MCQs

The medulla oblongata contains vital autonomic centers controlling respiration, cardiac activity, and vasomotor functions. The cerebrum is involved in higher mental functions, the cerebellum coordinates movement, and the hypothalamus regulates homeostasis but does not directly control these vital reflexes.

Group A fibers (like somatic motor fibers and large sensory fibers for touch and position) are the thickest, most heavily myelinated fibers in the body, providing the fastest nerve conduction velocities.

Ependymal cells are ciliated, cuboidal-to-columnar glial cells that line the fluid-filled cavities of the CNS (ventricles and central canal) and help secrete and circulate cerebrospinal fluid.

Irritability (excitability) is the physiological ability of a neuron or muscle cell to respond to a stimulus by generating an electrical signal. Conductivity refers to its ability to transmit that signal.

Hyperkalemia reduces the concentration gradient for K⁺. As a result, less K⁺ leaves the cell through leak channels, leaving more positive charge inside and shifting the resting membrane potential toward depolarization.

In the PNS, a bundle of axons is called a nerve. In contrast, a bundle of axons in the CNS is called a tract. A ganglion is a cluster of cell bodies in the PNS.

Endorphins and enkephalins are endogenous opioids that bind to opioid receptors in the brain and spinal cord, powerfully blocking the transmission and perception of pain signals.

Axoaxonic synapses occur when one axon terminal synapses onto another axon terminal. They regulate (facilitate or inhibit) the amount of calcium that enters the second terminal, modulating neurotransmitter release.

Microglia act as the resident macrophages of the central nervous system, migrating to sites of injury or infection to clear away dead tissue, plaques, and pathogens.

The autonomic (visceral) nervous system controls involuntary physiological functions by regulating the activity of smooth muscles, cardiac muscles, and secretory glands.

Unlike acetylcholine which is destroyed by enzymes, monoamine neurotransmitters (like serotonin, dopamine, and norepinephrine) are primarily cleared by active reuptake transporters bringing them back into the presynaptic knob.

Drugs like local anesthetics (e.g., lidocaine) or neurotoxins (e.g., tetrodotoxin) block voltage-gated Na⁺ channels, preventing the massive sodium influx required to generate and propagate an action potential.

The threshold potential (usually around -55 mV) is the tipping point where voltage-gated Na⁺ channels open en masse, triggering the explosive positive feedback loop of an action potential.

Following peak depolarization, voltage-gated K⁺ channels open and K⁺ rushes out of the cell down its electrochemical gradient, restoring the negative internal charge during repolarization.

Because a neuron cannot fire another action potential during the absolute refractory period regardless of stimulus strength, this period sets a strict upper limit on how frequently the neuron can fire.

Electrical synapses do not use neurotransmitters; instead, they utilize gap junctions (connexons) that physically connect the cytoplasm of adjacent cells, allowing instantaneous, bidirectional current flow.

Neuromuscular junctions (motor end plates) are the highly specialized synapses where motor neurons release acetylcholine to trigger skeletal muscle contraction.

Norepinephrine (and epinephrine from the adrenal medulla) acts on sympathetic target organs to mediate the 'fight or flight' stress responses, such as increased heart rate and bronchodilation.

Depolarization refers to any shift in membrane potential toward a less negative (or more positive) value, generally caused by the influx of positive ions like sodium.