205 Questions
What is the only form of transport that is not carrier mediated?
Simple diffusion
What is the primary function of gap junctions between cells?
To permit intercellular communication and electrical coupling
What is the direction of flux in the equation J=-P × A × (C1 - C2)?
From high to low concentration
What is the unit of measurement for permeability (P) in the diffusion equation?
em/sec
What is the surface area (A) used in the sample calculation?
100 cm^2
What is the primary component of cell membranes?
Phospholipids
What is the function of the hydrophobic tails of phospholipids in the lipid bilayer?
They face each other to form the bilayer
Which type of substances can dissolve in the hydrophobic lipid bilayer?
Lipid-soluble substances
What is the function of integral proteins in the cell membrane?
They are channels for ions and molecules
What is the main difference between integral and peripheral proteins in the cell membrane?
Integral proteins are imbedded in the cell membrane, while peripheral proteins are not
What is the function of tight junctions between cells?
They allow for the passage of solutes depending on their size and charge
What determines the permeability of tight junctions?
The size and charge of the solutes
What characterizes 'leaky' tight junctions?
They are permeable to certain solutes
What is the primary difference between electrochemical gradient and carrier-mediated transport?
Direction of transport
What determines the penneability of a solute through a membrane?
Oil-water partition coefficient and solute size
What type of transport involves the use of metabolic energy to transport solutes against their concentration gradient?
Primary active transport
What is the role of the Na+-K+ pump in cell physiology?
To establish a concentration gradient for Na+ and K+ ions
Which type of solute has the highest permeability in lipid membranes?
Small hydrophobic solutes
What is the effect of increasing membrane thickness on diffusion?
It decreases the diffusion coefficient
What is the primary difference between cotransport and countertransport?
Direction of transport
What is the role of water-filled channels or pores in cell membranes?
To transport hydrophilic solutes
What is a characteristic of carrier-mediated transport that is not shared with simple diffusion?
Stereospecificity
Which of the following is an example of a competitive inhibitor of glucose transport?
Galactose
What is the term for the maximum transport rate of a carrier-mediated transport system?
Transport maximum
Which type of transport does not require metabolic energy?
Facilitated diffusion
What is the direction of the electrochemical gradient in facilitated diffusion?
Down an electrochemical gradient
What is a characteristic of facilitated diffusion that is not shared with primary active transport?
Requirement of metabolic energy
What is the term for the process of carrier-mediated transport that requires metabolic energy?
Primary active transport
Why is galactose not transported by the same carrier as glucose?
Because it is a competitive inhibitor
Which of the following characteristics is NOT shared by facilitated diffusion and primary active transport?
Requiring metabolic energy
What is the primary reason for impaired glucose uptake by muscle and adipose cells in diabetes mellitus?
Lack of insulin
Which ion is transported from intracellular to extracellular fluid by the Na+, K+-ATPase pump?
Na+
What is the energy source for the Na+, K+-ATPase pump?
ATP hydrolysis
Which of the following is a characteristic of primary active transport?
Transport against an electrochemical gradient
What is the direction of glucose transport in muscle and adipose cells?
Downhill
Which of the following is an example of facilitated diffusion?
Glucose transport in muscle and adipose cells
What is the effect of galactose on glucose transport in muscle and adipose cells?
Inhibition
What is the function of the Ca2+-ATPase pump in the sarcoplasmic reticulum?
To transport calcium ions into the sarcoplasmic reticulum
What is the effect of inhibiting the Na+, K+-ATPase pump?
Decreases the transmembrane sodium gradient
What is the characteristic of secondary active transport?
Indirectly uses metabolic energy from the sodium gradient
What is the function of the H+-ATPase pump in gastric parietal cells?
Transports protons into the lumen of the stomach
What is the type of transport where two or more solutes are coupled?
Secondary active transport
What is the effect of inhibiting the H+-ATPase pump in gastric parietal cells?
Decreases acid secretion in the stomach
What is the type of transport that occurs in the renal thick ascending limb?
