muscle physiology

what are the three different types of muscle found in the human body?

skeletal, cardiac, and smooth muscle

how does each muscle type differ from one another structurally and functionally?

1. skeletal muscle is attached to the skeleton, it is striated, and it is under voluntary control

2. cardiac muscle is the muscle of the heart, it is striated, and it is under involuntary control

3. smooth muscle is found in places like the GI tract and the vasculature, it is not striated, and it is under involuntary control

what does the term striated refer to?

striations seen in muscle under a light microscope are the "A" and "I" bands of the sarcomeres in myofibrils

how is a tendon related to muscle?

the tendon attaches the muscle to the bone

how is a tendon formed?

when epimysium and perimysium converge

what is the function of a tendon?

tendons help in transmitting the force from the muscle to the load

what connective tissue surrounds skeletal muscle (organ)?

epimysium

what are skeletal muscle (organs) composed of?

muscle fascicles

what connective tissue surrounds muscle fascicles?

perimysium

what are muscle fascicles composed of?

muscle fibers/skeletal muscle cells/skeletal muscle myoblasts/myofibers

what connective tissue surrounds muscle fibers?

endomysium

what are muscle fibers composed of?

myofibrils

which of the three connective tissues are dense and which are loose?

epimysium and perimysium are dense connective tissue whereas endomysium is a loose connective tissue

what is the sarcolemma?

plasma membrane of a skeletal muscle cell

what is the sarcoplasm?

cytoplasm of a skeletal muscle cell

unlike a typical soma cell, what unique things are located in the sarcoplasm of a skeletal muscle cell?

myofibrils, glycogen, and multiple nuclei are all located in the sarcoplasm of a skeletal muscle cell

can you identify some important things located in the endomysium?

terminal knobs (synaptic knobs), capillaries (vasculature), and satellite cells (stem cells) are located in this loose connective tissue

what is the name of the connective tissue immediately parietal to sarcolemma?

the connective tissue immediately parietal to the sarcolemma is the endomysium

what type of cell makes endomysium?

fibroblasts

you are a teacher charged with explaining the structural organization of a muscle organ and the connective tissues associated with it (you decide to start from the smallest level of organization and work your way up, constructing the entire muscle organ) so you take a single strand of red licorice and wrap a clear piece of plastic wrap around - what does this layer of clear plastic wrap represent?

the layer of clear plastic wrap represents the endomysium

you then make several more of these single strands of licorice wrapped with clear plastic wrap, bundle them all together and then wrap the bundle with a layer of green plastic wrap - what does this layer of green plastic wrap represent?

the layer of green plastic wrap represents the perimysium

you make several more of these bundles of licorice wrapped by plastic wrap, bundle them together and then wrap the entire bundle with a layer of white plastic wrap - what does this layer of white plastic wrap represent?

the layer of white plastic wrap represents the epimysium

what are the features of a skeletal muscle cell?

skeletal muscle cells are long, cylindrical, and multinucleate cells

what is true syncytium?

when multiple individual cells (myoblasts) fuse together to form one cell that has only one plasma membrane and one cytoplasm

what is the name for a skeletal muscle stem cell?

myoblast

what germ layer is the skeletal muscle stem cell derived from?

mesoderm

are skeletal muscle cells classified as a true syncytium? why or why not?

yes, because skeletal muscle fibers are formed by the fusion of several individual cells

what is the resting membrane potential of a typical skeletal muscle cell?

-85mV

is the resting membrane potential of a typical skeletal muscle cell more or less polarized than a typical neuron?

it is more polarized (i.e., more negative) than a typical neuron

how is the resting membrane potential of a typical skeletal muscle cell established?

the typical RMP of -85mV is established by potassium leak channels and ClC-1 chloride channels

what is a motor-end plate?

the region of the sarcolemma that is involved in a nerve-muscle synapse (i.e., neuromuscular junction - NMJ)

how is the surface area of a motor-end plate changed?

surface area is increased by convolutions (junctional folds) within the sarcolemma

what type of receptors does the motor-end plate have?

contains nAChRs (i.e., ligand-gated Na+ channels) and undergoes graded potentials

what is a T(traverse)-tubule?

T-tubules are extensions/invaginations of the sarcolemma

if you were microscopic and inside of a T-tubule, where would you technically be?

if you were microscopic and inside of a T-tubule, you would technically be on the outside of the cell, since a T-tubule is an extension of the sarcolemma

what is the function of a T-tubule?

the function of a T-tubule is to allow action potentials to propagate as close to the inside of the cell as possible - this allows the sarcoplasmic reticulum to release its calcium stores

how would you describe a myofilament?

myofilaments are the protein fibers that constitute a myofibril

what are the two major proteins that make up the two major types of filaments?

thin filaments are primarily made of actin and thick filaments are made of myosin

what is a myofibril?

a myofibril is a linear, repeating sequence of sarcomeres

what is a myofiber?

a skeletal muscle cell

which structure of a typical somatic cell is the sarcoplasmic reticulum most analogous to?

the sarcoplasmic reticulum is most analogous to the smooth endoplasmic reticulum

what is the specialized function of the sarcoplasmic reticulum in skeletal muscle cells?

the function of sarcoplasmic reticulum is to store calcium in skeletal muscle

what is the smooth endoplasmic reticulum ATPase, abbreviated as SERCA, where is it located, and what is its function?

