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Background to the Skeletal System




MAJOR BONES
Above are listed the main bones that you should be able to recognize in the human skeleton and use in any description for an A.S. exam
 * 1) There are approximately 208 bones in the human body (between 300 and 350 when we are born)
 * 2) There are five classifications of bone: Long bones, short bones, flat bones, irregular bones and Sesamoid bones
 * 3) The human skeleton is composed of fused bones and individual bones supported by ligaments, cartilage, tendons and muscles
 * 4) There are two distinct divisions within the body - the axial and the appendicular

Function of the skeleton:
 * 1) The Skeletal System serves as a framework for tissues and organs to attach themselves to.
 * 2) The Skeletal System provides a protective structure for vital organs
 * 3) Located in long bones are two distinctions of bone marrow (yellow and red). The yellow marrow has fatty connective tissue and is found in the marrow cavity. During starvation, the body uses the fat in yellow marrow for energy.The red marrow of some bones is an important site for blood cell production, approximately 2.6 million red blood cells per second in order to replace existing cells that have been destroyed by the liver.
 * 4) Bones also store important minerals such as calcium and phosphorus. These help to regulate mineral balance in the bloodstream. The bone makes up a bank of stored minerals for either deposit or withdrawl during times of low mineral count.

Backbone The backbone or spine is divided into 4 sections: THe cervical vertebrae (7 unfused bones), the thoractic vertebrae (12 unfused bones), the lumbar vertebrae (5 unfused bones) and the sacrum (5 fused) and coccyx (4 fused bones). The size of each vertebral body increased from the cervical vertebrae to the lumbar vertebrae in order to support the weight of the body. The neural arch (within the vertebrae) enables muscles to attach via the transverse and the spinous processes, while the articular processes liink to adjacent vertebrae.

Picture source In between each vertebrae is a disc of fibro-cartilage, a tough resilient tissue which helps to absorb shock and allows a small amount of movement between the vertebral bodies. The vertebral column exhibits four curves. Two of the curves are concave in shape and two are convex in shape. These curves increase the strength of the structure as well as absorbing shock from jumping or walking.

Skeletal tissue consists of bone and cartilage.
 * SKELETAL TISSUE**

Cartilage is soft, slightly elastic tissue. It does not possess a blood supply and receives nutrition via diffusion from the capillary network outside the tissue. All bones start out as cartilage in the developing foetus, before being gradually replaced by bone.

This is found on the articulating surfaces of bones that form joints. It is bluish in colour and is composed of a fine network of collagen fibres. Joint movement improves the nutrition supplied to this tissue and can encourage growth. Hyaline cartilage often thickens as a result of ecercise which further protects the joints. During exercise, hyaline cartilage will soak up synovial fluid released from the synovial membrane, thus improving mobility in the joint
 * Hydraline cartilage**

Much denser tissue. It is tough and its shock absorption properties mean that it is often found in areas of the body where high amounts of stress are imposed. (knees).
 * White fibrocartilage**

This is a much more pliant and flexible tissue giving support and also flexibility. The external ear is an example of this.
 * Yellow elastic cartilage**

Bone on the other hand is a rigid tissue, non-elastic composed of approximately 65% minerals (calcium phosphate, magnesium salts) and 35% organic tissue such as collagen. This collagen gives the bone some resilience and prevents the bones from breaking on the slightest of impacts. Bone tissue can be categorized into compact or cancellous.

This is hard bone and it forms the surface layers of all bones and the whole of the cylindrical shaft of long bones. It goes some way towards protecting the bones from external forces or impacts and has great weight-bearing properties. Surrounding the compact bone is the **periosteum** which is fibrous and extremely vascular tissue. The periosteum enables tendons to attach to bones, which transit the muscular 'pull' and therefore allows movement to take place.
 * Compact Bone**

This is the spongy bone that lies beneath and alongside compact bone. It has a honeycomb appearance. The bone is constantly re-organised in response to the various stresses placed upon the bone. The matrix within the bone is the best way to combine strength with the minimum of weight, so that bones can take much stress yet are light and easily moved. In addition the space is filled with marrow, and this is where red blood cells are manufactured.
 * Cancellous Bone**

