Neuromuscular Factors and Spasticity Overview

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Questions and Answers

What primarily initiates muscle contraction?

Pyramidal tract

Cerebellum

Primary motor cortex (correct)

Extrapyramidal tract

Which of the following symptoms is NOT typically associated with muscle spasticity?

Loss of muscle tone (correct)

Clonus

Scissoring

Hypertonia

What is the role of the extrapyramidal tract?

Modulating sensory inputs for movement

Initiating voluntary movements from the motor cortex

Facilitating discrete muscle movements

Controlling posture and background levels of neuromuscular tone (correct)

Which motor unit type is characterized as fatigue-resistant?

Type I fibers Signup and view all the answers

What can increase the force generation capacity of a muscle?

Increasing the number of motor units recruited Signup and view all the answers

Which type of muscle fiber is primarily recruited during high force activities?

Type II (fast twitch) Signup and view all the answers

Which condition is NOT a cause of damage to upper motor neurons?

Bacterial infection Signup and view all the answers

What is the 'all or none' principle in muscle contraction?

Once activated, a motor unit always contracts all its fibers to full capacity Signup and view all the answers

What occurs when a second stimulus is added to a motor unit before it has completely relaxed?

Summation Signup and view all the answers

What structure within myofibrils contains actin and myosin?

Sarcomere Signup and view all the answers

Which proprioceptor is responsible for monitoring muscle tension during contraction?

Golgi Tendon Organ Signup and view all the answers

What is the optimal joint angle for maximal force production in muscles?

90-130 degrees Signup and view all the answers

What happens to a muscle's force-generating capacity as the shortening velocity increases?

It declines rapidly Signup and view all the answers

Which type of contraction is associated with the highest force production?

Eccentric contraction Signup and view all the answers

What is the primary determinant of strength based on muscle structure?

Cross-sectional area Signup and view all the answers

What does DOMS primarily arise from?

Microtears and inflammation Signup and view all the answers

What is the purpose of using dynamometers in strength testing?

To measure maximum force generating capacity Signup and view all the answers

What is the elastic component's role in muscle performance during the stretch-shortening cycle?

It stores energy for subsequent contractions Signup and view all the answers

Which of the following factors does NOT affect strength production?

Heart rate Signup and view all the answers

What role does the size principle play in motor unit recruitment?

Small fibers are recruited for endurance before large fibers Signup and view all the answers

What type of flexibility benefits from increased extensibility of musculo-tendon structures?

Static flexibility Signup and view all the answers

What is a significant result of neuromuscular fatigability?

Decline in muscle tension with repeated stimulation Signup and view all the answers

Study Notes

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.

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Description

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.