Lecture 6 - Muscle 2 Flashcards
What are the 3 phrases for a twitch contraction
Latent period
Contraction phase
Relaxation phase
What is the latent period
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
What is the contraction phase
This is the time that tension is developing due to the cross-bridge cycling.
What is the relaxation phase
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
What is a tetanic contraction
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
What determines how much force a muscle produces in a tetanic contraction
Length of the muscle fibres (sarcomeres) at that point in time
Contraction velocity (how quickly cross bridges are having to cycle)
How is muscle force dependent on the length of the muscle fibre
The force-length relationship - perfect overlap of cross-bridges (slide 8)
Determines how many binding sites there are for myosin
how does velocity affect muscle force
More force can be generated at quicker speed until Vmax is achieved
What determines how much force a muscle produces
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
What is a motor unit
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)
How are skeletal muscle fibres differentiated
Their maximal velocities of shortening (slow, fast or very fast)
The major pathway they use to form ATP—oxidative or glycolytic
How do Myosin isoforms work - Classifying skeletal muscle
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
What are the 3 pathways in which ATP can be synthesised in a muscle fibre cell
Phosphorylation of ADP by creatine phosphate
Oxidation phosphorylation of ADP in mitochondria (Aerobic)
Glycolytic phosphorylation of ADP in the absence of oxygen (Anaerobic)
What are oxidative fibres
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
What are glycolytic fibres
few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen
What are the three principal types of skeletal muscle fibres
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)
How do we measure Muscle activation
Electromyography (EMG)
What is a concentric of a muscle movement
Contraction/shortening of the muscle
What is the eccentric of a muscle movement
Lengthening of a muscle
What is an isometric of muscle movement
Muscle remains the same length
Where are the higher centres of motor control located
The brain
Describe motor control
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
What are proprioceptors
Sensory receptors within muscle and joints
What are in muscle proprioceptors
Muscle spindles – provide feedback on muscle length
Golgi tendon organs – provide feedback on muscle force
Responsible for reflex actions of muscle
Describe the stretch reflex
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
What is the key feature of involuntary movement
REFLEXES are unconscious or automatic movements
Often protective
Describe the withdrawal reflex
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
What is Exercise Associated Muscle Cramp
Painful, spasmodic and involuntary contraction of skeletal muscle that occurs during or immediately after exercise
occurs in Working muscle groups
What are the two hypotheses to exercise associated muscle cramp
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
What is Delayed onset muscle soreness
Microdamage to muscle which results in minor inflammation and pain, but is normal and important for muscle adaptation
What is the main cause of Delayed Onset Muscle soreness
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
What is Muscular Dystrophy
Muscle wasting – weakening and breakdown of muscle over time
Describe the genetics behind Duchennes muscular dystrophy
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
What are some Neuromuscular junction or central nervous system disorders
Toxins (Botunlinum toxin)
Autoimmune conditions (eg Myasthenia gravis)
Multiple sclerosis
Cerebral palsy
Motor neuron disease
Read Chapter 9 in Vanders
