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BIOL1040
Module 4: Support and Movement
Skeletal Muscle
Describe the basic structure and roles of muscle
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Convert chemical energy into mechanical
energy
Movement (bones, blood, food)
Support (bony and soft tissue)
Protection (guarding of orifices)
Body temperature regulation
Nutrient store
Skeletal: voluntary, striated (repeating
sarcomeres)
Cardiac: involuntary, striated
Smooth (gut, bladder, vessels): non-striated
Properties of muscle tissue
 Excitability/conductivity
 Ability to respond to stimuli and produce
action potentials
 Contractility
 Ability to shorten and thicken
 Extensibility
 Ability to stretch without damage
 Elasticity
 Strain energy storage
Muscle fibre structure
 A-band (dark zone)
 M-line (binding of myosin)
 H-zone (myosin, no actin)
 Overlap zone
 I-band (light zone)
 Actin, no myosin
 Z-line (actinin proteins)
Describe the organisation of skeletal muscle and
its relation to the skeletal system
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Muscle origin
Muscle belly
Muscle insertion
Connective tissue
Endomysium
Perimysium
Epimysium
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Joints
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Hinge joint
Nonaxial
Gliding
Monoaxial/uniaxial
Hinge
Pivot
Biaxial
Ellipsoid
Saddle
Triaxial (multiaxial)
Ball and socket
Movement of synovial joints
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Flexion/extension in sagittal plane
Flexion decreases joint angle, extension
increases
Adduction/abduction in coronal plane
Adduction moves body part to midline, abduction moves away
Rotation along a long axis
Explain the intracellular basis and the sliding
filament theory of muscle contraction
Sliding filament theory
 Actin and myosin arranged in sarcomeres
orientated in one direction
 Contraction occurs by shortening of
sarcomeres which pull on attachment
points of muscle
Excitation-contraction coupling
 Motor unit – motor neuron plus all muscle
fibres it innervates
 More motor units with fewer fibre
innervations per unit leads to greater force
control and position control
Muscle contraction
1. Axon terminal of a motor neuron releases
acetylcholine
2. Acetylcholine diffuses across synaptic cleft
and binds to muscle fibre membrane
3. Depolarises muscle fibre membrane and
the action potential travels to the
sarcoplasmic reticulum (SR) via T tubules
4. Ca+ released from the SR binds to troponin
5. Troponin and tropomyosin undergo
conformational change and expose myosin
binding sites on actin
Pivot joint
Sagittal
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6. Actin and myosin form linkages
7. Myosin pulls actin filaments toward each other (inward)
8. Sarcomere shortens and contraction occurs
Explain how cellular level events relate to gross force production and movement
Action potentials across sarcolemma
1. ACh at motor end plate binds to nicotinic ACh receptors and opens ion channels
2. Na+ diffuses into fibre faster than K+ moves out
3. Interior becomes slightly less negative (local depolarisation of sarcolemma)
4. Charge change opens voltage-gated sodium channels so Na+ enters nearby (propagation of action
potential)
5. Na+ channels close, K+ channels open (repolarisation)
6. ATP dependent Na+/K+ pump restores resting concentrations of Na+/K+
 Muscle force dependent on number and timing of stimulation
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Tension generation depends on sarcomere length (muscle fibre length) and passive tension of noncontractile connective tissue
Skeletal muscle operates at lengths where tension is high
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Connective tissue resists stretch the more it’s stretched
Prevents muscle over-extension
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Electromyography (EMG)
 Records electrical activity of muscles
 Surface electrodes (superficial muscles)
 Fine-wire electrodes (deep or small muscles)
Death
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Blood stops circulating
Ca2+ leaks out of sarcoplasmic reticulum
ATP (produced anaerobically from glycogen) used in sustained muscle contraction due to excess Ca2+
Myosin heads cannot unbind after ATP is used up, stiffening body (rigor mortis)
Instantaneous (cadaver spasm)
Disappears with tissue decay
Rhabdomyolysis
 Disintegration or dissolution of muscle, associated with excretion of myoglobin in urine
 Caused by vigorous exercise, drugs, heatstroke etc.
 Symptoms include dark urine, weakness, renal failure
 Treated by drinking more fluids and diuretics
Skeletons
Distinguish between skeletal designs
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Skeleton is the framework of biological organisms
Does not always comprise bones
Skeletal features important for taxonomy
Hydroskeletons
 Fluid held under pressure in a closed,
semi-rigid body compartment
 Muscles attached to compartment
wall and alter shape
 E.g. earthworms have longitudinal
and circular muscles enabling
elongation and contraction
Exoskeletons
 Calcium carbonate shells or cuticle
 Muscles attached to inside of skeleton
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E.g. arthropods enlarge or shed and replace exoskeleton as they grow
Endoskeletons
 Vertebrates
 Organs
 Bones
 Cartilages
 Ligaments
 Bone marrow
 Tissues
 Connective tissues
 Cells
 Osteocytes
 Osteoblasts
 Osteoclasts
 Molecules/chemicals
 E.g. Ca2+ in extracellular matrix
List the basic roles of skeletons
Mechanical roles
 Support
 Protection
 Movement
Metabolic roles
 Nutrient store
 Minerals and lipids
 Blood cell formation
 Haematopoiesis
Describe the purposes and processes of growth and adaptation of
skeletal structures
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Growth changes shape and size
Undergoes bone apposition and removal
Primary bone
Original bone laid down during growth
Secondary bone
Secondary osteons formed as replacement bone
List the composition, structure and function of vertebrate bone
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206 bones in axial and appendicular skeletons of humans
Long bones
Spongy bone
Shaft with ends
Compact bone
Leverage/movement
Compact (lamellar or cortical bone) or trabecular (spongy bone)
E.g. femur, phalanges
Short bones
Square shaped
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Movement
E.g. carpals, tarsals, sesamoid bones
Flat bones
Protection/haematopoiesis
E.g. sternum, scapula, ribs
Irregular bones
Support, movement and haematopoiesis
E.g. vertebrae, os coxae, pneumatic bones
Bone matrix
 Hydroxyapatite
 Resistance to compression
 Comprises 65% of bone tissue
 Contains nearly all of body’s calcium
 Bone brittleness
 Collagen
 33% of bone tissue
 Bone flexibility
 Limited mineralisation
 Osteocytes
 Maintain matrix
 Osteoblasts
 Create matrix
 Osteoclasts
 Breakdown matrix
Describe why and how bone is modelled and remodelled
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Bones are hollow to resist bending and fracturing
Larger hollow area increases strength of bony element
Cortical remodelling
 Bone modelling units (BMU) are cells responsible
 Osteoclasts excavate tunnel parallel with diaphysis and osteoblasts refill tunnel with osteoid
 Osteoid gradually mineralises
Trabecular remodelling
 Resorption occurring faster than deposition leads to loss of bone tissue and structural change
 Increases risk of bone fracture
 Osteopenia
 Bone mass 1-2.5 standard deviations below mean for young adults
 Osteoporosis
 Bone mass less than 2.5 SDs below mean