Structure And Functions Of The Peripheral Nervous System

Introducing the Brain and Autonomic nervous system

Overview of the Nervous System

Question:

Discuss about the Introducing the Brain and Autonomic nervous system.

The nervous system is known to be the most complex and the most organized system in the body. It has three main functions which are responsible for the functioning of the whole body. These functions include sensory input, information processing and motor output. Sensory input is the gathering of input from the sensory receptors distributed all over the body and its transmission to the brain where the second function occurs. Information processing is the interpretation of sensory input to information that can be understood by the body. Motor output is the transmission of the interpreted and integrated information back to the organs and glands for a response to be initiated. For these functions to be executed efficiently there is need for organization.

There are two divisions in the nervous system; the central nervous system and the peripheral nervous system. The brain and the spinal comprises the central nervous system and it is responsible for controlling most of the bodily functions. The peripheral nervous system is comprised of nerves that have their origins in the brain (the cranial nerves) and the spinal cord (the spinal nerves). Its functions are to convey afferent information from sensory receptors to the brain and relay efferent information back to the muscles and organs (Noback, Strominger, Demarest, & Ruggiero, 2005). This paper will focus more on the structural nature and functioning of the peripheral nervous system.

The peripheral nervous system is majorly involved in transmission of neural information. Due to this, there is need for its organization. The PNS has two divisions; the somatic nervous system and the autonomic nervous system (Noback, Strominger, Demarest, & Ruggiero, 2005). Figure 1.0 demonstrates the organization of the nervous system.

Figure 1.0 Structure of the nervous system

This is the branch of the nervous system that is responsible for the innervation of the skin, joints and skeletal muscles. It is composed of the skeletal muscles and the somatosensory area of the brain (Rains, 2003). The somatic nervous system is characterized by its function of transmission and the processing of conscious and unconscious sensory information to the CNS and the relay of motor control information to the voluntary muscles. It is composed of two nerve fibers; the afferent and the efferent nerve fibers. The afferent nerve fibers are responsible for the transmission of neural signals from the periphery to the central nervous system while the efferent nerve fibers transmit information from the brain and spinal cord to the periphery (Weiten, 2012). Postures and movements influence the skeletal muscles to relax or contract. It is through this influence that this system is able to express itself (Noback, Strominger, Demarest, & Ruggiero, 2005). This system is also responsible for reflexes. Reflexes are automatic responses to stimuli that do not require conscious thought. This occurs at the spinal cord level. Figure 1.1 shows an example of a reflex that is modulated at the spinal cord.

Figure 1.1 spinal reflex

The somatic nervous system is also composed of the brainstem which is characterized by the origin of descending motor pathways that help in the modulation of spinal reflexes. At the cerebral cortex area, there are two areas of importance; the somatosensory cortex and the somatomotor cortex. These two help us understand the somatotopy of the body as there is a homunculous representation of the body. The somatosensory area in the cerebral cortex is importance in the characterization of a sensory stimulus while the somatomotor cortex is responsible for provision of motor commands that activate skilled movement like writing by the brainstem and spinal cord (Noback, Strominger, Demarest, & Ruggiero, 2005).

Structural Components of the Peripheral Nervous System

The autonomic nervous system is responsible for regulating involuntary functions that bring about homeostasis and adaptation in the body. It is involved in the maintenance of blood pressure, temperature of the body, digestive functions etc (Benarroch, 2007). It does this through the transmission of both sensory and motor information of the visceral organs to the brain and to the involuntary muscles respectively. The ANS has three divisions; sympathetic division, parasympathetic division and the enteric nervous system.

The enteric nervous system works hand in hand with the CNS for the innervations of the digestive system. It has two components; the myenteric or Auerbach and the submucosal or Meissner plexuses (Nezami & Srinivasan, 2010). A plexus is known to be a network of blood vessels or nerves. The myenteric plexus which runs from the esophagus to the internal anal sphincter causes the instestinal wall to either contract or relax (Furness, Callaghan, Rivera, & Cho, 2014). It is situated in the middle of the circular and longitudinal smooth muscle layers. The Meissner plexus is located in the small and large intestines and it regulates their environment by controlling the blood flow and the secretions produced (Nezami & Srinivasan, 2010). The enteric nervous system is denoted by three neurons, the sensory neurons (intrinsic primary afferent neurons), interneurons and motor neurons.

