Endochondral Ossification: produces the vertebrae, pelvic bones and bones of the limbs. This is a process where the connective mesenchyme tissue turns into hyaline cartilage. Hyaline cartilage consists of specialized cells that produce a matrix. These cells called Chondroblasts and when they are surrounded by matrix they become a chondrocytes. These are rounded cells that occupy a space within the matrix called a lacuna. The matrix contains collagen, which provides strength, and proteoglycans, which make cartilage resilient by trapping water.
The Role of Cartilage
Cartilage plays a number of roles related to bone growth and repair, support and joint movement. Although there are three types of cartilage in the body, and cartilage, most of the bones in the body develop from hyaline cartilage growth. Furthermore, the repair of damaged bone often involves hyaline cartilage. Hydraline cartilage resembles the shape of the bone yet to come – a flexible rubbery matrix – which is then broken down, reorganised, and calcified to form a bone.
The joint surface were one bone meets another is covered with a layer of hyaline cartilage called the articular cartilage. Together with a lubricating fluid secreted between the bones, this cartilage enables a joint to move far more easily than it would if one bone rubbed directly against the other.
Calcium Homeostasis
The skeleton is also the main storage site for calcium. Calcium, as we have described, is an important mineral for healthy bone growth – calcium moves into bone as osteoblasts and builds new bone.
The skeleton also plays an important part in regulating blood calcium levels. It is important that there is a balance of oesteoblast and oesteoclast activity – i.e. Calcium Homeostasis. The movement of calcium into and out of bone helps to determine the blood calcium levels. When osteoblast and osteoclast activity is balanced, the movement of calcium into and out of a bone is equal. Bone calcium levels must be maintained or new bone growth will be impaired. Similarly, blood calcium levels must be maintained within slim limits for functions such as muscle contraction and membrane potentials to happen normally.
The factors, which cause the skeleton to undergo a spurt of growth in puberty but then for growth to stop
Sex hormones, estrogen (a class of female sex hormones) and testosterone (a male sex hormone) initially stimulate bone growth. This accounts for the burst of growth that takes place at the time of puberty when production of these hormones increases. Both hormones stimulate ossification of epiphyseal plates, however, and thus the cessation of growth.
Estrogens cause a quicker closure of the epiphyseal plate than does testosterone; therefore females usually stop growing earlier than males. Because a female’s growth period is somewhat shorter, they do not usually reach the same height as males. However decreased levels of testosterone or estrogen can prolong the growth phase of the epiphyseal plates, even though the bones grow more slowly. Growth is very complex, and however is influenced by many factors in addition to sex hormones, such as other hormones, nutrition and genetics.
Task 2 – Synovial Joint
When working with clients it is important to describe clearly and accurately the movement’s possible at all the synovial joints. To do this, therapists use special terms. I will therefore describe clearly the movements possible at the knee and hip joints, which are both synovial joints.
Synovial Joints: Joints are classified according to the way that they are joined to the adjacent bones. Synovial joints such as the hip and the knee are diarthrosis - freely moving joints. The bones of the hip and knee joints are separated by a joint cavity, lubricated by synovial fluid, and enclosed in a fibrous joint capsule. There are 6 types of synovial joints: ball and socket; hinge; pivot; gliding and condyloid. The hip is a ball and socket joint and, the knee is a hinge joint.
The Knee Joint or tibiofemoral joint:
Are the largest and most complex diarthrosis joint in the body, it is located between the femur and the tibia bones of the leg, and is classified as a hinge joint. At a hinge joint, one bone has a convex surface (in this case the femur) that fits into a concave depression (in this case the tibia) of the other one. Hinge joints are monaxial, which means they can only move one way or on one plane, like a door hinge for example. The knee joint allows flexion and extension, but when the knee is flexed it is also capable of slight rotation and lateral gliding.
The Hip Joint or Coxal Joint:
Is a ball-and-socket joint, the smooth, convex head of the femur fits into the acetabulum – a concave socket in the os coxae (one of the bones in the pelvic girdle) - to form the coxal, or hip joint. The hip joint is a multiaxial joint, which means it can move in three ways or on 3 planes. The hip is capable of a wide range of movement, including flexion, extension, abduction, adduction, rotation and circumduction. Therefore this is why there is much more range of movement in the hip joint (a ball and socket joint) than the knee joint (a hinge joint). Because the ball-and-socket joint is multiaxial, it allows a wide range of movement in almost any direction, whereas a hinge joint can only really move in two directions.
Flexion: Flexion is movement that decreases the angle of a joint (the closing of a joint), usually in a sagittal plane. A sagittal plane extends vertically and divides the body or an organ into right and left portions. Both the hip and knee joints are capable of Flexion. Examples include raising the thigh and bending the knee, as in lifting the foot up onto a step.
Extension: Extension is movement that straightens a joint (the opening of a joint) and generally returns a body part to anatomical position – i.e. extended straight up and down. Both the hip and the knee joints are capable of extension. Examples include straightening the knee and lowing the thigh to place the foot back down to the floor.
