Muscle Physiology!!!

Now that we’ve covered bones, let’s move onto muscles and how they work. I will be focusing on the physiology. To try to explain this clearly, we’re going to go through a set of steps that cause the muscle to contract.

First there is a neuron. This neuron transmits neurotransmitters, which are hormones that cause chemical reactions. They release the neurotransmitters when the dendrites receive a signal and shoot it down the axon and to the synaptic bulb. In the bulb, the neurotransmitters are floating around in synaptic vesicles. They are then released through the pre-synaptic membrane into the synapse. In our case now, the synapse is located between the neuron and a muscle fiber.

Once the neurotransmitters are released in to the synapse, they vibrate until they attach to their neurotransmitter specific receptors which are located on the sarcolemma (cell membrane of the muscle fiber). In our case, the neurotransmitter to activate a muscle fiber is acetylcholine. Once acetylcholine binds to the muscle fiber, sodium specific channels open. Sodium rushes into the interior of the muscle fiber. Once its threshold is reached, the sodium gates close. Then potassium gates open and potassium rushes out of the muscle fiber into the synapse. This causes an action potential which activates the muscle fiber. This signal travels along the muscle fiber cause sodium voltage gates channels to open and this action repeats down the sides of the muscle fiber with a wave of voltage.

For the muscle to makes it’s movements, there is a presence of calcium. In the muscle fibers, there is a t-tubule system which takes this wave of voltage and charges the sarcoplasmic reticulum. This open the calcium channels and allows calcium to rush out. This causes thick and thin filaments, myosin and actin, respectively, to be shifted to make a movement. This happens because the actin and the myosin are very close together. There are myosin heads located on the myosin. The myosin heads bind to a receptor on a thin filament and shoot ATP which drives the thin filaments forward. The myosin heads, when driven forward, pop out of the receptor and latch onto another one. This repeats as the heads move down a line, causing a contraction.



Picture taken from: http://en.wikipedia.org/wiki/Myofibril

Bones!!

I’m finally into the school routine to begin sharing my learning process with you and the blogosphere. I am now taking an Anatomy and Physiology class which is going on the summer, so I will have the opportunity to share what I have learned about the human body throughout the next three months.

For starters, I’m going to let you in on some information about bones. This will be all about their formation, growth, and repair.

Bones begin developing at the embryological stage of life. This happens in two processes. These processed allow for a smooth delivery through the birth canal and lay out the outline for the rest of life. The first process is called endomembraneous ossification. This process includes the development of the skill and the jaw. The skull forms a connective tissue membrane through exchanges of soft tissue and cells. This leads to eventual complete ossification postpartum. The reason the ossification only completes after birth is to allow for fontanelles, soft spots, for the birth process. These fontanelles allow for the skill to form to the shape of the birth canal. If the skull completely ossified in the womb, then both the baby and the mother would die in labor because the head would not be able to fit through.

Another process of bone development is called endochondral ossification. This involves the skeleton. Here the skeleton is mapped out by cartilage. It is made of a very flexible material until it is later ossified into its final shape. This material contains very little calcium, is not hard or dense, and is made of a mineral matrix containing a lot of water. The cartilage is produced by condroblasts which bring together the necessary proteins to create the condrocytes, which make up the solidified cartilage. Through cartilage proliferation, the skeleton quickly begins to take on shape. As blood vessels then begin to invade the cartilage matrix, they bring in hormones which turn this matrix into a bone matrix. In this newly forming matrix, small densities appear in which there are primary bone ossification centers. The fetus is now ready for birth, as most of the structure has turned to bone.

At this point the tips of the bones are still cartilage as well as a small section in between the long part and the ends. This allows room for joint cartilage and growth, respectively. The small sections that I referred to, are bands and are known as epiphyseal plates, growth plates. When bones are at their full growth size, these bands are no longer existent and the only cartilage on bones remaining is the joint cartilage.

Bones grow in two directions. They grow in length and they grow in width. The growth in length takes place in the growth plates. The part of the growth plate which is closer to the inside of the bone is called the diaphyseal side and the part towards the outside is the epiphyseal side. Cartilage proliferation occurs in which the epiphyseal side is pushed away with new cartilage. This activity is stimulated by nutrition and hormones. As the new cartilage is created, the old cartilage turns to ossified bone. Eventually, when the bones are at their complete size, the grow hormones will decrease and there will be an epiphyseal closure.


When bones grow in diameter, this is a process involving the endosteum and periosteum, inner and outer, parts of the bone. The periosteum lays down layers of new bone, making the bone wider. At the same time the endosteum eats away bone from the inside, to maintain a correct ratio of inner and outer bone.

Bone is made up of many different parts. The epiphyses is the structure at the ends of the bones. On one side, the epiphyses has a layer of cartilage. This cartilage is called the articular cartilage. It reduces friction and absorbs shock. This is where the bone forms a joint with another bone. On the other side of the epiphyses is the epiphyseal line. This is where bone length is developed. The epiphyseal line also connects the epiphyses to the diaphysis, shaft, of the bone. Within the diaphysis is the medullary canal. This is a space which contains fatty yellow bone marrow in adults. The endosteum, a thin membrane, lines the inside of the medullary canal. On the outside of the diaphysis, is the periosteum, a tough connective tissue. This contains the bone developing cells which allow for the bone to grow in thickness, which was mentioned earlier. In the bone there are two types of marrow. Red marrow produces blood cells and platelets, which is most seen in fetal bone development. Yellow marrow is made of adipose cells and acts as a storage for potential chemical reserve.

Bones continually remodel. This occurs through bone resporption and bone deposition, removal of minerals and collagen fibers and addition of these materials, respectively. This process is controlled by osteoclasts and osteoblasts. Osteoclasts are bone cells that release enzymes that digest protein, while helping regulate blood calcium levels. Osteoblasts are bone building cells which secrete the building component necessary for tissue development, as well as initiate calcification. The strength of a bone is related to the stress that it undergoes. So if a person exercises a lot, ten the new bone will grow in thicker. New bone is always more resistant to damage than old bone.


Last, but not least, I will talk about fractures and how the bone heals itself. When there is a fraction, this causes bleeding and then a hematoma forms. This formation of a hematoma is the first step of the healing process. Then cartilage cells begin to invade the hematoma and make a bridge between the two parts of the bone. This is called cartilaginous callous formation. This is followed by bony callous formation in which the cartilage is slowly replaced by a mass of spongy bone. The bone then goes through a process of remodeling in which the spongy bone is replaced with dense bone. When you go to the doctor for a broken bone, you will see that each of these processes of repair is paired with some sort of change in your own care for the healing. At first you are given a cast, to allow for a stillness so that the cartilage cells can build a secure bridge. Once the spongy bone is forms, it is attached enough for you to have the cast removed and to wear a brace or a splint.

I hope that this has been helpful and educational for you. More to come, readers!

Picture taken from : http://en.wikipedia.org/wiki/Bone