidal Tracts:

  • Motor tracts that originate from the cerebral cortex and descend to the spinal cord.

  • They pass through the medullary pyramids.

  • Examples: Corticospinal tract, Corticonuclear (corticobulbar) tract, and Corticopontine tract.   

• Extrapyramidal Tracts:

  • All other descending motor pathways that do not travel through the medullary pyramids.

  • Examples: Rubrospinal, Reticulospinal, Olivospinal, and Vestibulospinal tracts.

The Pyramidal System: Direct Activation Pathway

• Corticospinal Tract Divisions:

  • Lateral Corticospinal Tract: Consists of fibers that cross in the medulla ($80\%$ to $90\%$ of fibers). They supply all levels of the spinal cord and allow for fine movements, speed, and agility (e.g., individual finger control, writing, needlework).

  • Anterior Corticospinal Tract: Consists of uncrossed fibers (about $10\%$) that descend ipsilaterally and cross near the level of the synapse with $LMNs$. They primarily supply the neck and upper limbs and are involved in axial or postural movements.

• Course of the Corticospinal Tract:

  1. Fibers descend from the primary motor area of the cerebral cortex.

  2. Pass through the corona radiata and collect in the posterior limb of the internal capsule.

  3. Move through the middle portion of the cerebral peduncles in the midbrain.

  4. Separated by transverse pontine fibers in the pons.

  5. Form the medullary pyramids.

  6. Decussation (crossing) occurs at the lower region of the medulla (medullary decussation).

• Corticobulbar Tract:

  • Innervates the head and face.

  • Most fibers terminate in the reticular formation near cranial nerve nuclei.

  • Synapses with association neurons that meet with $LMNs$ in the motor nuclei of cranial nerves:

  • $III$ and $IV$ (Midbrain)

  • $V$, $VI$, and $VII$ (Pons)

  • $IX$, $X$, $XI$, and $XII$ (Medulla)

  • Provides bilateral innervation to most nuclei, except for the lower part of the facial nerve nucleus ($VII$) and the hypoglossal nerve nucleus ($XII$), which receive only contralateral innervation.

• Corticopontine Tract:

  • Links the motor cortex to the cerebellum through the pons (pontocerebellar input).

The Extrapyramidal System: Indirect Activation Pathway

• Rubrospinal Tract:

  • Origin: Red nucleus in the midbrain.

  • Inputs: Cerebellum and cerebral motor cortex.

  • Output: Crosses to the contralateral side at the level of the nucleus. Axons are located in the lateral spinal white matter.

  • Function: Moves distal limbs (hands and feet), regulates tone and posture. It is excitatory to flexors and inhibitory to extensors.

• Vestibulospinal Tract:

  • Origin: Vestibular nuclei in the pons and medulla.

  • Inputs: Inner ear vestibular section and cerebellum.

  • Output: Remains ipsilateral.

  • Function: Excitatory to ipsilateral extensors and inhibitory to flexors. Maintains balance in response to head movement.

• Reticulospinal Tract:

  • Origin: Reticular formation (central gray matter of brainstem).

  • Inputs: Spinal cord, vestibular nuclei, cerebellum, sensory-motor cortex, globus pallidus, and hypothalamus.

  • Pontine Reticulospinal Tract: Runs ipsilaterally; excitatory to axial extensor muscles.

  • Medullary Reticulospinal Tract: Runs mostly ipsilaterally (some cross); mediates larger movements of the trunk and limbs that do not require fine balance.

• Tectospinal Tract:

  • Origin: Superior colliculus in the midbrain.

  • Inputs: Visual stimuli.

  • Output: Contralateral skeletal muscles.

  • Function: Controls neck muscles to move the head and eyes in response to visual stimuli.

Neuromuscular Control and Sensors

• The Motor Unit:

  • Defined as one motor neuron and all the muscle fibers it innervates.

  • Nerve-to-fiber ratio depends on the task: low ratio for fine movements (e.g., eyes) and high ratio for gross movements (e.g., jumping).

  • Unit Types:

  • SO, Type I (Small): Slow oxidative; generates $2\,g$ of force for $60\,minutes$.

