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Muscles, Nerves and Movement

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Barbara Tyldesley MEd FCOT, TDipCOT

and

June I. Grieve BSc MSc

Muscles, Nerves and Movementin human occupation

Third edition

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Preface to the third edition viiAcknowledgements viii

SECTION I: INTRODUCTION TOMOVEMENT 1

Chapter 1: Basic Units, Structure andFunction: Supporting Tissues, Muscle and Nerve 3Framework and support: the connective

tissues 3Articulations 8Skeletal muscle 10Basic units of the nervous system 15Muscle tone 22Summary 24

Chapter 2: Movement Terminology 25The anatomical position 25Planes and axes of movement 26Structure and movements at synovial joints 27Group action and types of muscle work 29Biomechanical principles 31Summary 38

Chapter 3: The Central Nervous System:The Brain and Spinal Cord 39Part I: The Brain 39Introduction to the form and structure 39Cerebral hemispheres 44Basal ganglia 50Thalamus 51Hypothalamus and limbic system 51Brain stem 53Cerebellum 54Summary of brain areas: function in

movement 55Part II: The Spinal Cord 56Position and segmentation of the

spinal cord 56Spinal reflex pathways 60Summary of the functions of the

spinal cord 62Summary 63

Chapter 4: The Peripheral Nervous System: Cranial and Spinal Nerves 64Spinal nerves 65Peripheral nerves 68Cranial nerves 70Autonomic nervous system 72Summary 75Section I Further Reading 75

SECTION II: ANATOMY OF MOVEMENT IN EVERYDAY LIVING 77

Chapter 5: Positioning Movements of the Shoulder and Elbow 79Part I: The Shoulder 80The shoulder (pectoral) girdle 80The shoulder (glenohumeral) joint 81Muscles of the shoulder region 83Part II: The Elbow 92Elbow position and function 92The elbow joint 92Muscles moving the elbow 93Summary of the shoulder and

elbow in functional movements 97Summary 97

Chapter 6: Manipulative Movements: The Forearm, Wrist and Hand 98Functions of the forearm and

wrist 98The forearm 99The wrist 101Functions of the hand 105Movements of the hand: fingers

and thumb 105Muscles moving the hand: fingers

and thumb 107Types of grip 117Summary of muscles of the forearm

and intrinsic muscles of the hand 119

Summary 120

Contents

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Chapter 7: The Nerve Supply of the Upper Limb 121The brachial plexus 121Terminal branches of the brachial plexus 123Axillary nerve: shoulder movement 123Spinal segmental innervation of the

upper limb 130Summary 130

Chapter 8: Support and Propulsion: The Lower Limb 131Joints and movements of the pelvis,

thigh and leg 131Muscles of the thigh and leg in support,

swing and propulsion 137Functions of the foot 145Summary of the lower limb muscles 151Summary 152

Chapter 9: Nerve Supply of the Lower Limb 153Lumbar plexus: position and formation 154Terminal branches of the lumbar plexus 154Sacral plexus: position and formation 157Terminal branches of the sacral plexus 157Spinal segmental innervation of the

lower limb 160Summary 160

Chapter 10: Upright Posture and Breathing: The Trunk 161Upright posture 162Breathing 169Pelvic tilt and the pelvic floor 173Nerve supply of the muscles of the neck

and trunk 174Summary of the muscles of the trunk 175Summary 175Section II Further Reading 176

SECTION III: SENSORIMOTOR CONTROL OF MOVEMENT 177

Chapter 11: Sensory Background to Movement 179Somatosensory system 180Vestibular system 187Visual system 189Regulation of posture 190Summary 191

Chapter 12: Motor Control 193Spinal mechanisms 193Descending motor system 197Planning, co-ordination and motor

learning 201Summary 204Section III Further Reading 204

SECTION IV: HUMAN OCCUPATION: COMPONENTS AND SKILLS 205

Chapter 13: Performance of Functional Movements 207Multiple factors in movement control 207Core positions and movement patterns 210Summary 222

Chapter 14: Occupational Performance 223Framework for understanding human

occupation 223Case histories 225Part I 226Example case history 226Further case histories 228Case history 1: Elderly person 228Case history 2: Parkinson’s disease 228Case history 3: Traumatic brain injury 230Case history 4: Hand injury 230Case history 5: Spinal cord injury 231Case history 6: Chronic pain 231Part II 232Case history 1: Elderly person 232Case history 2: Parkinson’s disease 233Case history 3: Traumatic brain injury 234Case history 4: Hand injury 235Case history 5: Spinal cord injury 236Case history 6: Chronic pain 238Conclusion 240Section IV Further Reading 240

Appendix I: Bones 241

Appendix II: Segmental Nerve Supply of Muscles 252

Glossary 256

Index 263

Index of Clinical Notepads 269

vi Contents

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This book is written primarily for students enter-ing the paramedical professions, particularly thosewith a limited scientific background. Clinicians willuse the book as a reference source for acquiring afurther understanding of the basic movement prob-lems encountered by their clients. This third edi-tion of Muscles, Nerves and Movement extends thestudy of movement in daily activities, established inearlier editions to include the wider domain ofhuman occupation.

Section I describes the structure and function ofthe basic components of the musculoskeletalsystem. An introduction to the location, organisa-tion and functions of central and peripheralnervous systems is given, which is developedfurther in Section III. The terms to describe themovements at joints and the types of muscle workare defined and the mechanical principles relatedto the stability of the body are outlined.

Section II is a functional approach to the anato-my of movement. The structure and movements ofthe major joints of the body are described. Theposition, attachments and nerve supply of the mus-cles are given, together with their use in namedactivities of daily living.

Section III augments the knowledge of thenervous system from Section I to formulate anaccount of the sensory and motor systems in move-ment control.

