patho exam 3 Flashcards

Question

SKELETAL MUSCLE MECHANICS

Answer 1

A single AP produces a brief, weak contraction: twitch, too short & too weak to be useful * Muscles work cooperatively to produce useful contractions of varying strength by adjusting the following two factors: – Number of muscle fibers contracting within a muscle: depends on the extent of motor unit recruitment – Tension developed by each fiber: frequency of stimulation, length & thickness of fiber, extent of muscle fatigue

Answer 2

Motor unit: one motor neuron & innervated muscle fibers, distributed evenly * Muscle tension depends on: – Size of the muscle – Extent of motor units involved – Size of motor units (fine vs coarse) * Muscle fatigue prevented by asynchronous recruitment of motor units (shifts)

Answer 3

3 motor units the muscle with more motor units generates more strength than the muscle with no motor units

Answer 4

Frequencyofstimulation – Twitch summation: two twitches from different APs added together to produce greater tension than one alone, possibly b/c of a discrepancy between the duration of AP & the resultant twitch – Tetanus: smooth, sustained contraction of maximal strength (3X-4X twitch); caused by rapid stimulation of muscle fiber allowing no time for relaxation between stimuli – Results from a sustained elevation in Ca2+, CB cycling

Answer 5

single twitch - if a muscle fiber is restimulated after it has completely relaxed, the second twitch has the same magnitude as the first twitch twitch summation - if a muscle fiber is restimulated before it has completely relaxed. the second twitch is added on to the first twitch, resulting in summation tetanus - if a muscle fiber is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli, a maximal sustained contraction known as tetanus occurs

Answer 6

Length of fiber at onset of contraction (resting muscle length is the optimal length): – Offers the maximal opportunity for cross-bridge interaction – At lengths other than the optimal length, not all cross bridges are able to interact for muscle shortening, so less optimal * Extent of fatigue * Thickness of the muscle fiber

Answer 7

Isometric contraction: – Muscle tension developed is less than its opposing load – Muscle cannot shorten and lift the object with that load – No change in length, despite developing tension * Isotonic contraction: – Muscle tension developed is greater than its opposing load – Muscle usually shortens and lifts an object – Muscle changes length, but maintains a constant tension throughout the period of shortening – Two types: concentric (muscle shortens) and eccentric (muscle lengthens)

Answer 8

Skeletal muscle contraction & relaxation requires ATP (energy) for the following three steps: – Power stroke of cross bridges by myosin ATPase – Detachment of cross bridges from actin filaments at the end of power stroke – Active transport of Ca2+ back into SPR during relaxation Limited amount of ATP present, additional pathways: – From creatine phosphate – From oxidative phosphorylation – From glycolysis

Answer 9

3 types of skeletal muscle fibers: based on the pathway used for ATP synthesis (oxidative/glycolytic) & the rapidity by which they split ATP & contract (fast/slow) * Slow-oxidative (type I) fibers - Red,  myoglobin & mitochondria * Fast-oxidative (type IIa) fibers * Fast-glycolytic (type IIb) fibers - White (little myoglobin)  glycogen conc. - Fast fibers have higher myosin ATPase activity than slow fibers Most humans have a mixture of all three types, genetics play a role

Answer 10

Composes the internal, contractile organs (the walls of hollow organs and tubes) except the heart Exert pressure and regulates forward movement Elongated, spindle shaped, single nucleus & small Arranged in sheets, unstriated, filament types: Thick myosin filaments (longer than skeletal muscles) Thin actin filaments (more than skeletal), have tropomyosin, no troponin Intermediate size filaments (support, not contractile) Cellular arrangement is not organized into sarcomere pattern of skeletal muscle, so, do not show banding (they are “smooth”) Dense bodies have same proteins as Z-lines of skeletal muscles Innervated by both sympathetic and parasympathetic NS (ANS)

Answer 11

has: thin/thick filament dense bodies actin (thin filament) myosin head myosin (thick filament) actin (thin filament)

Answer 12

Smooth muscle cells contract when Ca2+ ions enter the cells from the ECF and from intracellular stores Ca2+ release activates signaling cascade leading to myosin cross bridge movement

Answer 13

Based on level of ongoing contractile activity: – Phasic SM: * Contracts in bursts * AP  Ca2+ levels * Pronounced increase in contractile activity * Walls of hollow organs e.g. digestive organs – Tonic SM: * Partially contracted at all times (tone) * Low RMP (-55 to -40 mV), so some VGCCs opened * Tone dependent on Ca2+ levels, not on AP activity * Walls of arterioles: maintains blood pressure

Answer 14

Based on how the muscle becomes excited: – Single-unit SM: * Most common SM, aka visceral SM * Forms a functional syncytia, works as a unit * Myogenic (self-excitable): pacemaker or slow-wave potentials * May be phasic or tonic – Multi-unit SM: * Properties similar to skeletal and smooth muscles * Multiple discrete units, functioning independently * Neurogenic: nerve produced * Phasic only

Answer 15

Single-unit smooth muscle: most common, visceral – Form functional syncytia: a group of interconnected cells (gap junctions) * When an AP develops in one cell, it quickly spreads to other cells * Contract as a single, coordinated unit – Myogenic (self-excitable): does not require nervous stimulation for contraction * Pacemaker cells generate AP * Pacemaker cells display spontaneous electrical activity of one of two types: – pacemaker potentials – slow-wave potentials

Answer 16

Pacemaker potentials: – Automatic shifts in ion concentrations in the ECF and ICF cause spontaneous depolarizations to threshold potential * Slow-wave potentials: – Gradually alternating hyperpolarizing and depolarizing swings in potential caused by automatic cyclical changes in rate that sodium ions are actively transported across membrane – If threshold is reached, a burst of AP occur at the peak of depolarization

Answer 17

Multi-unit smooth muscle – Consists of multiple discrete units functioning independently – Properties partway b/w skeletal & single-unit smooth muscle * Neurogenic (“nerve produced” excitation, similar to skeletal muscle) * Supplied by the involuntary ANS – Found in: walls of large blood vessels, large airways of lungs, ciliary muscle, iris of the eye, base of hair follicles

Answer 18

Graded contraction possible in single-unit smooth muscle mechanism different from skeletal muscle – Varying the level of – Increased Ca2+ concentrations cause increased numbers of Ca 2+ ions in the cytosol cross bridges formed and increased tension – Many factors influence Ca2+ concentration * Tone: low level of tension maintained by many single-unit smooth muscle cells that have enough calcium in the cytosol, occurs in the absence of APs * ANS and hormones: alter the strength of self-induced, smooth muscle contractions via signaling * Other factors (e.g., local metabolites and certain drugs): alter the contraction of both single unit and multi-unit smooth muscle

Answer 19

Smooth muscles can develop maximal tension even when stretched 2.5x their resting length – Resting length is shorter than lo – Thin & thick filaments overlap even when stretched – Hence, smooth muscles can exist at different lengths with little change in tension * Stretching a smooth muscle inherently relaxes it – Hence, smooth muscles do not exert any pressure on the contents until it is ready to be emptied * Smooth muscle is slow & economical – Slow CB cycling maintains tension with less ATP use

Answer 20

Found in the walls of the heart, has properties of skeletal and smooth muscle – Skeletal: * Highly organized and striated * Troponin and tropomysoin * Clear tension-length relationship * T tubules and SR * Mitochondria and myoglobin – Smooth: * Can generate APs which spread throughout the walls of the heart, i.e., pacemaker potentials * Innervated and modified by ANS (hormones/local factors)