Physiology

• Motor neuron & the muscle fiber it innervates.  The smallest amount of muscle that can be activated voluntarily
• Graduation of force in skeletal muscle is coordinated largely by the nervous system
• Recruitment of motor units is the most important means of controlling muscle tension
• All fibers in the motor unit contract simultaneously

All fibers in a motor unit are of the same fiber type, slow with slow, fast with fast...

• Neuromuscular junction
• Excitation-contraction coupling
• Sliding filament theory

• 1. Alpha motor nuerons innervate skeletal muscle fibers at specialized junsctions call "neuromuscular junction" or "myoneural junction"
• 2. One neuron can innervate multiple fibers: "synaptic bouton"
• 3. The sarcolemma in the neuromuscular junction is called the "motor end plate"
• 4. The neurotransmitter released is "Ach" which binds to specialized cholinergic receptors called "nicotinic receptor" that causes an opening of ligand gated Na+ ion channel production of postsynaptic potential - resulting in action potential in muscle fiber.

• Acetyl CoA binds with choline acetyl transferase creating Acetyl choline
• Add Ca+ and the acetyl choline packages fuse with the membrane, causing the release of the Ach (this is where Clostridium botulinum toxin attacks)
• The Ach goes to the Ach receptor, allowing the Na+ to flood into the muscles (this is where Curare M. gravis attacks)
• From here the Acetyl choline esterase releases the acetate and choline separate.   (Neostigmine (m. gravis) will attack here)
• The choline is then absorbed back into the nerve ending where is again binds with Acetyl CoA

• Curare: (poison from SA Frogs) Blocks Nicotinic receptors-prevents Ach induced muscle contractions. Curare derivatives - to relax muscles prior to surgery
• Clostridium: Toxin prevents Ach release from somatic motor neurons. Death by paralysis of respiratory muscles.
• Ach E (Acetylcholine esterase) inhibitors: Myasthenis gravis - autoimmune disease - reduced Ach receptor function - treated with AchE inhibitor Neostigmine
• Downer cows (hypocalcemia): decreased Ach release, hypothermia, weak pulse, decreased rumen contractions

AP travels down T tubule. Ca+ release into sarcoplasm -> Ca+ binds to Troponin - conformation change - active sites of Tropomyosin exposed -> "power stroke" myosin head binds to and pulls actin towards M line (sliding filament theory) -> Myosin head detaches from actin with ATP binding

• Muscle length and tension
• Relationships: length vs tension and force vs velocity
• Isometric vs Isotonis
• Twitch - Treppe - Wave summation - Tetanus
• Energy Supply
• Skeletal muscle metabolism

Muscle "adapts" to meet the habitual level of demand placed on it. This is affected by frequency of recruitment and the load.

• Muscle generates force while muscle length remains constant
• Postural muscles - maintain constant body position while opposing gravity

Muscle shortens under a constant load. Muscle length changes as force is generated = movement. Common in animal locomotion.

Decrease muscle length (cross bridges formed - thick & thin filaments interdigitate to overcome resistance)

Increase muscle length (tension developed < load on muscles) For example, when an animal walks down a steep incline - animal controls rate of elongation of muscles as the legs stretch to the next location.

• Passive process
• Elastic forces (connective tissue, muscle proteins-titan), opposing muscles & gravity - return muscle to resting length

• Latent phase
• Contraction phase
• relaxation phase

Muscle stimulated 2nd time immediately after the relaxation phase of 1st twitch - results in increased tension known as Treppe. This increased tension is due to gradual increase in SR Ca+ between because the Ca+ pumps in SR are unable to sequester all Ca between twitches. If the repetitive stimuli & contractions continue - tension will eventually decrease and fatigue occurs

2nd stimulation occurs before complete relaxation = 2nd twitch will cause greater tension than original twitch. This occurs at stimulation @ 50/sec and SR can no longer sequester the Ca+ between twitches.

Tetan = Rigid): individual twitches are distinguishable

• Individual twitches indistinguishable- because of high frequency of stimulations - no relaxation phase. This is the normal state observed during muscle contraction. Ca+ are released from SR faster than they are pumped back in - Ca+ conc in cytoplasm is above the threshold required for muscle contraction
• No relaxation phase

• Excess energy stored as Creatine Phosphate - exclusively in muscle: Conc is 3-6 times more than ATP
• Creatine Kinase: key enzyme for rapid ATP generation
• ATP & creatine Phosphate - enough for 10-15 sec

• 2 ATP molecules/glucose - enough for ~ 2 min
• Type IIB fibers
• Glycogen ~ 1% of muscle weight

• ~36 ATP molecules/glucose
• Type I fibers
• Lipids: prolonged endurance training