How many neurons are in a monosynaptic reflex arc

Privacy Policy. Skip to main content. Peripheral Nervous System. Search for:. Components of a Reflex Arc A reflex arc defines the pathway by which a reflex travels—from the stimulus to sensory neuron to motor neuron to reflex muscle movement.

Learning Objectives Describe the components of a reflex arc. Key Takeaways Key Points Reflexes, or reflex actions, are involuntary, almost instantaneous movements in response to a specific stimulus. Reflex arcs that contain only two neurons, a sensory and a motor neuron, are considered monosynaptic. Examples of monosynaptic reflex arcs in humans include the patellar reflex and the Achilles reflex.

Most reflex arcs are polysynaptic, meaning multiple interneurons also called relay neurons interface between the sensory and motor neurons in the reflex pathway. Key Terms motor neuron : A neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly control muscles. There are two types of reflex arcs: autonomic reflex arc affecting inner organs and somatic reflex arc affecting muscles. Spinal Reflexes Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex.

Learning Objectives Distinguish between the types of spinal reflexes. Key Takeaways Key Points The stretch reflex is a monosynaptic reflex that regulates muscle length through neuronal stimulation at the muscle spindle. The alpha motor neurons resist stretching by causing contraction, and the gamma motor neurons control the sensitivity of the reflex. The stretch and Golgi tendon reflexes work in tandem to control muscle length and tension. Both are examples of ipsilateral reflexes, meaning the reflex occurs on the same side of the body as the stimulus.

The crossed extensor reflex is a contralateral reflex that allows the body to compensate on one side for a stimulus on the other. The withdrawal reflex and the more-specific pain withdrawal reflex involve withdrawal in response to a stimulus or pain. 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. Reflexes do not require involvement of the brain, although in some cases the brain can prevent reflex action.

Reflex arc : The path taken by the nerve impulses in a reflex is called a reflex arc. This is shown here in response to a pin in the paw of an animal, but it is equally adaptable to any situation and animal including humans. There are two types of reflex arcs:the autonomic reflex arc, affecting inner organs, and the somatic reflex arc, affecting muscles. When a reflex arc consists of only two neurons, one sensory neuron, and one motor neuron, it is defined as monosynaptic.

Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes patellar reflex, achilles reflex , brief stimulation to the muscle spindle results in the contraction of the agonist or effector muscle. After the reflex action has happened, the brain does become aware and tells you what happened. At this point, the brain might even add to the action. For example, you might have ducked as an involuntary response to a very loud noise, but when the brain becomes involved you learn why you ducked down and the brain sends the voluntary action to respond—maybe to stand back up.

In order for reflexes to work, messages need to move around the body. These messages are action potentials , and they travel along the neurons and send messages, special parts of the neurons are involved.

The neuron has three different parts that allow signals to be sensed, to travel, and then move to another neuron or muscle. These three parts are called the dendrites, the axon, and the nerve ending Figure 1. The dendrites receive information from the sensor or other neurons. This information then moves to the axon, which travels to or from the spinal cord. The action potential travels from the nerve endings at one end of the neuron to the next neuron.

Many reflexes start at the muscle or skin and go to the spinal cord. When the action potential reaches the nerve ending, the signal is transferred to another neuron, such as an interneuron or motor neuron. The action potential then travels outside the spinal cord to a muscle.

But the neurons do not touch each other in the spinal cord and do not touch at the muscle. There are tiny spaces called synapses that the action potential must jump across. Doctors will perform a test to make sure reflexes are working properly because reflexes can change if you are sick and as you grow.

Imagine you are sitting up on the exam table and the doctor taps you just below the knee with a rubber hammer. Hopefully, the doctor moved out of the way!

The response to the tap of the rubber hammer is called a knee-jerk reflex, but scientists and doctors call it a monosynaptic reflex —the simplest reflex that occurs inside your body [ 2 ].

Monosynaptic is an important word because it describes how the reflex works. When broken into two parts, the word is easier to remember. That means, in the knee-jerk reflex, there is only one point where the message transfers between neurons, so it is monosynaptic.

This monosynaptic reflex is called simple because it works through only four separate parts, whereas most reflexes work through five parts.

The five parts of most reflexes are:.