Muscle Physiology and Motor Units Quiz
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Questions and Answers

What are the primary components of muscles that contribute to energy conservation and power transfer?

Muscles contain both contractile and elastic components.

Explain the stretch-shortening cycle and its advantage over isolated concentric contraction.

The stretch-shortening cycle produces more force than an isolated concentric contraction.

What is the all-or-none principle in relation to motor units?

The all-or-none principle states that all muscle fibers within a motor unit contract or none contract.

Describe the role of synapses in the nervous system.

<p>Synapses are sites of connection where information is transmitted from one neuron to another or from a neuron to a muscle.</p> Signup and view all the answers

Distinguish between intramuscular and intermuscular coordination.

<p>Intramuscular coordination refers to the efficiency of muscle fibers within a single muscle, while intermuscular coordination involves the coordination between different muscles.</p> Signup and view all the answers

What are the primary functions that the nervous system regulates to control force output from motor units?

<p>The three primary functions are rate coding of motor units, recruitment of motor units, and coordination of motor units and muscles.</p> Signup and view all the answers

How does the metabolic power of Type IIx motor units compare to that of Type I motor units?

<p>Type IIx motor units have higher metabolic power than Type I motor units.</p> Signup and view all the answers

In terms of muscle fiber count within motor units, how do Type I, Type IIa, and Type IIx motor units rank?

<p>Type I motor units have the fewest muscle fibers, followed by Type IIa, and Type IIx which has the most.</p> Signup and view all the answers

What role does calcium ion (Ca2+) play in muscle contraction and relaxation?

<p>Calcium ions are involved in both contraction, where they are released, and relaxation, where they are re-uptaken.</p> Signup and view all the answers

Describe the speed of activation of Type I, Type IIa, and Type IIx motor units.

<p>Type II motor units activate faster than Type IIa, which in turn activate faster than Type I motor units.</p> Signup and view all the answers

What is a primary method used to measure intermuscular coordination?

<p>Electromyography (EMG)</p> Signup and view all the answers

Which mechanism primarily dominates the low force range of muscle contractions?

<p>Recruitment</p> Signup and view all the answers

What is the primary purpose of discharge patterning in intramuscular coordination?

<p>To manipulate firing rate of motor units for task demands</p> Signup and view all the answers

In the context of muscle contraction, what is compartmentalization?

<p>Grouping muscle fibers with shared functions</p> Signup and view all the answers

During high force demand, how do motor units typically respond?

<p>They fire simultaneously to maximize force</p> Signup and view all the answers

What factor affects the mix of rate coding and recruitment in muscle contraction?

<p>Type of muscle and movement</p> Signup and view all the answers

What is a key characteristic of fast-twitch (Type II) muscle fibers compared to slow-twitch (Type I)?

<p>They produce more force but fatigue quickly</p> Signup and view all the answers

What indicates a well-trained muscle in terms of EMG signals?

<p>Higher EMG magnitude with coordinated firing</p> Signup and view all the answers

Which statement best describes the recruitment of motor units during maximal contraction?

<p>Higher threshold motor units are recruited last</p> Signup and view all the answers

How does the firing rate change as muscle contraction intensity increases?

<p>It increases to meet higher force demands</p> Signup and view all the answers

Match the type of coordination with its description:

<p>Intermuscular coordination = Measured as EMG and biomechanical efficiency Intramuscular coordination = Expressed through motor unit firing and recruitment Discharge patterning = Manipulating firing rate of motor units Compartmentalization = Independently controlled groups of muscle fibers</p> Signup and view all the answers

Match the mechanisms with their predominate force range usage:

<p>Recruitment = Predominant in low force range Firing rate = Predominant in high force range Rate coding = Used throughout the force range Synchronization = Occurs during maximal force requirements</p> Signup and view all the answers

Match the EMG signal representation with its corresponding muscle condition:

