Lecture #24:
Muscle Contraction and Animal Movement
I. INTRODUCTION
1. Modes of
movement include swimming, crawling, walking, running, hoping, flying.
2. At the
cellular level (micro), animal movement is based on two basic contractile
mechanisms:
a. microtubules
move cilia (oviduct) and flagella (sperm).
b. microfilaments
are important in ameboid (fibroblast) movement and muscle contr.
3. Skeletons important
for support, protection, and movement (give muscles something). (Fig. 49.28a)
II. ANATOMY OF SKELETAL MUSCLES
1. Muscle belly,
single muscle fiber (a cell), myofibrils, and sarcomere. (Fig 49.31)
2. The filaments
within a sarcomere:
a. thick
filaments consist of overlapping filaments of myosin protein
containing up to 350 "heads".
(Fig 49.31)
b. thin
filaments consist of helical configurations of two strands of actin
protein and one strand of tropomyosin.
(Figs 49.31)
3. Microanatomy
of a sarcomere and the sliding filament model. (Fig 49.32)
III. CHEMISTRY OF SKELETAL MUSCLE CONTRACTION
1. Ca++
reacts with troponin that is attached to the tropomyosin. (Fig 49.34)
2. This leads to a
configurational change in the tropomyosin, and exposes cross-bridge
binding sites on the actin strands.
3. In its
low-energy configuration, the myosin head does not attach to the actin binding
site. (Fig. 49.33.1)
4. There is a
natural affinity between the ATP-energized head of a myosin molecule
and the binding site on an actin filament.
(Fig 49.33.2
& 33.3)
5. When the
"cross-bridge" forms, there is a release of ADP+*P from the myosin
head, and a bend (conformational change) in the cross-bridge. (Fig. 49.33.4)
6. ATP then
reacts with the myosin head and causes a momentary release of the head. (Fig. 49.33.4)
7. Hydrolysis
of ATP into ADP and *P returns the myosin head to original position.
8. The myosin head
can now react with a new actin binding site. (ratchet effect).
(9) Replenishment
of ATP is provided by a vertebrate phosphagen (creatine phosphate).
(10) Reconsider
all the chemical activity it takes to lift a finger.
IV. ELECTROCHEMICAL EXCITATION OF SKELETAL MUSCLE FIBERS
1. Axon
terminals at neuromuscular junctions release acetylcholine. (Fig 49.35)
2. Depolarization
travels in both directions from the motor end-plate.
3. Action
potential travels even into the traverse (T) tubules at Z-lines. (Fig 49.36)
4. Depolarization
of the T-tubules extends to depolarization of sarcoplasmic reticulum.
5. This depolarization
opens Ca++ channels in the sarcoplasmic reticulum and releases
stored Ca++ ions among myofilaments.
6. Integrated
action of motor units in a vertebrate muscle. (Fig.
49.38)