Reflex Arc, Research Paper Example

Reflex arc, being a structural basis of reflex action, represents a serious of units through which impulses have to pass in order to bring about the reflex response. The main components of the arc are receptors, sensory (afferent) neurons, intermediate neurons, motor (efferent) neurons and effectors. The receptors may be scattered sensory cells in the skin or organs of special sense. Stimulation of the receptors creates impulses in sensory neurons which are located in the peripheral nerves. These neurons convey impulses to the CNS where they make synaptic connection to the intermediate neurons. These in turn connect with motor neurons which extend into peripheral nerves and reach the effectors, usually glands or muscles (Roberts, 2002).

Let us take a look at two of the common human reflexes – patellar and pupillary. To better understand the differences between the two of them, first of all, it is essential to distinguish the ways both of the reflexes function and the purpose they have for the human organism.

Patellar reflex is a part of the peripheral nervous system (PNS), which connects the CNS to organs and limbs. Patellar reflex is the contraction of the quadriceps femoris muscle, with resulting extension of the knee, in response to percussion of patellar tendon. A firm tap on the tendon stimulates stretch sensory receptors, most importantly, muscle spindles, that let off an afferent impulse in a sensory nerve fiber of the femoral nerve which leads to the lumbar region of the spinal cord. In that region the sensory neuron synapses directly with a motor neuron that conducts an efferent impulse to the quadriceps femoris muscle, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle, causes the leg to kick (Campbell, DeJong, Haerer, 2005). The patellar reflex is an example of the monosynaptic reflex arc, meaning that only one synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest reflex arc and the integration center is the synapse itself. There is no interneuron in the pathway leading to contraction of the quadriceps muscle – instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord (Campbell, DeJong, Haerer, 2005). The primary purpose of the patellar reflex is to prevent the stretching of the quadriceps. Being a type of stretch reflexes, patellar reflex is important in maintaining the upright position of the human body. The presence of the patellar response indicates that the muscle spindle, sensory and motor neurons, neuromuscular junctions, and the muscle are working appropriately, that there is an appropriate balance of excitatory and inhibitory inputs from the higher brain levels as well as the existence of the integrity of the L2-L4 vertebral segments of the spinal cord. Neural activity at other sites in the body may influence the patellar reflex response. Some evidence suggests that the magnitude of the patellar reflex can be increased when the contractile tone of the quadriceps muscle is increased. Mental activity, which increases muscle tone, may increase the magnitude of the response. Decreased mental activity can decrease the magnitude of the patellar reflex while physical activity and mental stress may make the reflex less sensitive (Reflexes, 2009).

Pupillary reflex belongs to the autonomic nervous system (ANS), which functions automatically to maintain homeostasis and carry out many involuntary functions of the body (Van De Graaff, Rhees, 1997). Pupillary reflex controls the size of the pupil, which depends on the intensity of the light that falls on the retina of the eye. The afferent arm of the pupillary reflex consists of fibers passing from the retina, optic nerve, optic chiasm, and optic tract that bypass the lateral geniculate body to synapse on the pretectal nuclei in the midbrain. Most fibers from the pretectal nuclei cross back to the Edinger-Westphal nuclei within the midbrain. These nuclei form the beginning of the efferent arm of the pupillary reflex. Parasympathetic fibers arise from the Edinger-Westphal nuclei and pass to the eye with close association with the motor efferent fibers of the oculomotor nerve. These parasympathetic fibers synapse on the ciliary ganglion, located lateral on the optic nerve. Short ciliary nerves arise from the ganglia to innervate the ciliary muscles and constrictor of the pupil (Slatter, 2002). Pupillary reflex is an example of the polysynaptic reflex arc, meaning that more than one associated neuron is needed to provide the synapses of the sensory and motor neurons. There are two responses to eye light stimulations: direct reflex, referring to the eye that is being stimulated, and consensual reflex, referring to not stimulated eye. The neural pathway of the direct reflex consists of the following: light passes through the optic nerve to optic tract ipsilaterally, from there to ipsilateral pretectal nuclei, then to ipsilateral Edinger-Westphal nuclei and through ipsilateral oculomotor nerve to ciliary ganglion, from where short post-ganglionic ciliary nerves activate the pupil constrictor. On the other hand, with the consensual reflex two simultaneous pathways are identified: one crossing at chiasm and another crossing from pretectal nuclei. In the first case, light goes through the optic nerve, crossing at chiasm to contralateral optic tract, from their passes to pretectal nuclei and Edinger-Westphal nuclei both contralateral to light stimulus, then out through the oculomotor nerve contralateral to light stimulus travels to ciliary ganglion, from which ciliary nerves activate the constrictor. At the same time, light passes through the optic nerve to optic tract ipsilaterally and from there to ipsilateral pretectal nuclei; pretectal cells send axons across midline in posterior commisure to Edinger-Westphal nuclei, then out through the oculomotor nerve both contralateral to light stimulus; from there it passes to ciliary ganglion and papillary constrictor (Van De Graaff, Rhees, 1997). Under normal circumstances, the responses in both eyes should be identical; if not, certain eye-related problems are to be identified by the specialists. Normally, pupil size is tonically modulated by parasympathetic and sympathetic systems. Pharmacological testing may help to localize preganglionic and postganglionic sympathetic and parasympathetic lesions. The miotic pupil is seen with uveitis, Horner’s syndrome, and midbrain lesions, whereas a mydriatic, large pupil may be seen with iris atrophy or lesions in the optic nerve (Slatter, 2002).

The main differences between patellar and papillary reflexes can be summarized in the following table:

References

Campbell, W. W., DeJong, R. N. and Haerer, A. F. (2005). The Reflexes. DeJong’s the neurologic examination (6^th ed., p. 476). Lippincott Williams & Wilkins.

Van De Graaff, K. M. and Rhees, R. W. (1997). Peripheral and Autonomic Nervous Systems. Schaum’s outline of theory and problems of human anatomy and physiology (2^nd ed., pp.204-220 ). McGraw-Hill Professional.

Reflexes. Retrieved on June 20, 2009 from http://csm.jmu.edu/biology/danie2jc/reflex.htm

Roberts, M. B. V. (2002). Nervous and Hormonal Communication. Biology (19^th ed., p. 276). Nelson Thornes.

Slatter, D. H. (2002). Textbook of Small Animal Surgery (3^rd ed., p. 1098). Elsevier Health Sciences.