MNB.05 Muscle Anatomy and Physiology PDF

Musculoskeletal System, Nervous System & Bioelectricity MNB.5 Muscle anatomy and physiology P r o f Wa r r e n T h o m a s DATE: 30 Oct 2024 Learning outcomes At the end of this lecture, the learner will be able to Discuss the three main muscle types: cardiac, smooth and striated. Describe t...

Musculoskeletal System, Nervous System & Bioelectricity MNB.5 Muscle anatomy and physiology P r o f Wa r r e n T h o m a s DATE: 30 Oct 2024 Learning outcomes At the end of this lecture, the learner will be able to Discuss the three main muscle types: cardiac, smooth and striated. Describe the structure and arrangement of anatomical muscles Outline the innervation of the motor unit Describe the physiology of whole muscle action including twitch and summation Differentiate between isometric and isotonic contraction MN B.5 Mu scle a n at omy an d p hysi o lo g y 2 Muscle About 40% of the body is skeletal muscle There are approximately 650 skeletal muscles in the human body. There are over 150 surface (anatomical) muscles in the body MN B.5 Mu scle a n at omy an d p hysi o lo g y 3 Muscle Muscle: tissue with the ability to contract Tendon: tough band of fibrous connective tissue that connects muscle to bone Ligament: Attaches bone to bone MN B.5 Mu scle a n at omy an d p hysi o lo g y 4 Muscle Usually attached to 2 different bones of the skeleton - one attachment is fixed and called the origin - The other called the insertion is drawn towards the origin The point of insertion is drawn towards the origin However, as the skeleton is mobile in practice the action is equal at both ends Muscles rarely act alone but tend to work in groups or sets Any movement is brought about by the contraction of a number of muscles working together or in sets Muscles can only contract and never push - Arranged in antagonistic pairs Flexor closes a joint Extensor opens a joint MN B.5 Mu scle a n at omy an d p hysi o lo g y 5 Muscle Arm: Biceps Brachii flexes the elbow Triceps Brachii extends the arm Simplification: Action is complex – not just flexion or extension; many more muscles involved… MN B.5 Mu scle a n at omy an d p hysi o lo g y 6 Muscle Each muscle is composed of millions of muscle fibres In most anatomical muscles the fibres extend the entire length of the muscle Each muscle is bound by a loose connective tissue sheath called an epimysium MN B.5 Mu scle a n at omy an d p hysi o lo g y 7 Muscle Each muscle is composed of bundles of fascicles Each fascicle is composed of many muscle fibers Each fiber is composed of smaller fibers called myofibrils The myofibers contain the contractile apparatus – the sarcomere MN B.5 Mu scle a n at omy an d p hysi o lo g y 8 Muscle Types Three muscle types: Skeletal muscle - Attached to bones Cardiac muscle - Walls of the myocardium Smooth muscle - Organs MN B.5 Mu scle a n at omy an d p hysi o lo g y 9 Muscle Types 1. Striated Muscle Also called striped or skeletal Voluntary muscle i.e. under direct nervous control Low endurance muscle MN B.5 Mu scle a n at omy an d p hysi o lo g y 10 Striated Muscle Muscle Biopsy Wide-bore Needle inserted into muscle, and a small ‘tube’ of muscle removed. Muscle analysed using a microscope and histological stains. MN B.5 Mu scle a n at omy an d p hysi o lo g y 11 Striated Muscle Hexagonal Nuclei on outside (peripheral) Fibers same size MN B.5 Mu scle a n at omy an d p hysi o lo g y 12  Muscle types 2. Cardiac Muscle Only in the heart A type of striated muscle but related to smooth muscle Involuntary muscle High endurance muscle MN B.5 Mu scle a n at omy an d p hysi o lo g y 13  Muscle types 2. Cardiac Muscle: Striated, intercalated discs (gap junctions) 1- 2 nuclei per cell Centrally placed Innervated by the pace-maker MN B.5 Mu scle a n at omy an d p hysi o lo g y 14 Muscle types 3. Smooth Muscle Lines the digestive tract, bronchus and blood vessels Involuntary muscle i.e. not under direct nervous control High endurance muscle MN B.5 Mu scle a n at omy an d p hysi o lo g y 15 Muscle types 3. Smooth Muscle: 1 centrally placed nucleus per cell No striations Sustained contraction MN B.5 Mu scle a n at omy an d p hysi o lo g y 16 Motor Unit The functional unit of skeletal muscle is the motor unit (MU) Composed of: - A single motor neuron - The group of muscle fibres innervated by it As there are considerably more muscle fibres than there are motor neurons some motor neurons must innervate more than one fibre MN B.5 Mu scle a n at omy an d p hysi o lo g y 17 Motor Unit The number of muscle fibres per motor unit varies according to the fineness of the control required. Fewer muscle fibres per motor unit