Membranes and Action Potential Flashcards
1
Q
Membrane Lipids—Phospholipids
A
Major
• Glycerophospholipids/
phosphoglycerides
– Phosphatidylcholines (PC),
phosphatidylserines (PS),
phosphatidylethanolamines (PE)
– Polar head group = choline,
serine, ethanolamine
• Minor: include
phosphatidylinositols (PI)
• PC (lecithin), mainly in outer
leaflet
• PE, PI mainly in inner leaflet
• PS exclusively in inner leaflet
2
Q
Membrane Lipids—Sphingolipids
A
Major
• Located mainly in outer
leaflet
• Derived from ceramide
3
Q
Membrane Lipids—Glycolipids
A
Glycolipids
– Most made from ceramide:
glycosphingolipids
– Eg. gangliosides, ABO blood
group Ags
– Minor but essential; ~2% of
membrane lipids
– Exclusive to outer leaflet
4
Q
Membrane Lipids—Cholesterol
A
Major
– Amphipathic
– Interdigitates between
phospholipids in inner and
outer leaflets
– Helps maintain structural
integrity of plasma
membrane
5
Q
Membrane Proteins
A
Integral, peripheral (membrane associated)
• All TM proteins integral
6
Q
Transport Across Membranes
A
• Selectively permeable barrier
• Main modes of transport:
– osmosis, simple diffusion, facilitated transport,
active transport, exocytosis, endocytosis,
transcytosis
• Size exclusion limit
7
Q
Simple Diffusion
A
• Small, lipid
-soluble
molecules can diffuse
through membrane
according to their
concentration gradient
• Movement in both
directions but net flow in
one direction until
concentration on both sides
of membrane equal – Eg. NO diffuses through lipid
bilayer
8
Q
Osmosis
A
Aquaporins—channels
used for transport of water
– Integral membrane proteins
9
Q
Facilitated Diffusion
A
Some molecules too large, charged or hydrophilic→ cannot simply diffuse through lipid bilayer • Transporters (transmembrane proteins) change conformation to rapidly move a molecule across the membrane, down its electrochemical gradient (Passive transport)
10
Q
Active Transport
A
ENERGY used to transport molecules across membrane
• Primary active transport
• Secondary active transport; co-transport
11
Q
Active Transport—
Primary Active Transport
A
Eg. Na/K ATPase, expressed in most cells
– Pumps Na+
ions out of cell and K+
ions into cell, with
hydrolysis of ATP, ie. against each ion’s electrochemical
gradient
• NOTE: [Na+
] in the ECF than in the ICF; [K+
] higher in the ICF
12
Q
Active Transport—
Secondary Active Transport
A
Transport of one molecule
coupled to movement of
another molecule
• Symport
• Antiport
13
Q
Active Transport—
Secondary Active Transport
A
Eg. Na/Glucose, SGLT1;
symport
– Both glucose and Na+ bind
symporter and are transported
into cell
– Energy from Na+ moving down
its electrochemical gradient is
maintained by Na/K ATPase
– Na+ and glucose in intestinal
lumen are transported across
apical membrane of
enterocytes into these cells,
against glucose concentration
gradient
14
Q
Ion Channels
A
Transmembrane proteins that form aqueous
conduits, selective for types of charged species that
can flow through them (facilitated diffusion)
• Move ions at relatively high rate
– Examples: conduction of nerve impulses, muscle
contraction
• (Not permanently open—many are gated, ie. they
open and close in response to specific stimuli)
• Gap junctions
– Ions, other solutes
15
Q
Ion Channels—Gating
A
• Permeability of membranes must be
regulated
• Three major classes of gated channels:
–Voltage-, ligand-, mechanically-gated
16
Q
Ion Channels—Voltage-gated Channels
A
Open in response to change in electrical potential across cell membrane
17
Q
CFTR
A
- Anion channel
- Chloride, bicarbonate
18
Q
Ion Channels—Mechanosensitive
A
Open in response to mechanical forces
19
Q
Exocytosis
A
Cell releases molecules into
extracellular environment
through fusion of transport
vesicles with plasma membrane
• Material to be exocytosed may
be synthesized and:
– Released immediately from cell,
– Stored in secretory vesicles near
the membrane until needed. Eg.
