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Clinical Practice Keywords Musculoskeletal health/ Musculoskeletal system/Skeletal system Systems of life This article has been Skeletal system double-blind peer reviewed In this article... ● The structure and function of muscles and ● How pathology can affect the musculoskeletal system ● Ways to promote good musculoskeletal health

Skeletal system 2: structure and function of the musculoskeletal system

Key points Author Jennie Walker is principal lecturer, Nottingham Trent University. The musculoskeletal system comprises Abstract Understanding the structure and purpose of the musculoskeletal system , , enables practitioners to understand common pathophysiology and consider the most , appropriate steps to improve musculoskeletal health. This article, the second in a and muscles that two-part series, considers the structure and function of the musculoskeletal system, form a framework reviews the structure of muscles and joints and identifies some of the common for the body pathology occurring at these structures.

The structure of a Citation Walker J (2020) Skeletal system 2: structure and function of the determines its musculoskeletal system. Nursing Times [online]; 116: 3, 52-56. plane and range of movement he musculoskeletal system is Joints Maintaining a made up of bones, cartilage, liga- The joints are the articulating surfaces healthy diet and ments, tendons and muscles, between two bones and may be classified physical are Twhich form a framework for the according to how much movement they essential for good body. Tendons, ligaments and fibrous allow: musculoskeletal tissue bind the structures together to create l  – a fixed, unmovable joint; health stability, with ligaments connecting l  – a joint in which some to bone, and tendons connecting muscle to movement is possible; Musculoskeletal bone. There are 206 bones in the adult skel- l Diarthrosis – a freely movable joint pathophysiology eton; male and female are almost (Moini, 2020). can significantly the same, but the female has a They can also be classified according to reduce functional broader to accommodate the components that unite the bones (such as ability and quality and the male skeleton is typically taller fibrous structures, cartilaginous structures of life with greater . The skeleton is and synovial structures), as shown below. divided into the: Understanding the l Axial skeleton – comprising the , Fibrous joints and and the cage; Fibrous joints are articulating surfaces physiology of the l Appendicular skeleton – consisting of linked together with tough fibrous con- musculoskeletal the pelvic and pectoral girdles, and the nections. One example is the suture lines system allows upper and lower limbs (Cedar, 2012). in the skull, where bones that were initially practitioners to Coordinated movement is made pos- separate have become fused together (syn- consider signs and sible through the combination of pur- ostosis) to form one bone (Danning, 2019). symptoms, and poseful and synchronised movements As the suture line does not permit move- determine across the relevant muscles and bones to ment once fusion has occurred, this is con- appropriate create articulation of the joints. The con- sidered to be a synarthrotic joint. management figuration of the joint surface determines Syndesmoses are another type of the movement possible. Planes of move- , in which ligaments and ment include flexion, extension, abduc- connect the joint tion, adduction, rotation and circumduc- to create a firm structure. One example is tion (Table 1). the inferior tibiofibular joint, in which

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interosseous, tibiofibular and transverse ligaments connect the distal and Table 1. Planes of movement at the joint of the lower leg. Another is the Movement Description Illustration radioulnar joint, where an intraosseous Flexion Bending the joint: membrane connects the distal and decreases the angle ulnar bones of the . This can also between the bones be classified as an amphiarthrotic joint as it permits some movement to allow for Extension Straightening the limb (continuing the extension pronation and supination of the and joint Flexion past the anatomical forearm. straight line may be described as Extension Cartilaginous joints hyperextension) These joints are connected by tough carti- lage between the bone and can be classified Abduction Movement away from the as primary (synchondroses) or secondary midline (symphyses). Adduction Movement towards the midline Synchondroses Abduction Synchondroses are cartilaginous joints formed of , and are mainly found in the growing skeleton as the ossifi- cation centres of growing bone that will Adduction ossify over time (), such as the epiphyseal growth plate. Cartilaginous joints are usually immo- Rotation Movement around a longitudinal bile but, in a rare condition in children and adolescents, the attachment of the epiph- ysis loosens, allowing the femoral to slip down the femoral neck. This is known Rotation as a slipped upper femoral and often presents with the child developing an unexpected limp (Robson and Synder- Circumduction Movement in a circular combe Court, 2019). motion In the mature skeleton, an example of a is the first sternocostal joint (between the first rib and the manu- brium); all other are synovial. Circcumductionumduction Symphyses Pronation For example. turning the These are permanent cartilaginous joints, palm of the hand Supination Pronation in which the bones are connected through downwards ; interestingly, these are all at the body’s midline (Robson and Synder- Supination For example, turning the combe Court, 2019). The intervertebral palm of the hand upwards discs between the vertebral bodies of the spine are an example of bones connected by fibrocartilage. These fibrous joints allow Opposition Movement of the relatively limited movement individually across the palm to touch but extensive movement can be achieved the tips of the on Opposition collectively across the whole spine. the same hand Another example of a is the symphysis in the pelvis, which helps maintain pelvic stability. In pregnancy, the symphysis pubis is softened by Inversion Turning the sole of the to allow for expansion during delivery. inwards This, together with the unfused bones of the baby’s skull, allows passage of the Eversion Turning the sole of the baby’s head through the birth canal. foot outwards As symphyses allow slight movement between the articulating surfaces, they are Eversion Inversion

