MCQs

Parkinson's disease is characterized by the selective death of dopaminergic neurons in the substantia nigra of the midbrain, leading to tremors, rigidity, and loss of motor control.

Acetylcholinesterase (AChE) is the enzyme located in the synaptic cleft that rapidly hydrolyzes ACh into acetate and choline, terminating the synaptic signal to prevent continuous stimulation.

Spatial summation is the additive effect of graded potentials occurring at different physical locations (synapses) on the neuron at the same time. Temporal summation occurs at a single synapse over time.

Opening Cl⁻ channels (allowing Cl⁻ influx) or K⁺ channels (allowing K⁺ efflux) leads to hyperpolarization, moving the membrane potential further away from the threshold and inhibiting the neuron.

An EPSP involves the opening of cation channels (usually Na⁺), leading to a localized, graded depolarization that makes the postsynaptic neuron more likely to fire an action potential.

Neurotransmitters bind to specific ligand-gated receptors on the postsynaptic membrane, which undergo a conformational change to open ion channels and generate graded potentials.

Acetylcholine (ACh) is the primary neurotransmitter of the parasympathetic nervous system (both pre- and postganglionic), while the sympathetic system predominantly relies on norepinephrine for postganglionic signaling.

The arrival of an action potential at the synaptic knob opens voltage-gated Ca²⁺ channels. The resulting Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane and release their contents.

At rest, sodium leak channels are very rare compared to abundant potassium leak channels, making the resting membrane highly impermeable to Na+ and highly permeable to K+.

Once the threshold voltage is reached, the action potential occurs with a constant, maximum amplitude independent of stimulus strength. Stronger stimuli increase the frequency of impulses, not their amplitude.

A nerve fiber consists of an axon (axis cylinder) along with its surrounding myelin sheath and neurilemma (Schwann cell sheath). The term does not typically include the cell body or dendrites.

Cranial nerves carrying taste information (VII, IX, X) project first to the gustatory nucleus within the nucleus of the solitary tract in the medulla oblongata before relaying to the thalamus.

Nearsightedness (myopia) occurs when the eyeball is too long or the lens is too strong, causing images of distant objects to focus in front of the retina, resulting in blurred distance vision.

The organ of Corti sits atop the basilar membrane. Sound waves displace the basilar membrane, causing the hair cells to push against the overlying stationary tectorial membrane, resulting in stereocilia deflection.

Cold receptors are most active between 25°C and 30°C. Below 10°C, cold receptors become less active and act as a local anesthetic, while temperatures above 45°C activate pain receptors (nociceptors) rather than warm receptors.

The otolithic membrane is the gelatinous layer studded with heavy otoliths. When the head tilts, gravity pulls this membrane, bending the hair cells of the macula. The cupula is in the semicircular canals, and the tectorial membrane is in the cochlea.

Severed afferent nerve pathways in the stump can generate spontaneous action potentials. The brain, relying on the labeled line code, interprets these signals as originating from the receptors that used to be in the amputated limb.

Conduction velocity increases with axon diameter and myelination. A-alpha fibers (carrying proprioceptive information from muscle spindles) are the largest and most heavily myelinated, allowing the fastest signal transmission.

The dark pigment epithelium absorbs stray light (preventing reflection/blurring) and plays a critical biochemical role in storing Vitamin A and reconverting all-trans-retinal back to 11-cis-retinal for the photoreceptors.