Neuromuscular Physiopathology 1 Week 5 + 6

T or F: Type 2b and 2x muscle fibers are the same thing with different names

True

Skeletal muscles

Pulls on skeletal bones

Voluntary contractions

Cardiac muscles

Pushes blood through arteries and veins

Rhythmic contractions

Functions through intercalated discs and gap junctions

Smooth muscles

Pushes fluids and solids along the digestive tract

Involuntary contraction

Four basic properties of muscles

Excitability (ability to respond to stimuli)

Contractibility (ability to shorten and exert a pull or tension)

Extensibility (ability to continue to contract over a range of resting lengths)

Elasticity (ability to rebound towards its original length)

Functions of skeletal muscle

Produce skeletal movement (Pulls on tendons to move bones)

Maintain posture and body position (Stabilize joints to aid in posture)

Support soft tissue (support weight of visceral organs)

Regulate entering and exiting of material (voluntary control over swallowing, defecation, and urination)

Maintain body temperature (some of the energy used for contraction is converted to heat

Epimysium

covering of entire muscle

Perimysium

covering of a fascicle

Endomysium

covering of a muscle fiber

What is the function of all the bundling of muscle fibers?

Gives more surface area than one big solid band would

Allows vessels and nerves between the muscle fibers

Muscle fiber

Can be 30-40cm long

Multinucleated

Nuclei are located just deep to sarcolemma

Myofibrils

thread like structures which have a contractile function

Sarcoplasmic reticulum (SR)

Membranous sacs which encircles each myofibril, stores and releases calcium for contraction

Filaments

function in the contractile process

Sarcomere

compartments of arranged filaments

Z-line/disc

marks the end/edge of the sarcomere

Thick and thin filaments overlap each other

I Band

lighter, only contains thin filaments, Z discs pass through the center of each band

A band

Darker middle part of the sarcomere where thick and thin filaments overlap

H band/zone

center of each A band which contains only thick filaments

Uncrossed myosin

M line

the supporting proteins that hold thick filaments together in the H zone

Triad relationship

T tubules conduct impulses deep into muscle fibers

T tubules

voltage sensor which weaves between each muscle fiber and innervates the inner fibers

Actin

globular protein that forms two fibrous strands twisted around

Tropomyosin

protein that covers the myosin binding sites on actin when the muscle is relaxed

Troponin

protein that holds tropomyosin

Myosin filament

composed of 6 polypeptide chains

Titin

a core that connects myosin to the Z line, holds the myosin in place

T or F: Actin is the motor and myosin is the track along which it moves

False (myosin is the motor and actin is the track)

Sliding filament theory

widely accepted, binding of calcium to troponin results in a conformational change in the tropomyosin, allowing myosin to bind

What is released to form actin-myosin cross bridges

inorganic phosphate

What causes the power stroke?

Release of ADP

What causes Myosin to be released from actin?

Binding ATP

What causes cocking of myosin?

ATP splitting into ADP and P

Rate code

an increase in the rate of action potentials causes an increase in force generated

Size principle

with increasing strength of input onto motor neurons, smaller neurons are recruited and fire before larger motor neurons are recruited

Motor unit

a single neuron along with any and all muscle fibers it innervates

Precise control

a motor neuron controlling two or three muscles (e.g. eye muscles)

Less precise control

a motor neuron controlling perhaps 2,000 muscle fibers

Back muscles

1:100 nerve to muscle fibers

Finger muscles

1:10 nerve to muscle fibers

Eye muscles

1:1 nerve to muscle fibers

Motor neuron pool

all motor neurons that innervate one whole muscle

Graded strength principle

muscles contract with varying degrees of strength at different times

Strength of muscle contraction is dependent on metabolic condition, amount of load, recruitment of motor units, and initial length of muscle fibers

Fast fibers

"Type II-B/X, Fast fatigue, white"

Has no myoglobin (the protein which holds oxygen)

Can only perform glycolysis or lactic acid fermentation

Produce powerful contractions quickly

Slow Fibers

"Type I or Red"

Half the diameter of fast fibers

Takes 3x longer to contract after stimulation

Intermediate fibers

"Type II-A, fast resistant, pink"

Similar to fast and slow fibers

Low myoglobin content, high glycolytic content

Contract using anaerobic metabolism

Lots of mitochondria

Higher capillary supply

Resist fatigue

Tone (Tonic contraction)

continual, partial contraction of a muscle

Flaccid

muscles with less than normal tone

Spastic

muscles with more than normal tone

Toned muscles

muscles in a constant state of partial contraction which keeps them firm

Muscle twitch

a quick jerk if a muscle after an action potential hits

Latent phase

before contraction, action potential travels through sarcoplasmic reticulum, releasing calcium