Na+-K+-2Cl- cotransport
What is the function of SERCA in the sarcoplasmic reticulum?
Transports calcium ions into the sarcoplasmic reticulum
What is the energy source for Na+-glucose cotransport?
The energy derived from the 'downhill' movement of Na+
What happens to Na+-glucose cotransport when the Na+-K+ pump is poisoned?
It decreases
What is the direction of glucose transport in Na+-glucose cotransport?
Uphill
What is the location of the carrier for Na+-glucose cotransport?
Luminal membrane
What is the direction of Na+ movement in Na+-glucose cotransport?
Downhill
What is the purpose of the Na+-K+ pump in Na+-glucose cotransport?
To maintain the inwardly directed Na+ gradient
What is the direction of Ca2+ movement in Na+-Ca2+ exchange?
Uphill
What is the energy source for Na+-Ca2+ exchange?
The energy derived from the 'downhill' movement of Na+
What is the effect of poisoning the Na+-K+ pump on Na+-Ca2+ exchange?
It decreases
What is the major difference between the Na+ diffusion potential and the Cl- diffusion potential?
The permeability of the membrane to the ions
What is the purpose of the Nernst equation in the context of electrochemical equilibrium?
To calculate the equilibrium potential of an ion
What happens to the net diffusion of Na+ at electrochemical equilibrium?
It becomes zero
What is the relationship between the chemical and electrical driving forces on an ion at electrochemical equilibrium?
They are equal and opposite
What is the driving force on an ion?
The difference between the actual membrane potential and the ion's equilibrium potential
In which direction does current flow occur?
In the same direction as the driving force
What determines the magnitude of current flow?
Both the size of the driving force and the permeability of the ion
What is the condition for current flow to occur?
There must be a driving force on the ion, and the membrane must be permeable to the ion
What is the resting membrane potential?
The potential difference across the cell membrane in millivolts
What determines the resting membrane potential?
The concentration differences of all permeant ions
What is the direction of the intracellular potential relative to the extracellular potential in a resting membrane potential of -70 mV?
70 mV, cell negative
What is the equilibrium potential of an ion?
The potential the ion would 'like' the membrane to be, as calculated by the Nernst equation
What is the primary condition for an action potential to occur?
Net inward current is greater than net outward current
What is the approximate resting membrane potential of a nerve cell?
-70 mV
What is the primary reason for the inactivation of Na+ channels at rest?
The inactivation gates are open
What is the equilibrium potential for sodium ions?
+65 mV
What is the primary factor that determines the size and shape of an action potential?
The membrane potential at threshold
What is the relationship between the net inward current and the net outward current at threshold?
The net inward current is greater than the net outward current
What is the primary characteristic of an action potential?
It is an all-or-none response
What is the primary function of the high resting conductance to potassium ions?
It drives the membrane potential towards the potassium equilibrium potential
What is the primary contributor to the resting membrane potential?
Potassium ions
What is the effect of the Na+-K+ pump on the resting membrane potential?
It indirectly contributes to the resting membrane potential
What is the direction of the electrogenic contribution of the Na+-K+ pump?
Outward current
What is the effect of depolarization on the membrane potential?
It makes the membrane potential less negative
What is the characteristic of an action potential?
A rapid depolarization followed by repolarization
What is the main factor that determines the resting membrane potential?
Potassium equilibrium potential
What is the direction of the potassium equilibrium potential?
Towards the negative direction
What is the indirect contribution of the Na+-K+ pump to the resting membrane potential?
It maintains the sodium and potassium concentration gradients
What is the unit of measurement for the equilibrium potential (E) calculated using the Nernst equation?
millivolts (mV)
What is the significance of the charge (z) in the Nernst equation?
It is the valence of the ion, which can be positive or negative.
What is the purpose of using the 'intuitive approach' in the Nernst equation?
To decide the sign of the equilibrium potential (E).
What is the relationship between the intracellular and extracellular concentrations of an ion at electrochemical equilibrium?
The intracellular and extracellular concentrations are equal.
