FUNCTIONS OF MUSCULAR SYSTEM Body movement Maintain posture Respiration Produce body heat Communication Constriction of organs and blood vessels Heartbeat
Connective Tissue Sheaths in Skeletal Muscle Figure 10.1a
TYPES OF MUSCLE PATTERNS • PARALLEL • PENNATE • CONVERGENT • CIRCULAR
PARALLEL MUSCLE The fascicles are parallel. They are long fibers, which can contract to 75% of their length. They contract a long way, but they are relatively weak, because there are relatively few fascicles. E.g. Sternocleidomastoid.
Arrangement of Fascicles in Muscles Figure 11.3
PENNATE PENNATE (means “feather shape”) MUSCLES: three types: • UNIPENNATE; looks like half a feather. The fascicles are short, but there are more of them. They are stronger, but do not have the same length contraction ability of the parallel muscles. • BIPENNATEare fascicles that insert into the tendon from both sides; they are stronger than unipennate. • MULTIPENNATE are the strongest (biceps femoris). The fascicles are in multiple bundles inserting on one tendon
CONVERGENT CONVERGENT MUSCLE has more fibers than parallel, but contracts a greater distance than pinnate. E.g. Pectoralis major.
CIRCULAR MUSCLE CIRCULAR MUSCLE (Sphincter) is arranged in a circle, with a small area of tendon on the sides. It allows closure of the eyes, mouth, etc. They are not very strong, but they don’t need to be.
TERMS: • ORIGIN = The region which usually doesn’t move when the muscle contracts. Look at the biceps brachii; does the shoulder move when I bend my arm? No; the shoulder = origin. • INSERTION= The point of attachment that moves; bend arm, radial tuberosity = attachment. • AGONIST = The main muscle for a particular action; bend arm, biceps = agonist. • ANTAGONIST = Does the opposite action; bend elbow, antagonist extends. Every muscle in the body has to have an antagonist. • SYNERGIST = The muscle that helps the agonist. There are several muscles that assist when the arm is bent.
Muscle Types • Skeletal: elongated • striated • Voluntary • Moves the skeleton • Smooth: spindle shaped • no striations • Involuntary • Found in organs and lining of blood vessels • Cardiac: cylindrical shaped • striated • involuntary (only responds to direct electrical stimulation)
SKELETAL MUSCLE • Theses are very long fibers (biceps muscle can be 8-10 cm). • They have thousands of nuclei because they start from many stem cells that fuse together into one skeletal muscle fiber.
Skeletal Muscle • Myoblasts exist in adults, so muscle heals well. • A muscle cell torn in half can regenerate. • There are almost no muscle diseases for this reason (muscular dystrophy is the main muscle disease). • Muscles Overview Video • http://www.youtube.com/watch?v=ren_IQPOhJc
Skeletal Muscle: Longitudinal section In skeletal muscle fibers, there are light and dark stripes called striations, which can be seen under a microscope.
A cross section of skeletal muscle looks like bundles of circles because you are looking at cut fascicles.
Skeletal Muscle • The plasma membrane of muscles is called a SARCOLEMMA. • The cytoplasm of muscle cells is called SARCOPLASM. • Muscle cells contain many mitochondria and other organelles. • One type of unusual organelle found only in muscle cells is called a myofibril. They are packed in bundles and fill up most of the cell.
MUSCLE MYOFIBRILS • Cylindrical organelles found within muscle cells • Contain actin and myosin myofilaments • Extend from one end of the muscle fiber (muscle cell) to the other • Contain sarcomeres joined end to end.
Every dark band + light band is one sarcomere
SARCOMERES The striations result from the internal structure of SARCOMERES within the sarcoplasm. The sarcomere is the basic structural and functional unit of skeletal muscle. The sarcomere is what contracts.
Actin and Myosin Sarcomere model video • Sarcomeres consist of two types of myofilaments made out of protein: • thin (ACTIN) myofilaments • Look like two strands of beads twisted together. • thick (MYOSIN) myofilaments. • Both ends of a thick filament are studded with knobs called myosin heads (look like little golf clubs).
Actin Myosin Actin Myosin
MECHANISM OF CONTRACTION The Sliding Filament Theory • Contraction results as the myosin heads of the thick filaments attach like hooks to the thin actin filaments at both ends of the sarcomere and pull the thin filaments toward the center of the sarcomere. • The myosin head is like a hook with a hinge. After a myosin head pivots at its hinge, it draws the actin closer, then lets go, springs up again to grab the actin filament again, pulls it closer, and it keeps repeating this until the entire actin filament has been drawn in as far as it can go. • The sites where the myosin heads hook onto the actin are called cross-bridges.
Sarcomere Contraction • The complete process of contraction of the sarcomere takes only a fraction of a second. • The actin and myosin filaments do not shorten; they merely slide past each other. • The energy required is ATP. • Muscle Contraction • http://www.youtube.com/watch?v=CepeYFvqmk4
Muscle Contraction A muscle TWITCH is one single muscle fiber contraction. • It takes 1/20th of a second. • How is it that I can pick up and hold a chair if the fiber only contracts for 1/20th second? • There are ten thousand fibers per muscle; each one contracts at different intervals, so contraction is maintained, just like tug-of-war. One person in ten can drop the rope and get a better grip because the others are maintaining the tension.