  • FOG, Type IIA (Medium/FFR): Fast oxidative glycolytic; generates $10\,g$ of force for $15\,minutes$.

  • FG, Type IIB (Large/FF): Fast glycolytic; generates $20\,g$ of force for $6\,minutes$.

• Muscle Sensors:

  • Muscle Spindle: Contains intrafusal muscle fibers and sensory fibers in a fibrous capsule. Detects muscle stretching (tonic and phasic info).

  • Golgi Tendon Organ ($GTO$): Located between the muscle and tendon. Consists of sensory nerve endings interwoven among collagen fibers. Protects the musculoskeletal system from injury.

Basal Ganglia Circuitry

• Movements are influenced by the basal ganglia via the thalamus to the motor cortex.

• Direct Pathway: Caudate/Putamen $\rightarrow$ Internal Globus Pallidus/Substantia Nigra. It releases the thalamus from tonic inhibition, increasing excitatory drive to the SMA. Result: Turns up motor activity.

• Indirect Pathway: Caudate/Putamen $\rightarrow$ External Globus Pallidus $\rightarrow$ Subthalamic Nucleus $\rightarrow$ Internal Globus Pallidus/Substantia Nigra. Result: Turns down motor activity.

Movement Disorders: Parkinson's and Huntington's

• Parkinson’s Disease (PD):

  • Pathology: Severe loss ($60\%$ to $80\%+$) of dopaminergic neurons in the substantia nigra pars compacta ($SNc$).

  • Mechanism: Excessive inhibition of the thalamus leads to decreased cortical excitation.

  • Cardinal Symptoms:

  • Resting Tremor ("pill rolling").

  • Bradykinesia (slowness, loss of automatic movement).

  • Rigidity (Cogwheel or Lead-pipe).

  • Postural Instability.

• Huntington’s Disease (HD):

  • Pathology: Neuronal death in the Striatum ($Caudate$ and $Putamen$).

  • Mechanism: Loss of GABA, ACh, and metenkephalin neurons leads to diminished inhibition of the thalamus, causing excessive cortical excitation.

  • Symptoms:

  • Choreic movements (brief, purposeless, involuntary, random).

  • Personality changes (apathy, depression, explosive behavior).

  • Dementia, Dysarthria (speech), and Dysphagia (swallowing).

Comparison of Motor Neuron Lesions

• Upper Motor Neuron ($UMN$) Lesion:

  • Effect: Paralysis affects movements rather than individual muscles.

  • Tone: Increased (Spasticity/"Clasp-knife" type), Hypertonia.

  • Reflexes: Increased (Hyperreflexia), Clonus present.

  • Babinski Sign: Positive (Big toe extends upward, others fan out). Normal only in infants under $1\,year$ old.

  • Atrophy: Slight, only from disuse.

• Lower Motor Neuron ($LMN$) Lesion:

  • Effect: Individual muscles or groups of muscles are affected.

  • Tone: Decreased (Hypotonia/Flaccidity).

  • Reflexes: Diminished or absent (Areflexia).

  • Atrophy: Pronounced muscle wasting.

Clinical Terminology and Movement Definitions

• Plegia and Paresis:

  • Hemiplegia: Paralysis of half the body.

  • Hemiparesis: Partial loss of power on half the body.

  • Paraplegia: Paralysis in both legs.

  • Quadriplegia: Paralysis in all four limbs.

  • Monoplegia: Paralysis in one limb.

• Movement Descriptions:

  • Tremor: Rhythmic, oscillatory movement of body parts.

  • Ataxia: Breakdown of coordinated execution; leads to unsteadiness and wide-based gait. Often involves Dysmetria (misjudging distance).

  • Athetosis: Slow, writhing, continuous involuntary movements, usually in distal limbs.

  • Chorea: Irregular, rapid, unsustained, jerky movements that flow between body parts.

  • Dystonia: Twisting, repetitive movements that result in abnormal static postures.

  • Myoclonus: Sudden, shock-like contractions or inhibitions.

  • Akathisia: Inner restlessness; inability to sit still.

  • Ballism: Large amplitude, flailing movements of proximal limbs. Hemiballism is often caused by stroke or levodopa overdose.