PHYS20008 - Week 3 (Muscles)

Structure of muscles

1) Muscle belly - has tendons at either end

2) Fascicles (are inside muscle belly)

3) Muscle Fibres (are inside fascicles) - are single muscle cells.

4) Myofibrils: are hundreds of muscle fibres packed together

Why are myofibril striated?

The stripes happen because proteins are arranged in patterns.

What’s the basic contractile unit of muscle & what does it contain?

Basic contractile unit of muscles = sarcomere

• goes from Z line → Z line

Contains:

• thick filaments = myosin

  • myosin usually described as 2 headed golf club

  • has head, tail and hinge region (hinge is very imp as it moves during contraction)

  • has actin binding site ('lets myosin attach to actin) & ATP binding site (ATP provides the energy for contraction)

• thin filaments = actin

  • actin molecule are arranged in helical structure

  • are binding sites where myosin attaches

What are the 2 important regulatory proteins that regulate contraction?

• tropomyosin (At rest, Tropomyosin covers binding sites on actin, which prevents myosin from attaching to stop unnecessary contraction)

• troponin

What’s calcium’s role regarding actin-myosin attachment sites?

When calcium is released:

• calcium binds troponin

• tropomyosin moves away

• binding sites become exposed

• myosin can attach

Where is Calcium stored?

Stored inside Sarcoplasmic Reticulum.

At rest:

• calcium levels are tightly regulated

• calcium stays inside storage

What’s the role of cytoskeletal proteins?

Give the sarcomere stability.

These include:

• nebulin

• titin

  • The largest protein in the body

  • It acts like a spring & helps keep everything together during contraction.

Muscle Diseases/Myopathies

Many myopathies happen because cytoskeletal proteins are missing.

Without these proteins:

• muscle becomes fragile

• contractions cause damage

• repeated damage occurs

Replacing these proteins can restore stability.

Tendons and the Series Elastic Component

Muscles attach to tendons.

At the Myotendinous Junction, there is a Series Elastic Component, which behaves like a spring.

• spring is imp as Before lifting a load, the springiness must first be overcome.

• This springiness:

  • protects muscle

  • allows repeated contractions

  • helps prevent damage

Action Potentials and T-Tubules

When muscle contracts, an action potential travels along membrane & goes down the T-tubular system. This causes calcium release from the sarcoplasmic reticulum.

What happens when contraction stops? (relaxation)

• calcium leaves troponin

• calcium pumped back into sarcoplasmic reticulum -(ATPase pumps do this)

Problems With Calcium

If calcium leaks:

• unwanted contraction occurs

• muscle degradation can occur

High calcium levels can damage muscle.

Sliding Filament Theory

When muscle contracts, Sarcomeres shorten, but filaments do NOT change length. Instead, the Thick and thin filaments just slide between one another.

What determines how much force is produced?

The amount of overlap between actin and myosin.

What does Excitation-Contraction Coupling mean and what are the steps?

Means Electrical signal → muscle contraction.

Sequence of Events

1. Action potential generated

2. Travels along membrane

3. Travels down T-tubules

4. Calcium released

5. Calcium binds troponin

6. Tropomyosin moves

7. Myosin binds actin

8. Cross bridge cycling begins

Voltage Sensor and Calcium Release

There’s a voltage sensor which detects the action potential.

At rest, a “plug” blocks calcium release.

When voltage changes, plug opens & calcium floods out.

Plug closes again when stimulation stops.

What does cross bridge cycling explain?

Explains how muscles produce force.

What’s rigor state/mortis?

At death:

• ATP disappears

• myosin heads cannot detach

This causes Rigor Mortis.

What happens when ATP binds to myosin?

Myosin detaches from actin. ATP splits into ADP & inorganic phosphate. This energises the myosin head.

What’s the power stroke?

The myosin head moves because of the hinge region.

This movement pulls the sarcomere toward the centre. Thousands of myosin heads work together.

Central & Peripheral mechanisms:

Central Mechanisms involve:

• brain

• spinal cord

• motivation

• volition

Peripheral Mechanisms involve:

• muscle

• fuel sources

• electrical events

• metabolic problems

Sprint vs Endurance

Sprinting

• high frequency fatigue

• fast fatigue

• fast recovery

Endurance

• slower fatigue

• longer recovery

• better fatigue resistance

What is muscle force proportional to?

Force is mostly proportional to Cross-sectional area - Big muscles usually produce more force.

Concentric, Isometric and Eccentric

Concentric / Myometric:

• muscle force > load, so Muscle shortens (ex// lifting a dumbbell)

Isometric:

• force = load, so muscle stays same length (ex// trying to lift something immovable)

Eccentric / Plyometric:

• load > force, so Muscle lengthens (ex// lowering a heavy weight)

• produce high force

Ascending, plateu & descending

Ascending Limb:

• Short sarcomeres

• overlap not optimal

• force reduced

Plateau Region:

• Optimal overlap = Maximum force

Descending Limb:

• Sarcomeres stretched

• little overlap

• low force

Where does active force and passive force come from?

Active Force comes from Cross bridge formation.

Passive force comes from Stretching connective tissue & Series Elastic Component

The more you stretch:

• passive force increases

• active force decreases

How is total force calculated?

Active force + passive force

Force-velocity relationship

• No load = maximum velocity (shorten fastest when no load)

• Heavy load = slower lift

• Immovable load = zero velocity (isometric)

The latency period

When a muscle first gets stimulated, it does not shorten instantly - there’s first a latency period.

During this time, muscle is building force trying to match the external load & taking up the “slack”.

