Neurobiology — simple reflex arcs and their chemical messengers In human physiology, simple nerve reflexes (e.g., the knee-jerk reflex) transmit signals across synapses using which class of signaling molecules?

Introduction:
Reflexes are rapid, involuntary responses that rely on fast communication between neurons. The chemical messages that jump the tiny synaptic gap from one neuron to the next are called neurotransmitters. Understanding this core concept distinguishes synaptic messengers from intracellular second messengers or signaling enzymes.

Given Data / Assumptions:

• Simple spinal reflex arcs involve sensory neurons, interneurons (often), and motor neurons.
• Neurons communicate at chemical synapses found in the spinal cord and at neuromuscular junctions.
• Signal transfer across a synapse requires a chemical released from the presynaptic terminal that binds receptors on the postsynaptic membrane.

Concept / Approach:

Neurotransmitters (such as acetylcholine, glutamate, GABA, and glycine) are stored in synaptic vesicles. An arriving action potential triggers Ca2+ influx, vesicle fusion, and transmitter release into the synaptic cleft, where it binds postsynaptic receptors to excite or inhibit the next cell, enabling the split-second timing needed for reflexes.

Step-by-Step Solution:

Verification / Alternative check:

Electrophysiology shows miniature end-plate potentials caused by quantal neurotransmitter release. Pharmacology of reflexes (e.g., effects of curare on acetylcholine receptors) further supports the central role of neurotransmitters.

Why Other Options Are Wrong:

Nitric oxide is a gaseous modulator but is not the principal fast synaptic transmitter in simple reflex arcs.

G proteins are intracellular transducers coupled to receptors; they are not released across synapses.

Proteases are enzymes that break down proteins, not primary neural synaptic messengers.

Second messengers like cAMP act inside cells; they are not secreted into blood to mediate reflex synapses.

Common Pitfalls:

Confusing neurotransmitters (intercellular at synapses) with second messengers (intracellular), or assuming all signaling molecules in neurons are transmitters.

Final Answer:

Neurotransmitters