What is the value of RT (in mV) at 37°C, as mentioned in the text?
60 mV
What is the purpose of the Nernst equation in cell physiology?
To determine the equilibrium potential of an ion.
What is the significance of the logarithmic term in the Nernst equation?
It allows calculation of the equilibrium potential (E) in millivolts (mV).
What is the advantage of using the Nernst equation to calculate the equilibrium potential (E)?
It enables calculation of the equilibrium potential (E) for any ion.
What is the primary cause of rapid depolarization during the upstroke of the action potential?
Inward Na+ current
What is the effect of depolarization on the inactivation gates of the Na+ channels?
They close slowly
What is the effect of Tetraethylammonium (TEA) on voltage-gated K+ channels?
It blocks them
What is the net effect of closing Na+ channels and opening K+ channels during repolarization?
The K+ conductance becomes higher than the Na+ conductance
What is the consequence of the Na+ conductance becoming higher than the K+ conductance during the upstroke?
The membrane potential is driven toward the Na+ equilibrium potential
What is the effect of tetrodotoxin on voltage-sensitive Na+ channels?
It blocks them
What is the characteristic of the brief portion at the peak of the action potential?
It is positive
What is the combined effect of the closure of Na+ channels and the opening of K+ channels during repolarization?
The membrane potential is repolarized
During which period is the membrane potential driven very close to the K+ equilibrium potential?
During repolarization
What is the reason why no action potential can occur during the absolute refractory period?
The inactivation gates of the Na+ channels are closed
What happens to the inactivation gates of the Na+ channels during repolarization?
They open
During which period can an action potential be elicited only if a larger than usual inward current is provided?
Relative refractory period
What is the cause of accommodation?
The depolarization of the membrane potential
What is the effect of the higher K+ conductance during the relative refractory period?
The membrane potential is driven closer to the K+ equilibrium potential
What is the duration of the absolute refractory period?
The entire duration of the action potential
What is the reason why an action potential cannot be elicited during the absolute refractory period?
The inactivation gates of the Na+ channels are closed
In hyperkalemia, why do action potentials not occur in skeletal muscle despite the membrane potential being closer to threshold?
Inactivation gates on Na+ channels are closed by depolarization
What is the primary mechanism by which conduction velocity is increased in myelinated nerves?
Saltatory conduction at nodes of Ranvier
What is the role of myelin in propagating action potentials?
Myelin acts as an insulator, increasing conduction velocity
What is the sequence of events that occurs during neurotransmission at a chemical synapse?
Action potential in the presynaptic cell, then depolarization of the presynaptic terminal
What is the functional significance of nodes of Ranvier in myelinated nerves?
Nodes of Ranvier are the site of action potential propagation
What is the primary mechanism by which action potentials propagate in unmyelinated axons?
Local current flow
What is the effect of increasing the diameter of a nerve fiber on conduction velocity?
Conduction velocity increases
What is the functional significance of myelination in increasing conduction velocity?
Myelination reduces the capacitance of the nerve fiber
What is the primary role of Ca2+ in the neuromuscular junction?
To facilitate the release of ACh from the presynaptic terminal
What is the purpose of the choline acetyltransferase enzyme?
To synthesize and store ACh in synaptic vesicles
What is the effect of depolarization of the presynaptic terminal?
Ca2+ channels open, allowing ACh release
What is the primary role of ACh in the neuromuscular junction?
To facilitate muscle contraction
What is the role of the nicotinic receptor in the postsynaptic membrane?
To bind ACh, causing muscle contraction
What is the effect of inhibitory neurotransmitters on the postsynaptic membrane?
They hyperpolarize the postsynaptic membrane
What is the primary component of synaptic vesicles in the presynaptic terminal?
ACh and proteoglycan
What is the primary function of the motoneuron in the neuromuscular junction?
To release ACh into the synaptic cleft
What triggers the release of ACh into the synaptic cleft?
Increase in Ca2+ permeability
What is the net result of the binding of ACh to nicotinic receptors on the postsynaptic membrane?