<p>Trained (coordinated) = Lower EMG magnitude compared to untrained Untrained (uncoordinated) = Higher EMG magnitude Agonist = Primary mover muscle group Antagonist = Opposing muscle group</p> Signup and view all the answers

Match the factor affecting muscle coordination with its description:

<p>Muscle type = Influences rate coding vs recruitment Type of movement = Affects coordination strategy Muscle morphology = Determines structure and function relationship Neural recruitment = Modulates activation based on task demands</p> Signup and view all the answers

Match the EMG measurement significance with its application:

<p>Magnitude of EMG = Indicates muscle activity level Timing of EMG = Reflects synchronization of motor unit firing Rate of EMG = Indicates force production capability Signal patterns = Reveal recruitment strategies used</p> Signup and view all the answers

Match the muscle fiber type with its characteristic:

<p>Type I = Slow twitch muscle fibers Type IIa = Intermediate twitch muscle fibers Type IIx = Fast twitch muscle fibers Type IIB = Highest force output, low endurance</p> Signup and view all the answers

Match the concept of motor unit recruitment with its feature:

<p>Full recruitment = Occurs at low force for small muscles Partial recruitment = Common in large muscles at lower intensities High-threshold units = Recruited last during maximal effort Low-threshold units = Activated first during muscle contractions</p> Signup and view all the answers

Match the type of muscle contraction with its EMG behavior:

<p>Smooth contraction = Requires synchronous firing of motor units Alternate contraction = Involves individual unit firing Maximal force = All available motor units recruited Submaximal force = Some units may remain inactive</p> Signup and view all the answers

Match the description of EMG signals to their muscle conditions:

<p>High EMG = Indicative of uncoordinated muscle activity Low EMG = Sign of trained muscle efficiency Synchronized firing = Indicates coordinated motor unit action Variable EMG patterns = Seen in mixed training states</p> Signup and view all the answers

Match the elements of coordination with their definitions:

<p>Motor unit firing rate = Rate of activation for muscle contraction Recruitment patterns = Order of muscle units activated Synchronization = Coordination among motor units Task demands = Influences discharge patterning behavior</p> Signup and view all the answers

What is the predominant mechanism used in the low force range of muscle contractions?

<p>Recruitment</p> Signup and view all the answers

Intramuscular coordination only relies on the recruitment of motor units.

<p>False</p> Signup and view all the answers

What is the primary outcome measured by electromyography (EMG)?

<p>Muscle electric activity</p> Signup and view all the answers

In trained muscles, EMG signals indicate increased muscle __________.

<p>coordination</p> Signup and view all the answers

Match the type of muscle coordination with its definition:

<p>Intermuscular coordination = Coordination between different muscle groups Intramuscular coordination = Coordination within a single muscle Rate coding = Modulating firing rate of motor units Recruitment = Activating additional motor units to increase force</p> Signup and view all the answers

Which of the following factors can influence the mix of rate coding and recruitment in muscle contraction?

<p>Size of the muscle</p> Signup and view all the answers

Discharge patterning allows motor units to fire at different times to meet task demands.

<p>True</p> Signup and view all the answers

In the context of EMG, what does a higher signal magnitude typically indicate?

<p>Greater muscle activation</p> Signup and view all the answers

In maximal contraction, motor units fire __________ to achieve the required force.

<p>synchronously</p> Signup and view all the answers

What can compartmentalization in muscle fibers lead to?

<p>Specific responses to different tasks</p> Signup and view all the answers

Study Notes

Motor Learning - PHYL 4518

  • Course code: PHYL 4518
  • Course name: Motor Learning
  • Instructor: Zoe Chan, PhD
  • Email: [email protected]
  • Semester: Fall 2024
  • Week: 5

Reminders

  • Online Quiz #1 grades posted
  • Question on constant error, graded manually

Question 16

  • A person shoots three arrows at a bullseye.
  • Shot 1: 4 cm to the right of bullseye
  • Shot 2: 4 cm to the left of bullseye
  • Shot 3: Hits the bullseye
  • Calculate the constant error (CE) of the shots.
  • Negative value indicates error to the left, correct to 1 decimal place.
  • Answer: 0.0 cm

Summary & Review (Muscles)

  • Muscles contain both contractile and elastic components.
  • Conserve energy.
  • Amplify/transfer power.
  • Absorb power.
  • CE adjusts tone (tension) to act on the SE and then bones.

