Oral exam 1

Motor unit

composed of one motor neuron + its 1 axon + all the muscle fibers it innervates

• one motor neuron only has one axon that extends toward a muscle however as it approaches and enters the muscle, it splits into many fine collateral branches (axon branching)

• each individual branch end-terminal forms a synapse (NMJ) onto one specific muscle fiber

All-or-none rule

When that single motor neuron fires an action potential, the signal travels down its single axon, spreads through all its branches simultaneously, and causes every single muscle fiber in that motor unit to contract at once (if that reaches threshold, RECRUITMENT)

when you increase the stimulus you activate more motor neurons (bc have different thresholds) which recruits more muscle fibers to contract

• think one motor neuron synapses to mant muscle fibers so if AP occurs all fibers that it innervates will also reach threshold and AP occurs

Individual muscle fiber

all-or-none: cannot change single twitch size; either twitches fully or doesn’t at all (this is bc its motor neuron did not generate an AP)

Whole muscle

Graded: recruits more or fewer muscle fibers (via more motor units reaching threshold and propagating an AP) to scale total force!!

this is what we saw on the screen

Neuromuscular junction

The synapse between the motor neuron (presynaptic cell) and the muscle fiber (postsynaptic cell)

Sequential opening of Na+ channels

an AP propagates down the axon by virtue of sequential opening of Na+ channels

1. AP started at axon hillock (lots of v-gated Na+ channels) by the neuron receives excitatory input

2. causes a depolarization of the cell and then opening of Na+ ion channels which leads to an AP (Na+ influx, down its concentration gradient)

3. creates a positive feedback loop bc will cause even higher depolarization which causes more channels to open

4. this depolarization spread to the next section of axon, causing next second to reach threshold and AP continues down axon

Muscle fiber AP

when ACh binds nicotinic ACh receptors on the membrane of the muscle fiber this leads to a depolarization and an AP which causes Ca2+ to be released from internal storage organelles called sarcoplasmic reticulum, and an increase in Ca2+ in muscle fiber triggers the contractile machinery/movement.

when sarcoplasmic reticulum resequesters (isolates again) the Ca2+ ions and the cytoplasmic Ca2+ concentration decreases, contraction is terminated

true

true or false

The relaxation period of a muscle twitch is directly determined by how long it takes for cytoplasmic Ca2+ to return to baseline and back into the sarcoplasmic reticulum (driven by Ca2+-ATPase)

Stimulus intensity (voltage)

when increase stimulus intensity (voltage) you recruit more axons (a nerve, like sciatic nerve, contains many axons) to fire APs

if apply a weak electrical stimulus then maybe only the easiest-to-excite axons reach threshold and fire, but when use a stronger stimulus more axons will reach threshold and fire (RECRUITMENT)

false

true or false

increasing stimulus intensity makes an individual AP bigger

AP frequency effect

If you stimulate the nerve repeatedly, increasing the stimulus can also allow axons to fire more frequently, but there is a limit

after an AP, the Na+ channels need time to reset/recover. this is related to the refractory period, so axon cannot immediately fire another AP over and over

true

true or false

each muscle fiber either twitches or not; twitches are not graded in intensity

Once a muscle fiber generates and AP and triggers release of Ca2+ this causes the muscle fiber to twitch and being contracting

true

true or false

Cannot just make one muscle fiber’s individual twitch much stronger just by increasing the stimulus intensity

instead more force=recruit more muscle fibers

we will see an increase in tension of the twitch (this is muscle contraction strength as a whole)

Time requirment

the requestering of Ca2+ by the sarcoplasmic reticulum requires time, and mechanical relaxation of the fiber takes much longer than does termination of the muscle fiber action potential

if a new muscle fiber action potential is generated during this time, the new twitch can add onto this residual tension (temporal summation)

Tetanic contraction

at maximum frequency (limited by refractory period), the twitches fuse with one another, producing a long, strong contraction called tetanus

Low-frequency AP stimulation

Because a single AP causes just one twitch at baseline tension, even when low-frequency APs occur, they are conducted along a muscle fiber; each successive twitch begins before the muscle has time to return to its fully relaxed condition following the previous twitch, so tension builds up moderality (increases in tension but not max tension like tetanus)

piggybacks off of the relative refractory period

KNOW

since this is muscle physiology, increasing the voltage of an electrical stimulus is not necessarily the same as increasing the receptor potential but it only increases the number of motor units that are recruited leading to more muscle fibers generating an AP which causes the release of Ca2+ from SR and tell the muscle to contract

Why is there no tension development at very low stimulation voltages

At very low stimulation voltages, the stimulus does not reach the threshold required to generate an action potential in the motor nerve. Therefore, no AP travels down the motor axon in the NMJ, and ACh is not released to stimulate muscle fibers.

