L5: Physiology of Muscle Contraction

Question 1

how does troponin work

Answer 1

1) 4 calcium ions bind to troponin C
2) in heart TnC only binds to 3 Ca2+
3) causes troponin to change its conformation
4) Conformational changes in troponin shut off TNI
5) Tropomyosin-troponin leaves F-actin groove and unmasks the myosin binding site on actin
6) Next myosin heads make cross bridges cycling to actin
7) myosin breaks down Atp
8) myosin pulls thin filaments

Question 2

what does the significance of the total TNI marker

Answer 2

Total TnI = marker for total muscle breakdown

Question 3

what does the cardiac TNI signify

Answer 3

Cardiac TnI = marker for myocardial infarct

Question 4

describe cross bridge cycling

Answer 4

contraction depends on the binding of myosin heads to thin filaments at specific sites

-In resting state of sarcomere, myosin heads are blocked from binding to actin by tropomyosin

this occupies the specific binding sites -( in F-actin double helical groovwe)

Question 5

describe the relationship between force generation and sarcomere length (study slide 7)

Answer 5

-Increase in overlap of thick and thin filaments provide an increase in force

Question 6

core memorisation of cross bridge cycling

Answer 6

1) myosin releases actin
2) myosin head cleaves ATP
3) Myosin head binds actin
4) power stroke performed

Question 7

what occurs in rigor mortis

Answer 7

• Atp depleted after death
• muscle cell does not requester ca2+ into SR, so increase in cytosolic ca2+
• Ca2+ allows cross bridge cycle contraction until ATP and creatine P runs out
• w/O ATP- myosin stops just after power stroke, so actin still bound to myosin, and this ends when muscle tissue degrades in 3 days

Question 8

Info on creatine

Answer 8

1- creatine found in muscle fibres

2- Phosphorylated to creatine phosphate

3- this is how energy is stored in the muscle

Question 9

what occurs to creatine phosphate during cross cycling

Answer 9

When cross bridge cycling hydrolyses ATP to ADP + pi, creatine phosphate donates a high energy phosphate to ADP restoring it to ATP

ATP levels must be kept stable - buffering and regeneration

the reaction is catalysed both ways by creatine phosphokinase

Question 10

difference between creatine and creatinine

Answer 10

Creatine is a small molecule that can accept high energy phosphate bonds from ATP

• creatine-phosphate is the above molecule after phosphate has been added to it

-creatinine is a diagnostic marker for kidney function
and is the breakdown product of creatine

Question 11

what is CPK a biomarker for

Answer 11

Muscle destruction

detected by antibodies

Question 12

describe the calcium gradients

Answer 12

there are 2 calcium gradients;
- extracellular vs cytosolic free calcium

• SR vs cytosolic free calcium

It is the efflux of calcium from the SR to the cytoplasm that provides most of the calcium

Question 13

the depolarisation of muscle

Answer 13

1- ACh leads to muscle depolarisation

2- active nicotinic AChR leads to net inward current

3- depolarisation spread via T-tubules

4- local action potentials trigger calcium efflux from terminal cisternae across membrane of sarcoplasmic reticulum into fibre cytoplasm

Question 14

describe E-C coupling

Answer 14

Ryanodine receptor (RyR)

in SR membrane releases ca2+ from SR triggered by voltage sensor on ca2+ channel

SERCA in SR membrane pumps ca2+ back into SR and requires ATP

Question 15

what is the molecular basis of tetany

Answer 15

• single AP leads to calcium release from SR which leads to twitch
• Ca2+ ions are rapidly pumped back into SR leading to the end of twitch
• Frequent Aps lead to insufficient calcium resequestration and summation of contraction

Question 16

study slide 16

Answer 16

how was it

Question 17

what are the 2 main types of muscle fibres

Answer 17

1- slow twitch (type 1 red oxidative, small diam, )

high myoglobin, many mitochondria

2- fast twitch (type 2 white- non- oxidative wide diam lower myoglobin, increases energy from glycolysis

