LECTURE 13: MUSCLE CONTRACTION & MOTOR UNITS

1. LECTURE 13: MUSCLE CONTRACTION & MOTOR UNITS REQUIRED READING: Kandel text, Chapter 34 Skeletal muscle is made up of long, multinucleated muscle fibers arranged in parallel and usually connected on one or both sides to bones through connecting tendons and aponeuroses Each muscle fiber is 50-100 mm diameter and 2-6 cm in length Each adult muscle fiber is innervated by only one motor axon, while each motor axon branches to innervate 100-1000 muscle fibers. The muscle fibers innervated by a single motor neuron is called a MOTOR UNIT Motor neuron cell bodies are arranged in nuclei (longitudinal columns)….Each muscle is innervated by motor neurons from a single motor nucleus

2. COMPOUND MUSCLE ACTION POTENTIALS CAN BE RECORDED WITH EXTRACELLULAR ELECTRODES Although extracellular tissues and fluids have very low resistance, the extracellular longitudinal current flow during an action potential produces a very small DV between two points near muscle endplates The near-simulataneous activation of many nearby muscle fibers induced by firing of one or more motor units gives a compound muscle action potential with an easily recorded extracellular DV The technique of recording compound muscle action potentials is called Electromyography (EMG). EMG is used clinically by neurologists to detect even small defects in: Myelination (resulting in slowed conduction) Synaptic transmission (pre- or post-synaptic defects)

3. SARCOMERIC ARCHITECTURE OF MUSCLE FIBERS Each myofibril composed of sarcomeres linked by Z-disks Overall length of muscle reflects width of sarcomeres, which can change by passive or active sliding of thin actin filaments between thick myosin filaments The sarcoplasmic reticulum is system of membranous invaginations which position calcium-rich lumen in tight proximity to all thick and thin filaments Myosin heads along thick filaments bind actin on thin filaments, and myosin neck flexion provides power stroke to drive thin filaments in direction promoting sarcomere contraction

4. CONTRACTION: THE THICK/THIN FILAMENT BINDING - POWER STROKE - UNBINDING CYCLE CHEMICAL ENERGY IS CONVERTED TO MECHANICAL ENERGY Myosin:ADP head in cocked position can bind to actin subunit if cytoplasmic calcium is available to bind troponin and expose actin’s myosin binding site. Myosin/actin binding triggers myosin neck flexion (power stroke) ATP binding to myosin head causes detachment from actin filament ATP hydrolysis by myosin’s ATPase activity recocks the myosin head

5. RELATIONSHIP BETWEEN MOTOR AXON FIRING AND CONTRACTILE FORCE Motor axon firing induces muscle action potential that propagates throughout sarcoplasmic reticulum, triggering coordinated calcium influx and initiating contraction cycle Calcium reuptake terminates cycle Frequency of axon firing determines type of contractile response

6. MAXIMAL CONTRACTILE STRENGTH WITHIN A RANGE OF MUSCLE LENGTH In highly extended muscle, fewer actin-myosin adhesions can be formed upon excitation In highly compressed muscle, thin filament overlaps obstruct adhesion formation A broad intermediate extension range is optimal for contractile force generation

7. ACTIVE FORCE OF MUSCLE DEPENDS ON VELOCITY OF MUSCLE LENGTH CHANGE Rapidly shortening muscle cannot exert much active force on a load (many myosins at any time are detached from thin filament as part of contractile cycle, and many others are near end of power stroke which is less powerful) Lengthening muscle can exert maximal active force on load (Even as myosin-filament bonds are broken by extension, they are immediately reformed) E.g., arm wrestling matches can be long because muscles can resist extension more easily than they can apply force during contraction; each person can more easily resist the opponent’s forward force than to generate sufficient forward force of his(her) own

8. MUSCLE FATIGUE CAUSED BY ATP DEPLETION Fatigue is the property whereby the power-stroke cycle of contraction slows down or stops due to depletion of ATP energy stores. Early in fatigue, compensation achieved because ATP-ADP exchange does not occur at end of a power stroke and myosin-actin interaction persists. Different muscle fiber TYPES have different contractile properties, including different rates of fatigue. All muscle fibers in a single motor unit are of the same fiber type FATIGUE - SENSITIVE STEP

9. SLOW-TWITCH AND FAST-TWITCH MUSCLE FIBERS

10. MOTOR UNITS ARE RECRUITED IN A FIXED ASCENDING ORDER AS REQUIRED FOR A TASK

11. MOTOR NEURON SIZES DETERMINE THEIR ORDER OF RECRUITMENT As higher order spinal neurons fire at increasing rates, equal IEPSPs in small and large motor neurons give larger EPSPs in smaller motor neurons, so threshold EPSP is first achieved in smaller motor neurons which serve smaller motor units ADVANTAGES OF ORDERED RECRUITMENT Provides a greater dynamic range of force regulation, allowing a muscle to perform lighter or heavier tasks with sensitivity Lower-force tasks can be performed by smaller motor units, expending far less total energy and using smaller fatigue-resistant motor units Most technically difficult motor tasks are those requiring fine muscle function immediately after a period of heavy muscle function, since fatigued large fast-twitch motor units can resist attempted movements by subsequent commands to small motor units

12. SIMULTANEOUS FORCE ON OPPOSING MUSCLES CAN CREATE STIFFNESS AND MAINTAIN JOINT ANGLE IN RESPONSE TO SUDDEN EXTERNAL FORCES The relationship between muscle force production and velocity of muscle extension vs. compression can be exploited as a very rapid restoring mechanism for maintaining fixed joint position E.g., when standing in a subway car that can lurch suddenly to one side or another, we stabilize our position by stiffening the ankle using the opposing lateral muscles at equal force How does this work? When a sudden motion moves our body to one side, one of the two stiffened ankle muscles extends while the other one shortens. Since a shortening velocity reduces muscle force efficiency while lengthening velocity does not, the two muscles forces become unequal, with the extended muscle now exerting more force and acting to restore original joint angle

13. NEXT LECTURE: AUTONOMIC NERVOUS SYSTEM READING: Kandel text, Chapter 49