Question

multiple choice question

which reflex is mediated by the brain?

tendon
crossed-extensor
stretch

Explanation:

Identify the nature of the listed reflexes

The question asks which reflex is mediated by the brain. Reflexes can be categorized based on the integration center of their reflex arcs:

• Spinal Reflexes: Integrated in the spinal cord.
• Cranial Reflexes: Integrated in the brain.

Let's analyze the options provided:

1. Tendon reflex: A somatic reflex integrated in the spinal cord that prevents muscles from contracting with too much tension.
2. Crossed-extensor reflex: A somatic spinal reflex that helps maintain balance when a limb is withdrawn from a painful stimulus.
3. Stretch reflex: A somatic spinal reflex (such as the patellar reflex) that monitors and regulates muscle length.

All three options listed in the image (Tendon, Crossed-extensor, and Stretch) are classic examples of Spinal Reflexes integrated within the spinal cord, not the brain.

Evaluate the completeness of the options

Since all visible options (Tendon, Crossed-extensor, Stretch) are spinal reflexes, none of them are mediated by the brain. In typical anatomy and physiology curricula, reflexes mediated by the brain (cranial reflexes) include pupillary light reflexes, blinking, salivation, or swallowing.

However, looking closely at the image, there is a large blank space below the third option ("Stretch"), suggesting that the options list might be scrollable or partially cut off, or that the system expects us to identify if one of these has a cranial component. Since none of the visible options are cranial reflexes, and we must choose from the visible options if forced, we should double-check if any of these can be modulated by the brain. While the brain can influence spinal reflexes via descending tracts, their primary integration centers remain in the spinal cord.

If there is a missing option (such as a pupillary or corneal reflex), we must note that based on the visible options, all of them are spinal reflexes. However, in some simplified contexts, if no other options exist, we must evaluate if any of these have brainstem-mediated pathways. None do; they are purely spinal. Let's state the physiological classification of each visible option clearly to show they are spinal reflexes.

Answer:

Based on the visible options, all of the listed choices are integrated in the spinal cord rather than the brain:

• (A) Tendon
• (B) Crossed-extensor
• (C) Stretch