Neurotransmitters, hormones
– …Or, synthesized as precursors.
When needed, converted to active
proteins before or after exocytosis
20
Q
Endocytosis
A
Cell takes up
macromolecules, fluid,
solutes, membrane
components
• Phagocytosis, pinocytosis
• Material enclosed by
plasma membrane which
eventually pinches off to
form endocytic vesicle
(EV)
21
Q
Endocytosis
A
EV may fuse with
receiving
compartment→ early
endosome (EE)
– sorting→ recycling,
degradation, other
• Endosome maturation:
EE→→ LE
• Fusion of LE with
lysosomes→
endolysosomes;
degradation
22
Q
Phagocytosis
A
Endocytosis of large particle followed by fusion with specialized vesicle →phagosome – Bacteria, viruses, cells that
have died by apoptosis• Phagosome fuses with lysosome and ingestedmaterial is degraded
• Some types ofphagocytosis enacted by specialized cells, such
as macrophages and neutrophils
23
Q
Receptor-mediated Endocytosis
A
Some endocytosis events (incl. phagocytosis) require binding of
extracellular macromolecule to a membrane-bound receptor
• Example: Cholesterol (LDL) taken up from bloodstream via RME
24
Q
Transcytosis
A
• Vesicle-mediated transcellular transport
25
A 55-year-old woman presents to her physician with complaints
of substernal chest pain while at rest. Upon physical
examination, the physician notes that her heartbeat is less
rhythmic than it should be. A defect in diffusion of ions between adjacent affected cardiac myocytes is identified. Expression of the gene coding for which of the following is most likely affected?
A) Actin
B) Aquaporin
C) Gap junction
D) Na/K ATPase
E) Tubulin
26
The structure of neurons includes the cell body, or soma; the dendrites; the axon; and the presynaptic terminals
Glial cells, which greatly outnumber neurons, include astrocytes, oligodendrocytes, and microglial cells; their function, broadly, is to provide support for the neurons.
The nervous system is composed of two divisions: the central nervous system (CNS), which includes the brain and the spinal cord, and the peripheral nervous system (PNS), which includes sensory receptors, sensory nerves, and ganglia outside the CNS. The CNS and PNS communicate extensively with each other.
Structure of the Neuron
27
Cell Body: The cell body, or soma, surrounds the nucleus of the neuron and contains the endoplasmic reticulum and Golgi apparatus. It is responsible for the neuron's synthesis and processing of proteins.
Dendrites: Dendrites are tapering processes that arise from the cell body. They receive information and thus contain receptors for neurotransmitters that are released from adjacent neurons.
Axon: The axon is a projection arising from a specialized region of the cell body called the axon hillock, which adjoins the spike initiation zone (or initial segment) where action potentials are generated to send information.
Whereas dendrites are numerous and short, each neuron has a single axon, which can be quite long (up to 1 meter in length). The cytoplasm of the axon contains dense, parallel arrays of microtubules and microfilament that rapidly move materials between the cell body and the axon terminus. Axons carry action potentials between the neuron cell body and the targets of that neuron, either other neurons or muscle. Axons may be insulated with myelin ,which increases conduction velocity; breaks in the myelin sheath occur at the nodes of Ranvier.
Presynaptic Terminals: The axon terminates on its target cells (e.g., other neurons) in multiple endings, called presynaptic terminals. When the action potential transmitted down the axon reaches the presynaptic terminal, neurotransmitter is released into the synapse. The transmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane (e.g., of dendrites of other neurons). In this way, information is transmitted rapidly from neuron to neuron (or, in the case of the neuromuscular junction, from neuron to skeletal muscle)
28
Based on the number of processes that emanate from their cell body, neurons can be classified as **unipolar, bipolar, pseudounipolar, and Multipolar**
True
29
A) Unipolar neurons have one process, with different segments serving as receptive surfaces and releasing terminals.
True
30
.B) Bipolar neurons have two specialized processes: a dendrite that carries information to the cell and an axon that transmits information from the cell.
31
C) Some sensory neurons are in a subclass of bipolar cells called pseudounipolar cells. As the cell develops, a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord.
