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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
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:
1)
Myelination (resulting in slowed conduction)
2)
Synaptic transmission (pre- or post-synaptic defects)
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
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
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 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
MUSCLE FATIGUE CAUSED BY ATP DEPLETION
Fatigue is the property whereby the powerstroke 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
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 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
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
NEXT LECTURE: AUTONOMIC NERVOUS SYSTEM
READING: Kandel text, Chapter 49