Parasympathetic nervous system activation responses

The sympathetic or adrenergic nervous system operates in juxtaposition to the parasympathetic nervous system to maintain homeostasis in response to physical activity and physical or psychological stress. Sympathomimetic neurotransmission is generally mediated by norepinephrine [51-41 -2] (1), CgH NO, released from presynaptic storage granules upon stimulation. A second endogenous sympathomimetic agent, epinephrine [51-43-4] (2),... 

Beside this there are some major differences with the neurotransmission in the autonomous nervous system The contractile activity of the skeletal muscle is almost completely dependent on the innervation. There is no basal tone and a loss of the innervation is identical to a total loss in function of the particular skeletal muscle. In contrast to the target organs of the parasympathetic nervous system the skeletal muscle cells only have acetylcholine receptors at the site of the so-called end-plate, the connection between neuron and muscle cell with the rest of the cell surface being insensitive to the transmitter. The release of acetylcholine results in a postjunctional depolarization which is either above the threshold to induce an action potential and a contraction or below the threshold with no contractile response at all. In contrast to the graduated reactions of the parasympathetic target organs, this is an all or nothing transmission. 

Noradrenergic model. This model suggests that the autonomic nervous system of anxious patients is hypersensitive and overreacts to various stimuli. The locus ceruleus may have a role in regulating anxiety, as it activates norepinephrine release and stimulates the sympathetic and parasympathetic nervous systems. Chronic noradrenergic overactivity down regulates 02-adrenoreceptors in patients with generalized anxiety disorder (GAD) and posttraumatic stress disorder (PTSD). Patients with social anxiety disorder (SAD) appear to have a hyperresponsive adrenocortical response to psychological stress. 

The slow phase of the stress system is characterized by processes that promote recovery from, and adaptation to, the stressful conditions that prompted the response. At the level of the hypothalamus, this phase is probably mediated by the urocortins acting through CRHR2 receptors (Reul and Holsboer, 2002 de Kloet et al., 2005). In contrast to the fast phase, the slow phase is associated w ith activation of the parasympathetic nervous system, w hich promotes the appetitive and metabolic functions, w hich help to restore homeostasis. As cortisol levels... 

Anticholinesterase insecticides phosphorylate the active site of cholinesterase in all parts of the body. Inhibition of this enzyme leads to accumulation of acetylcholine at affected receptors and results in widespread toxicity. Acetylcholine is the neurohormone responsible for physiologic transmission of nerve impulses from preganglionic and postganglionic neurons of the cholinergic (parasympathetic) nervous system, preganglionic adrenergic (sympathetic) neurons, the neuromuscular junction in skeletal muscles, and multiple nerve endings in the central nervous system (Fig. 10-5). 

A multivariate approach of analyzing the psychophysiological measures obtained from heart period has been proposed as a method for obtaining cardiac autonomic information (Backs, 1995,1998). The multivariate approach attempts to improve the sensitivity and diagnosticity of heart rate by identifying the neurogenic activity of the sympathetic and parasympathetic nervous systems responsible for the observed heart rate in a task. Principal components analysis (PCA) was used in the present study to extract information about the sympathetic and parasympathetic nervous systems common to RSA, low-frequency HRV, residual heart period, and heart period. Details of how the components were derived are presented in the Method section. 

The autonomic system can be further divided into (1) the sympathetic nervous system, and (2) the parasympathetic nervous system. The sympathetic system is generally associated with the traditional fight or flight response, and the parasympathetic system is usually associated with routine integration of normal activity. 

Because baroreceptors respond to stretch or distension of the blood vessel walls, they are also referred to as stretch receptors. A change in blood pressure will elicit the baroreceptor reflex, which involves negative feedback responses that return blood pressure to normal (see Figure 15.6). For example, an increase in blood pressure causes distension of the aorta and carotid arteries, thus stimulating the baroreceptors. As a result, the number of afferent nerve impulses transmitted to the vasomotor center increases. The vasomotor center processes this information and adjusts the activity of the autonomic nervous system accordingly. Sympathetic stimulation of vascular smooth muscle and the heart is decreased and parasympathetic stimulation of the heart is increased. As a result, venous return, CO, and TPR decrease so that MAP is decreased back toward its normal value. 

Figure 19.17 The biochemistiy and physiology responsible for penile erection. Sexual activity itself begins with a state of arousal that leads to erection. Arousal results in part from stimulation of the sense organs. The hypothalamus coordinates the sensations and activates the autonomic nervous system. Sensory nerves from the skin of the penis and other erogenous zones stimulate the parasympathetic system. This activates nitric oxide synthase and the resultant nitric oxide, via cyclic GMP, causes vasodilation of the arterioles. This increases blood flow through the corpora cavernosa which then expands producing an erection. Pheromones secreted by the female can stimulate the odour detecting system in the nasal cavity of the male (Chapter 12 and see above). Stress, however, activates the sympathetic system releases cyclic AMP which can result in vasoconstriction of the arterioles. Other factors that can interfere with an erection are physical fatigue and alcohol.

The sympathetic nervous system (SNS) plays a role in the "fight-or-flight response." Set in motion by activity in the limbic system and hypothalamus, the sympathetic system mobilizes the body to take action in response to dangerous situations. When the fight-or-flight response is activated, there is a sudden, massive increase in metabolic rate increased blood pressure and heart rate and the increased blood flow to the heart, brain, and muscles. Conversely, the parasympathetic system which is activated at times of relaxation and quiescence, acts to reduce heart rate and blood pressure in an overall attempt to conserve energy (see figure 3-1). 

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