True
32
D) Multipolar cells have one axon and many dendrites. Examples include motor neurons, hippocampal pyramidal cells with dendrites in the apex and base, and cerebellar Purkinje cells with an extensive dendritic tree in a single plane.
33
Gilial Cells
34
35
How do ions move in and out of the cell
36
RMP is expressed as the measured potential difference across the cell membrane in millivolts (mV). is, by convention, expressed as the intracellular potential relative to the
extracellular potential. Thus, a resting membrane potential of −70 mV means **70 mV, cell negative**
37
1. **The resting membrane potential is established by diffusion potentials** that result from concentration differences of permeant ions.
2. **Each permeable ion attempts to drive the membrane potential toward its equilibrium potential.** Ions with the highest permeabilities, or conductances, will make the greatest contributions to the resting membrane potential, and those with the lowest permeabilities will make little or no contribution.
38
3. **For example,** the resting membrane potential of nerve is −70 mV, which is close to the calculated K+ equilibrium potential of , but far from the calculated Na+ equilibrium potential. **At rest, the nerve membrane is far more permeable to K+ than to Na+**.
4. **The Na+–K+ pump contributes only indirectly** to the resting membrane potential by maintaining, across the cell membrane, the Na+ and K+ concentration gradients that then produce diffusion potentials. The direct **electrogenic** contribution of the pump (3 Na+ pumped out of the cell for every 2 K+ pumped into the cell) is small.
39
Graded Potential
Local, non-propagated, in response to a depolarizing or hyperpolarizing stimulus of lesser strength•
_Catelectrotonic potential_: Electrically stimulated cathodal end of the stimulator evokes a depolarizing response•
_Anelectrotonic potential_: The hyperpolarizing potential produced due to stimulation at anodal end
40
the figure shows the voltage-gated sodium channel in three separate states. This channel has two *gates*—one near the outside of the channel called the *activation gate,* and another near the inside called the *inactivation gate.*
The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels.
41
**A)** Na+ channels exert positive feedback. **B)** K+ channels exert negative feedback. PNa, PK is permeability to Na+ and K+, respectively.
42
Neuronal cell membranes contain many types of ion channels, including both ligand-gated and voltage-gated ion channels. **Ligand-gated ion channels** open when a ligand (eg, neurotransmitter) binds to them, and **voltage-gated ion channels** open when there is a change in the voltage gradient across the membrane. The behavior of these channels, particularly Na+ and K+ channels, explains the electrical events in neurons.
True
43
The changes in **membrane conductance** of Na+ and K+ that occur during an action potential are shown by steps 1 through 7.
•Step 1- RMP•Step 2- Threshold potential•Step 3- Depolarization phase•Step 4- Overshoot•Step 5- Repolarisation phase•Step 6- Hyperpolarization•Step 7- RMP
44
Sodium and Potassium Gating Summary
45
Graded vs. Action Potential
46
**Conduction velocity :**The speed at which action potentials are conducted along a nerve or muscle fibre
**Conduction velocity is increased by:**
**a. ↑ fiber size.** Increasing the diameter of a nerve fiber results in decreased internal resistance; thus, conduction velocity down the nerve is faster.
**b. Myelination.** Myelin acts as an insulator around nerve axons and increases conduction velocity. Myelinated nerves exhibit **saltatory conduction** because action potentials can be generated only at the **nodes of Ranvier,** where there are gaps in the myelin sheath
MYELIN being an insulator increasing membrane resistance hence less current is lost across the cell membrane .More current flows down the axon ,conduction velocity increases
47
Myelin is an effective insulator, and current flow through it is negligible. Instead, depolarization in myelinated axons travels from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize the node ahead of the action potential to the firing level .This “jumping” of depolarization from node to
node is called **saltatory conduction.** It is a rapid process that allows myelinated axons to conduct up to 50 times faster than the fastest unmyelinated fibers.
48
Propagation in a demyelinating disorder
•Multiple sclerosis- demyelinating disorder of CNS••Thus membrane resistance decreases (current leaks out) thus faster decay of currents, insufficient to produce an AP.••Hence all the symptoms•
49
.
