The Initiation and Propagation of Impulses - Essay Example

The body performs numerous voluntary and involuntary activities all of which are controlled by the peripheral and the autonomic nervous system respectively.” The initiation and propagation of impulses in excitable tissues has long been considered chiefly from the point of view derived from the study of peripheral nerve fibers” (Bishop,1976) Conduction of nerve impulses along a neuron generates response to stimuli. The basis of all CNS activity is reflexes which occur at brain or spinal cord levels. It is really surprising that most of our important bodily functions are nothing but reflexes, for example, breathing or digesting food. Sometimes we are well aware of the reflex action but in some cases these actions happen almost without our knowledge. Therefore, conduction of nerve impulses along nerve fibers forms an important part in controlling functions and body activities. The path taken by nerve impulses to generate a reflex is known as a reflex arc. In humans, several reflex arcs may make up one reflex action. The reflex arc depicts the simplest route taken by a single nerve impulse to generate response.” One of the most fundamental properties of the spinal reflex is that of reciprocal innervations first described by Sherrington. Where one set of muscles in a reflex act is activated, the activity of a set of antagonistic muscles is reciprocally depressed” (Brown, 1914). The components of the impulse circuits consists of – receptors, afferent nerves, brain or spinal cord, Efferent nerves and effectors. The stimulus is first received by the receptor tissue, organ or cell. The receptor conducts the stimulus impulse to the efferent (sensory) nerves. The sensory nerves transmit the stimulus to the CNS which generates the appropriate response. Interneurons are present with the CNS to transmit the impulse from the sensory to the motor nerve, thereby creating a continuous neural network. This response is transmitted by the motor nerves or efferent nerves to the effector muscle, gland or organ which finally responds to the applied stimulus. The most important part of this entire circuit is the conduction of the impulses along the neurons. We may notice that excluding the receptors or the effectors, the transmission of the impulse is done by nerve fibers which make them the most crucial components in controlling the body systems. Neurons consisting of dendrites and axons are linked to one another by a synapse forming a nerve fiber. Calcium channels open when the impulse depolarizes the presynaptic membrane. The ions help in the release of neurotransmitters like acetylcholine, GABA etc. into the synaptic cleft. With the help of these neurotransmitter chemicals the impulses travel from one neuron to the next and along the entire neural path. Depolarization of the cell membrane excites the cell and initiates nerve impulse. After the impulse crosses over the cleft, the neurotransmitters involved are removed with the help of enzyme action and re-absorbed and re-cycled for transmission of the next nerve impulse. Reflexes induced by nerve impulse conduction, help us to maintain balance and posture. It also assists in movement of the body. 2) Reflexes help us respond immediately to harmful stimuli and also relieve the brain from too much of work. Therefore, we can understand the importance of impulse transmission in the body. The role played by the nervous system in the body is of extreme importance because it helps to maintain a steady state. The autonomic nervous system which controls all the involuntary actions of the body is a vital system because without this system maintenance of homeostasis in the body would have been impossible. For example, if we touched a hot iron and we had no reflex to remove our hands, the skin would be burnt and we would have an injury. To understand the role of nervous system in the body we must understand biological process like “those involving the functioning of the central nervous system, which behaves as self-regulatory devices or servo- mechanisms. The pupil reflex to light is an example of such a process” (Stark and Sherman, 1956). To see what exactly pupil accommodation is, you may look into the mirror and direct a bright torch at your eyes and almost immediately you will notice a change in the size of the pupil. This constriction happens as a response to the external stimuli-light. It is interesting to notice that even when excessive light enters just one eye, causing constriction in that eye, the pupil of the other eye automatically constricts, responding to the stimuli incident on the other eye. This can be seen in the diagram below. Such a reflex action is known as consensual reflex action. The ANS, has two antagonistic components- Sympathetic and Parasympathetic (Mandal, 2013). “Pupil size and dynamics are controlled by two synergistic pathways that operate on the smooth muscles of the pupil” (Privitera et al,). In case of pupil constriction or light reflex, the sympathetic ANS dilates the pupil while the Parasympathetic pupil constricts it. When light falls on the retina, the receptors present on the retina, transmit the impulse from the optic nerve to the Pretectal nucleus, which is present in the mid-brain of the right and left sides. The impulse is then passed to the accessory oculomotor or the Edinger-Westphal nucleus which forms a part of the third cranial nerve on either sides of the brain. The response generated is conducted via the efferent nerve fibers that causes contraction of the sphincter muscles of around the pupils of the eye, leading to the constriction of the pupils within one second. The decrease in the size of the aperture in the eyes, also known as miosis, allows less light to enter and hence protects the eyes against damage of the retina owing to excessive stimulation. The lesser amount of light also helps to create a better, sharper and clearer image even in bright light. In a similar pattern the nervous system plays an important part in controlling other important bodily function like heart rate, expansion and contraction of lungs, peristalsis, constriction of blood vessels, secretion of body fluids etc. BIBLIOGRAPHY 1) A. Mandal. 2013. Function of Nervous system. [ONLINE] Available at: http://www.news-medical.net/health/Function-of-the-Nervous-System.aspx. [Accessed 06 March 13]. 2) Bishop, G, 1976. Natural history of Nerve impulse. Science, 193, 114-120. 3) Brown, G, 1914. On the nature of the fundamental activity of the nervous centers; together with an analysis of the conditioning of rhythmic activity in progression, and a theory of the evolution of function in the nervous system. Journal of Physiology, 48, 18-46. 4) Privitera, M, 2008. The pupil dilation response to visual detection. SPIE, 6806, 3-10. 5) Stark and Sherman. 1956. A servoanalytical study of consensual pupil reflex to light. [ONLINE] Available at: http://www.montefiore.ulg.ac.be/systems/SYST002/Stark57.pdf. [Accessed 08 March 13]. 6) . 2013. [ONLINE] Available at: http://nerve.bsd.uchicago.edu/TheNerveImpulse05.pdf. [Accessed 07 March 2013]. Read More