Mammalian Physiology- Support and locomotion

Includes info. on skeletal structure, muscles and movement, structure and function of striated muscle and effects of ageing

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Support and Locomotion

Structure of Bone

  • Compact bone forms outer layers of shafts in long bones in limbs and flat bones, i.e. ribs
  • Haversian system makes up compact bone. Concentric circular arrangements. Contain dark patches called lacunae with branches called canaliculi are filled with living cells called osteocytes
  • Osteocytes start as bone forming cells called osteoblasts. Osteoblasts synthesise and secrete tropocollagen. Tropocollagen molecules link up to form collagen. Amongst collagen an inorganic molecule consisting of calcium phosphate is deposited. The osteocytes/blasts then become totally surrounded by a matrix of organic and inorganic molecules.
  • Bone is a composite material. The organic component gives it high tensile strength. The inorganic component = compressive strength.
  • The osteocytes maintain contact with eachother through cytoplasmic processes in canaliculi. In centre of Haversian system is a canal containing an artery, a vein and a lymphatic vessel
  • Living bone is also broken down by osteoclasts
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Structure of Collagen

  • Hyaline cartilage found covering the ends of bones at moveable joints, in the ribs and in the trachea
  • Living cells in cartilage are called chondrocytes. Chondrocytes are responsible for producing and maintaining their matrix. They have a large golgi body and extensive E.R.
  • Cartilage does not contain blood vessels thus chondrocytes obtain nutrients through diffusion from surrounding tissues
  • Hyaline cartilage is flexible, smooth and slippery. Slipperiness is achieved by a thin layer of cartilage which is covered in a thin glycoprotein rich layer

The Axial Skeleton

  • Comprises of bones lined along the midline of the body, i.e. skull, vertebral column, ribs and sternum
  • Vertebral column is made up of 33 vertebraes, including 7 lumbar vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused sacral vertebrae (sacrum) and 4 tail vertebrae (coccyx)
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The Axial Skeleton continued...

  • Main load bearing part of the vertebrae is the centrum. Along this is the neural arch, beneath which the spinal cord lies in the neural canal
  • Projecting outwards and sideways from neural arch are bony processes to which muscles are attached. Sideways = transverse processes, backwards = neural spine. Also has articular processes
  • Lumbar vertebrae are the most solid, large centrum and large transverse processes. Needed because powerful back muscles are attached here. Neural spine is shorter and sturdier than thoracic vertebrae which articulate with the ribs
  • A place where two bones meet is called a joint and joints between vertebrae are known as intervertebral joints.
  • Each vertebrae is joined to the next by ligaments.
  • Between each vertebrae there are discs of cartilage known as intervertebral discs.
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The Appendicular Skeleton

Muscles and movement

  • Muscles are highly specialised tissues which exert a lot of force when they contract
  • Smooth muscle: able to contract slowly and strongly for long periods of time
  • Cardiac muscle: found only in the heart where it contracts and relaxes rhythmically
  • Skeletal muscle: muscle attached to the skeleton, responsible for all movements under conscious control
  • Striated muscle: "stripey"- includes cardiac and skeletal

Structure of a joint

  • Ball and socket joints- allow a rotary movement
  • Hinge joints- allow movement in one plane only
  • Joints at which the bones move substantially are called synovial joints. The bones are held together by a capsule made of strong connective tissue containing collagen.
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Structure of a Fibril

  • The broad dark bands are A bands and lighter bands are I bands. Each I band has a thin dark Z line in its centre. This is called the M line in A bands. The part of a fibril lying between 2 Z lines is called a sarcomere
  • Proteins in the fibrils form filaments which run lengthways along the fibril. One is myosin, which forms thick filaments and the other is actin, which forms thin filaments.
  • Actin is globular but many actin bind to form long chains. 2 chains twist to form the filament. Actin chains are anchored in the Z lines
  • Two other proteins also involved in the structure are troponin and tropomyosin

How muscles contract

  • The thin filaments slide between the thick filaments, shortening the sarcomere.
  • This is done by the myosin heads binding to the actin. The heads then tilt 45 degrees. The actin gets pulled along to the centre of the sarcomere
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Structure of a joint continued...

