Muscle Physiology Exam

list the 5 universal characteristics of muscles

1. excitability

2. conductivity

3. contractility

4. extensibility

5. elasticity

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 alongs the muscle fiber

contractility

shortens when stimulated

extensibility

capable of being stretched between contractions

elasticity

returns to its original rest length after being stretched

describe skeletal muscle

voluntary striated muscle usually attached to bones

what are striations in skeletal muscles

• alternating light and dark transverse bands

• results from arrangement of internal contractile proteins

are skeletal muscles voluntaru or involuntary

• voluntary

• Usually subject to conscious control

what are muscle cells usually called

• muscles fibers called myofibers

• as long as 30cm

list connective tissues of skeletal muscles

• endomysium → connective tissue around a muscle cell

• perimysium → connective tissue around muscle fascicle

• epimysium → connective tissue surrounding entire muscle

What happens to connective tissue when a muscle fibre contracts

• collagen is somewhat extensible and elastic

• stretches slightly under tension and recoils when released

• resists excessive stretching and protects the muscle from injury

• returns muscle to its resting length

• contributes to power output and muscle efficiency

is collagen contractile

No, collagen is non-contractile and provides structural support.

does collagen have extensible and elastic properties

Yes, collagen is somewhat extensible and elastic, stretching slightly under tension and recoil when released.

What is important about elastic recoil

It aids in returning muscles to their resting length after contraction.

define myoblast

stem cells that fused to form each muscle fiber in early development

what are satellite cells

• unspecialized myoblasts remaining between the muscle fiber and endomysium

• play a roll in regeneration of damged skeletal muscle tissue

what is most muscle repair made up of

satellite cells that differentiate into myoblasts to regenerate muscle fibers.

what is the plasma membrane known as

sarcolemma

what is the cytoplasm known as

sarcoplasm

what occupies the sarcoplasm

• myofibrils → long protein cords occupying most of sarcoplasm

• glycogen → carbohydrate stored to provide energy for exercise

• myoglobin → red pigment provides some oxyegn needed for muscle activity

the smooth ER of a muscle fiber is called

sarcoplasmic reticulum (SR)

where are the terminal cisternae located

sarcoplasmic reticulum

Where are the transverse tubules located

• sarcoplasmic reticulum

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

what is the triad made up of

• a T tubule

• two terminal cisterns associated with it

define myofiliment

a basic contractile unit of muscle fibers, consisting of actin and myosin proteins.

list the three types of myofilaments

1. thick filaments

2. thin filaments

3. elastic filaments

myofilaments: thick filaments

• made of several hundred myosin molecules

• each molecule is shaped like a golf club

  • two chains intertwined to form a shaft-like tail

  • double globular head

• heads directed outward in a helical array around the bundle

  • heads on one half od the thick filament angle to the left, while heads on oher half angle to the right

  • bare zone with no heads in the middle

myofilaments: thin filaments

• fibrous (F) actin: two intertwined strands

  • string of globular (G) actone subunits each with and active site that can bind to head of myosin molecule

• tropomyosin molecules

  • each blocking six or seven active sites on G actine subunits

• troponin molecules: small, calcium binding protein on each tropomyosin molecule

myofilaments: elastic filaments

• titin: hige springey protein

• run through core of thick filament and anchor it to Z disc and M lone

• help stabilize and position the thick filament

• prevent overstretching and provide recoil

what does dystrophin do

• clinically important protein

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

• transfers forces of muscle contraction to connectove tissue ultimately leading to tendon

• genetic defects in dystrophin produce disabling diease → muscular dystrophy

list the contractile proteins

• Myosin

• actin

list the regulatory proteins

• Troponin

• Tropomyosin

A band

• “a” stands for anisotropic

  • The darkest part is where thick filaments overlap a hexagonal array of thin filaments

H band

not as dark; middle of A band; thick filaments only

M line

middle of H band

I band

• light “I” stands for isotropic

  • the way the bands reflect polarized light

Z disc

• provides anchorage for thin filaments and elastic filaments

• bisects I band

sarcomere

• segment from Z disc to Z disc

• functional contractile unit of muscle fiber

thick filaments

• made of several hundred myosin molecules

• Each molecule is shaped like a golf club

  • two chains intertwined to form a shaft-like tail

  • double globular head

• heads directed outward in a helical array around the bundle

  • heads on one half of the thick filament angle to the left, while heads on other half angle to the right

  • bare zone with no heads in the middle

thin filaments

• fibrous (F) actin: two intertwined strands

  • string of globular (G) actin subunits each with an active site that can bind to head of myosin molecule

• tropomyosin molecules

  • each blocking six or seven active sites on G actin subunits

troponin molecule

A small calcium-binding protein on each tropomyosin molecule

tintin

• A massive protein found in striated muscle (skeletal and cardiac muscle) that acts as a molecular spring responsible for the muscle's passive elasticity.

