MUSCULAR SYSTEM

Honors Biology

Muscles

• From the Latin mus meaning little mouse (Flexing muscles looked like mice scurrying under the skin)

• Have ability to transform ATP into mechanical energy

• Muscles can only pull, never push, which allows them to exert a force

ADP + P + Energy ---> ATP

ATP --> ADP + P + Energy

MUSCLE TYPES

• Skeletal - Slow to Fast twitch• Cardiac - Fast twitch• Smooth - Slow twitch

• Twitch = contraction

• Skeletal & smooth muscle cells are elongated & called fibers

• All have contractile myofilaments actin & myosin

Myosin Actin

Muscle Fxn’s

-Produce movement• Skeletal – locomotion & manipulation in

response to the environment• Cardiac – moves blood• Smooth – propels (squeezes) stuff through the

digestive, urinary, circulatory, and reproductive systems

-Maintaining posture

-Stabilizing joints

-Generating heat (40% of your body heat)

Skeletal Muscle

• Striated• Multinucleate• Voluntary muscles• Can generate great power but fatigue quickly• Non-rhythmic contraction

Sarcoplasmic Reticulum

Sarcolemma (Plasma membrane)

Sarcoplasm = (Cytoplasm with lots of glycogen stored)

Sarcomere – Contractile Unit of Muscle

Nerve & Blood Supply

Each muscle is served by:• 1 nerve• 1 artery• 1 or more veins

Skeletal Muscle

Attachment Skeletal muscles attach to bones in at least 2 places

• When the muscle contracts, the moveable bone (Insertion), moves toward the immovable or less-moveable bone (Origin)

Sliding Filament Theory

• Hugh Huxley 1954 proposed that during contraction actin will slide past myosin which result in overlapping filaments

1. Cross bridge attachment

2. Power stroke (Myosin head pivots pulling actin)

3. Cross bridge detachment (ATP binds to myosin head loosening the bond to actin)

4. “Cocking” the myosin head – ATPase hydrolyzes ATP to ADP & Pi returning the myosin head to it’s cocked position

REGULATION OF CONTRACTION

• Muscle contraction is stimulated by an action potential from a nerve

• The neuromuscular junction in skeletal muscle is regulated by acetylcholine (ACh)

• Ach needs to be broken down as soon as it is used; the enzyme acetlycholinesterase serves this function.

a. b. c.

Generating an Action

Potential• Depolarization (Na channels open)

• Repolarization (Na channels close K channels open)

• Refractory Period (K channels close)

• Na/K pump

Homeostatic Imbalance• Myasthenia gravis – autoimmune disease where ACh

receptors are broken down by ACh antibodies resulting in drooping eyelids and general muscle fatigue

• Curare – arrowhead poison used in South America which blocks ACh receptors resulting in respiratory arrest & death

• Cobra venom – same as curare

• Botulinum toxin prevents ACh release

• Black widow spider venom releases all Ach

• Nerve gasses inhibit AChase which keeps cleft flooded with ACh

ACh destruction

• After ACh initiates the action potential the ACh is broken down by acetylcholinesterase

• This prevents continued muscle contraction in the absence of additional nerve stimuli

Mysostatins control muscle growth

Absence of results in enlarged muscle development