L20 Spinal Cord Reflexes

In this BSL Spinal Cord Reflexes lesson, students examine properties of some simple neuromuscular reflexes commonly tested in physical diagnosis.

They will record and compare

• Subject’s contractile force vs. stimulus strength
• influence of the Jendrassik maneuver
• voluntary vs. involuntary (reflex) effector responses
• Achilles’ tendon reflex and patellar tendon reflex latent periods

Physicians and other clinicians test for the functional integrity of reflex pathways to obtain objective evidence regarding the function of muscles, peripheral nerves (motor and sensory), and the central nervous system. Conclusions drawn from testing reflexes are not the sole consideration in diagnosis but rather are considered in conjunction with other symptoms and signs of pathophysiology.

Experimental Objectives

1. To become familiar with anatomical and physiological elements of simple spinal reflexes.
2. To examine properties of some simple neuromuscular reflexes commonly tested in physical diagnosis.
3. To measure and compare latent periods and reaction times of extensor and flexor reflexes.
4. To elicit an extensor reflex and compare contractile force vs. stimulus strength.
5. To apply the Jendrassik maneuver and observe exaggeration of an extensor reflex.
6. To measure and compare reaction times of voluntary activation of skeletal muscle vs. involuntary (reflex) activation of skeletal muscle.

Tasks Performed by the Student

Students will note the spinal cord reflexes in the following six scenarios:

• Knee Jerk Reflex—Subject sits with legs hanging at 90 degrees
• Knee Jerk Reflex—Subject sits and perform the Jendrassik maneuver
• Knee Jerk Reflex—Subject performs mental math with three-digit numbers
• Flexor Withdrawal Reflex—Subject is pricked by a pin or other cutaneous stimulus
• Voluntary Knee Jerk Reflex—Subject sits with eyes closed and voluntarily jerks the knee at the sound of the reflex hammer hitting a flat surface
• Ankle Jerk Reflex—Subject rests the knee and shin on the seat of a chair while Achilles tendon is struck

BSL Lessons are designed to allow at least four students to record and save data in a normal lab period (60-90 minutes). Typically, labs work most efficiently with three or more students working together at each BSL station.

Lesson Hardware

This lesson requires a Biopac Student Lab (BSL) System and the following hardware: Reflex Hammer Transducer (SS36L); the response can be recorded with electrodes (such as EL503) and/or a goniometer (such as SS21L with TAPE1 adhesive). If your BSL System does not include all hardware items, expand your system by selecting the required items below. For more details, launch BSL or BSL Student to review the BSL Lab Manual for Lesson 20 Setup.