![]() These are referred to as the sympathetic trunks. Most of the sympathetic ganglia lie in close proximity to the spinal cord forming two chains on either side of the body. The sympathetic nervous system arises from the thoracolumbar region of the spinal cord. It also arises from the sacral plexus at the levels of the second to fourth sacral segments (S2–4). Specifically, the parasympathetic division arises from four of the cranial nerves-the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. The parasympathetic division arises from cranial and sacral origins. The sympathetic division arises from the thoracolumbar region from the first thoracic to the second lumbar level (T1–L2). The ANS is subdivided into sympathetic and parasympathetic divisions based on physiological and anatomical differences. After the synapse in the autonomic ganglion, the second fiber is referred to as the postganglionic fiber as it passes to the effector organ, in this case cardiac or smooth muscle, glands or gastrointestinal neurons. This part is defined as the preganglionic fiber. The origin of the first neuron of the ANS is found in the CNS, with the first synapse occurring in an autonomic ganglion. This is different to the single neuron of the somatic nervous system. Within the ANS generally, it is comprised of two neurons and a synapse. However, part of the ANS supplies the gastrointestinal system and is referred to as the enteric nervous system as the neurons are found supplying the glands and smooth muscle in the actual wall of the tract. The ANS can be thought of as that part of the nervous system supplying all other structures apart from skeletal muscle (supplied by the somatic nervous system). There are 8 in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the sacral region, and 1 coccygeal nerve. There are 31 pairs of nerves that connect with the spinal cord as the spinal nerves. The details of each of these nerves, their functions, and clinical applications will be dealt with in turn in subsequent chapters. There are 12 pairs of cranial nerves and these emerge from either the brain as fiber tracts (olfactory (I) and optic (II) nerves) or the brainstem (all other cranial nerves (III–XII)). Within the muscle fibers, they release the neurotransmitter acetylcholine and are only excitatory, that is, result only in contraction of the muscle. They are commonly referred to as motor neurons due to their termination in skeletal muscle. They consist of large diameter fibers and are ensheathed with myelin. They have an extremely long course as they do not synapse after they leave the CNS until they are at their termination in skeletal muscle. The somatic nervous system consists of the cell bodies located in either the brainstem or the spinal cord. Simply put, the somatic nervous system innervates skeletal muscle, whereas the ANS innervates glands, neurons of the gastrointestinal tract, and cardiac and smooth muscles of glandular tissue. The further classification used of the efferent division is the somatic and autonomic nervous system (ANS). It is worth noting that the efferent division is subclassified into what they ultimately supply. Efferent information however transmits information from the CNS externally to, for example, glands and muscles. Their axons are found outwith the CNS, but then enter the CNS. With spinal nerves, they contain both afferent and efferent information, whereas some cranial nerves like the olfactory and optic nerves contain only afferent information (for smell and sight, respectively).Īfferent information transmits impulses from receptors to the CNS. Therefore, the nerves of the PNS can be classified as belonging to either afferent (taking information to the CNS) or efferent (away from the CNS). Whether they have this myelin or not, they do have the same general feature in that a nerve contains nerve fibers with axons of either afferent or efferent neurons. The nerves of the PNS can be either myelinated (formed by the surrounding Schwann cells) or unmyelinated in nature. In total, there are 43 pairs of nerves in the PNS-12 cranial nerves and 31 spinal nerves. The nerves in the PNS transmit information from all parts of the body to and from the CNS. Paul Rea, in Clinical Anatomy of the Cranial Nerves, 2014 Peripheral Nervous System ![]()
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