C5
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Questions and Answers

The sarcolemma is a thick membrane that surrounds myofibrils.

False (B)

Titin molecules play a role in the elastic properties of myosin and actin filaments during muscle contraction.

True (A)

Calcium ions released from the sarcoplasmic reticulum are responsible for initiating the repulsive forces between actin and myosin filaments.

False (B)

The neurotransmitter acetylcholine is released at the endings of motor nerves to stimulate muscle contraction.

<p>True (A)</p> Signup and view all the answers

The action potential in muscle fibers causes the sarcoplasmic reticulum to absorb large quantities of calcium ions.

<p>False (B)</p> Signup and view all the answers

Magnesium enhances the function of voltage-dependent channels by competing with Ca2+.

<p>False (B)</p> Signup and view all the answers

Acetylcholine esterase is found in the postsynaptic motor end plate of the neuromuscular junction.

<p>False (B)</p> Signup and view all the answers

In a synapse, one action potential in a presynaptic neuron can lead to an action potential in the postsynaptic neuron without any summation.

<p>False (B)</p> Signup and view all the answers

The binding of neurotransmitters at the neuromuscular junction is always excitatory, resulting in an end plate potential.

<p>True (A)</p> Signup and view all the answers

Smooth muscle cells possess a motor end-plate region for neurotransmitter reception.

<p>False (B)</p> Signup and view all the answers

The resultant change in membrane potential at the neuromuscular junction is considered a graded potential.

<p>False (B)</p> Signup and view all the answers

Smooth muscle contraction is regulated through direct innervation by motor neurons via a motor end-plate.

<p>False (B)</p> Signup and view all the answers

Inhibition of skeletal muscles can be achieved at the neuromuscular junction level.

<p>False (B)</p> Signup and view all the answers

The receptor potential is typically a depolarization of the afferent neuron's receptor initiated by changes in channel permeability resulting from physical injury.

<p>False (B)</p> Signup and view all the answers

Summation of EPSPs involves either temporal or spatial summation in efferent neurons and interneurons.

<p>True (A)</p> Signup and view all the answers

The end-plate potential occurs in smooth muscle due to binding of neurotransmitter to receptors on the surface membrane.

<p>False (B)</p> Signup and view all the answers

Pacemaker potential refers to a gradual depolarization of the membrane in smooth muscle and cardiac muscle due to automatic channel permeability changes.

<p>True (A)</p> Signup and view all the answers

Slow-wave potential is characterized by rapid, sustained depolarization swings in potential that always reach threshold in gastrointestinal smooth muscle.

<p>False (B)</p> Signup and view all the answers

Myasthenia gravis primarily enhances the function of acetylcholine receptor-channels.

<p>False (B)</p> Signup and view all the answers

The process of depolarization in skeletal muscle is not influenced by the binding of acetylcholine (ACh).

<p>False (B)</p> Signup and view all the answers

Efferent neurons can only experience spatial summation of EPSPs, not temporal summation.

<p>False (B)</p> Signup and view all the answers

Calcium entering at the NMJ is responsible for the release of neurotransmitters like acetylcholine.

<p>True (A)</p> Signup and view all the answers

The gradual changes in channel permeability in pacemaker potential are primarily responsible for the oscillatory nature of the depolarization.

<p>True (A)</p> Signup and view all the answers

Cardiac muscle fibers are made up of individual cells connected only in parallel with one another.

<p>False (B)</p> Signup and view all the answers

The atrial syncytium constitutes the walls of the two ventricles.

<p>False (B)</p> Signup and view all the answers

At the NMJ, the driving force of sodium is weaker than that of potassium.

<p>False (B)</p> Signup and view all the answers

Smooth muscle in the gastrointestinal tract only exhibits slow-wave potential without any influence from external factors.

<p>False (B)</p> Signup and view all the answers

The electrical activity of the heart is reflected in the ECG, which also provides information about mechanical activity.

<p>False (B)</p> Signup and view all the answers

An action potential in muscle fibers can occur below the threshold level.

<p>False (B)</p> Signup and view all the answers

The specialized conductive system responsible for conducting potentials from the atrial syncytium to the ventricular syncytium is called the A-V bundle.

<p>True (A)</p> Signup and view all the answers

The acetylcholine-gated channels allow potassium ions to enter the muscle fibers.

<p>False (B)</p> Signup and view all the answers

The end plate potential can increase the electrical potential in the muscle fiber by as much as 50 to 75 millivolts.

<p>True (A)</p> Signup and view all the answers

Pacemaker cells in the heart are incapable of generating rhythmic potentials independently.

<p>False (B)</p> Signup and view all the answers

The SA node is responsible for initiating the excitation sequence in the heart.

<p>True (A)</p> Signup and view all the answers

End plate potentials A and C are strong enough to initiate an action potential.

<p>False (B)</p> Signup and view all the answers

Acetylcholinesterase functions to cleave acetylcholine into acetate and choline.

<p>True (A)</p> Signup and view all the answers

The electrocardiogram shows simultaneous action potentials at different levels of the heart conduction system.