Circumduction: Circumduction is movement in which one end of an appendage remains relatively stationary while the other end makes a circular motion. Only the hip is capable of Circumduction, hinge joints such as the knee or elbow are not. For example, if a person were to extend their leg and draw a circle in the air with their big toe, the hip would stay fairly stationary while the ankle would move in a circular motion. However the knee is not capable of moving in a circle at all. Circumduction is actually a sequence of flexion, abduction, extension, and adduction.
Abduction: Abduction is movement of a body part away from the midsagittal line, which is the midline of the body, which runs vertically and divides into equal right and left halves. For example standing spread-legged. To abduct the legs is to spread them apart. The knees cannot be abducted.
Adduction: Adduction is movement toward the midsagittal line, returning the body part to anatomical position. For example standing with the legs back together again. .
Rotation: Rotation is a movement where a bone turns on its longitudinal axis. Twisting the leg so that the foot and knee face outwards or inwards is rotation of the leg on its axis. As mentioned earlier the knee when flexed is capable of slight rotation of the leg.
Task3 – Muscle Structure and Contraction
The Structure of a Skeletal Muscle
A skeletal muscle consists of both muscular tissue and connective tissue. Muscle fibres are associated with smaller amounts of connective tissue, blood vessels, and nerves. Skeletal muscle fibres are around 10 to 100 um in diameter and up to 30 cm long. The endomysium surrounds the muscle fibres with a delicate network of loose connective tissue called areolar; this allows room for blood capillaries and nerve fibres to reach each muscle fibre. Fasciculus are grouped bundles of muscle fibres, which are visible to the naked eye as parallel strands. The perimysium is a connective tissue sheath, which separates each fasciculus from its neighbouring one. The perimysium is usually thicker than the endomysium. The epimysium surrounds the muscle as a whole with a dense, collagenous connective tissue sheets called fascia. Deep fascia runs between neighbouring muscles, and superficial fascia is the connective tissue between the muscles and skin.
How muscles are attached to the bone: Muscle can attach to the bone in two different ways. The first way is by a deep fleshy attachment, where the collagen fibres of the epimysium runs into, or appears to merge with the periostium, which is the fibrous sheath around the bone.
The second way is an indirect attachment, the collagen fibres of the epimysium continue as a strong fibrous tendon that merges into the periostium of a nearby bone.
The Events leading up to the Contraction of a Muscle Fibre
Excitation is the process in which action potentials in the nerve fibre lead to action potentials in the muscle fibre. An action potential is a quick up-and-down voltage movement, from the negative voltage called the resting membrane potential (RMP), to a positive value and then back to a negative value again.
Excitation happens in five smaller steps:
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A nerve signal arrives at the synaptic knob and stimulates voltage-gated calcium channels to open, and this is where the calcium ions enter the synaptic knob.
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Calcium ions stimulate the exocytosis of the synaptic vesicles; they in turn release acetylcholine (ACh) into the synaptic cleft.
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ACh diffuses across the synaptic cleft and binds to receptor proteins on the sarcolemma.
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These receptors are ligand-gated ion channels. When ACh (the ligand) binds to them, they change shape and open an ion channel through the middle of the receptor protein. Each channel allows Na+ to diffuse quickly into the cell and K+ to diffuse outward. As a result of these ion movements, the sarcolemma reverses polarity. This means that as Na+ enters, its voltage quickly jumps from the RMP of –90 mV to a peak of +75 mV, and then falls back to a level close to the RMP as K+ diffuses out. The end-plate potential (EPP) is this rapid fluctuation in membrane voltage at the motor end plate.
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Areas of sarcolemma next to the end plate have voltage gated ion channels that open in response to the EPP. Some of the voltage-gated channels are specifically for K+ and allow it to leave. These ion movements create an action potential; therefore the muscle fibre is now excited.
Excitation-Contraction Coupling Process refers to the events that link the action potentials on the sarcolemma to activation of the myofilaments - protein microfilaments in the muscle, mainly made up of myosin or actin - thereby preparing them to contract.
There are 4 steps in the coupling process:
- A wave of action potentials spreads from the end plate in all directions, like ripples on water. When the wave of excitation reaches the
T tubules, it continues down them into the sarcoplasm.
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Action potentials open voltage-regulated ion gates in the T tubules. These are physically linked to calcium channels in the terminal cisternae of the sarcoplasmic reticulum (SR), so gates in the SR open as well and calcium ions diffuse out of the SR, down their concentration gradient and into the cystol.
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The calcium ions then bind to the troponin of thin filaments.
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The troponin-tropomyosin complex change shape and moves to a different position. This therefore exposes the active sites on the actin filaments and makes them available for binding to myosin heads. Contraction then occurs.
Bibliography
Thibodeau A. Gary, Patton T. Kevin, Anatomy and Physiology Fourth Edition, 1999
Saladin S. Kenneth, Anatomy and Physiology second edition the unity of form and function, 2001
Tate Stephens Seeley, Anatomy and Physiology fifth edition, 2000