Without muscle action potentials, the muscle fibers do not contract, so no tension develops

bc even a single AP produce tension SO no AP was generated

a single muscle action potential can produce a twitch and generate tension. however if the stimulus is too weak to active a moto axon, no muscle action potential occurs and no tension develops

Does the maximum contraction strength (tension) continue to increase as the voltage increases (stimulus strength)

No. As the stimulus increases, more motor axons reach threshold, recruiting more motor units and muscle fibers. This increases the total tension produced by the muscle. however contraction strength eventually reaches a max when all recruitable motor axons and muscle fibers have been activated

when the nerve is stimulated repeatedly at increasing frequencies, individual muscle twitches can also summate because the muscle has not completely relaxed before the next stimulus. At sufficiently high frequencies, this can produce tetanus, a sustained contraction with greater tension than a single twitch.

BUT AP frequency is limited by the refractory period of the nerve, while muscle relaxation and calcium handling also influence the degree of summation

Maximum tension from increasing stimulus voltage occurs when all recruitable motor units are activated. Tetanus is a separate way of increasing tension by increasing stimulation frequency

Is there a voltage at which contraction strength continues to increase as the voltage increases

Yes, there is a threshold voltage at which tension first begins to develop, and as the stimulus voltage increases, tension generally increases as more motor axons and muscle fibers are recruited. Eventually, a maximum stimulus voltage is reached at which all recruitable motor axons are activated.

REMEMBER: the voltage at which maximum recruitment occurs is not necessarily the voltage at which tetanus occurs. Tetanus depends on the frequency of stimulation, not simply the stimulus voltage.

which hooks cathode (-) or anode (+) is actually eliciting the nerve action potential

the cathode (negative electrode) generally produce the nerve action potential more effectively. the negative electrode makes the extracellular environment near the nerve more negative, which makes the inside of the nerve membrane less negative. This depolarizes the membrane and can bring it to threshold, opening voltage-gated Na+ channels and initiating an action potential

. A negative cathode makes the outside near the electrode more negative, making the MEMBRANE POTENTIAL less negative (depolarizes it)

Cathode does NOT cause negative ions to leave the cell. The key is the change in the electrical potential across the membrane

Does the relative strength of contraction increase from increasing frequency

Yes, increasing the frequency of stimulation can increase the strength of muscle contraction through temporal summation. when the second muscle action potential occurs before the muscle has fully relaxed from the previous twitch, Ca2+ remains elevated in the muscle cell, allowing the twitches to overlap and produce greater tension. At sufficiently high stimulation frequencies, individual twitches may fuse into a sustained contraction called tetanus (summation of contraction)

muscle APs themselves do not merge into one long AP (bc all-or-none). Instead, the mechanical twitches overlap because the muscle has not fully relaxed

What does it mean if tension development does not return to baseline level between each of the pulses

it means temporal summation of muscle tension has occurred. The next stimulus arrives before the muscle has completely relaxed from the previous twitch, so the tension from successive twitches adds together. As stimulation frequency increases, this can progress toward a sustained tetanic contraction

plausible explanation for what the external solution contained (or lacked)

one of these drugs was present, bc after the sciatic nerve was exposed to the solution, the amplitude of the muscle tension recording decreased, indicating a reduction in contraction strength

these drugs block v-gated Na+ channels in the motor axons, preventing or reducing propagation of action potential along the nerve

when the nerve is stimulated, the cathode normally depolarizes the nerve membrane, opening V-gated Na+ channels and initiating an AP

however when these channels are blocked, the action potential may fail to propagate down some or all of the motor axons (bc each neuron has a different excitatory threshold)

if an AP reaches the axon terminal, it normally opens V-gated Ca2+ channels, allowing Ca to enter and trigger release of ACh, which binds to nicotinic ACh receptors on the muscle membrane, producing depolarization that initiates a muscle AP and then contraction

when Na+ channels in some motor axons are blocked, fewer action potential reach the axon terminals, resulting in less ACH release from those terminals. Consequently, fewer muscle fibers are activated reducing the total tension produced by the muscle and decreasding the amplitude of the recording

Relaxation period of twitch

The relaxation period of a muscle twitch is directly determined by how long it takes for Ca2+ to be pumped back into the sarcoplasmic SR

Mechanism of TTX/STX/mircoconotoxin

1. These drugs block susceptible voltage-gated Na+ channels

2. few motor axons successfully generate or propagate APs

3. fewer axon terminals release ACh in response to stimulation

4. Few muscle fibers generate AP and contract (less recruitment)

5. Lower total muscle tension, decreased recording amplitude

threshold

minimum stimulus needed to trigger an AP

each neuron of the sciatic nerve has different threshold; therefore some are easier to excite than others

recruitment

more motor axons are activated (AP generated in them bc they reach threshold) cause more muscle fibers to contract

stimulus voltage

increasing voltage generally increases recruitment until maximum recruitment (all muscle fibers are being used up, like all aer twitching)

stimulus frequency

Increasing frequency can cause twitches to summate

temporal summation

The muscle has not fully relaxed before the next twitch

tetanus

sustained contraction caused by high-frequency stimulation (tetanic contraction)

cathode

generally depolarizes the nerve/neurons membrane and initiates an AP