FRANCESCA CORRA FRANCESCA considered to be amphiarthroses.

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Table 2. Types of

Joint type Description Example Illustration Planar Two flat or slightly curved articulating ● Intercarpal joints surfaces that slide against each other, ● Intertarsal joints allowing a side-to-side and -and-forth ● Vertebral facets movement Carpals

Hinge Typically has convex part of one bone that ● Elbow sits inside the concave part of another and ●  allows a uni-axial movement, such as flexion ●  and extension

Radius

Pivot One bone pivots around another to create a ● Atlantoaxial joint between rotational movement around a single axis C1 () and C2 (axis) of the spine Humerus ● Radioulnar joint between the radius and the ulnar, Ulna allowing them to pronate Radius and supinate the hand

Condyloid An ovoid convex part sits in the ellipsoidal ● Radiocarpal joint of the cavity of the other bone, which permits Radius Ulna flexion/extension, abduction/adduction and ● Metacarpophalangeal circumduction joints in the hand Lunate Scaphoid

Saddle Allows articulation through reciprocal ● Carpometacarpal joint of reception as the bones have concave-convex the thumb surfaces that interlock. Permits flexion/ extension, abduction/adduction and circumduction First metacarpal

Ball and The spheroid structure (ball) sits in the ● Glenohumeral socket socket of another bone. This allows multiaxial ● Acetabulofemoral () movement and has the greatest range of movement. The depth of the socket and the of fibrocartilaginous labrum are the limiting Head of factors for motion

Synovial joints muscles around the joint maintains move- attached to the , allowing Synovial joints are designed to allow free ment, while stability is maintained movement, maintaining tensile strength movement of the joint and are classified as through the use of structures, and helping to prevent dislocation. Inside diarthroses. Characterised by a gap such as ligaments, labra, fat pads and the capsule are sensory fibres, which between the articulating bones, they are menisci (Danning, 2019). detect pain and identify the joint’s position held in close proximity by a . The joint has an outer fibrous capsule (Moini, 2020). The capsule’s inner layer is

FRANCESCA CORRA FRANCESCA Contraction of the infrastructure of that encapsulates the entire joint and is highly vascularised and innervated by slow/

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small nerve fibres that, if stimulated, may Fig 1. Anatomy of cause a diffuse burning or aching sensation (Danning, 2019). This layer also contains the (synovium), which Muscle fibre vessels is composed of synoviocytes of which there () are two types: Perimysium l Type A – mediate cytokine release and are involved in generating an immune response (Robson and Syndercombe Epimysium Court, 2019); l Type B – produce the .