Section IV has two new chapters. One chapterexplores the multiple factors in the performance offunctional movements, and develops a system forthe analysis of core body positions and the transi-tions between them. A framework for the under-

standing of human occupation forms the basis ofthe final chapter and several case histories aredescribed to demonstrate the application of theframework to specific clinical problems.

Each chapter begins with a contents list and endswith a summary. The exercises which direct thereader to observe colleagues in the classroom andpeople in their daily lives have been retained fromearlier editions. Some of the clinical note-pads havebeen revised and expanded.

The first edition was written during many yearsof experience of teaching and examining at theLiverpool (Barbara Tyldesley) and London (JuneGrieve) Schools of Occupational Therapy. For thisthird edition, two lecturers in occupational thera-py have been recruited to the team of authors.Linda Gnanasekaran of Brunel University has writ-ten and edited Chapter 13 on the Performance ofFunctional Movements. Ian McMillan of QueenMargaret University College, Edinburgh, devisedand developed the framework for the Under-standing of Human Occupation in Chapter 14. Healso revised the section on the interpretation ofpain in Chapter 11. Both Linda and Ian have givenvaluable advice on the updating of Sections I to III.

Basic anatomy, physiology and neurology arenow frequently integrated into modules of study ofthe theory and practice of occupational therapy.We hope that students will use this book as areference source at appropriate levels in thecourse. If the knowledge gained from this bookleads a student to ask questions and to seekanswers in other reading, then our objectives havebeen achieved.

Preface to the third edition

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We very much appreciate the time and energy givenby Linda Gnanasekaran and Ian McMillan to thisnew edition. Their advice and support have madeour task easier and we have learnt from them. JoCreighton has combined her drawing skills with herunderstanding of movement to produce the figuresfor Chapter 13, for which we thank her. We are alsograteful to clinical occupational therapists Ronnie

Bentley, Linda Gwilliam and Louise Hogan whohave contributed to the case history exercises.Caroline Connelly has been an enthusiastic andencouraging editor at Blackwell Science.

June Grieve Barbara Tyldesley

Acknowledgements

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Section I

Introduction to movementComponents of the musculoskeletal and nervous system, movement terminology

• Basic units, structure and function: supporting tissues, muscle and nerves

• Movement terminology

• The central nervous system: the brain and spinal cord

• The peripheral nervous system: cranial and spinal nerves

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The foundation for the study of movement lies inthe musculoskeletal and nervous systems. Whilemuscle action is the basis of movement of the skele-ton at joints, the nervous system is an essentialcomponent of all movement. Nervous tissue sendscommands to the muscles for action, andresponds to changes in the external and internalstimuli that affect movement during performance.The nervous system regulates the activity in the car-diovascular and respiratory systems to meet thedemands of the muscles for the release of energy.The combined activity of muscle and nerve main-tains muscle tone. This is the background muscleactivity which holds the static positions of the bodyin preparation for movement.

This chapter covers the basic components ofstructure that are organised to produce a movablejoint, a contractile muscle and nerves that areexcitable, with the connective tissues supporting allof these. The three basic functional units are:

• connective tissues which provide the frameworkfor stability and support while body parts movein particular directions;

• skeletal muscle which changes in length and pullson bones to produce movements at joints, pow-ered by the energy released in the the muscles;

• neurones and nerves forming the nervous systemwhich conducts information between theenvironmental sensors, the control centres formovement and the muscles. In this way appro-priate movement is initiated, executed andregulated.

The overall function of connective tissue is to uniteor connect structures in the body, and to give sup-port. Bone is a connective tissue which provides therigid framework for support. Where bones articu-late with each other dense fibrous connective tis-sue, rich in collagen fibres, surrounds the ends ofthe bones, allowing movement to occur while main-taining stability. Cartilage, another connective tis-sue, is also found associated with joints, where itforms a compressible link between two bones, orprovides a low-friction surface for smooth move-ment of one bone on another. Connective tissueattaches muscles to bone, in the form of either acord (tendon) or a flat sheet (fascia). The connec-tive tissues may be divided into:

• dense fibrous tissue• cartilage• bone.

FRAMEWORK AND SUPPORT: THE CONNECTIVE TISSUES

1Basic Units, Structure and Function: Supporting

Tissues, Muscle and Nerve

Framework and support: the connective tissuesDense fibrous tissue, cartilage and bone

ArticulationsFibrous, cartilaginous and synovial joints

Skeletal muscle Structure and formAdaptation of muscles to functional use

Basic units of the nervous systemThe neurone: excitation and conduction Motor and sensory neuronesThe motor unitReceptors

Muscle tone

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Dense fibrous tissue

Dense fibrous connective tissue unites structures inthe body while still allowing movement to occur. Ithas high tensile strength to resist stretching forces.This connective tissue has few cells and is largelymade up of fibres of collagen and elastin that givethe tissue great strength. The fibres are producedby fibroblast cells that lie in between the fibres (Fig.1.1). The toughness of this tissue can be felt whencutting through stewing steak with a blunt knife.The muscle fibres are easily sliced, but the cover-ing of white connective tissue is very tough. Exam-ples of this tissue are as follows:

• The capsule surrounding the movable (synovial)joints which binds the bones together (see Fig. 1.7).

• Ligaments form strong bands that join bone tobone. Ligaments strengthen the joint capsules inparticular directions and limit movement.

• Tendons unite the contractile fibres of muscle tobone.

In tendons and ligaments, the collagenous fibres liein parallel in the direction of greatest stress.

• An aponeurosis is a strong flat membrane, withcollagen fibres that lie in different directions toform sheets of connective tissue. An aponeuro-

4 Muscles, Nerves and Movement

Fig. 1.1 Dense fibrous connective tissue seen covering bone as periosteum, and forming the tendon of a skeletalmuscle.

