Introduction to Motor Control

This section introduces the fundamental principles of how the nervous system controls muscle contraction, a key aspect of skilled performance and motor learning. Understanding these mechanisms is essential for students of personal health, kinesiology, and related fields.

Learning Objectives

• Describe how the nervous system controls muscle contraction.

• Explain the characteristics of motor units and motor neuron pools.

• Differentiate between extrafusal and intrafusal muscle fibres.

• Distinguish between gamma and alpha motor neurons.

Neuronal Communication and Action Potentials

Synaptic Transmission and Electrical Gradients

Neurons communicate via specialized junctions called synapses, where electrical or chemical signals are transmitted from one neuron to another. The movement of ions across the neuronal membrane creates electrical gradients that are essential for signal transmission.

• Synapse: The site where a neuron communicates with another cell.

• Electrical Gradient: The difference in charge across the neuronal membrane, primarily due to the distribution of sodium (Na+) and potassium (K+) ions.

Action Potentials

An action potential is a rapid, temporary change in membrane potential that travels along the neuron, enabling communication.

• Resting Potential: The baseline membrane potential, typically around -70 mV.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Depolarization: Na+ channels open, allowing Na+ to enter the cell, making the inside more positive.

• Repolarization: Na+ channels close, and K+ channels open, restoring the negative membrane potential.

Key Equation:

Local Potentials: EPSP and IPSP

Local changes in membrane potential, called excitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP), determine whether an action potential will occur.

• EPSP: Depolarizes the membrane, increasing the likelihood of an action potential.

• IPSP: Hyperpolarizes the membrane, decreasing the likelihood of an action potential.

• These potentials must summate to reach threshold and trigger an action potential.

Control of Muscle: How Do We Get Our Muscles to Move?

Muscle movement is initiated by signals from the nervous system, which are transmitted to muscle fibers via motor neurons. This process involves complex integration of neural inputs and muscle properties.

Inputs to Motor Neurons

Motor neurons receive inputs from various sources, including the brain, spinal cord interneurons, and sensory feedback from muscle spindles. The response of a motor neuron to synaptic input depends on its size, surface area, and axon diameter.

• Smaller motor neurons: Larger postsynaptic potentials, slower conduction velocity, fewer muscle fibers innervated.

• Larger motor neurons: Smaller postsynaptic potentials, faster conduction velocity, more muscle fibers innervated.

Motor Units and Motor Neuron Pools

Definition and Characteristics

A motor unit (MU) consists of a single motor neuron and all the extrafusal muscle fibers it innervates. The number of muscle fibers per motor neuron is called the innervation ratio.

• High innervation ratio: Muscles requiring large force (e.g., gastrocnemius: up to 2000 fibers per neuron).

• Low innervation ratio: Muscles requiring fine control (e.g., eye muscles: as few as 5 fibers per neuron).

Each muscle fiber is innervated by only one motor neuron, but a motor neuron may innervate multiple muscle fibers. The fibers of a single motor unit are distributed throughout the muscle and intermixed with fibers from other motor units.

Motor Unit Recruitment

To produce the appropriate amount of force, the nervous system recruits motor units in a specific order, typically from smallest to largest (the size principle).

• Small motor units: Recruited first, produce less force, allow for fine control.

• Large motor units: Recruited as more force is needed.

• The level of force at which all motor units are recruited varies between muscles.

Motor Neuron Pool

The motor neuron pool refers to all the motor neurons that innervate a single muscle. These neurons are clustered in the spinal cord and may span several spinal segments.

Muscle Fibre Types

Extrafusal Muscle Fibres

Extrafusal muscle fibers are the standard contractile fibers responsible for generating force and movement. They are innervated by alpha motor neurons and are the primary contributors to muscle power.

• Located throughout the muscle.

• Responsible for voluntary movement.

Intrafusal Muscle Fibres

Intrafusal muscle fibers are specialized fibers found within muscle spindles. They are fewer in number and are involved in detecting changes in muscle length, contributing to proprioception (the sense of body position and movement).

• Located deep within skeletal muscles, alongside extrafusal fibers.

• Grouped in small numbers (4-7 per spindle) and wrapped in a capsule.

• Do not generate significant force; act as sensors rather than movers.

Types of Motor Neurons

Alpha (α) Motor Neurons

Alpha motor neurons innervate extrafusal muscle fibers and are responsible for initiating muscle contraction and movement. They are sometimes referred to as "lower motor neurons."

• Control voluntary muscle contraction.

• Essential for generating force and movement.

Gamma (γ) Motor Neurons

Gamma motor neurons innervate intrafusal muscle fibers within muscle spindles. Their primary role is to regulate the sensitivity of muscle spindles to stretch, thus contributing to proprioceptive feedback.

• Adjust the tension of intrafusal fibers.

• Help maintain muscle spindle sensitivity during muscle contraction.

Muscle Force and Neural Control

Factors Influencing Muscle Force

Muscle force output is influenced by several factors, including the properties of the muscle fibers and the pattern of neural activation.

• Force-Length Relationship: The degree of overlap between actin and myosin filaments affects force production.

• Force-Velocity Relationship: The faster a muscle shortens (concentric contraction), the lower the force it can produce.

• The same neural activation can produce different force outputs depending on muscle length and contraction speed.

The nervous system must account for these properties when converting brain commands into muscle force during movement and motor learning.

Summary Table: Types of Muscle Fibres and Motor Neurons

Key Takeaways

• Each motor neuron innervates multiple muscle fibers, forming a motor unit.

• The size of the motor unit influences its recruitment and the precision of muscle control.

• There are two main types of muscle fibers (extrafusal and intrafusal) and two corresponding types of motor neurons (alpha and gamma).

• Alpha motor neurons control movement; gamma motor neurons regulate sensory feedback.

Additional info: This summary expands on the provided notes with standard academic definitions and context for clarity and completeness, suitable for exam preparation in a Personal Health or Kinesiology course.