Skeletal muscles

Neuronal Control of Skeletal Muscles

• Neurons control skeletal muscles through motor axons that transmit signals to muscle fibers.

Muscle Fiber Innervation

• Each muscle fiber is innervated by a single motor axon.

• One motor axon can innervate multiple muscle fibers.

Structure of Muscle Fibers

• Each muscle fiber consists of multiple myofibrils.

• Myofibrils are surrounded by a network of sarcoplasmic reticulum, crucial for calcium ion storage and release.

Striations and Sarcomeres

• Skeletal muscle fibers exhibit a striped appearance due to the alignment of myofibrils.

• Myofibrils are divided into repeating segments known as sarcomeres, which contain actin and myosin filaments essential for muscle contraction.

Sliding Filament Model of Muscle Contraction

• Muscle contraction occurs through the sliding of thin filaments (actin) past thick filaments (myosin).

• This process is driven by ATP hydrolysis, which facilitates interactions between actin and myosin, generating muscle force.

Role of Calcium in Muscle Contraction

• Binding of acetylcholine (Ach) to cholinergic receptors on muscle endplates opens sodium (Na+) channels, leading to depolarization and the generation of an endplate potential.

• Depolarization spreads through T tubules to the sarcoplasmic reticulum, resulting in Ca++ release that triggers muscle contraction by unveiling binding sites on myosin heads.

• Calcium levels must decrease for muscle relaxation, aided by parvalbumin and calcium-ATPase that pump Ca++ back into the sarcoplasmic reticulum.

• Continuous motor unit activation, if exceeding the recovery rate, leads to muscle tetanus.

Cholinergic Receptors

• Nicotinic receptors: Ionotropic receptors activated by nicotine, blocked by curare.

• Muscarinic receptors: Metabotropic receptors stimulated by muscarine, blocked by atropine.

Cholinergic Neuropharmacology

• Various substances affect acetylcholine signaling:

  • Botulinum toxin: Antagonist preventing acetylcholine release.

  • Black widow spider venom: Triggers acetylcholine release.

  • Hemimicholinium: Inhibits choline uptake.

  • Neostigmine: Inhibits acetylcholinesterase, prolonging acetylcholine action.

  • Atropine: Blocks muscarinic receptors.

  • Curare: Blocks nicotinic receptors.

The Neuromuscular Junction

• Motor axons terminate at neuromuscular junctions rich in ACh receptors, allowing neuromuscular transmission and muscle activation.

Muscle Fiber Structure

• Junctional folds and T-tubules connect with the sarcoplasmic reticulum for efficient transmission of action potentials throughout muscle fibers.

Definition of Motor Units

• A motor unit is defined as a motor axon and all muscle fibers it innervates, allowing for coordinated muscular activity.

Variability in Motor Units

• Motor units differ in their twitch force and fatigue resistance, showing variations in physiological properties:

  • Fast, fatiguable units: High energy output, prone to fatigue.

  • Fast, fatigue-resistant units: Moderate energy output, more endurance.

  • Slow, highly fatigue-resistant units: Low power output but sustain activity for longer periods.

Recruitment of Motor Units

• Additional motor units are recruited as muscle contractions increase in strength.

• Later-recruited motor units tend to drop out earlier during relaxation, indicating a hierarchy of recruitment based on contraction demands.

Larger Motor Neurons

• Motor neurons that are larger can produce stronger and faster contractions due to more significant activation of muscle fibers, enhancing force production.

Alpha Motor Neuron Properties

• Alpha motor neurons typically innervate muscle fibers of the same type to ensure coordinated and efficient muscle contractions.

Sensory and Gamma Motor Innervation

• Spiraling sensory and gamma motor axons signal intrafusal fibers within muscle spindles, contributing to proprioception and muscle stretch awareness.

Monosynaptic Stretch Reflex

• A reflex action where muscle contracts quickly in response to its stretch, highlighting the efficiency and simplicity of the reflex arc with minimal synaptic connections.

Postural Adjustments

• Muscle lengthening often occurs through antagonistic muscle activity, enabling reflexive responses to maintain posture during dynamic movements.

Alpha-Gamma Coactivation

• Allows muscle spindles to sense stretch during muscle contractions, maintaining accurate feedback for motor control.

Golgi Tendon Reflex Circuit

• Involved in preventing excessive muscle contraction by inhibiting alpha motor neurons during high tension, ensuring safety and balance in muscle activity.

Jackknife Reflex

• A reflex observed during decerebrate rigidity, where resistance during limb flexion or extension is replaced by relaxation due to Golgi tendon organ inhibition.

Peripheral Motor Neuron Innervation

• Mammalian motor neurons innervate either intrafusal or extrafusal muscle fibers exclusively, ensuring specialization and efficiency in motor control.

Muscle Lengthening Mechanisms

• Most muscle lengthening post-contraction is facilitated by antagonistic muscles that afford necessary stretching force.

Opponent Organization of Muscles

• Agonist muscles facilitate movement while antagonists oppose it, arranged in functional pairs to provide balance and controlled movement during physical activities.

Fast Wing Muscles in Insects

• Some insects utilize antagonistic muscle arrangements allowing rapid wing beats via passive stretch without requiring synchronous neural firing for each contraction.

Cardiac Muscle Dynamics

• Cardiac muscle fibers have unique structural properties, including branching and intercalated disks for coordinated contractions across the heart.

Natural Pacemaker of the Heart

• The sinoatrial node regulates heart rhythm through autonomic influences from acetylcholine slowing and norepinephrine/adrenaline speeding the heartbeat.

Action Potentials in Cardiac Muscle

• The extended duration of action potentials in cardiac muscle is crucial for effective contraction and preventing arrhythmias.