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sis can form the attachment of a muscle, such asthe oblique abdominal muscles, which meet inthe midline of the abdomen (see Chapter 10, Fig.10.6). In the palm of the hand and the sole of thefoot an aponeurosis lies deep to the skin andforms a protective layer for the tendons under-neath (see Chapter 8, Fig. 8.21).

• A retinaculum is a band of dense fibrous tissuethat binds tendons of muscles and prevents bow-string during movement. An example is the flex-or retinaculum of the wrist, which holds thetendons of muscles passing into the hand in posi-tion (see Chapter 6, Fig. 6.15).

• Fascia is a term used for the large areas of densefibrous tissue that surround the musculature ofall the body segments. Fascia is particularly dev-eloped in the limbs, where it dips down betweenthe large groups of muscles and attaches to thebone. In some areas, fascia provides a base forthe attachment of muscles, for example thethoracolumbar fascia gives attachment to the longmuscles of the back (see Chapter 10, Fig. 10.6).

• Periosteum is the protective covering of bones.Tendons and ligaments blend with the perio-steum around bone (see Fig. 1.36).

• Dura is thick fibrous connective tissue protect-ing the brain and spinal cord (see Chapter 3, Fig. 3.22).

Cartilage

Cartilage is a tissue that can be compressed and hasresilience. The cells (chondrocytes) are oval and lie in a ground substance that is not rigid like bone. There is no blood supply to cartilage, so thereis a limit to its thickness. The tissue has great resist-ance to wear, but cannot be repaired whendamaged.

Basic Units, Structure and Function 5

Fig. 1.2 Microscopic structure of hyaline and fibrocartilage, location in the skeleton of the trunk.

Clinical note-pad 1A: ContractureFibrous connective tissue loses its strength andelasticity when there is a loss of movement forany reason over a period of time. When musclesand tendons remain the same length for anylength of time, e.g. due to muscle weakness instroke, or joint pain in rheumatoid arthritis, per-manent shortening occurs (contracture) whichleads to joint deformity. See also Dupuytren’scontracture, Clinical note-pad 6B.

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Hyaline cartilage is commonly called gristle. It is smooth and glass-like, forming a low-frictioncovering to the articular surfaces of joints. In theelderly, the articular cartilage tends to becomeeroded or calcifies, so that joints become stiff.Hyaline cartilage forms the costal cartilages whichjoin the anterior ends of the ribs to the sternum(Fig. 1.2). In the developing foetus, most of thebones are formed in hyaline cartilage. When the cartilaginous model of each bone reaches acritical size for the survival of the cartilage cells,ossification begins.

• LOOK at some large butcher’s bones to see thecartilage covering the joint surfaces at the end. Note that it is bluish and looks like glass.

Fibrocartilage consists of cartilage cells lying inbetween densely packed collagen fibres (Fig. 1.2).The fibres give extra strength to the tissue whileretaining its resilience. Examples of where fibro-cartilage is found are the discs between the bonesof the vertebral column, the pubic symphysis join-ing the two halves of the pelvis anteriorly, and themenisci in the knee joint.

Bone

Bone is the tissue that forms the rigid supports forthe body by containing a large proportion of calci-um salts (calcium phosphate and carbonate). Itmust be remembered that bone is a living tissuecomposed of cells and an abundant blood supply.It has a greater capacity for repair after damagethan any other tissue in the body, except for blood. The strength of bone lies in the thin plates(lamellae), composed of collagen fibres with cal-cium salts deposited in between. The lamellae liein parallel, held together by fibres, and the bonecells or osteocytes are found in between. Each bonecell lies in a small space or lacuna, and connectswith other cells and to blood capillaries by finechannels called canaliculi (Fig. 1.3a).

In compact bone, the lamellae are laid down inconcentric rings around a central canal containingblood vessels. Each system of concentric lamellae(known as a Haversian system or an osteon) lies ina longitudinal direction. Many of these systems areclosely packed to form the dense compact bonefound in the shaft of long bones (Fig. 1.3b).

In cancellous or trabeculate bone, the lamellaeform plates arranged in different directions to forma mesh. The plates are known as trabeculae and thespaces in between contain blood capillaries. Thebone cells lying in the trabeculae communicate witheach other and with the spaces by canaliculi (Fig.1.3c). The expanded ends of long bones are filledwith cancellous bone covered with a thin layer ofcompact bone. The central cavity of the shaft oflong bones contains bone marrow. This organisa-tion of the two types of bone produces a structurewith great rigidity without excessive weight (Fig.1.4). Bone has the capacity to remodel in shape in


Clinical note-pad 1B: OsteoporosisOsteoporosis is literally a condition of porousbones, largely due to a depletion of calcium from the body. For a number of reasons, calcium loss exceeds calcium absorption fromthe diet, causing bone mass to decrease exces-sively. This leads to fractures occurring as aresult of normal mechanical stresses upon theskeleton which it would normally withstand.Spontaneous fractures may occur for no appar-ent reason.

The whole skeleton is affected, but problemsmanifest most frequently in bones that bearweight or transmit large forces. Hence, the vertebrae may shrink and crumble or fracture.This causes height loss, kyphosis and back pain.A range of motor and sensory problems mayoccur owing to compression of the spinalnerves or the spinal cord. Fractures the neck ofthe femur and the wrist are common, oftenresulting from a fall.

Osteoporosis commonly affects people in thelatter half of life, particularly elderly women whoare at increased risk because of the sudden dropin oestrogen levels occurring at the menopause,combined with a lower bone mass than men.Other risk factors are poor dietary intake of calcium, vitamin D deficiency, physical inactivity,low body weight, smoking, high alcohol intakeand some medications, e.g. long-term cortico-steroid use. Osteoporosis is thought to affect onein three women in the UK, and can be a sourceof major disability.

