Axon
a long, skinny tubular extension of a neuron.
neuromuscular junction
the terminus of an axon, the surface of the muscle cell, and the small space between the two cells.
Membrane potential
quantifies the overall separation of charged particles on opposite sides of a membrane.
Polarization
of charges is the separation of ions and the cause of membrane potential.
ligand-gated ion channels
open or close when they bind their ligands and allow particular ions to cross a membrane
Electrical current
is the movement of charged particles, such as Na+ and K+ ions.
Neurotransmitters
are secreted by neurons to stimulate their target cells.
Depolarized
membranes have less separation of charges than a polarized membrane at resting potential.
Threshold
the minimum level of depolarization required to activate a neuron.
Action potential
a moving wave of electrical current traveling one direction down an axon.
Voltage-gated ion channels
open or close when their local environment is depolarized to threshold level or more.
Refractory period
when ion channels cannot reopen a second time until they have rested for some time.
Patch clamp method
removes a tiny circle of plasma membrane and measures the electrical current in one or a few ion channels.
Myelin sheaths
are fatty layers of insulation surrounding some nerve axons.
Nodes of Ranvier
are 1 �m gaps between Schwann cells that permit faster electrical conductance in neurons.
Exocytosis
The secretion of cellular products when a vesicle fuses with the plasma membrane.
sarcomere
composed of overlapping rods of myosin and actin proteins and are responsible for muscle cell contraction.
Longitudinal sections
are cut along the long axis of a structure.
Cross sections
Are cut perpendicular to the long axis of a structure.
Actin
form long twisted polymer rods that appear lighter in micrographs of sarcomeres.
Myosin
are longer and bundle together to form thicker rods in micrographs of sarcomeres.
Hypertrophy
occurs when individual cells grow in size.
Hyperplasia
the proliferation, or division, of cells.
Summarize the two different types of communication systems used by neurons and where each of these takes place in a neuron. Which of these communication systems ultimately triggers muscle contraction?
Neurons use electrical communication to move information down their lengths as rapidly as possible. Chemical communication informs a neuron what action it should take and what action it should pass on, such as telling a muscle cell to contract
Where is sodium more abundant, inside or outside of a cell? Where is potassium more abundant? For each ATP consumed, how many ions does the Na+/K+ pump move across the plasma membrane? What is the change in electrical charge for a cell when the pump compl
more sodium outside
more potassium inside
For every ATP consumed, the Na+/K+ pump moves three sodium ions out of the cell and two potassium ions into the cell. This 3:2 ratio is a vital ion ratio, because it produces the electrical potential every animal
Look at the Na+/K+ pump cycle in Figure 9.3B. Describe the type of modulation, covalent or allosteric, in each step and the functional consequence of each shape change caused by the modulation
The Na+/K+ pump changes its shape in response to covalent modulation when it is phosphorylated or dephosphorylated. Figure 9.3B illustrates the steps. The pump (1) is allosterically modulated when it binds three sodium ions (2), which leads to ATP binding
Look up the drug digitalis on the Internet, which was originally purified from a flower called foxglove. To what protein does this heart medication bind, and what happens to this protein once digitalis binds?
The heart medication, digitalis, works by stalling a dose-dependent subset of your Na+/K+ pumps so that they cannot transport ions any more. The net result is a reduction of membrane potential closer to -50 mV with more sodium inside the cells than normal
Describe how the ligand-gated ion channel in Figure 9.4 works. What causes the gate/door to open and close?
When the ligand binds, the ion channel allows only sodium ions to flow down their chemical and electrical concentration gradients into the cytoplasm. Ion channels are not as specific as enzymes, but they are selective. Only ions of a particular charge and
What happens to a neuron that is stimulated below threshold depolarization? What happens when a neuron is stimulated above threshold? In Figure 9.5, what is the approximate threshold for this particular human neuron?
When a cell was depolarized by only 10 mV (red inset), the measuring electrode (purple graph) detected exactly what was applied to the cell, and the voltage decayed over time.
However, when the neuron was depolarized by 20 mV, something very different hap
Explain the difference in outcomes between the Depolarization at one point in time VS. Depolarization over time given that the equipment was unchanged between the two experiments. What role did threshold play?
Depolarization at one point in time did give the expected results because the action potential travels through the membrane and does not depolarize the entire membrane at on point in time but rather over an amount of time.
once the membrane is locally dep
Compare the duration of the input stimulus and the neuron's response in the right half of Figure 9.5B. Speculate how the duration of the neuronal depolarizations can be greater than the duration of the activating electrical impulse.
Through experiments similar to Figure 9.5, physiologists had discovered that neurons require a threshold level of stimulation before they will propagate their own electrical impulse down their axons. When a neuron is stimulated at least as high as the thr
Speculate how action potentials are prevented from traveling back up the axon once they reach the nerve terminus.
To prevent an echo of the action potential from bouncing back to the site of its origin, all voltage-gated sodium channels have a refractory period of time when they cannot open even if the local threshold depolarization is exceeded. The area of depolariz
In Figures 9.5 - 9.7, notice that the depolarization disappears and the cell returns to the resting membrane potential. Speculate how cells reset their membrane potentials
through repolarization (K+ channels opening to repolarize the cell)
What is the purpose of the refractory period for the sodium ion channels?
To prevent the "echo" of the electrical signal form going back to where it began.
If it were to stay open then the signal may not travel down
why do K+ channels not immediately open after depolarization?
To prevent the signal from being killed too quickly
where are ligand-gated channels and voltage-gated channels located?
Ligand-gated Na+ channels are found on plasma membrane where chemical signal is first delivered
Voltage-gated ion channels cover the vast majority of the surface area on a neuron
What causes the threshold depolarization?
ligand-gated Na+ channels open and cause the initial +20 mv
What causes full depolarization?
