- 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
Physiological profiles of motor units
All fibers in a motor unit are of the same fiber type, slow with slow, fast with fast...
3 parts of muscle contraction
- 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 choline metabolism
- 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
Acetyl choline metabolism: disease
- 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
Factors affecting skeletal muscle contraction
- 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 is "plastic"
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
Muscle twitch stages
- 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
Summation (wave summation)
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
Creatinine Phosphate (anaerobic)
- 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
Oxidative metabolism (TCA cycle & oxidative phosphorylation)
- ~36 ATP molecules/glucose
- Type I fibers
- Lipids: prolonged endurance training