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response to the stresses on it, so that the structurelines of the trabeculae at the ends of the bonefollow the lines of force on the bone. For example,the lines of trabeculae at the ends of weight-bearing bones, such as the femur, provide maxi-mum strength to support the body weight againstgravity. Remodelling of bone is achieved by theactivity of bone-forming cells known as osteoblasts,

and bone-destroying cells known as osteoclasts;both types of cell are found in bone tissue. Thecalcium salts of bone are constantly interchangingwith calcium ions in the blood, under the influence of hormones (parathormone and thyrocalcitonin).Bone is a living, constantly changing connectivetissue that provides a rigid framework on whichmuscles can exert forces to produce movement.


Fig. 1.3 Microscopic structure of bone: (a) an osteocyte (enlarged) and the organisation of osteons in compact bone seen in transverse section; (b) a section of the shaft of long bone; (c) cancellous bone showing trabeculae withosteocytes.

(a)

(b)

(c)

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• LOOK at any of the following examples of con-nective tissue that are available to you:(1) Microscopic slides of dense fibrous tissue, car-

tilage and bone, noting the arrangement of thecellular and fibre content.

(2) Dissected material of joints and muscleswhich include tendons, ligaments, aponeuro-sis and retinaculum.

(3) Fresh butcher’s bone: note the pink colour(blood supply), and the central cavity in theshaft of long bones.

(4) Fresh red meat to see fibrous connective tissuearound muscle.

Where the rigid bones of the skeleton meet, con-nective tissues are organised to bind the bonestogether and to form joints. It is the joints thatallow movement of the segments of the body rel-ative to each other. The joints or articulations

between bones can be divided into three typesbased on the particular connective tissuesinvolved. The three main classes of joint arefibrous, cartilaginous and synovial.

Fibrous joints

Here, the bones are united by dense fibrous con-nective tissue.

The sutures of the skull are fibrous joints thatallow no movement between the bones. The edgeof each bone is irregular and interlocks with theadjacent bone, a layer of fibrous tissue linking them(Fig. 1.5a).

A syndesmosis is a joint where the bones arejoined by a ligament that allows some movementbetween the bones. A syndesmosis is foundbetween the radius and the ulna (Fig. 1.5b). Theinterosseous membrane allows movement of the forearm.

A gomphosis is a specialised fibrous joint thatfixes the teeth in the sockets of the jaw (Fig. 1.5c).

Cartilaginous joints

In these joints the bones are united by cartilage.A synchondrosis or primary cartilaginous joint

is a joint where the union is composed of hyalinecartilage. This type of joint is also called primarycartilaginous. The articulation of the first rib withthe sternum is by a synchondrosis. During growthof the long bones of the skeleton, there is a syn-chondrosis between the ends and the shaft of thebone, where temporary cartilage forms the epi-physeal plate. These plates disappear when growthstops and the bone becomes ossified (Fig. 1.6a).

A symphysis or secondary cartilaginous joint isa joint where the joint surfaces are covered by athin layer of hyaline cartilage and united by a discof fibrocartilage. This type of joint (sometimescalled secondary cartilaginous) allows a limitedamount of movement between the bones by com-pression of the cartilage. The bodies of the verte-brae articulate by a disc of fibrocartilage (Fig. 1.6b).Movement between two vertebrae is small, butwhen all of the intervertebral discs are co pressedin a particular direction, considerable movement ofthe vertebral column occurs. Little movementoccurs at the pubic symphysis, the joint where theright and left halves of the pelvis meet. Movement

ARTICULATIONS


Fig. 1.4 Gross structure of long bone: longitudinal andtransverse sections.

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is probably increased at the pubic symphysis in thelate stage of pregnancy and during childbirth, toincrease the size of the birth canal.

Synovial joints

Synovial joints are the mobile joints of the body.There is a large number of these joints, which showa variety of form and range of movement. The com-mon features of all of them are shown in the sec-tion of a typical synovial joint (Fig. 1.7) and listedas follows.• Hyaline cartilage covers the ends of the two

articulating bones, providing a low-frictionsurface for movement between them.

• A capsule of dense fibrous tissue is attached tothe articular margins, or some distance away, oneach bone. The capsule surrounds the joint likea sleeve.

• There is a joint cavity inside the capsule whichallows free movement between the bones.

• Ligaments, bands or cords of dense fibrous tis-sue, join the bones. The ligaments may blend


Fig. 1.5 Fibrous joints: (a) suture between bones of the skull; (b) syndesmosis between the radius and ulna; (c) gomphosis: tooth in socket.

Fig. 1.6 Cartilaginous joints: (a) synchondrosis in a child’smetacarpal bone, as seen on X-ray; (b) symphysisbetween the bodies of two vertebrae.

(a)

(b)

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with the capsule or they are attached to thebones close to the joint.

• A synovial membrane lines the joint capsule andall non-articular surfaces inside the joint, i.e. anystructure within the joint not covered by hyalinecartilage.

One or more bursae are found associated withsome of the synovial joints at a point of frictionwhere a muscle, a tendon or the skin rubs againstany bony structures. A bursa is a closed sac offibrous tissue lined by a synovial membrane andcontaining synovial fluid. The cavity of the bursasometimes communicates with the joint cavity. Pads

of fat, liquid at body temperature, are also presentin some joints. Both structures have a protectivefunction.

All of the large movable joints of the body, forexample the shoulder, elbow, wrist, hip, knee andankle, are synovial joints. The direction and therange of their movements depend on the shape ofthe articular surfaces and the presence of ligamentsand muscles close to the joint. The different typesof synovial joint are described in Chapter 2 when thedirections of movement at joints are considered.

Skeletal muscle is attached to the bones of theskeleton and produces movement at joints. Thebasic unit of skeletal muscles is the muscle fibre.Muscle fibres are bound together in bundles toform a whole muscle, which is attached to bonesby fibrous connective tissue. When tension devel-ops in the muscle, the ends are drawn towards thecentre of the muscle. In this case, the muscle iscontracting in length and a body part moves. Alter-natively, a body part may be moved by gravityand/or by an added weight, for example an objectheld in the hand. Now the tension developed in themuscle may be used to resist movement and holdthe object in one position.

