What is a function of muscles?

The marked structure is the pinna (auricle), the visible external part of the ear composed of elastic cartilage covered by skin. It forms the most lateral component of the external ear and is responsible for collecting and directing sound waves into the external auditory canal. Its characteristic folds (helix, antihelix, concha, tragus) help in sound localization by modifying sound wave direction and frequency filtering before transmission to the tympanic membrane. It plays an important role in auditory spatial awareness.

A. Tympanic membrane: The tympanic membrane (eardrum) is a thin, semi-transparent membrane located at the end of the external auditory canal. It vibrates in response to sound waves and transmits mechanical energy to the ossicles of the middle ear. Unlike the pinna, it is not externally visible and lies deep within the external ear canal, separating the external and middle ear.

B. External auditory canal: The external auditory canal is a tubular passage that extends from the pinna to the tympanic membrane. It is lined with skin containing ceruminous glands that produce earwax for protection. Its function is to conduct and slightly amplify sound waves toward the eardrum. Compared to the pinna, it is a deep canal rather than an external visible structure.

C. Pinna (auricle): The pinna is the external, cartilaginous portion of the ear that is visible on the side of the head. It functions to collect sound waves and funnel them into the external auditory canal while also aiding in sound localization by altering sound wave direction. Its unique ridges and depressions help differentiate sounds coming from different directions. Because it is the most external ear structure shown, it is the correct answer.

D. Cochlea: The cochlea is a spiral-shaped structure located in the inner ear within the temporal bone. It contains the organ of Corti, which converts mechanical sound vibrations into electrical nerve impulses. Unlike the pinna, it is deeply embedded within the skull and is responsible for hearing transduction rather than sound collection.

Neuronal excitability depends on changes in membrane potential, which is the electrical difference across the cell membrane. At rest, neurons maintain a negative resting membrane potential due to ion gradients established by the sodium-potassium pump and selective membrane permeability. Depolarization is the process by which the membrane potential becomes less negative and moves toward zero or positive values. This event is essential for initiating action potentials and allowing nerve impulse transmission along neurons.

A. Na⁺ channels close and Na⁺ ions cannot enter the cell: closure of sodium channels would prevent sodium influx and therefore maintain or reinforce the resting membrane potential. Without sodium entry, the inside of the neuron remains negatively charged relative to the outside. Depolarization specifically requires an influx of positive ions, not their exclusion. This describes inhibition of depolarization rather than its initiation.

B. K⁺ channels open and K⁺ diffuses into the cell: potassium movement typically involves efflux, not influx, during neuronal activity. When potassium channels open, K⁺ generally leaves the cell, contributing to repolarization or hyperpolarization rather than depolarization. The movement of positive ions out of the cell increases negativity inside the membrane.

C. Na⁺ channels open and Na⁺ ions diffuse into the cell: depolarization occurs when voltage-gated sodium channels open and allow Na⁺ ions to flow into the neuron. Sodium ions enter the cell down their electrochemical gradient, making the inside of the membrane less negative. This rapid influx of positive charge initiates the rising phase of the action potential. It is the fundamental event that triggers neuronal firing.

D. Chloride ions enter the cell causing hyperpolarization: chloride influx typically makes the inside of the neuron more negative, leading to hyperpolarization rather than depolarization. Increased negativity moves the membrane potential further from the threshold required for action potential generation. Chloride entry stabilizes or inhibits neuronal firing.