Visceral reflexes have been more fully discussed in the section on the autonomic nervous system. In contrast, somatic reflexes involve unconscious skeletal muscle motor responses. In doing so, these reflexes utilize some of the same lower motor neurons alpha motor neurons used to control skeletal muscle during conscious movement. Because reflexes are quick, it makes sense that somatic reflexes are often meant to protect us from injury. As examples, reflexes contribute to the maintenance of balance and rapid withdrawal of the hand or foot from damaging stimuli.
Somatic reflexes can either be intrinsic present at birth or learned. We will be focusing on intrinsic reflexes, which occur as the result of normal human development. Learned reflexes are much more complicated in their anatomical structure and result from repetitive actions, such as athletic training. One of the simplest reflexes is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle in the belly of the muscle is activated.
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.
At birth, primary motor infant reflexes arm the newborn with:. Once active, all primary infant motor reflexes advance first through basic patterns of development and than through more complex variant patterns of development. Once all levels of development have been mastered, a primary infant motor reflex is considered mature and is ready to integrate.
Once integrated, a primary infant motor reflex does not disappear, but instead becomes a subordinate component of related, more complex innate reflexes, learned reflexes, and learned reflexive activity. While the order and general time period within which a primary infant motor reflex emerges remains consistent across populations, despite cultural, environmental, and socio-economic conditions, the advancement, refinement and maturation of an innate primary infant motor reflex can be qualitatively enriched by a number of factors.
When an innate primary infant motor reflex emerges and functions as its genetic program dictates, caretakers in the environment remain nurturing and supportive without being overindulging, and an infant is provided encouragement and opportunity to actively engage, practice, and refine an innate reflex — an innate motor reflex is more likely to mature and integrate as it should.
When any one of these factors is not in place, a primary infant motor reflex might not mature and integrate, as it should. Given the integrative nature of reflexes, when any one primary infant motor reflex does not mature and integrate as it should, more complex innate and acquired reflexes, and learned reflexive activities requiring the simpler reflex to play a subordinate role in their respective function will, in turn, also become compromised.
If this chain of events is understood, restorative techniques like those created by Dr. Masgutova can be used to improve or return function by integrating the challenged reflex. Once integrated, primary motor infant reflexes are no longer actively present as they were during infancy. There are at least three reasons primary motor reflexes can re-surface after integrating life-threatening trauma physical or emotional , disease, or prolonged chronic or intermittent stress.
When this occurs, emotional and behavioral instability can occur, making it difficult for an individual to function. The same MNRI techniques used to integrate reflexes that never integrated also work to re-integrate reflexes that have re-surfaced.
Regardless of the reflex challenges that a person might face, the more you understand regarding the advancement, maturation, and integration of each primary infant motor reflex pattern, as a professional, parent, or caregiver, the more prepared you will be to identify and address reflex issues. Whether the challenges are relatively simple or complex, the earlier primary motor infant reflex issues are identified and addressed, the greater the likelihood challenges can be minimized or eliminated.
Innate motor reflex schemes are the most complex form of innate motor reflexes. They include whole body activities that involve the coordination of a particular class of less complex reflexes.
The class of motor responses included in a reflex scheme is connected by an innate set of rules called schema. The schema determines the parameters for producing different versions of the same reflex scheme. Innate somatic reflex schemes include rolling over, sitting up, crawling, walking, running, jumping and more — all maturational milestones as a child develops.
For example, as you are reading this you may be experiencing some drowsiness. We will assume that is because you have stayed up way too late! As you get tired you may have experienced the feeling of nodding off, where your head starts to fall forward followed by an almost violent jerking motion as you bring your head upright again. Your muscle spindles are key in maintaining posture, whether we are talking about nodding off in class or whether we are talking about staying upright as you walk down the street.
So, now that the muscle that was being stretched is shortened, what happens to the muscle spindle? Does it become insensitive to further changes in that muscle's length?
Remember, we said that gamma motor neurons innervate the contractile ends of the muscle spindle. As the alpha motor neurons activate extrafusal fibers, causing shortening of the muscle, gamma motor neurons activate the muscle spindle.
We refer to this as alpha-gamma co-activation. This causes the tapered ends to contract, thus maintaining a baseline tension on the central region of the muscle spindle that is sensitive to stretch. It is in this manner that the muscle spindle is able to maintain its sensitivity through a wide range of muscle length. In fact, even when a muscle is at rest the muscle spindle sends out a relatively steady stream of action potentials which helps to maintain a low level of muscle activity.
This constant tension of the muscle is what we refer to as muscle tone. Up to this point we have only addressed activation of the muscle group that is being stretched.
This is important but body movement is controlled by opposing muscle groups, the agonist and antagonist muscles. The agonist muscle is the muscle that contracts to cause a certain movement to happen and the antagonist is the muscle group that would do the opposite action.
In the example of the knee jerk reflex the quadriceps would be the agonist and the hamstring would be the antagonist.
In order to extend the leg at the knee we must contract the quadriceps, which we do via activation of the alpha motor neurons, but we must also relax, or inhibit, the hamstring. We accomplish this through a phenomenon called reciprocal inhibition. The sensory neuron that synapses with and excites alpha motor neurons supplying the quadriceps also synapses with an inhibitory interneuron. The inhibitory interneuron effectively shuts down the alpha motor neurons to the hamstring.
This allows the leg to extend at the knee. Whereas muscle spindles respond to stretch another type of sensory system responds to tension. You might think that stretch and tension are pretty much the same thing but they are not. Have you ever tried tying your shoes really tight and as you are pulling on the laces, which increases tension, one of the laces snaps?
It is pretty inconvenient when you have to replace a shoelace but think if that was your muscle! At times our muscles are capable of generating sufficient power to damage tendons or even break bones.
They can cause avulsion, where the tendon tears off a piece of the bone at its attachment site. In order to prevent this we have a safety mechanism in place called the Golgi tendon organ. Where we could consider the stretch reflex to be excitatory and cause contraction of the stretched muscle group the Golgi tendon reflex would be considered inhibitory and causes relaxation of the affected muscle.
Therefore the result of activation of a GTO would be the opposite of the activation of a muscle spindle. The main purpose of GTOs is to prevent excessive tension on tendons and thus prevents injury.
Golgi tendon organs are composed of encapsulated nerve endings that are found interwoven with collagen fibers near the transition from muscle to tendon.
These nerve endings monitor tension on the tendon rather than muscle length as muscle spindles do. As a muscle contracts it develops tension on the tendon which is detected by the GTO.
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