LECTURE 13: MUSCLE CONTRACTION

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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
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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)

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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
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CONTRACTION THE THICK/THIN FILAMENT BINDING -
POWER STROKE - UNBINDING CYCLE CHEMICAL ENERGY IS
CONVERTED TO MECHANICAL ENERGY
MyosinADP head in cocked position can bind to
actin subunit if cytoplasmic calcium is
available to bind troponin and expose
actins 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 myosins ATPase activity recocks the myosin
head
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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
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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
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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) Lengtheni
ng 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 opponents forward force
than to generate sufficient forward force of
his(her) own
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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
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SLOW-TWITCH AND FAST-TWITCH MUSCLE FIBERS
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MOTOR UNITS ARE RECRUITED IN A FIXED ASCENDING
ORDER AS REQUIRED FOR A TASK
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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-f
orce 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
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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
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NEXT LECTURE AUTONOMIC NERVOUS SYSTEM READING
Kandel text, Chapter 49