 The Skeletal and Muscle Systems Putting your body in motion.

Locomotion  Movement is essential to all animals for finding and capturing food.  On land, if an animals wants to move, it needs to be able to support its body and move against gravity.  Also, animals must be able to maintain their balance. Humans, which are bipedal animals, keep part of one foot on the ground at all times in order to stay balanced.  Movement results from muscles working against some type of skeleton. Microfilaments are the contractile elements of muscle cells.

The Function of the Skeleton  The skeleton has three functions: support, protection, and movement.  Humans have an endoskeleton consisting of a combination of cartilage and bone.  This skeleton is made up of 206 bones connected by ligaments at joints to allow for movement.

Bones  Almost all of the bones in the human body have the same components:  Periosteum – a thin but dense membrane containing nerves and blood vessels to nourish the bone. The inner part of the periosteum creates new bone and adapts existing bone to new conditions.  Compact bone – this layer of bone tissue is smooth and very hard and it is located beneath the periosteum.  Cancellous bone – resembles a sponge. It is softer than compact bone but still strong and it protects the bone marrow.  Bone marrow – the innermost part of the bone. It has a jelly-like texture and there are two types: yellow and red bone marrow. Red marrow creates the components of blood (red and white blood cells as well as platelets). Yellow marrow stores fat. To see how these bone parts are arranged in a bone go to: kidshealth kidshealth

Skeleton The vertebral frame is broken down into two parts: the axial skeleton and the appendicular skeleton. AXIAL SKELETON: The components of the axial skeleton are: - the skull which serves to protect the brain from injury and also fixes the eyes and ears to allow for better sight and sound reception. - the vertebral column, or backbone, supports the head and arms, and it allows you to stand upright and maintain your balance. Also, it protects the spinal cord and is an attachment point for the ribs as well as other muscles and organs. - the rib cage which protects the heart and lungs APPENDICULAR SKELETON: The components of the appendicular skeleton are: - the limb bones - the pectoral girdle provides an attachment point between the arms and the axial skeleton as well as for muscles that allow for movement of the shoulder and elbow joints. - the pelvic girdle protects the bladder, reproductive organs and the developing fetus in a woman. Also it supports the weight of the vertebral column and the upper body. *each appendage has several types of joints to allow for a variety of movements.

Parts of the Skeleton

Muscle Contractions Result in Movement  The only action muscles can perform is contraction, they extend passively.  Muscles are arranged on the skeleton in antagonistic pairs where each works against the other.

Skeletal Muscle  This type of muscle is attached to the bones and is responsible for the movement of the skeleton.  Each muscle consists of a bundle of long fibers and each fiber is a single cell with many nuclei. This means that the fiber was created through the fusion of many embryonic cells.  Each fiber can further be broken down into a bundle of myofibrils and each myofibril is composed of 2 kinds of myofilament (thick and thin).

Skeletal Muscle (continued)  Skeletal muscle is also called striated muscle.  The basic functional unit of a muscle is the sarcomere, which is composed of thick and thin myofilaments bordered by Z lines.  When the muscle is at rest, there is a region only occupied by thin filaments and this region is called the I band.  The A band is the length of the thick filaments.  The H zone is in the center of the sarcomere and because the thin filaments don’t extend the whole length of the sarcomere, the H zone contains only thick filaments. (diagram on pg. 1015)

Sarcomere

Sarcomere

Muscle Contraction-How does it work?  When muscles contract, the length of the sarcomere is shortened.  However, the length of each A band does not change. Instead, the H zone disappears and the I bands shorten, brining the thin filaments of the sarcomere further into the center.  IMPORTANT NOTE: neither the thick nor thin filaments change in length during muscle contraction.  Muscle contraction can be explained by the sliding- filament model.

Sliding-Filament Model  This model maintains that neither type of filament is shortened during contraction. Instead, the degree of overlap between the two types of filaments increases, which means that the region only occupied by thin filaments and the region only occupied by thick filaments must decrease in length.  Muscle contraction through the sliding of filaments is made possible by the interaction of the actin and myosin that make up the myofilaments.

Myosin Structure  Myosin has a head region and a tail region  The tails of myosin molecules cohere and in this way form thick filament. The head is what powers muscle contractions.

The Process of Muscle Contraction  The process starts when ATP binds to the myosin head and is hydrolyzed into ADP and inorganic phosphate. This releases energy.  Some of the energy that is released is transferred to the myosin which changes shape to a high-energy configuration. The energized myosin then binds to a specific site on actin, forming a cross-bridge.  The energy the myosin obtained from the hydrolyzation of ATP is released and the myosin goes back to its low-energy configuration.