leads to increased dexterity and vice versa e.g. - Larynx muscle 2/3 fibres/MU - Eye muscles 10 fibres/MU - Biceps muscles 1000+/MU - Average for the body is 150 fibres/MU MN B.5 Mu scle a n at omy an d p hysi o lo g y 18 Motor Unit Neuromuscular Junction (NMJ) Also known as the motor endplate The connection between the muscle fibre and its motor neuron The membranes of the nerve and muscle cells come into close contact One NMJ per fibre MN B.5 Mu scle a n at omy an d p hysi o lo g y 19 Motor Unit When muscle fibre is stimulated at the NMJ it contracts; this is an "all-or- nothing" response The degree of contraction of a whole muscle is a function of the number of MUs which are stimulated at any time Maximal contraction is when all MUs are firing together In practice to ensure that contraction is a smooth non-jerky process, asynchronous firing of the motor units gives a graded response MN B.5 Mu scle a n at omy an d p hysi o lo g y 20 Types of Muscle Contraction Muscles contract in two ways: Isometric contraction - ‘same length’ Isotonic contraction - ‘same tension’ MN B.5 Mu scle a n at omy an d p hysi o lo g y 21 Types of Muscle Contraction Isometric contraction The length of the muscle does not change The tension on the muscle increases MN B.5 Mu scle a n at omy an d p hysi o lo g y 22 Types of Muscle Contraction Isotonic Contraction The muscle length changes The tension remains constant Two types of Isotonic contraction: - Concentric – Muscle shortens - Eccentric – Muscle lengthens MN B.5 Mu scle a n at omy an d p hysi o lo g y 23 Types of Muscle Contraction Concentric Isotonic Contraction Force generated by the muscle is greater than the load to be lifted Muscle shortens in length MN B.5 Mu scle a n at omy an d p hysi o lo g y 24 Types of Muscle Contraction Eccentric Isotonic Contraction Also occur to slow joint movement Muscles undergoing heavy eccentric loading suffer greater damage when overloaded leading to greater muscle necrosis Principle behind body building Muscles are approximately 10% stronger during eccentric contractions than during concentric contractions MN B.5 Mu scle a n at omy an d p hysi o lo g y 25 Muscle Physiology Whole muscle function is studied in laboratory preparations e.g. Frog gastrocnemius muscle When the muscle or its nerve is stimulated by a single electric shock it responds with a quick twitch - The twitch lasts about 0.1 sec in the Frog - The twitch lasts in about 0.05 sec humans - Requires specialized equipment to analyse MN B.5 Mu scle a n at omy an d p hysi o lo g y 26 Muscle Physiology Three phases to muscle contraction: Latent period (0.005 sec) between the stimulus and first visible reaction Contraction period (0.04 sec) when the muscle shortens ---within Relaxation period (0.05 sec) the muscle returns to its original length After initial stimulation there is a short period during which muscle will not respond to further stimulus called the Refractory period (0.002 sec) MN B.5 Mu scle a n at omy an d p hysi o lo g y 27 Muscle Physiology Single Twitch Stimulus tension 0.005s 0.04s Time 0.05s Latent Contraction Relaxation MN B.5 Mu scle a n at omy an d p hysi o lo g y 28 Muscle Physiology Muscle can respond to a second stimulation while still contracting Stimulation at frequency shorter than the twitch time results in the second stimulus being superimposed on the first This results in a greater shortening of the muscle called summation MN B.5 Mu scle a n at omy an d p hysi o lo g y 29 Muscle Physiology Summation Summation in whole muscle can occur in two different ways: - By increasing the number of motor units involved - called multiple motor unit summation (recruitment) - By increasing the rate of contraction of individual motor units - called wave summation In practice both these occur together during contraction helped by the asynchronous firing of the motor units MN B.5 Mu scle a n at omy an d p hysi o lo g y 30 Muscle Physiology As the impulse frequency increases the twitches become superimposed upon one another in wave summation (10 pulses/sec) Eventually increasing stimulus causes the successive contractions to fuse together in a state of maximal contraction called tetanus (>40 pulses/sec) Upon tetanisation further increased stimulation will only result in very slight shortening. MN B.5 Mu scle a n at omy an d p hysi o lo g y 31 Muscle Physiology MN B.5 Mu scle a n at omy an d p hysi o lo g y 32 References - Solomon et al, Ch 40. - Alberts et al, Ch 17 p 599 - 604 MN B.5 Mu scle a n at omy an d p hysi o lo g y 33 Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA N T N AM E: P ro f Warr en Tho ma s E MA IL: w ath omas@ rcs i-mub. com 34