Tendon Jerk Reflex Practical

A brisk tap is applied to the Achilles tendon. This excites triceps surae muscle spindles and causes a reflex contraction of the muscles via activity in a monosynaptic reflex arc. Such a response is usually called a tendon jerk or T-reflex (or in this particular case an ankle jerk)

Briefly describe Experiment 1 of the practical: Unconditioned Ankle Jerk (relaxed subject)

A brisk tap is applied to the Achilles tendon, eliciting the T-reflex whilst the subject contracts other muscle groups. Such a procedure is called conditioning and should lead to change in the activity in descending motor pathways (descending drive) which may alter the reflex response

Briefly describe Experiment 2 of the practical: Conditioned Ankle Jerk (subject clenching)

The subject makes a voluntary movement of the same muscle (triceps surae) without the initiation of the reflex. This enables comparisons to be made between the results from experiments 1 and 2 with voluntary activation of the triceps surae muscles.

Briefly describe Experiment 3 of the practical: Voluntary Response

The tap stretches the triceps surae muscles. This stretch is detected by muscle spindle organs which are embedded within these muscles. When the muscle spindle organs are stimulated, a reflex stimulation of the calf muscles occurs. The calf muscles respond with a brief contraction resulting in plantar flexion of the foot

Describe what occurs in the body when the knee jerk reflex has been initiated by tapping the Achilles tendon

There are 2 adhesive electrodes placed over the calf muscles. One is placed in the middle of the leg and the other is placed 5cm above where the Achilles tendon meets the muscle. There is a 3rd adhesive electrode placed just below the back of the knee to earth the subject

How are the adhesive electrodes used in this experiment

The longer the length of the afferent and efferent pathways (and therefore the longer the length of the individual's leg), the longer the response latency

How does the response latency of a monosynaptic reflex vary

The calf muscles contract to restore the original length of the stretched muscles

How is the stretch reflex arc achieved in the case of the ankle jerk

Quadriceps of the thigh (knee jerk) Calf muscles of the lower leg (ankle jerk)

In what muscles is it easiest to study stretch reflexes

A reflex pathway that only involves one synaptic relay in the spinal cord (CNS). The simplest stretch reflex pathway. Has a very short reflex latency

Monosynaptic Reflex

Myotatic Reflexes Tendon Jerks T-Reflexes)

Other names for stretch reflex

Knee jerk reflex

The tapping of the Achilles tendon initiates which reflex

The action potentials occurring in muscle fibres making up those motor units activated by the nerve stimulation (i.e. events happening before excitation-contraction coupling)

What does the electromyogram (EMG) represent

A record of the massed action potentials that are conducted relatively slowly (3-5 m/s) from the neuromuscular junction along skeletal muscle fibres

What is an electromyogram (EMG)

A negative feedback pathway that acts to control muscle length by preventing the overstretch of a muscle if its tendon (and therefore the muscle) is stretched

What is the stretch reflex arc

To clean the surface of the skin over the muscle before attaching the adhesive recording electrodes. This removes skin oil which conducts electrical signals poorly and therefore degrades the EMG signal

What is the surgical spirit used for in this experiment

Used to measure the approx distance in mm from the mid calf electrode to the spinal column at the level of the waist

What is the tape measure used for in this experiment

To ensure that a visible ankle jerk is readily elicited from the subject

What is the tendon hammer used for in this experiment

An electromyogram (EMG)

What would be used to monitor the level of activation of muscle cells in skeletal muscles

Embedded in the triceps surae muscles

Where are muscle spindle organs found

In all skeletal muscles

Where can stretch reflexes be elicited

The POS lead connects to the mid-calf electrode. The NEG lead connects near the ankle. The green EARTH lead attaches to the electrode just below the knee

Where do the recording electrodes connect in this experiment

Stretch reflexes are important in the maintenance of posture. They are responsible for maintaining a steady contraction in response to stretching of certain muscles as they support the body against the force of gravity

Why are stretch reflexes important