➿The autonomic nervous system is a division of the nervous system that controls the activity of internal organs.
➿The sympathetic division prepares the body for fight or flight reactions. The parasympathetic system promotes ‘rest and digest’ (restorative) functions.
➿Acetylcholine is the principal transmitter released by the preganglionic fibres of both the sympathetic and the parasympathetic nervous systems. The parasympathetic postganglionic fibres secrete acetylcholine onto their target organs, whereas norepinephrine is principally secreted by the postganglionic sympathetic fibres.
➿The central portions of the autonomic nervous system are located in the hypothalamus, brainstem and spinal cord. The limbic system and parts of the cerebral cortex send signals to the hypothalamus and lower brain centres, which can also influence the activity of the ANS
➿The posterior and lateral hypothalamic areas increase blood pressure and heart rate, whereas the preoptic area decreases blood pressure and heart rate. These effects are mediated by cardiovascular centres in the pontine and medullary reticular formation.
➿An autonomic nerve pathway involves two nerve cells. It is connected by nerve fibers to the other cell, which is located in a cluster of nerve cells (called an autonomic ganglion). Nerve fibers from these ganglia connect with internal organs.
➿In the ANS, the connection between the CNS and its effector consists of two neurons—the preganglionic neuron and the postganglionic neuron. The synapse between these two neurons lies outside the CNS, in an autonomic ganglion [The cell bodies of the post ganglionic neuron, located in chains alongside the vertebral column, in plexuses in the abdomen (Sympathetic) or within the innervated target organ (Parasympathetic)]. The axon of a preganglionic neuron enters the ganglion and forms a synapse with the dendrites of the postganglionic neuron. The axon of the postganglionic neuron emerges from the ganglion and travels to the target organ #TheLayMedicalMan
➿The sympathetic system has short preganglionic fibres and long
postganglionic fibres. As the parasympathetic ganglia are located near or within their effector organs, the parasympathetic postganglionic fibres are short.
➿The pre-ganglionic fibres are slow-conducting B or C fibres. The postganglionic fibres that originate from the ganglia and innervate target organs are largely slow-conducting, unmyelinated C fibres. #TheLayMedicalMan
➿There are more postganglionic fibres than preganglionic nerves and so the stimulation of a single preganglionic neuron can activate many postganglionic nerves, resulting in divergence. But in the superior cervical ganglion, numerous preganglionic fibres converge on a single postganglionic neuron, resulting in convergence.
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