muscle physiology & dynamics of work: how a working horse works

8/8/2019 Muscle Physiology & Dynamics of Work: How a Working Horse Works

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Muscle Physiology & Dynamics of Work

How a Working Horse Works

Lee Ann Swenson, MS, BSc, CEMMT



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Equine Muscle Physiology & Mechanics

Muscle Tissue Intro

Structure & Function

Muscle Microanatomy & Physiology

Dynamics of Work

Specific Muscle Fibers & Energy Substrates

Exercise & Effects on Muscle



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Muscle Tissue: Introduction

Muscles = Contraction

3 Types of Muscle

Visceral Muscles (Smooth Muscle) Involuntary GI Tract, Blood Vessels, Uterus, etc.

Cardiac Muscle Involuntary Heart

Skeletal Muscle (Striated Muscle) V oluntary

Movement of Joints, Limbs, etc.

± Explosive power

± Stamina

± Motor Control



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Skeletal Muscle: Structure & Function

Large part of body weight (up to 40% including H20)

Closely associated with the skeletal, nervous, and

circulatory systems

Manipulation impacts a range of tissues & systems Generates heat

Each muscle is a collection of fibers & associated tissues

Attached to bone via tendons & connective tissue

Least moveable attachment = origin

Most moveable attachment = insertion



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Microanatomy

& Physiology



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Muscle Fiber = Individual Muscle Cell

Multinucleated ± composed of fused cells Large cells

10 ± 100 µm diameter

Approx 20 cm in length



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Muscle Cells

Specialized to contract

Generate FORCE and MOVEMENT

Do not divide

Increased muscle size is due to Increased cell size

Key Qualities of Muscle Cells

Excitable

Conductive Contractile



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Muscle Cell Key Components

Membrane = Sarcolemma T-Tubules

Transmit Messages

Mitochondria

Generate Energy

Numerous

Myofibrils

2 Proteins in long strands

Heart of the contractile function

Sarcoplasmic Reticulum (Endoplasmic Reticulum)



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Muscle Cell & Associated Structures

As visible with a standard light microscope



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Skeletal Muscle

Electron Micrograph



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Dynamics of Work

Mechanism of Contraction

Stimulus of Contraction

Energy for Contraction



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Sarcomere = Smallest Unit of Contraction

Repeating Pattern of Striations

Thick and Thin Filaments

Actin (Thin) & Myosin (Thick)

Myofilaments arranged in a specific pattern

H-Zone

Z-Line

A-Band



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Actin & Myosin

2 Principal Muscle Proteins

Found in Myofibrils

Arranged in a Ring-like Structure

Usally 6 Actin strands around a Myosin fibril Run Parallel & Lengthwise

Myosin (Thick) has protrusions (Crossbridges)

Actin (Thin) is intertwined with thinner topomyosin and

troponin



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Mechanism of Contraction

1. Nerve Impulse Stimulation2. CA++ Released into Cytoplasm by Sarcoplasmic

Reticulum

3. CA++ Binds to Troponin, which Rotates

4. Tropomyosin Moves and Actin is Exposed to Myosin5. Myosin Crossbridge Binds to Actin

6. Crossbridge Drags Along Actin (Power Stroke)



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When all the crossbridges in a sarcomere act together,the whole sarcomere contracts



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Mechanism of Relaxation

7. Nerve Impulse Ends

8. SR Reabsorbs CA++

9. CA++ Dissociates from Troponin

10.ATP Binds to the Crossbridge

11.Crossbridge Disconnects from Actin

12.Actin Fibers Return to Previous Positions

13.Sarcomere Relaxes



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Contraction-Relaxation

A muscle cell may not go back to immediate completerelaxation

Contraction can continue through a series of stimulations

(Summation)

Summation increases the total force of contraction If the stimulus is great enough, many sarcomeres in

many fibers are recruited, and the muscle as a whole

contracts.

Allows for varying amounts of work

Muscle failure occurs when the maximum number of

fibers are stressed beyond their limits



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Stimulus of Contraction:Muscle Contraction is Controlled by Motor Nerves



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Interaction of Motor Nerves and Muscle Fibers

Each muscle is innervated by only one motor nerve

One nerve can innervate a number of muscles

Each nerve controls many fibers (motor units), the fewer

the fibers the more delicate the movement If nerve contact is lost, fibers shrink (atrophy)

The pattern of nerve activity determines the fiber type



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Feedback Loop

Feedback from the tendon and stretch receptorscontrols motor nerve activity

Motor nerve activity is also controlled by higher

centers (brain)



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Mechanism of Neuromuscular

Stimulation (Excitation)

1. Electrical depolarization occurs along the stimulated

nerve

2. Nerve end touching the muscle fiber releases a

neurotransmitter ( ACH)3. Depolarization of the muscle cell membrane ( Action

Potential)

4. T-tubules open in SR and Ca++ is released

5. Increased intracellular Ca++

allows actin and myosin tointeract and the crossbridge cycle starts



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Relaxation

When electrical activity stops, the calcium is removed

and contraction stops

Muscle must relax between each contraction by actively

pumping Ca back to SR Ion pumps in the cell membrane actively repolarize the

muscle cell membranes

All processes necessary for relaxation are active ±

require energy



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Each crossbridge requires ATP Each myosin strand has dozens of crossbridges

Each muscle fiber has hundreds of myosin strands

Muscle Contraction Requires Significant Energy

Basic Unit of Energy = ATP

ATP ADP & Pi ENERGY

( ATP + H2O ADP + Pi +H+ + Energy)

ATP= adenosine triphosphate; ADP=adenosine diphosphate;

Pi=Inorganic phosphate



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F or a horse to maintain exercise for more than a few

seconds, ATP stores in muscle must be replenished at

an appropriate rate.