Depolarization of the postsynaptic membrane to a value halfway between the Na+ and K+ equilibrium potentials
What is the term for the smallest possible EPP?
Miniature end plate potential
What is the ultimate result of the depolarization of the postsynaptic membrane?
Muscle contraction
What is the mechanism by which ACh is released from the presynaptic terminal?
Exocytosis
What is the direction of Ca2+ movement when Ca2+ permeability increases?
Into the presynaptic terminal
What is the role of the nicotinic ACh receptor in the postsynaptic membrane?
It acts as a ligand-gated channel
What is the effect of the binding of ACh to the nicotinic receptor on the conductance of the channel?
It increases the conductance of the channel to Na+ and K+
What percentage of choline is taken back into the presynaptic ending by Na+-choline cotransport?
One-half
What is the effect of AChE inhibitors on the action of ACh at the muscle end plate?
It prolongs the action of ACh
What is the characteristic of myasthenia gravis?
Reduced number of ACh receptors
What is the effect of treating myasthenia gravis with AChE inhibitors?
It increases the size of the EPP
What type of synapse is found at the neuromuscular junction?
One-to-one synapse
What is the effect of hamicholinium on the presynaptic endings?
It decreases the amount of ACh in the presynaptic endings
What is the term for the size of the end plate potential?
EPP
What is the effect of AChE inhibitors on the muscle end plate?
It increases the size of the EPP
What is the effect of the accumulation of Ca2+ in the presynaptic terminal?
Increased release of neurotransmitter
What is the primary neurotransmitter released from postganglionic sympathetic neurons?
Norepinephrine
What is the primary effect of excitatory postsynaptic potentials on the postsynaptic cell?
Depolarization
What is the enzyme responsible for metabolizing norepinephrine in the presynaptic terminal?
Monoamine oxidase (MAO)
What is the metabolite of norepinephrine associated with pheochromocytoma?
Vanillylmandelic acid (VMA)
What is the primary difference between excitatory and inhibitory neurotransmitters?
Excitatory neurotransmitters are permeable to Na+, while inhibitory neurotransmitters are permeable to Cl-
What is the site of norepinephrine synthesis?
Presynaptic terminal
What is the effect of spatial summation on the postsynaptic neuron?
It produces a greater depolarization
What is the effect of reuptake on norepinephrine in the synapse?
Decreases the amount of norepinephrine in the synapse
What is the membrane potential of the postsynaptic cell after the opening of Cl- channels?
Approximately -90 mV
What is the enzyme responsible for synthesizing epinephrine from norepinephrine in the adrenal medulla?
Phenylethanolamine N-methyltransferase
What is the receptor type that norepinephrine binds to on the postsynaptic membrane?
Alpha (α) receptor
What is the receptor subtype involved in Parkinson's disease?
D2 receptor
What is the primary function of inhibitory postsynaptic potentials?
To move the membrane potential away from threshold
What is the effect of temporal summation on the postsynaptic neuron?
It produces a greater depolarization
What is the phenomenon where the resulting postsynaptic depolarizations overlap in time and add in stepwise fashion?
Spatial summation
What is the primary function of dopamine in the hypothalamus?
Inhibiting prolactin secretion
What is the neurotransmitter formed from tryptophan?
Serotonin
What is the primary difference between EPSPs and IPSPs?
EPSPs are caused by opening of Na+ channels, while IPSPs are caused by opening of Cl- channels
What is the primary function of the postsynaptic cell?
To integrate excitatory and inhibitory inputs
What is the function of histamine in the hypothalamus?
Stimulating arousal and attention
What is the most prevalent excitatory neurotransmitter in the brain?
Glutamate
What is the type of receptor that NMDA is an example of?
Ionotropic receptor
What is the enzyme involved in the conversion of dopamine to norepinephrine?
Dopamine β-hydroxylase
Which neurotransmitter has two types of receptors, one that increases Cl- conductance and is the site of action of benzodiazepines and barbiturates, and another that increases K+ conductance?
