Muscle Structure and Function

Question 1

What are the three types of muscles? What are their functions? Are they under voluntary control?

Answer 1

Smooth: line hollow organs, not under voluntary control
Cardiac: in heart generates force to pump blood, not under voluntary control
Skeletal: applies force to bones to control posture and movements, under voluntary control

Question 2

Skeletal Muscle: Structural features

Answer 2

Skeletal muscle fibres are huge multinucleate cells containing large amounts of protein
Connective tissues ensheath muscle fibres and connect fibres to bones
Skeletal muscle is richly supplied in blood vessels and nerve fibres

Question 3

Muscle ______ are gathered into bundles called ______.
_______ are gathered into bundles called _______.
_______, ______ and ________ are each enclosed in __________ ______.
_________ ______ investments are gathered together to form _______ .
________ connect muscles to ________

Answer 3

Fibres, fascicles.
Fascicles, muscles.
Fibres, fascicles and muscles, connective tissue
Connective tissue, tendons
Tendons, bones

Question 4

A ______ _____ is comprised of bundles called ________.
__________ are made of repeating units known as _________.
__________ are made of contractile proteins or ___________.
Two _________ are ____ (thin) and _______ (thick)
The organisation of these ___________ give muscle its ________ appearance

Answer 4

Muscle fibre, myofibrils
Myofibrils, sacromeres
Sacromeres, myofilaments
Myofilaments, actin, myosin
Myofilaments, striated

Question 5

What are Transverse Tubules?

Answer 5

T-tubules are tubular extensions of the surface membranes. Their job is to conduct electrical signals deep into the core of the fibre

Question 6

What is the Sacroplasmic Recticulum?

Answer 6

The SR is an extensive membranous tubular network associated precisely with the T tubules at regular intervals
The terminal chambers of each SR structure associate with the T-tubules to form a triad (membrane triplet)

Question 7

What is SR’s job?

Answer 7

To take up and store calcium, then release calcium ions into cytoplasm along associated T-tubules

Question 8

What is Actin?

Answer 8

Actin is a globular protein. It is a thin filament twisted stand of 2 rows
Each thin filament is a twisted strand of 2 rows of F-actin terminating at one end at the z-line

Question 9

What is Myosin?

Answer 9

Myosin is a thick filament that has a thin tail and globular head
Formed from arrays of pairs of myosin molecules arranged with the tails pointing towards M line and forming complex double headed structure

Question 10

What are muscle contractions?

Answer 10

Muscle contractions are triggered by electrical events (AP) which arise in brain and are conducted out of the CNS along motor axons to muscle fibres

Question 11

What is the Neuromuscular Junction?

Answer 11

The myelinated axon of a motor neuron terminates at a single point on the muscle fibre - the NMJ
Each fibre receives contact from one motor neutron at one site
Arrival of AP initiates synaptic transmission resulting in AP muscle fibres triggering excitation contraction coupling

Question 12

What is the motor unit in NMJ?

Answer 12

A whole muscle is a collection of motor units, big motor units develop lots of force but less control, smaller motor units develop less force but have more control
Cell bodies in the ventral part of spinal cord

Question 13

What is recruitment?

Answer 13

When the amount of motor units activated is changed due to the amount of force produced

Question 14

What is troponins function?

Answer 14

Troponin binds tropomyosin and actin and regulates interaction between actin and myosin during cross bridge cycle

Question 15

What is Excitation-Contraction Coupling?

Answer 15

Term used to describe steps from plasma membrane excitation to calcium release to muscle a contraction

Question 16

Steps of Excitation-Contraction Coupling

Answer 16

1. Action potential triggers an AP in the muscle fibre
2. The AP spreads over the sarcolemmas surface and invades T-tubular system
3. Depolarisation triggers release of Ca2+
4. Ca2+ release into cytoplasm promotes binding of Ca2+ to troponin molecule causing change in troponin shape, exposing actin binding site, myosin binds to actin, filaments slide, causes contraction
5. Calcium pumped from intracellular space back to SR and unbinds from the contractile apparatus causing muscle to relax

Question 17

What are the steps involved in the Cross Bridge Cycle?

Answer 17

1. ADP bound myosin head is cocked and ready to bind to actin
2. In presence of Ca2+, Ca2+ binds to troponin exposing binding sites for myosin
3. The bound myosin rotates its head producing a ‘power stroke’, binds to site (cross bridge formed) - ADP released
4. ATP molecule binds to myosin head (cross bridge broken)
5. Actin and myosin detach - ATP hydrolysed

Question 18

What does muscle tension depend on?

Answer 18

1. The rate at which the muscle is stimulated at
2. The number of muscle fibres recruited

Question 19

How does a twitch in a muscle occur?

Answer 19

Single action potential results in pulse of Ca2+ release into cytoplasm developing a short period of tension

Question 20

How does a tetanus occur?

Answer 20

Multiple action potentials cause rapid release of Ca2+ from SR causing sustained period of actin-myosin interaction and sustained period of contraction

Question 21

Length-Tension relationship in muscles

Answer 21

Each muscle has an optimal length where it will be at its strongest, if it is longer or shorter than that length it will be weaker (less tension)

Question 22

Muscle Structure: Smallest to Biggest

Answer 22

Myofilaments -> Sacromeres -> Myofibril -> Muscle Fibres -> Fasicles -> Muscle

Question 23

What is endomysium, perimysium and epimysium?

Answer 23

Endomysium : surrounds each muscle fibre
Perimysiuem : surround each fascicle
Epimysium : surround entire muscle

Question 24

Sliding filament theory

Answer 24

Contractile proteins develop force by trigger molecular interaction that allows association of the myosin head with the nearby thin actin filament followed by the flexing of the myosin head to allow it to walk along the thin filament
In this process the interlaced thick and thin filaments slide past each other
When activated the ends of the sacromere are brought closer together by flexing of myosin heads (Z lines drawn closer to M line)