BIO NMDCAT

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

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.

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.

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.

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.