GABA
What is the function of NO synthase in presynaptic nerve terminals?
Convert arginine to citrulline and NO
What is the function of guanylyl cyclase in various tissues, including vascular smooth muscle?
Signal transduction
What is the organization of thick and thin filaments in a sarcomere?
Interdigitating thick and thin filaments
What is the region of the sarcomere where thick filaments are present?
A band
What is the primary function of transverse tubules (T tubules) in skeletal muscle fibers?
Invagination of myofibrils
Which neurotransmitter is synthesized from glutamate by glutamate decarboxylase?
GABA
What is the primary location of glycine in the body?
Spinal cord and brain stem
What is the role of troponin C in the thin filament?
Permits the interaction of actin and myosin when bound to Ca2+
What is the function of the transverse tubules in skeletal muscle?
To carry the depolarization from the sarcolemmal membrane to the cell interior
What is the site of Ca2+ storage and release for excitation-contraction coupling?
Sarcoplasmic reticulum
What is the function of the dihydropyridine receptor in the transverse tubules?
To cause a conformational change in response to depolarization
What is the location of the transverse tubules in skeletal muscle?
At the junctions of A bands and I bands
What is the composition of the thin filament?
Actin, tropomyosin, and troponin
What is the function of troponin I in the thin filament?
Inhibits the interaction of actin and myosin
What is the function of troponin T in the thin filament?
Attaches the troponin complex to tropomyosin
What is the organization of the thick and thin filaments in the A band?
The thin filaments are anchored at the Z lines, and the thick filaments are present in the A bands
What is the primary function of the Ca2+-AlPase pump in the sarcoplasmic reticulum?
To transport Ca2+ from the intracellular fluid into the SR
What is the effect of depolarization of the T tubules on the ryanodine receptors?
It opens the Ca2+ release channels
What is the role of calsequestrin in the SR?
To bind tightly to Ca2+ in the SR
What is the consequence of increased intracellular [Ca2+] on the muscle contraction?
Increased muscle contraction
What is the function of the ryanodine receptor in the SR?
To release Ca2+ from the SR into the intracellular fluid
What is the effect of Ca2+ binding to troponin C on the thin filaments?
It moves tropomyosin out of the way
What is the sequence of events that leads to muscle contraction?
Depolarization of T tubules -> opening of Ca2+ release channels -> Ca2+ binding to troponin C -> muscle contraction
What is the role of the dihydropyridine receptor in the T tubules?
To open the Ca2+ release channels in the SR
What is the function of Troponin C in the troponin complex?
Permits the interaction of actin and myosin when bound to Ca2+
What is the purpose of the dihydropyridine receptor in the T-tubules?
To cause a conformational change in response to depolarization
Where are the T-tubules located in the skeletal muscle?
At the junctions of A bands and I bands
What is the function of the thin filaments in the sarcomere?
To interact with myosin during muscle contraction
What is the purpose of the sarcoplasmic reticulum in the skeletal muscle?
To store and release Ca2+ for excitation-contraction coupling
What is the function of Troponin I in the troponin complex?
Inhibits the interaction of actin and myosin
What is the arrangement of thick and thin filaments in the A band?
Thin filaments are anchored at the Z lines and interdigitate with thick filaments
What is the structure that carries the depolarization from the sarcolemmal membrane to the cell interior?
T-tubules
What is the function of the troponin complex in the thin filaments?
To permit the interaction of actin and myosin when bound to Ca2+
What is the primary function of the Cai+-AlPase in the sarcoplasmic reticulum?
To transport Ca2+ from the intracellular fluid into the SR interior
What is the result of the conformational change in troponin after binding to Ca2+?
Tropomyosin moves out of the way of the actin filaments
What is the role of the dihydropyridine receptor in the T tubules?
To open Ca2+ release channels in the SR
What is the result of the increased intracellular Ca2+ concentration?
The muscle contracts
What is the role of calsequestrin in the SR?