In summary, the tension developed allows amuscle:

• to shorten to produce movement• to resist movement in response to the force of

gravity or an added load.

Furthermore, muscles may develop tension whenthey are increasing in length. This will be consid-ered in Chapter 2, muscle work.

Both muscle and fibrous connective tissue haveelasticity. They can be stretched and return to theoriginal length. The unique function of muscle isthe capacity to shorten actively.

• HOLD a glass of water in the hand. Feel the activ-ity in the muscles above the elbow by palpating themwith the other hand. The tension in the muscles isresisting the weight of the forearm and the water.

• LIFT the glass to the mouth. Feel the muscle activ-ity in the same muscles as they shorten to lift the glass.

SKELETAL MUSCLE


Clinical note-pad 1C: Osteoarthritis andrheumatoid arthritisOsteoarthritis is a degenerative disease occurringin the middle aged and elderly. There is a pro-gressive loss of the articular cartilage in the weight-bearing joints, usually the hip and the knees. Bonyoutgrowths occur and the capsule becomesfibrosed. The joints become stiff and painful.

Rheumatoid arthritis is a systemic disease thatcan occur at any age (average 40 years) and it ismore common in women. The peripheral joints(hands and feet) are affected first, followed bythe involvement of other joints. Inflammation ofthe synovial membrane, bursae and tendonsheaths leads to swelling and pain which isrelieved by drugs. Deformity is the result of ero-sion of articular cartilage, stretching of the cap-sule and the rupture of tendons.

Fig. 1.7 Typical synovial joint.

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Structure and form

The structure of a whole muscle is the combinationof muscle and connective tissues, which bothcontribute to the function of the active muscle. In a whole muscle, groups of contractile muscle fibres are bound together by fibrous connectivetissue. Each bundle is called a fasciculus. Further

coverings of connective tissue bind the fasciculitogether and an outer layer surrounds the wholemuscle (Fig. 1.8).

The total connective tissue element lying inbetween the contractile muscle fibres is known asthe parallel elastic component. The tension that isbuilt up in muscle when it is activated depends onthe tension in the muscle fibres and in the parallel


Fig. 1.8 Skeletal muscle: the organisation of muscle fibres into a whole muscle, and a sarcomere in the relaxed andthe shortened state (as seen by an electron microscope).

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elastic component. The fibrous connective tissue,for example a tendon, which links a whole muscleto bone is known as the series elastic component.The initial tension that builds up in an active mus-cle tightens the series elastic component and thenthe muscle can shorten. A model of the elastic andcontractile parts of a muscle is shown in Fig. 1.9.If the connective tissue components lose their elas-ticity, through lack of use in injury or disease, amuscle may go into contracture (see Clinical note-pad 1A). Lively splints are used to maintain elas-ticity and prevent contracture while the musclerecovers.

The individual muscle fibres lie within a musclein one of the following two ways.

• Parallel fibres are seen in strap and fusiformmuscles (Fig. 1.10a, b). These muscles have longfibres which are capable of shortening over theentire length of the muscle, but the result is a lesspowerful muscle.

• Oblique fibres are seen in pennate muscles. Themuscle fibres in these muscles cannot shorten tothe same extent as parallel fibres. The advantageof this arrangement, however, is that more mus-cle fibres can be packed into the whole muscle,so that greater power can be achieved.

The muscles with oblique fibres are known asunipennate, bipennate or multipennate, dependingon the particular way in which the muscle fibres arearranged (Fig. 1.10c, d). Some of the large musclesof the body combine parallel and oblique arrange-ments. The deltoid muscle of the shoulder (seeChapter 5, Fig. 5.9) has one group of fibres that aremultipennate and two groups that are fusiform,which combines strength to lift the weight of thearm with a wide range of movement. The form ofa particular muscle reflects the space available andthe demands of range and strength of movement.

Muscles have a limited capacity for repair,although a small area of damage to muscle fibresmay regenerate. In more extensive damage, theconnective tissue responds by producing more col-lagen fibres and a scar is formed. An intact nerveand adequate blood supply are essential for mus-cle function. If these are interrupted the musclemay never recover. Movement can then only berestored by other muscles taking over the functionsof the damaged muscles.

Microscopic structureA muscle fibre can just be seen with the naked eye. Each muscle fibre is an elongated cell withmany nuclei surrounded by a strong outer mem-brane, the sarcolemma.. If one fibre is viewed undera light microscope, the nuclei can be seen close to the membrane around the fibre. The chiefconstituent of the fibre is several hundreds ofmyofibrils, strands of protein extending from oneend of the fibre to the other (Fig. 1.8). The arrange-ment of the two main proteins, actin and myosin,that form each myofibril presents a bandedappearance. The light and dark bands in adjacentmyofibrils coincide, so that the whole muscle fibreis striated.


Fig. 1.9 Elastic components of muscle.

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Fig. 1.10 Form of whole muscle: parallel fibres (a) strap and (b) fusiform; oblique fibres (c) multipennate and (d) unipennate and bipennate.

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The electron microscope reveals the detail of thecross-striations in each myofibril. A repeating unit, known as the sarcomere, is revealed along thelength of the myofibril. Each sarcomere links to the next one at a disc called the Z-line. The thinfilaments of actin are attached to the Z-line andproject towards the centre of the sarcomere. Thethicker myosin filaments lie in between the actinstrands. The darkest bands of the myofibril arewhere the actin and myosin overlap in thesarcomere.

