Chapter 6 The Muscular System

Functions of Muscular System Produce movement and manipulate the environment Maintain posture Stabilize joints Generate heat Slide 6.8

The Muscular System Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle

Similarities of all Types of Muscles Muscle cells are elongated (muscle cell = muscle fiber) Contraction of muscles is due to the movement of microfilaments All muscles share some terminology Prefix myo refers to muscle Prefix mys refers to muscle Prefix sarco refers to flesh Slide 6.2

Differences in the types of muscles Slide 6.1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Differences in the types of muscles Slide 6.1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue Skeletal, striated, voluntary

Skeletal Muscle Attachments Tendon – cord-like structure Aponeuroses – sheet-like structure Sites of muscle attachment Bones Cartilages Connective tissue coverings Tendon Aponeuroses

Cardiac Muscle Characteristics Has striations Usually has a single nucleus Joined to another muscle cell at an intercalated disc Involuntary; the heart has a pacemaker Found only in the heart Figure 6.2b Cardiac, striated, involuntary

Smooth Muscle Characteristics Has no striations Spindle-shaped cells Single nucleus Involuntary – no conscious control Found mainly in the walls of hollow organs Arranged in layers 2 layers Figure 6.2a Smooth, no striations, involuntary

Microscopic Anatomy of Skeletal Muscle Cells are multinucleate Nuclei are just beneath the sarcolemma Sarcolemma (organelle) – specialized plasma membrane Figure 6.3a

Microscopic Anatomy of Skeletal Muscle Sarcoplasmic reticulum (organelle) Specialized smooth ER that surrounds all myofibrils Stores Ca ions to be able to release them on demand Figure 6.3a

Microscopic Anatomy of Skeletal Muscle Myofibril (organelle) Bundles of myofilaments Myofibrils are aligned to give distrinct bands I band = light band A band = dark band Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Sarcomere Chains of contractile units in myofibrils Two types of myofilaments Thick = myosin Thin = actin Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Thick filaments = myosin filaments Composed of the protein myosin Figure 6.3c

Microscopic Anatomy of Skeletal Muscle Thin filaments = actin filaments Anchored to the Z disc Composed of the protein actin Figure 6.3c

Microscopic Anatomy of Skeletal Muscle Myosin filaments have heads (extensions, or cross bridges) Myosin heads link the thick and thin during contraction Myosin and actin overlap somewhat Figure 6.3d

Microscopic Anatomy of Skeletal Muscle Thick (myosin) and thin (actin) filaments produce the striations in skeletal muscle Figure 6.3d

Organizational levels of Skeletal Muscle Organ – the muscle (biceps) Fiber – a muscle cell Myofibril – organelle composed of sarcomeres and myofilaments Sarcomeres – unit of myofibril Myofilament – actin and myosin

Skeletal Muscle Cell Nucleus – control center Sarcolemma – plasma membrane Sarcoplasm – cytoplasm

Skeletal Muscle Cell Sarcoplasmic reticulum – storage of calcium ions to be released when stimulated by an impulse T-tubules – surround the myofibrils and assist in delivering ions

Skeletal Muscle Cell Mitochondria (lots) – provide energy and ATP Myofibril – composed of thick and thin filaments and many sarcomeres

From nerve stimulus to sarcomere contraction Muscle Contraction From nerve stimulus to sarcomere contraction

Nerve Stimulus to Muscles 1. Skeletal muscles must be stimulated by a nerve to contract Motor unit is composed of: One neuron All muscle cells stimulated by that neuron One nerve cell branches into axonal terminals Figure 6.4a

Nerve Stimulus to Muscles Axonal terminals form junctions with sarcoplasm called neuromuscular junctions Nerve and muscle do not make contact there is a gap Synaptic cleft – gap between nerve and muscle Gap is filled with interstitial fluid Figure 6.5b

Nerve Stimulus to Muscles 2. Action potentials are sent down the neuron in response to a stimuli Because the two cells do not touch, the action potential cannot ‘jump the gap’ Figure 6.5b

Nerve Stimulus to Muscles 3. The neuron communicates indirectly with the muscle cell by releasing a neurotransmitter The neurotransmitter for skeletal muscle is acetylcholine (ACh) and it is stored in the axonal terminal 4. Sarcolemma has receptors for Ach and causes an action potential in muscle cell

Transmission of Nerve Impulse Wrap-up How does the neurotransmitter produced by the motor neuron cause the skeletal muscle to contract? What is the neurotransmitter in muscle cells? What is the synaptic cleft? What makes up a motor unit? List the steps of transmission of a nerve impulse.

The Sliding Filament Theory of Muscle Contraction Put your right palm on the back of your left hand. Now slide your right palm toward your left elbow What happened to the distance between your elbows? This is how muscle contraction works! Figure 6.7 Slide 6.17a

The Sliding Filament Theory of Muscle Contraction Activation by nerve causes calcium to be released by the sarcoplasm reticulum Calcium binds to actin and exposes myosin binding sites where the myosin heads attach and form a crossbridge ATP is used as the energy source Figure 6.7 Slide 6.17a

The Sliding Filament Theory of Muscle Contraction The thin filament slides past the thick filament towards the center as each myosin head attaches and detaches Distance between Z discs shortens as actin moves toward the center The result is that the entire muscle is shortened (contracted) Figure 6.7 Slide 6.17b

The Sliding Filament Theory of Muscle Contraction Length of the A band (myosin and actin) stays the same Length of the H zone (only thick filaments) shortens Length of the I band (thin filaments) shortens http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter42/animations.html# Figure 6.7 Slide 6.17b

The Sliding Filament Theory Go to figure 6.8 in your book and summarize the steps for the sliding filament theory and explain how a muscle contracts

Transmission of Nerve Impulse to Muscle Contraction 1. Nerve impulse sent and received by axonal terminal 2. Neurotransmitter (acetylcholine) is released upon arrival of nerve impulse 3. Diffuses across synaptic cleft and attaches to receptors on the sarcolemma 4. Triggers an action potential of the muscle cell 5. The action potential causes the SR to release calcium ions 6. The calcium ions bind with the actin filaments to open the binding site for myosin 7. Myosin heads bind to them with the help of ATP 8. The cell contracts