Motor Units A MOTOR UNIT is a single neuron and all of the muscle fibers on which it synapses. If one neuron sends a signal, only its muscle fibers contract (the motor unit). This allows for strength variations in lifting a chair vs. an eraser. For full strength, all the motor units contract. For half strength, half of the motor units contract.
Motor Units There are 3 motor units in this diagram; that allows for 3 different levels of contraction. The more motor units there are, the more precisely the muscle can respond.
Motor Units The action potential continues from one motor neuron to the next motor neuron until the last neuron lands on its target cells; in this case, skeletal muscle fibers. A single motor neuron and all the skeletal muscle fibers it innervates constitute a motor unit. A muscle in your tongue may only have a few muscle fibers innervated by a neuron to allow for precise movement. However, large thigh muscles may have as many as 1000 muscle fibers per neuron, since precision is not necessary.
Motor Units • The muscles of the back are larger motor units (many muscle fibers per neuron). Since there are fewer motor units present (one neuron for a thousand muscle cells, we get strength, but less precision. • The muscles that move the tongue have smaller motor units (one neuron for 10 muscle cells). Since there are many motor units present = less strength, more precision.
TETANUS TOXIN The bacteria that make this toxin live deep in the soil and cannot survive in air. If you step on something that imbeds soil deeply into your tissues (like a rusty nail), you might contract the bacteria. You will need a tetanus vaccine before the toxins accumulate. A toxin caused by a certain bacteria can cause muscle to remain contracted (in tetanus). It quickly results in death because the diaphragm and other respiratory muscles cannot function properly, and the person suffocates.
Muscle Tone Even when muscles are relaxed, some of their fibers are still contracting, giving the muscle some tone. Therefore, the normal state of a muscle, with some contraction, is called muscle tone. This is important in posture so you can stand upright but mostly relaxed. Muscle tone refers to the constant tension produced by muscles of the body over long periods of time. It is responsible for keeping the back and legs straight, the head held in an upright position, and the abdomen from bulging. it declines during REM sleep.
Muscle Tone • Upper Extremity Tone VIDEO • Lower Extremity Tone VIDEO
Motor Neurons • A neuron (nerve cell) that innervates (supplies) skeletal muscle is called a motor neuron (causes the body to move). There are 2 motor neurons involved in this task. • The Upper Motor Neuron has its cell body in the brain, and its axon (like a stem) lands on the cell body of the Lower Motor Neuron, which is in the spinal cord. • The axon of the Lower Motor Neuron leaves the spinal cord and innervates the muscle.
Upper and Lower Motor Neurons (in red) Upper motor neuron Lower motor neuron
Muscle Tone • Hypertonia • Can present clinically as either spasticity or rigidity. Seen in upper motor neuron diseases, such as multiple sclerosis or cerebral palsy. • Hypotonia • Seen in lower motor neuron diseases (spinal cord damage and ALS/Lou Gehrig Disease) • Can present clinically as muscle flaccidity, where the limbs appear floppy, stretch reflex responses are decreased, and the limb’s resistance to passive movement is also decreased.
Muscle Spasticity: Hypertonia • Clinically spasticity is defined as velocity dependent resistance to stretch. • Passively moving (the doctor does the movement) the patient’s elbow quickly will elicit spastic twitches, but passively moving elbow slowly is normal. • It mostly occurs from upper motor neuron lesions (scar, tumor, or other damage), but it can also present in multiple sclerosis, which is an autoimmune condition. Can also be seen in cerebral palsy (lack of oxygen at birth).
Muscle Spasticity • There is a difference in cause of two of the most common spasticity conditions, spastic diplegia (cerebral palsy) and multiple sclerosis. • In spastic diplegia, the upper motor neuron lesion arises often as a result of neonatal asphyxia (lack of oxygen in a newborn), while in conditions like multiple sclerosis, spasticity is from multiple sclerosis, which is an autoimmune destruction of the myelin sheaths around nerve endings.
Muscle Spasticity • Causes include • Spastic diplegia (Cerebral palsy) • Multiple sclerosis • Spinal cord injury • Stroke • Test for clonus to see if spasticity is present.
Muscle Clonus • Clonus (from the Greek for "violent, confused motion") is a series of involuntary muscular contractions initiated by a reflex. • Clonus is a sign of certain neurological conditions, and is particularly associated with upper motor neuron lesions such as in spastic diplegia, stroke, multiple sclerosis, spinal cord damage. • Clonus is most common in the ankles, where it is tested by rapidly dorsiflexing the foot. If the foot then jerks 5 times or more, clonus is present. A positive clonus test means the patient has spasticity, usually due to an UMN disorder. • VIDEO: Ankle clonus • http://www.youtube.com/watch?v=iWEJIVO85TI
Muscle Rigidity • Unlike spasticity, rigidity is velocity-independent resistance to passive stretch. • There is uniform increased tone whether the elbow is passively moved quickly or slowly.
Muscle Fasciculations • These are small, local, involuntary muscle contractions. • Fasciculations have a variety of causes, the majority of which are benign, but can also be due to disease of the lower motor neurons. • Fasciculations VIDEO • Tremor VIDEO
Muscle Fasciculations • Benign causes of fasciculations include: • Magnesium deficiency • Diarrhea • Overexertion • Inadequate intake from diet (almonds are a good source of magnesium) • Dehydration • Fatigue • A small neuron dying can also cause fasciculations.