Muscular System

Functions of the Muscular System

• Produce Movement (all movements of the human body and its parts result from muscle contraction)

• Maintain Posture (we are rarely aware of the skeletal muscles that maintain body posture yet these muscles function almost continuously making one tiny adjustment after another to counteract the never ending downward pull of gravity)

• Stabilize Joints (Even as muscles pull on bones to cause movement they stabilize and strengthen the joints of the skeleton)

• Generate Heat (Muscles generate heat as they contract this heat is vitally important in maintaining normal body temperature because the skeletal muscle accounts for at least 40 percent of body mass it is the muscle type most responsible for generating heat.

  NOTE: They also protect the more fragile internal organs, those that are not covered by the skeletal system we have those internal organs of the abdominal pelvic region or the abdominal region

  Skeletal muscles are responsible for all locomotion these will help us to move from place to place and manipulation they enable us to respond quickly to changes in the external environment EX. Jumping out of the way of a car, direct our eyes, smile or frown

  Cardiac muscles of the heart it is beating rhythmically to produce blood

  Smooth muscles in the wall of your blood vessels it helps maintain blood pressure. In other organs of the body in the digestive, urinary and reproductive tract also propel or squeezes substance through the organs along the tract. So aside from that, we have the smooth muscles. They form valves to regulate the passage of substances through internal body openings. They also dilate and constrict the eyes of your eyes and forms the erector pili muscles attached to the hair follicles.

  Muscles are responsible for all types of body movements so there are three types of um there are three basic muscle types that are found in the body we have skeletal cardiac and smooth.

  NOTE: The muscular system is composed of these types of tissues the tissues then are turned in to muscle fibers. The functional unit of a muscle is a muscle fiber

• Muscle Fiber is the cell of a muscular system

CHARACTERISTICS OF SKELETAL MUSCLES.

• Most are attached by tendons to bones.

• Muscle tissue-They are long cylindrical striated fibers. The striation is caused by the alternating light and dark bonds. They are multinucleated, meaning they have multiple nuclei in one muscle fiber, which is usually located at the periphery.

• Type of movement - it is voluntary, meaning it can be made to contract or relax by conscious control.

• Location - they are usually attached to bones by the tendons.

• Cells - are surrounded and bundled by several connective tissues

Endomysium - means within the muscle. So the endomysium is a connective tissue that surrounds a muscle fiber or the cell of the muscular tissue.

Fascicles - So within each skeletal muscle, the muscle fibers are grouped into fascicles. There are several muscle fibers within the fascicle and that fascicle is covered by the connective tissue we call the perimysium

Perimysium - means around the muscle and it covers the Fascicles with connective tissue (perimysium)

Epimysium - literally means outside the muscle the epimysium surrounding several muscle fibers, fascicles, surrounding several fascicles. The fascicles are covered by the perimysium. The muscle fiber is covered by an endomysium. And then the fascicles are in turn bundled together through a connective tissue we call the epimysium. So it covers the entire skeletal muscle. It blends with the deep fascia that lies between the neighboring muscles or the superficial

Fascia - deep to the skin is on the outside of the epimysium so usually fascia is colored white

Skeletal Muscle Attachments - muscle attachments whether origin or insertion may be direct or indirect there are two types

NOTE: when a muscle contracts the movable bone is the muscle's insertion then it moves toward the immovable or less movable bone which is the muscle's origin.

Aponeurosis and Tendons - these are termed as indirect attachments. So the muscle's connective tissue wrappings would extend beyond the muscle either as a rope like tendon or as a sheath like aponeurosis the tendon or aponeurosis anchors the muscle to the connective tissue surrounding the skeletal element either a bone or a cartilage, or the fascia of other muscles. So these indirect attachments are attached to these direct attachments, which are the bones, cartilages, and the connective tissue coverings

Differentiate

Direct/Fleshy attachments the epimysium of the muscle is fused to the periosteum of a bone or a perichondrium of a cartilage. And this will in turn be connected to the indirect attachments such as tendons and aponeurosis.

Indirect attachments are much more common because of their durability and small size. Tendons are mostly tough collagen fibers that can withstand the abrasion of rough bony projections that would tear apart the more delicate muscle tissues

Naming of the Skeletal Muscles

• some are named depending on their location across an imaginary line or what they call the axis. So the names of some muscles reveal the direction in which their fibers or their fascicles run in reference to some imaginary line or what they term as axis.