Voltage-gated Na+ channels open and cause the full depolarization of +50 mv
What happens when sodium ions enter the cell?
Membrane partially depolarized (becomes less negative)
Why do only sodium ions flow through the gated sodium channel?
Bcz of their size and charge of the ion
Channels are super specific for the ion they transport bcz of their structure
Bcz of the size and the charge of the ion
Channels are super specific for the ion they transport bcz of their structure
Are neurons really resting when they have this potential?
No, they are maintaining the resting potential with the use of active transport Na+/K+ pump
"resting" means no electrical impulses
-70 mV is the resting potential in neurons.
What is the net effect of the Na+/K+ pump on the charge across the membrane?
cells accumulate an overall membrane potential, which is a separation of charged particles in the form of Na+ and K+ ions. Neurons that communicate to your muscle cells at the neuromuscular junction have a resting membrane potential of approximately -70
How does the Na+/K+ pump work?
3 sodiums bind to pump, the Na+ allosterically alters the pump allowing ATP to bind, the pump with 3 Na+ ions phosphorylates itself (ADP waste product), covalent modulation causes a shape change in pump, Na+ leaves the pump to the outside of the cell, new
When is the Na+/K+ pump phosphorylated in the transport process?
It is only phosphorylated when Na+ is added.
Is the Na+/K+ pump active or passive transport?
The Na+/K+ pump is active trasport ( against concentration gradient) requires ATP
How do we tell our muscles to move?
The neuron, or nerve cell, receives the chemical input and converts the signal to an electrical current that races down the axon to its terminal adjacent to the muscle cell. At the neuromuscular junction, the neuron converts the electrical signal back int
Conduction of an impulse through a neuron
ligand-gated Na+ channels open due to a ligand binding to the channel
Partial Depolarization of membrane
causes voltage-gated Na+ channels to open
Fully Depolarized membrane
Na+ channels close
K+ voltage gated channels open
Repolarize cell membrane
Approximate how many more Na+/K+ pumps than voltage-gated sodium channels you would expect to find in the plasma membrane of a neuron. Base your answer on their different rates for transporting ions.
Neurons need up to 10,000 times more Na+/K+ pumps as voltage-gated sodium channels to match ion transport rates. This simple calculation tells you that different membrane proteins must be produced in different amounts, and their densities in the plasma me
Explain why an action potential only moves in one direction along an axon. Be sure to address both the initiation of the moving action potential and the repolarization of the neuron. Address both the immediate repolarization and the slower reset of ion co
Action potentials only move in one direction because after voltage-gated sodium channels open, the influx of Na+ causes depolarization. The Na+ channels then close and K+ channels open causing the cell to repolarize. The Na+/K+ pump resets the ion concent
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Explain how different channels opened for different durations as in Figure 9.10. Be sure to address why the number of open channels diminishes even though the stimulating signal is still on.
Ion channels flick open and closed with random timing so that you cannot know exactly when a channel will open next or how long it will stay open. However, while the channel is open, the current always reaches the same magnitude because ions are rushing t
Why do myelinated axons transmit electricity faster? Explain why nodes of Ranvier would not work if they were spaced too far apart.
The benefit of myelin is only realized if nodes of Ranvier are appropriately spaced. The spacing of the nodes allows waves of sodium to flood the axon cytoplasm and spread to the next node where more voltage-gated sodium channels are available to open. Th
are the the voltage-gated Ca2+ channels evenly distributed?
no, the voltage-gated Ca2+ channels are not evenly distributed
Summarize the three waves of communication through a neuron that will stimulate a muscle.
1) receive a chemical message;
2) produce an electrical current that causes an action potential to move down the axon;
3) produce a new chemical message to tell your muscles to contract
what type of protein allows a neuron to reduce its cytoplasmic Ca2+ after the action potential has dissipated.
Ca2+ pump
Study the electron micrographs in Figure 9.13. Where on the secretory vesicle is the calcium-binding protein located that controls exocytosis? Use soap bubbles as an analogy for secretory vesicles to explain how exocytosis leads to the secretion of neurot
Embedded in the membrane of a neuron's secretory vesicles are proteins that can bind Ca2+. Their Ca2+ binding sites must be located in the cytoplasm and not inside the lumen of the vesicles because the increased Ca2+ ions are located in the cytoplasm (Fig
List all of the voltage-gated ion channels you studied in this section.
Na+ voltage gated ion channels
K+ voltage gated ion channels
Ca+ voltage gated ion channels
Does every ion channel of the same type in a small region of a cell open and close the same way?
Ion channels flick open and closed with random timing so that you cannot know exactly when a channel will open next or how long it will stay open. However, while the channel is open, the current always reaches the same magnitude because ions are rushing t
What is myelin? What impact does it have on the function of nerve cells?
fatty layers of insulation surrounding some nerve axons that conduct electricity faster than non-myelinated cells.
The Schwann cell's multi-layered plasma membrane wrapping produces an insulator of sufficient thickness to ensure that the ions do not leak
What role does calcium play in a neuron's function?
Calcium flows into the axon terminal when an action potential reaches it. The influx of calcium through voltage-gated calcium channels bind the secretory vesicle's proteins which leads to a new protein shape and the fusion of secretory vesicles to the pla
Research question: How do muscles get bigger?
what was the experimental setup?
what animal was used?
The investigators conducted research using rats as their study organism. The scientists wanted to cause one muscle (plantaris) to grow rapidly by removing another muscle (gastrocnemius) from young male rats.
Twenty rats had their gastrocnemius removed fro