The arrangement of the myosin molecules in thethick myosin filaments forms cross-bridges that linkwith special sites on the active filaments when themuscle fibre is activated. The result of this linkingis to allow the filaments to slide past one another,so that each sarcomere becomes shorter. This, inturn, means that the myofibril is shorter, and sinceall the myofibrils respond together, the muscle fibreshortens.

• LOOK at Figure 1.8, starting at the bottom, toidentify the details of the structure of a muscle: (1) sarcomeres lie end to end to form a myofibril;(2) myofibrils are packed tightly together inside amuscle fibre; (3) muscle fibres are bound togetherin a fasciculus; and (4) fasciculi are bound to forma whole muscle.

In active muscles, the energy required to developtension is released by chemical reactions. Most ofthese reactions occur in structures called mito-chondria (Fig. 1.11). All cells have mitochondria,but they are more abundant in muscle fibres wherethey lie adjacent to the myofibrils. The breakdownof adenosine triphosphate (ATP) and a ‘back-up’phosphocreatine provide a high level of energy out-put in the muscle. The store of ATP is replenishedin the mitochondria using oxygen and glucosebrought by the blood in the network of capillariessurrounding muscle fibres (Fig. 1.11). In this way,the muscle fibres have a continuous supply ofenergy, as long as the supply of oxygen is main-tained (aerobic metabolism). Glycogen is anothersource of energy that is stored in muscle fibres.When there is insufficient oxygen to replenish ATP by oxidative reactions, energy is released from breakdown of glycogen to maintain the ATPlevels. This occurs during a short burst of high-levelmuscle activity.

Adaptation of muscles to functional use

Not all muscle fibres in one muscle are the same.Two main types have been distinguished.

• Slow fibres, known as type I fibres, are redbecause they contain myoglobin which storesoxygen, like the haemoglobin in the blood, andthey are surrounded by many capillaries. Ener-gy supply for the slow fibres (called SO) is mainlyfrom oxidative reactions. The slow fibres respondto stimulation with a slow twitch and they areresistant to fatigue.

• Fast fibres, known as type II fibres, are white withno myoglobin and have fewer capillaries perfibre. Energy is derived mainly from the break-down of glucose and stored glycogen withoutoxygen. The fast fibres (called FG) respond witha fast twitch, but they are easily fatigued whenthe glycogen stores are used up.

Slow fibres are adapted for sustained postural activ-ity, while the fast fibres are recruited for rapidintense bursts of activity, for example running,cycling and kitchen tasks such as cutting bread andchopping vegetables.

Skeletal muscle shows a remarkable capacity toadapt its structure to functional use. Both the rel-ative proportion of slow and fast fibres and thenumber of sarcomeres in the myofibrils canchange over time.


Fig. 1.11 Energy for muscle contraction (simplified). The‘slow’ pathway predominates in type I fibres, where ATPis replenished by aerobic reactions to provide the energyfor long periods of low-level activity. The ‘fast’ pathwaypredominates in type II fibres, where glycogen providesenergy without oxygen for short bursts of high-level activity.

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Muscle strength and bulk is increased by pro-gressive resistance training programmes usingweights or strength-training machines. The addedstrength is due to an increase in the number and sizeof the myofibrils, particularly in the fast musclefibres which hypertrophy most readily. Less increaseoccurs in the slow fibre type. There is little evidencethat similar training programmes can strengthen themuscles of patients with chronic degenerative dis-orders of the neuromuscular system. Any changemay depend on the number of remaining intactfibres. For these patients, improvement in staminarather than strength will be more useful for daily liv-ing in any case. Training for endurance in healthyyoung adults has the effect of changes in some fastfibres, which become more like slow fibres. Thepresence of these type IIA or FGO fibres increas-es the length of time that the muscle can performmovement without fatigue.

Studies of the effects of ageing have shown a pro-gressive decrease in the size of fast fibres with fewerchanges in slow fibres. These changes are most like-ly to be the response to a less active life. Fast fibrescan increase in size in elderly people, so that exer-cise programmes are beneficial when there are nopathological changes present.

Muscles also change the number of sarcomeresin the myofibrils if a muscle is held in a shortenedor lengthened position, for example by a plastercast. Sarcomeres are lost in the shortened positionand added in the lengthened position. This is anadaptation to changes in the functional length ofthe muscle. Any benefit, however, may be over-ridden by the changes in the muscle which lead tomuscle contracture.

The functions of the nervous system in movementare: to conduct motor commands from the brain tothe muscles; to regulate the activity in the cardio-vascular and respiratory systems which supply themuscles with essential nutrients and oxygen; and tomonitor changes in the environment that affectmovement.

The properties of neurones are:

• excitation: neurones generate impulses in res-ponse to stimulation

• conduction of impulses between neurones (inone direction only).

Neurones are organised in networks or centres in the brain and the spinal cord. Activity in onecentre is directed to a particular end, for examplethe location of a specific sensation. The outputfrom one processing centre is then conducted to one or many other centres in a series ofoperations, for example from motor centres in the brain to the spinal cord. Information can alsobe conducted in parallel between processingcentres.

The properties of neural networks are:

• processing of activity directed to a particular end

• relay of the output of processing to othercentres in the nervous system.

This section is primarily concerned with thestructure and the activity in the basic units of thenervous system, the neurones. Neural processing inspecific centres in the central nervous system willbe considered in Section III.

The neurone: excitation andconduction

Each neurone has a cell body and numerousprocesses extending outwards from the cell. Theprocesses are living structures and their membraneis continuous with that of the cell body (Fig. 1.12).(Think of the cell body like a conker with spinesprojecting out in all directions.) The projectionsvary in length: short processes are called dendrites,and each neurone has one long process, the axon.

BASIC UNITS OF THE NERVOUSSYSTEM


Clinical note-pad 1D: MyopathiesNeuromuscular disorders that are myopathicoriginate in the muscle, and may be inherited oracquired. There is muscle weakness in the prox-imal muscles, which is slowly progressive withmuscle wasting.

