Lecture 6 - Muscle 2

Question 1

What are the 3 phrases for a twitch contraction

Answer 1

Latent period
Contraction phase
Relaxation phase

Question 2

What is the latent period

Answer 2

This is the period of time from the action potential to the onset of contraction. The time delay is due to the excitation-contraction coupling

Question 3

What is the contraction phase

Answer 3

This is the time that tension is developing due to the cross-bridge cycling.

Question 4

What is the relaxation phase

Answer 4

This is the time that the tension is decreasing (i.e., relaxing) and is longer than the contraction phase. This is due to the amount of time it takes to get all the Ca2+ sequestered

Question 5

What is a tetanic contraction

Answer 5

In real life activities muscles rarely receive a stimulation which elicits a single action potential
Multiple action potentials result in a summation of tension (force)
If the frequency of the stimulation is high a tetanic contraction occurs where force remains constant (ish) for the period of activation
This is typical of muscle force in functional activities

Question 6

What determines how much force a muscle produces in a tetanic contraction

Answer 6

Length of the muscle fibres (sarcomeres) at that point in time

Contraction velocity (how quickly cross bridges are having to cycle)

Question 7

How is muscle force dependent on the length of the muscle fibre

Answer 7

The force-length relationship - perfect overlap of cross-bridges (slide 8)

Determines how many binding sites there are for myosin

Question 8

how does velocity affect muscle force

Answer 8

More force can be generated at quicker speed until Vmax is achieved

Question 9

What determines how much force a muscle produces

Answer 9

Individual muscle fibres are not operating in isolation
There are different types of motor units and how many of these different types that are activated also influences how much for the muscle produces
Therefore how much force a muscle produces is also influenced by (alongside muscle length and velocity):
Activation level of the muscle
Time since onset of activation

Question 10

What is a motor unit

Answer 10

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)

Question 11

How are skeletal muscle fibres differentiated

Answer 11

Their maximal velocities of shortening (slow, fast or very fast)

The major pathway they use to form ATP—oxidative or glycolytic

Question 12

How do Myosin isoforms work - Classifying skeletal muscle

Answer 12

Slow and fast fibres contain forms of myosin that differ in the maximal rates at which they use ATP

Myosin Heavy Chain I
Myosin Heavy Chain IIa
Myosin Heavy Chain IIx
Myosin Heavy Chain IIb (not expressed in human skeletal muscle) (SLIDE 14)!!!!!!!

This determines the maximal rate of cross-bridge cycling and thus the maximal shortening velocity

Question 13

What are the 3 pathways in which ATP can be synthesised in a muscle fibre cell

Answer 13

Phosphorylation of ADP by creatine phosphate
Oxidation phosphorylation of ADP in mitochondria (Aerobic)
Glycolytic phosphorylation of ADP in the absence of oxygen (Anaerobic)

Question 14

What are oxidative fibres

Answer 14

Some fibers contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation

Most of the ATP such fibres produce is dependent upon blood flow to deliver oxygen and fuel molecules

Question 15

What are glycolytic fibres

Answer 15

few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen

Question 16

What are the three principal types of skeletal muscle fibres

Answer 16

Slow-oxidative fibres (Type I) combine low myosin-ATPase activity with high oxidative capacity.
Fast-oxidative-glycolytic fibres (Type IIa) combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.
Fast-glycolytic fibres (Type IIx) combine high myosin-ATPase activity with high glycolytic capacity

(Note that the fourth theoretical possibility—slow-glycolytic fibers—is not found)

Question 17

How do we measure Muscle activation

Answer 17

Electromyography (EMG)

Question 18

What is a concentric of a muscle movement

Answer 18

Contraction/shortening of the muscle

Question 19

What is the eccentric of a muscle movement

Answer 19

Lengthening of a muscle

Question 20

What is an isometric of muscle movement

Answer 20

Muscle remains the same length

Question 21

Where are the higher centres of motor control located

Answer 21

The brain

Question 22

Describe motor control

Answer 22

Voluntary movement is initiated by the higher centres in the brain
These signals are then relayed to the middle areas which co-ordinate the movement
And onto the MOTOR NEURONS which activate the MUSCLES (efferent signals)
SENSORY FEEDBACK comes back from SENSORY RECEPTORS in the muscles and joints (afferent signals) to modulate the movements

Question 23

What are proprioceptors

Answer 23

Sensory receptors within muscle and joints

Question 24

What are in muscle proprioceptors

Answer 24

Muscle spindles – provide feedback on muscle length
Golgi tendon organs – provide feedback on muscle force

Responsible for reflex actions of muscle

Question 25

Describe the stretch reflex

Answer 25

When muscle is stretched afferent signal from muscle spindle to spinal cord
Synapes with motor neuron of stretched muscle which causes it to contract to resist the stretch
Also synapses with inhibitory interneuron which inhibits motor neurons to the flexor muscle
Afferent signal from muscle spindle also goes to higher centres (brain) so movement becomes ‘conscious’
PROTECTS MUSCLE FROM TOO MUCH STRETCH

Question 26

What is the key feature of involuntary movement

Answer 26

REFLEXES are unconscious or automatic movements
Often protective

Question 27

Describe the withdrawal reflex

Answer 27

Painful stimulus
Pain detected by nociceptor and signal sent to spinal cord
Synapses with motor neuron of flexor muscle to flex knee and withdraw foot
Inhibitory interneuron results in inhibition of ipsilateral extensor muscle to facilitate this movement
Reflex also crosses to contralateral side to increase weight support on that side
Excitatory interneuron to contralateral extensor
Inhibitory interneuron to contralateral flexor
Afferent signal from nociceptor also goes to higher centres (brain) and pain becomes conscious

Question 28

What is Exercise Associated Muscle Cramp

Answer 28

Painful, spasmodic and involuntary contraction of skeletal muscle that occurs during or immediately after exercise

occurs in Working muscle groups

Question 29

What are the two hypotheses to exercise associated muscle cramp

Answer 29

Electrolyte depletion and dehydration:
Change in sodium potassium, magnesium or calcium concentration in plasma
No prospective studies to support this theory

Altered neuromuscular control:
Altered reflex control due to fatigue
Excitatory input overwhelms inhibitory input

Question 30

What is Delayed onset muscle soreness

Answer 30

Microdamage to muscle which results in minor inflammation and pain, but is normal and important for muscle adaptation

Question 31

What is the main cause of Delayed Onset Muscle soreness

Answer 31

Occurs as a result of overload of muscle
Particularly due to eccentric exercise
Results in up regulation of protein synthesis and adaptation of muscle to new load

Question 32

What is Muscular Dystrophy

Answer 32

Muscle wasting – weakening and breakdown of muscle over time

Question 33

Describe the genetics behind Duchennes muscular dystrophy

Answer 33

Genetic condition caused by mutation in the gene for the protein dystrophin on X chromosome
Dystrophin important in linking myofibrils to sarcolemma
Lack of dystrophin results in muscle fibre disorganisation and death
Affects muscles of pelvis and lower limb first progressing to the upper limbs and respiratory muscles
Results in premature death

Question 34

What are some Neuromuscular junction or central nervous system disorders

Answer 34

Toxins (Botunlinum toxin)
Autoimmune conditions (eg Myasthenia gravis)
Multiple sclerosis
Cerebral palsy
Motor neuron disease

Question 35

Read Chapter 9 in Vanders