This reflex arc shows a __________.
Diagram of a cross section of the spinal cord. There is a blue neuron labeled “Primary and secondary afferents”, which reaches the spinal cord through the posterior root. It transmits action potentials towards the spinal cord. It forms a synapse with a red neuron labeled “alpha-motor neuron”, whose axon leaves the spinal cord through the anterior root. It transmits action potentials away from the spinal cord.
This reflex arc shows a __________.
somatic reflex
polysynaptic reflex
monosynaptic reflex
visceral reflex
The correct answer and explanation is:
This reflex arc shows a monosynaptic reflex.
Explanation:
A monosynaptic reflex is a type of reflex arc that involves only one synapse between a sensory neuron (afferent) and a motor neuron (efferent). This is the simplest and fastest type of reflex, as it requires minimal processing within the central nervous system (CNS).
In the provided diagram, the primary and secondary afferents (blue neuron) detect a stimulus, such as muscle stretch, and send action potentials to the spinal cord via the posterior root. Once inside the spinal cord, the afferent neuron directly synapses with the alpha-motor neuron (red neuron) in the anterior horn of the spinal cord. The motor neuron then sends an action potential through the anterior root, leading to a rapid contraction of the target muscle.
Key Characteristics of a Monosynaptic Reflex:
- Single Synapse: There is only one synapse between the afferent and efferent neurons, making the response very fast.
- Involves Alpha-Motor Neurons: The efferent pathway includes an alpha-motor neuron, which directly stimulates skeletal muscle contraction.
- Common Example – The Stretch Reflex: A well-known monosynaptic reflex is the patellar reflex (knee-jerk reflex). When a doctor taps the patellar tendon with a reflex hammer, the quadriceps muscle stretches. This stretch is detected by muscle spindles, which activate sensory neurons that directly synapse with motor neurons, causing the muscle to contract and produce the characteristic “knee jerk.”
Why It’s Not the Other Options:
- Polysynaptic Reflex: This involves more than one synapse, usually including interneurons, making it slower than a monosynaptic reflex.
- Somatic Reflex: This is a broader category that includes both monosynaptic and polysynaptic reflexes affecting skeletal muscles.
- Visceral Reflex: This controls involuntary functions such as digestion or blood pressure, involving autonomic neurons.
Thus, the correct answer is monosynaptic reflex.