Muscular System Exam 2

Functions of muscular system

Movement: contraction pulls on insertions where a tendon attaches to a bone or other tissue

Posture: muscles stabilize joints by producing muscle tone with continuous contractions

Protection: muscles support the walls of the abdominopelvic cavity and protect visceral organs

ex. rectus abdominis and transverse abdominis protect the intestines

Thermogenesis: metabolism can be regulated to produce heat and warm the body temperature

Contractility 

protein filaments within muscle cells pull and slide together producing a force called muscle tension that regulates movement at the organ level 

Muscle Tension

force a muscle exerts on an object when contracted

must overcome the load (opposing force exerted on the muscle) for the muscle to shorten

Excitability

muscles respond to stimuli producing electrical changes across the plasma membrane

stimuli can be mechanical, chemical, or electrical

Conductivity

when muscle cell is excited, the resulting electrical changes across the plasma membrane rapidly spread through the cell along the membrane

Distensibility

most cells will rupture when stretched, but muscle cells are distensible

stretched up to three times their resting length without damage 

Elasticity

muscle cells can also return to their original shape after being stretched

Cardiac Muscle

found in the muscular wlal of the heart (myocardium)

contains branching striated cells called cardiomyoctyes (fibers) that have a central round nucleus

form cell junctions called intercalated disks that contain desmosomes and gap junctions

In cardiac muscle, Contraction is

involuntary and stimulated by an internal pacemaker of the heart

regulated by the autonomic nervous system and endocrine system 

Striations

alternating light and dark bands (stripes) resulting from overlapping thick and thin myofilaments

Smooth Muscle

spindle shaped (fusiform) cells called fibers with a central nucleus and no striations

found in walls of hollow organs

involuntary contraction regulated by the autonomic nervous system and endocrine system

Dense bodies

proteins that anchor thin filaments to the sarcolemma

myosin and actin filaments are arranged diagonally to the length of the fiber, causing smooth muscle to contract in a corkscrew manner 

Skeletal Muscle

Contractions are

long, striated, cylindrical cells called fibers contain multiple nuclei

contraction is voluntary and stimulated and stimulated by the somatic division of peripheral nervous system

Fascile

Endomysium 

bundle of muscle fibers

loose connective tissue surrounding muscle fibers within a fasicle 

Perimysium

fibrous CT surrounding fascicles deep to epimysium (fibrous CT covering the surface of a skeletal muscle organ)

Tendons

dense regular connective tissue structures that connect muscles to bones

Action

movement of the body that is produced during contraction of a muscle

Origin

location where a muscle connects to a bone that remains stationary during the action produced by contraction of that muscle

Insertion

point of muscle attachment that moves during the action produced by a contraction

Sarcolemma

plasma membrane of a muscle fiber

Myofibrils

non membranous organelles within muscle gibers made of repeating structural and functional units called sarcomeres

contains overlapping myofilaments

Sarcoplasmic Reticulum 

membranous organelles similar to the smooth endoplasmic reticulum surrounding myofibrils 

primary function: storage and release of calcium

Transverse (T) Tubules

deep indentations of the sarcolemma forming a network of narrow spaces filled with extracellular fluid

sarcolemma here contains channels that allow facilitated diffusion of sidum ion from the extracellular fluid into the cytoplasm surrounding the sarcoplasmic reticulum

Terminal cisternae

expanded chambers of sarcoplasmic reticulum adjacent to tubules

Sarcomere

structural and functional unit of skeletal andcardiac muscle fibers

Myofilamts

fibrous proteins within myofibrils that produce contraction

Thick filamts

found in the center of the sarcomere primarily consisting of a motor protein that pulls on the thin filaments during contraction

Myosin

ATPase motor protein that converts the chemical potential energy of ATP to the kinetic energy of power strokes for muscle contraction

Thin filaments

Contains

found on both ends of the sarcomere

Actin (cytoskeletal protein with binding sites for myosin)

Troponin (calcium sensor protein)

Tropomyosin (protein that converts myosin binding sites when the muscle is relaxed

Z lines

ends of a sacromere that contain proteins to anchor thin filaments in place

A band

I band

dark staining region of the sarcomere that spans the entire length of the thick filaments

contains the M line: center of sarcomere

lighter staining region of the sarcomere that contains only thin filaments with no overlap of thick filaments

Axon terminal

end of motor neuron that releases the neurotransmitter acetylcholine to excite a skeletal muscle fiber

Motor endplate

surface of the sarcolemma that has nicotinic recpetors to detect the neurotransmitter acetylcholine released by somatic motor neurons

Synaptic Celft

small space where neurotransmitters are released

Excitation-Contraction coupling

linking the electrical signal to the activation of myofibrils to produce movement

When a nerve impulse reaches the end of an axon terminal at the neuromuscular junction:

Voltage-gated calcium channels

open and allow Ca2+ to enter axon

causes synaptic vesicles to fuse with the axon membrane and release the neurotransmitter acetylcholine into the synaptic cleft by exocytosis 

Neurotransmitter diffuses across the synaptic cleft and binds to receptors at the motor end plate that functions as ligand gated ion channels

Sodium enters the cell, producing depolarization leading to activation of voltage gated channels responsible for the action potential

action potential goes along sarcolemma down T-tubules, triggers Ca2+ release form terminal cisternae

Ca2+ binds to troponin causing tropomyosin to move off actin bindings sites, allows myosin to attach

Sliding Filament Theory of Contraction

contraction results from a crossbridge cycle, where thin myofilaments are pulled and then slide past the thick myofilaments within the sarcomeres.

