How many synapses are in the integrating center

Tapping the patellar tendon pulls and stretches the quadriceps muscle and causes the sensory receptor of the muscle, called a muscle spindle fiber , to send a signal along the afferent neuron to the spinal cord Figure 2. This causes the efferent neuron to return a signal to the quadriceps muscle to contract and lift the lower leg. This action resists the initial stretch and is a classic example of negative feedback.

Muscle spindles, sensory structures which contain intrafusal muscle fibers , are innervated by sensory neurons and are arranged in parallel to normal contractile , extrafusal , muscle fibers. Intrafusal fibers are not contractile where they associate with the sensory neuron. Intrafusal fibers respond to tension in the tissue by depolarizing a sensory neuron.

The sensory neuron synapses with a motor neuron in the spinal cord that innervates contractile extrafusal fibers. The contraction of the extrafusal fibers, that is, contraction of the belly of the muscle, releases tension on the intrafusal fibers, decreasing stimulation to neuron.

In the case of the monosynaptic knee-jerk reflex, hitting the patellar tendon with a mallet stretches the intrafusal fibers of the spindles in the quadriceps muscle, leading to contraction. In parallel, an inhibitory impulse is sent from the spinal cord to cause relaxation of the hamstring muscles, via a polysynaptic pathway.

It is important to note that, even with the simplest of reflexes, there are multiple inhibitory and stimulatory influences that can affect the excitability of the motor neuron.

These can amplify or suppress a reflex response and may somewhat vary from subject to subject. Lesions that damage the sensory or motor fibers, or damage to the spinal cord, generally diminish a reflex unless the spinal cord has been completely transected.

Additionally, neural activity at other sites in the body may influence a reflex arc. The Jendrassik maneuver JM is a special method for reinforcement that is applied in the clinical setting when it is difficult to initiate a reflex in a patient. Sometimes, though, outside of the clinical setting, experimental outcomes with the JM maneuver can result in no effect or inhibition of a reflex. Different results when implementing the JM may be due to differences in the physiology of healthy experimental subjects versus patients in the clinical setting who are being assessed for nerve problems.

Reflex testing is of clinical value. Testing of the patellar response indicates:. Tests for simple muscle reflexes, such as the patellar reflex, are basic to any physical exam when motor nerve or spinal damage is suspected. To determine this, doctors may also test stretch reflexes in the triceps muscle and the Achilles tendon ankle-jerk reflex and compare results.

The Achilles reflex is a monosynaptic stretch reflex similar to the patellar reflex. In the Achilles reflex, the hammer taps the Achilles tendon while the foot is dorsiflexed, and the foot, in response, should jerk toward the plantar surface. Thus, at all times the cerebellum is aware of the state of stretch in muscles, in other words the TONE of muscles. Coactivation of Gamma efferents. Whenever a motor command descends from the motor cortex and synapses on neural cell bodies which innervate muscles, collaterals from these descending fibers also synapse on the corresponding cell bodies gamma efferents which innervates the ends of the intrafusal muscle fibers.

This is important so that as the extrafusal muscle fibers contract and shorten, the intrafusal also shorten and remain taunt. This enable the MS to always respond to stretch even immediately after contraction of a muscle. In other words the coactivation of gamma efferents avoids 'silent periods' which would occur if the intrafusal muscle fibers did not contract simultaneously with the extrafusal muscle fibers. Thus with gamma drive, the spindle is ready to respond to unexpected perturbation The spindle activity generates a reflex response which compensates for the perturbation.

Gripping an object. Tendon jerk is reinforced by clenching fists or jaw as the Gamma pathway is centrally facilitated rendering spindle more sensitive to stretch. Hoffmann Reflex H-Reflex technique. The H-reflex and F-wave. The H-reflex is the electrical equivalent of the monosynaptic stretch reflex and is normally obtained in only a few muscles.

It is elicited by selectively stimulating the Ia fibers of the posterior tibial or median nerve. The stimulus travels along the Ia fibers, through the dorsal root ganglion, and is transmitted across the central synapse to the anterior horn cell which fires it down along the alpha motor axon to the muscle.

The result is a motor response, usually between 0. The H-reflex can normally be seen in many muscles but is easily obtained in the soleus muscle with posterior tibial nerve stimulation at the popliteal fossa , the flexor carpi radialis muscle with median nerve stimulation at the elbow , and the quadriceps with femoral nerve stimulation.

