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SKELETAL MUSCLE HUMAN FUNCTIONAL ANATOMY 213 Whole muscles Uniarticular muscles and muscle action Include connective tissue elements such as tendons Surrounded by epimysium = deep Objectives Muscle fascicles 1. To understand the muscles mechanism contraction The grain you can 2. To relate that to the function and structure of muscles see in meat 3. To understand types of muscle contraction Surrounded by 4. supply of muscles perimysium

In this lecture: Muscle fibres Are the multi- Muscle nucleated cells of Length – tension relationship muscle Architecture Surrounded by Nerve supply of muscles endomysium Uniarticular muscles Types of muscle action Muscle fibres are Soleus muscle structure and function composed of myfibrils Readings 1. Introductory section on muscle in any anatomy text Myofibrils are 2. Sensory receptors in the neurology section of Gray’s composed of myofilaments anatomy. 3. Carpenter’s “Human Neuroanatomy. Receptors and effectors. 4. Grants method of anatomy – section on the shoulder and The myofilaments arm. of muscle are the actin and myosine molecules which together make the functional unit of muscle => THE SARCOMERE

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THE SARCOMERE MUSCLE ARCHITECTURE Length – Tension relationship Length – Tension relationship Z disc H zone Muscles are composed of sarcomeres, so the same properties apply: Actin (thin filaments) A muscle fibre, or fascicle can shorten by about 40% Myosin (thick filaments) Whole muscles’ properties depend on the arrangement of muscle fibres Crossbridges on thin filaments Bond with thick filaments EXAMPLES USING 10 SARCOMERES Generate force which shortens sarcomere (muscle) 1. The strap muscle – many sarcomeres in series (end-to-end) Can shorten a lot, but is not very strong As the sarcomere changes length the number of effective crossbridges changes = LENGTH v TENSION relationship

2. A short, broad muscle has many sarcomeres in Tension parallel (side-by-side) (force) It can only shorten a little, but is much stronger (It is also very bulky and needs large attachments)

1.0 1.5 2.0 2.5 3.0 3.5 4.0 3. A bipennate muscle has fibres arranged around a tendon Length (microns) It can still only shorten a little but is strong, and compact

The sarcomere has an effective contraction of about 40% of its resting Most muscles length attach near joints and so do not need to shorten much But they do need to be strong.

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DELTOID MUSCLE 3 TYPES OF MUSCLE ACTION

The Deltoid muscle has three parts. 1. CONCENTRIC Each has a different action. The muscle develops sufficient tension to overcome the resistance and shorten, thus moving a part of the body. 1. The middle part of deltoid is the only muscle that can abduct the arm Eg. The action of brachialis when bringing a glass up towards the at the glenohumeral joint. The mouth. produce 90o abduction, the deltoid In this situation the muscle is the prime mover or agonist need only shorten by about 1cm. Therefore each muscle fibre needs to 2. ISOMETRIC be about 2.5cms long. The muscle develops tension equal to the opposing force. It does not change in length and the body part does not move. However because it is the only muscle that abducts the arm, middle deltoid needs to Eg. The action of brachialis holding the glass between drinks. be quite strong, so there are a large In this situation the muscle is acting as a stabiliser number of muscle fibres arranged around intermuscular tendons – ie 3. ECCENTRIC MULTIPENNATE . The muscle develops tension less than the opposing force. The opposing force overcomes the muscle force and the muscle lengthens. The part of the body moves against the muscle.

2. The anterior and posterior parts Eg. The action of brachialis when lowering the glass In this situation the muscle is acting as an antagonist The anterior and posterior parts assist in adduction, flexion and extension of the shoulder. They need to shorten a greater distance, so SYNERGIST the muscle fibres need to be quite long – these parts are STRAP A muscle acts as a synergist when it prevents the unwanted action of MUSCLES – they shorten quite a lot but they are not very strong. They another muscle. Also called stabilising or neutralising don’t need to be very strong because there are other muscles to assist in those movements. Eg. When the middle part of deltoid contracts there is a tendency for the humerus to be pulled up and bump into the underside of the acromion. The rotator cuff muscles hold the humeral head in place.

