"Fight or Flight" - Sympathetic nervous system.

Sympathetic Nervous System

In the course of phylogeny an efficient control system evolved that enabled the functions of individual organs to be orchestrated in increasingly complex life forms and permitted rapid adaptation to changing environmental conditions. This regulatory system consists of the CNS (brain plus spinal cord) and two separate pathways for two-way communication with peripheral organs, viz., the somatic and the autonomic nervous systems. The somatic nervous system comprising extero- and interoceptive afferents, special sense organs, and motor efferents, serves to perceive external states and to target appropriate body movement (sensory perception: threat _ response: flight or attack). The autonomic (vegetative) nervous system (ANS), together with the endocrine system, controls the milieu interieur. It adjusts internal organ functions to the changing needs of the organism. Neural control permits very quick adaptation, whereas the endocrine system provides for a long-term regulation of functional states. The ANS operates largely beyond voluntary control; it functions autonomously. Its central components reside in the hypothalamus, brain stem, and spinal cord. The ANS also participates in the regulation of endocrine functions.

The ANS has sympathetic and parasympathetic branches. Both are made up of centrifugal (efferent) and centripetal (afferent) nerves. In many organs innervated by both branches, respective activation of the sympathetic and parasympathetic input evokes opposing responses.

In various disease states (organ malfunctions), drugs are employed with the intention of normalizing susceptible organ functions. To understand the biological effects of substances capable of inhibiting or exciting sympathetic or parasympathetic nerves, one must first envisage the functions subserved by the sympathetic and parasympathetic divisions (A, Responses to sympathetic activation). In simplistic terms, activation of the sympathetic division can be considered a means by which the body achieves a state of maximal work capacity as required in fight or flight situations.

In both cases, there is a need for vigorous activity of skeletal musculature. To ensure adequate supply of oxygen and nutrients, blood flow in skeletal muscle is increased; cardiac rate and contractility are enhanced, resulting in a larger blood volume being pumped into the circulation. Narrowing of splanchnic blood vessels diverts blood into vascular beds in muscle.

Because digestion of food in the intestinal tract is dispensable and only counterproductive, the propulsion of intestinal contents is slowed to the extent that peristalsis diminishes and sphincteric tonus increases. However, in order to increase nutrient supply to heart and musculature, glucose from the liver and free fatty acid from adipose tissue must be released into the blood. The bronchi are dilated, enabling tidal volume and alveolar oxygen uptake to be increased.

Sweat glands are also innervated by sympathetic fibers (wet palms due to excitement); however, these are exceptional as regards their neurotransmitter (ACh).

Although the life styles of modern humans are different from those of hominid ancestors, biological functions have remained the same.

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Mukesh Patel

"Rest & Digest" - Parasympathetic nervous system.

Parasympathetic Nervous System

Responses to activation of the parasympathetic system.

Parasympathetic nerves regulate processes connected with energy assimilation (food intake, digestion, absorption) and storage. These processes operate when the body is at rest, allowing a decreased tidal volume (increased bronchomotor tone) and decreased cardiac activity. Secretion of saliva and intestinal fluids promotes the digestion of foodstuffs; transport of intestinal contents is speeded up because of enhanced peristaltic activity and lowered tone of sphincteric muscles.

To empty the urinary bladder (micturition), wall tension is increased by detrusor activation with a concurrent relaxation of sphincter tonus.

Activation of ocular parasympathetic fibers (see below) results in narrowing of the pupil and increased curvature of the lens, enabling near objects to be brought into focus (accommodation).

Anatomy of the parasympathetic system.

The cell bodies of parasympathetic preganglionic neurons are located in the brainstem and the sacral spinal cord. Parasympathetic outflow is channeled from the brainstem (1) through the third cranial nerve (oculomotor n.) via the ciliary ganglion to the eye; (2) through the seventh cranial nerve (facial n.) via the pterygopalatine and submaxillary ganglia to lacrimal glands and salivary glands (sublingual, submandibular), respectively; (3) through the ninth cranial nerve (glossopharyngeal n.) via the otic ganglion to the parotid gland; and (4) via the tenth cranial nerve (vagus n.) to thoracic and abdominal viscera. Approximately 75 % of all parasympathetic fibers are contained within the vagus nerve. The neurons of the sacral division innervate the distal colon, rectum, bladder, the distal ureters, and the external genitalia.

Acetylcholine (ACh) as a transmitter.

ACh serves as mediator at terminals of all postganglionic parasympathetic fibers, in addition to fulfilling its transmitter role at ganglionic synapses within both the sympathetic and parasympathetic divisions and the motor endplates on striated muscle. However, different types of receptors are present at these synaptic junctions.

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Mukesh Patel

The Cardiac Cycle

The cardiac cycle is the sequence of events in one heartbeat.
 

In its simplest form, the cardiac cycle is the simultaneous contraction of the two atria, followed a fraction of a second later by the simultaneous contraction of the two ventricles. Systole is another term for contraction. The term for relaxation is diastole.

You are probably familiar with these terms as they apply to blood pressure readings. If we apply them to the cardiac cycle, we can say that atrial systole is followed by ventricular systole. There is, however, a significant difference between the movement of blood from the atria to the ventricles and the movement of blood from the ventricles to the arteries.

Followings are links to further understand the subject matter.


bcs.whfreeman.com/thelifewire/content/chp49/49020.html







library.med.utah.edu/kw/pharm/hyper_heart1.html





anatimation.com/cardiac/intro.html





www.mhhe.com/biosci/ap/seeleyap/student/olc2/ap_animation-quizzes.html


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Mukesh Patel

Fabulous illustrative animations in Anatomy Physiology.

Dear Readers,

A charming & warming Good morning to everybody.

Please find very nice animations to understand concepts of Human Anatomy & physiology with special reference to medical science.




You are requested to follow this link of University of Pennsylvania - Medical Animation Library. The link is as follows: www.pennmedicine.org/health_info/animationplayer/

Happy Reading; Learn with Fun.

Thanks & Best Regards,

Mukesh Patel