Lab 2 Review

Objective: To record a human nerve reflex impulse and determine its speed in order to understand the dynamics of the stretch reflex and its implications in motor control.

Recommended Reading: Chapter 8 (sections from Organization of the Nervous System, Cells of the Nervous System, Electrical Signals in Neurons). This chapter provides a comprehensive overview of the neural mechanisms underlying reflex actions and the structure and function of various types of neurons in the peripheral and central nervous systems.

Background Information

Stretch Reflex: The stretch reflex is an important physiological mechanism that allows the body to maintain posture and balance by automatically responding to stretching of muscles through highly specialized receptors. It provides insights into stretch receptors, nerve conduction velocity, electromyograms (EMG), and motor control. The reflex is rapid and involuntary, involving a single synapse between sensory and motor neurons, thus enabling quick responses to stimuli.

Specialized receptors located within muscle fibers, known as spindle fibers, respond to changes in muscle length and tension, transferring neural signals to spinal motor neurons. This reflex mechanism involves a single synapse, facilitating immediate muscle contraction in response to the stretching of tendons and associated muscle fibers.

The Stretch Receptor

Muscle Spindles: Muscle spindles are specialized sensory receptors that detect muscle length, tension, and pressure, thereby providing the central nervous system (CNS) with crucial information about the status of muscle contraction. These spindles are arranged parallel to muscle fibers and are activated in response to muscle stretching.

Their primary role includes:

• Maintaining muscle tone during various physical activities.

• Contributing to antigravity reflexes that help prevent falls by eliciting rapid muscle contractions in response to unexpected changes in posture or position.

• Intrafusal fibers within the spindles regulate the activity of sensory afferent nerves that convey information back to the CNS; they are primarily innervated by gamma motor neurons that adjust spindle sensitivity.

Reflex Arc Structure

Monosynaptic Stretch Reflex: This reflex comprises a direct synaptic connection between sensory afferent nerves and motor neurons, devoid of interneurons, resulting in a direct transmission of reflex signals.

Example: A typical scenario involves the reflexive action occurring when a person lands from a jump. The landing creates a stretch in the extensor muscles of the leg, which sends signals through the reflex arc, leading to an automatic contraction of those muscles to stabilize the body and prevent injury.

Equipment Required for Experiment

Computer: A PC laptop is necessary to record and process data during the experiment. It should be equipped with appropriate software to analyze EMG signals.

Experiment Procedure: Achilles Tendon Reflex

Aim: The objective is to measure the conduction time from the tendon tap to the resultant response of the gastrocnemius muscle, thereby providing insights into the efficiency of the reflex pathway.

1. Have the subject sit or kneel comfortably in a relaxed posture.

2. Locate the Achilles tendon just above the heel, identifying the precise point to elicit consistent muscle contraction through a gentle tap.

3. Employ a reflex hammer to tap the tendon and trigger the stretch reflex, ensuring to record the EMG signal during both plantar flexion and dorsiflexion actions to capture the response.

4. Instruct the subject regarding the nature of the exercise to ensure they are prepared for the reflex response, minimizing any voluntary muscle contractions that could affect the measurement.

5. Conduct a minimum of ten trials to ensure reliable and reproducible data is collected and to analyze variations in reflex conduction times.

6. Save all collected data after concluding the trials for further analysis and review.

Data Analysis Guidelines

Time Measurement: Utilize two cursors to represent T1 and T2 values on the recorded EMG signals, measuring the reflex conduction time accurately for analysis purposes.

Conduction velocities can be calculated using the provided formulas:
$\text{Mean Reflex Conduction Time (ms)} = \frac{\text{Total Time (msec)}}{\text{Number of Trials}}$$$\text{Mean Reflex Conduction Time (ms)} = \frac{\text{Total Time (msec)}}{\text{Number of Trials}}$$
$\text{Conduction Velocity (m/s)} = \frac{\text{Total path length (mm)} \times 2}{\text{Mean reflex time (msec)} - 0.5 msec}$$$\text{Conduction Velocity (m/s)} = \frac{\text{Total path length (mm)} \times 2}{\text{Mean reflex time (msec)} - 0.5 msec}$$

Questions for Discussion

1. What muscle groups are primarily involved in plantar flexion and dorsiflexion of the ankle, and what are their respective roles in maintaining balance?

2. How does reflex time vary with the strength of the stimulus, and what physiological factors contribute to this variation?

3. Design a more precise measurement experiment for reflex time that incorporates varying age groups to assess the effects of aging on reflex speed.

4. What additional synaptic inputs and pathways may influence spinal motor neuron activity beyond the excitatory inputs from stretch receptors, considering the role of descending pathways and inhibitory interneurons?