2 - Mucle Physiology

How many skeletal muscles are in the human body?

Over 650

What is the function of skeletal muscle?

Enables voluntary movement and maintains posture

Compare skeletal, cardiac, and smooth muscle

Skeletal: elongated, multinucleated (peripheral), striated, voluntary; Cardiac: branched, 1–2 nuclei (central), striated, involuntary; Smooth: spindle-shaped, single nucleus, non-striated, involuntary

TISSUE ORGANISATION OF SKELETAL MUSCLE

What surrounds the whole muscle?

Epimysium

What are fascicles?

Bundles of muscle fibres

What surrounds fascicles?

Perimysium

What surrounds individual muscle fibres?

Endomysium

How are connective tissues related to tendons?

Epimysium, perimysium, and endomysium are continuous with tendons

STRUCTURE OF MUSCLE CELLS (MYOFIBRES)

What is the sarcolemma?

Plasma membrane of a muscle fibre

What are T-tubules?

Invaginations of the sarcolemma that carry action potentials deep into the cell

What is the sarcoplasmic reticulum (SR)?

Specialised ER that stores Ca2+

What are terminal cisternae?

Enlarged regions of SR near T-tubules

What is a triad?

One T-tubule flanked by two terminal cisternae

What is a myofibril?

Bundle of contractile filaments inside a muscle fibre

HIERARCHY OF MUSCLE ORGANISATION

Order of muscle organisation

Myofilaments → Myofibrils → Myofibres → Fascicles → Muscle

MOTOR UNITS

Main proteins in myofibrils

Actin (thin) and myosin (thick)

What is a sarcomere?

Functional unit of contraction between two Z lines

What is the A band?

Dark region representing length of thick filaments

What is the I band?

Light region with thin filaments only; shortens during contraction

What is the Z line?

Boundary of sarcomere; anchors thin filaments

What is the H zone?

Region with only thick filaments; shrinks during contraction

What is the M line?

Centre of sarcomere; anchors thick filaments

NEUROMUSCULAR JUNCTION (NMJ)

What is a motor unit?

One motor neuron and all the muscle fibres it innervates

Where are small motor units found?

Muscles requiring fine control (e.g. eye muscles)

Where are large motor units found?

Muscles requiring powerful contraction (e.g. quadriceps)

ACh RECEPTOR

Components of the neuromuscular junction

Presynaptic terminal, synaptic cleft (20–30 nm), postsynaptic membrane (motor end plate)

What neurotransmitter is used at the NMJ?

Acetylcholine (ACh)

What enzyme breaks down ACh?

Acetylcholinesterase

Structure of ACh receptor

5 subunits: 2 alpha, 1 beta, 1 gamma, 1 delta

How is the ACh receptor activated?

Two ACh molecules bind to alpha subunits

Which ions pass through ACh receptor channels?

Na+, K+, Ca2+ (mainly Na+ enters)

Why does mostly Na+ enter the muscle fibre?

Large concentration gradient and negative intracellular charge

STEPS OF NMJ TRANSMISSION

Steps of neuromuscular transmission

AP arrives → Ca2+ channels open → Ca2+ influx → vesicles release ACh → ACh binds receptors → Na+ influx → end plate potential → muscle AP

END PLATE POTENTIAL

What is the end plate potential (EPP)?

Local depolarisation (~50–75 mV) at motor end plate

What does EPP trigger?

A muscle action potential leading to contraction

ACh BREAKDOWN

What happens to ACh after release?

Broken down into acetate and choline and recycled

What is the safety factor at the NMJ?

EPP is about 3 times larger than needed to trigger an AP

What causes NMJ fatigue?

High frequency stimulation depletes ACh vesicles

SAFETY FACTOR + FATIGUE

Key proteins in excitation-contraction coupling

DHP receptor (T-tubule) and ryanodine receptor (SR)

Steps of excitation-contraction coupling

AP travels along sarcolemma → into T-tubules → DHP activated → ryanodine receptor opens → Ca2+ released → contraction

EXCITATION CONTRACTION COUPLING

Structure of myosin

Head, neck, and tail regions

What binds ATP on myosin?

The head (S1 region)

What causes the power stroke?

Flexion of the myosin head

MYOSIN STRUCTURE

Components of thin filament

Actin, tropomyosin, troponin

What blocks myosin binding sites?

Tropomyosin

What does Ca2+ bind to?

Troponin, causing tropomyosin to move

THIN FILAMENT

Where are thin filaments anchored?

Z line

Orientation of thick filaments

Heads face Z line, tails face M line

Proteins that stabilise filaments

Nebulin (thin), titin (thick)

FILAMENT ORGANISATION

SLIDING FILAMENT CYCLE

Steps of the sliding filament cycle

Myosin binds actin → power stroke → ATP binds causing detachment → ATP hydrolysed to re-cock head

Requirements for muscle contraction

ATP, Ca2+, actin, myosin, troponin, tropomyosin

ATP ROLES

Functions of ATP in muscle

Energises myosin, detaches myosin from actin, pumps Ca2+ into SR

How long does stored ATP last?

About 10 seconds

ENERGY SOURCES

What is creatine phosphate used for?

Rapid regeneration of ATP

How large is creatine phosphate store?

About 4 times ATP pool

GLYCOLYSIS

ATP yield from glycolysis

2 ATP per glucose

Does glycolysis require oxygen?

No

What is produced without oxygen?

Lactic acid causing fatigue

AEROBIC RESPIRATION

ATP yield from aerobic respiration (glucose)

About 32–36 ATP

ATP yield from fatty acids (palmitate)

About 105 ATP

MYOGLOBIN AND OXYGEN

What is the function of myoglobin?

Stores oxygen and facilitates diffusion in muscle

How does myoglobin compare to haemoglobin?

Higher affinity for oxygen

RECOVERY AFTER EXERCISE

What happens to lactic acid after exercise?

Converted to glucose in the liver (Cori cycle)

What is oxygen debt used for?

Restore myoglobin, regenerate ATP and creatine phosphate, synthesise glycogen

MSCLE FIBRE TYPES

Features of red muscle fibres

High myoglobin, many mitochondria, many capillaries, low glycogen

Function of red fibres

Slow, sustained contraction (postural muscles)

WHITE FIBRES

Features of white muscle fibres

Low myoglobin, few mitochondria, high glycogen

Function of white fibres

Fast, powerful contractions

SUMMARY

What determines muscle performance?

Availability of ATP

Short-term vs long-term energy sources

Short: glycolysis; Long: oxidative metabolism