Bone is formed during the process of **ossification**

Source source Ligaments are tough fibrous connective tissue that is composed almost wntirely of thick bundles of collagen fibres. Their main role is to attach **one bone to another and therefore they typically occur at joints.** They are respobsible for stabilising joints and to enable effective movement to take place. Source
 * Types of Bones**
 * Ligaments**
 * Classification of Joints**

These are very stable joints and allow no observable movement to occur. Bones are often joned by strong fibres called sutures.
 * Fixed Joints**

These are joined by a tough fibrous cartilage which provides stability and possesses shock absorption properties. However a small amount of movement is available, for example between the lumbar bones intervertebral discs of cartilage.
 * Cartilaginous or slightly moveable joints**

These are the most common type of joint in the body and the most important in terms of physical activity.
 * Synovial Joint**

The joint is enclosed in a fibrous joint capsule, which is lined with a synovial membrance. Lubrication is provided by the synovial fluid which is secreted into the joint by the synovial membrane. In addition, where the bones come into contact with each other, they are lined with smooth, yet hard wearing hyaline or artiular cartilage.

Synovial joint stability is provided by the strength of the muscles crossing the joint, which are supported by ligaments that may be inside or outside the capsule. Ligaments are very elastic and lose effectiveness to some degree when torn or stretched.

Some synovial joints possess sacs of synovial fluid known as bursae, which are sited in areas of increased pressure or stress and help to reduce friction as tissues and structures move past each other. Pads of fat help to absorb shock and improve the fit of the articulating bones. Joints (picture source )

A movement of a limb or body part will always have a beginning and an ending point. Through analysing the position of the finishing point relative to the starting point we can form a classificaiton of movement.
 * Movement Patterns occuring at the joints**

Flexion occurs when the angle between the articulating bones is decreased. e.g raising the lower arm up to touch the folder. Flexion occurs in the median plane about the horizontal axis.
 * Flexion**

Extension of a joint occurs whent he angle of the articulating bones is increased. When standing up from a seated position, the angle between the femur and tibia increases. Extreme extension is known as hyper-extension and this is at an angle greater than 180 degrees. Extension occurs in the median plane about the horizontal axis.
 * Extension**

This is movement of a body part away from the midline of the body or the other part. Abduction occurs when the arms are raised laterally from your side. Abduction occurs in the frontal plant about an anterio-posterior axis,
 * Abduction**

Adduction is the opposite of abduction and concerns movement towards the midline of the body. Adduction occurs in the frontal plane about an anterio-posterio axis.
 * Adduction**

Circumduction occurs when a circle can be described by the body part. This is usually a combination of flexion and extention. Trye circumduction normally occurs at a ball and socket joint. This occurs in the median and frontal planes
 * Circumduction**

Pronation occurs at the elbow and involves internal rotation between the radius and humerus. It typically occurs when the palm of the hand is rotated to face down. Pronation occurs in the horizontal plane about a longitudinal axis
 * Pronation**

Supination is the opposite of pronation.
 * Supination**

Rotation of a joint occurs where the bone turns about its axis within the joint. Rotation towards the body is termed internal or medial and rotation away from the body is termed lateral or external. Rotation occurs in the horizontal plane, about a longitudinal axis.
 * Rotation**

This occurs at the ankle joint and is typified by the pointing of the toes. This occurs in the median plane about an horizontal axis
 * Plantarflexion**

This also occurs at the ankle joint and occurs when the foot is raised.
 * Dorsiflexion**

This occurs when the sole of the foot is turned inwards towards the mid-line of the body. Inversion occurs on the frontal plane.
 * Inversion**

Eversion occurs when the sole of the foot is turned laterally outwards.
 * Eversion**

=Muscles=



=Types of Muscle=

(source)



Skeletal Muscles are those which attach to bones and have the main function of contracting to facilitate movement of our skeletons. They are also sometimes known as striated muscles due to their appearance. The cause of this 'stripy' appearance is the bands of Actin and Myosin which form the Sarcomere, found within the Myofibrils. Skeletal muscles are also sometimes called voluntary muscles, because we have direct control over them through nervous impulses from our brains sending messages to the muscle. Contractions can vary to produce powerful, fast movements or small precision actions. Skeletal muscles also have the ability to stretch or contract and still return to their original shape.

