Week 9- Motor System

Objectives of the Motor System

  • List the structures involved in the top-down control of voluntary movement.

  • Define and understand clinical terminology including:

    • Muscle tone

    • Cocontraction

    • Contracture

    • Paresis

    • Paralysis

    • Hemiplegia

    • Paraplegia

    • Tetraplegia

    • Hypertonia

    • Spasticity

    • Rigidity

    • Clonus

  • Analyze the difference between upper motor neurons and lower motor neurons and their role in the motor system.

  • Analyze the differences in lower motor and upper motor neuron lesions and give examples of common causes.

  • Describe the motor pathways related to posture and gross movements and selective motor control.

  • Identify OT’s role in working with patients with dysfunction with upper and lower motor neurons.

Skeletal Muscle Structure and Function

  • Characteristics of Skeletal Muscle:

    • Excitable

    • Contractile

    • Extensile

    • Elastic

  • Proteins in Skeletal Muscle:

    • Structural proteins

    • Contractile proteins

Muscle Contraction

  • Mechanism of Contraction:

    • Produced when actin slides relative to myosin.

    • Involves repeated attachment, swiveling, and detachment of myosin heads leading to contraction of the muscle.

Resistance to Stretch and Movement

  • Factors Influencing Resistance to Stretch:

    • Muscle length

    • Active contraction

    • Titin

    • Connective tissue

    • Weak actin-myosin bonds

  • Clinical Definition of Muscle Tone:

    • Resistance to stretch in a resting muscle.

  • Joint Stability:

    • Determined by elastic and contractile forces.

    • Cocontraction: Simultaneous contraction of antagonist muscles; stabilizes joints.

Lower Motor Neurons (LMNs)

  • Function of LMNs:

    • Convey signals to skeletal muscle fibers.

  • Types of LMNs:

    • Alpha motor neurons (large diameter, fast twitch)

    • Gamma motor neurons (smaller diameter, slow twitch)

  • Location of LMNs:

    • Cell bodies located in the ventral horn of the spinal cord; exit via the ventral root.

  • Motor Pools:

    • Clusters of axons innervating a single muscle.

    • Medial (proximal/axial muscles)

    • Lateral (distal muscles)

    • Anterior (extensors)

    • Posterior (flexors)

  • Myotomes:

    • Groups of muscles innervated by a single spinal nerve.

Motor Units

  • Definition:

    • A single alpha motor neuron and the muscle fibers it innervates.

  • Types of Muscle Fiber Contraction:

    • Fast twitch (larger diameter alpha motor neuron)

    • Slow twitch (smaller diameter alpha motor neuron)

Spinal Cord Coordination

  • Mechanisms:

    • Organize and synchronize muscle contractions.

  • Key Mechanisms Include:

    • Reciprocal Inhibition:

    • Involves interneurons in the spinal cord controlling opposing muscle groups.

    • Muscle Synergies:

    • Typical synergy and pathological synergy.

    • Proprioceptive Input:

    • Body schema awareness contributing to movement.

    • Stepping Pattern Generators (SPGs):

    • Reflexive arc mechanisms for gait and walking patterns.

Spinal Reflexes

  • Definition:

    • Involuntary motor response to external stimuli.

  • Components of a Reflex Arc:

    • Receptor

    • Afferent limb

    • Synapse

    • Efferent limb

    • Effector

  • Brain Input:

    • Some reflexes can operate without brain input.

  • Common Types:

    • Phasic Stretch Reflexes:

    • Response to a quick muscle stretch.

    • Cutaneous Reflexes:

    • E.g., stepping on a nail elicits a withdrawal reflex.

Contracture

  • Definition:

    • Adaptive shortening of muscle-tendon unit where connective tissue loses elasticity and thickens.

  • Effects:

    • Sarcomeres may disappear from the ends of myofibrils, leading to decreased muscle function.

Involuntary Muscle Contractions

  • Types of Involuntary Muscle Contractions:

    • Muscle cramps

    • Fasciculations (small twitching of muscles)

    • Myoclonus (rapid, involuntary muscle jerks)
      - Pathological Movements:

    • Generated by dysfunctional basal ganglia affecting movement control.

    • Includes fibrillations (spontaneous contraction of individual muscle fibers) and tremors (rhythmic oscillations of a body part).

Signs of Lower Motor Neuron Lesions

  • Symptoms:

    • Decrease or loss of reflexes

    • Paresis or paralysis

    • Atrophy

    • Decrease or loss of muscle tone

    • Types of Tone:

      • Hypotonia (abnormally low muscle tone)

      • Flaccidity (complete lack of muscle resistance)

    • Fibrillations (muscle twitches)

Poliomyelitis

  • Overview:

    • A viral disease affecting only LMNs that selectively invade and destroy cell bodies.

  • Post-Polio Syndrome Symptoms:

    • Muscle weakness, joint and muscle pain, fatigue, and breathing difficulties.

    • Pain attributed to overworked muscles and ligamentous stress due to weakness.

Upper Motor Neurons to the Spinal Cord

  • Function of Upper Motor Neurons (UMNs):

    • Control all motor signals from the brain to the spinal cord.

