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Ch3 Motor System

Core idea

  • A muscle contracts when motor neurons fire action potentials (APs).
  • Motor neurons connect to muscle fibers via the neuromuscular junction (NMJ).

Neuromuscular Junction (NMJ)

  • Neurotransmitter (NT): ACh (acetylcholine)
  • ACh binds nAChR (nicotinic ACh receptor), which allows Na\(^+\) and K\(^+\) to flow through.
  • The nAChR channel reversal potential is approximately \(E_{\text{channel}} \approx 0,\text{mV}\).
  • This depolarization propagates along the muscle fiber membrane via voltage-gated Na\(^+\) channels.

Skeletal Muscle

  • Responsible for voluntary movement.
  • Muscle fibers are cells.
  • A muscle fiber contracts only when recruited by a motor neuron.

Innervation and motor units

  • 1 muscle fiber is controlled by 1 motor neuron.
  • A motor unit = one motor neuron + all the muscle fibers it innervates.

Muscle Microanatomy

Myofibrils and sarcomeres

  • Myofibrils contain the contractile proteins.
  • Z line: where thin filaments (actin) anchor.
  • Sarcomere: region between two Z lines.
  • The sarcomere connects to the thick filament via titin.

Banding pattern

  • I band: only thin filament
  • A band: with thick filament (myosin)
  • H zone: only thick filament
  • M line: middle of sarcomere

Filaments

Thin filament

Contains:

  • troponin
  • actin
  • tropomyosin

Thick filament

Contains:

  • heavy chain
  • light chain
  • ATP binding site
  • actin binding site

Excitation–Contraction Coupling (Skeletal Muscle)

T-tubules and sarcoplasmic reticulum

  • The fiber contains transverse (T) tubules that facilitate AP propagation.
  • The sarcoplasmic reticulum (SR) is close to T tubules.

DHP receptor and ryanodine receptor

  • T-tubules have the DHP receptor (does not need Ca\(^{2+}\) to flow through to cause contraction in skeletal muscle).
  • DHP is connected to the ryanodine receptor (RyR).

  • When activated by voltage:

  • DHP triggers RyR opening,

  • which causes more Ca\(^{2+}\) to flow into the cell (released from SR).

Ca\(^{2+}\) → troponin → tropomyosin shift

  • Ca\(^{2+}\) binds to troponin.
  • This causes tropomyosin to shift and uncover the myosin-binding site on actin.
  • Myosin cross-bridges bind to actin and contraction occurs.

Relaxation

  • Ca\(^{2+}\)-ATPase pumps Ca\(^{2+}\) back to the SR and stops contraction.

Cross-Bridge Cycle

  • When the muscle is relaxed and with Ca\(^{2+}\), then the myosin cross-bridge connects to the actin binding site.
  • ADP releases and the cross-bridge moves.
  • ATP detaches the cross-bridge from actin.
  • Myosin hydrolyzes ATP and is ready to attach again.

Muscle Types (high level)

Smooth muscle

  • around hollow spaces

Cardiac muscle

  • only in the heart

Fiber Types and Recruitment

Fiber types

  • Fast glycolytic fibers: Type 2X
  • Fast oxidative glycolytic fiber: Type 2A
  • Slow oxidative fiber: Type 1

Key notes:

  • All fibers within a motor unit are the same.
  • Fibers are recruited in order from slow (thinner) to fast (thicker).

Recruitment order

  1. Slow oxidative fiber Type 1 (1st) — oxidative phosphorylation ATP
  2. Fast oxidative glycolytic fiber Type 2A (2nd) — oxidative phosphorylation ATP
  3. Fast glycolytic fibers Type 2X (3rd) — glycolysis ATP