Muscle Physiology-Skeletal Muscle:Mechanics, control of force of contraction, energy sources Flashcards
Describe Skeletal Muscle Mechanics and organiation
- Whole muscles are groups of muscle fibers bundled together and attached to bones
Tendons attach the muscle to bones
- Muscle tension is transmitted to bone as the contractile component tightens the series-elastic component
Tension is produced internally within the sarcomeres which is the contractile component of the muscle
Explain Origin & Insertion of Muscles using the bicepts brachii and tricepts brachii as examples
- The function of skeletal muscles is often indicated by their origin and insertion.
- In the example below, the biceps brachii has its origin on the scapula and its insertion on the radius.
- The fulcrum or pivot is the elbow joint.
- As the force is exerted on the same side of the fulcrum as the load this arrangement is known as a third class lever.
- The triceps brachii has its insertion on the ulna, on the opposite side of the fulcrum to the load, which acts as a first class lever.
From the diagram:
Bicepts brachi contract to flex the elbow joint
Tricepts brachii contract to extend the elbow joint
Biomechanics - Levers: Give examples of
- First class levers
- Second class levers
- Third class levers
- An example of a first class lever would be the muscles in the posterior region of the neck attached to the skull acting on the joint. The load and the force are on opposite sides of the fulcrum (like a see-saw). The fulcrum is fairly central at the base of the skull.
- In a second class lever the load and force are on the same side of the fulcrum, with the load between the force and the fulcrum (like a wheelbarrow).
- In the example shown the gastrocnemius muscle exerts a force on the calcaneus and flexes the foot.
- Most muscles in the body are involved in third class levers, which are the least efficient. Here the force and load are on the same side of the fulcrum, with the force applied between the load and the fulcrum (as in a fishing rod).
- This area of biomechanics is important in sports medicine.
What are the 2 main types of Muscle Contraction?
What type of contraction is associated with DOMS?
Isotonic contraction (constant tension, variable muscle length) - the muscle changes length and moves the load
(the muscle changes length while exerting a constant force)
- Concentric – muscle shortens e.g. lifting a load with the arm. As your muscle shortens, it generates enough force to move an object e.g. bicep curl
- Eccentric - the muscle exerts a force while being extended e.g. walking downhill or lowering a barbell in a biceps curl
N.B. can cause delayed-onset muscle soreness (DOMS). This is soreness in the muscle which usually develops between 1 to 3 days after the exercise, and the cause of it is unknown.
- Isokinetic contraction (i.e., constant motion): velocity remains constant as the muscle fibers shorten e.g. and breaststroke in swimming, where the water provides a constant, even resistance to the movement of adduction
Builds up muscle stregth quickly-used in training. Doesnt usually naturally occur in the body
- Isometric contraction (constant length of the muscle, variable tension) the muscle is exerting a force while maintaining a constant length. This type of contraction produces a disproportionate increase in heart rate and mean arterial pressure for the muscle mass involved. This is probably due to reduced blood flow through the contracted muscle causing the building up of metabolites which stimulate nerve endings in the muscle. However, the exact mechanism is still not knowne.g. hand grip
Explain the factors in Contraction and Stimulation
- The number of fibers contracting within a muscle depends on the extent of motor unit recruitment
stronger contractions = more motor units are recruited
- Frequency of stimulation can influence the tension developed by each muscle fiber
Frequency of stimulation, fiber length at contraction onset, extent of fatigue, and fiber thickness
What are Graded Contractions?
- Contractions of a whole muscle can be of varying strength
- Two primary factors can be adjusted to produce graded contractions:
Number of muscle fibres contracting
Tension developed by each contracting fibre
Explain Twitch Summation
- Twitch summation and tetanus
Twitch summation: the increase in tension accompanying repetitive stimulation of a muscle fiber
Tetanus: a smooth, sustained contraction of maximal strength
- Twitch summation results primarily from a sustained elevation in cytosolic Ca2+
Additional factor: more time to stretch the series-elastic component
A motor unit is a motor neurone and the muscle fibres it innervates
What happens if motor neurones dies?
- Skeletal muscle is organised into motor units. A single motor unit is a motor neurone and the muscle fibres that it innervates. One motor neurone can innervate between 2 and 2,000 muscle fibres, depending on the degree of fine control required.
- In some degenerative diseases, such as motor neurone disease, the motor neurones die and the motor units are gradually lost , causing paralysis.
Communication between Neurones and Muscle‘Crosstalk’
Give examples
‘Crosstalk’
- Signalling molecules communicate between the nerve and muscle
- Atrophy of nerve or muscle can lead to atrophy of the corresponding neurone or muscle
►Neurotrophins: Neurotrophin-3 (very important in the NMJ)
►Cytokines: Cardiotrophin-1
►Insulin-like growth factors: IGF 1
Explain research on the Communication between Neurones and Muscle
- Research has shown that as well as neurones stimulating muscle fibres there is also communication between neurones and muscle fibres.
- This involves signalling molecules and is known as crosstalk. This is now known to be very important.
- Three examples of the signalling molecules involved are the neurotrophins, cytokines and insulin-like growth factors.
- Neurotrophin-3 is now known to be very important in the neuromuscular junction.
- Regular communication between neurones and muscle fibres is vital and if it ceases then atrophy will occur.
