MCB 244 exam 3

Functions of Muscles

Movement, Stability, Control of body openings and passages, Heat production (thermogenesis), Hormone secretion, Glycemic control

Endomysium

thin sleeve of loose connective tissue around each fiber, allows room for capillaries and nerve fibers; provides chemical environment for muscle fiber and nerve ending

Perimysium

thicker layer of connective tissue that wraps fascicles (bundles of muscle fibers wrapped together), Carries nerves, blood vessels, and stretch receptors

Epimysium

fibrous sheath surrounding entire muscle; blends with fascia and deeper connective tissues

Fascia

sheet of connective tissue between muscles or muscle groups

Fusiform muscles

thick in the middle with fascicles converging on each tapered end

Parallel muscles

uniform width and parallel fascicles

Triangular (convergent) muscles

broad at one end with fascicles converging on the other, narrower end

Pennate muscles

feather-shaped; fascicles attach obliquely on a tendon that runs full length of muscle

Circular muscles (sphincters)

fascicles form rings around body openings and passages

Intrinsic muscle

entirely contained within a region

Extrinsic muscle

acts on a designated region but arises from another region

Excitability

responsiveness to chemical signals, stretch, and electrical changes across the plasma membrane

Conductivity

local electrical excitation sets off a wave of excitation that travels along the muscle fiber

Contractility

shortens when stimulated

Extensibility

capable of being stretched between contractions

Elasticity

returns to its original rest length after being stretched

Skeletal muscle

voluntary, striated muscle usually attached to bones

Sarcolemma

plasma membrane of a muscle fiber

Sarcoplasm

cytoplasm of a muscle fiber

Myofibrils

long protein cords occupying most of sarcoplasm

Glycogen

carbohydrate stored to provide energy for exercise

Myoglobin

red pigment; provides some oxygen needed for muscle activity

Sarcoplasmic reticulum (SR)

smooth ER that forms a network around each myofibril

Terminal cisterns

dilated end-sacs of SR which cross the muscle fiber from one side to the other

Transverse (T) tubules

tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side

Myoblasts

stem cells that fused to form each muscle fiber early in development, each contributing one nucleus

Satellite cells

unspecialized myoblasts remaining between the muscle fiber and endomysium

Structure of a Skeletal Muscle Fiber

Thick filaments

made of several hundred myosin molecules (myosin is a motor protein)

Fibrous (F) actin

thin filament; two intertwined strands of globular (G) actin subunits, each with an active site that can bind to head of myosin molecule

Tropomyosin

thin filament; each blocks six or seven active sites on G actin subunits

Troponin

thin filament; small, calcium-binding protein on each tropomyosin molecule

Elastic filaments

made of a huge, springy protein called titin

regulatory proteins

Tropomyosin and troponin

contractile proteins

myosin and actin

Dystrophin

links actin in outermost myofilaments to membrane proteins that link to endomysium

Striations and Sarcomeres

Sarcomere

segment from Z disc to Z disc; functional contractile unit of muscle fiber

Motor unit

one nerve fiber and all the muscle fibers innervated by it; they behave as a functional unit

Synapse

point where a nerve fiber meets its target cell

neuromuscular junction (NMJ) or motor end plate

Synapse when target cell is a muscle fiber

Axon terminal

swollen end of nerve fiber; Contains synaptic vesicles with neurotransmitter called acetylcholine (ACh), Nerve impulse causes synaptic vesicles to undergo exocytosis releasing ACh into synaptic cleft

Synaptic cleft

narrow gap between axon terminal and sarcolemma

Postsynaptic membrane folds (junctional folds) of sarcolemma

increase surface area, maximize number of ACh receptors

basal lamina

thin layer of collagen and glycoprotein separating muscle fiber and nerve ending from surrounding connective tissue

acetylcholinesterase (AChE)

enzyme that breaks down ACh, allowing for relaxation

electrical potential, or voltage

Any difference in charge between two points

resting membrane potential (RMP)

The plasma membrane is electrically polarized with a negative charge

Spastic paralysis

a state of continual contraction of the muscles; possible suffocation

Tetanus (lockjaw)

a form of spastic paralysis caused by toxin Clostridium tetani

Botulism

type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum

Excitation

action potentials in motor nerve fiber lead to action potentials in the muscle fiber

