Motor Neuron Disorders

• The somatic motor system includes all CNS and PNS structures contributing to motor activity.

Upper Motor Neuron (UMN)

• Descending projections converge on alpha lower motor neurons (LMN).
• Originate from motor areas of the cerebral cortex, responsible for voluntary movement.
• Brainstem nuclei influence voluntary movement, posture, tone, and regulation of reflexes.
• Fibers cross at the decussation of pyramids in the brainstem.

Lower Motor Neuron (LMN)

• Also known as alpha motor neurons, the final common pathway.
• Cell bodies (nuclei) are located in the anterior horn of the spinal cord or cranial nerve nuclei of the brainstem.
• Axons exit the CNS to innervate striated skeletal muscle.

Decussations and Laterality

• Most descending tracts cross the midline (decussate).
• Lesions rostral to the decussation result in contralateral symptoms.
• Lesions caudal to the decussation result in ipsilateral symptoms.

Motor System Modulation

• Modulation of the UMN systems is performed by the thalamus, basal ganglia, and cerebellum.
• Cortical motor areas, descending fibers, and nuclei have topographical organization.
• UMNs exert simultaneous influences on the final common pathway (LMNs) through parallel operations.

Control Systems

• Feedforward movements need anticipatory relationships between the motor system and the environment for preparation.
• Feedback movements need ongoing sensory feedback from the body and environment for reaction.

Coordination of Movement

• Coordination uses reciprocal innervation of agonist and antagonist muscles.
• Purposeful movements involve multiple muscles working together synergistically.
• Central pattern generators (CPGs) reduce the complexity of signals needed for routine purposeful movement.

Muscle Synergies

• Synergists are groups of muscles that commonly work together to perform a motor act.
• Synergists include the prime mover (agonist), stabilizers, and accessory muscles.
• Loss of normal synergies causes significant functional problems.

Types of Motor Activity

• Tone refers to background muscular activity at rest.
• Reflexes are simple, stereotyped overt movements set into motion by a peripheral stimulus.
• Voluntary movement involves a decision to act, is learned, and exploits hard-wired subroutines.

Peripheral Components

• The motor unit is a single alpha motor neuron (LMN) and the group of muscle fibers it innervates.
• Slow twitch fibers are red.
• Fast twitch, fatigable fibers are white.
• Intermediate-fast-twist, fatigue-resistant fibers exist.

Neuromuscular Junction

• The neuromuscular junction is a chemical synapse between the axon terminal of an alpha motor neuron and the sarcolemma of the striated muscle fiber.
• Acetylcholine released from the presynaptic terminals is excitatory.

LMN Syndrome

• LMN syndrome is caused by damage to the LMN cell body or axon.
• Characterized by hyporeflexia, hypotonia, paralysis or paresis, atrophy, and denervation (fasciculations, fibrillations).

Diseases of the Motor System

• Four sites of the motor unit can be attacked by disease: the cell body of the LMN, the axon, the NMJ, and the muscle fibers.
• Neurogenic diseases affects the cell body or axons (e.g., polio, neuropathies).
• Myopathic diseases affects striated muscle or NMJ (e.g., myasthenia gravis).

Polio

• Polio is a viral infection, preventable by vaccination.
• Polio attacks cell bodies of the LMNs.
• Cardinal clinical signs include weakness/paralysis, atrophy, and decreased reflexes.
• Postpolio syndrome may appear 20-30 years after the acute infection.

Neuropathies

• Neuropathies involves pathological changes of the peripheral nerves.
• Diabetes mellitus (DM) and acute inflammatory demyelinating polyneuropathy (AIDP) are examples.

Myasthenia Gravis

• Myasthenia Gravis is an autoimmune disorder of neuromuscular transmission resulting in weakness.
• Eye muscles are typically affected first, resulting in ptosis and diplopia.
• Muscles of facial expression, mastication, swallowing, and speech are typically affected next, resulting in altered facial expression, dysphagia, and dysarthria.

Myopathies

• Myopathies are progressive hereditary diseases of skeletal muscle.
• Duchenne muscular dystrophy is caused by a defective gene coding for dystrophin, a sex-linked recessive trait transmitted from mother to male children
• Lack of dystrophin renders sarcolemma vulnerable, leading to weakness, tearing, and loss of muscle fibers and shortens life expectancy to late adolescence or early adulthood.

Amyotrophic Lateral Sclerosis (ALS)

• ALS is the most frequent motor neuron disease, affecting LMN and UMN signs..
• Amyotrophy (weakness, denervation, atrophy) indicates LMN signs.
• Lateral sclerosis indicates UMN signs.
• ALS is relentlessly progressive, with about 50% of cases progressing to death within 2-3 years of diagnosis.

