Significance of Autonomic Nervous System to Psychologists

                        

Significance of Autonomic Nervous System to Psychologists

The complex human nervous system is divided broadly into two major parts based upon the location of the nervous tissue. The central nervous system (CNS) is the first part which has a centrally located nervous tissue and consists of the brain and the spinal cord. The second part is the peripheral nervous system (PNS) which includes the remaining nervous tissue of the body. The PNS contains 12 pairs of cranial nerves and their branches which arise from the brain stem and 31 pairs of spinal nerves which transmit messages between the spinal cord and the body. The nerves which form a part of the PNS can transmit messages to the CNS, and send instructions from CNS to the end organs like muscles and glands. The peripheral nervous system is further divided into two sub systems which are the somatic nervous system and the autonomic nervous system (ANS). Out of the two, the ANS controls some of the vital involuntary jobs like maintenance of homeostasis, digestion, breathing, maintenance of posture, etc, which are not noticed by us (The Nervous System (Chapter 11), n.d.). This article provides information about the components of the somatic nervous system and autonomic nervous system.

The Somatic Nervous System

This system consists of nerves which carry sensory information from skin to the CNS and instructions from the CNS to the end organs like skeletal muscles and sensory receptors located in the head and extremities.

Sensory neurons. They are the afferent neurons which carry information from the receptors present on the skin to the brain or the spinal cord. They have short axon and long dendrites (which bring information to the cell body) and the cell body is located in the dorsal root ganglion.

Motor neurons. They are the efferent neurons which carry messages from the brain and the spinal cord to muscles and other end organs. These neurons contain long axons and short dendrites. The cell body and the dendrites are located in the spinal cord, whereas the axon lies outside to the spinal cord. One motor neuron can be connected to many muscle fibers through collateral branches of the axon forming a “motor unit” (Swenson, 2006) (The Human Nervous System, 2004).

Interneurons are the third type of neurons which transmit the messages from the sensory neurons to the motor neurons. Upon activation of the sensory receptors, the sensory information gets carried by the sensory neurons to the CNS where it synapses to the interneurons, which in turn synapse on motor neurons. Finally, the motor neurons carry the instructions to the effectors organs like muscles, which respond by contracting. This simple monosynaptic pathway involving the flow of information is known as the reflex arc (The Human Nervous System, 2004).

The Autonomic Nervous System

John Langley was the first person to give the name “autonomic nervous system” to the complex network of peripheral nerves and ganglia which control the functioning of the smooth muscles and the glands of the viscera. This involuntary control system can regulate the functioning of the cardiac and smooth muscles and control the glandular secretions (Berntson, Sarter, & Cacioppo, n.d.). The somatic nervous system helps in connecting the external sensory organs to muscles through brain whereas, ANS controls the direct connection between the visceral organs/glands and brain or the spinal cord. ANS is controlled by the hypothalamus and medulla oblongata, containing neurons bundled together with somatic system neurons in the spinal and cranial nerves. ANS consists of two main divisions which work in opposition to each other while regulating the involuntary processes of the body and they are the sympathetic and the parasympathetic nervous systems (The Nervous System (Chapter 11), n.d.).

The Sympathetic Nervous System (SNS)

The primary function of this system is to mobilize the body during stressful situations to generate fight-or-flight response. The central origin of this system lies in the thoracic and lumbar regions of the spinal cord, hence is considered to have a thoracolumbar outflow. The neurons of this system release a neurotransmitter known as norepinephrine, which can excite its target muscles as a part of neuronal response. These nerves can also trigger the adrenal glands stimulating the release of hormones, epinephrine and norepinephrine during stressful situation under hormonal response. This system raises precise organ specific reactions. For example, during a situation of emergency, it stimulates the acceleration of the heart rate, constriction of blood vessels, increase in blood pressure, etc. These activities meet the demand for extra energy by the skeletal muscles. But a few other processes which are not immediately required under stress are suppressed. For example, digestion takes place at a slower rate; sphincter controlling the bladder gets constricted, etc (The Nervous System (Chapter 11), n.d.). The neurotransmitter acetylcholine is released by the preganglionic sympathetic neurons which stimulates the action potential in the postganglionic neurons. However, the final transmission of messages to the end organs through the sympathetic nerves takes place in the presence of norepinephrine released by the postganglionic neurons. This system maintains a bidirectional flow of messages and controls various internal organs like heart, blood vessels, lungs, eyes, kidney, sweat glands, penis and the digestive system. Sensations like heat, cold and pain are felt through this system. Disorders in sympathetic nervous system can lead to symptoms like slurred speech, headache, hypertension, loss of muscle strength, breathing problems, etc. It generally counteracts the parasympathetic nervous system. The diseases caused by the malfunctioning of SNS are sympathicotonia, complex regional pain syndrome or reflex sympathetic dystrophy syndrome, fibromyalgia, Parkinson’s disease and diabetic neuropathy. (Berntson, Sarter, & Cacioppo, n.d.) (Sympathetic Nervous System & All About It, n.d.). Some drugs like Caffeine can stimulate the SNS.

