lecture recording on 12 March 2025 at 10.35.19 AM

Introduction to Motor Neuron Signaling and Muscle Recruitment

• Discussion of where signals from motor neurons go and how they recruit motor units based on required force.

• Connection to physiological adaptations of muscle fibers in unit three.

Activity to Understand Muscle Recruitment

• First Task:

  • Sit and extend your legs to recruit necessary muscle fibers for leg extension.

• Second Task:

  • Introduce resistance by having a partner push against your leg while you try to extend it, leading to more muscle fibers being activated.

• This demonstrates muscle recruitment differences based on resistance:

  • More tension required when resistance is applied.

  • Increased effort leads to greater muscle fiber recruitment.

Understanding Muscle Recruitment Changes

• Muscle Recruitment Scenarios:

  • Comparison of muscle engagement in low resistance (body weight only) vs high resistance (partner's resistance).

    • Increased force means more muscle fibers activated.

• Question for Reflection:

  • Did your muscle recruitment change with added resistance?

    • Affirmative responses noted an increase in muscle recruitment during resistance.

Neural Pathway for Voluntary Movement

• Overview of Neural Pathway:

  • Perception of information (sensory input) is integrated within the spinal cord and then sent to specific brain areas:

    • Spinal cord input

    • Integration in posterior parietal cortex (PPC)

    • Planning in premotor cortex

    • Execution in motor cortex

  • Action begins with perception, signals sent out cause muscle contractions.

Proprioceptive Information Processing

• Proprioception from Muscle Spindles and Golgi Tendon Organs:

  • Information about muscle length and tension is processed by these sensory receptors.

  • The signaling for muscle length results in different firing rates:

    • Nuclear Chain Fibers: Sensitivity to muscle length.

    • Nuclear Bag Fibers: Sensitivity to the rate of muscle length change (velocity).

Pathway of Proprioceptive and Visual Information

• Proprioceptive inputs go to S1 (primary somatosensory cortex) before reaching the PPC for integration.

• Visual input follows a similar pathway from V1 (primary visual cortex) to PPC.

• Combined information enables the formation of action intentions for movement plans.

Movement Planning and Execution

• Movement Plan Generation:

  • After forming an action intention, plans are created in the premotor cortex.

  • The motor cortex (M1) then executes the movement plan, sending signals down the spinal cord to motor units.

Concept of Motor Units

• Definition of Motor Unit:

  • A motor neuron plus all muscle fibers it innervates.

  • Not a one-to-one ratio; one motor neuron can control multiple muscle fibers.

• Size Principle of Motor Units:

  • Small motor units need for precise movements (e.g., eye movements).

  • Large motor units for gross movements (e.g., leg muscles).

Recruitment of Motor Units During Contraction

• Recruitment Process:

  • Smaller motor units activated first for lighter tasks; larger units engaged as resistance increases.

  • This recruitment depends on the degree of contraction needed to overcome resistance.

Types of Muscle Fibers

• Slow Twitch (Type I) Fibers:

  • High fatigue resistance, utilized during prolonged aerobic activities.

• Fast Twitch (Type II) Fibers:

  • Quick to fatigue, used for rapid and powerful movements, suitable for activities like sprinting.

• Training can adapt muscle fibers based on the type of activity, influencing muscle performance and endurance capabilities.

Upcoming Class Activities

• Students will visualize the central nervous system pathway involved in locomotion, from sensory inputs to motor outputs.

• Engage in physical activity (bicep curls) to directly observe motor unit recruitment changes based on resistance.