**Skeletal muscle fibre type**
Not all fibres within Skeletal muscles are the same. Different fibre types contract at different speeds, are suited to different types of activity and vary in colour depending on their Myoglobin (an oxygen carrying protein) content. Skeletal muscle is external and used primary for movement.

//Properties://
 * Extensibility:** Skeletal muscle can lengthen when contracting and provide effort required to move the lever system.
 * Elasticity:** The ability of muscle to return to its resting state and shape once it has been stretched
 * Contractility:** This is the ability of the muscle to contract or shorten when stimulated by nerves

//Functions:// Heat Production:** The contraction of skeletal muscle involves the production of energy. This is a byproduct of the breaking down of glycogen to release energy. Shivering is an involuntary muscle when the muscle is cold which is used to generate heat.
 * Movement:** Skeletal muscles attach to bones, against which they pull and enable movement.
 * Support:** Muscles are seldom fully relaxed and are often slightly contracted. A key term with support refers to **muscle tone

Smooth muscle
Smooth muscle is also sometimes known as Involuntary muscle due to our inability to control its movements, or Unstriated as it does not have the stripy appearance of Skeletal muscle. Smooth muscle is found in the walls of hollow organs such as the Stomach, Oesophagus, Bronchi and in the walls of blood vessels. This muscle type is stimulated by involuntary neurogenic impulses and has slow, rhythmical contractions used in controlling internal organs, for example, moving food along the Oesophagus or contricting blood vessels during Vasoconstriction.

Cardiac muscle
This type of muscle is found solely in the walls of the heart. It has similarities with skeletal muscles in that it is striated and with smooth muscles in that its contractions are not under conscious control. However this type of muscle is highly specialised. It is under the control of the autonomic nervous system, however, even without a nervous imput contractions can occur due to cells called pacemaker cells. Cardiac muscle is highly resistant to fatigue due to the presence of a large number of mitochondria, myoglobin and a good blood supply allowing continuous aerobic metabolism.



Skeletal Muscle Structure


source =Structure of Skeletal Muscle=


 * Epimysium:** Protective layer of connective tissue surrounding the entire surface of the muscle
 * Fasiculi**: These are bundles of muscle fibres
 * Myofibrils:** Each fibre within a fasiculi contains many smaller fibres called myofibrils. These provide the contractile unit of the muscle

Myofibrils have characteristic light and dark bands (striations) which represent a **sarcomere.** This is repeated along the length of the myofibril.

Sarcomeres have a highly organised structure, and at the most fundamental level the sarcomere is composed of two protein based **myofilaments

1) A thick myosin filament 2) A thin actin filament**

The interaction and overlapping of these two myofilaments enables muscles to contract through the **sliding filament theory**

=Sliding Filament Theory= Myosin filaments are composed of many myosin molecules, which are made up of two parts: a rod and a head. This in turn forms a golf-club shaped molecule. The head of each molecule contains **ATPase,** an enzyme which is used to break down adenosine triphosphate (ATP), which in turn releases energy for muscular contraction. This energy is used to bind the myosin cross-bridge onto the actin filament, thereby allowing muscular contraction. The sliding of the filaments allow the myosin cross bridges to continually attach, detach, and reattach. It is the sweeping action or the power stroke of the myosin head which causes the actin filaments to be pulled towards the centre, and slide past the myosin filaments. The use of ATP that releases energy allows this all to take place.

The action of the sliding filaments during contraction causes the shortening of all sarcomeres and therefore all muscle fibres. //Relaxation: Muscles relax passively - the cross bridges uncouple and allowing the sarcomere to return to its original length Contraction: Requires an impulse from the central nervous system. The motor neurone, and the fibres it stimulates is called a motor unit which is the functional unit of skeletal muscle.//

Muscle Types
Muscle fibres are composed of thousands of individual muscle fibres which are held together by connective tissue. Muscle fibres may differ in their physiological make-up and it is the type of fibre which exists that explains differences in performance.

