BIO NMDCAT

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.

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.

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.

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

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 efferent (motor) pathway carries signals away from the CNS to the target effectors (muscles or glands). The afferent (sensory) pathway carries signals toward the CNS.

The patellar reflex is a monosynaptic spinal stretch reflex mediated specifically by the femoral nerve and the lumbar spinal cord segments L2, L3, and L4.

The Na⁺/K⁺ pump is an active transport ATPase that hydrolyzes ATP to move sodium and potassium against their concentration gradients, consuming a massive portion of the neuron's metabolic energy.

Conduction velocity is positively correlated with both axon diameter (reducing internal electrical resistance) and the presence of myelination (allowing fast saltatory conduction).

Synaptic fatigue occurs when the rate of neurotransmitter release exceeds the rate of its synthesis and recycling, causing a temporary failure of synaptic transmission despite incoming action potentials.

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.

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.

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.

Nodes of Ranvier are the unmyelinated gaps between adjacent myelin segments (internodes) where action potentials are regenerated during saltatory conduction.

Because the membrane segment immediately behind the advancing action potential is in its refractory period (Na⁺ channels are inactivated), the impulse cannot spread backward, ensuring unidirectional propagation.

Multiple sclerosis is an autoimmune disorder characterized by the progressive demyelination of axons in the CNS, which disrupts saltatory conduction and leads to severe neurological deficits.

Temporal summation occurs when a single presynaptic neuron fires multiple times in rapid succession, allowing the resulting graded potentials to add together before the previous ones dissipate.

GABA is the major inhibitory neurotransmitter in the brain, functioning primarily by opening chloride channels on postsynaptic neurons to induce hyperpolarization. Glutamate is the major excitatory neurotransmitter.

Contralateral reflex arcs (e.g., the crossed extensor reflex) involve interneurons that cross the spinal cord midline to produce a motor response on the side of the body opposite to the stimulus.