Duchenne muscular dystrophy is an inheritedmyopathy that affects boys only. There is a rapidprogression of muscle weakness that begins inchildhood.

Acquired myopathy can result from infections,or endocrine disorders, or as a complication ofsteroid drug treatment.

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The dendrites are adapted to receive signals orimpulses and pass them on to the cell body. Someneurones, particularly in the brain, have thousandsof complex branching dendrites, so that signalsfrom a large number of other neurones can bereceived.

The axon is the output end of every neurone.The length of an axon varies from a few millimetresto 1 m. Cell bodies of motor neurones in the spinalcord in the lower back have long axons that extenddown the leg to supply the muscles of the foot. Theaxon may be surrounded by a sheath of myelin, afatty material, which increases the rate at whichimpulses are conducted down the axon. Themyelin is laid down between layers of membraneof Schwann cells that wrap around the axon. Gapsin the myelin occur between successive Schwanncells forming nodes of Ranvier (Fig. 1.12).

At the end of the neurone the axon branches,and each branch is swollen to form a bouton orsynaptic knob. The boutons lie near a dendrite orcell body of another neurone. A typical motor neu-rone may have as many as 10,000 boutons on itssurface which originate from other neurones.Axons also terminate on muscle fibres at a neuro-muscular junction, on some blood vessels and inglands.

An impulse is a localised change in the mem-brane of a neurone. When a neurone is excited, themembrane over a small area allows charged parti-

cles (ions) to pass across the membrane, a processknown as depolarisation, and an impulse is gener-ated. The area of depolarisation then moves to theadjacent area and the impulse travels down themembrane in one direction only. Each impulse isthe same size, like a morse code of dots only, butthe signals carried can be varied by the rate andpattern of the impulses conducted along theneurone.

A synapse is the junction where impulses passfrom one neurone to the next. Impulses always trav-el in one direction at a synapse, i.e. from the axonof one neurone to the dendrites and cell body ofthe next neurone. This ensures the one-way trafficin the nervous system.

When an impulse arrives at the end of the axon,a chemical is released from the boutons into thegaps between them and the next neurone. Thechemical is known as a neurotransmitter and thegap is the synaptic cleft (Fig. 1.12). Each moleculeof the neurotransmitter has to match special pro-tein molecules, known as receptor sites, on the nextneurone. When the transmitter locks on to thereceptor site, the combination triggers the depo-larisation of the membrane of the second neuroneand impulses are conducted down it. Next, thetransmitter substance is broken down by enzymes,taken up again by the boutons, re-formed andstored.

Each neurone has a threshold level of stimula-tion. The level of excitation reaching a neuronemust be sufficient to depolarise the membrane, sothat impulses are generated. Some impulses reach-ing a neurone affect the membrane in such a waythat no impulses are propagated, this is known asinhibition. The source of inhibitory effects may bethe presence of small neurones, the activity ofwhich always produces inhibition, or the release ofdifferent transmitter substance from the boutonsof the axon. The mechanism of inhibition will bediscussed in more detail in Chapter 12.

Various transmitter substances have been iden-tified in the nervous system. These include acetyl-choline, adrenaline, dopamine and serotonin.Acetylcholine is the neurotransmitter released atmost of the synapses in the pathways involved inmovement, and also at the neuromuscular junc-tions. Drugs that prevent the release of acetyl-choline at synapses are used as relaxants formuscles, for example in abdominal surgery.


Clinical note-pad 1E: Multiple sclerosis (MS)In multiple sclerosis, changes in the myelinsheath around axons result in the formation ofplaques, which affects the rate of conduction of nerve impulses. Axons in the central nervoussystem (brain and spinal cord) are affected, while those in the peripheral nervous system are not. The visual system seems to be mostsensitive to plaque formation. Disturbance ofboth movement and sensation occurs. Fatigueand cognitive impairment are other commonclinical features that affect function. The num-ber of plaques and their sites vary between indi-viduals and with time in the same individual, sothat the disease sometimes follows a course ofrelapse and remission. In some patients there isprogressive deterioration.

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Neuroplasticity The total number of neurones in the braindecreases after early adult life. Despite this loss, thebrain retains its capacity to learn new skills and touse knowledge in different ways. Brain injurydestroys neurones, either by direct damage to theneurones or as a result of reduced blood flow to theaffected area of the brain. Recovery and rehabili-tation may produce a, sometimes remarkable,return of function. These facts suggest that thenervous system has the capacity to be modified andnew connections can be made, although the reasonsare not entirely understood.

Biochemical changes have been explored as a possible explanation for the plasticity of neurones.During the early development of the basic networksof the brain, protein substances called nervegrowth factors (NGFs) are present, but these

are absent in the adult brain. Attempts to re-introduce NGFs in patients with degenerativediseases of the nervous system have been largelyunsuccessful.

Evidence from animal experiments has demon-strated that structural changes in neurones canoccur in a damaged area in some parts of the brain.These changes include new synaptic connectionsmade by undamaged neurones and the sproutingof the axons to form synapses at sites that were pre-viously activated by injured axons. The time whenthe fibres make new connections coincides with thereturn of function.

Current knowledge supports some plasticity ofneurones in response to learning new skills andafter injury. The cell body of each neurone is rel-atively fixed, but the synaptic connections that itmakes with other neurones can be modified.


Fig. 1.12 Neurone and synapse; synaptic cleft enlarged.

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Motor and sensory neurones

So far, the structure and properties of a typicalneurone have been described. Electrochemicalchanges in the dendrites and cell body result inimpulses that are propagated in one direction only,down the axon. In the organisation of the nervoussystem in development, the cell bodies of neuroneslie in the central nervous system (brain and spinalcord) and the axons lie in the peripheral nerves that leave it to be distributed to all parts of thebody.

