MNB.5 Muscle Anatomy and Physiology PDF

Musculoskeletal System, Nervous System & Bioelectricity MNB.5 Muscle anatomy and physiology N A M E : D r. To m H o d g k i n s o n 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 struct...

Musculoskeletal System, Nervous System & Bioelectricity MNB.5 Muscle anatomy and physiology N A M E : D r. To m H o d g k i n s o n 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 MNB.5 Muscle anatomy and physiology 2 Muscle Types Muscles are the tissues of movement in the body. Some are under voluntary control and some are involuntary. Three muscle types: Skeletal muscle - Attached to bones Cardiac muscle - Walls of the myocardium Smooth muscle - Organs MNB.5 Muscle anatomy and physiology 3 Skeletal 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. MNB.5 Muscle anatomy and physiology 4 Skeletal Muscle and associated tissues Muscle: tissue with the ability to contract Tendon: tough band of fibrous connective tissue that connects muscle to bone Ligament: Attaches bone to bone MNB.5 Muscle anatomy and physiology 5 Skeletal Muscle A skeletal muscle usually attaches to 2 different bones of the skeleton, i.e. crosses a joint. - one attachment is fixed and called the origin - The other called the insertion is drawn towards the origin With contraction, 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. Any movement is brought about by the coordinated contraction of a number of muscles working together or in groups. Muscles can only contract and never push - Arranged in antagonistic pairs Flexor closes a joint Extensor opens a joint MNB.5 Muscle anatomy and physiology 6 Skeletal Muscle Arm: Biceps Brachii flexes the arm at the elbow Triceps Brachii extends the arm at the elbow This is a simplification: Action is more complex – Not just flexion or extension, and many more muscles involved… MNB.5 Muscle anatomy and physiology 7 Skeletal 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. epimysium MNB.5 Muscle anatomy and physiology 8 Skeletal Muscle Each muscle is composed of bundles of fascicles. Each fascicle is composed of many muscle fibers. Each fiber contains many smaller fibers called myofibrils. The myofibers contain the contractile apparatus – the sarcomere MNB.5 Muscle anatomy and physiology 9 Skeletal Muscle Striated Muscle, also called striped Voluntary muscle i.e. under direct nervous control Low endurance muscle Muscle Biopsy - Wide-bore Needle inserted into muscle, and a small ‘tube’ of muscle removed. Muscle analysed using a microscope and histological stains. Longitudinal section Cross section Hexagonal Nuclei on outside (peripheral) Fibers same size MNB.5 Muscle anatomy and physiology 10 Cardiac Muscle Only found in the heart. A type of striated muscle but also related to smooth muscle. Involuntary muscle. High endurance muscle. Striated, intercalated discs (gap junctions). 1- 2 nuclei per cell, which are centrally placed. Innervated by the pace-maker. MNB.5 Muscle anatomy and physiology 11 Smooth Muscle Lines the digestive tract, trachea/ bronchus and blood vessels Involuntary muscle i.e. not under direct nervous control High endurance muscle 1 centrally placed nucleus per cell No striations Sustained contraction MNB.5 Muscle anatomy and physiology 12 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. 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 MNB.5 Muscle anatomy and physiology 13 Motor Unit Neuromuscular Junction (NMJ), also known as the motor endplate is 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 muscle fibre. MNB.5 Muscle anatomy and physiology 14 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 Motor Units which are stimulated at any time. Maximal contraction is when all Motor Units 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. MNB.5 Muscle anatomy and physiology 15 Types Of Muscle Contraction Muscles contract in two ways: Isometric contraction - ‘same length’ Isotonic contraction - ‘same tension’ Isometric contraction Isotonic Contraction The length of the muscle does not change The muscle length changes. The tension on the muscle increases The tension remains constant Two types of Isotonic contraction: - Concentric – Muscle shortens - Eccentric – Muscle lengthens MNB.5 Muscle anatomy and physiology 16 Types Of Muscle Contraction Concentric Isotonic Contraction Eccentric Isotonic Contraction Force generated by the muscle is Also occur to slow joint movement greater than the load to be lifted Muscles undergoing heavy eccentric loading suffer greater Muscle shortens in length damage when overloaded leading to greater muscle damage. Muscles are approximately 10% stronger during eccentric contractions than during concentric contractions. MNB.5 Muscle anatomy and physiology 17 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 MNB.5 Muscle anatomy and physiology 18 Muscle Physiology Three phases to muscle Stimulus contraction: Latent period (0.005 sec) between the stimulus and first visible reaction tension Contraction period (0.04 sec) when the muscle shortens Relaxation period (0.05 sec) the muscle returns to its original length 0.005s 0.04s Time 0.05s After initial stimulation there is a short Latent Contraction Relaxation period during which muscle will not respond to further stimulus called the Refractory period (0.002 sec) MNB.5 Muscle anatomy and physiology 19 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 MNB.5 Muscle anatomy and physiology 20 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 MNB.5 Muscle anatomy and physiology 21  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. MNB.5 Muscle anatomy and physiology 22 References - Solomon et al, Ch 40. - Alberts et al, Ch 17 p 599 - 604 MNB.5 Muscle anatomy and physiology 23 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 A M E : D r. To m H o d g k i n s o n EMAIL: [email protected] 24