Rest, parasympathetic autonomic nervous system

Figure 14.1 Effect of autonomic nervous system stimulation on action potentials of the sinoatrial (SA) node. A normal action potential generated by the SA node under resting conditions is represented by the solid line the positive chronotropic effect (increased heart rate) of norepinephrine released from sympathetic nerve fibers is illustrated by the short dashed line and the negative chronotropic effect (decreased heart rate) of acetylcholine released from parasympathetic nerve fibers is illustrated by the long dashed line.

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. 

The autonomic nervous system is divided into the sympathetic and parasympathetic components, which typically exert opposing effects. The sympathetic system is involved in the fight or flight reaction (increased blood pressure and heart rate, and accommodation for increased vision, for example) that prepares the organism for stressful situations. The parasympathetic system conversely establishes a more relaxed situation, for instance, the rest period after a meal. The autonomic nervous system that is responsible for the independent control of the mechanical and secretory functions of the gastrointestinal tract is sometimes called the enteric system. 

It is well established that increased sympathetic nerve activity is associated with chronic heart failure (CHF) (Porter et al. 1990 Singh 2000 Olshansky 2005 Brodde et al. 2006 Watson et al. 2006). The increase in sympathetic activity is a compensatory mechanism that provides inotropic support to the heart and peripheral vasoconstriction. However, it promotes disease progression and worsens prognosis (Watson et al. 2006). The autonomic nervous system (ANS) is a very complex, balanced system that influences the initiation, termination, and perpetuation of atrial fibrillation (AF), and the AF affects the ANS (Olshansky, 2005). At rest, sympathetic and parasympathetic outflows are related reciprocally heart failure patients had high sympathetic and low parasympathetic outflows, and healthy subjects had low sympathetic and high parasympathetic outflows (Porter et al. 1990). 

The autonomic nervous system is divided into two branches. The sympathetic branch produces the physiological effects that accompany emotional arousal, and the parasympathetic branch controls the body when at rest. 

Purkinje cells is demonstrated in Figure 12.1 and, like all cardiac myocytes, can be divided into four phases. Phase 4 (pacemaker potential) involves the slow influx of sodium ions, depolarizing the cell until the threshold potential is reached. Once the threshold potential is reached, the fast sodium current is activated, resulting in a rapid influx of sodium ions causing cell depolarization (phase 0 rapid depolarization). Phase 1 (partial repolarization) involves the inactivation of sodium channels and a transient outward current. Phase 2 (plateau phase) results from the slow influx of calcium ions. Repolarization (phase 3) occurs as a result of outflow of potassium ions from the cell and restores the resting potential. There are variations between the different areas of the heart, specifically the nodal tissues do not possess fast sodium channels and slow L-t5rpe calcium channels generate phase 0 current (Fig. 12.1). Phase 4 activity varies between nodal areas the sinoatrial node depolarizes more rapidly than the atrioventricular (AV) node. Automaticity is under autonomic nervous system control. Parasympathetic neurons... 

Heart rate can be altered by the autonomic nervous system to meet changes in demand for blood supply. Sympathetic stimulation increases the rate and force of contraction in response to exercise, threats and emotions. Parasympathetic stimulation reduces heart rate and force of contraction under resting conditions. 

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