Neuromuscular Physiology

How do signals travel along the axon?

A signal (binding of neurotransmitter to dendrite) is received on one end, relayed over the axon, and upon reaching the other end, releases a signal (neurotransmitter) that communicates with other neurons

How do we establish a signal over the length of the cell?

Establish a charge across the plasma membrane (create polarized membranes) then trigger depolarization on one end of the cell; depolarization will spread as a wave over the length of the axon and trigger the release of neurotransmitters upon reaching the end of the axon

As the depolarization wave moves down a nerve, it maintains the same ___________

strength

Frequency modulation

a more intense contraction of muscle occurs with a higher firing frequency

The strength of a muscle contraction depends on (2):

the number of motor units activated AND the frequency of nerve firing

Electric potenial

ability of electrons to due work based on a concentration gradient

Name of two equations used to determine electrical potential

Nernst equation and Goldman equation

Membrane potential

difference in electrical charge across a cell's plasma membrane; the voltage difference between the inside and outside of the cell

The membrane potential is the charge _______ the cell minus the charge _______ the cell

inside; outside

If the inside of the cell is negative compared to the outside (as it usually is), the membrane potential is ________

negative

If the inside becomes positive compared to the outside (like during an action potential), the membrane potential becomes _________

positive

Threshold potential

miniumum potential that will elicit excitation; will not reestablish resting potential past this point

Depolarization

Increase in the membrane potential

Polarization

Decrease in the membrane potential

Repolarization

Return of the cell to resting state, caused by reentry of potassium into the cell while sodium exits the cell

What is the threshold potential?

-55mV

What happens when a stimulus is powerful enough to reach the threshold potential?

membrane will rapidly depolarize and cause an action potential

action potential

the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell

The difference between the threshold value and resting membrane potential is called ___________

excitability

If there is low extracellular potassium, there will be a _______ concentration gradient

higher

If there is a higher concentration gradient between intra and extracellular concentration of K, a ___________ stimulus is required for depolarization

higher

A higher concentration gradient will cause a __________ membrane potential

decreased (further from threshold)

Hyperpolarization

membrane potential becomes more negative due to higher extracellular potassium

If there is high extracellular potassium, there will be a _______ concentration gradient

lower

A lower concentration gradient will cause a __________ membrane potential

higher (closer to the threshold value)

If there is a lower concentration gradient between intra and extracellular concentration of K, a ___________ stimulus is required for depolarization

smaller

Calcium will __________ a nerve

stabilize

What effect does calcium have on the threshold value?

it raises the threshold value

If there is high extracellular Ca, the threshold value will ___________

increase

If there is higher extracellular Ca, a ________ stimulus will be required to reach the threshold value

greater

______ potassium or _______ calcium will cause a lower excitability

low; high

______ potassium or _______ calcium will cause a higher excitability

high; low

Excitability

ease with which a stimulus can initiate an action potential

Excitable cells have a ___________ resting membrane potential

higher

Resting membrane potential

the electrical charge of a neuron when it is not active (-90mV)

What ion is the main contributor to the resting membrane potential of a neuron?

Potassium

Why is potassium the main contributor to the resting membrane potential?

it has the ability to travel across the membrane very readily when the membrane is resting; it is very permeable

There are high concentrations of __________ inside the cell and high concentrations of _________ outside the cell

Potassium; sodium

The longer a cell is in a resting the state, it loses its charge. Why?

1.) the cell loses potassium since it is highly permeable

2.) some sodium cells will leak into the cell

Potassium Leak channels

allow a small amount of potassium to leak out of the cell down the concentration gradient

What establishes the resting membrane potential?

sodium potassium pump

Na/K exchange pump

pumps potassium into the cell and sodium out of the cell

The Na/K exchange pump requires _____. Why?

ATP; potassium and sodium are being pumped against their concentration gradients

The sodium-potassium pump moves ________ sodium molecules ________ the cell and _________ potassium molecules __________ the cell

Three; out

Two; into

Action potential

Change in the membrane potential that reverses the polarity of the cell (such as going from a negative to a positive membrane potential)

How is depolarization accomplished?

Since it is an increase in the membrane potential, an influx of positive ions into the neuron

Depolarization is initiated by allowing extracellular _________ to flow back into the cell

Sodium

Voltage gated Na channel

Channel with a gate that is closed between -90 and -50 mV, but opens above -50 mV, allowing Na to flow back into the cell along its concentration gradient

As the membrane potential becomes closer to the ________ value, more Voltage gated Na channels will __________

threshold; open

The opening of the voltage gated Na channel is a __________ feedback loop

positive

Two channels that help to reestablish the resting potential after depolarization

1.) potassium leak channels

2.) Na/K pump

How do the Na-K pump and K leak channels work to establish the resting potential?