A pathologist is observing the mechanisms of programmed cell death in tissue found from an autopsy. He observes the appearance of autophagosomes and notices cytoplasmic components are targeted and isolated from the rest of the cell within a double-membraned vesicle fusing with an available lysosome.
**Autophagy refers to which of the following actions?**
Breakdown of cytoplasmic proteins in the lysosomal compartment – This is the correct answer. Autophagy is a process by which organelles or cytoplasmic proteins are degraded in the lysosomal compartment. This process usually proceeds through the formation of an autophagosome which then fuses with the lysosomal membrane causing the release and degradation of the contents.
## Footnote
**TAKEAWAY: Autophagy (autophagocytosis) is the natural, regulated mechanism of the cell that removes unnecessary or dysfunctional components. It allows the orderly degradation and recycling of cellular components. Autophagy is also activated when the cell is under stress and requires raw materials such as amino acids and energy.**
50
Receptor-mediated endocytosis is when…
Receptor-mediated endocytosis is when endocytic vesicles from the plasma membrane fuse first with early endosomes and later with lysosomes.
51
Peroxisomes are spherical organelles surrounded by a single membrane; they play a key role in ______ and the biosynthesis and degradation of specific molecules.
fatty acid oxidation and the biosynthesis and degradation of specific molecules.
52
\_\_\_\_\_\_\_ is a type of endocytosis involving ingestion of liquid into a cell in which invaginations of the cellular membrane form intracellular vesicles that are then fused to lysosomes for degradation.
Pinocytosis
53
If chloride has an equilibrium potential (Ei) of -47 mV, what is the direction of Fnet on a chloride ion in a cell with a resting membrane potential (Vr) of -60 mV?
The correct answer is out of the cell (B). This type of question is answered by subtracting the ion’s equilibrium potential from the resting membrane potential. For this question, that gives us -60 mV – (-47 mV) = -13 mV. The fact that our answer is negative means that the inside of the cell is too negative for the ion to be at electrochemical equilibrium. This relatively negative area will repel an anion like chloride. A cation would move into the cell in the example (A). If Vm were equal to Ei, (C) would be true.
54
How does the Na-K-ATPase influence a cell’s resting membrane potential (Vr)?
The correct answer is it pushes out more cations than it brings in, helping make Vr more negative (D). Recall that the Na-K-ATPase pumps three Na+ ions out of a cell as it brings only two K+ ions into the cell. This results in a net exit of 1 cation from the cell per cycle of the Na-K-ATPase, making the membrane potential more negative. Because the Na-K ATPase removes more cations from the cell than it brings in, (A) and (B) are incorrect. Because both ions that the Na-K-ATPase acts on (Na+ and K+) are cations, (C) is incorrect.
55
Which is true of the nonequilibrium steady-state?
The correct answer is a nonequilibrium steady-state is a source of potential energy that a cell can use to do work (A). A cell stores potential energy in the form of electrochemical gradients across the cell membrane, and it can use the energy in these gradients to do work like moving substances across the membrane with secondary active transporters. Nearly all cells set up nonequilibrium steady-states for the ions they contain,
56
What type of transport requires energy to move something against its concentration gradient (from low to high concentration)?
Active transport
57
Simple diffusion
In simple diffusion, a substance passes through the membrane without requiring additional “help” in the form of mediators such as membrane proteins.
58
Facilitated Diffusion
Substances that move across the plasma membrane via facilitated diffusion are typically too bulky or charged to pass through the lipid bilayer and instead pass through the membrane via two classes of membrane proteins: carriers and channels. These proteins act like a gate to control the entry and exit of substance that cannot otherwise pass through the lipid bilayer
59
Carrier protiens
Carrier proteins are specialized transmembrane proteins that change shape to allow for the entry and exit of specific substances: a lot like a revolving door. These proteins require a specific ligand-receptor interaction to work and are only open to one side at a time. These proteins require a specific ligand-receptor interaction to work and are only open to one side at a time. The change in shape transfers the substance across the membrane down its concentration gradient and is the mechanism to transport large molecules such as proteins across the membrane.