One muscle force exceeds the load and muscle shortens. As the load gets heavier, latency period becomes longer, because the muscle needs more time to develop enough force.

Formula for power:

Power=Load×Velocity (power depends on both force & velocity)

What’s an isometric twitch response?

Is the response to one electrical stimulus.

The muscle:

• Contracts

• Relaxes

3 muscle fibre types:

1) Fast fatigable (FF): control fast glycolytic fibres

They:

• Contract very quickly

• Produce lots of force

• Fatigue rapidly

2) Slow (S): control slow oxidative fibres

They:

• Contract slowly

• Produce less force

• Resist fatigue for a very long time

3) Fast fatigue resistant (FR): These are in between.

They:

• Contract quickly

• Resist fatigue better than FF fibres

• Use both oxidation and glycolysis

Fast vs slow fibres

Fast fibres:

• Strong

• Explosive

• Tire quickly

Slow fibres:

• Endurance-based

• Fatigue resistant

• Lower force

Colour of muscle fibres

Scientists long ago noticed some muscles were Pale or Dark red.

This comes from Myoglobin content & Enzyme differences

• Fast fibres are usually Pale/white

• Slow fibres are usually Dark/red

Human fibre types

In humans:

• Type 2X = fastest fibres - ex// sprinters

• Type 2A = intermediate

• Type 1 = slow oxidative - ex// marathon runners

Other animals like mice may also have Type 2B fibres, which are even faster.

Are fibre types genetic?

Mostly yes. We are born with an inherent fibre distribution. Training can modify some muscle properties, but it cannot completely change your inherited fibre makeup.

Can new neuromuscular junctions form?

Yes, as nerves can Sprout new branches & Form new neuromuscular junctions - This may help preserve muscle innervation during ageing.

Do we create new muscle cells?

Mostly muscle growth is hypertrophy (Existing muscle fibres get larger) NOT usually Hyperplasia (creating many brand-new fibres).

However, muscle stem cells can help regeneration after damage & some evidence suggests limited hyperplasia may occur in special situations.

Types of movements:

Reflexes:

• is the simplest type of movement (ex// coughing or knee jerk)

• reflexes are very simple, very rapid (because many reflexes protect us from harmful stimuli, and sending information all the way to the brain would take too much time) & integrated only at the level of the spinal cord

More complex:

• These are conscious actions that become learned traits over time. (ex// playing piano)

• requires learning, practise & training

Rhythmic Movements:

• ex// walking/repeated step patterns

• require some involvement from brainstem

What’re the 3 phases of voluntary movement?

• Planning - think about the movement first (requires high centres of brain)

• Initiation - (motor cortex becomes involved to prepare the movement)

• Execution - Muscles contract to perform the task.

  • During execution, feedback mechanisms help correct movements, obstacles can be avoided & movements can be adjusted

  • Body also uses feed forward mechanisms: anticipating what will happen/preparing before something occurs

Reflex Pathways

Stimulus → receptor (detects something) → spinal cord (processes it) → muscle response (immediately)

Categories of Reflexes

A. Somatic vs Autonomic Reflexes:

• Somatic reflexes control skeletal muscles

• Autonomic reflexes control smooth muscle, bladder, heart & endocrine glands

B. Spinal vs Cranial Reflexes:

• Spinal Reflexes are integrated in the spinal cord

• Cranial reflexes involve brain & are more complex

C. Innate vs Learned Reflexes:

• Innate reflexes: born with them (ex// newborn reflexes or knee jerk)

• Learned reflexes develop thru conditioning or training.

D. Monosynaptic vs Polysynaptic Reflexes:

• Monosynaptic Reflex: only 1 synapse - very fast & simple

• Polysynaptic Reflex: Has 2 or more synapses & interneurons (can involve excitatory & inhibitory interneurons)

What are the 2 muscle receptors?

1) Muscle spindle:

• detects changes in muscle length, especially stretching

• its job it to trigger a reflex contraction

2) Golgi tendon organ:

• detects force

• protects muscles from excessive load

Unlike the spindle:

• it causes reflex relaxation

So:

• Muscle spindle → reflex contraction

• Golgi tendon organ → reflex relaxation

Intrafusal vs Extrafusal Fibres:

Intrafusal fibres: are inside spindle

• have connective tissue sheath & nerve ending wrapped around middle

• middle region has NO sarcomeres & is NON contractile

• ends DO have sarcomeres and ARE contraction

Extrafusal Fibres: These are the normal large muscle fibres.

Stretch Reflex

When a muscle stretches:

• the spindle detects the change

• signals go to the spinal cord

• alpha motor neurons activate

• the muscle contracts

So: Muscle stretch → spindle activation → spinal cord → muscle contraction

Then, the muscle returns to normal length.

If the muscle shortens during contraction, how does the spindle keep detecting length?

alpha gamma coactivation.

During muscle contraction,

• alpha motor neurons activate extrafusal fibres

• gamma motor neurons activate intrafusal fibres

The contractile ends of the spindle pull on both sides. This stretches the middle non-contractile region again so the spindle:

• keeps functioning

• maintains tone

• continues detecting length changes

Golgi Tendon Organ

The Golgi tendon organ:

• detects force

• protects muscles from excessive load

Unlike the spindle:

• it causes reflex relaxation

So:

• Muscle spindle → reflex contraction

• Golgi tendon organ → reflex relaxation

What’s Reciprocal Inhibition?

Around a joint:

• one muscle contracts

• the opposing muscle relaxes

In the knee jerk reflex:

• quadriceps contract

• hamstrings relax