Question

Question was provided via image upload.

Explanation:

Analyze the question and visible options

The question asks: "Which correctly describes a stretch reflex?"
Let's evaluate the visible options:

1. "It is primarily mediated by the brain."

• This is incorrect. A stretch reflex (such as the patellar reflex) is a spinal reflex. It is mediated primarily by the spinal cord, allowing for an extremely rapid response without requiring conscious brain processing.

1. "It cannot occur is the spinal cord is severed." (Note: "is" is a typo in the system for "if")

• Let's analyze this statement. A stretch reflex is a localized spinal reflex arc. The sensory neuron synapses directly (monosynaptically) onto the motor neuron in the spinal cord at a specific segmental level. Because the reflex arc is entirely contained within the spinal cord segment and the peripheral nerves, it does not require pathways traveling up to or down from the brain. Therefore, if the spinal cord is severed above the level of the reflex arc (for example, a cervical injury when testing a lumbar patellar reflex), the stretch reflex still occurs (and can actually become hyperactive or hyperreflexic due to the loss of inhibitory descending signals from the brain). Thus, saying "it cannot occur if the spinal cord is severed" is physiologically incorrect because the local arc remains intact.

Let's consider the standard physiology of a stretch reflex to determine the correct answer choice that is likely cut off or represented by the options:

• A stretch reflex is a monosynaptic reflex.
• It is initiated by stretching of the muscle spindle (stretch receptors).
• It causes contraction of the stretched muscle (the agonist) and relaxation of the antagonist muscle (via reciprocal inhibition).
• It is a spinal reflex, meaning its integration center is in the spinal cord, not the brain.
• Because it is integrated in the spinal cord, it can still occur even if the spinal cord is severed (specifically, severed above the level of the reflex arc).

Let's re-evaluate the second option: "It cannot occur is the spinal cord is severed." Since this statement is false, and the first statement "It is primarily mediated by the brain" is also false, the correct answer must be one of the other standard choices for this McGraw-Hill SmartBook question.

Common correct options for this specific question in anatomy and physiology databases include:

• "It is a monosynaptic reflex."
• "It is mediated by the spinal cord."
• "It is initiated by stretch receptors (muscle spindles)."

Let's provide a clear explanation of why the visible options are incorrect, and identify the correct physiological description of a stretch reflex.

Answer:

Question 1

• (A) It is primarily mediated by the brain.
• (B) It cannot occur is the spinal cord is severed.
• (C) It is a spinal reflex integrated in the spinal cord (often monosynaptic, initiated by muscle spindles). (Correct answer)