  • The capsule is lined by a thin synovial membrane. The cells in the synovial membrane secrete a clear watery fluid- reduces friction
  • Synovial fluid also provides chondrocytes with their nutrients
  • Ends of bones covered with a thin layer of hyaline cartilage

Movement of the Elbow joint

  • Brought about by the contraction of muscles. Muscles are attached to bones by tendons
  • Muscle tissue is made up of fibres. Pairs of muscles are called antagonistic muscles
  • The elbow however, has more than two muscles that cause its movement. The biceps muscle is attached to the scapula and the radius. The brachialis muscle is attached to the humerus and the alna. Both of these muscles cause the elbow joint to flex (bend) thus are called flexor muscles
  • Their antagonistic opposite is the triceps. This is attached to the scapula, humerus and the alna. Its contraction causes the elbow joint to straighten thus its called an extensor muscle
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Bones as levers

  • We use a lever to help us exert a large force. This kind of lever is called a force multiplier. The opposite is a distance multiplier (see diagrams).
  • Biceps- distance multiplier as force needed is greater than the weight of what needs lifting

Structure and Function of Striated Muscle

Muscle Histology

  • Made up of many fibres. Fibres are formed from many cells whose plasma membranes have broken down
  • Thus it is a large multi-nucleated cell called a syncitium. The syncitium is surrounded by a plasma membrane called a sarcolemma.
  • Fibres are filled with parallel structures, made up of light and dark staining bands. These are called fibrils/or myofibrils
  • Inbetween fibrils are lots and mitochondria and extensive S.R.
  • Sarcolemma is deeply infolded to form channels called T tubules
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How muscles contract continued...

  • As the myosin head tilts the ADP + Pi are released. ATP then binds to the head where it is converted into ADP + Pi. The energy released here provides energy for the removal of the myosin head from the actin
  • The myosin head returns to its original position, binds to the actin, pulls it along, is released and so on...

How a nerve impulse causes muscle contraction

  • Caused by the arrival of APs along the axons of motor neurones
  • The small gap between the membrane of the axon and the sarcolemma of the muscle fibre is called a neuromuscular junction
  • The motor neurone axon divides into several branches forming a motor end plate. One neurone can form neuromuscular junctions with several muscle fibres
  • An AP arrives at the pre-synaptic membrane of neuromuscular junction
  • Ca2+ ions flood into the axon causing vesicles of acetylcholine to release contents into the synaptic cleft.
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How a nerve impulse causes muscle contraction cont

  • Acetylcholine binds to receptor proteins in the sarcolemma
  • Na+ channels open thus Na+ ions depolarise the sarcolemma
  • This depolarisation spreads across the whole sarcolemma, down the T tubules and into the centre of the muscle fibre. It is transmitted to membranes of the S.R.
  • Ca2+ ions flood out of the cisternae and diffuse in amongst the fibrils
  • Calcium ions bind to the troponin in actin filaments. The troponin changes shape thus making the tropomyosin molecules move. This exposes a site on the actin to where the myosin can bind

Energy for muscle contraction

  • ATP provides energy. Only a small amount found in muscles thus more required when exercising. Muscles may use their store ofcreatine phosphate (donate a phosphate to ADP)
  • If more required then glycogen is broken down in muscles and liver aerobically
  • Glucose can be broken down anaerobically, however this produceslactic acid which is toxic
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Twitch and Tonic muscle fibres

  • All vertebrates have 2 different types of muscle fibres
  • Twitch fibres used for fast muscle contractions and are adapted for movements requiring quick and strong contractions such as sprinting. Thus they use ATP very fast from glycolysis. They contain few mitochondria and small amounts of myoglobin (thus white not red). More extensive S.R. than tonic and actively transport Ca+ ions faster then tonic
  • Tonic fibres are used for slow and prolonged contractions, such as posture and long distance running. Thus they use ATP slowly from aerobic respiration. They have large no.s of mitochondria and amounts of myoglobin (thus are red). They have a shorter diameter thus a short diffusion distance for oxygen.
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Effects of ageing

Osteoarthritis

  • Happens when rate of break down of cartilage outweighs the rate of renewal
  • affects the joints, causing pain and loss of movement
  • cartilage becomes rougher, due to changes in the glycoproteins and collagen that give it its resilience
  • can be partly relieved by anti-inflammatorys/also sugery

Osteoporosis

  • Loss of bone mass, when rate of breakdown by osteoclasts outweighs the rate of renewal by osteoblasts
  • High bone density when young, through exercise/calcium, can reduce the risk in later life
  • Loss of bone mass speeds up in women after the menopause due to a lack of oestrogen, thus hormone replacement therapy is used
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