The sympathetic nervous system is known as the thoracolumbar flow as the preganglionic neuron originates from the T1 to L3 (first thoracic segment to the third lumbar segment) (Bankenahally & Krovvidi, 2016). It is the part of the autonomic nervous system that is important in activating the flight or fight reflex with the aim of protecting the body from perceived harmful/ dangerous situations. It is characterized by short preganglionic fibers and the utilization of norepinephrine. This earns it its name adrenergic system. The preganglionic fibers go through the ventral roots, the spinal nerves, white rami communicantes and the sympathetic trunk (Noback, Strominger, Demarest, & Ruggiero, 2005). Figure 1.3 shows as the sympathetic nervous system.

The flight or fight response triggered by the sympathetic nervous system as a response by the body towards a perceived threat. The flight or fight response is also described as sympathoadrenal system which was coined by Walter B Cannon. The amygdala is the part of the brain that processes whether a situation is a threat or not. If a threat is perceived by the amygdala, it sends signals to the hypothalamus which commands the pituitary gland to release adrenocorticotrophine hormone (ACTH). This hormone stimulates the adrenal glands to release epinephrine that is released to the blood for circulation. The epinephrine/adrenaline causes a number of physiological changes like increased heart rate so as to increase the amount of blood being pumped to the extremities to facilitate either flight or fight, increased respiration rate in order to increase the amount of oxygen reaching the muscles, decreased digestive activity so as to focus and shunt blood to the muscles, dilated pupils to facilitate better vision and other responses (Harvard Medical School, 2016).

The parasympathetic nervous system works contrary to the sympathetic system in that it seeks to relax the body so as to conserve its resources in preparation of a dangerous situation. This division is also known as the cranio-sacral division due to its origins arising in the brain and the sacral portion of the spinal cord. It is unique in that it is comprises of four cranial nerves. These are cranial nerves III, the oculomotor; VII, facial nerve; IX, glossopharyngeal nerve and X vagus nerve (Noback, Strominger, Demarest, & Ruggiero, 2005). Focusing on the cranial nerves, the oculomotor nerve is involved in the constricting of the pupils and bringing about accommodation of the lens. The facial nerve is responsible for increasing secretion production in the nasal, submandibular, lacrimal and sublingual glands and  the vagus nerve is associated with stimulate digestive functions. The sacral portion of this system originates from S2-S4 levels of the spinal cord and they are important in the innervation of the abdominal and pelvic area. One major function that depends on this portion is micturation (Noback, Strominger, Demarest, & Ruggiero, 2005). The parasympathetic nervous system is known for its long preganglionic axons and short postganglionic axons.

The Somatic Nervous System

Physiological effects triggered by the sympathetic nervous system and the parasympathetic nervous system

Table 1.0 physiological effects of the autonomic nervous system

For us to better understand how neurons transmit information, we need to understand their structure. A neuron is mainly composed of a cell body, dendrites and an axon.  The cell body is composed of the nucleus and nucleic acids that are important in facilitating neural signaling due to their active metabolism. The dendrites are short extensions that are responsible for the reception of signals and impulses. They are widespread which helps them receive more signals. An axon is a long extension that is responsible for the transmission of neural signals. Myelin sheath is present along the length of the axons and helps in increasing the speed of neural transmission.

Communication between neurons can be done in two ways, either electrical or chemical. Electrical signaling is the transmission of neural information through the generation of electrical signals. This is achieved through the use of action potentials that depend upon the flow of ions across the plasma membrane. A neuron is composed of an electrically charged solution. If the charge of this solution compared to the one surrounding the neuron is different, a voltage will be generated. This voltage is determined by ions which can be positively charged or negatively charged. The flow of ions is determined by two processes diffusion and electrostatic pressure (Stanford University, 2012).

K⁺ ions are located inside the neuron while Na⁺ ions are located outside the neuron. The membrane of a neuron is selectively permeable meaning that it permits specific substances enter or leave the cell and prohibits some. The membrane prohibits Na⁺ ions and proteins that are negatively charged to either enter or leave. Since the outside environment does not have the proteins, the intrinsic environment of the cell has a negative charge. This causes the Na⁺ ions on the outside to move towards to membrane and the negatively charged proteins inside the neuron to move towards the membrane as opposites attract. This creates a resting potential (Macmillan, 2015).

