Muscles
Learning Objectives, Topics, & Key Words
Fall Semester, 2003
Readings: Martini,6 th edition, pp. 135 ‚ 136; Chapter 10
Objectives: 1. Describe the 3 types of muscle cells, their properties and functions.
2. Describe the microscopic structure of muscle.
3. Understand the mechanism of muscle contraction and its cellular control
4. Describe the macroscopic structure of muscle.
5. Describe the structure and function of the neuromuscular junction. List the events during neuromuscular transmission.
6. Describe the types of contraction.
7. Distinguish between the three types of skeletal muscle motor units
Types of Muscle Cells
skeletal (striated)
smooth
cardiac (striated)
General Properties of Muscle
irritability (respond to electrical stimuli)
contractility (shorten/develop force/resist lengthening)
extensibility (easily stretched when relaxed)
elasticity (recover length when released, like a rubber band)
Gross Anatomy of skeletal muscle
tendon
aponeurosis
belly (also called, head)
epimysium - the connective tissue surrounding whole muscle (also called, deep fascia)
perimysium - CT surrounding a fascicle
endomysium - CT surrounding a fiber
general principles of muscle architecture
origin
insertion
action
innervation
Microscopic Structure
skeletal muscle
fibers (=cells)
sarcolemma
sarcoplasm
multinucleate (subsarcolemmal)
myofibrils
myofilaments
thick filaments: myosin
thin filaments: actin + tropomyosin + troponin
connecting filaments: titin
sarcomere (the repeating unit that forms the stripes)
Z line to Z line
transverse tubules (T-tubules)
sarcoplasmic reticulum (SR)
mechanism of muscle contraction
myosin heads attach to actin filaments forming cross-bridges
myosin heads change shape
actin filaments are pulled toward middle of sarcomere
"sliding filament mechanism"
cross bridge cycles repeat
one ATP hydrolysed per cycle at each myosin head
Functional Anatomy
motor unit = a motor neuron plus the muscle cells that it innervates
motor unit size (no. of muscle fibers/neuron) is highly variable
Neuromuscular Transmission
neuromuscular junction (NMJ) is a synapse (one NMJ/fiber)
transmission of AP from nerve axon to muscle cells at endplate by chemical means
NMJ is a model for synaptic transmission in the central nervous system
Sequence of Events During Neuromuscular Transmission
[pre-synaptic] action potential invades axon terminals
--> calcium ions enter axon terminal through voltage-sensitive calcium channels
--> calcium triggers vesicles of acetylcholine (ACh) to fuse with pre-synaptic membrane
--> exocytosis --> diffusion of ACh across synaptic cleft
[post-synaptic] --> binding of ACh to receptors --> receptor open its channel for sodium ion ("chemically-gated ion channel")
--> end plate potential--> muscle action potential
large safety factor because EPP much greater than threshold
ACh hydrolysis by acetylcholinesterase ends its effect
pathology of neuromuscular transmission
myasthenia gravis (autoimmune attack on ACh receptors)
botulism (block presynaptic release mechanism)
Excitation-Contraction Coupling
AP --> T-tubule --> calcium release from SR -->binding to troponin --> tropomyosin moves
-->exposing myosin-binding sites on actin subunits--> cross-cycling (contraction)
Muscle Relaxation
SR calcium pump moves Ca2+ from myoplasm into SR; uses ATP hydrolysis as energy source
Types of Muscle Contractions
isotonic
isometric
twitch - response to a single AP in motor nerve
unfused (incomplete) tetanus
tetanus
gradation of muscle force
variation of action potential frequency
recruitment of motor units
types of skeletal muscle fibers (motor unit types)
red - slow (postural motor units; fatigue-resistant) -- oxidative metabolism
white - fast (high power motor units; easily fatiguable) -- glycolytic metabolism
red - fast (fatigue-resistant, powerful motor units) -- oxidative metabolism
Muscle Growth
cells get wider = more myofibrils
cells get longer = more sarcomeres
promoted by growth hormone, anabolic steroids such as testosterone
atrophy (wasting)
disuse atrophy (muscle proteins have approximate 17 day turnover halftime)
denervation atrophy
hypertrophy - exercise effect (max strength increase is 6%/week)
Structure and Function of Other Muscle Cell Types
cardiac muscle (heart)
smooth muscle (viscera, blood vessels)
both of these cell types connected by desmosomes (mechanical) and gap junctions (electrical)
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Durham Technical Community College
Durham, NC 27703
Last updated 1 October 2003