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Course: PG Pathshala-Biophysics Paper 13: Physiological biophysics Module 04: Force, mechanics and muscle-metabolism
Principal
Investigator:
Dr. Moganty R. Rajeswari, Professor
AIIMS, New Delhi
Co-Principal
Investigator:
Dr. T. P. Singh, Professor
AIIMS, New Delhi
Paper Coordinator: Dr. K. K. Deepak, Prof. & HOD, Physiology
AIIMS, New Delhi
Content Writer: Dr. Bhawna Mattoo, Senior resident
AIIMS, New Delhi
Content Reviewer: Dr. Renuka Sharma, Professor
VMMC & SJH, New Delhi
Quadrant I:
Learning Objectives
- To understand mechanical model of muscle
- To define a motor unit
- To know the types of muscle fibers
- To differentiate the types of muscle contraction
- To understand muscle metabolism
Overview
- Introduction
- Mechanical model of muscle
- Motor unit
- Fast twitch and slow twitch fibers
- Types of muscle contractions
- Isometric contraction
- Isotonic contraction
- Pre-loading and after loading
- Length tension relationship in isometric contraction
- Force velocity relationship in isotonic contraction
- Factors controlling force generation by muscle
- Twitch force and tetanic force
- Recruitment of motor units
- Muscle metabolism
- Sources of energy substrates and temporal availability
- Aerobic vs Anaerobic metabolism
- Summary
1. Introduction Force is a push or a pull on an object causing its displacement while mechanics is concerned with behaviour of physical bodies when they are subjected to force. In the human body muscle is the basis of generating any force and the skeletal muscles controls the body’s voluntary movements. It is thus imperative to understand the force generation by muscular system and the mechanics behind it by using simple concepts of elasticity, length, velocity and force. We will also attempt to understand the fuel required and effect of different energy substrates on actual work done. Learning of such concepts has important applications especially in the fields of sports and exercise physiology. 2. Mechanical model of the skeletal muscle
Mechanics of skeletal muscle contraction is better understood using a 3-component model (Fig 1) consisting of 1) contractile element 2) series elastic element and 3) parallel elastic element. Contractile elements consist of the thick and thin filaments that interact through cross bridge cycling and generate force or active tension. Elastic elements contribute to the development of passive tension in the muscle when they are stretched. Elastic elements are classified into two types based on their pattern of arrangement with respect to the contractile elements. o Parallel elastic elements consist of sarcolemmal and connective tissue components disposed parallelly along the muscle fibers o Series elastic elements consists of tendons that are serially connected to the contractile elements of the muscle fiber
3. Motor Unit Every muscle is supplied by a motor nerve that electrically excites the muscle fibers to produce contraction. A motor nerve consists of a bundle of motor nerve fibers which are the axons of the alpha motor neurones located in the anterior columns of spinal cord. Every axon when it reaches close to the zone of innervation divides and forms multiple axonal branches that form terminal boutons and synapse with the muscle fibers. An alpha motor neuron along with the set of muscle fibers innervated by its axon is collectively called a motor unit. The fibers constituting a specific motor unit could be spatially scattered with in the muscle belly. With every action potential fired by the alpha motor neuron, the muscle fibers forming the corresponding motor unit contract in an ‘All or None’ fashion.
Fig1 : Shows the mechanical model of skeletal muscle depicting the arrangement of the three different components
Pre-loaded contractions are in which the load is already acting on the muscle before it starts to contract, where as in after-loaded contractions , the load starts acting on the muscle only after it shortens by a finite length.
6. Length tension relationship in isometric contraction
The relationship between initial length of the muscle and the peak tension it develops during isometric contraction can be studied by fixing the two ends of the muscle and stretching it in a graded manner to alter the length. The relationship between the two variables is as shown in Fig 4. The tension exerted by the muscle when it is in the relaxed state is called passive tension. Passive tension is contributed primarily by the series and parallel elastic elements in the muscle when they are stretched. The additional tension developed by the muscle when it contracts due to acto-myosin cross bridge cycling
Fig 3 : Shows the changes in Tension/Length (change in length when the muscle shortens during isotonic contraction is plotted instead of absolute length) of the muscle as a function of time when it contracts isometrically or isotonically.
is called the active tension. Total tension refers to the sum of the active and passive tensions exerted by the muscle.
When muscle is detached from its bony attachments, it attains a shorter length known as the resting length (L). Stretching of the muscle in a graded manner initially leads to an increase in the active tension generated which peaks at around 1.2 times the resting length of the muscle (1.2). Further stretching of the muscle leads to a reduction in active tension and an exponential rise in the passive tension exerted by the elastic elements. The molecular basis of the peaking of active tension at 1.2L has been attributed to attainment of maximal acto-myosin interactions because of optimal overlap and increased proximity due to compression of myofibril lattice because of stretching.