Skeletal muscle has two main fibre types:slow twitch and fast twitch. Muscle proportions tend to be inherited. Slow twitch muscles are designed for long periods of low intensity work (long distance swimmer) Fast twitch fibres can generate vast amounts of force, but tire easily (100m sprinter)

Fast Twitch type 2a (fast oxidative glycolytic fibres (FOG)) can pick up characteristics through endurance training. These have a greater resistance to fatigue. An 800m run might rely on type 2a fibres Fast Twitch type 2b (fast twitch glycolytic (FTG)) are used for activities of very high intensity and have a much stronger force of contraction. These muscle fibres tend to be larger and thicker. Power lifters or sprinters have a large proportion of these fibres. =Exercise and the Skeletal System= How does exercise benefit the skeletal system? 1) Skeletal tissue becomes stronger due to the stresses placed upon it 2) Cartilage thickens which aids cushioning 3) Tendons thicken which allows them to withstand greater force 4) Greater range of movement at the joints due to increased flexibility

=Connective Tissues= Connective tissue is required and responsible for holding all of the individual muscle fibres together. It surrounds the individual muscle fibres and encases the whole muscle, forming tendons, which attach the muscles to bones and transmit the 'pull' of the muscle to the bones to cause movement and harness the power of muscle contractions. Tendons vary in length and are composed of parallel fibres of collagen. They attach directly to the periosteum of the bone via a tough tissue known as **Sharpey's fibres source** The point of attachment for each muscle are termed the **origin and the insertion** Origin: This is the end of the muscle attached to the nearest stable bone (usually a flat bone) which the muscle can pull Insertion: This is the muscle attachment on the bone that the muscle puts into action (e.g. the bicep is a flexor muscle as it raises the lower arm. Flexion takes place at the elbow joint. Since the bicep raises the lower arm, it must be attached to that body part via the insertion. The bicep has its insertion on the radius). It is not unusual for a muscle to have several origins, while maintaining a common insertion. Biceps have two heads for example. It has only one insertion on the radius. source =Antagonistic Muscle Groups= Muscles never work alone. The elbow is an example of flexion taking place. Flexion of the arm at the elbow, the muscle responsible for the flexion is the //biceps brachii// and the musce which produces the desired joint movement is the **agonist or prime mover** However, for the muscle to shorten when contracting, the tricep muscle must lengthen. The tricep in this instance is known as the **antagonist** since the action is opposite to the bicep.


 * Fixator** muscles or stabilisers also work in this movement. Their role is to stabilise the origin so that the agonist can achieve maximum and effective contraction. In this case the trapezius contracts to stabilise the scapula to create a rigid platform. **Neutralisers or synergist** muscles in this movement prevent any undesired movements which may occur, particularly at the shoulder where the bicep works over two joints.

=Types of Muscle Contraction= In order to produce the vast range of movements of which it is capable, the body's muscles either shorten lengthen or remain the same length while contracting.


 * __Isotonic__** contractions refer to those instances when the muscle is moving while contracting (these can be divided into concentric and eccentric)


 * //Concentric//** contractions involve the muscle shortening while contracting as happens in the bicep brachii during the upward phase of a bicep curl.


 * //Eccentric//** contractions involve the muscle lengthening while contracting. This can be seen in the bicep during the downward phase of the bicep curl. THis is used to counteract the force of gravity. **PLYOMETRICS** is a type of exercise strength training which is based on muscle contracting eccentrically.


 * __Isometric__**

Isometric contraction is when the muscle is neither lengthening or shortening. The muscle remains the same length while contracting. Remaining upright is an example of muscles working isometrically. Handstands are an example of this, or holding a weighted position.


 * __Isokinetic__**

Normally when a muscle contracts the angular velocity of the muscle shortening or lengthening varies throughout the contraction. Machines have been devised so that it is possible to keep the speed at which the muscle lengthenes or shortens constant, but not necessarily the resistance applied. The speed of the movement cannot be increased. Any attempt to increase the velocity results in an equal reaction force from the machine. Isokinetic training is excellent for strength training.

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