Why reflex arc is important

The relay neuron in turn makes a synapse with one or more motor neurons that transmit the impulse to the muscles of the limb causing them to contract and pull away from the sharp object. 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.

By contrast, in polysynaptic reflex arcs, one or more interneurons connect afferent sensory and efferent motor signals. For example, the withdrawal reflex nociceptive or flexor withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli.

It causes the stimulation of sensory, association, and motor neurons. Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex. The stretch reflex myotatic reflex is a muscle contraction in response to stretching within the muscle. This reflex has the shortest latency of all spinal reflexes.

It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length. When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching.

A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length. The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. The tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.

Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral. The sensory receptors for this reflex are called Golgi tendon receptors, and lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by a passive stretch or muscular contraction.

Jendrassik maneuver : The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together note the hands of the patient in the chair. This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex. The tap below the knee causes the thigh muscle to stretch.

Information is sent to the spinal cord. After one synapse in the ventral horn of the spinal cord, the information is sent back out to the muscle Tonicity of Skeletal Muscle. Therefore, to understand the control of tone it is imperative to understand function of the muscle spindle. The functional value of reflexes. The Stretch Reflex. As briefly described above the muscle spindle plays an integral role in the stretch reflex.

In brief:. As a muscle lengthens the MS is stretched. Impulses are conducted towards the CNS spinal cord where the afferent fiber divides into several colateral fibers. One of these colateral fibers stimulates the homonymous muscle same muscle that was stretched causing it to contract which in turn relieves the stretch stimulus to the muscle spindle. Simultaneously, another afferent collateral synapses with an inhibitory interneuron Renshaw cell secreting GABA which in turn synapses on the neuron innervating the antagonistic muscle opposing muscle to which was stretched.

Highly recommended — See it in action — animation of tendon tap. Animation 1. Innervation of the Muscle Spindle. Type II Fibers: 8 Microns in diameter. Gamma motor fibers. Ia primaries afferents have powerful excitatory effect on a motoneurones of same muscle and synergists in adjacent spinal segments.

May be monosynaptic or polysynaptic. Reciprocal inhibition: Ia also inhibit a motoneurones of antagonistic muscles via inhibitory interneurone and corresponding contralateral muscles. Ia afferents also have a weak polysynaptic excitatory action on dynamic and static gamma motoneurones.

Classical stretch reflex 'the capacity of a muscle to resist extension' is sum of these spindle projections to muscle. The monosynaptic Ia component is responsible for the 'tendon jerk'. The 'tonic stretch reflex' is mainly disynaptic or polysynaptic. Alpha motor neurons and motor units contribute to muscle contraction.

Cerebellar 'awareness':. After Muscle Spindle stimulation stretch and the afferent fiber enters into the spinal it divides into several colaterals. Some of these colaterals synapse on the cell bodies of neurons which ascend to the cerebellum anterior and posterior spinocerebellar tracts. 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.

There are three main types of neuron: sensory, motor and relay. These different types of neurons work together in a reflex action. A reflex action is an automatic involuntary and rapid response to a stimulus, which minimises any damage to the body from potentially harmful conditions, such as touching something hot. Reflex actions are therefore essential to the survival of many organisms. A reflex action follows this general sequence and does not involve the conscious part of the brain.