Muscles and tendons

Describe the typical vasculature of muscles and tendons

Muscles: - generous blood supply (1+ arteries)

• more than 1 artery is advantageous for metabolic demand (doesn’t reduce risk of damage if supply is interrupted)

• Arteries branch in the perimysium and capillaries follow the endomysial sheaths of the individual fibres.

• Veins are satellite (paired) to arteries.

• When muscles contract it causes the vessels to be squeezed, if prolonged it can interrupt circulation

Tendons

• poorly vascularised - advantage = no haemorrhaging

• disadvantage = slow healing

what is it called where the artery meets the muscle tissue

anastomose

Describe the typical innervation of muscles and tendons.

Muscles:

• motor neurones are found in ventral horns of grey matter of the spinal cord.

• Reach the muscle in a single nerve

• ramify within the fascia

• variable ratio of motor to sensory fibres

tendons: aneural

what are the different kinds of muscle motor fibres?

1. Large alpha (supply main mass)

2. smaller gamma (supply modified muscle cells w/in muscle spindles)

3. non-myelinated vasomotor fibres (supply blood vessels)

4. sensory fibres (supple spindles, tendon organs and other receptors)

What do muscles do when activated, what happens to tension?

try to shorten

tension will increase/decrease/stay the same depending on external forces

what is meant by isometric activity

• if the muscle can’t shorten, tension increases e.g. trying to move a static object such as a wall

examples:

• Yielding - contraction opposed by resistance e.g. holding a weight steady at arms length

• Overcoming = muscle contraction opposed by an immovable object e.g. pushing against a wall

What role do muscles and joints play in locomotion?

• muscles attach to fulcra (joints)

• joints act as levers

• position of muscles around a joint affect suitability for starting/completing movement

what is meant by actively insufficient muscles

• muscles that are incapable of resulting in a full range of movement over more than one joint that they’re spread over

• can’t fully shorten

Outline the muscles and their roles in lifting the dogs head

Sternocephalicus - moves head up and down, side to side

Rhomboideus muscle - lifts neck (draws scapula up, backward and forward)

brachiocephalicus - pulls leg forwards, neck and head down and to one side

Omotransverarius muscle - moves limb forwards and neck to the side

Splenius muscle - move head backwards and forwards

Why are the main muscle groups in the proximal limb rather than the distal limb

• decreases overall muscle mass enabling a higher capacity for both force and power

• a small movement of the upper limb will result in a large movement of the lower limb (less effort needed to move the whole limb?)

What is meant by an antagonistic pair, give an example

• One muscle acts as an agonist/prime mover producing a certain effect

• The other muscle is an antagonist - opposes the movement of the agonist.

• brachialis and triceps brachii (extend and contract the elbow).

what is meant by a synergist muscle

• a muscle that may modify the action of the agonist but not directly be agonistic/antagonistic.

what is meant by a fixator

• a muscle that stabilises a joint.

• Often involves co-contraction of muscles that oppose each other when the joint moves

what does ‘origin’ mean

• a proximal/central attachment

what does insertion mean

• denotes a more distal/peripheral attachment

What is the action of epaxial muscles

extension of the vertebral column

what is the function of the hypaxial muscles

flexion of the neck and tail

what is the function of the sternocephalicus

flexion of the neck

what are the functions of the supraspinatus, infraspinatus and subscapularis

• initiates the movement of the arms away from the body

• externally rotate the humerus

• internal rotation of the humerus and shoulder joint

what is the function of the triceps brachii

extension of the forearm at the elbow

what is the function of the biceps brachii

contraction of the forearm at the elbow

what is the function and location of the extensors of the carpus and digit

• in the forelimb

• extension of the carpus and digit

what is the function of the flexors of the carpus and digit

• contraction of the wrist and fingers (carpus and digit)

what are the functions of the gluteal muscles

hip extension, abduction and rotation

what are the functions of the semimembranous and semitendinosus

• extension of the hip and knee

• assists the popliteus muscle rotating the leg internally

what is the function of the quadriceps femoris

• moves the femur cranially and caudally

what is the function of the extensors of the tarsus

• extension of the tarsus

what’s the role of the flexors of the tarsus

• flexion of the tarsus

what is the role of the flexors of the digits

• flexion of digits

what is meant by isotonic contraction

• if an animal is able to move an object, the activated muscles will shorten as they develop force

what kinds of isotonic contraction are there

Isotonic = same force

Eccentric = muscle lengthens e.g. breaking down a steep slope, the horse extends its forelimbs to slow itself down

Concentric = muscle shortens e.g. bicep curl/tugging in dogs.