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Fuels

Intramuscular Triglycerides & Glycogen

Extracellular FFAs from Adipose Deposits and Glucose from the

Liver

Total amount of fuel stored in a 1,000 lb horse



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AEROBIC

ANAEROBIC

Two Main Pathways For Energy Metabolism



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Aerobic Metabolism

Occurs in Mitochondria

For low energy demands of slow speed exercise

Primary pathway for endurance exercise

Gallop speeds < 18sec/200m can usually be met byaerobic metabolism in fit horses

Training can increase capacity to generate energyaerobically

Enhanced oxygen delivery to muscle Increased mitochondrial density

Increased enzyme concentrations



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Aerobic Metabolism

Oxidative Phosphorylation Fats & CHO oxidized to produce ATP

Fats ± stored in depots around body

CHO ± stored as glycogen in liver & muscle

(glycogen metabolizes to glucose)

Aerobically metabolized approx 2x as fast as fat



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Aerobic Metabolism

Limitations

Primarily limited by availability of oxygen in working

muscles

Upper airway obstructions Cardiovascular system impairment

Hemoglobin concentration



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Anaerobic Metabolism

High intensity exercise of short duration (2-3 min in

horses)

At start of fast exercise, O2 delivery does not

immediately reach the level required to support aerobicmetabolism

Approx 30-45 seconds of exercise is required before maximal

rate of oxygen use is achieved

During this time, anaerobic metabolism supplies energy

Fast Exercise not totally anaerobic; makes up the deficit Horse galloping over 1200 m, aerobic metabolism provides

approximately 70% of energy



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Anaerobic Metabolism

Glycolysis = Degradation of muscle glycogento lactate

Results in increases in lactate, hydrogen ions

and Pi in the cells

Lactic acid accumulation and fatigue developas muscle pH falls

At pH < 6.4 glycolysis and contraction are

inhibited



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Different Muscles have Fibers with

Different PropertiesType I & Type IIA

High Oxidative Capacity

Store Triglycerides & Glycogen

Standing and posture: Slow contracting fibers that are well suppliedwith oxygen ± example stay apparatus

Type I aka ³Slow Twitch´ Fibers ³Red Fibers´

Type IIB

Low Aerobic Capacity

Store Glycogen

Athletic Movements: Muscles that generate rapid movement containfast fibers and can work for short periods without oxygen

Type II aka ³White´ Fibers, ³Fast Twitch´ Fibers



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Walking Primarily Type I Fibers

primarily aerobic energy, primary substrate is fat

Transition from Walk to Trot and Cancer Type IIA Fibers Recruited

Primarily aerobic energy, substrate is both fat and glycogen

Transition to Gallop Type IIB Fibers Recruited

Energy no longer purely aerobic,

Fiber Type Recruitment Based on Energy

Requirements



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Exercise

Concentric Exercise

Isometric ± constant length

Isotonic ± constant force

Or a mixture of the two

Eccentric Exercise

Lengthening contractions



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Effects of Exercise on Muscle

Lack of exercise leads to fiber atrophy

Gentle exercise maintains muscle mass & flexibility

Moderate long term activity increases fatigue resistance

High load exercise leads to muscle fiber hypertrophy



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Muscle Fatigue

Prolonged and/or strong contraction > Fatigue

Inability of contractile and metabolic processes to

continue supplying the same work output

Nerve sends electric stimulation, NMJ transmits, actionpotentials spread over muscle fibers

However contraction becomes progressively weaker due

to reduced ATP in the muscle fibers

Interruption of blood flow through a contracting muscle

leads to almost complete fatigue in less than a minute

due to loss of nutrient supply



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Muscle Fatigue

Endurance Horses

Most often due to glycogen depletion, as most work is

performed aerobically

Race Horses

Most often due to lactic acid accumulation



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Lactic Acid or Lactate

By product of anaerobic glycolysis

A potential cause of late onset muscle soreness 24 ± 48

hours after intense exercise

Sent from muscle to blood and removed via liver

Removal requires oxygen and is hastened by light work

during recovery



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Muscle Atrophy

Results anytime a muscle is not used or used only for

weak contractions

Denervated muscle begins immediate atrophy

Example: Sweeney Injury to Suprascapular N causing atrophy in supraspinatus &

infraspinatus



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Muscle Hypertrophy

Diameter of individual muscle fibers increase

Sarcoplasm increases

Fibers gain in nutrient and intermediary metabolic

substances ( ATP, creatine phosphate, glycogen,intracellular lipids, additional mitochondria)

Myofibrils may also increase in size

Hypertrophy increases both power of the muscle and the

nutrient mechanisms to maintain that power



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Conclusion

Muscle Microanatomy & Physiology

Dynamics of Work

Specific Muscle Fibers & Energy

Substrates

Together IMPACT> Exercise &

Its Effects on Muscle

muscle physiology & dynamics of work: how a working horse works

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