Contraction phase

calcium binds to troponin and sliding of filaments occur, tension builds to peak

Relaxation phase

sliding of filaments ceases (active sites on actin are covered), calcium levels fall and tension falls to resting levels

Force summation

an increase in contraction intensity as a result of additive effect of individual twitch contractions

Multiple fiber summation

results from an increase in the number of motor units contracting simultaneously

Frequency summation

results from an increase in the frequency of contractions of a single motor unit

Treppe

the staircase phenomenon, gradual steplike increase in the strength of contraction that is seen in a series of twitch

Tetanus

a prolonged contraction with no relaxation

Multiple wave summation

multiple twitch waves are added together

Incomplete tetanus

very short periods of relaxation occur between peaks of tension

Complete tetanus

twitch waves fuse together into a single, sustained peak

What is the only source of energy used directly for muscle contractile activity?

ATP

Direct phosphorylation

Creatine phosphate system

Hydrolysis of ATP yields energy required for muscular contraction

The first ATP store to be used

Anaerobic glycolysis

Lactic acid system

Glucose is broken down into pyruvic acid and 2 ATP

Results in the formation of lactic acid

Neuromuscular junction

the point where the neuron and muscle meet

Acetylcholine

the neurotransmitter released into the NMJ which stimulates receptors and an impulse in the sarcolemma

Axon terminal

the end of a motor neuron's branches

Motor end plate

specialized region of the muscle cell plasma membrane adjacent to the axon terminal

Usually one per fiber

Synaptic trough

invagination of the motor end plate

Synaptic cleft

20-30 nm wide, contains large quantities of acetylcholinesterase

Subneural clefts

increases the surface area of postsynaptic membrane

ACh channels at the top

Voltage gated Na channels at bottom half

Voltage gated Ca+ channels

opened by voltage gated channels, cause the release of vesicles

Muscle RMP

80-90 mV

Muscle action potentials

1-5msec in duration, about 5x as long as in large myelinated nerves

Muscle action potential velocity

3-5m/sec, about 1/13 the velocity of large myelinated fibers

NAChr

nicotinic acetylcholine receptors

Gated sodium channels found in the bottom of the subneural cleft, creates a fast response

Creates an end plate potential which can stimulate threshold

Transverse tubules

extensions of the cell membrane that penetrate into the center of skeletal muscle

Permit rapid transmission of action potential through a muscle cell

Sarcoplasmic reticulum

terminal cisternae and longitudinal tubules

Stores intracellular calcium, which is used to initiate muscle contraction

Dihydropyridine receptor

calcium channel on the T-tubule

When it is stimulated, it pulls on the RyR channels, allowing calcium to diffuse out of the sarcoplasmic reticulum

Ryanodine receptor

protein on the sarcoplasmic reticulum

Keeps calcium in the sarcoplasmic reticulum (the cork)

When released, calcium can bind to troponin

Hypercalcemia

There is more calcium in the ECF

Calcium competes for the sites of sodium channels (especially at the terminal end)

The calcium plugs the sodium channels, blocking the sodium channel and causing less neurotransmitter to be released

Hypocalcemia

Leads to over-release of ACh due to lower calcium competition for sodium channels

Magnesium

competitively binds calcium binding sites

Functions as a calcium channel blocker

Prevents the release of NTs

ATP needs magnesium

most ATP in the cell forms a complex with magnesium to become biologically active

Rigor

when there is no ATP available, the muscle cannot relax

Rigor mortis

state of contracture that occurs following death due to a loss of ATP

ACh-like drugs

methacholine, carbachol, nicotine

Bind and activate nACh receptors, causes a prolonged effect because they are not broken down by AChE

Direct agonists, stimulates muscle contractions

Anti-AChE drugs

neostigmine, physostigmine, diisopropyl fluorophosphate "sarin nerve gas"

Blocks the degeneration of ACh, prolonging its effect

Indirect agonists, Stimulates muscle contractions

Curariform drugs

D-tubocurarine

Blocks nACh channels by competing ACh binding site

Prevents end-plate potentials

Direct antagonist, inhibits muscle contractions

Botulinum toxin

Decreases the release of ACh from nerve terminals

Insufficient stimulus to initiate action potential

Defective release of ACh

pre-junctional pathology

Destruction of ACh receptors

post-junctional pathology

LEMS

blocks calcium channels

Improves throughout the day

MG

blocks sodium channels

Worsens throughout the day

Tensilon test

a fast acting anticholinesterase agent is injected causing a temporary dramatic reduction in symptoms