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NERVE SUPPLY OF MUSCLES SENSORY OF MUSCLES

Muscles have both motor and sensory nerve endings. Neuromuscular spindles also have a motor supply that causes little muscles inside the spindle to contract – thus stretching the sensory The main motor endings are large myelinated nerves that connect to endings. This also causes something like the -jerk reflex to the muscle fibres at motor end plates (MEP). – Every muscle fibre has increase stimulation to the whole muscle – MUSCLE TONE. This an MEP prepares the muscle for action like when Motor units – the nerve fibres that supply the muscle branch and you are in the starting supply more than one muscle fibre. blocks for a race. Some muscles have small motor units, where each nerve supplies only They also increase a few muscle fibres – these muscles are very precise (eg extra-ocular muscle tone (tension) muscles). when you are thinking Other muscles have large motor units where each nerve fibre supplies about the exams! many muscle fibres – these muscles have gross actions (eg. ). Golgi tendon organs These are found in the tendons and are also sensitive to stretching of the Sensory nerves of muscles tendon. They work in a protective reflex that switches the muscle off if End in receptors within the muscle or its tendon and provide feedback the tendon is stretching dangerously. about muscle tension and joint position (proprioception). Some participate in reflexes. Pacinian and Ruffini endings are sensitive to pressure and tension. They are found in the capsules of joints, and provide information about Neuromuscular spindles are joint position. specialised muscles fibres that have sensory endings that are sensitive to the All these sensory nerves (neuromuscular spindles, golgi tendon organs, length of the muscle. When the muscle Pacinian and Ruffini corpuscles) provide sensory information that does is stretched they send messages back the not reach your consciousness but give feedback to spinal and cerebellar spinal cord that trigger a reflex reflexes. contraction of the whole muscle. Eg. Knee-jerk reflex. Free nerve endings These endings are sensitive to pain and this does reach your consciousness. Human Functional Anatomy 213 9 Human Functional Anatomy 213 10

UNIARTICULAR MUSCLES The SOLEUS MUSCLE Uniarticular muscles cross only one joint (eg deltoid) With Deltoid we found that the muscle fibres SHOULDER were the right length to move the joint through Deltoid the full range of motion of the joint. Rotator cuff group Teres major We assumed that the same would be the case Coracobrachialis for the other uniarticular muscles – like soleus

ELBOW MUSCLES: Triceps (medial and lateral heads) Gastrocnemius crosses the knee and ankle. Brachialis Soleus crosses only the ankle.

FOREARM 1. We assumed that it would move the ankle Supinator joint through its full range of motion. Pronator quadratus 2. The () moves about 50mm between flexion and extension. HIP 3. So we expected the muscles fibres in Gluteals soleus to be about 125mm long 6 lateral rotators 4. But all the muscle fibres in the Pectineus soleus muscle are about 30mm Adductor group long 5. Therefore they can shorten by KNEE about 12mm Vastus group 6. So the soleus muscle can only Popliteus move the ankle through 24% of its range ANKLE Soleus

All other muscles cross more than one joint – they are Biarticular

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SOLEAL ACTION RATIO Is the percentage of ankle flexion that the soleus can perform by active contraction.

In humans it is 24% and this suggests that the soleus is important for fine tuning ankle posture. Required for maintaining balance

The Soleal Action Ratio in other primates…

Different primates with different Soleal Action Ratio locomotor styles 70 Leapers 60 50 Quadrupeds 40 Biped 30 20 10 0 Homo Macaca Lemur Siamang Gibbon Langur

Different leverages at the (ie longer Soleal Action Ratio for power: 70 shorter heels for 60 Leapers 50 speed) 40 Quadrupeds 30 Biped 20 10 0 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Speed Heel / Foot ratio Power