SERCA is a calcium pump found in the sarcoplasmic reticulum membrane (located in the longitudinal region) - its function is to transfer calcium from the sarcoplasm to the interior of the sarcoplasmic reticulum so the muscle isn't always contracting (THIS PUMP IS ALWAYS ON)

what is calsequestrin (CSQ), where is it located, and what is its function?

calsequestrin is a membrane-bound, calcium-binding protein found in the lateral sacs (junctional SR) of the sarcoplasmic reticulum - its function is to hold calcium in high concentration in the sarcoplasmic reticulum

what is the sarcoplasmic reticulum triad?

lateral sac - T-tubule - lateral sac (2 lateral sacs: 1 T-tubule)

what is the function of a triad?

the function of a triad is to turn the excitation of the sarcolemma into the release of calcium from the sarcoplasmic reticulum - there are 2 triads associated with a single sarcomere

what is a sarcomere?

the smallest structural and functional unit of a myofibril in striated muscle

what is a sarcomere composed of?

actin, myosin, stabilizing proteins, and regulating proteins

what is the A band? what can you find there

the A band is the length of the thick filaments - myosin, actin, the M line, titin, and myomesin can be found in the A band

what is the I band? what can you find there?

the I band is the distance between thick filaments between two adjacent sarcomeres - actin, actinin, titin, and the Z line can be found in the I band

what is the H zone? what can you find there?

the H zone is the distance between thin filaments in the center of the sarcomere - myosin, myomesin, titin, and the M line can be found in the H zone

what is the zone of overlap? what can you find there?

the zone of overlap is the region of the sarcomere where actin and myosin overlap one another - myosin, actin, and titin can be found in the zone of overlap

what is the M line?

the M line is the point of attachment of the thick filaments

what is the Z line?

the Z line is the point of attachment of the thin filaments

why is an A band also referred to as a dark band?

the A band is referred to as the dark band because it contains thick and thin filaments, and therefore picks up a lot of stain - less light can pass through

why is an I band also referred to as a light band?

the I band is referred to as the light band because it contains only thin filaments and does not pick up as much stain as the A band - more light can pass through

why is the zone of overlap arguably the most important region of a sarcomere?

it is the place where actin and myosin interact

how many I bands are present in ONE sarcomere?

there are zero full I bands present in one sarcomere (but there is 1/2 of an I band)

what is actinin?

a protein that connects actin filaments to the Z line

what is myomesin?

a protein that links myosin filaments to the M line

what is titin?

an elastic protein that runs from the Z line to the M line - flows between the thin filaments (actin) but through the thick filaments (myosin)

how would you describe an A band, I band, H zone, Z line, M line and zone of overlap in a compressed, or contracted, sarcomere?

- the A band will not change in length

- the I bands will get shorter

- the H zone will get shorter

- the Z lines will move closer together

- the zone of overlap will get longer

what is the structural organization of actin?

G-actin monomers dimerize and these dimers assemble into two strings of intertwining beads referred to as F-actin

what are the two regulatory proteins found within actin?

troponin and tropomyosin

what is the function of tropomyosin?

tropomyosin prevents the interactions between thin and thick filaments by blocking the active sites on the G-actin monomers

what is the function of troponin?

troponin is attached to the protein tropomyosin and binds calcium - with bound calcium, troponin "pulls" tropomyosin out of the way of the G-actin active sites permitting the myosin heads to bind to actin.

what is the ratio between G-actin and tropomyosin?

one tropomyosin protein runs the length of ~7 G-actin molecules

what is the structural organization of myosin?

myosin looks like a golf club since it contains a head, neck, and tail domain - thick filaments are composed of multiple myosin proteins

what is the function of myosin?

the function of myosin is to convert chemical energy in the form of ATP to mechanical energy, thus generating force and movement - the myosin head binds to an active site of a G-actin, forming a cross-bridge

in a relaxed sarcomere, is the myosin head cocked towards the M line or the Z line?

in a relaxed sarcomere, the myosin head is cocked towards the Z line

in a contracted sarcomere, is the myosin head cocked towards the M line or the Z line?

in a contracted sarcomere, the myosin head is cocked towards the M line

can you explain the sliding filament theory in your own words?

