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