Chapter 8

Muscular System

Introduction

• Three types of muscles:–Skeletal–Smooth–Cardiac

Structures of Skeletal Muscle

• Tissues present in skeletal muscle–Muscle, nervous, connective (including

blood)• Skeletal muscle is held in place by layers of

fibrous connective tissue called fascia*fascia covers the muscle

Structures of Skeletal Muscle

• Fascia extends beyond the end of a skeletal muscle to form a cord like tendon (muscle to bone)

• When connective tissues extend beyond the muscle to form a sheet-like structure, it is called an aponeuroses – Attach to bone or to adjacent muscles

Structures of Skeletal Muscle

• Fascicles are bundles of skeletal muscle fibers• The connective tissue that covers each muscle

fiber within a fascicle is the endomysium*the muscles are enclosed and separated into

compartments by epimysium and perimysium (figure 8.1)

*blood vessels and nerves pass through layers

Analogy Time!• Pretend you are going to play a joke on

someone and give them 100 pencils. The pencils will represent muscle fibers. First you wrap each individual pencil in tissue paper (dense tissue paper of course!). This would be endomysium. Then you take about 10 pencils in a bundle (a fascicle) and wrap them in paper (Perimysium). Then you take all the bundles and wrap them in gift wrap (epimysium). But you are going to mail this joke, so you also have to wrap it in brown paper representing the fascia.

Skeletal Muscle Fiber

• Single muscle fiber– Single cell, thin, elongated, contracts– Sarcoplasm (cytoplasm) contains

–Many nuclei and mitochondria–Myofibrils (parallel)

Skeletal Muscle Fiber

• Make note of this!• Myofibrils– Fundamental role in contraction–2 kinds of protein filament• Thin – actin• Thick – myosin

Skeletal Muscle Fiber

• What are sarcomeres? (figure 8.3)– Functional units of muscle contractions– 1st are light bands of actin filaments which are

attached to Z lines. – 2nd are A bands which contain myosin filaments -

overlap actin filaments. – The segment of a myofibril from one Z line to the

next Z line is known as a sarcomere– Myofibrils are sarcomeres joined end to end

Skeletal Muscle Fibers

• The network of membranous channels in the cytoplasm of muscle fibers is the sarcoplasmic reticulum. A second set of channels is the tranverse tubules which extend inward from the fibers membrane.

• These channels activate muscle contraction when the fiber is stimulated.

Questions to Answer

• What is tendinitis?• What causes two of the most common forms

of muscular dystrophy?• What causes muscle strain?– What is the difference in a mild form and a severe

form?

*Write your answers on a piece of scrap paper

Labeling

• Part D of “Structure of a Skeletal Muscle”

Neuromuscular Junction

• Write these notes below “D” on page 58• Motor neurons – neurons that control

effectors• Synapse – functional connection between a

motor neuron and a skeletal muscle fiber • Muscle fibers and motor neurons not directly

connected – a gap between them called synaptic cleft

Neuromuscular Junction

• Neurotransmitters – chemicals released by the neurons to allow communication with the cells (tell fibers when to move)

• Neuromuscular Junction – connection between motor neuron and muscle fiber

Neuromuscular Junction

• End of Motor Neuron – rich in mitochondria and synaptic vesicles (store neurotransmitters)

• Motor end plate – muscle fiber membrane is folded and contains many nuclei and mitochondria

• Nerve impulse end of motor neuron release neurotransmitters motor end plate muscle contraction!

Skeletal Muscle Contraction“Sliding Filament Theory”

• Roles of Actin and Myosin–Actin - have a binding site for the myosin

cross bridges–Myosin - have cross bridges that “pull”

actin for contraction

*Part B is answered by notes on page 58

Skeletal Muscle Contraction“Sliding Filament Theory”

• Use highlighter and Figure 8.8 printout to highlight key steps

• The sliding filament model gets its name from the way the sarcomeres shorten

• The filaments do not change, they slide past one another – Actin slides toward the center of the sarcomere

from both ends

Skeletal Muscle Contraction“Sliding Filament Theory”

• Acetylcholine (neurotransmitter) and Calcium ions (Ca+)Nerve impulse motor neuron releases

acetylcholine diffuses to muscle fiber causes muscle impulse allows diffusion of Ca+ into muscle fiber binding sites open muscle contraction begins

Skeletal Muscle Contraction“Sliding Filament Theory”

• What are cross bridges?–formed when the projecting parts of

the myosin filament that will occupy a binding site on the actin filament and pull

Skeletal Muscle Contraction“Sliding Filament Theory”

• The action of acetylcholine is halted by the enzyme acetylcholinesterase

• The enzyme decomposes the neurotransmitter

Homework

• Please write the answers to the homework handout on a separate sheet of paper

• Due tomorrow!• Worth 20 points