Neuroscience Chapter: Membrane Potential

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Questions and Answers

What characterizes graded potentials in neurons?

They involve voltage-gated channels.

They require a threshold potential to occur.

They can vary in size and fade over distance. (correct)

They are restricted to action potentials only.

Which of the following statements about graded potentials is true?

They occur only in axons.

They have an absolute refractory period.

They always cause action potentials.

They can sum algebraically. (correct)

What causes hyperpolarization in a neuron?

Efflux of K+ ions only.

Influx of Na+ ions.

Influx of Cl- ions bringing excess negative charge. (correct)

Depolarization from stimuli.

What potential must be reached for an action potential to be generated?

-55 mV Signup and view all the answers

Which of the following best describes the passive conduction of graded potentials?

They are conducted locally through current without voltage channels. Signup and view all the answers

When multiple stimuli are too far apart, what happens?

Action potentials cannot arise. Signup and view all the answers

What is primarily responsible for the generation of action potentials?

Voltage-gated Na+ channels. Signup and view all the answers

What happens to ion permeability when the threshold is reached?

It increases dramatically. Signup and view all the answers

What is the primary function of leakage channels in neuronal membranes?

To maintain resting membrane potential Signup and view all the answers

What effect does the Na+/K+ pump have on the resting membrane potential?

It helps maintain the resting membrane potential by pumping Na+ out Signup and view all the answers

What triggers the opening of voltage-gated ion channels?

A change in the potential difference across the membrane Signup and view all the answers

What results from the opening of sodium ion channels during neuronal stimulation?

Depolarization of the membrane Signup and view all the answers

How do chemically-gated channels operate?

They open when a chemical binds to a receptor Signup and view all the answers

What is meant by graded potentials?

They can either stimulate or inhibit neuronal activity Signup and view all the answers

What is the ratio of Na+ ions pumped out to K+ ions pumped in by the Na+/K+ pump?

3 Na+ out for every 2 K+ in Signup and view all the answers

What occurs during hyperpolarization of the membrane?

K+ ions flow OUT of the cell Signup and view all the answers

What is the smallest functional unit of muscle contraction?

Sarcomere Signup and view all the answers

Which of the following describes the role of actin in muscle tissue?

It serves as the binding site for myosin. Signup and view all the answers

How do myosin and actin interact to cause muscle contraction?

They slide past each other without shortening. Signup and view all the answers

What is the function of the sarcoplasmic reticulum in muscle fibers?

To store calcium ions. Signup and view all the answers

Which component of myosin is responsible for ATP splitting?

Myosin ATPase site Signup and view all the answers

What is the function of Z-lines in the muscle fiber structure?

They separate sarcomeres. Signup and view all the answers

What type of muscle fibers are described as striated and multi-nucleated?

Skeletal muscle fibers Signup and view all the answers

Which of the following statements about muscle fiber organization is incorrect?

The I-band contains only myosin. Signup and view all the answers

What is the primary effect of graded potentials on action potentials?

They increase the frequency of action potentials. Signup and view all the answers

What primarily prevents action potentials from traveling backwards along the axon?

Absolute refractory period. Signup and view all the answers

How does the amplitude of action potentials change with increased graded potentials?

It remains constant. Signup and view all the answers

What is responsible for making action potentials self-propagating?

The influx of Na+ ions at the axon hillock. Signup and view all the answers

What process occurs at nodes of Ranvier during saltatory conduction?

Action potentials jump from one node to the next. Signup and view all the answers

What is the result of Na+ ions flowing into the neuron during depolarization?

It triggers additional action potentials. Signup and view all the answers

During an absolute refractory period, which configuration do Na+ channels have?

Activation gate is closed, inactivation gate is open. Signup and view all the answers

What is the maximum frequency of action potentials that can be achieved?

Approximately 1000 / sec. Signup and view all the answers

What is the primary function of tropomyosin in muscle contraction?

To cover the binding sites on actin when the muscle is at rest Signup and view all the answers

What happens to the troponin-tropomyosin complex during muscle contraction?