The Parasympathetic Nervous System (PNS)

This system gets activated when the body is in a resting state. The main purpose of this system is to restore and conserve energy. It allows the body to recover from a stressful situation as it stimulates the release of endorphins, a hormone that is called “feel good” hormone. This system stimulates the rest-and-digest response. PNS consists of 4 cranial nerves originating from the brain stem. PNS is also termed as craniosacral branch as its activity originates within the head to the sacral region. PNS also contains peripheral ganglia similar to SNS, but they do not get collected into coherent ganglionic chains. Instead, they lie near the visceral organs. Hence PNS is considered to raise a more localized response. Vagus nerve is mostly involved in the PNS activity. The afferent fibers of the vagus nerve convey the stimuli associated with the entry of food into the stomach to the vagus in the brain (which acts like a command station or nucleus). Later, the efferent fibers of the vagus nerve convey the messages from the brain to stomach to stimulate digestion. Both preganglionic neurons and post ganglionic neurons of PNS release the neurotransmitter, acetylcholine. PNS antagonizes the action of SNS and performs functions like lowering the heart rate, decreasing the blood pressure, constriction of the pupils, promotion of the digestion of food and functioning of the reproductive organs by dilating the blood vessels reaching the genitals (Berntson, Sarter, & Cacioppo, n.d.) (Sympathetic Nervous System & All About It, n.d.)

Opposing Versus Synergistic Actions of SNS and PNS

Many visceral organs are innervated by both PNS and SNS branches which are opposing in their actions. For example, sympathetic nerves stimulate the β-adrenergic receptors and increase the heart rate whereas, acetylcholine is released due to parasympathetic innervation, which acts on muscarinic receptors situated on the SA (sinoatrial node) and stimulates the lowering down of the heart rate. A perfect balance between these two systems leads to a balanced autonomic control of the target organs. These two systems perform synergistic action in some organs. For example, co-activation of sympathetic system along with parasympathetic system stimulates the salivary secretions when we eat something potentially noxious, like hot chilies. Even the penile erection and ejaculation requires the co-activation of both the systems (Berntson, Sarter, & Cacioppo, n.d.).

Conclusion

Changes in the activities of ANS can lead to diseases. Examining of the ANS is of special significance to psychologists, as ANS responses can be correlated with shifts in emotion, motivation, preferences and attention. They may also be linked to mental and physical health vulnerabilities. Autonomic nervous system responses can be measured with the help of electrodermal activity (EDA) (which measures the responses in the eccrine sweat glands), Electrocardiogram (ECG) (which measures the electric signal produced by the heart), impedance cardiography (which estimates the blood flow changes in the heart) and blood pressure detecting devices (Mendes, n.d.). ANS helps the body to respond to different stimuli and recover from it with time. A perfect functioning of the ANS is very essential for the maintenance of our day-to-day life.

References 

Berntson, G, G., Sarter, M., Cacioppo, T, J. (n.d.). Autonomic nervous sytem. Retrieved July 12, 2013, from http://psychology.uchicago.edu/people/faculty/cacioppo/jtcreprints/bsc.ans.03.pdf

Mendes, B,W. (n.d). Autonomic Nervous System. Retrieved July 12, 2013, from http://www.wendyberrymendes.com/cms/uploads/Mendes%20-%20Autonomic%20nervous%20sys.pdf

Swenson, R. (2006). Chapter 3 - Peripheral nervous system. Dartmouth.edu. Retrieved July 12, 2013, from http://www.dartmouth.edu/~rswenson/NeuroSci/chapter_3.html

Sympathetic Nervous System & All About It. (n.d.). Retrieved July 12, 2013, from http://www.sympatheticnervoussystem.net/

The Nervous System (Chapter 11). (n.d.). Retrieved July 12, 2013, from http://highered.mcgraw-hill.com/sites/dl/free/0070960526/323541/mhriib_ch11.pdf

The Human Nervous System. (2004). Biologymad.com. Retrieved July 12, 2013, from http://www.biologymad.com/nervoussystem/nervoussystemintro.htm

 

 

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