Steps of the crossbridge cycle 

Crossbridge formation: myosin head binds to actin

Power stroke: myosin head pivots pulling on actin 

Crossbridge detachment: myosin head releases actin and binds to a molecule of ATP

Energizing myosin heads: myosin head catalyzes hydrolisis of ATP and some of the energy released is used to move myosin head back to high energy position. 

2 metabolic pathways to supply ATP

Direct phosphorylation: creatine kinase transfers a phosphate from creatine phosphate to ADP forming ATP (energy for 10-15 seconds)

Anaerobic respiration: Glycolytic muscle metabolism generates 2 ATP from each glucose converted to lactic acid

Aerobic respiration: oxidative muscle metabolism uses oxidative phosphorylation to generate large amounts of ATP (30 ATP from 1 glucose)

ETC needs O2

ATP synthase on inner membrane of mitochondria performs last step of oxidative phosphorylation, converts ATP to ADP

Fatigue

decline in muscle tension as a result of previous contractility fatigue

How are muscle fibers classified

how quickly their mysoin ATPases work (fast or slow twitch)

metabolic pathways they primarily use for ATP production (oxidative-aerobic or glycolitic-anaerobic)

Slow oxidative muscle fibers

produce slower less forceful contractions but can sustain activity for longer without fatigue 

slow myosin ATPase activity (slower to produce contractions, but resists fatigue)

uses aerobic respiration to produce ATP

numerous mitochondria and rich supply of capillaries

small diameter: increases rate of nutrients and waste exchange but fewer myofibrils produce a smaller force of contraction

High myoglobin content: provides a depot of oxygen to fuel aerobic respiration and gives muscle fibers a dark red color

Fast Oxidative muscle fibers

produce rapid contractions, moderate ability to resist fatigue 

fast myosin ATPase activity (allows fiber to rapidly contract when excited)

uses anaerobic respiration but can also uses increasing levels of aerobic respiration with endurance exercise training 

less mitochondria, capillaries, and myoglobin 

Fast Glycolitic muscle fibers

produce more forceful contractions but fatigues quickly

fast myosin ATPase activity (allows fiber to rapidly contract when excited)

primarily uses anaerobic respiration to produce ATP

fewer mitochondria

large diameter, enables high glycogen content providing a depot of glucose to fuel anaerobic respiration

provides more space for myofibrils enabling FG fibers to produce more forceful contractions

Twitch

single contraction in a muscle resulting from a single action potential from a motor neuron

Summation

when impulses are delivered in a rapid succession the second twitch will add tension to teh preceding twitch

Tetanus

rapid stimulation results in sustained contraction

Motor Unit Recruitment

nervous system regulated the number of muscle fibers activated as another mechanism to regulate the amount of tension produced. Proceeds in a standardized sequence:

1. weak stimulus directed into somatic motor neurons in CNS activated neurons with lowest threshold (control slow twitch fibers, more force)

2. more neurons with higher thresholds start to fire. neurons stimulate motor units (fast twitch fibers). Greater force generated in muscle.

3. Highest threshold motor neurons fire (glycolytic fast twitch fibers). Muscle contraction reaches max force. Fast twitch fatigue first but give more force

4. Nervous system avoids fatigue by asynchronous recruitment of motor units. Dif motor neurons take turns maintaining muscle tension.

Isometric contraction

“same length:

muscle tension increases with contraction, but muscle length remains constant

occurs when contraction is unable to produce enough force to overcome the load 

ex. pushing against stationary wall

Isotonic contraction

“same tension”

Muscle tension remains constant during contraction as muscle length changes

Isotonic concentric contraction

muscle shorten

ex. pick up pencil, kick soccer ball

Isotonic Eccentric contractions

muscle contracts as it lengthens

ex. squats

Smooth Muscle Tissue

found in walls of hollow organs (except the heart_ 

composed of spindle-shaped fibers 

innervated by autonomic efferent fibers that have bulbous swellings called varicosities where neurotransmitters are released (instead of axon terminals)

can either excite or inhibit contraction

Smooth Muscle Contraction mechanism

excitation can be autonomously generated (produced spontaneously) or neurogenic (stimulated by neurotransmitter)

both lead to increased calcium concentration of cytosol

Calmodulin (CaM)

protein directly activated by Ca2+ to stimulate smooth muscle contraction

activation leads to activation of myosin light chain kinase (enzyme that transfers phosphate from ATP to activate myosin motor protein)

actin and myosin interact according to the sliding filament mechanism with the same 4 basic steps as skeletal muscle

smooth muscle relaxes when intracellular Ca2+ levels decline leading to inactivation CaM and MLCK while myosin is dephosphorylated by myosin light chain phosphatase

smooth muscle DOES NOT contain troponin

Single Unit (visceral) smooth muscle

Autonomous excitation mechanism

smooth muscle fibers are electrically coupled to one another via gap junctions and contract rhythmically as a unit

found in the walls of esophagus, stomach, intestine, uterus, urinary bladder, ureters

intrinsic pacesetter cells display rhythmic, spontaneous variations without nervous stimulation

depolarization triggers opening of L-type calcium channels (type of voltage gated calcium channel that stay open for a while)

contraction can be regulated by autonomic nervous system and endocrine system

Multi-unit smooth muscle

Excitation mechanism

individual smooth muscle fibers and independently innervated by autonomic motor neurons

found in skin forming arrector pili (attached to hair follicles) and inside eye (eye muscles of iris)

acetylcholine or norepinephrine released by the varicosities of autonomic efferent fibers stimulates smooth muscle contraction, involved second messenger pathways that regulate calcium influx.