<p>True (A)</p> Signup and view all the answers

Nicotinic receptors are found exclusively in the brain.

<p>False (B)</p> Signup and view all the answers

The atrioventricular (A-V) node is located between the atria and the ventricles.

<p>True (A)</p> Signup and view all the answers

Repolarization of the muscle fiber occurs without the need for potassium current.

<p>False (B)</p> Signup and view all the answers

Gap junctions in cardiac muscle fibers prevent the diffusion of ions.

<p>False (B)</p> Signup and view all the answers

The Purkinje fibers are part of the heart's mechanical activity rather than electrical activity.

<p>False (B)</p> Signup and view all the answers

The summation of graded potentials is necessary for action potential initiation in muscle fibers.

<p>False (B)</p> Signup and view all the answers

Flashcards

Sarcolemma

The thin membrane surrounding a skeletal muscle fiber. It plays a crucial role in transmitting the nerve impulse that initiates muscle contraction.

Myofibrils

Thread-like structures within muscle fibers, composed of actin and myosin filaments. These filaments are responsible for muscle contraction.

Titin

A protein that anchors the myosin filament within the sarcomere, acting like a spring to maintain muscle structure.

Sarcoplasm

The fluid within the muscle fiber that surrounds the myofibrils. It contains nutrients, electrolytes, and other components essential for muscle function.

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Sarcoplasmic reticulum

A specialized type of endoplasmic reticulum found within muscle fibers, responsible for storing and releasing calcium ions that trigger muscle contraction.

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Depolarization

The process of bringing a neuron's membrane potential to a level that triggers an action potential.

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EPSPs (Excitatory Postsynaptic Potentials)

Slight depolarizations of a neuron's dendrite and cell body caused by excitatory neurotransmitters.

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Summation of EPSPs

The combination of multiple EPSPs occurring close together in time or space, leading to depolarization.

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Receptor Potential

A depolarization of a sensory neuron's receptor caused by its specific stimulus.

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End-Plate Potential

The depolarization of the motor end plate in skeletal muscle, caused by the neurotransmitter acetylcholine binding to its receptors.

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Pacemaker Potential

A gradual, spontaneous depolarization of the membrane in smooth muscle and cardiac muscle.

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Slow-Wave Potential

Alternating depolarizations and hyperpolarizations in the membrane potential of smooth muscle in the gastrointestinal tract.

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Myasthenia Gravis

A condition where the body's immune system attacks and destroys acetylcholine receptors on muscle cells.

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Threshold Potential

The point at which a neuron's membrane potential reaches a threshold that triggers an action potential.

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Action Potential

A rapid and transient electrical signal that propagates along the neuron's axon.

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Neurotransmitter Release at the NMJ

The process where calcium enters the nerve terminal and triggers the release of acetylcholine (ACh) into the synaptic cleft.

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Acetylcholine Binding

ACh binds to specific receptors located on the muscle fiber membrane, called nicotinic receptors.

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Ion Channel Opening

The binding of ACh to nicotinic receptors opens ion channels, allowing sodium (Na+) ions to rush into the muscle fiber and potassium (K+) ions to flow out.

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End-Plate Potential (EPP)

The influx of sodium ions causes a localized depolarization of the muscle fiber membrane, known as the end-plate potential (EPP).

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Muscle Fiber Action Potential

The EPP can trigger an action potential in the muscle fiber, leading to muscle contraction. This occurs if the EPP reaches the threshold for triggering an action potential.

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Neurotransmitter Clearance

The enzyme acetylcholinesterase breaks down ACh into acetate and choline, terminating its signaling effect.

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Nicotinic Receptors

Nicotinic receptors are ionotropic receptors located in the muscle fiber membrane at the neuromuscular junction.

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Graded Nature of the EPP

The EPP is a graded potential, meaning its amplitude varies depending on the amount of ACh released.

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Localized Nature of the EPP

The EPP is localized to the end-plate region of the muscle fiber, where the nerve terminal meets the muscle fiber.

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EPP Triggering Action Potential

Once the EPP reaches the threshold, it triggers an action potential that spreads through the muscle fiber, leading to muscle contraction.

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Cardiac Muscle Syncytia

Cardiac muscle cells are connected in a network, allowing electrical signals to spread rapidly, creating two syncytia: the atrial and ventricular syncytia.

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Intercalated Discs

Specialized junctions between cardiac muscle cells that allow rapid diffusion of ions, enabling synchronized contraction.

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Atrioventricular (AV) Bundle

A specialized conducting system in the heart that transmits electrical impulses from the atria to the ventricles.

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Electrocardiogram (ECG)

A recording of the electrical activity of the heart, reflecting the global activation of the heart muscle.

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Pacemaker Cells

Specialized cardiac muscle cells that have the ability to generate spontaneous rhythmic electrical impulses, controlling the heart's rhythm.

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Sinoatrial (SA) Node

The primary pacemaker of the heart, located in the right atrium, initiating the heart's electrical activity.

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Atrioventricular (AV) Node

A specialized region in the heart that delays the conduction of electrical impulses from the atria to the ventricles, ensuring proper contraction timing.

