Lecture 5 - Muscle 1

Question 1

What are some key properties of muscle

Answer 1

Extensibility – it can be stretched

Elasticity – it will return to its original length

Question 1

How does muscle generate force

Answer 1

Changes in shape and pressure
Changes in length and pulling on levers

Question 2

What is autorhythmicity

Answer 2

Node cells (pacemaker) produce spontaneous action potentials

Question 3

What is the refractory period in cardiac muscle and why

Answer 3

250ms to prevent tetanic contraction

Question 4

What mechanism does cardiac muscle operate

Answer 4

sliding filament

Question 5

Where is smooth muscle located

Answer 5

Surrounding hollow structures

Question 6

Cardiac muscle extras

Answer 6

Electrical coupling between cells

Actin and myosin cross bridges form

Question 7

How is smooth muscle controlled

Answer 7

Amount of calcium in cell - released by autonomic nervous system

Question 8

Smooth muscle extras

Answer 8

Spontaneous action potentials or drifting of polarity in some smooth muscle cells

Change in length changes shape of cells

Change in length changes shape of cells

Question 9

How are skeletal muscles controlled

Answer 9

Motor neurons - reflex and voluntary

Question 10

Skeletal muscle extras

Answer 10

Change in length of skeletal muscle moves skeleton
Actin and myosin form cross-bridges
Sliding filament mechanism

Question 11

How does skeletal muscle create and control movement?

Answer 11

Skeletal muscle generates force to act on the skeleton
Skeletal muscle changes length whilst generating force to move and control the skeleton
Skeletal muscle is voluntarily and involuntarily controlled

Question 12

What is the job of tendons

Answer 12

Attach muscle to bone, containing a aponeurosis

Question 13

What are tendons made of

Answer 13

Primarily collagen

Question 14

What is the architecture of a muscle

Answer 14

the arrangement of the muscle fibres and the tendon

Question 15

How does transmission of muscle force to create force occur

Answer 15

Force is generated within the fibres of the muscle belly (more on how this happens shortly….)

Force is transmitted from the muscle fibres to the connective tissue (aponeurosis) with the muscle belly

The sheets of aponeurosis come together to form the tendons of the muscle
Force is transmitted through these to the skeletal

If the muscle changes length in the contraction this will be translated to the skeleton and the bone (lever) will move

Question 16

How does transmission of muscle force to resist or control occur

Answer 16

This process also happens in reverse

Situations where an external load is applied to a region or the body and muscles resist the effect that force would have

In these situations the tendons will stretch to allow joints to flex and the muscles will generate force to absorb the energy of the impact

There are less extreme examples of this in everyday locomotion
The stretch and recoil of tendon within certain muscle-tendon units is vital for economical locomotion

Question 17

What are the defining characteristics of skeletal muscle cells

Answer 17

1. multinucleated
2. contains many mitochondria
3. Transverse tubules (T tubules)
4. myofibrils and sarcomeres
5. specific terms for some of the intracellular structures:
   Sarcolemma = Plasma membrane
   Sarcoplasm = Cytoplasm
   Sarcoplasmic reticulum = Smooth ER

Question 18

What are the myofibrils and what are they arranged as

Answer 18

Structures that give skeletal and cardiac muscle their characteristic striated appearance.

Actin (thin)
Myosin (thick)

Question 19

Refer to A level notes and sliding filament theory

Question 20

What is actin and what does it do

Answer 20

Contractile protein

Structure is like pearls strung together on a string and then the strands of pearls are twisted together

Each actin has a binding site for myosin

Question 21

What is tropomyosin and what does it do

Answer 21

Regulatory protein

Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state

Question 22

What is Troponin and what does it do

Answer 22

Regulatory protein

Troponin binds Ca2+ reversibly and once bound changes conformation to pull tropomyosin away from the myosin interaction sites

Ca2+ binding to troponin regulates skeletal muscle contraction because it moves the tropomyosin away and allows myosin to interact with the actin.

Question 23

How does sarcoplasmic reticulum aid in propagating a voltage potential

Answer 23

The sarcoplasmic reticulum (SR) in muscle is homologous to the endoplasmic reticulum found in most cells.

Ca2+ is stored and is released following membrane excitation.

The T-tubules and SR are connected with junctions.
These junctions involve two integral membrane proteins, one in the T-tubule membrane, and the other in the membrane of the sarcoplasmic reticulum.

The T-tubule protein is a modified voltage-sensitive Ca2+ channel known as the dihydropyridine (DHP) receptor, which acts as a voltage sensor.

Question 24

Describe the process of Excitation-contraction coupling

Answer 24

Muscle action potential propagated into T Tubules

Ca2+ released from lateral sac of sarcoplasmic reticulum

Ca2+ binding to troponin removes blocking action of tropomyosin

Cross-bridges form between actin and myosin and generate force
Cross-bridge moves through power stroke sliding the actin passed the myosin
ATP causes release of myosin head and its return to original state
If Ca2+ and ATP still present myosin head will attach again to new actin binding site

When action potentials cease Ca2+ taken back into sarcoplasmic reticulum

Ca2+ removal from troponin restores tropomyosin which blocks myosin binding sites on actin

Question 25

What is the only mechanism by which action potentials are initiated in skeletal muscle

Answer 25

Stimulation of nerve fibres

Question 26

Where are motor neurons that innervate skeletal muscle and their cell bodies located

Answer 26

Brainstem or spinal cord

Question 27

How is a charge propagated from neural input to skeletal muscle

Answer 27

Large diameter myelinated axons propagate action potentials at high velocities, allowing signals from the central nervous system to travel to skeletal muscle fibres with minimal delay

Question 28

What is a motor unit

Answer 28

the motor neuron and the skeletal muscle fibres it innervates

(One motor neuron innervates many muscle fibres, but one muscle fibre is innervated by only one motor neuron.

Within a whole muscle there are many motor units.) (slide 30)

Question 29

Describe the process of an action potential reaching a neuromuscular junction

Answer 29

Motor neuron action potential
Ca2+ enters voltage-gated channels
Acetylcholine released
Acetylcholine binding opens ion channels in motor end plate of muscle fibre
Na2+ entry across motor end plate resulting in depolarisation
Current spreads from depolarised end plate to adjacent sarcolemma
Muscle fibre action potential initiated
Action potential propagates along sarcolemma

Question 30

All or nothing response a level notes

Question 31

What do synaptic junctions contain

Answer 31

Neurotransmitter acetylcholine