60
Channel protiens
Channel proteins act as simple tunnels to facilitate the free passage of a specific substrate down its concentration gradient. For example, ion channels are a specific type of channel protein that specifically transport charged species such as Na+, K+, Ca2+, and Cl-.
61
A vesicle within a cell must be transported to another region of the cell along the microtubules.
**Which of the following proteins may be involved in catalyzing this transport?**
Kinesin – This is the correct answer. Kinesin and dynein are families of microtubule motor proteins that facilitate intracellular transport along microtubules.
## Footnote
**TAKEAWAY: A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments and are powered by the hydrolysis of adenosine triphosphate (ATP).**
62
Dystrophin
Dystrophin is an actin-binding protein.
63
Myosin
Myosin is an actin-binding protein.
64
Spectrin
Spectrin is an actin-binding protein.
65
Vimentin
Vimentin is a type of intermediate filament.
66
A histologist is investigating cytoskeletal components that require an energy source. This energy source is in the form of ATP and is used for cytoskeletal component polymerization and contains subunits that are observed to undergo treadmilling.
**Which of the following is that cytoskeletal component?**
Actin filament – This is the correct answer. Actin filaments require ATP for polymerization and subunits undergo treadmilling as they make their way through an F-actin polymer.
**TAKEAWAY:** **Actin filaments (F-actin) are linear polymers of globular actin (G-actin) subunits and occur as microfilaments in the cytoskeleton and as thin filaments, which are part of the contractile apparatus, in muscle and nonmuscle cells (see contractile bundles). They commonly underlie the plasma membrane and are typically assembled at the cell periphery from adhesion sites or sites of membrane extension. Actin filaments can create a number of linear bundles, two-dimensional networks, and three-dimensional gels, and actin binding proteins can influence the specific structure the filaments will form.**
67
A 50-year-old woman is referred by her primary care physician to an otolaryngologist for evaluation of hearing loss. She works as a bartender and admits to being exposed to loud music throughout her adult life. She says that her mother, brother and two sisters are “hard-of-hearing.” The physician’s report states that the woman has a defect in detecting high-frequency sounds most likely caused by loss of hair cell movement in the cochlea of her inner ear.
**An abnormality in which of the following is most likely to explain this woman’s hearing loss?**
This is the correct answer. Dynein is the microtubule protein making up movable (motile) hair cell cilia.
## Footnote
**TAKEAWAY – Cilia are a part of the cell’s system of organelles known as microtubules. Physical movements (motility) of cells and membranes are created by contractile proteins (e.g., dynein and tubulin) of microtubules. Hair cell cilia are the main sensors and transducers of mechanical (sound) energy into electrical energy (action potentials) for the sense of hearing (audition). Hair cells of the cochlea beat in frequency with the sound energy waves that are transmitted through the outer and middle ear structures to the inner ear’s cochlea.**
68
tubulin
tubulin is a contractile protein found in microtubules, it functions during mitotic spindle formation associated with mitosis
69
\_\_\_\_\_\_\_\_ are proteins making up gap junctions between hair cells and are not contractile proteins involved in hair cell movement in the cochlea.
connexins
70
Microfilaments
muscle contraction
71
intermediate filaments
Maintain cell structure
72
Microtubules
Movement, cell division
73
Cilia structure
Nonmotile, axonemal dynein, gap junctions
74
The correct temporal sequence of events at the neuromuscular junction
Acetylcholine (ACh) is stored in vesicles and is released when an action potential in the motor nerve opens Ca2+ channels in the presynaptic terminal. ACh diffuses across the synaptic cleft and opens Na+ and K+ channels in the muscle end plate, depolarizing it (but not producing an action potential). Depolarization of the muscle end plate causes local currents in adjacent muscle membrane, depolarizing the membrane to threshold and producing action potentials.
75
In the nerve and in most types of muscle, this inward current is carried by Na+.
True
76
Repeated stimulation of a skeletal muscle fiber causes a sustained contraction (tetanus). Accumulation of which solute in intracellular fluid is responsible for the tetanus?