Explain what is meant by tetanic contraction

• A series of rapid succession action potentials that result in persistent contraction of the muscle fibres

• AP lasts 1-2ms, causing a twitch once twitch subsides a new AP is generated

• If AP intervals are shorter than the duration of a single twitch, fibre remains contracted (sustained constant force) = Complete Tetanic contraction

• incomplete tetanic contraction = partly fused contractile force fluctuates at same fq as APs

• maximum contraction = Ca ion concentration i cytosol is maintained above level required to unlock all actin binding sites

• maximum force = 3 - 5x higher than during a single twitch (Ca ion level is higher)

what must happen before the muscle can develop external force?

the elastic elements must be taut

single contraction is too brief for complete tautening

why is it important to routinely vaccinate horses against tetanus (MSK relevance)

What does tetanus do?

• releases toxins that block the neural pathway from spine to muscles

• disrupts nervous system and causes spasms = pathological tetanic contractions

• high mortality - spasms in any muscle, affect digestion, walking, breathing etc.

Why vaccinate?

• weakens tetanus

• results in a faster immune response

• horses are at high risk due to environment

• horses are more sensitive to this disease

Cause:

• clostridium tetani

Explain the relationship between contractile force and sarcomere length

Overstretched muscle: - reduces overlap of myosin and actin, resulting in less contraction and less force produced. (tendons that attach muscles to bone help reduce the risk of this)

Too short muscles: - actin and myosin overlap too much preventing myosin binding to actin (may occur post injury if muscle isn’t used)

What is muscle power output

Work per unit of time

• product of velocity and muscle shortening and external load

Why is power output as a function of the external load an inverted U-shape?

• when external load requires less than the maximum contractile force the power output will be generated

• Power output will increase with external load until max optimum output is reached (when load is about 1/3 of max contractile force)

• If the external load continues to increase, the power output will decrease due to fatigue

How can you use the relationship between power output and external load to your advantage?

• the larger the animal’s mass, the greater the power output needed to move the animal (independent of external load)

• There’s an optimal muscle mass that enables optimum power output (load should be around 1/3 of max contractile force)

• Important to consider for certain animals e.g. race horses - they’re trained for max optimum power output with a specific weight of jockey.

Describe the interaction of muscle, tendon and bones in the movement of the limb e.g. stifle joint

• work in antagonistic pairs

In the stifle joint:

• semi-tendinous muscle contracts so the lateral vastus stretches to bend the hind leg at the stifle (opposite to straighten the leg). Must be in pairs to stretch as it requires contraction

• When muscle shortens, attachment points of tendons move together to move the bones

• Semi-tendinous muscle is connected to the tibia by pes anserinus tendon.

• Quadriceps tendon connects the lateral vastus muscle to tibial tuberosity

• Origin moves the least, insertion moves the most.

What is a musculoskeletal exam?

A thorough and systematic physical examination of the MSK system that includes:

• taking a history

• static evaluation

• dynamic evaluation

• diagnostics

what is meant by a static and dynamic evaluation?

Static = assessment of the animal at rest

Dynamic = assessment of the animal in motion

what are the principles of equine MSK examination

For it to be adequate:

• horse must be safe to handle and appropriately restrained

• shod/have appropriately trimmed feet for a dynamic exam

Area:

• quiet, away from other people/horses/traffic

• flat, even, non-slippy surface, ideally a hard and soft area for lunging

Before starting:

• detailed history (NO POSSIBLE TRAUMA/FRACTURE (can lead to catastrophic injury))

• if in doubt do a fully physical exam first

• don’t trot if clearly severely lame/obvious injury

What does a static exam involve (outline)

Assess:

• conformation

• hoof conformation and balance

• muscle development and symmetry (back and pelvis)

• palpation

• joint range of motion

• palpation of limbs when weight bearing/non-weight bearing

What do we palpate in a horse

Forelimb: Shoulder, elbow and antebrachium, carpus

Distal foreblimb: Extensor and flexor tendons, metacarpophalangeal (fetlock) joint, proximal interphalangeal (pastern) joint and distal interphalangeal (coffin) joint

Foot: Hoof, frog, sole, white line (can use hoof testers)

Hind limbs: pelvis and coxofemoral joint, stifle, tarsal

Distal hindlimb: same as forelimb, except metatarsophalangeal joint instead.