To bind Ca2+ loosely
What is the sequence of events that leads to muscle contraction?
Depolarization of the T tubules → Ca2+ release from the SR → Ca2+ binding to troponin C → Cross-bridge cycling
What is the role of the ryanodine receptor in the SR?
To release Ca2+ from the SR into the intracellular fluid
What is the result of the binding of Ca2+ to troponin C?
Tropomyosin moves out of the way of the actin filaments
What is the location of the Ca2+ release channels in the SR?
In the SR membrane
Study Notes
Cell Membranes and Transport
- Cell membranes are composed primarily of phospholipids and proteins.
- Lipid bilayer:
- Phospholipids have a glycerol backbone (hydrophilic head) and two fatty acid tails (hydrophobic).
- Hydrophobic tails face each other, forming a bilayer.
- Lipid-soluble substances (e.g., O2, CO2, steroid hormones) can cross cell membranes because they can dissolve in the hydrophobic lipid bilayer.
- Proteins:
- Integral proteins: anchored to and imbedded in the cell membrane through hydrophobic interactions, may span the cell membrane, and include ion channels, transport proteins, receptors, and GTP-binding proteins.
- Peripheral proteins: not imbedded in the cell membrane, not covalently bound to membrane components, and loosely attached to the cell membrane by electrostatic interactions.
Intercellular Connections
- Tight junctions (zonula occludens): attachments between cells (often epithelial cells), may be an intercellular pathway for solutes, and may be "tight" (impermeable) or "leaky" (permeable).
Transport Across Cell Membranes
- Simple diffusion:
- Characteristics: non-carrier mediated, occurs down an electrochemical gradient, does not require metabolic energy, and is passive.
- Example: urea concentration gradient between blood and proximal tubular fluid.
- Carrier-mediated transport:
- Characteristics: stereospecificity, saturation, and competition.
- Types: facilitated diffusion, primary active transport, and secondary active transport.
- Facilitated diffusion:
- Characteristics: occurs down an electrochemical gradient, does not require metabolic energy, and is carrier mediated.
- Example: glucose transport in muscle and adipose cells.
- Primary active transport:
- Characteristics: occurs against an electrochemical gradient, requires direct input of metabolic energy, and is carrier mediated.
- Examples: Na+, K+-ATPase (Na+-K+ pump), Ca2+-ATPase (Ca2+ pump), and H+-ATPase (proton pump).
- Secondary active transport:
- Characteristics: transport of two or more solutes is coupled, one solute is transported "downhill" and provides energy for the "uphill" transport of the other solute(s), and metabolic energy is not provided directly.
- Examples: Na+-glucose cotransport (symport) and Na+-Ca2+ exchange (antiport).
Diffusion Potentials and Equilibrium Potentials
- Diffusion potential: a potential difference established across a membrane due to the diffusion of ions down their concentration gradients.
- Equilibrium potential: the potential difference at which the chemical and electrical driving forces on an ion are equal and opposite, and there is no net diffusion of the ion.
- Nernst equation: used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane.
Action Potentials
- Definitions:
- Depolarization: makes the membrane potential less negative.
- Hyperpolarization: makes the membrane potential more negative.
- Inward current: flow of positive charge into the cell.
- Outward current: flow of positive charge out of the cell.
- Action potential: a rapid depolarization followed by repolarization of the membrane potential.
- Driving force and current flow:
- The driving force on an ion is the difference between the actual membrane potential and the ion's equilibrium potential.
- Current flow occurs if there is a driving force on the ion and the membrane is permeable to the ion.
- Resting membrane potential:
- Established by diffusion potentials that result from concentration differences of permeant ions.
- Each permeable ion attempts to drive the membrane potential toward its equilibrium potential.
- Ionic basis of the nerve action potential:
- Resting membrane potential: approximately -70 mV, cell negative, due to high resting conductance to K+.
- Depolarization: Na+ channels open, and Na+ conductance increases.
- Repolarization: Na+ channels close, and K+ conductance increases.
