Chapter 12 Muscle Physiology

key functions of muscles

generate force and generate motion

skeletal muscle generates

heat for thermal regulation

skeletal muscle is

striated, voluntary, and controlled by motor neurons

cardiac muscle is 

striated, involuntary, intrinsic, hormonal and autonomic control 

smooth muscle is

not striated, involuntary, intrinsic, hormonal and autonomic control

tendons are composed of

dense regular connective tissue

tendons are a

collagen attachment of muscle to bone

origin 

attachment point closer to midline/trunk or more stationary bone 

insertion

distal attachment

flexor

moves bones closer together

Extensor

moves bones away from each other

muscle fiber  = 

myofibers 

sarcoplasm =

cytoplasm

satellite cells =

stem cells

muscle fibers are

long multinucleated cells running parallel in length

fascicles 

collections of muscle fibers surrounded by connective tissue 

sarcolemma

cell membrane

sarcoplasmic reticulum

modified ER and longitudinal tubules wrap each myofibril

terminal cisternae

ends of SR and concentrates calcium with Ca2+ ATPase

transverse tubule 

t-tubule that carries AP throughout entire muscle and the lumen is continuous 

triad

2 terminal cisternae and a t-tubule

sarcomere

proteins in the myofibrils form a contractile unit

contractile proteins

interactions between muscle protein function for contraction

types of contractile proteins 

myosin, actin, tropomyosin, and troponin 

myosin

thick filament with a head, neck, and tail region

myosin head

motor domain that binds ATP and can bind with actin to form crossbridge

actin

thin filaments and polymers form chains 

tropomyosin 

blocks actin binding site 

troponin

calcium binds and controls tropomyosin

titin and nebulin

elastic protein and stabilizes positions of actin and myosin

I band

only thin filaments, Z-disk and light

Z-disk 

protein attachment sites for actin 

A band

thick and thin filaments overlap, dark band

H-zone

only thick filaments

M-line

center proteins attachment for thick filament

NMJ signaling uses 

acetylocholine 

NMJ signaling process

ACh → AP → Ca2+ release → troponin binds to Ca → crossbridge cycling

sliding filament theory

actin and myosin sliding by each other and shortens sarcomere resulting in a contraction

role of calcium in muscle contraction

binds troponin, moves tropomyosin, and allows cross-bridge formation

role of ATP in muscle contraction 

ATP detaches myosin 

rigor mortis

myosin head is bound to actin

contractile cycle involves

ATP, ADP, Pi, myosin, actin, tropomyosin and troponin

contractile cycle

rigor state → resting state → power stroke → contraction

resting state 

myosin hydrolyzes ATP → ADP + Pi 

power stoke

Calcium influx causes it to bind to troponin and move tropomyosin. Myosin binds to actin releasing Pi and moves actin toward M line

muscle relaxation

calcium ATPase pumps calcium into sarcoplasmic reticulum

central muscle fatigue

CNS origins, subjective feelings, and psychological basis

Peripheral muscle fatigue 

anywhere between NMJ and contraction process 

classification of skeletal muscle is based on

contraction speed and fatigue resistance

skeletal muscle includes

oxidative fibers, mitochondria, blood supply, and myoglobin

Types of skeletal muscle

Type I, Type IIA, and Type IIB/X

Slow twitch/ Type I 

used for posture and oxidative 

Fast Twitch/Type IIA

oxidative, faster movements, and fatigue resistant 

Fast twitch/Type IIB/X

fastest, fatigable, and glycolytic

how length tension affects muscle mechanics

tension generated by muscle is directly proportional to the number of cross bridges that form

single twitch

muscle relaxes completely between stimuli

summation 

stimuli closer together and do not allow muscle to relax fully 

tetanus

entire muscle contraction and reaches steady tension

incomplete tetanus

slight relaxation between stimulus

complete/fused tetanus

no relaxation and fatigue

motor unit 

single motor neuron and all muscle fibers it innervates 

fine motor

only a few muscle cells and greater level of control

gross motor

many fibers it innervated by individual neurons

muscle recruitment is controlled by

nervous system

generic sequence of activation of muscle fibers by type 

1. type I 

2. Type IIA

3. Type IIB/X

type I activation

recruited first due to neurons low threshold

Type IIA activation

generates greater force T

Type IIB/X activation

highest threshold

isotonic contractions 

creates force to move loads 

types of isotonic contractions

concentric and eccentric

concentric contractions

shortening contractions to move load

eccentric contractions

lengthening contractions

isometric contractions 

creates force but no movement of load (muscle doesn’t change in length)

locations of smooth muscle

vascular, GI, Urinary, respiratory, reproductive, and ocular

Vascular smooth muscle

blood vessels

GI smooth muscle

digestive tract and gall bladder

urinary smooth muscle 

bladder and ureters 

respiratory smooth muscle

airways

reproductive smooth muscle

uterus

smooth muscle phasic control

cyclical contraction and relaxation

tonic control of smooth muscle 

always some level of contraction 

microscopic organization of smooth muscle

dense bodies for anchoring actin and layers move in different directions

skeletal muscle contraction

fast, Ca2+, troponin

smooth muscle contraction

slow, Ca2+, calmodulin

process of smooth muscle contraction 

1. calcium binds to calmodulin and initiates cascade

2. ends by phosphorylation of myosin light chain by MLC kinase

3. increases ATPase activity of myosin head