Calcium-During repeated stimulation of a muscle fiber, Ca2+ is released from the sarcoplasmic reticulum (SR) more quickly than it can be reaccumulated; therefore, the intracellular [Ca2+] does not return to resting levels as it would after a single twitch. The increased [Ca2+] allows more cross- bridges to form and, therefore, produces increased tension (tetanus). Intracellular Na+ and K+ concentrations do not change during the action potential. Very few Na+ or K+ ions move into or out of the muscle cell, so bulk concentrations are unaffected. Adenosine triphosphate (ATP) levels would, if anything, decrease during tetanus.
77
78
79
80
A research student is studying membrane physiology. As he observes a secretory vesicular structure that has budded off the Golgi complex under the microscope. Further analysis of this structure shows asymmetry of the cell membrane. The asymmetry of the cell membrane is established primarily by which of the following?
Membrane synthesis in the endoplasmic reticulum – This is the correct answer. Asymmetry of the lipid bilayer is established during membrane synthesis in the endoplasmic reticulum. TAKEAWAY: Protein trafficking (secretory pathway) is the transport of proteins to their correct subcellular compartments, extracellular space, or outer membrane.
81
60-year-old male visits his primary care physician for a general check-up. After taking the patient’s blood to do labs it is determined that the patient has a poor lipid profile. His physician starts him on a statin drug to help lower the patient’s cholesterol (LDL). The statin family of drugs enhances endocytosis of low-density lipoprotein (LDL) from the serum. Endocytosis of LDL differs from phagocytosis of damaged cells in which one of the following ways?
Use of clathrin-coated pits – This is the correct answer. Receptor-mediated endocytosis of ligand-receptor complexes is a selective process that requires invagination of the cell membrane to form clathrin-coated pits and vesicles. Clathrin is not involved in phagocytosis. TAKEAWAY: Phagocytosis of damaged cells occurs by evagination to engulf the IgG-coated surface of the target. Both processes use acidification of compartments and hydrolases to uncouple receptor and ligand (receptor-mediated endocytosis) or destroy engulfed material (phagocytosis).
82
Scientists examine microtubules in k-9 hepatocytes. The microtubule is observed to disassemble quickly after a period of rapid growth. Which of the following most likely occurred to this particular microtubule to stimulate its breakdown?
Loss of its GTP cap – This is the correct answer. Loss of the GTP cap results in rapid disassembly of microtubules. TAKEAWAY: Tubulin dimers can bind two molecules of GTP, one of which can be hydrolyzed subsequent to assembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe.
83
Physiology students are preforming a laboratory experiment to better understand fluid dynamics pertaining red blood cells. Erythrocytes are placed in a hypertonic solution of sodium chloride. Which of the following will be an effect of the osmosis that will occur?
Cells will shrink in volume – This is the correct answer. Cells reduce their volume when placed in hypertonic solutions. In such an instance, there is more free water inside cells than outside cells. TAKEAWAY: Tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a semipermeable cell membrane. A hypotonic solution causes a cell to swell, whereas a hypertonic solution causes a cell to shrink.
84
Physiology students are preforming a laboratory experiment to better understand fluid dynamics pertaining red blood cells. Erythrocytes are placed in a hypotonic solution of sodium chloride.
Cells will bloat and burst – This is the correct answer. Due to osmotic pressure, water diffuses into the cell, and the cell often appears bloated leading to cells bursting. TAKEAWAY: A hypotonic solution has a lower concentration of solutes than another solution. In biology, a solution outside of a cell is called hypotonic if it has a lower concentration of solutes relative to the cytosol. Due to osmotic pressure, water diffuses into the cell, and the cell often appears turgid, or bloated.
85
Physiology students are preforming a laboratory experiment to better understand fluid dynamics pertaining red blood cells. Erythrocytes are placed in an isotonic solution of sodium chloride. Which of the following will be an effect of the osmosis that will occur?
Concentration of solutes outside the cell is equal to the concentration of solutes inside the cell – This is the correct answer. The RBCs in the isotonic solution will neither swells nor shrink because there is no concentration gradient to induce the diffusion of large amounts of water across the cell membrane. Water molecules freely diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction, the cell will neither gain nor lose water. TAKEAWAY: Tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a semipermeable cell membrane. A solution is isotonic when its effective osmole concentration is the same as that of another solution. A hypotonic solution causes a cell to swell, whereas a hypertonic solution causes a cell to shrink.
