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Muscles

Learning Objectives, Topics, & Key Words

Fall Semester, 2003



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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
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Last updated 1 October 2003