Outline what’s involved in a dynamic exam

• walking and trotting in straight lines in hand

• lunging

• ridden?

How do we test the flexing of the joints of the distal limb

• start with the distal limb and work up the leg (for testing all joints)

• looking at the 3 distal joints - fetlock, pastern and coffin

• apply pressure just above the fetlock (so rest of leg doesn’t affect it), and bring the toe up as far as you can

• hold for 30-60s, should get the heel bulb back onto the fetlock.

• greater flexion in hindlimb

How do we do a flexion test of hock and stifle

• these two joints work together called reciprocal apparatus, linked so flex and extend together - peroneus tertius muscle

• Bend knees and straighten back, bring up gently so the horse gets used to it, want to get the metatarsus parallel to the ground, fetlock tends to have some flexion (hold just above)

• should be able to get a degree of lateral movement, caused by the ball and socket joint of the hip

• hold for 30-60s part of a lameness exam/vetting

How do we flexion test the carpal, elbow and upper forelimb?

carpus separately:

• let distal limb hand loose, bring metacarpus up as close as possible to radius and ulna, should be able to bring them together completely (no gap_

Elbow and upper forelimb:

• Let lower limb hand as loosely as possible

• bring up in front of the horse

• should be able to get the radius parallel

• do gradually (tend to not enjoy this as much)

What points of a horse are we looking at for conformation?

• look for vertical lines through the centre of each joint, thus equal loading is present

• unequal loading can predispose to injury

Look at:

• different angles

Legs: fore and hind

Hip: tuber coxae

What does hoof conformation involve

Viewed from the front:

• Medio-lateral balance (slope of medial and lateral walls is identical)

Viewed from the side:

• hoof pastern angle should be ~45 degrees to the ground

• ground hoof wall angle parallel at heed and toe

What is some terminology for hoof balance when it’s not quite right

• broken back = BB

• broken forward = BF

  see image for what they look like

Outline the process of palpating a forelimb

1. Find scapula - cartilage, bridge and point (use finger pads and 4 flat fingers)

2. Find the humerus at the scapular point, find the cranial division and cusp (working ventrally)

3. follow the humerus down (find the deltoid tuberosity where apex of the ‘A’ shape of muscles)

4. Follow to the elbow: feel the radius and the olecranon (extension of the ulna)

5. Feel down radius (feel muscle difference on the lateral and medial sides) to the carpus

6. Feel down to the metacarpus and styloid process, just above the fetlock (on either side there are accessory carpal bones, remnant of previous digits)

7. Feel around the fetlock joint, down the phalangeal joints.

Same applies for the distal limb, except looking at the hip, femur, tibia, hock and tarsus.

How do we palpate a hoof?

• Use hoof palpaters

• Hold the hoof between your knees

• Use the tool to apply pressure along the white line, across the heel bulbs, on the hoof and opposite side of the frog (on both sides).

• bash the hoof too

• observing for any tenderness, swelling or localised heat.

Define conformation and outline what lines of weight-bearing, symmetry and posture we’re looking at

1. Physical appearance and outline of a horse as dictated by bone and muscle structure

2. from the front - does the weight run vertically through the middle of the limb

3. side: does vertical line down back of hock, straight metatarsal, back of fetlock?

4. don’t be too judgemental - legs may be ok even if not perfectly straight.

What is done for a dynamic physical exam

Observe the horse in:

• walk, trot and canter

• in a straight line in walk and trot, in hand

• on the lunge (both directions then on a hard then a soft surface, walk, trot and canter)

• From the lateral aspects

• From cranial/caudal aspects

• always in hand (often ridden too)

what are we looking at in a dynamic exam?