  Examples:

  Rectus (Straight) It means straight because the fibers are running parallel right angles to that line. (Rectus abdominis, rectus femoris)

  Oblique (slanted) indicates that the fibers run obliquely to it or it's slanted to that imaginary line.

  Transversus abdominis

• we have the relative size of the muscle.

  Maximus - The largest

  Minimus - The smallest

  Longus - Long

  Brevis - short

  Examples : Gluteus Maximus/Gluteus minimus

• Location of the muscle many muscles are named for bones some muscles are named according to their point of attachment.

  Examples : Temporalis which is close to the temporal bone

  occipitalis close to the occipital bone

• Number of origins when biceps, triceps, or quadriceps forms part of a muscle name, you can assume that the muscles has two, three, or four origins respectively. Hence, we have the term biceps brachii, meaning it has two origins or two heads. Triceps for three heads, quadriceps for four heads.

• Location of the muscle’s origin and insertion some muscles are named according to their points of origin and insertion

  NOTE: That the origin is always named first

  Example : the sternocleidomastoid the origin here or this is a muscle of the neck which has dual origin on the sternum hence we use the word sterno first then the second attachment the clavicle hence the word cleido and then it’s insertion in the mastoid process of the temporal bone = Sternocleidomastoid

• Shape of the muscle some muscles are named for their distinctive shape

  Example : Deltoid (Triangular), Posterior Trapezius (Trapezoidal)

• Action of the muscle when muscles are named for the movement they produce actions such as flexor extensor adductor abductor are used.

Smooth Muscle Characteristics

• It does not contain striations.

• The white and dark bonds are absent in this type of muscle.

• They are uni-nucleated, meaning they only have one nucleus. and that nucleus is usually centrally located.

• Type of action is involuntary. It is not controlled consciously, meaning it is dictated by the brain.(EX. Digestion)

• Location - it can be located in the iris of the eye. Walls of internal hollow structures like the stomach and urinary bladder, etc.

• Main function - motion as in constriction of the blood vessels, propulsion of food in the gastrointestinal tract, and the contraction of the urinary bladder.

Cardiac Muscle Characteristics

• Striated Fibers

• Usually uni-nucleated, but in some cases they may have 2 nuclei.

• Their Nucleus or their nuclei centrally located.

• It has the intercalated discs will attach one cardiac muscle fiber to another. Intercalated disc has desmosomes and gap junctions for communication and diffusion between cells.

• The type of movement is involuntary.

• Location in the hear wall.

• Function is pumping blood to all parts of the body.

Microscopic Anatomy of Skeletal Muscle

• Cells are multinucleate

• Nuclei are just beneath the sarcolemma(specialized plasma membrane of the muscle fiber that actively transports substrates into the muscle cell. It serves as a docking location for proteins originating in the basement membrane in cytoskeleton. And it also transmits neural excitatory impulses that lead to muscle contraction.)

• Sarcoplasmic reticulum is a specialized SER. This functions or this is dedicated to calcium ion handling necessary for muscle contraction and relaxation

  Note: Important for sliding filament theory

Dissection of muscle fiber

• Myofibril - a single muscle fiber contains hundreds to thousands of rod-like myofibrils that run parallel to its length. Myofibrils contain the contractile elements of the skeletal muscle cells, the sarcomeres, which even contain smaller rod-like structures called the myofilaments.

• Myofilaments - are a repeating of a dark and light bond. So you have the A-bond and the I-bond. In an intact muscle fiber, the dark A-bond and the light I-bonds are nearly perfectly aligned, giving the cell its striated appearance.

• A-bond - So, each A band has a lighter region in its midsection. It’s entire length is composed of the myosin filament and they are connected at the midline by the m line

• Light region (H zone) - each H zone is bisected by a dark line called the M zone.

• M line - is in the middle of the H zone. Then the M line is formed by molecules of the protein myomesin.

• I-band - the light band, it has a midline interruption, which is the darker area, which is called Z disc or Z line

• Sarcomere - the region of a myofibril is between two successive z-disks which means muscle segment averaging

  of two micrometers long a sarcomere is the smallest contractile unit of a muscle fiber meaning that is the functional unit of a skeletal muscle it contains a bond flanked by half of the I band at each end.

• Organization of the sarcomere So myofilaments or filaments are the muscle equivalents of the actin or myosin-containing microfilaments. The central thick filaments containing the red ones are what we call the thick filament or myosin filament. It is composed of the protein myosin and it has ATPase enzymes.