Synovial fluid The synovial fluid helps protect the joint from mechanical and contains hya- Fascicle luronic acid and lubricin (Danning, 2019). (wrapped by In a healthy joint, the synovial fluid is very perimysium) viscous and clear, and is either colourless or a pale straw colour. Water is able to enter the joint very easily during inflam- Endomysium mation but, once it mixes with hyaluronic (between fibres) acid, it cannot leave as quickly (Robson and Syndercombe Court, 2019) – as such, although it may only take a few hours for the joint to swell, it can take a few days for Bone that swelling to subside. The synovial fluid may become infected by a haematogenous (blood-borne) spread of bacteria, extension of an adjacent infec- tion or direct inoculation following 206 The stability of the joint depends on its trauma or an invasive procedure. This is QUICK Number of bones in an shape, the number and position of sup- known as septic and can damage FACT adult porting ligaments around it, their the synovium or cartilage. strength and the tension they exert (Tor- tora and Derrickson, 2009). Supporting surface and reduces friction during move- ligaments are described according to their This is an autoimmune inflammatory ment. This helps distribute the weight position in relation to the capsule (extra- that affects the synovium. It across the joint, reducing friction and capsular or intracapsular). Excessive ten- occurs more often in smokers and is three damage to the bone surface (Robson and sion on ligaments, such as moving the times more common in women than men Syndercombe Court, 2019). joint beyond its functional range of move- (Ralston and McInnes, 2014). is a degenerative condi- ment, can cause them to stretch and may Clinical onset is characterised by the tion involving focal loss of the articular mean they or tear. abnormal production of cytokines and cartilage, so the cartilage becomes less damage can compromise joint stability inflammatory mediators such as inter- efficient at protecting the ends of the bone and function. leukin 1, interleukin 6, interleukin 15 and (Ralston and McInnes, 2014). Over time, Prolonged disuse of the joint, for tumour necrosis factor (Ralston and McI- this can cause bony surfaces to rub example due to immobilisation in a cast or nnes, 2014). This causes the synovium to together on movement, causing pain and through bed rest, often gives reduced flex- become inflamed and hypertrophied so the audible . As the bone attempts to ibility of the ligaments and tendons, as synovial villi become thickened and fuse compensate for the loss of articular carti- well as muscular atrophy (Tortora and Der- together to form a pannus. The pannus lage, it produces new bone to try and sta- rickson, 2009). This may lead to reduced invades the surrounding tissue (such as the bilise the joint. This results in bone thick- mobility of joints and difficulties with cartilage, ligaments and joint capsule), ening under the remaining cartilage functional activity. which this can lead to progressive destruc- (sclerosis) and formation of tion of the joint (Danning, 2019). at the joint margins, which can reduce the Muscle Rheumatoid arthritis can also affect range of movement of the joint. There are three types of muscle in the periarticular structures, including tendon body: sheaths and bursas, as well as having extra- Supporting ligaments l Smooth; articular manifestations. Synovial joints are designed to permit l Cardiac; movement while, at the same time, main- l Skeletal. Osteoarthritis taining balance, strength and stability. Unlike skeletal muscle, smooth and car- The articulating surfaces in synovial joints They vary in structure and the type of diac muscles are not under voluntary con- are coated with approximately 2-3mm of movement they permit – Table 2 summa- trol (Soames and Palastanga, 2019). Skel-

FRANCESCA CORRA FRANCESCA hyaline cartilage, which provides a smooth rises the different types. etal muscle is innervated by the somatic