A stimulus causes a change in the flow of ions in that the sodium ion channels will open hence an overflow of Na⁺ ions into the cell making it positively charged. Due to this, the K⁺ ions will be forced to move out through the potassium channels. This change in voltage (depolarization) creates an action potential. The generation of action potential in the first segment of the axon, makes more sodium ions to enter the cell. These ions flow to the next segment which results to the firing of the next segment. To prevent the firing back of the first segment, the sodium and potassium pumps restore the cell to its resting potential by expelling the sodium ions out and pulling the potassium ions into the cell. This process continues until the neural signal is transmitted throughout the axon (Macmillan, 2015). Image 1.3 summarizes the events that occur in one segment of an axon

Neurons communicate with each other using chemicals called neurotransmitters. This process is known as synaptic transmission. This process is important as neural signal cannot jump from one neuron from another. It is comprised of five stages. First the neurotransmitter is synthesized in the presynaptic terminal. Second the synthesized neurotransmitter is stored in secretory vesicles. Third, the neurotransmitter is released to the synaptic cleft. Fourth step is the binding of the neurotransmitter to their receptors at the post synaptic terminal and lastly is the termination of the effects of the neurotransmitter that was released (Brady, Siegel, Albers, & Price, 2005).

The Autonomic Nervous System

We are going to use acetylcholine to explain the stated process. Acetylcholine is produced and stored in their secretory vesicles. One an action potential is generated, the calcium ion channels open resulting to an inflow of calcium ions into the cell. This influx triggers the secretory vesicles to move towards the membrane and facilitates their binding to the presynaptic membrane. After the binding, the neurotransmitters are released from the vesicles to the synaptic cleft. The neurotransmitters bind to the receptors at the postsynaptic terminal. This causes the opening of the sodium and potassium ion channels. This results in the depolarization of the postsynaptic terminal. At the cleft, there is an enzyme called cholinesterase which is responsible for the inactivation of the excess acetylcholine. This helps is prevention of continuous firing as only a little amount of the acetylcholine is used to stimulate the post synaptic terminal and it helps in the recycling of choline through the breaking down of acetylcholine (Noback, Strominger, Demarest, & Ruggiero, 2005). Figure 1.4 summarizes the neural transmission process.

Conclusion

The peripheral nervous system is a complex and organized system due to its diverse functions in the sensory, transmission and integration of impulses. It is important for the transmission of both sensory and motor signals for the skeletal muscles through the somatic nervous system and the viscera through the autonomic nervous system. The autonomic nervous system helps protect our bodies from threats through the fight-flight response that brings about a number of physiological changes. All these functions are made possible through the efficient electrical and chemical signal transmission of the neurons.

References

Bankenahally, R., & Krovvidi, H. (2016). Autonomic nervous system: anatomy, physiology, and relevance in anaesthesia management  and critical care medicine. BJA Education , 381-387.

Benarroch, E. E. (2007). The Autonomic Nervous System: Basic management and Physiology.Continuum: Lifelong Learning in Neurology , 13-32.

Brady, S., Siegel, G., Albers, R. W., & Price, D. (2005). Basic Neurochemistry: Molecular,  Cellular and Medical Aspects. Massachusetts: Academic Press.

Furness, J. B., Callaghan, B. P., Rivera, L. R., & Cho, H.-J. (2014). The Enteric Nervous System and Gastrointestinal management Innervation: Integrated Local and Central Control. In M. Lyte, & J.          Cryan, Microbial Endocrinology: The Microbiota-Gut-Brain Axis in Health and Disease.       Advances in Experimental Medicine and Biology (pp. 39-71). Springer: New York.

Harvard Medical School. (2016, March 18). Understanding the stress response. Retrieved March 11, 2018, from Health Harvard Publishing: https://www.health.harvard.edu/staying-healthy/understanding-the-stress-response

Macmillan. (2015, April 11). Introducing the Brain and Neural Signalling. Retrieved March 11, 2018, from Macmillan Learning:   https://www.macmillanlearning.com/Catalog/uploadedFiles/Licht1e_Ch02_Final_hr8.pdf

Nezami, B. G., & Srinivasan, S. (2010). Enteric Nervous System in the Small Intestine: Pathophysiology management and Clinical Implications. Curr Gastroenterol Rep , 358-365.

Noback, C. R., Strominger, N. L., Demarest, R. J., & Ruggiero, D. A. (2005). The Human Nervous System: Structure and Function. New Jersey: Humana Press Inc.

Rains, D. G. (2003). Principles of Human Neuropsychology. Pennsylvania: McGraw Hill.

Stanford University. (2012). Chapter 5: Synaptic Integration. Retrieved March 11, 2018, from Stanford University:https://web.stanford.edu/class/cs379c/archive/2010/stanford.lecture.02.pdf

Weiten, W. (2012). Psychology: Themes and variations. Las Vegas: Wadsworth Publishing.

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