• gait symmetry

• symmetrical movement of poll

• symmetrical movement of tuber coxae

• joint flexion

• hoof landing

• hoof tracking up

How can the head position be used to identify lameness

• should be held in a steady position

• If the horse does swing its head whilst moving, as long as it’s symmetrical it’s ok

Does the horse raise its head on one limb and lower on the other?

• raise up = ouch

• raise down = sound

How does the position of the tuber coxae tell us about lameness

• as long as there’s symmetry it’s generally ok

• is one side dropping more than the other?

• the side that’s dropping/swinging more= the lame limb

Why is observing at a trot better than at a walk?

• trot = 2 beat footfall therefore 50% chance of spotting

• walk = 4 footfall beat.

why do we hold flexion tests for 30-60s?

• aim is to walk/trot the horse immediately after

• if there’s discomfort, it will be easier to see after having held it in this way.

why do we use lunging?

• the limb on the inside of the circle take more load bearing

• the limbs on the outside must extend more

what tendons can we palpate in the distal limb of the horse

• flexor tendon - runs down the back just under the skin, DDFT is just underneath

• extensor tendon runs down the side

• suspensory ligament runs down both sides

What are 3 examples of diseases/problems that cause recumbency in cows

calving paralysis

fracture of the femur

toxic E.coli mastitis

What does DAMNIT-V stand for

D- degenerative

A - anomalous

M - metabolic

N - neoplastic, nutritional

I - idiopathic, infectious, inflammatory

T - toxic, traumatic

V - vascular

What do we use DAMNIT-V for?

classifying diseases

what does degenerative mean?

• a disease that’s characterised by progressive deterioration and loss of function of tissues/organs

what does anomalous mean?

a congenital anomaly, birth defect or congenital malformation

what does metabolic in DAMNIT-V mean

disrupts the body’s ability to convert food/drink into energy

what does Neoplastic mean in the DAMNIT-V

a tumour, benign or malignant

what does idiopathic mean in DAMNIT-V

a disease of which the cause is unknown

What are some examples of diseases in cows that we can classify into DAMNIT-V?

metritis - uterine infection

acidosis - abnormally low pH

blaot - excessive gas accumulation or rumen, leading to distension and potential organ damage

exhaustion - a metabolic disorder that affects fed cattle

peritonitis - inflammation of the lining of the abdominal cavity (often caused by a compromised gut wall enabling opportunistic bacteria to enter).

Bacterial Spongiform Encephalopathy - mad cow disease

what is the most common cause of recumbency in recently calved dairy cows?

Hypocalcaemia - a metabolic condition

why does hypocalcaemia occur in recently calved cows?

a large amount of calcium is diverted to the udder for milk production

what is hypocalcaemia also know as and why does it cause recumbency?

• milk fever

• cows can’t mobilise calcium from the bone or increase absorption from the gut quick enough

• Ca ions are needed for muscle contraction, so with the reduced amount, muscles can’t contract as much.

what clinical signs may we see with milk fever

• recumbency

• weakness

• bloat

• dystocia

• ataxia

• stiff, dry faeces

• retained foetal membranes

• dry muzzle

• lack of rumen contractions

• ‘S’ shaped bend in the neck

• weak heart sounds

how can we diagnose milk fever?

• clinical signs (the abnormal findings you identify on an examination)

• clinical pathology (results from samples collected from the patient and analysed).

When taking a blood sample, why do the vacutainers contain a vacuum?

• physical mechanism to draw blood out

• sterile

What are some considerations when taking a blood sample from a cow tail vein?

• keep needle away from me (zoonotic risk)

• keep needle in its sterile tube until ready to insert

• once needle is inserted, don’t remove until all vacutainers are filled

• be careful removing vacutainers so the needle isn’t pulled out

• may need to subtly adjust the tail position to ensure continuous blood flow.

what method is the best way to administer calcium salts for milk fever treatment?

