Why isnt smooth muscle striated

Joints can be mobile or non-mobile. Mobile joints synovial have a fluid-containing cavity to lubricate movements of the bones. Non-mobile joints connect bone to bone with cartilage or fiber. Ball and socket joint: shoulder, hip. Hinge joint: elbow. Gliding joint: wrist. Immobile joint: plates of the skull, rib-to-sternum.

Osteoblasts' role in lengthwise bone growth is to add bone tissue at the bone ends. By itself, osteoblasts will lengthen the knobs at the ends of the bone. Osteoclasts' role in bone growth is to remodel bone tissue by chipping away the knobs until it's the right size and shape.

Growth in diameter: Osteoblasts' role in diameter growth of bones is to add bone tissue to the outside of the bone. This is collectively known as the neuromuscular system. A typical muscle is serviced by anywhere between 50 and or more branches of specialised nerve cells called motor neurones. These plug directly into the skeletal muscle. The tip of each branch is called a presynaptic terminal.

The point of contact between the presynaptic terminal and the muscle is called the neuromuscular junction. To move a particular body part: The brain sends a message to the motor neurones. This triggers the release of the chemical acetylcholine from the presynaptic terminals. The muscle responds to acetylcholine by contracting. Shapes of skeletal muscle Generally speaking, skeletal muscles come in four main shapes, including: Spindle — wide through the middle and tapering at both ends, such as the biceps on the front of the upper arm.

Flat — like a sheet, such as the diaphragm that separates the chest from the abdominal cavity. Triangular — wider at the bottom, tapered at the top, such as the deltoid muscles of the shoulder. Circular — a ring-shape like a doughnut, such as the muscles that surround the mouth, the pupils and the anus. These are also known as sphincters. Muscle disorders Muscle disorders may cause weakness, pain, loss of movement and even paralysis. The range of problems that affect muscles are collectively known as myopathy.

The three main types of muscle include skeletal, smooth and cardiac. The brain, nerves and skeletal muscles work together to cause movement — this is collectively known as the neuromuscular system. More information here. Give feedback about this page. Was this page helpful? Yes No. View all bones muscles and joints.

Related information. These fibers are not arranged in orderly sarcomeres hence, no striations but instead are anchored to dense bodies which are scattered throughout the cytoplasm and anchored to the sarcolemma. A network of intermediate fibers run between the dense bodies providing an internal framework for contractile proteins to work against. A dense body is analogous to the Z-discs of skeletal muscle, anchoring the thin filaments in position.

Calcium ions are supplied primarily from the extracellular environment. T-tubules are absent but small indentations, called calveoli , in the sarcolemma represent locations where there are a high density of calcium channels present to facilitate calcium entry.

Sarcoplasmic reticulum is present in the fibers but is less developed than that observed in skeletal muscle. Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. Myosin light chain kinase in turn, activates the myosin heads by phosphorylating them converting ATP to ADP and P i , with the P i attaching to the head.

The heads can then attach to actin-binding sites and pull on the thin filaments. When the thin filaments slide past the thick filaments, they pull on the dense bodies, which then pull on the intermediate filaments networks throughout the sarcoplasm.

This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion Figure However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscle slightly contracted, which is important in certain functions, such as maintaining pressure in blood vessels.

Because most smooth muscles must function for long periods without rest, their power output is relatively low to minimize energy needs. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges , keep the thick and thin filaments linked together for a prolonged period, without the need for ATP. For smooth muscle stimulated by neurons, the axons from autonomic nervous system neurons do not form the highly organized neuromuscular junctions as observed in skeletal muscle.

Instead, there is a series of neurotransmitter-filled bulges, called varicosities , along the axon of the neuron feeding the smooth muscle that release neurotransmitters over a wide synaptic cleft. Also, visceral muscle in the walls of the hollow organs except the heart contains pacesetter cells.