Motor (efferent) neurones carry impulses away from the central nervous system to all partsof the body, or from the brain down to the spinalcord.

Sensory (afferent) neurones develop in a differ-ent way. The cell bodies of the sensory neuronesare found in ganglia just outside the spinal cord. There are no synaptic junctions on the cell bodies, and the axon divides into two almostimmediately after it leaves the cell. The two branch-es formed by this division are a long process in a peripheral nerve that ends in a specialisedsensory receptor, for example in the skin or amuscle, and a short process that enters the

spinal cord and terminates in the central nervoussystem.

Figure 1.13a shows the arrangement of a typicalsensory neurone. It is sometimes called ‘pseudo-unipolar’, since it has one axon but appears to bebipolar. Compare this with the multipolar motorneurone shown in Figure 1.12. Figure 1.13b showsthe position of a sensory neurone in relation to thespinal cord, a spinal nerve and its branches. Notethe cell body lying in a ganglion (swelling) and theaxon entering the spinal cord. Sensory neuronescarry impulses from the body towards the centralnervous system, or from the spinal cord up to thebrain.

Interneurones are those which lie only in thecentral nervous system and their axons do notextend into the nerves leaving it.

The motor unit

The motor neurones in the spinal cord, which acti-vate the skeletal muscles, lie in a central H-shapedcore of grey matter. These lower motor neuronesare found in the anterior (ventral) limb of the greymatter. Neurones that activate a particular groupof muscles lie together and form a motor neurone


Fig. 1.13 Sensory neurones: (a) typical sensory neurone; (b) the position of a sensory neurone in a spinal nerve andthe spinal cord.

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pool (Fig. 1.14). The axons of these neurones liein spinal nerves that branch to form the nerve sup-plying the muscle. There are fewer motor neuronesin the pool than muscle fibres in the muscle, andtherefore each neurone must supply a number ofmuscle fibres.

A motor unit consists of one motor neurone in the anterior horn of the spinal cord, its axon

and all the muscle fibres innervated by thebranches of the axon (Fig. 1.15). The number ofmuscle fibres in one motor unit depends on the function of the muscle rather than its size.Muscles performing large, strong movementshave motor units with a large number of musclefibres. For example, the large muscle of the calf hasapproximately 1900 muscle fibres in each motorunit. In muscles that perform fine precision move-ments, the motor units have a small number ofmuscle fibres (e.g. up to 100 in the muscles of thehand). The muscle fibres of one motor unit do notnecessarily lie together in the muscle, but may bescattered in different fasciculi. The number ofmotor units that are active in a muscle at any onetime determines the level of performance of themuscle.


Clinical note-pad 1F: PeripheralneuropathiesNeuromuscular disorders that are neurogenicoriginate in the nerve supply to the muscles,either in the spinal cord, in the nerve roots or inthe peripheral nerves (see Chapter 4). Neuro-pathies of peripheral nerves affect sensory andmotor axons, usually commencing distally, andare known as ‘glove and stocking’. Muscle weakness and sensory loss occur. Peripheral neuropathy can occur as a complication of diabetes that is not under control.

Guillain–Barré syndrome is an acute peri-pheral neuropathy that affects motor axons. Itusually follows a viral infection, and the result-ing motor weakness involves the trunk and prox-imal limb muscles, mainly in the lower limbs.Recovery is nearly always complete unless thereis severe involvement of the respiratory musclesor axonal damage.

Fig. 1.14 Motor neurone pool in the spinal cord.

Fig. 1.15 Motor unit.

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There are two types of motor unit.

• Low-threshold motor units supplying slow type I muscle fibres are involved in the sustained muscle activity that holds the postureof the body. The number of active motor units remains constant, but activity changesbetween all the low-threshold neurones. Theslow type I muscle fibres do not fatigue easily and the activity is maintained over long periods.

• High-threshold motor units with large-diameteraxons supplying fast type II muscle fibres areinvolved in fast, active movements, which movethe parts of the body from one position toanother. These motor units soon fatigue, butthey are adapted for fast, strong movements suchas running and jumping.

In a strong purposeful movement, such as pushingforwards on a door, the motor units are activatedor recruited in a particular order. The slow unitsare active at the start of the movement and thenthe fast units become active as the movementreaches its peak.

All muscle activity includes a combination ofslow and fast motor units. The slow units contributemore to the background postural activity, while thefast units play a greater part in rapid phasic move-ments. In manipulative activities the shoulder mus-cles have sustained postural activity to hold thelimb steady, while the hand performs rapid preci-sion movements, such as writing, sewing or usinga tool.

Receptors

Receptors are specialised structures that respondto a stimulus and generate nerve impulses in sen-sory neurones. They are collectively the source ofthe sensory information that is transmitted into thecentral nervous system. While there is awarenessof some receptor stimulation, a large amount ofsensory processing and the resulting responseoccurs below consciousness. I can feel the pressureof the fingertips on the computer keys as I write andhear the telephone when it rings. At the same time,I am unaware of the receptors in the muscles in theneck and the balance part of the ear responding tochanges in the position of the head so that my pos-ture is adjusted to keep my eyes on the keyboard.

A system of receptors that respond to a specificstimulus is known as the modality of sensation, forexample tactile modality. Several receptors of thesame type may give input into one sensory neurone.The area covered by all the receptors activating onesensory axon is called a receptive field. There maybe overlap in receptive fields, so that stimulation


Clinical note-pad 1G: Motor neurone diseaseThis is a progressive disorder of the motorneurones in the spinal cord. Muscle weaknessand fatigue of the muscles of the limbs and thetrunk occur, which become generalised to affectswallowing and speech. There is no sensory loss.Onset is usually around the age of 40 years, withrapid deterioration over 3–5 years.

Fig. 1.16 Receptors in the receptive fields of two neurones.

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