It changes conformation, allowing binding sites to be exposed Signup and view all the answers

What is the role of calcium ions (Ca2+) in muscle contraction?

They bind with troponin, facilitating the movement of tropomyosin Signup and view all the answers

How is a motor unit defined?

The motor neuron and all the muscle fibers it innervates Signup and view all the answers

What is the significance of the 'all-or-nothing' principle in muscle contraction?

All fibers in a motor unit must contract if the motor neuron fires Signup and view all the answers

What does the neuromuscular junction functionally resemble?

A synapse between two neurons Signup and view all the answers

Where is calcium stored in muscle cells?

In the sarcoplasmic reticulum Signup and view all the answers

What structural feature is essential for the process of muscle contraction?

The sarcomere Signup and view all the answers

Study Notes

Membrane Structure and Function

Membranes contain leakage channels that are always open, allowing ions to flow in and out.
Sodium ions (Na+) have both a chemical and electrical gradient driving them into the cell.
Na+/K+ pumps actively transport Na+ out of cells against its gradient, maintaining resting membrane potential.
The Na+/K+ pump moves 3 Na+ out for every 2 potassium ions (K+) brought in.

Graded Potentials

Resting membrane potential primarily depends on leakage channels.
Gated ion channels open/close in response to stimuli (e.g., light, sound, chemicals).
Graded potentials result from deviations in resting membrane potential and can either depolarize or hyperpolarize the neuron.
Amplitudes of graded potentials vary, hence the term "graded".
Graded potentials arise on dendrites and cell bodies, unlike action potentials which occur along axons.
These potentials do not require a threshold potential, can fade over distance, and can sum algebraically.

Action Potentials

Action potentials are triggered when graded potentials reach the threshold potential of approximately -55mV.
Voltage-gated Na+ and K+ channels are essential for the generation of action potentials.
Once threshold is achieved, Na+ channels open, causing rapid depolarization as Na+ flows into the cell.
Following depolarization, K+ channels open leading to repolarization as K+ exits the cell.
Action potentials are self-propagating, creating a chain reaction along the axon.

Refractory Periods

An absolute refractory period ensures action potentials only travel in one direction, preventing backflow.
Frequencies of action potentials can increase to about 1000 per second but not their size (amplitude).

Saltatory Conduction

Occurs in myelinated axons where action potentials jump between nodes of Ranvier, increasing transmission speed.

Muscle Fiber Structure

Muscle fibers are multi-nucleated cells formed from fused muscle cells containing myofibrils, which are bundles of myofilaments.
Special names for muscle cell organelles: sarcolemma (membrane), sarcoplasm (cytoplasm), and sarcoplasmic reticulum (SR).

Myofibrils and Sarcomeres

Myofibrils consist of actin (thin) and myosin (thick) filaments arranged in a banded pattern.
Key structures: Z-lines, A-bands (actin and myosin overlap), I-bands (actin only), H-bands (myosin only), M-line (middle), and sarcomere (functional unit).

Myosin and Actin Interaction

Myosin molecules have globular heads with binding sites for actin and ATP. They bind to Z-lines via titin proteins.
Actin molecules create a helical structure and possess binding sites blocked by the regulatory proteins troponin and tropomyosin.
Calcium ions (Ca2+) released from the sarcoplasmic reticulum enable muscle contraction by allowing myosin to bind to actin.

Motor Units and Neuromuscular Junction

A motor unit consists of one motor neuron and all muscle fibers it innervates, functioning collectively.
The 'all-or-nothing' principle dictates that when a motor neuron fires, all fibers in the unit contract.
Each muscle fiber connects at a neuromuscular junction, which functions similarly to chemical synapses with acetylcholine as the neurotransmitter.

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Description

This quiz covers the structure and function of membranes, focusing on leakage channels, the role of sodium ions, and the functioning of Na+/K+ pumps. Additionally, it explores graded potentials, their characteristics, and how they differ from action potentials in neurons.