Question 18

what are the differing properties of the fibres

Answer 18

• aerobic vs anaerobic
• faster calcium reuptake
• maximum tension produced

fatigue resistance

Question 19

what are the basis of muscle fibre type

Answer 19

Slow -fast

type 1 2A 2X 2B

Question 20

how are the fibre types distributed -ie

Answer 20

Soleus= 80% type 1 (slow), 20% type 2A

vastus lateralis = mixture of type 1 2A and 2X

Question 21

study slide 20

Answer 21

how was it

Question 22

what are the 3 types of muscle coordination

Answer 22

Motor Units
Recruitment & size principle

Tetany

Fusion of myocytes into long myofibres

Question 23

define motor units

Answer 23

a single alpha motor neuron and all muscle fibres it innervates

functions as a single contractile unit of skeletal muscle

all muscle fibres in a single motor unit are of the same type

Question 24

describe motor units in large muscles

Answer 24

-responsible for large muscle contractions that are powerful and synapse onto 1000 fibres

Question 25

describe a small muscle motor unit

Answer 25

Mediate precision movement- synapse onto 2/3 muscle fibres

Question 26

what does the type and function of the lower motor neuron determine

Answer 26

The muscle fibres

Question 27

what is isometric contractile force generation

Answer 27

generates a variable force while length of muscle remains unchanged

Question 28

isotonic force generation?

Answer 28

generates a constant force while the length of the muscle changes

Question 29

describe the sequence for force generation in muscle fibre

Answer 29

stage 1 : isometric- force increases, joint doesn’t move, as muscle force - force increases -ie biceps and brachioradialis

stage 2: isotonic : force remains the same, arm moves, glass moves upward in response to force- overcoming gravitational and inertia forces keeping glass on table

glass starts to rise as muscles shorten and the elbow bends and force generated by the muscle is constant as glass moves

Question 30

what are the types of muscular force generation

Answer 30

Concentric- force during contraction - tossing a ball into air

eccentric: force during muscle elongation - catching a ball

Question 31

what controls force generation

Answer 31

proprioception

Question 32

what is the recruitment size principle

Answer 32

As the initial contraction occurs, more and more motor units are recruited starting with smaller ones and progressively adding larger ones

allows fine graduation of force for small movements

until it becomes contraction=isotonic

Question 33

what is the difference between upper and lower MND

Answer 33

LMND :

• weakness
• muscle atrophy

UPMND:
spasticity and hypertonia

Question 34

describe and give an example of a stretch reflex

Answer 34

Patellar reflex which functions for posture and balance

it controls muscle length and increases muscle force

Question 35

what fibres are in sensory organs

Answer 35

intrafusal fibres

Question 36

what fibres ae in traction organs

Answer 36

Extrafusal fibres

Question 37

what does a lack of patellars reflex cause

Answer 37

westphals sign

Question 38

describe spindle

Answer 38

A spindle consists of 3-12 intrafusal fibres
Gamma motor neurons increase sensitivity
Drive contraction of edge of intrafusal fibres
Sensors from muscle spindle are:
Called Type 1a and Type 2
Wrap around the intrafusal fibres
Detect stretch (ie length) of central non-contracting region using stretch receptors
Spindle is like a thermostat that regulates the relationship between muscle length and muscle contractility
ie the relationship between neural drive and force generation

Question 39

what does westphels sign mean clinically

Answer 39

Receptor damage
Femoral nerve damage
Peripheral nerve disease
e.g. Peripheral Neuropathy

In upper motor neuron disease
Can lead to hypertonia and spasticity
UMN inhibits normal descending inhibitory input to spinal interneurons
The spindle reflex becomes over-sensitive
Can attempt to contract muscle all the time

Question 40

what is the tendon reflex

Answer 40

Decreases Muscle Force  dropping the load
Sensor firing -  less- contraction

Sensor to Spinal Cord

Interneuron to motor neuron

Motor neuron inhibited

Motor neuron to muscle

Question 41

give examples of a tendon reflex

Answer 41

Sensor = Golgi Tendon Organ
Detects Tension
In series with muscle
In tendon
Near border with muscle

Disynaptic
Ipsilateral Spinal reflex

Question 42

give an example of patellar stretch reflex

Answer 42

Detects Stretch
i.e. Length
Proprioception

Spindle is Parallel to other muscle fibres

Ipsilateral Spinal reflex

Monosynaptic