Summary & Review (Muscle Length & Force)

  • The contractile element produces the most active force at optimal length.

Summary & Review (Stretch Shortening Cycle)

  • The stretch-shortening cycle produces more force than an isolated concentric contraction.
  • Resistance training can increase stiffness.
  • Plyometric training can improve the SSC.
  • Stretching may be counterproductive to SSC.

Summary & Review (Nervous System)

  • The central nervous system (CNS) includes the brain and spinal cord.
  • The peripheral nervous system (PNS) transmits signals.
  • Sensory/afferent signals transmit to the CNS.
  • Motor/efferent signals transmit from the CNS.
  • The somatic nervous system controls skeletal muscle (voluntary).
  • The autonomic nervous system controls smooth muscle and glands (involuntary).
  • Sympathetic and parasympathetic divisions make up the autonomic nervous system.
  • Visceral sensory endings transmit sensory info from internal organs.
  • Somatic sensory endings transmit sensory info from external environment.

Summary & Review (Neurons)

  • Dendrites receive information.
  • Axons send information.
  • Sensory, interneurons, and motor neurons transmit info.
  • Neurons depolarize once they reach a threshold.

Summary & Review (Motor Units)

  • One motor unit (MU) consists of a motor neuron and all its innervated muscle fibers.
  • All-or-none principle: All fibers within a motor unit contract or none.

Today's Learning Objectives

  • Describe a synapse and its connection
  • Detail the 3 ways the nervous system regulates the MU to regulate force output.
  • Describe and distinguish between intramuscular and intermuscular coordination.

Synapse

  • Site of connection:
    • Neuron to neuron (presynaptic to postsynaptic neuron).
    • Neuron to muscle (neuromuscular junction).

Types of Motor Units

  • Motor unit type categorized by the histochemical profile of their fibers:
    • Type IIx: Fast Glycolytic (FG)
    • Type IIa: Fast Oxidative Glycolytic (FOG)
    • Type I: Slow Oxidative (SO)
  • Motoneuron soma size: Type I < IIa < IIx
  • Motoneuron axon diameter: Type I < IIa < IIx

  • of muscle fibers within MU: Type I < IIa < IIx

Types of Motor Units (Speed & Activation)

  • Speed of activation: Type IIx > IIa > I
  • Metabolic power: Type IIx > IIa > I
  • Fatigue resistance: Type I > IIa > IIx
  • Twitch response (grams): Data graphs of muscle fiber activation over time (ms).

Muscle Twitch

  • Contractile response generated by a single action potential.
  • Single twitches are rare in normal function.
  • Evoked artificially in research to study muscle function.

MU Behavior & Force Control

  • The nervous system controls 3 basic functions to control force output.
    • Rate coding of motor units (summation)
    • Recruitment of motor units
    • Coordination of motor units and muscles.

1. Rate Coding - Twitch Summation

  • Faster firing rate of action potentials = more tension.
  • Rate coding regulates firing rate to modify force output.
  • Human muscle firing rates (per second):
    • Isometric: 5-60 Hz
    • Ballistic: up to 120 Hz
  • Strongest contraction: fused tetanus

1. Rate Coding - Twitch Summation (Cont.)

  • Not enough time to relax elastic elements = increased tension.
  • Not enough time for Ca++ re-uptake = abundance of Ca++ in muscle cell = maximum number of cross-bridges forming.

2. Recruitment of Motor Units

  • Though experiment: Progressively harder squeezing of a handgrip dynamometer.
  • MUs recruited according to the size of their soma.
  • Size principle of recruitment: From small to large motor units; orderly recruitment.

