Reflexes
Reflexes are actions directed by the nervous system that do not require conscious direction or control. Reflexes are characterized as involuntary or voluntary. Reflexes designed to protect the body from injury are termed nociceptive reflexes. Such reflexes include the twitching or winking of the eye, coughing and sneezing to dislodge foreign bodies from the respiratory tract, and gagging (pharynegeal reflex) to as a mechanism to protect the respiratory and digestive tracts.
Under certain conditions it is possible for the body, as a result of experience, to develop rapid responses to certain stimuli so that rapid coordinate movements, such as those associated with running, are possible. These reflexes, however, are not the result of simplified neural pathways, but are the result of conditioning that allows the nearly automatic coordinated movements of many muscles.
Most reflexes require three different neural components within including a sensory neuron, a transmitting neuron, and a motor or effector neuron. The combined function of these neurons creates a reflex arc. Reflex arcs are most commonly associated with motor actions that allow a rapid avoidance of pain. For example, a quick jerk of an arm away from a hot object that avoids a burn is a classic example of a withdrawal reflex.
Although such actions do not require conscious thought, the transmitting neurons involved send messages to the brains pain perception neurons via the spinothalamic tract. In the case of a hot object, the voluntary muscle reflex arc begins with the mechanical receptors (thermal receptors) that provide stimulus to the transmitting neuron or neurons that, in turn, pass the neural impulse on to the motor neuron that causes the appropriate muscle movements.
The transmission of neural reflexes in reflex arcs uses the same electrical and chemical mechanisms of transmission as all other impulses. Within the neural cell body (axon), the impulse travels electrically as an action potential. At the synapse, the intercommunicating gap or space between neurons, the neural impulse is conducted through the release, diffusion, and binding of neurotransmitters. Accordingly, reflex impulses are limited to the same conduction speeds as any other form of neural impulse. Because reflexive actions are the result of simplified neural pathways as opposed to a special form of neural transmission, reflexes are also subject to rebound and fatigue.
One of the simplest forms of reflex involves a stretching response that involves only a sensory and a motor neuron. When stimulated by the extension of a particular muscle or the contraction of the muscle's antagonist muscle, the sensory neurons located in muscle spindles transmit a nerve impulse that, at the level of the spinal cord, directly stimulates the motor neurons that cause the muscle to contract. These contraction counter effects are vital to maintaining normal balance, and to prevent muscular damage from hyperextension.
Although simple in design, reflex pathways can achieve sophisticated results. In the case of the crossed extensor reflex, rapid contractions (e.g., jerking withdrawal movements) in one muscle or group of muscles also involve specific inhibition of other muscles or muscles groups required to maintain proper balance. If, for example, one places a foot on hot asphalt, jerking reflexive withdrawal of the left foot occurs via the inhibition of the quadriceps and extensors involved in advancing the foot and the rapid contraction of the hamstring muscles to withdraw the foot. This rapid shift from extension to flexion in one leg would then be uncoordinated with the already flexing right leg. Crossed extensor reflexes allow the rapid change from flexion to extension in the right leg, an reflex needed to maintain proper balance via inhibition of the right leg flexion muscles and the excitation of the extensor muscles.
Such stretching reflexes are also the involved in the automatic response to the contusion or stretching of knee (patellar) tendons. Because a muscle's agonist must contract as a muscle stretches, separate neural signals branch from the sensory neuron to via inhibitory neural pathways to inhibit the agonist in a process termed reciprocal inhibition. Tendons attached to muscles also respond to muscle stretching and contracting. In the deep tendon reflex, there is a process of reciprocal activation wherein receptors within the tendons send signals via transmitting neurons to agonist motor neurons that cause the agonist muscle or muscles to contract. This balancing of or complementation of muscle contraction and relaxation is a critical feature of normal body movement and coordination.
There are hundreds of defined reflexive actions, some normal and well known, other occurring only in conjunction with injury or diseases.
A blow to the Achilles tendon results in the plantar extension of the foot characterized by the familiar jerk of the ankle. The Babinski reflex involving the dorsiflexion (arching) of the big toe results from a scraping pressure drawn across the bottom of the foot.
The auditory reflex shuts both eyes as a familiar protective response to loud noise. Ciliary reflexes allow the pupil to rapidly accommodate to changing light conditions that could potentially damage the retina or optic nerve. Conjunctival reflexes cause the reflexive closure of the eyelids when the eye is touched.
Reflexes are often tested to assure normal neural functions and intact neural pathways. A common test involves the pupillary reflex that results in the constriction of the pupil when light is shown in the opposite eye.
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