Internal Organization

• Anatomical organization involves hard-wired neural connections.
• Descending UMN pathways originate in the brainstem and cortex.
• The cerebral cortex modulates activity of descending brainstem UMNs and LMNs.

Behavioral Organization

• Behavioral organization involves the selection of the intended goal of movement.
• Behavioral organization originates in pre-motor areas, where movements are programmed that are the sent to the primary motor cortex.
• Motor programs are a part of the behavioral organization.

Somatotropic Organization

• In the spinal cord, motor neuron pools are areas of alpha motor neurons that go out to certain muscle groups, located in the same areas of the gray matter.
• The distal-proximal rule states that distal areas are more lateral, and proximal areas are more medial.
• The flexor-extensor rule states that the posterior area is more for flexors, and the anterior is more for extensors.

Brainstem Systems

• The dorsolateral system includes the rubrospinal tract.
• The ventromedial system includes the tectospinal tract, medial and lateral vestibulospinal tracts, and medial and lateral reticulospinal tracts.

Cerebral Cortex

• The motor homunculus represents the body's motor map in the cortex.

Spinal Cord Reflexes

• Spinal cord reflexes are involuntary, relatively short-latency, and stereotyped responses triggered by a stimulus.
• The pathway involves a receptor, afferent fiber, reflex center in the CNS (spinal cord), efferent fiber, and effector.

Stretch (Monosynaptic) Reflex

• The stretch reflex is the simplest and best-studied reflex.
• The stretch reflex is elicited by tapping the tendon of the muscle with a reflex hammer, causing a brief and brisk contraction of the stretched muscle.
• The stretch reflex relies on the sensitivity of the muscle spindle, commonly tested at the Achilles, patellar, bicep, and tricep tendons.
• Involves reciprocal inhibition via interneurons to the antagonist.
• Tension change is detected by the muscle spindle in muscle fibers, not the tendon where you tap.

Reciprocal Inhibition

• Reciprocal inhibition involves theinhibition of antagonist muscles during agonist contraction.
• Reciprocal inhibition is facilitated by inhibitory interneurons in the spinal cord and extensively used during voluntary motion and the stretch reflex.

Inverse Myotatic Reflex

• The inverse myotatic reflex is mediated by Golgi tendon organs (GTOs), located in tendons that respond to muscle tension.
• Autogenic inhibition generally results in relaxation of the agonist and contraction of the antagonist.
• The purpose of this reflex is uncertain.

Flexion Reflex

• The flexion reflex is a response to a suddenly applied noxious stimulus.
• The flexion reflex causes an adapted withdrawal of the limb from the offending stimulus.
• Excitatory discharges spread through many segments via the spinospinal system.
• Reciprocal inhibition of the antagonists of the ipsilateral limb occurs.

Crossed Extension Reflex

• The crossed extension reflex involves flexion of the ipsilateral limb accompanied by extension of the contralateral limb.
• The crossed extension reflex is mediated by interneurons that cross the midline of the spinal cord.
• The crossed extension reflex prevents one from falling over.

Stepping/Central Pattern Generators (CPGs)

• CPGs help reduce the complexity of signals required for specific purposeful movements.
• CPGs are executed via central motor programs in the spinal cord and brainstem ex: Gait.
• Networks of spinal interneurons activate LMNs to alternate flexion and extension of the hips and knees.
• Each lower extremity has a dedicated SPG and two SPGs are coordinated in the spinal cord.
• Descending input is required to activate SPGs.
• Cortical input is required for directing ankle dorsiflexion.
• Brainstem input is necessary for postural control.

Corticospinal (Pyramidal) Tract

• Extrapyramidal structures influence the motor system but lack direct connections to this tract, ex; BG & Cerebellum.
• The Corticospinal tract influences conscious movement.
• Fractionated movements gives the ability to perform isolated movements and fine motor movement: Hands, fingers, ankles, toes.
• The descending "UMN tract" travels from the Cerebral cortex, descends through the pyramids of the medulla & terminates on LMNs in the spinal cord.
• 60% originate in the primary motor cortex (Brodmann’s area 4).
• The rest originate in the premotor and supplementary motor cortex (Brodmann’s area 6) and the postcentral gyrus (Brodmann’s area 3,1,2).

Corticobulbar Tract

• The corticobulbar tract is the functional equivalent of the corticospinal tract, but goes to the brainstem.
• The corticobulbar tract innervates the head/neck through LMNs of cranial nerves.

