Motor Unit Recruitment and Muscle Fiber Types

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Questions and Answers

What effect does the unbinding of Ca2+ from calmodulin (CaM) have on MLCK?

It increases the activity of MLCK.

It deactivates MLCK. (correct)

It activates MLCK permanently.

It has no effect on MLCK.

What role does myosin phosphatase play in muscle contraction?

It binds to myosin to enhance ATPase activity.

It increases the concentration of calcium ions.

It activates MLCK to promote contraction.

It removes phosphate from myosin, decreasing its activity. (correct)

How does decreased myosin ATPase activity affect muscle tension?

It keeps muscle tension unchanged.

It increases muscle tension.

It leads to decreased muscle tension. (correct)

It causes muscle tension to fluctuate unpredictably.

What is the effect of calcium ions (Ca2+) on calmodulin (CaM)?

Ca2+ binds to CaM, activating it. Signup and view all the answers

What is a direct result of increased myosin phosphorylation?

Enhanced myosin ATPase activity. Signup and view all the answers

What are the characteristics of slow oxidative muscle fibers?

Slow twitch speed and high mitochondrial density Signup and view all the answers

Which motor unit is associated with the smallest muscle fibers?

Small Motor Unit Signup and view all the answers

What is a critical aspect of the order of motor unit recruitment?

Recruitment follows a specific sequence from small to large Signup and view all the answers

What distinguishes multi-unit smooth muscle from single-unit smooth muscle?

Multi-unit smooth muscle allows for greater control and fine movements Signup and view all the answers

How does calcium contribute to muscle contraction in smooth muscle tissue?

Calcium can come from extracellular sources as well as the sarcoplasmic reticulum Signup and view all the answers

Which muscle type is characterized by being striated and under involuntary control?

Cardiac muscle Signup and view all the answers

What is the primary function of myoglobin in muscle fibers?

Storing oxygen for use during contraction Signup and view all the answers

Which type of smooth muscle typically exhibits pacemaker potentials?

Single-unit smooth muscle Signup and view all the answers

What is the role of varicosities in smooth muscle contraction?

They release neurotransmitters affecting smooth muscle contraction Signup and view all the answers

What best describes the function of fast glycolytic muscle fibers?

They are ideal for short bursts of intense activity but fatigue quickly Signup and view all the answers

How do the calcium pumps in different muscle fiber types vary?

Fast glycolytic fibers have fewer calcium pumps Signup and view all the answers

What effect does the unstable membrane potential in smooth muscle cells have?

It enhances their ability to respond to various stimuli Signup and view all the answers

Which factor does NOT influence the recruitment of muscle fibers during contraction?

The age of the individual Signup and view all the answers

In terms of endurance, which muscle fiber type exhibits the highest resistance to fatigue?

Slow oxidative fibers Signup and view all the answers

Study Notes

Motor Unit Recruitment

Motor unit: comprises a single motor neuron and all the muscle fibers innervated by it
Motor unit recruitment follows the "size principle": Smaller motor units are recruited first, followed by larger motor units. (Order of recruitment: Small, Intermediate, Large)
Smaller motor units have slower twitch speeds and are more fatigue resistant due to their higher oxidative capacity. Larger motor units have faster twitch speeds and are less fatigue-resistant due to their high glycolytic capacity.

Muscle Fiber Types

Slow Oxidative (SO) fibers:
- Slow twitch speed
- High myoglobin content
- High capillary density
- High mitochondrial density
- Fatigue resistant
- Primarily used for sustained, low-intensity activities
Fast Oxidative (FO) fibers:
- Intermediate twitch speed
- Moderate myoglobin content
- Moderate capillary density
- Moderate mitochondrial density
- Moderately fatigue resistant
- Used for activities requiring both power and endurance
Fast Glycolytic (FG) fibers:
- Fast twitch speed
- Low myoglobin content
- Low capillary density
- Low mitochondrial density
- Easily fatigued
- Primarily used for short-duration, high-intensity activities

Smooth Muscle

Characteristics:
- Found in internal organs
- Not striated
- Uninucleated
- Contracts slowly
- Controlled by the autonomic nervous system
Contractile dynamics:
- Contraction is based on changing cell shape rather than shortening sarcomeres
- Ca2+ signal comes from both the SR (Sarcoplasmic Reticulum) and external sources
Membrane Potential:
- Unstable, making smooth muscle responsive to various signals

Smooth Muscle Types

Single-unit:
- Cells are electrically coupled via gap junctions
- Contract as a single unit
- Found in internal organs like the digestive tract
Multi-unit:
- Cells are not electrically coupled
- Individual cells are innervated
- Allow for fine motor control
- Found in areas like the eye

Calcium Signal in Smooth Muscle

Sources of Ca2+:
- Ligand-gated calcium channels
- Stretch-activated calcium channels
- Voltage-gated calcium channels
- Sarcoplasmic reticulum (IP3-R)
Mechanism:
- Ca2+ influx increases intracellular calcium levels
- Leads to activation of myosin light chain kinase (MLCK)
- MLCK phosphorylates myosin
- Phosphorylated myosin interacts with actin
- Resulting in smooth muscle contraction

Membrane Potentials in Smooth Muscle

Slow wave potentials:
- Oscillations in membrane potential that can trigger action potentials
- Observed in smooth muscle tissues like the digestive tract
Pacemaker potentials:
- Spontaneous depolarizations that generate rhythmic contractions
- Found in tissues like the heart

Relaxation in Smooth Muscle

Mechanism:
- Ca2+ is pumped out of the cytosol by various pumps
- Ca2+ unbinds from calmodulin (CaM)
- Myosin phosphatase removes phosphate from myosin
- Myosin light chain kinase (MLCK) activity decreases
- Myosin ATPase activity decreases
- Reduced muscle tension and relaxation occur

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Description

Explore the dynamics of motor unit recruitment and the classification of muscle fiber types in this comprehensive quiz. Understand the size principle and the characteristics of slow oxidative and fast oxidative fibers. Test your knowledge on how these factors impact muscle performance.