Neuromuscular Factors and Spasticity Overview

Neuromuscular Factors During Physical Work

• Motor impulse initiation originates in the primary motor cortex.
• The cerebellum receives information about intended movement, relays it back to the motor cortex, and refines muscular activity.
• Extrapyramidal tracts manage posture and baseline muscle tone.
• Pyramidal tracts control precise muscle movements.

Muscle Spasticity

• Spasticity is a CNS disorder causing sustained muscle contraction.
• It results from damage to upper motor neurons (corticospinal tract).
• Common causes include spinal cord injury, multiple sclerosis, cerebral palsy, brain trauma, and metabolic diseases.
• Spasticity affects gait, movement, and speech.
• Symptoms include hypertonia (increased muscle tone), clonus (rapid muscle contractions), exaggerated deep tendon reflexes, muscle spasms, scissoring (leg crossing), and fixed joints.
• It significantly increases energy expenditure during daily activities, leading to fatigue.

Motor Unit

• A motor unit comprises a single motor nerve and all the muscle fibers it innervates, forming the functional unit of movement.
• The anterior motor neuron and its connected muscle fibers are included.
• The neuromuscular junction (motor end plate) transmits nerve impulses to muscle fibers via acetylcholine.
• Motor units are categorized based on twitch and tension characteristics:
  • Fast twitch/high-force fibers (Type II) are high-fatigue fibers, crucial for high-intensity situations.
  • Slow twitch/low-tension fibers (Type I) are fatigue-resistant, essential for sustained activity.
  • Fatigue susceptibility is determined by the muscle fiber's metabolic characteristics.

Motor Unit and Strength Gradation

• The "all-or-none" principle applies; once a motor neuron activates, all muscle fibers within the unit contract simultaneously.
• Strength increases via:
  • Recruiting more motor units (larger axon motor units are progressively recruited for more force).
  • Increasing the frequency of motor unit discharge (stimulating before complete relaxation for summation).

Skeletal Muscle Structure

• Myofibrils contain contractile units called sarcomeres, housing actin and myosin.
• The sliding filament theory describes muscle tension development through fiber shortening and force generation.
• Stretch reflexes monitor and adjust motor behavior through proprioceptors (receptors sensitive to stretch, tension, and pressure) in muscles and tendons.
  • Muscle spindles (common proprioceptors) respond to stretch, especially in muscles undertaking complex movements. This provides mechanosensory information about changes in fiber length and tension.
  • Stretch reflexes control movement and posture through afferent nerve fibers carrying impulses to the spinal cord, triggering responses from efferent neurons to the stretched muscle fibers.
• Golgi tendon organs (GTOs) are proprioceptors detecting tension generated by active muscle contraction, not length. GTO activation through excessive stretch triggers reflex inhibition to protect muscles.
• Pacinian corpuscles detect changes in movement or pressure, rather than the magnitude.

Muscle Strength and Endurance

• Strength is the maximum force a muscle can generate (isometric, isotonic, or isokinetic).
• Endurance is the capacity for repeated contractions over time.

Mechanical Factors Affecting Strength

• Length-tension-angle relationship: optimal sarcomere length for maximum tension (e.g., 90-130 degrees elbow angle for maximum force).
• Force-velocity relationship: increasing shortening velocity rapidly decreases force generation. Eccentric contraction (lengthening) yields the highest force.
• Power-velocity relationship: the relationship between maximum power and movement speed during concentric contractions. Power output has an inverted U shape, peaking at an optimal speed.
• Elasticity-force relationship: the stretch-shortening cycle (eccentric followed by concentric contraction) uses stored energy from the pre-stretch for increased strength.

Size Principle

• Two major types of activity: power and endurance. Different types of motor units are recruited in differing ways based on size.

  • Larger motor units recruit for power-type activities, smaller for endurance.

• Other factors—muscle size, body size, fiber composition, age/gender, training, position/stability, number of trials, learning, motivation—also influence strength.

Measuring Strength and Endurance

• Dynamometers (e.g., hand grip dynamometer) measure maximum force.
• 1-repetition maximum (1RM) assesses the maximum load a muscle group can lift once.

Neuromuscular Fatigability

• Decline in muscle force with repeated stimulation.
• Contributing factors to reduced force generation include: changes in neurotransmitter levels, reduced glycogen, increased blood/muscle lactate.

Delayed-Onset Muscle Soreness (DOMS)

• Two major causes:
  • Local ischemia, producing swelling and inflammation (shortly after exercise).
  • Mechanical trauma, involving microtears in muscle fibers leading to inflammation (24-48 hours post-exercise).

Muscle Atrophy (Wasting)

• Reduction in muscle mass due to immobility and reduced energy production.

Flexibility

• Flexibility affected by bone structure, age, sex, chronic activity, temperature, and soft tissue contributions to joint resistance.
  • Static flexibility is joint range of motion.
  • Dynamic flexibility is the rate of torque/resistance developed through stretching.

Implications for Occupational Therapists

• Altered motor unit recruitment patterns in disease states impact energy expenditure and ADLs.
• Therapists should tailor rehabilitation based on the type of contractions in the affected muscle groups.

Explore the roles of various brain regions in motor control and understand the implications of muscle spasticity as a CNS disorder. This quiz covers motor unit function, spinal conditions, and the impact of spasticity on movement and daily activities.