• Thin Filament found at the lateral sides or the actin filaments it is named after the composition of the protein and other regulatory proteins are found in the thin filaments and myosin filaments.

• Myosin Filaments have heads or extensions or cross bridges.

  During contraction, myosin heads will form a cross bridge with the actin filament above and this will swivel around at their point of attachment. Each thick filament will contain 300 myosin molecules bundled together together with their tails forming the central part of a nine thick filament and their heads facing outward at the end of each thick filament as a result the central portion of a thick filament such as in the h zone is smooth meaning there is a bare zone it is smooth but its ends are studded with staggered array of myosin heads. Then the heads bear actin and ATP binding sites and also have intrinsic ATPase activity that splits ATP and generate energy for muscle contraction. If there is contraction the bare zone is where the myosin filaments and the actin filament meet in the middle. M zone - Bare zone if contract if at rest it lacks actin filaments - H zone.

• Sarcoplasmic reticulum (SR) - For storage of calcium

Properties of skeletal muscle activity

• Irritability - This is also known as excitability or responsiveness. This is ability to receive and respond to a stimulus.

• Stimulus - is any change in the environment, either inside or outside the body.

• Contractility - this is the ability to shorten forcibly when adequately stimulated. This ability sets muscles apart from all other types of tissues.

• nerve stimulation to the muscles skeletal muscles must be stimulated by a nerve to contract.

• Somatic motor neurons - The neurons that stimulate the skeletal muscle to contract; each somatic motor neuron has a thread-like axon that extends from the brain, so from the brain or spinal cord, to a group of skeletal muscle fibers.

• Axon - the axon of a somatic motor neuron typically branches many times, each branch extending to a different skeletal muscle fiber.

Nerve Stimulus to Muscles

• Neuromuscular junction(NMJ) - this is the synapse between the motor or between the somatic motor neuron and the skeletal muscle fiber that is the space between the axon terminal of the neuron and the invaginations of the sarcolemma at the motor end plate.

• Synaptic Cleft - synapse is a region where communication occurs between two neurons or between a neuron and the target cell in our case is the muscle fiber at most synapses a small gap is observed so we have here the synaptic cleft. Nerve in the muscle do not make contact the action potential cannot jump the gap from one cell to another. What happens is that it releases neurotransmitter such as the ACH or the acetylcholine in our case

  NOTE: The Action potential arrives at the axon terminal at the neuromuscular junction but before the ACH is released in the synaptic Cleft is that there will be a reaction first in the voltage gated calcium channels in the axon terminal of the neuron then the voltage gated calcium channels will open the calcium ions will now enter the axon terminal moving down its gradient. Then through the movement or through the entry of the calcium ions, this causes the neurotransmitter ACH or acetylcholine to be released at the terminal button through exocytosis. Then the ACH will now bind to the receptors of the sarcolemma then the ACH binding opens ion channels in the receptors that allows simultaneous passage of sodium and potassium. The sodium will enter the muscle fiber then the potassium will move out of the muscle fiber more sodium ions will enter than potassium ions this will now produce a local change in the membrane potential called the end plate potential

Transmission of Nerve Impulse to Muscle

• Neurotransmitter - is a chemical released by acetylcholine or ACH and neurotransmitter attaches to receptor on the sarcolemma

• Sarcolemma becomes temporarily permeable to sodium that rushes into the cell giving it a positive charge

  sodium rushing to the cell generates an action potential once started the muscle contraction cannot be stopped through the action potential there will be an contraction action potential will only be terminated once the ACH or the acetylcholine found in the junction will be disintegrated or

  will break down in the synaptic cleft by the acetylcholinesterase(AChE).

• To return to resting state

  Potassium ions (K⁺) diffuse out of the cell

  Sodium-potassium pump, pumps sodium and potassium back to their original positions.

The sliding filament theory of muscle contraction

the sliding filament theory of muscle contraction is activated by the nerve, causing now the myosin heads to form cross bridges with the actin filament to attach to the binding sites found in the thin filament. this will start upon the action potential propagation along the sarcolemma and down to the T-tubules. T-tubules is found in the Sarcoplasmic reticulum. Calcium ions will be released through the transmission of the action potential along the T- tubules of the triad causes now a voltage-sensitive 2-buo protein to change shape. This shape change opens the calcium release channels in the terminal cisterns of the sarcoplasmic reticulum, allowing now calcium to flow into the cytosol. Then how does the cross-bridge occur? The calcium will bind to troponin which is in our case initial not present