7. Force velocity relationship in isotonic contraction
The mechanical performance of the muscle contracting isotonically is assessed by studying the relationship between the load or force of contraction (as load equals the force of contraction) and the velocity of shortening and expressed as the force velocity relationship. In isotonic contraction, force of contraction is found to be inversely related to the velocity of shortening as shown in Fig 5. Maximal velocity of shortening (Vmax) is attained when the muscle contracts against zero load or in an externally unloaded state. As the load increases the velocity of shortening decreases and becomes zero against a load (Fmax) that cannot be moved when the contraction becomes completely isometric.
Fig 4 : Shows the relationship between total, active and passive tensions and the length of the muscle. With the graded increase in length active tension rises and peaks at the optimal length (OL) and then decreases with further stretching of the muscle. Passive tension rises once the muscle is stretched beyond its resting length (L) and increases exponentially when the muscle is stretched further.
9. Muscle metabolism
Contraction of the muscle requires supply for acto-myosin cross bridge cycling, pumping back the calcium released with every excitation back to the sarcoplasmic reticulum and maintenance of the ionic gradients across the cell membrane. The rate of energy requirement is heavily dependent on the power output or rate of work done by the muscle.
9.1 Sources of energy substrates and temporal availability
Contracting muscle derives energy from the sources listed below:
Stored ATP – can meet the energy requirements for 1 second Creatine Phosphate – Creatine phosphate combines with ADP and rephosphorylates it to ATP. Creatine phosphate meet the energy requirements of a maximally contracting muscle for 3 seconds. Glucose derived from stored muscle glycogen – Stored muscle glycogen can provide glucose through glycogenolysis which can meet the energy requirements of the muscle for minutes. Plasma Glucose and body fats – Sustenance of the contractile activity of muscle beyond a couple of minutes will require continuous supply of glucose. In the post- absorptive state, plasma glucose is maintained primarily through liver glycogenolysis. With increase in duration of the muscle activity, free fatty acids mobilised from the adipose tissues undergo oxidative metabolism to supply ATP for muscle contractions. 9.2 Aerobic vs Anaerobic metabolism
Glucose can be utilised to generate ATP by exclusively anaerobic pathways or by both aerobic and anaerobic pathways. Anaerobic glycolysis drives the energy production especially when the oxygen supply to the muscle cannot catch up with the rate of energy requirement. However, the number of molecules of ATP generated per molecule of glucose metabolised anaerobically is significantly low
Fig 6 : Shows the contractile response of an isolated nerve muscle to single and multiple stimuli. 6A shows the contractile response to a brief electrical stimulus applied to the (simple muscle twitch) and the force generated by the muscle is referred to as twitch force. If the same preparation is stimulated repeatedly with a train of stimuli with inter-stimulus interval longer than the refractory period, we may observe partial (6B) or complete fusion of the contractions (6C) depending on the frequency of stimulation leading to smooth sustenance of tension generated by the muscle as shown in Fig (6A & B).
when compared to aerobic pathways. Exclusive usage of anaerobic pathways can sustain the energy requirements of heavy bouts of exertions lasting only for seconds as in 100m sprint.
Muscle fibers differ in their ability to derive energy through anaerobic and aerobic pathways. Slow twitch fibers have greater aerobic capacity than fast twitch fibers because of greater abundance of mitochondria, higher capillary density and myoglobin levels. Fast twitch fibers have higher glycolytic capacity than slow twitch fibers.
10. Summary 3-component model of muscle consists of 1) contractile element 2) series elastic element and 3) parallel elastic element. An alpha motor neuron along with the set of muscle fibers innervated by its axon is collectively called a motor unit. Muscle fibers that develop peak tension in a very short duration of time falling less than 10 milliseconds are classified as fast twitch fibers while the ones taking 40 – 100 milliseconds are labelled as slow twitch fibers. Isometric, the length of muscle remains constant while the tension developed changes. In Isotonic contraction, the tension developed by the muscle remains constant and equivalent to the load applied while the muscle shortens during the phase of contraction and lengthens back to its original length during the phase of relaxation. Force generated by the muscle in vivo is determined primarily by 2 factors: 1) Intrinsic contractile properties of the muscle fibers 2) Pattern of activation of motor units in the muscle by the nervous system. The physiological mechanism by which gradation of force of muscle contraction is ensured in vivo is through a phenomenon known as motor unit recruitment. Contracting muscle derives energy from stored ATP, creatine phosphate, glucose derived from stored muscle glycogen and plasma Glucose and body fats. Glucose can be utilised to generate ATP by exclusively anaerobic pathways or by both aerobic and anaerobic pathways.
