What is the peripheral nervous system
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The peripheral nervous system refers to parts of the nervous system outside the brain and spinal cord. It includes the cranial nerves and spinal nerves from their origin to their end. The anterior horn cells, although technically part of the central nervous system (CNS), are sometimes discussed with the peripheral nervous system because they are part of the motor unit. Show Motor neuron dysfunction results in muscle weakness or paralysis. Sensory neuron dysfunction results in abnormal or lost sensation. Some disorders are progressive and fatal. A motor unit consists of
The anterior horn cells are located in the gray matter of the spinal cord and thus are technically part of the CNS. In contrast to the motor system, the cell bodies of the afferent sensory fibers lie outside the spinal cord, in dorsal root ganglia. Nerve fibers outside the spinal cord join to form anterior (ventral) motor nerve roots and posterior (dorsal) sensory nerve roots. The ventral and dorsal roots combine to form a spinal
nerve. Thirty of the 31 pairs of spinal nerves have dorsal and ventral roots; C1 has no sensory root (see figure Spinal nerve
Spinal nerve Spinal nerveThe spinal nerves exit the vertebral column via an intervertebral foramen. Because the spinal cord is shorter than the vertebral column, the more caudal the spinal nerve, the further the foramen is from the corresponding cord segment. Thus, in the lumbosacral region, nerve roots from lower cord segments descend within the spinal column in a near-vertical sheaf, forming the cauda equina. Just beyond the intervertebral foramen, spinal nerves branch into several parts. Branches of the cervical
and lumbosacral spinal nerves anastomose peripherally into plexuses, then branch into nerve trunks that terminate up to 1 m away in peripheral structures (see figure
Plexuses
Plexuses PlexusesThe term peripheral nerve refers to the part of a spinal nerve distal to the root and plexus. Peripheral nerves are bundles of nerve fibers. They range in diameter from 0.3 to 22 mcm (1 mcm equals 1000 circular mils). Schwann cells form a thin cytoplasmic tube around each fiber and further wrap larger fibers in a multilayered insulating membrane (myelin sheath). The myelin sheath enhances impulse conduction. The largest and most heavily myelinated fibers conduct quickly; they convey motor, touch, and proprioceptive impulses. The less myelinated and unmyelinated fibers conduct more slowly; they convey pain, temperature, and autonomic impulses. Because nerves are metabolically active tissues, they require nutrients, supplied by blood vessels called the vasa nervorum. Peripheral nerve disorders can result from damage to or dysfunction of the one of the following:
Peripheral neuropathies may affect Because sensory and motor cell bodies are in different locations, a nerve cell body disorder typically affects either the sensory or motor component but rarely both. Toxic-metabolic or genetic disorders usually begin symmetrically. Immune-mediated processes may be symmetric or, early in rapidly evolving processes, asymmetric. Damage to the axon transport system for cellular constituents, especially microtubules and microfilaments, causes significant axon dysfunction. First affected are the smaller fibers (because they have greater metabolic requirements) at the most distal part of the nerve. Then, axonal degeneration slowly ascends, producing the characteristic distal-to-proximal pattern of symptoms (stocking-glove sensory loss, followed by weakness). After axonal damage, the fiber regrows within the Schwann cell tube at about 1 mm/day once the pathologic process ends. However, regrowth may be misdirected, causing aberrant innervation (eg, of fibers in the wrong muscle, of a touch receptor at the wrong site, or of a temperature instead of a touch receptor). Regeneration is impossible when the cell body dies and is unlikely when the axon is completely lost.
History should focus on type of symptom, onset, progression, and location, as well as information about potential causes (eg, family history, toxic exposures, past medical disorders).
Physicians should suspect a peripheral nervous system disorder based on the pattern and type of neurologic deficits, especially if deficits are localized to particular nerve roots, spinal nerves, plexuses, specific peripheral nerves, or a combination. These disorders are also suspected in patients with mixed sensory and motor deficits, with multiple foci, or with a focus that is incompatible with a single anatomic site in the CNS. Physicians should also suspect peripheral nervous system disorders in patients with generalized or diffuse weakness but no sensory deficits; in these cases, peripheral nervous system disorders may be overlooked because they are not the most likely cause of such symptoms. Clues that a peripheral nervous system disorder may be the cause of generalized weakness include the following:
Clues that the cause may not be a peripheral nervous system disorder include
Clinical assessment narrows diagnostic possibilities and guides further testing.
Patients with weakness but no sensory deficits can be evaluated with electrodiagnostic testing. Other testing, such as imaging, depends on whether a CNS lesion must be ruled out (eg, MRI if all limbs are affected, to rule out cervical spinal cord compression). Nerve biopsy is occasionally done to help differentiate demyelinating from vasculitic large-fiber neuropathies. If vasculitis is a consideration, the biopsy specimen should include skin and muscle to increase the likelihood of a definitive diagnosis. If a small-fiber neuropathy is suspected, skin punch biopsy can be done; loss of nerve endings supports that diagnosis. Genetic testing is indicated if a hereditary neuropathy is suspected.
Treatment of a peripheral nerve disorder is directed at the underlying disorder when possible. Otherwise, treatment is supportive. A multidisciplinary team approach helps patients cope with progressive neurologic disability:
What is the peripheral nervous system and its functions?Your peripheral nervous system (PNS) is that part of your nervous system that lies outside your brain and spinal cord. It plays key role in both sending information from different areas of your body back to your brain, as well as carrying out commands from your brain to various parts of your body.
What is peripheral in the nervous system?Peripheral nerves reside outside your brain and spinal cord. They relay information between your brain and the rest of your body. The peripheral nervous system is divided into two main parts: Autonomic nervous system (ANS): Controls involuntary bodily functions and regulates glands.
What are the 3 parts of the peripheral nervous system?Parts of the PNS. The PNS can be divided into two components: the somatic nervous system and the autonomic nervous system. The somatic nervous system (SNS) and the autonomic nervous system (ANS) are both part of the peripheral nervous system.
Which best describes the peripheral nervous system?Which best describes the central and peripheral nervous systems? The central nervous system processes stimuli information the body receives, and the peripheral nervous system senses stimuli and carries out the reaction.
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