Gross skeletal muscle anatomy

All muscle has primary muscle belly - most joins to bone via tendon, may sometimes join directly via Apennine process (which acts as bridge between muscle/bone or muscle/tendon)

tendon → epimysium → endomysium → perimysium → fascicle/sarcolemma → muscle fibre/cell

• tendon

  • aponeurosis connects the muscle belly to tendon (which joins it all to bone)

  • may sometimes skip the tendon & connect straight to bone

• endomysium

  • connective tissue surrounding the perimysium

• epimysium

  • binds the muscle belly together

• perimysium

  • collection of muscle fibres are found within perimysium

  • each fascicle is bound by perimysium (membrane)

• muscle fibre

  • muscle fibre = muscle cell

  • packed with contractile proteins (myofibrils)

• fascicle

  • surrounded by membrane (sarcolemma)

  • each muscle belly is comprised of fascicles

microstructure of skeletal muscle

Cardiac muscle

• Striated

• Single central nucleus

• Involuntary

• Irregular arrangement w/ intercalated disks

• Located in heart

Smooth muscle

• No striations

• Single nucleus (nervous signal not required for all contractions within muscle)

• Involuntary

• Longer contractions

• Located in e.g. walls of uterus, oesophagus, bronchi, arteries

Roles of muscle

• Continence

• Mastication

• Swallowing

• Digestion

• Birthing

• Vaso-dilation/constriction

• Bronchodilation/constriction

• Pupil dilation/constriction

• Maintaining cardiac rhythm

• Joint movement

• Prevent joint movement (joint stabilisation)

• Postural control

• Generating heat (shivering)

tendons during locomotion

• Muscle = proximal

• Tendon = distal

• Most muscles associated w/ some kind of tendon

• Proximal tendons tend to be shorter & fatter

• Distal tendons tend to be longer

  • having longer tendons means you can have shorter muscle fibres (long tendons are usually coupled w/ pennate muscles)

  • short muscle fibres don’t shorten (contract very much) - reducing energy cost of developing muscle force

tendon structure:

• hierarchy: fascicles → sub-fascicle → collagen fibre → collagen fibrils

• tenocytes → formation & turnover of the ECM

  

Roles of tendons:

• Minimising distal limb mass (e.g. horse)

• Joins muscle to bone (transmits muscle force to skeleton)

• Elastic energy storage

  • legs behave like pogo sticks, tendons act to store & release elastic potential energy (animals with long tendons are more economical)

  • Act to store & release elastic energy, thus reducing cost of running at steady speed

  • Animals w/ long substantial organs are therefore economical - expend little energy when running

• Energy conservation

  • Long tendons = short muscle fibres

  • Tendons can stretch/shorten ~10% during galloping

  • Short muscle fibres don’t shorten (contract) v much, reducing energy cost of developing muscle force

  • Some muscles have v short muscle fibres so they look vestigial - interosseus; SDF & DDF in horse

• Power amplification

  • Muscles shorten to perform work - doing this quickly = power

    • muscles generate power, tendons store power as they stretch

  • POWER = RATE OF DOING WORK

  • Stretched tendons recoil faster than muscle shortens = more power

  • Only small amount of work done but in shorter time, so output is higher

  • E.g. horses require >2000W power to swing legs quickly during fast gallop, 50kg muscle needed for active protraction

Muscle design

• Size, shape (long/thin vs short/fat), number of bellies (biceps/triceps), tendinous origins or insertions, architecture

• Architecture - arrangement of muscle fibres (relative to axis of force generation)

• Encompasses - muscle volume, muscle moment arms, tendons

• Muscle function - in terms of force, work & power

  • Force - push/pull on an object w. mass, causing it to change velocity

  • Work = force x distance

  • Power = Δwork/Δtime (rate of performing work)

Muscle fibre arrangement

• Pennate muscles

  • Short fibres at angle to internal tendon/aponeurosis

  • Increases muscle physiological cross sectional area (PCSA) - muscle force is proportional to this

  • Shorter fibres = shorter distance to contract = economical

    • packs more muscle in smaller area

    • shorter contraction distance → less energy used to generate same movement

  • E.g. serratus ventralis (attaches scapula to ribcage → synsarcosis)

    

• Parallel muscles

  • Fibres run in parallel to line of pull of muscle

  • found where where a limb needs movement

  • More sarcomeres in series = greater total muscle fibre shortening = more potential for performing muscle work

  • Work = force x distance

  • Muscles can move joints through large range of motion

  • Potential for increased velocity of contraction (since speed = distance/time)

  • e.g. proximal forelimb or hamstring