• It plays a crucial role in muscle structure and function by anchoring the thick filament (myosin), preventing overstretching, and allowing the muscle to recoil after being stretched

what are skeletal muscles innervated by

motor neurons of the somatic nervous system

How many motor neurons innervate one muscle fiber

One motor neuron

What is the difference between fine motor movement and when more strength is needed

• fine degree of control

  • three to six muscle fibres per neuron

  • eye and hand muscles

• more strength than control

  • powerful contractions supplied by large motor units with hundreds of fibers

  • The gastrocnemius of the calf has 1,000 muscle fibres per neuron

what is an advantage of having multiple motor units

• Effective contraction usually requires the contraction of several motor units at

• powerful movements

What is a synapse

The point where a nerve meets its target cell

Define neuromuscular junction

when the target cell is a muscle fibre

synaptic knob

a specialized structure found at the end of a neuron's axon.

motor endplate

a specialized area on a skeletal muscle fiber's membrane (sarcolemma) where a motor neuron synapses to transmit a signal for muscle contraction

synaptic cleft

The tiny gap between a presynaptic neuron's axon terminal and a postsynaptic cell's membrane.

myelin

a fatty substance that acts as an insulator, protecting and speeding up the transmission of electrical signals along nerve fibers in the brain and spinal cord

synaptic vesicles

small, sac-like organelles within neurons that store and release neurotransmitters at the synapse

schwann cell

• Glial cells in the peripheral nervous system (PNS) support and myelinate axons, ensuring fast and accurate communication.

• They are crucial for nerve regeneration and can also contribute to noncanonical functions like regulating pain and influencing cancer. 

sarcolemma

the cell membrane of a muscle fiber, specifically the plasma membrane of skeletal muscle

motor nerve fiber

a type of nerve fiber that carries signals from the brain and spinal cord to muscles and glands, causing them to contract or secrete

ach receptor

• integral membrane proteins that respond to the neurotransmitter acetylcholine.

• These receptors are crucial for muscle contraction and various other functions, including those of the autonomic nervous system.

junctional folds

infoldings of the muscle cell membrane at the neuromuscular junction

define electrophysiology

the study of the electrical activity of cells

Describe the muscle cells when they are unstimulated; what is going on with their electrical charge? what and where are the ions involved

• There are more anions (negatively charged particles) on the inside of the membrane than on the outside

• These anions make the inside of the plasma membrane negatively charged by comparison to ots outer surface

• the plasma membrane is electrically polarized (charged) with a negative resting membrane potential (RMP)

• there are excess sodium ion (Na+) in the extracellular fluid (ECF)

• there are excess potassium ions (K+) in the intracellular fluid (ICF)

define electrical potential

• AKA voltage

• a difference in electrical charge from one point to another

define resting membrane potential (RMP)

• the electrical potential difference across the plasma membrane of a cell when the cell is at rest, meaning it's not actively signalling or undergoing electrical activity

• about -90 mV in skeletal muscle cells

what maintains the RMP

maintained by the sodium-potassium pump

Explain what happens when the muscle cells are stimulated

• Na+ ions gate open in the plasma membrane

• Na+ flows into cell down its electrochemical gradient

• these cations override the negative charges in the ICF

• depolarization: insides of plasma memebrane becomes positive

• immediately Na+ gates close and K+ gates open

• K+ rushes out of the cell partly repelled by positive sodium charge and partly because of its concentration gradient

• loss of positive potassium ions turns the membrane negative again (repolarization

• this quick up and down voltage shift (depolarization and repolarization) is called an action potential

What are the four major phases of the process of muscle contraction/ relaxation

• excitation

• excitation-contraction coupling

• contraction

• relaxation

excitation

process in which nerve action potentials lead to muscle action potentials

excitation-contraction coupling

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

contraction

step in which the muscle fiber develops tension and may shorten

relaxation

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

list and explain the steps of excitation

1. Arrival of nerve signals

2. acetylcholine (ACh) release

3. Binding of ACH to the receptor

4. opening of ligand-regulated ion gate creation of end plate potentialal

5. opening of voltage regulated ion gates creation of action potential

list and explain the steps of excitation-contraction coupling

• The action potential propagated down the T tubules

• calcium released from the terminal cistern

• binding of calcium to troponin

• shifting of tropomyosin exposure of active sites on actin

list and explain the steps of contraction

• hydrolysis of ATP to ADP +Pi; activation and cocking of myosin head (recovery stroke)

• formation of myosin-actin cross-bridge

• binding of new ATP breaking of cross-bridge

• power stoke; sliding of thin filament over thick filament

List and explain the steps of relaxation

• cessation of nervous stimulation and ACh release

• ACh breakdown by acetylcholinesterase (AChE)

