Question

Using approximately 400-500 words, describe what happens when the doctor strikes your knee with the rubber hammer tool. What concepts are applied? Where else can a reflex be tested and what nerves are being activated?

Answer 1

When the doctor strikes the knee with the rubber hammer, the knee undergoes extension. This is an example of tendon reflex which is a deep stretch reflex. Stretch Reflex is the most simplest reflex in the body. They are invoked because of stroking a tendon, therefore they are deep. Stretch reflex is the monosynaptic reflex which means stretch induced contraction. As in the case of knee jerk, when the tendon of the quadriceps femoris is stopped midway between the origin and insertion with the hammer, quadriceps femoris undergoes contraction which leads to extension of the knee. The nerve involved in knee jerk is Femoral nerve (L2-L4).

• Type: monosynaptic
• Stimulus: stretch
• Reaction time: 19-24 msec.
• Central delay: time taken for the reflex activity to traverse the spinal cord in a stretch is 0.6-0.9 msec
• Stretch reflex is well developed in antigravity muscles such as extensor group of muscles of legs and flexor groups of muscles of arm.
• Examples are knee-jerk, ankle jerk, biceps jerk and triceps jerk.

Reflex arc of stretch reflex:

• Afferent limb: consist of receptor and afferent nerve. Receptor for stretch reflex is muscle spindle. It detects the degree and rate of muscle stretch. Afferent nerve Ia and II supply the muscle spindle. They travel along the spinal nerve and enter the spinal cord through the dorsal root and the centre branches to to every Alpha motor neuron that goes to the muscle from which the Ia is originated.
• Centre: Ventral Gray Horn of spinal cord.
• Efferent limb: the axons of Alpha motor neurons form the different nerve fibres which leave the spinal cord through the ventral root and supply the skeletal muscle fibres.
• Effector organs: both extensor and flexor muscles exhibit stretch reflexes and this form effector organ.

Muscle spindle have nuclear bag fibres and nuclear chain fibres. That are innervated by plate endings and trail endings respectively of gamma motor neurons. Nuclear bag fibres are used for dynamic muscle length changes. Nuclear chain fibres are used for stretched length of muscle.

The stretch reflex is characterized by the reciprocal innervation i.e. excitation of one group of muscles is associated with the inhibition of the antagonistic group of muscles on the same side, allowing the agonistic muscles to contract without interference. A collateral from each Ia fibre passes in the spinal cord to an inhibitory interneuron (Golgi bottle neuron) that synapses directly on one of the motor neurons supplying the antagonistic muscles.

Ankle jerk: With foot slightly elevated and dorsiflexed strike on the tendo-Achilles. Plantar flexion of the foot occurs due to contraction of the calf muscles. Due to S1 and S2.

Triceps jerk: keep the foream of the subjected to rest across his chest. Then type the triceps tendon with the broader side of the patellar hammer. Contraction of the triceps with extension of elbow. Due to C6 and C7.