  • Types of UMNs:

    • Medial UMNs (involved with postural and girdle muscles)

    • Lateral corticospinal tract (innervates distal muscles)

    • Nonspecific UMNs (e.g., raphespinal and ceruleospinal tracts).

Postural and Gross Movements: Medial Upper Motor Neurons

  • Key Tracts:

    • Reticulospinal Tract:

    • Facilitates anticipatory postural adjustments.

    • Medial Vestibulospinal Tract:

    • Controls neck and upper back muscles.

    • Lateral Vestibulospinal Tract:

    • Helps maintain the center of gravity.

    • Medial Corticospinal Tract:

    • Provides voluntary control, mainly over neck, shoulder, and trunk muscles.

Lateral Corticospinal Tract

  • Function:

    • Essential for selective motor control, the ability to activate individual muscles for movements critical for hand function.

  • Action Mechanism:

    • Activates inhibitory neurons to prevent unwanted muscle contractions during operation.

    • Descends laterally in the spinal cord, synapsing with laterally located LMN pools in the ventral horn.

Corticobrainstem Tracts

  • Function:

    • Responsible for voluntary control of head and neck muscle movements.

    • Facilitate LMNs that innervate the muscles of the face, tongue, pharynx, larynx, trapezius, and sternocleidomastoid muscles.

Cortical Motor Areas

  • Primary Motor Cortex:

    • Located anterior to the central sulcus in the precentral gyrus.

    • Exhibits precise, predominantly contralateral control over movement.

    • Homunculus:

    • A map illustrating the correspondence between cortical areas and body parts.

  • Areas Involved in Movement Preparation:

    • Premotor area

    • Supplementary motor area

Upper Motor Neuron Syndrome

  • Common Causes:

    • Stroke

    • Spinal Cord Injury (SCI)

    • Brain injury

    • Infection
      - Loss of Function:

    • Paresis and paralysis

    • Impaired selective motor control

    • Absent or decreased muscle tone

    • CNS shock and subsequent tone changes.

  • Gain of Function:

    • Abnormal muscle tone:

    • Hypertonia:

      • Spasticity (velocity-dependent hypertonia with increased resistance to faster movements)

      • Rigidity (velocity-independent hypertonia).

Paresis and Paralysis

  • Definitions:

    • Hemiplegia: Affects one side of the body.

    • Paraplegia: Affects the body below the arms.

    • Tetraplegia: Affects all four limbs.

    • Paresis: Partial paralysis.

    • Paralysis: No voluntary movement.

Abnormal Muscle Tone Review

  • Muscle Tone Continuum:

    • Flaccid: Complete lack of resistance.

    • Hypotonia: Abnormally low resistance.

    • Normal Tone

    • Spasticity: Velocity-dependent high resistance.

    • Rigidity: Velocity-independent high resistance.

Rigidity Types

  • Decerebrate Rigidity:

    • Rigid extension of the spine and limbs, typically associated with brainstem damage.

  • Decorticate Rigidity:

    • Flexed upper limbs, extended neck and lower limbs, and plantarflexion of the feet.

Abnormal Muscle Synergies

Abnormal Reflexes

  • Types:

    • Hyperreflexia: Overactive reflexes.

    • Abnormal Cutaneous Reflexes:

    • Example: Babinski’s sign.

    • Clonus: Involuntary, rhythmic muscle contractions in response to sustained stretch.

    • Clasp-Knife Response: Occurs during slow passive stretching of a paretic muscle, where resistance drops at a specific point.

Abnormal Reflexes: Hyperreflexia

  • Mechanism:

    • Neurons below injury level become overly excitable; spontaneous contractions can be stimulated by mechanical stimulus.

    • May have both negative impacts and benefits on movement.

Abnormal Reflexes: Babinski’s Sign

  • Testing Method:

    • Stroke the lateral sole of the foot from heel to ball and across to the great toe.

    • Normal Response: No movement or flexion of toes.

    • Babinski’s Sign: Extension of the great toe; often accompanied by fanning of other toes, indicative of UMN lesion.

Abnormal Reflexes: Clonus

  • Definition:

    • Involuntary repeated rhythmic contractions of a single muscle group in response to sustained stretch.

    • Unsustained clonus fades with maintained agonist stretch; sustained clonus (>5 beats) indicates pathology due to loss of UMN control.

Abnormal Reflexes: Clasp-Knife Response

  • Definition:

    • Resistance that drops when a paretic muscle is passively stretched slow enough; compares to a pocketknife opening.

Cocontraction

  • Definition:

    • Simultaneous contraction of agonist and antagonist muscles.

    • Abnormal cocontraction typically occurs in conditions like spastic cerebral palsy, interfering with functional movement goals.

Amyotrophic Lateral Sclerosis (ALS)

  • Overview:

    • A neurodegenerative disease destroying both LMNs and UMNs.

  • Signs of ALS:

    • Paresis

    • Hyperreflexia

    • Babinski’s sign

    • Atrophy

    • Hypotonia

    • Fibrillations

    • Fasciculations