Excitation–contraction coupling

events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract

Relaxation

when stimulation ends, a muscle fiber relaxes and returns to its resting length

Length–tension relationship

the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated

rigor mortis

hardening of muscles and stiffening of body beginning 3–4 hours after death

Myogram of a Muscle Twitch

Threshold

minimum voltage that causes a muscle twitch

Recruitment or multiple motor unit (MMU) summation

the process of bringing more motor units into play with stronger stimuli

temporal (wave) summation

Higher frequency stimuli (for example, 20 stimuli/s) produce these

incomplete tetanus

Only partial relaxation between stimuli resulting in fluttering

complete (fused) tetanus

Unnaturally high stimulus frequencies (in lab experiments) cause a steady, contraction called

Isometric contraction

contraction without a change in length

Isotonic contraction

contraction with a change in length but no change in tension

Isotonic contraction

contraction with a change in length but no change in tension

Concentric contraction

muscle shortens as it maintains tension (example: lifting weight)

Eccentric contraction

muscle lengthens as it maintains tension (example: slowly lowering weight)

Isometric and Isotonic Phases of Contraction

Anaerobic fermentation

enables cells to produce ATP in the absence of oxygen; yields little ATP and lactate, which needs to be disposed of by the liver

Aerobic respiration

produces far more ATP; does not generate lactate; requires a continual supply of oxygen

Myokinase

transfers Pi from one ADP to another, converting the latter to ATP

Creatine kinase

obtains Pi from a phosphate-storage molecule creatine phosphate (CP) and gives it to ADP

Anaerobic threshold (lactate threshold)

point at which lactate becomes detectable in the blood

Potassium accumulation in the T tubules

reduces excitability

ADP and Pi accumulation

slows cross-bridge movements, inhibit calcium release and decrease force production in myofibrils

Maximum oxygen uptake (VO2 max)

the point at which the rate of oxygen consumption plateaus and does not increase further with added workload

Excess post-exercise oxygen consumption (EPOC), or oxygen debt

elevated rate of oxygen consumption following exercise

Slow-twitch, slow oxidative (SO), red fibers, or type I fibers

Thin cells with abundant mitochondria, capillaries, myoglobin (deep red color) and contain a form of myosin with slow ATPase, and a SR that releases calcium slowly

Fast-twitch, fast glycolytic (FG), white fibers, or type II fibers

Contain a form of myosin with fast ATPase and a large SR that releases calcium quickly, Fibers are thick and strong; low myoglobin gives them pale color

Factors that affect muscular strength

Muscle size, Fascicle arrangement, Size of active motor units, Multiple motor unit summation, Temporal summation, The length–tension relationship, Fatigue

Cardiac Muscle

Striated, relatively short and thick, and surrounded by endomysium (but no peri or epimysium), Cardiomyocytes branch and join to other cells at connections called intercalated discs

Multiunit smooth muscle

individual myocytes respond independently of all the others

Unitary (single-unit) smooth muscle

myocytes coupled by gap junctions, so contract as a unit

Muscular dystrophy (MD)

group of hereditary diseases in which skeletal muscles degenerate and weaken, and are replaced with fat and fibrous scar tissue

Myasthenia gravis

autoimmune disease in which antibodies attack neuromuscular junctions and bind ACh receptors together in clusters

Central nervous system (CNS)

brain and spinal cord

Peripheral nervous system (PNS)

nerves and ganglia

Nerve

a bundle of nerve fibers (axons) wrapped in fibrous connective tissue

Ganglion

a knot-like swelling in a nerve where neuron cell bodies of PNS are concentrated

Sensory (afferent) division

carries signals from receptors (sense organs) to CNS

Somatic sensory division

carries signals from receptors in the skin, muscles, bones, and joints

Visceral sensory division

carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

Motor (efferent) division

carries signals from CNS to effectors (glands and muscles that carry out the body’s response)

Somatic motor division

carries signals to skeletal muscles; causes voluntary muscle contraction and automatic reflexes

Visceral motor division (autonomic nervous system, ANS)

carries signals to glands, cardiac and smooth muscle; no voluntary control; responses called visceral reflexes

Sympathetic division of ANS

stimulates and prepares the body for action

Parasympathetic division of ANS

has a calming effect on the body

An enteric plexus within digestive tract wall

enables coordination and communication within digestive tract

Secretion

when signal reaches end of axon, the neuron secretes a neurotransmitter that stimulates the next cell