Chapter 9-Muscular System

The Muscular System Moves Body Parts and Maintains Posture The muscular system produces movement and maintains posture There are three types of muscles: skeletal, cardiac, and smooth All muscles are excitable, contractile, extensible, and elastic Excitable: respond to stimuli Contractile: shorten Extensible: stretch Elastic: return to their original length after being shortened or stretched

FIGURE 6.2 Some major muscles of the body

Skeletal Muscles Work in Pairs Skeletal muscles are voluntary muscles responsible for moving our body Each muscle is attached to a bone by a tendon The origin of the muscle remains stationary during movement while the insertion is attached to the bone that moves Most muscles are arranged in pairs, called antagonistic pairs, that work in opposition to one another

FIGURE 6.1a The antagonistic action of the triceps and biceps muscles during flexion and extension. The origins and insertions of the muscles are shown.

FIGURE 6.1b The antagonistic action of the triceps and biceps muscles during flexion and extension. The origins and insertions of the muscles are shown.

Sacromeres Are the Contractile Units of Muscle When skeletal muscles are viewed under a microscope, they have distinct bands called striations They are formed by the arrangement of myofibrils within the muscle cell Each myofibril contains groups of long myofilaments Each myofilament is composed of myosin and actin filaments

FIGURE 6.3 The structure of a skeletal muscle

FIGURE 6.3a,b The structure of a skeletal muscle

FIGURE 6.3b,c The structure of a skeletal muscle

FIGURE 6.3c,d The structure of a skeletal muscle

Sarcomeres Sarcomeres are the contractile units of muscle They shorten as the actin filaments slide along the myosin filaments Muscle contraction occurs at the molecular level According to the sliding filament model, muscle contracts when actin filaments slide past myosin filaments

FIGURE 6.4 Each myofibril is packed with actin filaments and myosin filaments. When a muscle contracts, actin filaments slide past myosin filaments. Movements of the heads of myosin filaments pull actin filaments toward the center of a sarcomere.

FIGURE 6.5 part 1 The sliding filament model of muscle contraction. (The regulatory proteins on the actin are described in the following subsection.)

FIGURE 6.5 part 2 The sliding filament model of muscle contraction. (The regulatory proteins on the actin are described in the following subsection.)

FIGURE 6.5 part 3 The sliding filament model of muscle contraction. (The regulatory proteins on the actin are described in the following subsection.)

FIGURE 6.5 part 4 The sliding filament model of muscle contraction. (The regulatory proteins on the actin are described in the following subsection.)

FIGURE 6.5 part 5 The sliding filament model of muscle contraction. (The regulatory proteins on the actin are described in the following subsection.)

Sarcomeres The myosin head attaches to the actin filament forming a cross bridge Then it bends and swivels, pulling the actin filament toward the midline of the cell The tropomyosin-troponin complex and calcium ions regulate muscle contraction at the actin-myosin binding sites Contraction is triggered when a nerve impulse travels down a motor neuron until it reaches the neuromuscular junction

FIGURE 6.6 The availability of calcium ions controls muscle contraction.

FIGURE 6.6 The availability of calcium ions controls muscle contraction.

FIGURE 6.7 The connection between a motor neuron and a muscle cell is called a neuromuscular junction.

Sacromeres At the neuromuscular junction it causes the release of acetylcholine from vesicles in the motor neuron The acetylcholine causes changes in the permeability of the muscle cell, resulting in an electrochemical message similar to a nerve impulse

Muscle Contraction Depends on the Stimulation of Motor Units A motor neuron and all the muscle cells it stimulates are called a motor unit The strength of muscle contraction depends on the number of motor units that are stimulated The muscle cells of a motor unit are spread throughout the muscle resulting in even, whole muscle contraction

FIGURE 6.8 A motor unit includes a motor neuron and the muscle cells it stimulates.

Myofibril Contains protein filaments – ACTIN (thin) and MYOSIN (thick)   These filaments overlap to form dark and light bands on the muscle fiber A band = dArk • thick (myosin) I band = lIght • thIn (actin) In the middle of each I band are Z lines. A sarcomere is on Z line to the other

It is important to remember the heirarchy myosin myofibrils fasicles myofilaments actin

It is important to remember the heirarchy fasicles myofibrils myofilaments actin myosin

muscle fiber myofilament myofibrils epimysium muscle sarcomere

myofilament muscle sarcomere epimysium myofibrils muscle fiber

Muscles & Nervous System

Motor Unit or Neuromuscular Junction 1.  Neuron         2.  Sarcolemma   (or motor end plate)        3.  Vesicle      4.  Synapse        5.  Mitochondria

The neurotransmitter that crosses the gap is ACETYLCHOLINE. This is what activates the muscle. Acetylcholine is stored in vesicles

Motor Unit   The muscle fiber  and  the motor neuron 

SLIDING FILAMENT THEORY (MODEL) The theory of how muscle contracts is the sliding filament theory. The contraction of a muscle occurs as the thin filament slide past the thick filaments. The sliding filament theory involves five different molecules plus calcium ions.   The five molecules are:   myosin actin tropomyosin troponin ATP

Sliding Filament Handout

Sliding Filament Handout

ANIMATION OF SLIDING FILAMENT http://www.blackwellpublishing.com/matthews/myosin.html

Muscle Stimulation Increases the Strength of Contraction All of the muscle cells innervated by a single neuron contract at once causing a muscle twitch If a second stimulus is received before the muscle is fully relaxed, the second twitch will be stronger than the first due to wave summation Taken to the extreme, a sustained powerful contraction is called tetanus Fatigue sets in when a muscle is unable to contract even when stimulated

FIGURE 6.9 Muscle contraction shown graphically: (a) muscle twitch, (b) summation, (c) tetanus.

FIGURE 6.9 Muscle contraction shown graphically: (a) muscle twitch, (b) summation, (c) tetanus.

ATP for Muscle Contraction Comes from Many Sources The ATP for muscle contraction comes from many sources The initial source is the ATP stored in the muscle cells and then the ATP formed from the creatine phosphate reserves When those sources are depleted, the muscles depend upon stored glycogen, which is converted to glucose and then to ATP through aerobic respiration or lactic acid fermentation

FIGURE 6.10 Energy sources for muscle contraction

Slow-Twitch and Fast-Twitch Muscle Cells Differ Slow-twitch cells are loaded with mitochondria and therefore deliver prolonged, strong contractions Fast-twitch cells contract rapidly and powerfully but with much less endurance. They rely on lactic acid fermentation as their source of energy and therefore tire quickly

FIGURE 6.11 Slow- and fast-twitch muscle cells

Aerobic Exercise; Resistance Exercise Aerobic exercise increases endurance and coordination Resistance exercise builds strength