Neuromuscular Junction
- Synapse between axons of motoneurons and skeletal muscle.
- Neurotransmitter: acetylcholine (ACh).
- Postsynaptic membrane: contains a nicotinic receptor.
- Synthesis and storage of ACh in the presynaptic terminal:
- Choline acetyltransferase catalyzes the formation of ACh from acetyl coenzyme A and choline.
- ACh is stored in synaptic vesicles with ATP and proteoglycan for later release.### Repolarization and Refractory Periods
- Repolarization is caused by an outward K+ current and undershoot (hyperpolarizing afterpotential)
- During this period, the membrane potential is driven very close to the K+ equilibrium potential
- There are three types of refractory periods:
- Absolute refractory period: no action potential can be elicited, no matter how large the stimulus
- Relative refractory period: an action potential can be elicited, but only if a larger than usual inward current is provided
- Accommodation: depolarization closes inactivation gates on Na+ channels, making it difficult to generate an action potential
Propagation of Action Potentials
- Action potentials occur by the spread of local currents to adjacent areas of the membrane
- Local currents depolarize adjacent areas of the membrane to threshold, generating action potentials
- The conduction velocity is increased by:
- Increasing fiber size
- Myelination: myelin acts as an insulator around nerve axons, increasing conduction velocity
Neuromuscular and Synaptic Transmission
- Chemical synapses have the following characteristics:
- An action potential in the presynaptic cell causes depolarization of the presynaptic terminal
- Ca2+ increases, which releases ACh into the synaptic cleft
- ACh binds to nicotinic receptors on the postsynaptic membrane, opening Na+ and K+ channels
- The postsynaptic membrane is depolarized to a value halfway between the Na+ and K+ equilibrium potentials
- End plate potential (EPP) is a depolarization of the postsynaptic membrane
- The EPP is not an action potential, but a depolarization that can generate an action potential if it reaches threshold
Synaptic Transmission (continued)
- ACh is removed from the synapse by:
- Diffusion
- Breakdown by AChE
- Reuptake by the presynaptic terminal
- Diseases that affect synaptic transmission:
- Myasthenia gravis: antibodies to the ACh receptor reduce the number of receptors, leading to muscle weakness and fatigability
- Treatment: AChE inhibitors (e.g., neostigmine) increase the size of the EPP
Neurotransmitters
- ACh:
- Released from the presynaptic terminal
- Binds to nicotinic receptors on the postsynaptic membrane
- Norepinephrine:
- Released from postganglionic sympathetic neurons
- Binds to a or β receptors on the postsynaptic membrane
- Dopamine:
- Released from midbrain neurons
- Binds to D1 or D2 receptors on the postsynaptic membrane
- Serotonin:
- Released from brain stem neurons
- Binds to 5-HT receptors on the postsynaptic membrane
- Glutamate:
- Released from excitatory neurons
- Binds to NMDA or AMPA receptors on the postsynaptic membrane
- GABA:
- Released from inhibitory neurons
- Binds to GABAA receptors on the postsynaptic membrane
- Glycine:
- Released from inhibitory neurons
- Binds to glycine receptors on the postsynaptic membrane
Skeletal Muscle
- Structure:
- Each muscle fiber is multinucleate and behaves as a single unit
- Contains bundles of myofibrils, surrounded by SR and invaginated by T tubules
- Filaments:
- Thick filaments: present in the A band, contain myosin
- Thin filaments: anchored at the Z lines, contain actin, tropomyosin, and troponin
- Excitation-contraction coupling:
- Action potentials in the muscle cell membrane initiate depolarization of the T tubules
- Depolarization of the T tubules causes a conformational change in the dihydropyridine receptor, opening Ca2+ release channels in the SR
- Ca2+ binds to troponin C on the thin filaments, causing a conformational change in troponin that moves tropomyosin out of the way
- Cross-bridge cycling occurs, producing shortening and force generation
Learn about the composition and structure of cell membranes, including the lipid bilayer and its properties. Understand how different substances interact with the membrane and how they cross it.
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