-Na/K pump moves 3 NA out the cell and 2 K in of the cell using ATP

-this creates a high intracellular concentration of K and low intracellular concentration of Na inside the cell

-then, the K leak channel allows only K to move out of the cell, which creates the resting potential of -90mV

Refractory period

a period of inactivity after an action potential occurs where the cell membrane is unable to mount another action potential

Why does the refractory period take place?

once Na channels close, they cannot be opened again for a period of time

Absolute refractory period

no impulse can cause an action potential

When does the absolute refractory period occur?

during the action potential

Relative refractory period

maximal (subthreshold) impulse can cause an action potential, but threshold stimulus will not elicit action potential

When does the relative refractory period occur?

during repolarization

Total refractory period

absolute refractory period + relative refractory period

Positions of the gates of Na gated channel at rest

outside gate is closed

inside gate is opened

Positions of the gates of Na gated channel during depolarization

outside gate is opened

inside gate is opened

Positions of the gates of Na gated channel during refractory period

outside gate is opened

inside gate is closed

switch terms and definitions

switch terms and definitions

total refractory period

black arrow

relative refractory period

blue box

absolute refractory period

yellow box

During the relative refractory period, another action potential can be fired if...

the stimulus is large enough; has to be above the threshold value

How do signals travel along the axon?

A signal (binding of neurotransmitter to dendrite) is received on one end, relayed over the axon through several different action potentials, and upon reaching the other end, releases a signal (neurotransmitter) that communicates with other neurons

Action potentials travel in what direction?

unidirectional

What keeps the action potential from heading in the opposite direction?

the refractory period

As one part of a neuron is depolarized...

it triggers depolarization in the adjacent part of the cell

Myelinated nerves

nerves that are wrapped in myelin sheaths that act as an insulator

Nodes of Ranvier

Gaps in the myelin sheath to which voltage-gated sodium channels are confined

Why are action potentials transported down the nerve faster in a myelinated neuron?

1.) the action potential jumps from one node of ranvier to the next without having to travel across the entire nerve fiber

2.) insulation prevents ions from leaking out

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane

Neuromuscular junction

Synapse between motor neuron and muscle fiber

Neurotransmitter

chemical used by a neuron to transmit an impulse across a synapse to a muscle cell

Most common neurotransmitter

acetylcholine

Terminal nerve fiber

end of the nerve that synapses with a muscle fiber

Motor end plate

specialized part of a muscle fiber membrane at a neuromuscular junction

Where is acetylcholine found?

in vesicles at the end of a nerve fiber

What causes acetylcholine to be released?

-the action potential reaches the end of the nerve

-this opens voltage gated calcium channels to open

-as calcium rushes into the terminal nerve fiber, acetylene vesicles are released through exocytosis

What space is acetylcholine released into?

synaptic cleft

synaptic cleft

The narrow gap that separates the presynaptic neuron from the postsynaptic cell

After being released into the synaptic cleft, acetylcholine will...

bind to receptors on the motor end plate

When acetylchoine binds to receptors on the motor end plate, this causes...

voltage gated sodium channels to open

voltage gated sodium channels on motor end plate

when opened cause sodium to rush into the muscle cell

What happens when sodium rushes into the muscle cell?

end plate potential; analogous to action potential in neuron

Two things that can happen to acetylcholine after it binds:

1.) it can bind to the neurotransmitter again

2.) it is degraded by acetylcholinesterase

Muscle fibers are arranged into bundles called _____________

Myofibrils

Fascicle

bundle of muscle fibers

Each muscle fiber is made up of __________

Myofibrils

Myofibrils

Individual sarcomeres joined together in a chain; made up of myofilaments

Myofilaments

The contractile proteins, actin and myosin, of muscle cells

Arrangement of muscle components from small to large

Myofilaments —> myofibrils —> muscle fiber —> fascicle —> muscle

Three levels of connective tissue of skeletal muscle

1.) Epimysium 2.) Perimysium 3.) Endomysium

Epimysium

Wrap the entire muscle in a sheath of connective tissue, holding the fasciles in a group

Perimysium

Connective tissue that surrounds each individual fascicle

Endomysium

Connective tissue surrounding an individual muscle fiber

Sarcomere

a structural unit of a myofibril in striated muscle; contractile unit of a muscle cell that contains actin and myosin myofilaments