lecture 13: muscle contraction & motor units
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
LECTURE 13: MUSCLE CONTRACTION & MOTOR UNITSREQUIRED 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 m diameter and 2-6 cm in length
Each adult muscle fiber is innervated by only one motor axon, while each motor
axonbranches to innervate 100-1000 muscle
fibers. The muscle fibers innervatedby 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
COMPOUND MUSCLE ACTION POTENTIALS CAN BE RECORDED WITH EXTRACELLULAR ELECTRODES
Although extracellular tissues and fluids have very low resistance, the extracellularlongitudinal current flow during an action potential produces a very small V
between two points near muscle endplates
The near-simulataneous activation of many nearby muscle fibers induced by firingof one or more motor units gives a compound muscle action potential with an
easily recorded extracellular V
The technique of recording compound muscle action potentials is calledElectromyography (EMG).
EMG is used clinically by neurologists to detect even small defects in:1) Myelination (resulting in slowed conduction)
2) Synaptic transmission (pre- or post-synaptic defects)
SARCOMERIC ARCHITECTURE OF MUSCLE FIBERS
Each myofibril composed of sarcomereslinked by Z-disks
Overall length of muscle reflects widthof sarcomeres, which can change by
passive or active sliding of thinactin filaments between thick
myosin filaments
The sarcoplasmic reticulum is systemof membranous invaginations
which position calcium-richlumen in tight proximity to all
thick and thin filaments
Myosin heads along thick filamentsbind actin on thin filaments, and
myosin neck flexion providespower stroke to drive thin filaments
in direction promotingsarcomere contraction
CONTRACTION: THE THICK/THIN FILAMENT BINDING - POWER STROKE - UNBINDING CYCLECHEMICAL 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
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
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
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 moreeasily 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
MUSCLE FATIGUE CAUSED BY ATP DEPLETION
Fatigue is the property whereby the power-stroke cycle of contraction slows
down or stopsdue to depletion of ATP energy stores.
Early in fatigue, compensation achieved because ATP-ADP exchange does not
occur atend 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
SLOW-TWITCH AND FAST-TWITCH MUSCLE FIBERS
MOTOR UNITS ARE RECRUITED IN A FIXED ASCENDING ORDER AS REQUIRED FOR A TASK
MOTOR NEURON SIZES DETERMINE THEIR ORDER OF RECRUITMENT
As higher order spinal neurons fire atincreasing rates, equal IEPSPs in small
and large motor neurons give larger EPSPs in smaller motor neurons,
so threshold EPSP is first achieved insmaller motor neurons which
serve smaller motor units
ADVANTAGES OF ORDERED RECRUITMENT
Provides a greater dynamic rangeof force regulation, allowing a muscle
to perform lighter or heavier taskswith sensitivity
Lower-force tasks can be performedby smaller motor units, expending
far less total energy and usingsmaller fatigue-resistant motor units
Most technically difficult motor tasksare those requiring fine muscle function
immediately after a period ofheavy muscle function, since
fatigued large fast-twitch motorunits can resist attempted movements
by subsequent commands to smallmotor units
SIMULTANEOUS FORCE ON OPPOSING MUSCLES CAN CREATE STIFFNESSAND MAINTAIN JOINT ANGLE IN RESPONSE TO SUDDEN EXTERNAL FORCES
The relationship between muscle force production and velocity of muscle extension vs. compressioncan 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 stabilizeour 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 extendswhile 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
NEXT LECTURE: AUTONOMIC NERVOUS SYSTEM
READING: Kandel text, Chapter 49