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Physiology

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Reflex action It is a spontaneous or automatic response to a stimulus ,reflexes form the basis of most of our action, they have functionally classified which appeared more purposeful
-protective:-they protect the subject
-postural:-they are responsible for the maintenance of posture
-Righting:-they maintain equilibrium of the body
-conditioned:-they form the basis of learning
-Visceral:-these reflexes control the visceral activity

Reflex arc:-reflex action is mediated through a specific path and any damage to it, abolishes the reflex response, the essential components of a reflex path are receptor, sensory nerve, centre, motor nerve,effector organ most of the reflexes are present from birth and are unconditioned reflex. however some of them can be acquired as a result of training and they are known as conditioned reflexes.

Characteristics of reflex action

reaction time:-it is the time lag between the application of the stimulus and the appearance of the response the interval ranges from(20-40)millisecond depending upon the number of synapses involved in the reflex
summation:-in the motor neuron one can observe both temporal and spatial summation. if the sub minimal stimuli are applied in rapid succession to produce a response then it is termed as temporal summation. while the production of a response by simultaneous application to sub minimal stimuli constitutes spatial summation.

Reciprocal innervation :-in the reflex action when an agonist muscle contracts .the antagonist muscle on the same side is inhibited
Cross extensor:-the flexion of one limb causes extension of the contra lateral limb, this helps in postural support when there is flexion
Stretch reflex:-type Ia-sensory fibers enter the spinal cord through the dorsal roots and give rise to branches that either terminate in the cord near their level of entry or ascend to the brain, those that terminate in the cord synapse directly(monosynaptic)with alpha motor neurons in the ventral horn that innervate extrafusal fibers in the same muscle in which the primary sensory fibers originated, this circuitary is the substrate for the stretch reflex. an important function of stretch reflex is its damping effect on jerky movement. in the absence of normally functioning spindle sensory mechanisms an unusual repetitive contraction of muscles appears(clonus)


Clinical

If a person standing on the tip ends of the feet suddenly drops his or her body downward and stretches the gastrocnemius muscles, stretch reflex impulses are transmitted from the muscle spindles into the spinal cord. These impulses reflexively excite the stretched muscle, which lifts the body up again. After a fraction of a second, the reflex contraction of the muscle dies out and the body falls again, thus stretching the spindles a second time. Again, a dynamic stretch reflex lifts the body, but this too dies out after a fraction of a second, and the body falls once more to begin a new cycle. In this way, the stretch reflex of the gastrocnemius muscle continues to oscillate, often for long periods; this is Clonus ordinarily occurs only when the stretch reflex is highly sensitized by facilitatory impulses from the brain. For instance, in a decerebrate animal, in which the stretch reflexes are highly facilitated, clonus develops readily. To determine the degree of facilitation of the spinal cord, neurologists test patients for clonus by suddenly stretching a muscle and applying a steady stretching force to it.

N.B
(Decerbrate animal in which the brain stem is transected in the middle to lower part of mencephalon,which blocks normal inhibitory signals from higher centres of the brain to the pontile and vestibular muscle control nuclei this allows the nuclei to become tonically active)

Applications of the Stretch Reflex

Almost every time a clinician performs a physical examination on a patient, he or she elicits multiple stretch reflexes. The purpose is to determine how much background excitation, or tone, the brain is sending to the spinal cord. This reflex is elicited as follows.

Knee Jerk and Other Muscle Jerks.

Clinically, a method used to determine the sensitivity of the stretch reflexes is to elicit the knee jerk and other muscle jerks. The knee jerk can be elicited by simply striking the patellar tendon with a reflex hammer; this instantaneously stretches the quadriceps muscle and excites a dynamic stretch reflex that causes the lower leg to jerk . Similar reflexes can be obtained from almost any muscle of the body either by striking the tendon of the muscle or by striking the belly of the muscle itself. In other words, sudden stretch of muscle spindles is all that is required to elicit a dynamic stretch reflex. The muscle jerks are used by neurologists to assess the degree of facilitation of spinal cord centers. When large numbers of facilitatory impulses are being transmitted from the upper regions of the central nervous system into the cord, the muscle jerks are greatly exaggerated. Conversely, if the facilitatory impulses are depressed or abrogated, the muscle jerks are considerably weakened or absent. These reflexes are used most frequently in determining the presence or absence of muscle spasticity caused by lesions in the motor areas of the brain or diseases that excite the bulboreticular facilitatory area of the brain stem. Ordinarily, large lesions in the motor areas of the cerebral cortex but not in the lower motor control areas (especially lesions caused by strokes or brain tumors) cause greatly exaggerated muscle jerks in the muscles on the opposite side of the body.

Flexor Reflex and the Withdrawal Reflex:-

the flexor reflex is elicited most powerfully by stimulation of pain endings, such as by a pinprick, heat, or a wound, for which reason it is also called a nociceptive reflex, or simply a pain reflex. Stimulation of touch receptors can also elicit a weaker and less prolonged flexor reflex. If some part of the body other than one of the limbs is painfully stimulated, that part will similarly be withdrawn from the stimulus, but the reflex may not be confined to flexor muscles, even though it is basically the same type of reflex. Therefore, the many patterns of these reflexes in the different areas of the body are called withdrawal reflexes.

Neuronal Mechanism of the Flexor Reflex:

The left-hand portion of Figure shows the neuronal pathways for the flexor reflex. In this instance, a painful stimulus is applied to the hand; as a result, the flexor muscles of the upper arm become excited, thus withdrawing the hand from the painful stimulus. The pathways for eliciting the flexor reflex do not pass directly to the anterior motor neurons but instead pass first into the spinal cord interneuron pool of neurons.

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About 0.2 to 0.5 second after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend. This is called the crossed extensor reflex. Extension of the opposite limb can push the entire body away from the object causing the painful stimulus in the withdrawn limb.

Neuronal Mechanism of the Crossed Extensor Reflex. The right-hand portion of Figure shows the neuronal circuit responsible for the crossed extensor reflex, demonstrating that signals from sensory nerves cross to the opposite side of the cord to excite extensor muscles. Because the crossed extensor reflex usually does not begin until 200 to 500 milliseconds after onset of the initial pain stimulus, it is certain that many interneurons are involved in the circuit between the incoming sensory neuron and the motor neurons of the opposite side of the cord responsible for the crossed extension. After the painful stimulus is removed, the crossed extensor reflex has an even longer period of after discharge than flexor reflex.








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