What is the correct sequence of events that occurs along an axon after the membrane reaches threshold?
The membrane depolarizes.
Sodium channels open and sodium ions diffuse inward.
The membrane repolarizes.
Potassium channels open and potassium ions diffuse outward.
The Correct Answer is B,A,D,C
An action potential is a rapid, transient electrical signal that travels along the axon of a neuron. It is initiated when the membrane potential reaches a threshold level, triggering voltage-gated ion channels. This process involves a precise sequence of ion movements that change the membrane potential from resting state to depolarization and back to resting state. The coordinated opening and closing of sodium and potassium channels ensures unidirectional propagation of the nerve impulse.
1. The membrane depolarizes: This step is part of the rising phase of the action potential, where the inside of the neuron becomes less negative. However, depolarization does not occur first after threshold is reached; it is the result of sodium influx. Therefore, it follows the opening of sodium channels rather than preceding it in the sequence.
2. Sodium channels open and sodium ions diffuse inward: This is the correct initiating event after threshold is reached. Voltage-gated sodium channels open rapidly, allowing Na⁺ to flow into the axon down its electrochemical gradient. This influx of positive charge triggers the rapid depolarization phase of the action potential. It is the first event in the sequence.
3. The membrane repolarizes: Repolarization occurs after the peak of the action potential when sodium channels become inactivated and potassium channels remain open. The membrane potential returns toward its resting negative value. This step happens after potassium efflux has begun, not before it.
4. Potassium channels open and potassium ions diffuse outward: This step is responsible for restoring the negative membrane potential after depolarization. Voltage-gated potassium channels open more slowly than sodium channels, allowing K⁺ to exit the cell. This outward movement of positive ions contributes to repolarization and often leads to a brief hyperpolarization before resting potential is restored.
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Related Questions
Correct Answer is A
Explanation
The nucleus is a membrane-bound organelle found in eukaryotic cells that serves as the control center of the cell. It contains the cell’s genetic material in the form of DNA organized into chromosomes. Through gene expression and regulation, the nucleus directs cellular growth, metabolism, protein synthesis, and division. It is essential for maintaining cellular identity and coordinating all major cellular functions.
A. To store genetic information and control cellular activities: the nucleus houses DNA, which contains the genetic instructions required for all cellular functions. It regulates gene expression through transcription, producing messenger RNA that guides protein synthesis. The nucleus also controls the cell cycle and coordinates replication during cell division. As the central regulatory organelle, it determines overall cellular activity and inheritance.
B. To produce energy for the cell: energy production occurs primarily in the mitochondria, not the nucleus. Mitochondria generate ATP through oxidative phosphorylation and the electron transport chain. The nucleus does not participate directly in metabolic energy production.
C. To synthesize proteins for the cell: protein synthesis occurs in ribosomes, either free in the cytoplasm or attached to the rough endoplasmic reticulum. While the nucleus contains the genetic instructions for protein synthesis, it does not directly assemble proteins. Instead, it produces mRNA that is transported to ribosomes for translation.
D. To break down waste materials: waste degradation is primarily the function of lysosomes. Lysosomes contain hydrolytic enzymes that digest cellular debris, damaged organelles, and foreign material. The nucleus does not participate in catabolic processes or cellular digestion.
Correct Answer is D
Explanation
The marked structure is the lens, a transparent, biconvex, avascular structure located posterior to the iris and anterior to the vitreous body. It is suspended by zonular fibers (suspensory ligaments) attached to the ciliary body. The lens plays a critical role in vision by providing fine focusing of light onto the retina through the process of accommodation. Its curvature changes depending on whether the eye is focusing on near or distant objects, allowing precise image formation.
A. Cornea: The cornea is the transparent, dome-shaped anterior surface of the eye that provides most of the eye’s refractive power. It is the first structure that light passes through and contributes significantly to bending light toward the retina. Unlike the lens, it is external, fixed in shape, and continuous with the sclera, serving both protective and optical roles.
B. Retina: The retina is the innermost neural layer lining the posterior aspect of the eye. It contains photoreceptor cells (rods and cones) that convert light into electrical signals sent via the optic nerve to the brain. Unlike the lens, it does not focus light but instead processes visual information after image formation occurs.
C. Iris: The iris is the pigmented muscular structure located anterior to the lens and posterior to the cornea. It regulates pupil size to control the amount of light entering the eye through contraction and relaxation of its smooth muscles. Unlike the lens, it does not contribute to focusing light but only regulates light entry.
D. Lens: The lens is a transparent, flexible, biconvex structure located directly behind the iris. It fine-tunes the focusing of light rays onto the retina through accommodation, changing its curvature via the ciliary muscles. It plays a key role in sharp image formation at varying distances. Its central posterior position relative to the iris makes it the correct structure.
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