Lateral Corticospinal Tract (LCST)

• 75-90% of the corticospinal tract fibers are in the LCST
• Fibers cross the midline in the pyramidal decussation.
• Descends in the contralateral lateral funiculus

Anterior Corticospinal Tract (ACST)

• The ACST includes a smaller contingent of uncrossed fibers.
• The ACST descends in the ventral funiculus.

Suprasegmental Control of Reflexes

• Brain structures can influence the amount of inhibition, excitation, or the balance of inhibition and excitation existing in the pool of interneurons in the spinal gray matter.
• Cerebral cortex, reticular formation (brainstem), and cerebellum is involved
• Higher brain centers (brainstem) dampen the reflexes at the spinal level through a tonic descending inhibitory pathway (TDIP).

Muscle Tone

• Muscle tone is the resting tension of the muscle.
• Hypotonia is decreased resting tension in a muscle, often developing after a LMN injury (herniated disc, polio, peripheral nerve lesions).
• Hypotonia can also occur with cerebellar lesions.
• Hypertonia is increased resting tension in a muscle which is involuntarily contracting.

Spasticity

• Spasticity is a velocity-dependent increase in muscle tone.
• Spasticity often develops with UMN damage such as Stroke, MS, spinal cord injury, TBI.

Rigidity

• Rigidity is not velocity dependent (always the same resistance). Lead pipe is the most extreme form and is seen with severe brain injuries and basal ganglia damage.

Spinal Shock

• Spinal shock is the initial motor system response following SC injuries and involves transient hypotonia and hyporeflexia.
• All sensation and voluntary movement below the lesion is lost.
• Reflex activity below the lesion is lost.
• Bowel and bladder function are lost, leading to fecal and urinary incontinence.

Emergence of Spasticity

• After the period of spinal shock, UMN signs emerge (spasticity, hyper-reflexia).
• Emergence of spasticity is due to altered suprasegmental influences.

Reflexes

• Hyper-reflexia is increased DTRs, occurring with UMN damage.
• Hyporeflexia is decreased DTRs, occurring with LMN damage.

Strength

• Strength is measured using MMTs.

Patterns of Involvement of Muscles

• patterns of weaknesses are different in LMN versis UMN damage.

LMN Damage

• LMN damage causes flaccid paralysis or flaccid paresis of individual muscles or groups of muscles, ipsilateral to the lesion.

UMN Damage

• UMN damage causes spasticity and paresis.
• Damage is contralateral to the lesion if damage is rostral to the decussation.
• Damage is ipsilateral to the lesion if damage is caudal to the decussation.

Pathological Reflexes

• Pathological reflexes only occur if something is happening pathologically in the UMNs in particular.

Babinski Reflex

• The Babinski reflex is elicited by stroking from heel up to the lateral border across the ball of the foot.
• The Babinski reflex is a normal reflex in babies, elicited as a bilateral plantar response during the first year of life (due to unmyelinated axons).
• When elicited beyond infancy, it is a reliable sign of corticospinal tract damage (UMN).

Flexion Reflex

• The flexion reflex is normally elicited with a noxious stimulus.
• Following UMN damage, the flexion response may be seen with an innocuous stimulus due to disruption of descending suprasegmental inhibition.

Clasp-Knife Phenomenon

• The clasp-knife phenomenon involves the limb moving freely for a short distance, followed by a rapid increase in resistance, followed by a sudden giving way to movement.
• Accompanies spasticity following UMN damage and is elicited with passive movements of a limb.

Clonus

• Clonus is elicited with abrupt and sustained ankle dorsiflexion. Involves a series of rhythmic involuntary muscle contractions with UMN damage.

UMN Syndrome

• Structures involved include the UMN in the cerebral cortex and descending corticospinal tracts in the brainstem or spinal cord.
• Contralateral weakness if above the decussation; ipsilateral if below along with Hypertonia and Hyper-reflexia; pathological reflexes present.

LMN Syndrome

• Structures involved include CNS: spinal cord or brainstem alpha motor neurons (LMNs) and the PNS: Motor axons of LMNs.
• Weakness are always segmental and ipsilateral to lesion.
• Impacted muscles are innervated by the alpha motor neurons or axons that are damaged, also causes Hypotonia and Hyporeflexia.

Key Differences: UMN vs LMN

• UMNs arise in the cerebral cortex or brainstem, with axons traveling in descending tracts to synapse with LMNs and control circuits (BG+ cerebellum) adjust their activity.
• LMNs have cell bodies located in the spinal cord or brainstem and synapse with skeletal muscle.

Explore various motor neuron disorders, including hypotonia, hypertonia, spinal cord injuries, and upper/lower motor neuron lesions. Understand conditions like clasp-knife phenomenon and extensor plantar response.