More Muscle Contraction…  The action of reverting back to its low- energy configuration causes the myosin to bend back on itself and, in the process of bending, the myosin pushes the thin filaments toward the center of the sarcomere.  The cross bridge between myosin and acting breaks when a new ATP molecule binds to the myosin and the process starts over.  Each muscle cell only stores enough ATP for a few contractions. Most of the energy needed for repetitive muscle contractions is stored in creatine phosphate. This molecule can provide a phosphate group and change ADP to ATP.

The Sliding Filament Model

Regulation of Muscle Contraction  A skeletal muscle will only contract if it receives a stimulus from a motor neuron.  At rest, myosin binding sites on actin molecules are blocked by tropomyosin.  Also, there is tropomyosin on the thin filaments and its position is controlled by regulatory proteins called the troponin complex.  If the muscle cell is to contract, the myosin binding sites must be uncovered first.  Calcium ions can uncover the binding sites by binding to the troponin. This causes the tropomyosin-troponin complex to expose the binding sites by changing its shape.  THEREFORE: muscles can only contract when Calcium ions are present.

Tropomyosin-Troponin Complex

Sarcoplasmic Reticulum  The sarcoplasmic reticulum, a specialized endoplasmic reticulum, controls the calcium ion concentration in the cytoplasm.  The reticulum stores calcium ions by actively transporting them across its membrane and into its internal space  The calcium is stored in the sarcoplasmic reticulum until an action potential, the stimulus that causes muscle contractions, reaches the muscle cell.  The action potential spreads to the interior of the cell along transverse tubules, which are infoldings of the plasma membrane

More Sarcoplasmic Reticulum  The transverse tubules have contact with the sarcoplasmic reticulum at certain spots.  As the action potential reaches these areas, it changes the permeability of the sarcoplasmic reticulum, causing it to release calcium ions.  The ions then bind to troponin and the muscle can contract.  Contraction stops when the sarcoplasmic reticulum pumps the ions out of the cytoplasm.

Muscle Fiber

Diverse Body Movements Require Variation in Muscle Activity  Different actions require a differing degree of muscle contraction.  The nervous system has two main ways of creating graded muscle contractions.  One is through releasing the appropriate number of action potentials:  1 action potential will cause a 100 msec. contraction  If another action potential arrives before the first twitch is over, there will be a greater response  A continuous, overlapping series of action potentials will produce the greatest response, with the tension in the muscle dependent on how quickly the action potentials are released.  Tetanus happens when the stimulation from many action potentials is fast enough to blur individual twitches into one smooth, sustained contraction.

Motor Units  Another way to control the degree of muscle contraction is to organize muscle cells into motor units.  Each motor unit is made up of one motor neuron and all of the fibers it controls. When it fires an action potential, all of the fibers it controls react to it together. Therefore, the tension depends on how many muscle fibers are being controlled by the unit.  Tension can be increased by activating more motor units that are located in the muscle. This process is called recruitment.

Fast and Slow Muscle Fibers  Action potentials cause contractions, but the length of the contraction is controlled by the calcium ions and how long they are present in the cytoplasm.  This divides muscle fibers into fast and slow fibers.  Fast muscle fibers are used for quick, powerful contractions.  Slow muscles fibers are often found in posture muscles because they are used for long contractions. The slow fibers have less sarcoplasmic reticulum than the fast muscle fibers so the calcium ions can stay in the cytoplasm longer. Also, they have many mitochondria to provide a steady energy supply, a rich blood supply, and myoglobin.

Other Types of Muscle  Cardiac muscle  Found only in the heart.  This muscle is striated, just like skeletal muscle.  The main difference between cardiac and skeletal muscle is that there are gap junctions between cardiac muscle cells so that when an action potential reaches one cell, it spreads to all the other cells, and the whole heart contracts.  Also, cardiac muscle cells can generate action potentials on their own, something skeletal muscle cells cannot accomplish unless they receive a stimulus.  In cardiac muscle, action potentials last longer and they help control the duration of muscle contraction.

Other Types of Muscle (continued)  Smooth muscle  Not striated because it does not have as much myosin. Also, the arrangement of thick and thin filaments is different.  Contractions do not create as much tension, but they contract over a greater range of time.  There is no transverse tubule system and no sarcoplasmic reticulum. Instead, calcium ions enter via the plasma membrane during action potentials.  Therefore, contractions are slow but they can be controlled better.  This type of muscle is found in digestive organs and blood vessels.