• IV - transported around the body fastest and bypasses the slow digestive system

• also give a s/c - this enables a more long lasting effect (sudden introduction of lots of Ca could lead to problems in the brain)

what do we use to treat milk fever

20% calcium solution when s/c (a more gradual, prolonged effect, done in addition to IV)

40% calcium solution when IV

How do we administer treatment for milk fever

• via a ‘flutter valve’ to the jugular vein

• administer slowly (over 5-10mins), rapid administration can interfere with cardiac rhythm leading to death.

• Rate of flow can be controlled by raising or lowering the bottle

• long, large gauge needles should be used (14 gauge, 2” or 16 gauge 1.5”)

Why can milk fever cause calf death

• low calcium levels interfere with contraction of the uterus and the cows ability to strain. this delay can result in a still birth.

What 3 kinds of muscle are there

1. smooth

2. striated

3. cardiac

Outline skeletal muscle in mammals and some functions

• around 400-600 skeletal/striated muscles (40-60% total body weight)

Functions

• locomotion and breathing

• postural support

• heat protection during cold stress

• largest protein store

• energy store

Describe the structure of skeletal muscle fibres

• striated

• a true syncytium (single cells with multiple nuclei)

• nuclei are peripheral

• muscle fibres are very large (10-100micrometres diameter, very long length, arranged in bundles called fascicles.

• connective tissue: epimysium, perimysium, endomysium

Microstructure:

• sarcolemma, myofibrils, myofilaments

outline the connective tissue we find in skeletal muscle

1. Epimysium: surrounds the entire muscle

2. Perimysium: surrounds bundles of muscle fibres - fascicles

3. Endomysium: surrounds individual muscle fibres

Function:

• Functional: transfer information to the bone which affects movement

• Protective: Strong, helps support muscle when under force and tension

• too much connective tissue = dangerous e.g. can lead to fibrosis.

outline the microstructure of skeletal muscle

1. sarcolemma - muscle cell membrane

2. myofibrils - tubular structures that pack the fibres

3. myofilaments - threadlike strands within myofibrils, actin (thin filament, with troponin and tropomyosin) and myosin (thick filament)

Describe the interaction between actin and myosin

• found in repeat units called sarcomeres.

• Work together to cause muscle contraction

Outline the structure of sarcomeres

2 Z-lines = one sarcomere:

• Z-line = ends of sarcomere, boundary

• I-band = actin only

• A band = from edges of myosin, zone that contains whole of myosin

• H zone = only myosin

• M band = transverse line that binds the myosin filaments in the middle of the sarcomere

What do myosin and actin look like/structure

Myosin = fibrous tail and globular head

Actin = globular protein, 2 chains that are coiled around each other with tropomyosin twisted around and troponin spaced at regular intervals.

outline the sliding filament theory

• muscle shortening occurs due to the movement of the actin filament over the myosin filament

• formation of cross bridges between actin and myosin filaments

• reduction in the distance b/w Z-lines of the sarcomere

Give 3 reasons why knowing about skeletal muscles is helpful

• know how they work, so know when it goes wrong

• know which work in groups

• know how different drugs affect different muscles

How to the cross-bridges form?

1. Ca2+ ions bind to troponin which causes a conformation change in shape which exposes the binding sites on actin

2. ATP binds to the head of myosin causing the myosin head to move

3. ATP is hydrolysed to ADP + Pi and the energy released from this reaction causes the cross bridge to form

4. ADP + Pi detach from the myosin head, which causes the myosin head to slide back, which in turn pulls the actin with it.

what are the 2 groups of fibre types and examples of each

Slow fibres: - type 1, slow oxidative fibres

Fast fibres:

• Type IIa, fast-oxidative

• Type IIb - fast-glycolytic fibres

• Type IIx- intermediate properties

How is fibre type composition analysed?

• Immunochemistry

• each fibre has a specific myosin isoform

• ATPase activity

• fibres have specific metabolic profiles

How do the different fibre types look histologically?

Type IIa - white

Type IIb/x - brown, grainy

Type I - black

give one reason why fibre type composition may vary within species

• live at different altitudes e.g. highlander vs lowlander mice

Outline the properties of the fibre types

What is muscle fatigue

• a decrease in maximum contractile force

• ATP is required for formation of cross bridges, an increase in inorganic phosphates (often during sudden, strenuous work) and a decrease in Ca2+ ions

• A reduction in ATP - leads to rigor.