Reflex Arc and Sensory Receptors — Quick Reference
- Rubber mallet: the correct term for testing the patellar reflex; exam requires using “rubber mallet.” Do not write “reflex.”
- Two-point discrimination test: assess fine touch/sensory discrimination.
- Calipers: tool used for the test; be careful—the ends are sharp.
- Procedure: start at the fingertip and adjust distance until the subject feels two points; record the threshold.
- Terminology you must know for the exam:
- Patellar reflex (also called knee jerk in lay terms)
- Two-point discrimination test (not just “two point”)
- Calipers (for the test)
- Sensory receptors involved in basic tests:
- Meissner’s corpuscle (tactile corpuscle): light pressure, fine touch
- Pacinian/Lamellar corpuscle: deep pressure
- Free nerve endings: pain and temperature
- Muscle spindle: muscle stretch receptor (proprioception)
- Tendon organ (Golgi tendon organ): tendon tension receptor (proprioception)
- Sensory receptor adaptation:
- Definition: after a short period, a constant stimulus is less perceived
- Example context: penny on the skin; stimulus fades with time
- The reflex arc (five steps):
1) Receptor (site of stimulation)
2) Sensory (afferent) neuron
3) Integration center (spinal cord) – where the reflex is processed
4) Motor (efferent) neuron
5) Effector (muscle) - Interneurons and reflex types:
- Interneuron: neuron between sensory and motor neurons
- Polysynaptic reflex arc: includes an interneuron
- Monosynaptic reflex arc: no interneuron
- Most polysynaptic reflexes are in the spinal cord; monosynaptic might involve peripheral locations
- Key terms to distinguish in exams:
- Integration center vs interneuron: integrational center is in the spinal cord; interneuron is the connecting neuron in a polysynaptic arc
- Ipsilateral vs contralateral:
- Ipsilateral: same side as stimulus
- Contralateral: opposite side
- Visualizations you should know (per Canvas images):
- Meissner’s corpuscle: light touch
- Pacinian/Lamellar corpuscle: deep pressure
- Free nerve endings: pain/temperature
- Muscle spindle: stretch receptor in muscle
- Tendon organ: tension receptor in tendon
- Practical notes and exam readiness:
- Notes are legal documents; document observations carefully and contemporaneously
- If instructed to do something you disagree with, document under duress and note the supervisor’s directive
- Use precise terminology (no or minimal acronyms; e.g., motor neuron, sensory neuron; avoid “motor” alone or misspellings)
- Your practical will require identifying the correct terms from images and labeling according to textbook terms
- For the practical, you will be tested on all five steps of the reflex arc, differences between mono- and poly-synaptic arcs, and familiarity with the named receptors listed above
- Quick recap for memorize-and-recall:
- Five steps of reflex arc: Receptor → Sensory neuron → Integration center → Motor neuron → Effector
- Monosynaptic vs polysynaptic: presence of an interneuron determines polysynaptic status
- Receptors: Meissner’s (light touch), Pacinian/Lamellar (deep pressure), Free nerve endings (pain/temperature), Muscle spindle (stretch), Tendon organ (tension)
- Sensory adaptation: diminished response to a sustained stimulus
- Terminology: patellar reflex (knee jerk), two-point discrimination, calipers, rubber mallet, ipsilateral, contralateral