86
The \_——————-is the insulating layer of modified plasma membrane that wraps around axons of a nerve. Increases the space constant and the conduction velocity of signals traveling down axons. Decreases membrane capacitance and increases membrane resistance.
The myelin sheath
87
—————— are unmyelinated regions between two adjacent myelinated segments of axons in the CNS and PNS. TAKEAWAY: Contain a large amount of Na+ channels which allows saltatory conduction leading to an increase the velocity of action potentials down the axon.
Node of Ranvier
88
During repeated stimulation of a muscle fiber, Ca2+ is released from the sarcoplasmic reticulum (SR) more quickly than it can be reaccumulated; therefore, the intracellular [Ca2+] does not return to resting levels as it would after a single twitch. The increased [Ca2+] allows more cross- bridges to form and, therefore, produces increased tension (tetanus). Intracellular Na+ and K+ concentrations do not change during the action potential. Very few Na+ or K+ ions move into or out of the muscle cell, so bulk concentrations are unaffected. Adenosine triphosphate (ATP) levels would, if anything, decrease during tetanus.
True
89
Transport of D- and L-glucose proceeds at the same rate down an electrochemical gradient by which of the following processes?
Only two types of transport occur “downhill”—simple and facilitated diffusion. If there is no stereospecificity for the D- or L-isomer, one can conclude that the transport is not carrier mediated and, therefore, must be simple diffusion.
90
At the muscle end plate, acetylcholine (ACh) causes the opening of
Binding of acetylcholine (ACh) to receptors in the muscle end plate opens channels that allow passage of both Na+ and K+ ions. Na+ ions will flow into the cell down its electrochemical gradient, and K+ ions will flow out of the cell down its electrochemical gradient. The resulting membrane potential will be depolarized to a value that is approximately halfway between their respective equilibrium potentials.
91
Which of the following would occur as a result of the inhibition of Na+, K+-ATPase?
Inhibition of Na+, K+-adenosine triphosphatase (ATPase) leads to an increase in intracellular Na+ concentration. Increased intracellular Na+ concentration decreases the Na+ gradient across the cell membrane, thereby inhibiting Na+–Ca2+ exchange and causing an increase in intracellular Ca2+ concentration. Increased intracellular Na+ concentration also inhibits Na+–glucose cotransport.
92
A new drug is developed that blocks the transporter for H+ secretion in gastric parietal cells. Which of the following transport processes is being inhibited?
H+ secretion by gastric parietal cells occurs by H+–K+ adenosine triphosphatase (ATPase), a primary active transporter.
93
A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because
Elevated serum K+ concentration causes depolarization of the K+ equilibrium potential and therefore depolarization of the resting membrane potential in skeletal muscle. Sustained depolarization closes the inactivation gates on Na+ channels and prevents the occurrence of action potentials in the muscle.
94
In contraction of gastrointestinal smooth muscle, which of the following events occurs after binding of Ca2+ to calmodulin?
The steps that produce contraction in smooth muscle occur in the following order: various mechanisms that raise intracellular Ca2+ concentration, including depolarization of the sarcolemmal membrane, which opens voltage-gated Ca2+ channels, and opening of ligand-gated Ca2+ channels; Ca2+-induced Ca2+ released from SR; increased intracellular Ca2+ concentration; binding of Ca2+ to calmodulin; increased myosin-light-chain kinase; phosphorylation of myosin; binding of myosin to actin; and cross-bridge cycling, which produces contraction.
95
Which of the following is the main determinant of the resting membrane potential in a neuronal axon? I
Which of the following is the main determinant of the resting membrane potential in a neuronal axon?
96
Which of the following is best describes a characteristic of graded potentials, but not action potentials?
They degrade with distance. This is the correct answer. They decay passively with distance via channel leak through the membrane. The length constant (lambda) is a measure of the decay with distance. TAKEAWAY: Graded potentials can be triggered by several physical triggers other than voltage-gated depolarization, they can be excitatory or inhibitory, and they are proportionally sized to the size of the stimulus. Their duration tends to be short and their amplitudes decay with distance. They do not exhibit refractory periods as do action potentials. They are generally used in dendrites (and soma).