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Bundle of His

A bundle of specialized conductive fibers that transmit electrical impulses from the AV node to the ventricles, further spreading the signal.

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Purkinje Fibers

Specialized conductive fibers that branch out from the Bundle of His, distributing the electrical signal to the ventricular muscle fibers.

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Cardiac Conduction Pathway

The sequence of electrical events that take place during a heartbeat, starting at the SA node and progressing through the heart's conduction system.

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EPP Amplitude and Calcium/Magnesium Concentration

The relationship between the amplitude of the end-plate potential (EPP) and the concentration of calcium ions (Ca2+) is directly influenced by the concentration of magnesium ions (Mg2+). Magnesium competes with calcium for binding to the voltage-dependent calcium channels at the neuromuscular junction.

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Acetylcholine Esterase

The enzyme that breaks down acetylcholine (ACh) into its components (acetate and choline) at the neuromuscular junction (NMJ). It plays a crucial role in terminating the transmission of nerve impulses by removing ACh from the synaptic cleft.

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Neuromuscular Junction (NMJ)

The tiny gap between a motor neuron and a skeletal muscle fiber, where neurotransmission occurs. It is the site where the motor neuron releases acetylcholine to stimulate muscle contraction.

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Varicosities in Smooth Muscle

Specialized structures on the axon terminal of a motor neuron at the NMJ. Each varicosity contains vesicles filled with neurotransmitter, allowing for the widespread release of the neurotransmitter for coordinated muscle contraction.

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Absence of Motor End-Plate in Smooth Muscle

In smooth muscle cells, the absence of a motor end-plate, which is a specialized structure found in skeletal muscle cells.

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Neurotransmission

The process by which a nerve impulse reaches the end of a neuron and causes the release of neurotransmitters into the synaptic cleft, triggering a response in the next neuron or target cell.

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Postsynaptic Potential (EPP/EPSP/IPSP)

The change in membrane potential of a neuron or muscle cell caused by the binding of neurotransmitter to its receptors. In the NMJ, it is always excitatory, leading to muscle contraction. However, in synapses between neurons, it can be either excitatory or inhibitory.

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Synaptic Plasticity

The ability of synaptic connections to change their strength over time, allowing for learning and memory. This plasticity is not present in the NMJ.

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Study Notes

Skeletal Muscle Contraction

  • The sarcolemma is a thin membrane surrounding a skeletal muscle fiber
  • Myofibrils are composed of actin and myosin filaments
  • Titin filaments hold myosin and actin in place, extending from the Z disk to the M line
  • The sarcoplasm is the fluid between myofibrils
  • The sarcoplasmic reticulum is the specialized endoplasmic reticulum of skeletal muscle

Steps for Muscle Contraction

  • An action potential travels along a motor nerve to its endings on the muscle fiber
  • At each ending, acetylcholine is released
  • Acetylcholine acts on the muscle fiber membrane, opening acetylcholine-gated channels
  • Na+ ions diffuse into the muscle fiber, causing depolarization
  • This triggers voltage-gated sodium channels, initiating an action potential
  • The action potential travels along the muscle fiber membrane
  • The action potential depolarizes the muscle membrane, and the electricity flows through the center of the muscle fiber
  • The sarcoplasmic reticulum releases calcium ions, causing actin and myosin to interact and slide, leading to contraction
  • After this, calcium is pumped back into the sarcoplasmic reticulum, ending the contraction

Neuromuscular Junction (NMJ)

  • The NMJ is the synapse between a motor neuron and a skeletal muscle fiber
  • The motor unit is a motor neuron and the innervated muscle cells
  • A neuromuscular junction is formed from the large myelinated nerve fiber and the branching nerve terminals that invaginate into the muscle fiber surface
  • The synaptic gutter, the synaptic space or cleft in the invaginated membrane is 20 to 30 nanometers wide.

Events at the NMJ

  • Calcium enters the axon terminal and neurotransmitters are released (acetylcholine)
  • Acetylcholine binds to receptors, opening sodium and potassium channels
  • Depolarization is called an end-plate potential, a graded potential
  • Action potential in the muscle occurs once the threshold is reached. There is no summation needed
  • Neurotransmitters are cleared by acetylcholinesterase, breaking down acetylcholine into acetate and choline.
  • The muscle repolarizes, going back to the reversal potential.

End Plate Potential (EPP)

  • A sudden influx of sodium ions into the muscle fiber causes depolarization at the end plate.
  • The EPP creates a local potential called the end-plate potential.
  • It is graded (changes in amplitude), and it spreads out from the end plate.

Action Potential in Skeletal Muscle

  • Similar to nerve action potentials, but with quantitative differences
  • The resting membrane potential is about -80 to -90 mV
  • The duration of the action potential is 1 to 5 milliseconds

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C5 - Skeletal, Smooth and Cardiac Muscle (PDF)

Description

This quiz covers the essential aspects of skeletal muscle contraction, including the structure of muscle fibers and the biochemical steps leading to muscle contraction. Understand the roles of the sarcolemma, myofibrils, and neurotransmitters in the process. Gain insights into how electrical signals trigger muscle actions.

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