2. Recruitment of Motor Units (Cont.)

  • Type II: larger cell body (soma)
  • Type II: more difficult to "excite"
  • Type II: higher activation from nervous system needed to reach threshold
  • Type I: small soma
  • Smaller change in voltage compared to Type II

2. Recruitment of Motor Units (Cont.)

  • Small motor units are activated first and de-recruited last.
  • Fatigue resistant(type I fibers)
  • Most active during prolonged exercise

2. Recruitment of Motor Units (Properties)

  • Motor Unit Properties:
    • Neuron/axon size (Small, Med, Large)
    • Number of muscle fibers (Few, Med, Many)
    • Type of muscle fiber (Slow twitch, Mixed, Fast Twitch)
    • Energy needed for activation (Least, Mod, Most)
    • Recruitment order (First, Sec, Third)
    • Function (Endurance, Mixed, Force/Power)
    • Larger soma motoneurons preferentially innervate fatigable fibres (type IIa and IIx) during high force/speed.

3. Neuromuscular Coordination

  • Intermuscular coordination: Between 2+ muscle groups; coordination of muscle groups & body segments.
  • Intramuscular coordination: Within same muscle group; patterning & use of MUs within a muscle or across a muscle task group.

3.1 Intermuscular Coordination

  • Observed in the patterning and role-playing of different muscles.
  • The muscle's role may change from one moment to the next.
  • Sometimes a muscle functions as an agonist.
  • Sometimes a stabilizer.
  • Sometimes a neutralizer.
    • Example: Glute max:
      • Agonist during running.
      • Stabilizer during standing.
      • Neutralizing external rotation to counteract glut min's internal rotation.

3.1 Intermuscular Coordination (EMG)

  • Outcome often measured as EMG (Electromyography) and biomechanical efficiency.
  • Detects muscle electric activity (muscle action potential).

3.2 Intramuscular Coordination

  • Intramuscular coordination expressed in 3 fundamental ways:
    • Coordination between motor unit firing rate and recruitment
    • Discharge patterning (synchronization)
    • Compartmental coordination.

3.2 Intramuscular Coordination (Rate Coding & Recruitment)

  • Rate coding & recruitment, both mechanisms throughout force range.
  • Recruitment predominant in low force range; firing rate in high force range.
  • Muscle & type of movement, small muscle (hand): full MU recruitment at 30% MC; large muscle: rate coding early and recruits all units at 80-90% MC.

3.2 Intramuscular Coordination (Recruitment)

  • Recruitment is almost entirely dependent on the intensity of the effort.
  • 90% of MUs are recruited when force is needed.

3.2 Intramuscular Coordination (Discharge Patterning)

  • Discharge patterning - manipulating firing rate of MUs to meet task demands.
  • Smooth contraction (most of the time)
  • MU alternate contraction

3.2 Intramuscular Coordination (Discharge Patterning - Maximal Force)

  • MU fire at the same time with maximal force needed.

3.2 Intramuscular Coordination (Compartmentalization)

  • Compartmentalization: Smaller and independently controlled groups of muscle fibers within a muscle or group of muscles.
  • Compartments based on: muscle morphology (fast [Type II] vs slow twitch [Type I]); neural recruitment; biomechanical functions (angle of pull).
  • Compartmentalization of the deltoid (medial, anterior, posterior).

MU Behavior Adaptations to Training

  • Strength and power training increase motor unit activation to 100%.
  • Increases average or maximal firing rates.
  • Causes an earlier onset of high-threshold units to reach high firing rates sooner.

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Description

Test your knowledge on the primary components and functions of muscles, the stretch-shortening cycle, and the all-or-none principle in motor units. Delve into the roles of synapses, coordination types, and the metabolic properties of different muscle fiber types. This quiz will enhance your understanding of muscle physiology and nervous system interactions.

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