• Reabsorption of calcium ions by sarcoplasmic reticulum

• loss of calcium ions from troponin

• return of tropomyosin to position blocking active sites of actin

define recruitment or multiple motor unit (MMU) summation

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

define size principle

Weak stimuli (low voltage) recruit small units, while strong stimuli recruit small and large units for powerful movements

define temporal (wave) summation

The greater the frequency of stimulation the more strongly a muscle contracts

define incomplete tetanus

partial relaxation between stimuli resulting in fluttering

define complete (fused) tetanus

unaturally high stimulus frequencies (in lab experiments) cause a steady contraction

define isometric contraction

• muscle produces internal tension but external resistance causes it to stay the same length

• can be prelude to movement when tension is absorbed by elastic component of muscle

• important in postural muscle function and antagonistic muscle joint stabilization

define isotonic contraction

• muscle changes in length with no change in tension

define concentric contraction

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

define eccentric contraction

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

why is ATP important for muscle function

all muscle contraction depends on ATP

Define 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

define aerobic respiration

• produces far more ATP

• does not generate lactate

• requires a continual supply of oxygen

What is immediate energy

The phosphagen system is also known as the ATP-PC system. This system provides energy for very short, intense bursts of activity, like a sprint or a quick jump, lasting up to 10-15 seconds.

what and when does immediate energy occur

• Also known as the ATP-PC system, is the energy system used for activities that require very rapid and explosive movements lasting up to 10 seconds.

• It primarily relies on stored ATP (adenosine triphosphate) and phosphocreatine (PC) within muscle tissue. 

What is short-term energy

As the phosphagen system is exhausted, muscles shift to anaerobic fermentation

What and when does short-term energy occur

• muscles obtain glucose from blood and their own stored glycogen

• in the absense of oxygen, glycolysis can generate a net of 2 ATP for every glucose molecule consumed

• converts glucose to lactate

What is long-term energy

After about 40 seconds, the respiratory and cardiovascular systems start to deliver oxygen fast enough for aerobic respiration to meet most of muscle’s ATP demand

What and when does long-term energy occur

• Aerobic respiration produces more ATP per glucose than glycolysis does (another 30 ATP per glucose)

• An efficient means of meeting the ATP demands of prolonged exercise

• After 3-4 minutes, the rate of oxygen consumption levels off to a steady state where aerobic ATP production keeps pace with demand

• for 30 minutes, energy equally distributed from glucose and fatty acids

• beyond 30. Minutes, depletion of glucose causes fatty acids to become the more significant fuel

What are the limits (progressive weakness) of long-term energy

• fuel depletion as glycogen and glucose levels decline

• Electrolyte loss through sweat can decrease muscle excitability

• central fatigue occurs when fewer motor signals are issued from the brain

  • Brain cells are inhibited by exercising muscles release of ammonia

  • psychological will to persevere - not well understood

define VO2

• Maximum oxygen uptake (VO2 max) is a major determinant of one’s ability to maintain high-intensity exercise for more than 4 to 5 minutes

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

  • proportional to body size

  • peaks at around age 20

  • usually greater in males than females

  • can be twice as great in trained endurance athletes as in untrained person

define O2 debt

It is the difference between the elevated rate of oxygen consumption following exercise and the usual resting rate

O2 debt is needed for the following

• to aerobically replenish ATP (some of which helps regenerate CP stores)

• to replace oxygen reserves in myoglobin

• to provide oxygen to the liver that is busy disposing of lactate

• to provide oxygen to many cells that have elevated metabolic rates after exercise

• EPOC (O2 debt) can be six times basal consumption and last an hour

List the differences between slow-twitch fibres and fast-twitch fibres

• can predominate in certain muscle groups

• Muscles of the back contract relatively slowly (100 ms to peak tension), whereas muscles that move the eyes contract quickly (8 ms to peak tension)

slow twitch, slow oxidative (SO), red or type I fibres

• well adapted for endurance resists fatigue by oxidative (aerobic) ATP production

• important for muscles that maintain posture (e.g. erector spinae of the back, soleus or calf)

• abundant mitochondria, capillaries nyoglobin: deep red colour

• contain a form of myosin with slow ATPase and an SR that releases calcium slowly

• relatively thin fibres

• grouped in small motor units controlled by small, easily excited motor neurons, allowing for precise movements

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

• fibres are well adapted for quick responses

• important for quick and powerful muscles: eye and hand muscles gastrocnemius of calf and biceps brachii

• contain a form of myosin with fast ATPase and a large SR that releases calcium quickly

• utilize glycolysis and anaerobic fermentation for energy

  • abundant glycogen and creatine phosphate

  • lack of myoglobin gives them pale (white) appearance

• fibers are thick and strong

• grouped large motor units controlled by larger, less excitable neurons allowing for powerful movements

What is the difference between Type IIA and Type IIB fast twitch fibres

• fast twitch, intermediate or type IIA fibers

  • fast twitch but fatigue resistance

  • known in other animals but rare in humans

• fast twitch type IIB fibres

  • also known as type IIx, are fast-twitch glycolytic fibers that primarily rely on anaerobic glycolysis for energy.

  • These fibers are characterized by high strength and power output but relatively low endurance.

  • They are recruited for explosive activities like sprinting and low-repetition weightlifting.