C13- neural control of force

Neural Control of Force - Different Force Requests

• Force exerted by muscles acts on tendons, which act on bones, resulting in joint movement.
• The body must adjust muscle force to the task.
• Muscle force depends on two factors:
  • Mechanical properties of the muscle.
  • Nervous system control of muscle motor neurons.
• The brain needs to be aware of muscle mechanical properties to accurately control force.
• This lecture discusses nervous system control of muscle force recruitment, also considering muscle structure.

The Motor Unit

• The fundamental unit of the motor system.
• Formed by a spinal motor neuron and the muscle fibers it innervates.
• A single motor axon may branch to innervate multiple muscle fibers.
• Each muscle fiber is innervated by a single motor neuron.
• An entire muscle may receive input from hundreds of different motor neurons.

The Structure of a Motor Unit

• Myofibrils are tubular structures formed by connected sarcomeres in series.
• The arrangement of sarcoplasmic reticulum cisternae is parallel to sarcomeres and T tubules.
• T tubules are invaginations of the sarcoplasm, coinciding with Z disks.
• The sarcoplasmic reticulum is highly developed in muscle cells.
• Terminal cisternae are enlargements of the sarcoplasmic reticulum alongside T tubules.

The Sarcomere

• The functional unit of a myofibril.
• Defined by Z disks, where actin filaments attach.
• Actin filaments extend towards the sarcomere midline but never meet.
• Myosin filaments are in the sarcomere midline and go in opposite direction to the actin filaments.
• Sarcomere characterized by three zones:
  • Peripheral zone with only actin.
  • Central zone with only myosin.
  • Intermediate zone where actin and myosin overlap.
• Titin and nebulin filaments maintain the correct sarcomere geometry.

The Neuromuscular Junction

• Electrical (action potential) signals initiate muscle contraction.

• Motor neuron axons originated in the spinal cord (lamina IX of anterior horn) innervate muscles below the neck.

• Cranial nerve nuclei (trigeminal and facial) innervate muscles above the neck.

• The endplate has extensive synaptic contacts with nicotinic receptors, enabling action potential transduction to the muscle.

• Action potential steps:

  1. Voltage-gated calcium channels open.
  2. Acetylcholine (ACh) vesicle exocytosis.
  3. ACh binds to nicotinic receptors, opening Na+ and K+ channels, causing depolarization.

Mechanism of Contraction and Importance of Ca2+

• Action potential generated by the membrane, but the sarcomere is inside the cytoplasm.
• Calcium stored in the sarcoplasmic reticulum (SR) is crucial for linking the action potential to the mechanical contraction.
• DHPR (dihydropyridine receptor) is a transmembrane protein on the T tubule.
• RyR (ryanodine receptor) is a Ca2+ channel on the SR.
• Arrival of the action potential at the T tubule changes the DHPR conformation, opening RyR channels and releasing Ca2+ into the sarcoplasm.
• Calcium binding to troponin causes tropomyosin to move, allowing myosin to bind to actin, triggering contraction.
• Muscle relaxation occurs when Ca2+ is pumped back into the SR, and tropomyosin recovers the actin-binding sites.

Single Twitch and Tetanic Contraction

• A single twitch involves a latent period, contraction phase, and relaxation phase.
• During latent period, the action potential propagates.
• The contraction phase is characterized by Ca2+ binding, cross-bridge formation, and sarcomere shortening.
• During relaxation phase, Ca2+ ions are pumped back into SR and cross-bridge cycling stops.
• The tetanic force is the force generated by a muscle fiber stimulated with trains of action potentials.
• Tetanic force depends on stimulus frequency and is greater than a single twitch.

Tetanic Summation

• Increased discharge frequency leads to summation of contractions.
• Calcium concentration progressively increases, leading to maximal possible bridge formation.
• Tetanic summation is possible due to the shorter action potential duration compared to a single twitch duration.

Motor Neuron Location and Motor Nuclei

• Lower motor neurons are located in the anterior horn of the spinal cord (Lamina IX).
• Motor pools (nuclei) are distributed along neuromeres, innervating specific muscles.

Innervation Ratio

• Innervation ratio refers to the number of muscle fibers innervated by a single motor neuron.
• High ratio (e.g., large muscles) results in less precise movements.
• Low ratio (e.g., fine motor control) enables more precise movements.
• Muscle fibers innervated by a single motor unit are distributed homogeneously within the muscle, enabling synchronized contraction.

Functional and Biochemical Properties of Muscle Fibers

• Velocity, maximal force, and fatiguability of contraction are key properties.
• Slow and fast twitch fibers have differing properties.
• Key characteristics for classification, like ATPase activity, oxidative phosphorylation capacity, and glycolytic enzyme levels are examined.

Functional and Biochemical Properties of Motor Neurons

• Two main motor neuron types: α (skeletal muscle) and γ (intrafusal fibers).
• Tetanic summation is dependent on discharge frequency and directly correlates with muscle force.

Dynamic Sensitivity of Motor Neurons

• Muscle force is very dependent on motor neuron firing and tetanic summation.
• Motor neurons exhibit both static and dynamic sensitivity.
• Muscles have inertia which means initial fast firing required.
• Initial high-frequency firing overcomes inertia of the muscle, eventually leading to constant-frequency firing.

Clinical Point – Amyotrophic Lateral Sclerosis (ALS)

• ALS (Lou Gehrig's disease) is a progressive neurodegenerative disease.
• ALS results in the death of motor neurons, eventually leading to paralysis, difficulty swallowing, breathing problems.etc.