Typically, it is first seen at low stimulation strength without any motor response preceding it. As the stimulation strength is increased, the direct motor response appears. With further increases in stimulation strengths, the M response becomes larger and the H-reflex decreases in amplitude.

When the motor response becomes maximal, the H-reflex disappears and is replaced by a small late motor response, the F-wave. H-reflex latency can be determined easily from charts, according to height and sex or from published normal values. Whatever these values however, the best normal value in localized processes is the patient's asymptomatic limb.

If no facilitation maneuvers are performed, the difference in latency between both sides should not exceed l ms. The H-reflex is useful in the diagnosis of S1 and C7 root lesions as well as the study of proximal nerve segments in either peripheral or proximal neuropathies. Its absence or abnormal latency on one side strongly indicates disease if a local process is suspected.

Much controversy remains, however, on whether its absence bilaterally in otherwise asymptomatic individuals is of any clinical significance. The F-wave is a long latency muscle action potential seen after supramaximal stimulation to a nerve. Although elicitable in a variety of muscles, it is best obtained in the small foot and hand muscles. It is generally accepted that the F-wave is elicited when the stimulus travels antidromically along the motor fibers and reaches the anterior horn cell at a critical time to depolarize it.

The response is then fired down along the axon and causes a minimal contraction of the muscle. Unlike the H-reflex, the F-wave is always preceded by a motor response and its amplitude is rather small, usually in the range of 0. The F-wave is a variable response and is obtained infrequently after nerve stimulation. Commonly, several supramaximal stimuli are needed before an F-response is seen since only few stimuli reach the anterior horn cell at the appropriate time to depolarize it.

With supramaximal stimulation however, depolarization of the entire nerve helps spread the stimulus to the pool of anterior horn cells thus enhancing its chances to reach a greater number of anterior horn cells at the critical time and produce an F-wave.

Because different anterior horn cells are activated at different times, the shape and latency of F-waves are different from one another. Conventionally, ten to twenty F-waves are obtained and the shortest latency F-wave among them is used. The normal values can be determined from charts or published data and, in unilateral lesions, the best normal values remain those of the patient's asymptomatic limb. The difference between both sides' shortest latencies should not exceed l ms.

The data obtained from the F-wave have been used in many different ways to determine proximal or distal pathology. Those include the F-wave chronodispersion or difference in latency between the F-wave with the shortest and that with the longest latency, and the F-wave ratio.

We find the F-wave ratio very useful in routine clinical work. It is obtained by dividing the conduction time of the proximal nerve segment by that of the distal nerve segment and is performed as follows:. Obtain the F-wave latency from proximal F prox stimulation knee or elbow.

Obtain the motor response likewise from proximal stimulation M prox. Then determine the latency of the proximal nerve segment by this equation:. The latency of the distal segment is none other than the motor response latency obtained from proximal stimulation M prox.

The F-ratio is then obtained by dividing the proximal latency by the distal latency:. The axon from this receptor travels to the spinal cord where it synapses with the motor neuron controlling the muscle, stimulating it to contract.

This is a rapid, monosynaptic single synapse , ipsilateral reflex that helps to maintain the length of muscles and contributes to joint stabilization. A common example of this reflex is the knee jerk reflex that is elicited by a rubber hammer striking against the patellar tendon, such as during a physical exam. When the hammer strikes, it stretches the tendon, which pulls on the quadriceps femoris muscle. Along with the monosynaptic activation of the alpha motor neuron, this reflex also includes the activation of an interneuron that inhibits the alpha motor neuron of the antagonistic muscle.

This aspect of the reflex ensures that contraction of the agonist muscle occurs unopposed. Recall from the beginning of this unit that when you touch a hot stove, you reflexively pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. To avoid further damage, information travels along the sensory fibers from the skin and into the posterior dorsal horn of the spinal cord.

Once in the spinal cord, the sensory fibers synapse with a variety of interneurons that mediate the responses of the reflex. These responses included a strong initial withdrawal of the flexor muscle caused by activation of the alpha motor neurons , inhibition of the extensor muscle mediated through inhibitory interneurons , and sustained contraction of the flexor mediated by a spinal cord neuronal circuit. Because the integration center in this reflex arc has many synapses, it is a polynaptic reflex.

And as already discussed, the sensory information will also travel to the brain to develop a conscious awareness of the situation such that conscious decision-making can take over immediately after the reflex occurs. Imagine what would happen if, when you stepped on a sharp object, it elicited a strong withdrawal reflex of your leg. You would likely topple over.