It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves. Cancellous pronounced: KAN-suh-lus bone , which looks like a sponge, is inside compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae pronounced: truh-BEH-kyoo-lee. This is where bone marrow is found. How Do Bones Grow? Bone contains three types of cells: osteoblasts pronounced: AHS-tee-uh-blastz , which make new bone and help repair damage osteocytes pronounced: AHS-tee-o-sites , mature bone cells which help continue new born formation osteoclasts pronounced: AHS-tee-o-klasts , which break down bone and help to sculpt and shape it What Are Muscles and What Do They Do?
Humans have three different kinds of muscle: Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face. Skeletal muscles are called striated pronounced: STRY-ay-ted because they are made up of fibers that have horizontal stripes when viewed under a microscope.
These muscles help hold the skeleton together, give the body shape, and help it with everyday movements known as voluntary muscles because you can control their movement. They can contract shorten or tighten quickly and powerfully, but they tire easily.
Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. We can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically which is why they're also called involuntary. Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system.
Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure.
Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily. Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.
How Do Muscles Work? There are over muscles in the human body. Learning the muscular system often involves memorizing details about each muscle, like where a muscle attaches to bones and how a muscle helps move a joint. In textbooks and lectures these details about muscles are described using specialized vocabulary that is hard to understand.
Here is an example: The triceps brachii has three bellies with varying origins scapula and humerus and one insertion ulna. It is a prime mover of elbow extension. The anconeus acts as a synergist in elbow extension. What does all that textbook jargon mean?
The triceps brachii has four places where it attaches to the scapula, humerus, and ulna. A skeletal muscle attaches to bone or sometimes other muscles or tissues at two or more places. If the place is a bone that remains immobile for an action, the attachment is called an origin. If the place is on the bone that moves during the action, the attachment is called an insertion. The triceps brachii happens to have four points of attachment: one insertion on the ulna and three origins two on the humerus and one on the scapula.
The muscles surrounding synovial joints are responsible for moving the body in space. Hyperextension is the abnormal or excessive extension of a joint beyond its normal range of motion, thus resulting in injury. Similarly, hyperflexion is excessive flexion at a joint. Hyperextension injuries are common at hinge joints such as the knee or elbow. Abduction and adduction are motions of the limbs, hand, fingers, or toes in the coronal medial—lateral plane of movement. Moving the limb or hand laterally away from the body, or spreading the fingers or toes, is abduction.
Adduction brings the limb or hand toward or across the midline of the body, or brings the fingers or toes together. Circumduction is the movement of the limb, hand, or fingers in a circular pattern, using the sequential combination of flexion, adduction, extension, and abduction motions. Adduction, abduction, and circumduction take place at the shoulder, hip, wrist, metacarpophalangeal, and metatarsophalangeal joints.
Abduction and adduction motions occur within the coronal plane and involve medial-lateral motions of the limbs, fingers, toes, or thumb. Abduction moves the limb laterally away from the midline of the body, while adduction is the opposing movement that brings the limb toward the body or across the midline.
For example, abduction is raising the arm at the shoulder joint, moving it laterally away from the body, while adduction brings the arm down to the side of the body. Similarly, abduction and adduction at the wrist moves the hand away from or toward the midline of the body.
Spreading the fingers or toes apart is also abduction, while bringing the fingers or toes together is adduction. Adduction moves the thumb back to the anatomical position, next to the index finger. Abduction and adduction movements are seen at condyloid, saddle, and ball-and-socket joints see Figure 2.
Circumduction is the movement of a body region in a circular manner, in which one end of the body region being moved stays relatively stationary while the other end describes a circle.
It involves the sequential combination of flexion, adduction, extension, and abduction at a joint. This type of motion is found at biaxial condyloid and saddle joints, and at multiaxial ball-and-sockets joints see Figure 2.
Rotation can occur within the vertebral column, at a pivot joint, or at a ball-and-socket joint. Rotation of the neck or body is the twisting movement produced by the summation of the small rotational movements available between adjacent vertebrae.
At a pivot joint, one bone rotates in relation to another bone. This is a uniaxial joint, and thus rotation is the only motion allowed at a pivot joint. For example, at the atlantoaxial joint, the first cervical C1 vertebra atlas rotates around the dens, the upward projection from the second cervical C2 vertebra axis.
This joint allows for the radius to rotate along its length during pronation and supination movements of the forearm. Rotation can also occur at the ball-and-socket joints of the shoulder and hip.
Here, the humerus and femur rotate around their long axis, which moves the anterior surface of the arm or thigh either toward or away from the midline of the body. Movement that brings the anterior surface of the limb toward the midline of the body is called medial internal rotation. Conversely, rotation of the limb so that the anterior surface moves away from the midline is lateral external rotation see Figure 3.
Be sure to distinguish medial and lateral rotation, which can only occur at the multiaxial shoulder and hip joints, from circumduction, which can occur at either biaxial or multiaxial joints.
Turning of the head side to side or twisting of the body is rotation. So, your biceps is described as a "flexor" muscle. In the illustration below, the image on the right shows the biceps flexing. The opposing muscle of a flexor is called the "extensor" muscle. Your triceps is an extensor. When you contract your triceps your arm straightens and the angle between the forearm and the upper arm increases.
You may have already guessed but this is called "extension" and you can see that in the left illustration below. These designations are intrinsic, meaning they are an unchangeable property of the muscle.
This means that contracting a flexor muscle will always exhibits flexion and never extension and vis versa for contracting extensor muscles. Okay, so now that we have our terms of motion established we can discuss these antagonistic pairs properly! The two muscles in an antagonistic pair are in opposition.
That is, if one extends a limb during its contraction, the other will return the limb to its original position when flexed. In each pair, depending on the movement, one muscle plays the role of the "agonist" and the other muscle plays the role of "antagonist". The agonist is a muscle that contracts to cause the movement. The antagonist is an opposing muscle that relaxes relatively to stretch.
These two roles, agonist and antagonist, can be exchanged back and forth. To visualize this, let's jump back to our biceps and triceps example. Image waving at your best friend: when your hand is moving away from you, your triceps is an agonist, contracting to extend your arm. Your biceps is an antagonist, relaxing to allow elongation while possibly contracting ever-so-lightly to control the speed of that moving forearm.
When your hand is moving back in during your waving motion your biceps is an agonist, flexing your arm towards you. In this case, your triceps is an antagonist and must relax to stretch to allow the movement. So you can see that unlike the intrinsic designations of the flexors and extensors, the two roles of antagonistic pairs are dependent on the motion.
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