Nerves - Reflexes and the Reflex Arc

Nerves 
A nerve contains bundles of peripheral neuron fibers sur­rounded by connective tissue. Each nerve fiber within a nerve is surrounded by a white myelin sheath; there­fore, nerves have a white appearance. There are three types of nerves. Sensory nerves contain only the long fibers of sensory neurons, and motor nerves contain only the long axons of motor neurons. Mixed nerves, however, contain the long fibers of both sensory and motor neurons.
Within the CNS, bundles of fibers are called tracts. When these fibers are myelinated, they form the white matter of the CNS. Unmyelinated fibers and cell bOGies are gray and form the gray matter of the CNS.
Reflexes and the Reflex Arc
Reflexes are automatic, involuntary responses to changes occurring inside or outside the body. For example, a person does not have to consciously think about the heart rate, breathing rate, body temperature, or even food digestion because these processes are controlled by reflexes. Reflexes are also involved in swallowing, sneezing, vomiting, and urination. However, people are more aware of reflexes that involve reactions to painful stimuli.
Reflexes depend on the reflex arc. The reflex arc involves the spinal cord and a spinal nerve. The long fibers of a sensory neuron are located in the spinal nerve; the cell body of the sensory neuron is located in a dorsal-root ganglion (gang'gle-on). (Typically, cell bodies are located in the CNS; those that lie outside the CNS are located in ganglia.) Each dorsal-root ganglion contains the cell bodies of hundreds of sensory neurons. Axon endings of sensory neurons are located in the spinal cord-specifically, the gray matter's dorsal horn. The in­terneuron is located completely within the gray matter of the spinal cord (CNS). The cell body and dendrites of the motor neuron are located in the spinal cord-specifically, within the gray matter's ventral horn. The axon of the motor neuron is located in the spinal nerve. Spinal nerves are called mixed nerves because they contain sensory and motor fibers.
A sensory receptor is a structure specialized to receive information from the environment and to generate nerve impulses. When a sensory receptor at the end of a sensory neuron is stimulated-in this case, by injury-it generates nerve impulses that are conducted along the sensory neuron toward the cell body in a dorsal­root ganglion. From the cell body, the impulses are con­ducted along the axon of the sensory neuron. They are then transmitted from the sensory neuron to the interneuron, which passes them on to the motor neuron. The axon of the motor neuron conducts the nerve impulses to muscle fibers, which then contract so that the hand is withdrawn from the injury-causing object. The response to nerve im­pulses transmitted along a reflex arc is called a reflex.
Various other reactions usually accompany a reflex re­sponse: The person may look in the direction of the object, jump back, and utter appropriate exclamations. These responses are a result of the sensory neuron stimulating several interneurons, which take impulses to all parts of the central nervous system, including those responsible for consciousness.
Reflexes are essential to homeostasis. They keep the in­ternal organs functioning within normal bounds and pro­tect the body from external harm. Reflexes can also be used to determine if the nervous system is reacting properly. Two of these types of reflexes are:
Knee-jerk reflex (patellar reflex): This reflex is initiated by striking the patellar ligament just below the patella. The response is contraction of the quadriceps femoris muscles, which causes the lower leg to extend.
Ankle-jerk reflex: This reflex is initiated by tapping the Achilles tendon just above its insertion on the calca­neus. The response is plantar flexion due to contraction of the gastrocnemius and soleus muscles.
Some reflexes are important for avoiding injury, but the knee-jerk and ankle-jerk reflexes are important for normal physiological functions. For example, the knee-jerk reflex helps a person stand erect. If the knee begins to bend slightly when a person stands still, the quadriceps femoris is stretched, and the leg straightens.
Reflexes (automatic reactions to internal and external stimuli) depend on the reflex arc. Some reflexes are important for avoiding injury, and others are necessary for normal physiological functions.