Lecture Notes on Motor Output in the Somatic Nervous System

LO: 1.Define motor unit – a single alpha motor neurone and all the muscles it innervates 2. Understand how smooth and precise muscle contraction is achieved 3. Explain the principle fibre types in human skeletal muscle 4. Explain how the force of contraction is controlled by α-MNs 5. Explain the difference between α- and γ-motoneurones 6. Explain how muscle spindles and Golgi tendon organs regulate muscle contraction 7. Explain the operation of the gamma (γ)-loop

Overview

• Focus on motor output in the somatic nervous system
  • Skeletal muscle control: voluntary movements
  • Motor control at the spinal cord level
• Discussion points:
  • Muscle fibers and motor neuron innervation
  • Role of proprioceptors

Alpha Motor Neurons & Muscle Contraction

• Alpha motor neurons:
  • Largest motor neurons in the spinal cord
  • Cell bodies in the ventral horn
  • Axons extend towards skeletal muscles via neuromuscular junctions
• Motor unit:
  • Consists of an alpha motor neuron and all the muscle fibers it innervates
  • Single neuron can synapse with multiple fibers, but each fiber receives input from just one neuron (focal innervation)
• Synaptic Process:
  • Action potential triggers voltage-gated calcium channels, leading to neurotransmitter (acetylcholine) release
  • Acetylcholine binds to nicotinic receptors on muscle fiber, causing sodium influx and depolarization (end plate potential)
  • Acetylcholine breakdown by acetylcholine esterase
• Safety Mechanism:
  • Excess acetylcholine ensures reliable muscle contraction

Muscle Fiber Types

• Type 1 (Slow Twitch):
  • Slow contraction, low force
  • Oxidative metabolism, high fatigue resistance
  • Important for sustained contractions (e.g., posture)
• Type 2 (Fast Twitch):
  • Fast contraction, high force
  • Glycolytic metabolism, low fatigue resistance
  • Important for rapid movements (e.g., eye muscles)
• Alpha motor neurons innervate only one type of muscle fiber
• Recruitment of muscle fibers:
  • Slow twitch fibers recruited first; fast twitch recruited for heavier loads
  • Increased contraction force by recruiting more alpha motor neurons or increasing firing rate

Neuromuscular Control & Reflexes

• Smooth & Controlled Movements:
  • Muscle fibers of a single neuron spread throughout muscle
  • Asynchronous firing of motor neurons
• Innervation Ratio:
  • Low ratio = fine control (e.g., fingers)
  • High ratio = coarse control (e.g., abdominal muscles)

Gamma Motor Neurons & Proprioceptors

• Gamma Motor Neurons:
  • Innervate muscle spindles, not involved in contraction
  • Muscle spindles act as mechanoreceptors detecting muscle stretch
• Muscle Spindles:
  • Made of intrafusal fibers (static and dynamic nuclear bag, nuclear chain)
  • Innervated by type 1A and type 2 sensory fibers
  • Role in stretch reflex and gamma loop
• Golgi Tendon Organs:
  • Detect muscle tension
  • Innervated by 1B sensory fibers
  • Involved in Golgi tendon reflex preventing over-contraction

Reflex Arcs

• Stretch Reflex:
  • Triggered by muscle spindle stretch (e.g., knee-jerk reflex)
  • Involves alpha motor neuron activation and inhibitory interneurons (Renshaw cells)
• Gamma Loop:
  • Co-activation of alpha and gamma motor neurons ensures balance during unexpected loads
• Golgi Tendon Reflex:
  • Protects muscle from damage by reducing contraction

Summary

• Alpha motor neurons and muscle spindles play crucial roles in muscle control
• Different types of reflexes ensure proper muscle function and prevent damage
• Precision in muscle control is important for different muscular activities

Quiz Recap

• False statement about alpha motor neurons: They do not innervate both slow and fast twitch fibers
• Increasing contraction force: Done by recruiting type 2 fibers and increasing alpha motor neuron firing