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(motor) to simulate voluntary Skeletal muscles have several key func- muscle size and strength, may occur due movement, whereas cardiac and smooth tions, including: to inactivity or factors such as poor nutri- muscles are innervated by the autonomic l Maintaining posture and body tion or chronic illness. Although ageing, . position; and the accompanying reduction in l Producing movement; mobility, can reduce the quality of the con- Skeletal muscle l Helping return venous blood from nective tissue and cause ligaments to lose The anatomy of skeletal muscle is shown lower limbs to the right side of the some of their (Robson and Syn- in Fig 1. Skeletal muscle fibre cells are (skeletal muscle pump); dercombe Court 2019), it is important to narrow, but can be long (Danning, 2019) l Converting chemical energy to optimise musculoskeletal health by main- and each fibre has its own connective mechanical energy, which generates taining a healthy diet and continuing tissue covering called the endomysium heat and contributes to the body’s physical activity to the best of a person’s (Soames and Palastanga, 2019). The muscle temperature (Moini, 2020; Robson and ability (Rowe et al, 2019). fibres are in bundles known as fascicles, Syndercombe Court, 2019). which are held together by a layer of con- Many muscles are named according to Conclusion nective tissue called the perimysium. their various characteristics, such as: Muscles and joints are an important part of These are grouped together to form mus- brevis (short), longus (long), maximus the musculoskeletal system. The structure cles, bound by a sheath of fibrous connec- (large) and minimus (small). Their names of articulating surfaces and the type of con- tive tissue known as the epimysium. The may also indicate the direction of the nective tissues play a significant role in the epimysium merges with the perimysium muscle – for example, the transverse range and plane of movement permitted at to form the muscle tendon, which attaches abdominus runs transversely and the the joint. Skeletal muscles are responsible the muscle to the periosteum of the bone. rectus abdominis muscle runs vertically for movement and posture, and work best The site where tendons and ligaments (‘rectus’ means straight) – and can also when used regularly to prevent atrophy. insert to the bone is known as the ; indicate the function; as an example, flexor Pathology affecting the joints can have a this is the site commonly affected in seron- pollicis longus signifies a muscle that significant impact on function and range egative spondyloarthro-pathies (for flexes (Drake et al, 2020). of movement – understanding how example, , psoriatic Skeletal muscles are antagonistic – they pathology affects the musculoskeletal arthritis and ). Seronega- work in opposition to each other to create system allows practitioners to address tive are a type of arthritis that movement. When skeletal muscle receives signs and symptoms, and proactively pro- do not have rheumatoid factor . a signal from the somatic (motor) nerve, it mote musculoskeletal health. NT Bursas are fluid-filled sacs located at shortens, pulling one bone towards the sites where there may be shearing forces, other. As one muscle in the pair contracts, References Cedar SH (2012) for Health: Applying the such as when muscles and tendons pass the other muscle relaxes; the is Activities of Daily Living. London: Red Globe Press. over, or around the edge of, bone – for then reversed to straighten the bone joint. Danning C (2019) Structure and function of the example, in the (subacromial Skeletal muscle requires four key prop- musculoskeletal system. In: Banasik J and Copstead LED (eds) Pathophysiology: 6th Edition. London: bursa) or the hip (trochanteric bursa) erties: Elsevier. (Robson and Syndercombe Court, 2019). l Contractility – so it contracts to Drake R et al (2020) Gray’s Anatomy for Students: Bursas allow structures to glide smoothly produce forces sufficient to move bone; 4th Edition. London: Elsevier. over each other, reducing friction during l Extensibility – ensuring it is able to Moini J (2020) Bone tissues and the skeletal system. In: Anatomy and Physiology for Health Professionals. movement. On occasion, they may become stretch without being damaged; Burlington, MA: Jones & Bartlett Learning. inflamed and painful due to , l Elasticity – allowing it to return to its Ralston SH, McInnes IB (2014) Rheumatology and arthritis or repetitive motion and ‘overuse’ resting state after being stretched or bone . In: Walker B et al (eds) Davidson’s Principals and Practice of Medicine. Edinburgh: of the joint, a condition known as . contracted; Churchill Livingstone. Common examples include prepatellar l Excitability – so it is able to respond to Robson L, Syndercombe Court D (2019) Bone, bursitis (housemaid’s knee) and olecranon a stimulus (action potential). Muscle, skin and . In: Naish J, bursitis (student’s elbow). The neuromuscular junction is the Syndercombe Court D (eds) Medical Sciences. Edinburgh: Elsevier. chemical synapse formed between the Rowe G (2019) Bioscience. In: Rowe G et al (eds) nerve fibre and the muscle fibre. The nerve The Handbook for Nursing Associates and Assistant Nursing Times into the synapse between the muscle and Practitioners. London: Sage Publications. Self-assessment Soames R, Palastanga N (2019) Anatomy and the nerve fibre releases acetylcholine, Human Movement: Structure and Function. online which acts as a chemical neurotransmitter London: Elsevier. to convey the electrical impulse from the Tortora GJ, Derrickson B (2009) Principles of your knowledge with Nursing Anatomy and Physiology. Hoboken, NJ: John Wiley nerve to the receptors in the muscle. & Sons. Times Self-assessment after reading During , the neuromuscular junc- this article. If you score 80% or more, tion is an important site for action, as you will receive a personalised blocking the acetylcholine receptors pro- CLINICAL Skeletal system certificate that you can download and vides a neuromuscular blockade that SERIES store in your NT Portfolio as CPD or causes muscle paralysis. Part 1: Anatomy and physiology revalidation evidence. Skeletal muscle performs best when it of bones Feb is used regularly, and the use of targeted Bit.ly/NTSkeletal1 To take the test, go to: training or exercise can improve its endur- Part 2: Structure and function of nursingtimes.net/NTSASkeleton ance and power. Likewise, loss of muscle the musculoskeletal system Mar mass (atrophy), such as a decrease in

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