the sliding filament theory explains how the thick and thin filaments interact during muscle contraction

what is the special name for the motor neuron that controls skeletal muscle?

the alpha motor neuron

where is an alpha motor neuron's cell body located in the spinal cord?

the alpha motor neurons cell body is located in the anterior horn

how is skeletal muscle contracted?

by voluntary commands generated in the CNS

can you describe how the axon of an alpha neuron penetrates a muscle organ and eventually forms a neuromuscular junction (NMJ) with a skeletal muscle cell?

the axon first pierces the epimysium of the muscle organ, then courses through the perimysium and finally terminates in the endomysium resulting in the formation of a neuromuscular junction

what are the three components of a neuromuscular junction?

the terminal knob, the motor end plate, and the synaptic cleft

what is a motor unit?

an alpha motor neuron and all of the skeletal muscle fibers it innervates

starting with action potentials propagated down the axon of a motor neuron, can you describe all the events that result in the release of neurotransmitters like acetylcholine from a synaptic terminal?

an action potential is propagated down the axon of a motor neuron, which causes a graded potential to ensue in the terminal knob - this graded potential causes voltage-gated calcium channels to open, allowing calcium to enter and stimulate synaptic vesicles to undergo exocytosis and release acetylcholine into the cleft - acetylcholine then binds to nicotinic receptors within the junctional folds of the myofiber

other than nicotinic receptors, what are the receptors for acetylcholine referred to as?

the nicotinic receptors are also known as cholinergic receptors

are nicotinic receptors metabotropic or ionotropic?

ionotropic

where are the nicotinic receptors located in a motor-end plate?

within the junctional folds of the skeletal muscle cell

what is the name of the enzyme that degrades acetylcholine in a neuromuscular junction?

acetylcholinesterase (AChE) degrades acetylcholine into acetate & choline that is then taken back into the presynaptic neuron

upon stimulation by acetylcholine, what type of potential does the motor end plate undergo?

the motor end plate undergoes local/graded potentials

where do action potentials first emerge?

action potentials first emerge just outside the motor end plate in the sarcolemma

what are the channels called that are responsible for the generation of action potentials?

voltage-gated sodium channels are responsible for the generation of action potentials

what is a presynaptic blockade?

when a somatic motor neuron receives an action potential but the neuron can't release acetylcholine into the synaptic cleft due to something preventing the transmission of neurotransmitters to the skeletal muscle cell

what is a postsynaptic blockade?

when a somatic neuron receives an action potential and undergoes normal release of acetylcholine but there isn't a place for this neurotransmitter to bind on the skeletal muscle cell

how does botulinum toxin cause presynaptic blockade in a neuromuscular junction?

botulinum toxin is a bunch of proteases that degrade the proteins that are required for the synaptic vesicles to dock in the membrane and release acetylcholine

how does the blockade caused by botulinum toxin affect muscle function?

this blockade will prevent muscle contraction (cause muscle paralysis) since no acetylcholine can be released

how can botox be used to reduce the appearance of wrinkles?

botox (a form of botulinum toxin) reduces the appearance of wrinkles by relaxing tonic muscle contractions under the skin that cause skin to sag

are there any legitimate medicinal uses for botox? if so, what?

yes, there are legitimate medicinal uses for botox - it can be used to reduce tonic muscle contractions (spasticity) in patients with cerebral palsy

how does curare cause postsynaptic blockade in a neuromuscular junction?

curare blocks the ligand-gated sodium channels within the junctional folds in a neuromuscular junction

how does the blockade caused by curare affect muscle function?

since acetylcholine cannot bind to nicotinic receptors, the muscle is paralyzed

how is curare used by South American natives to hunt animals?

it is used in the tips of hunting arrows to paralyze animals so that the natives can catch up to them after they're shot

are there any legitimate medicinal uses for curare? if so, what?

yes, it can be used during surgeries to paralyze muscle

what is myasthenia gravis?

myasthenia gravis is an autoimmune disease that results in the production of antibodies against the nicotinic receptors (sodium ligand-gated channels) of skeletal muscle cells

how does myasthenia gravis cause muscle weakness?

myasthenia gravis causes muscle weakness by blocking nicotinic receptors

how can myasthenia gravis progress to muscle paralysis?

myasthenia gravis progresses to muscle paralysis when the nicotinic receptors are blocked by the antibodies and the skeletal muscle cell internalizes the receptors (downregulates them)

what are two treatment modalities for myasthenia gravis?

1. pyridostigmine inhibits acetylcholinesterase so that there is more acetylcholine within the synaptic cleft potentially causing increased activity in unimpacted nicotinic receptors on the muscle cell to uptake the acetylcholine (increase muscle contraction)

2. immunosuppressants can decrease the amount of antibodies produced by the disease by suppressing the entire immune system