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Autonomic Nervous System - Anatomy & Physiology (view source)
Revision as of 17:00, 7 March 2011
, 17:00, 7 March 2011→Sympathetic Nervous System
==Sympathetic Nervous System==
==Sympathetic Nervous System==
The SNS is activated in stressful or physically demanding situations and can temporarily enhance the physical performance of the body in order to better cope with the stressor. The preganglionic neurons (see Basics section) of the SNS are found within the lateral horns of the spinal cord within the full length of the thoracic vertebrae and the cranial lumbar vertebrae. The number of vertebral segments containing SNS preganglionic neurones has wide variation depending on the species in question; horses have 18 thoracic segments, canines have 13 and humans have 12. In most domestic species, the cranial most 4 to 6 lumbar vertebrae contain SNS segments. These preganglionic segments are located like a string of beads along either side of the spinal cord and are referred to as ''sympathetic chain ganglia''. In most cases the number of sympathetic ganglia exceeds the number of spinal cord vertebrae containing the preganglionic cell bodies. Within the abdominal cavity ventral to the lumbar vertebrae there are three sympathetic ganglia called the ''prevertebral ganglia''.
The SNS is activated in stressful or physically demanding situations and can temporarily enhance the physical performance of the body in order to better cope with the stressor. The preganglionic neurons (see Basics section) of the SNS are found within the lateral horns of the spinal cord within the full length of the thoracic vertebrae and the cranial lumbar vertebrae. The number of vertebral segments containing SNS preganglionic neurones has wide variation depending on the species in question; horses have 18 thoracic segments, canines have 13 and humans have 12. In most domestic species, the cranial most 4 to 6 lumbar vertebrae contain SNS segments. These preganglionic segments are located like a string of beads along either side of the spinal cord and are referred to as ''sympathetic chain ganglia''. In most cases the number of sympathetic ganglia exceeds the number of spinal cord vertebrae containing the preganglionic cell bodies. Within the abdominal cavity ventral to the lumbar vertebrae there are three sympathetic ganglia called the ''prevertebral ganglia''.
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These preganglionic fibres then branch out to form synapses with postganglionic neurons, but in doing this each preganglionic fibre actually interconnects between neighbouring sympathetic chain ganglia. Therefore presynaptic nerve fibres from one segment of the spinal cord terminate in several ganglia and therefore activity in individual ganglia may affect many areas of the body. Postsganglionic neurones then extend to target organs spread throughout the body. The exception to this rule is the ''adrenal medulla'' as this endocrine gland receives sympathetic input directly from preganglionic nerve fibres, directly from the spinal cord. In this case, the adrenal glands are effectively modified post-ganglionic sympathetic neurons that do not have axons.
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