- Research is progressing on using some of these agents to treat degenerative diseases such as motor neurone disease
What does Temporal summation of muscle fibres result in?
- Increased frequency of action potentials to muscle fibres causes
- summation:
Temporal summation can also occur, where an increase in the frequency of action potentials causes summation of contractions. When a high enough frequency is reached then a fused tetanus occurs, which is a continuous contraction of the muscle.
•tetanus (Clostridium tetani):
The disease ‘Tetanus’ is caused by a toxin from the bacterium Clostridium tetani. The toxin blocks inhibitory motor feedback control which leads to unopposed muscle contraction
Increase rate of firing of AP to generate a larger force from the muscle
What is the importance of the process Spatial summation (recruitment)?
- More motor neurons activated, hence more muscle fibres recruited to develop more force
- Reflex pathways from muscle spindles, joint receptors, Golgi tendon organs (a proprioceptive sensory receptor organ that senses changes in muscle tension. It lies at the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle) have a role in recruitment
In order to increase the force generated by a muscle more motor units can be activated , a process known as recruitment. This is termed spatial summation.
What is Electromyography (EMG)?
- The electrical activity in muscle can be measured by a method called electromyography (EMG).
- Electrodes can be placed on the skin superficial to a muscle or needle electrodes can be inserted into a target muscle.
- The recordings made are effectively compound action potentials within the muscle.
- As motor unit recruitment increases in response to increasing load, so the amplitude of the recording increases.
- This technique can be useful in diagnosis in neurology e.g. MND (motor neurone disease).
Feedback - Muscle Receptors
- Muscle receptors provide afferent information needed to control skeletal muscle activity
For effective control of motor output, the CNS needs continual information regarding ongoing changes in muscle length and tension
- Muscle spindles monitor muscle length
- Golgi tendon organs detect changes in tension
Describe muscle Spindles and the Stretch Reflex
- Muscle spindle structure:
Distributed throughout the fleshy part of a skeletal muscle
Consist of collections of specialized muscle fibers known as intrafusal fibers
- Stretch reflex:
Local negative-feedback mechanism to sense and resist changes in muscle length when an additional load is applied
Explain Proprioception
- Proprioception literally means awareness of self. Proprioceptors are specialised nerve endings which enable the brain to know the position of parts of the body in three dimensional space.
- In skeletal muscle there are specialised fibres known as muscle spindles which sense the degree of stretch and tension in the muscle.
- Muscle spindles give rise to some reflexes in the body which can be tested.
- Feedback control of movement
From the diagram:
Intrafusal fiber= spindles
Diagram of the muscle spindles in muscle
From the image:
Muscle receptors.
- (a) A muscle spindle consists of a collection of specialized intrafusal fibers that lie within a connective tissue capsule parallel to the ordinary extrafusal skeletal muscle fibers. The muscle spindle is innervated by its own gamma motor neuron and is supplied by two types of afferent sensory terminals, the primary (annulospiral) endings and the secondary (flower-spray) endings, both of which are activated by stretch.
- (b) The Golgi tendon organ is en-twined with the collagen fibers in a tendon and monitors changes in muscle ten-sion transmitted to the tendon.
Sources of Energy for Contraction: Phosphorylation of ADP by Creatine Phosphate
- Skeletal muscle uses creatine phosphate as a high energy storage molecule.
- The enzyme creatine kinase (CK) transfers phosphate groups from creatine phosphate to ADP to produce ATP. The store of creatine phosphate would provide energy for muscle contraction for about 15 seconds (estimates vary) , enough for 100 m sprint.
- If muscle is damaged then CK can leak into the blood and can be detected in the plasma by an assay (Serum CK - marker for muscle damage, amount of CK in blood proportional to the amount of muscle damge). This was widely used, but is largely replaced now by the detection of troponin in plasma
Sources of Energy for Contraction: Anaerobic Glycolysis and Lactate Formation
Under anaerobic conditions, skeletal muscle can use the glycolytic pathway to produce ATP: Glycolysis of blood glucose or glucose-6-phosphate from muscle glycogen
This can occur in the first 20 to 40 seconds of a bout of muscle activity.
The production of lactate leads to a build up of this metabolite locally which can stimulate nerve endings casing pain (muscle cramp).
Sources of Energy for Contraction: prolonged aerobic muscular events
- In prolonged aerobic exercise skeletal muscle generates ATP via oxidative phosphorylation.
- This depends on an adequate blood supply providing glucose and oxygen, and removing CO2.
Explain Muscle Fatigue
From the diagram:
5-HT= Serotonin
- Muscle fatigue is still not well understood and is the subject of research.
- Exercise may be terminated due to muscle fatigue (peripheral fatigue)-depletion of glucose or by mechanisms in the brain (central fatigue).
- There are many theories on muscle fatigue, one of which is that depletion of muscle glycogen stores is involved, if they cannot be replenished.
- This can occur fairly rapidly (within 1 minute) If the muscle is working and blood flow is interrupted.
- This is thought to occur in intermittent claudication which is the name given to pain in a leg muscle caused by exercise and which can be relieved by rest (usually due to narrowing of femoral artery in the leg).
- It is caused by ischaemia, often resulting from atherosclerosis.
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