Peripheral Nervous System Anatomy

Updated: Jun 30, 2016
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Thomas R Gest, PhD  more...
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Overview

Overview

The peripheral nervous system refers to parts of the nervous system outside the brain and spinal cord. It includes the cranial nerves, spinal nerves and their roots and branches, peripheral nerves, and neuromuscular junctions. 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.

In the peripheral nervous system, bundles of nerve fibers or axons conduct information to and from the central nervous system. The autonomic nervous system is the part of the nervous system concerned with the innervation of involuntary structures, such as the heart, smooth muscle, and glands within the body. It is distributed throughout the central and peripheral nervous systems.

An image depicting the peripheral nervous system can be seen below.

Major divisions of the peripheral nervous system. Major divisions of the peripheral nervous system.

Nerve cells are called neurones. A neurone consists of a cell body (with a nucleus and cytoplasm), dendrites that carry electrical impulses to the cell, and a long axon that carries the impulses away from the cell. The axon of one neurone and the dendrites of the next neurone do not actually touch. The gap between neurones is called the synapse.

Neuronal function is complex and involves numerous processes in nerve transmission. Generation of a nerve impulse (action potential) of a sensory neurone occurs as a result of a stimulus such as light, a particular chemical, or stretching of a cell membrane by sound. Conduction of an impulse along a neurone occurs from the dendrites to the cell body to the axon. Transmission of a signal to another neuron across a synapse occurs via chemical transmitter. This substance causes the next neurone to be electrically stimulated and keeps the signal going along a nerve.

Ganglia may be divided into sensory ganglia of spinal nerves (spinal or posterior root ganglia) and cranial nerves and autonomic ganglia. Sensory ganglia of spinal nerves are fusiform swellings situated on the posterior root of each spinal nerve just proximal to the root’s junction with a corresponding anterior root. They are referred to as spinal or posterior root ganglia. Similar ganglia that are also found along the course of cranial nerves V, VII, VIII, IX, and X are called sensory ganglia of these nerve. Autonomic ganglia, which are often irregular in shape, are situated along the course of efferent nerve fibers of the autonomic nervous system. They are found in the paravertebral sympathetic chains, around the roots of the great visceral arteries in the abdomen, and close to, or embedded within, the walls of various viscera.

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Gross Anatomy

Subdivisions of the peripheral nervous system

The sensory (afferent) division carries sensory signals by way of afferent nerve fibers from receptors in the central nervous system (CNS). It can be further subdivided into somatic and visceral divisions. The somatic sensory division carries signals from receptors in the skin, muscles, bones and joints. The visceral sensory division carries signals mainly from the viscera of the thoracic and abdominal cavities.

The motor (efferent) division carries motor signals by way of efferent nerve fibers from the CNS to effectors (mainly glands and muscles). It can be further subdivided into somatic and visceral divisions. The somatic motor division carries signals to the skeletal muscles. The visceral motor division, also known as the autonomic nervous system, carries signals to glands, cardiac muscle, and smooth muscle. It can be further divided into the sympathetic and parasympathetic divisions. [1, 2, 3, 4, 5]

The sympathetic division tends to arouse the body to action. The parasympathetic divisions tend to have a calming effect.

Nerve fibers of the PNS are classified according to their involvement in motor or sensory, somatic or visceral pathways. Mixed nerves contain both motor and sensory fibers. Sensory nerves contain mostly sensory fibers; they are less common and include the optic and olfactory nerves. Motor nerves contain motor fibers.

Peripheral nerve supply to the muscles in the lowe Peripheral nerve supply to the muscles in the lower extremity.
Peripheral nerve supply to the muscles in the uppe Peripheral nerve supply to the muscles in the upper extremity.

Anatomy of nerves and ganglia

A nerve is an organ composed of multiple nerve fibers bound together by sheaths of connective tissue. The sheath adjacent to the neurilemma is the endoneurium, which houses blood capillaries that feed nutrients and oxygen to the nerve. In large nerves, fibers are bundled into fascicles and wrapped in a fibrous perineurium. The entire nerve is covered with a fibrous epineurium.

A ganglion is a cluster of neuron cell bodies enveloped in an epineurium continuous with that of a nerve. A ganglion appears as a swelling along the course of a nerve. The spinal ganglia or posterior or dorsal root ganglia associated with the spinal nerves contain the unipolar neurons of the sensory nerve fibers that carry signals to the cord. The fiber passes through the ganglion without synapsing. However, in the autonomic nervous system, a preganglionic fiber enters the ganglion and in many cases synapses with another neuron. The axon of the second neuron leaves the ganglion as the postganglionic fiber.

Cranial nerves

The cranial nerves emerge from the base of the brain and lead to muscles and sense organs in the head and neck for the most part. The twelve pairs of cranial nerves with their functions are as follows:

  • Olfactory nerve (I): Sensory nerve that carries impulses for smell to the brain.
  • Optic nerve (II): Sensory nerve that carries impulses for vision to the brain.
  • Oculomotor nerve (III): Motor nerve that carries impulses to the extrinsic eye muscles, which help direct the position of the eyeball. This nerve also carries impulses to the muscles that regulate the size of the pupil.
  • Trochlear nerve (IV): Motor nerve that carries impulses to one extrinsic eye muscle (the superior oblique muscle). Once again, this muscle helps regulate the position of the eyeball.
  • Trigeminal nerve (V): A mixed nerve. The sensory fibers of this nerve carry impulses for general sensation (touch, temperature and pain) associated with the face, teeth, lips and eyelids. The motor fibers of this nerve carry impulses to some of the mastication muscles of the face.
  • Abducens nerve (VI): A mixed nerve, but primarily a motor nerve. This nerve carries impulses to the lateral rectus muscle of the eye. This muscle is an extrinsic eye muscle that is involved in positioning the eyeball.
  • Facial nerve (VII): A mixed nerve. The sensory fibers of this nerve carry taste sensations from the tongue. The motor fibers of this nerve carry impulses to many of the muscles of the face and they carry impulses to the lacrimal, submandibular, and sublingual glands.
  • Vestibulocochlear nerve (VIII): A sensory nerve that carries impulses for hearing and equilibrium from the ear to the brain.
  • Glossopharyngeal nerve (IX): A mixed nerve. The sensory fibers of this nerve carry basic sensory information and taste sensations from the pharynx and tongue to the brain. The motor fibers of this nerve carry impulses associated with swallowing to the pharynx.
  • Vagus nerve (X): A mixed nerve. The sensory fibers of this nerve carry impulses from the pharynx, larynx, and most internal organs to the brain. The motor fibers of this nerve carry impulses to internal organs of the chest and abdomen and to the skeletal muscles of the larynx and pharynx.
  • Accessory nerve (XI): A mixed nerve, but primarily motor. Carries impulses to muscles of the neck and back.
  • Hypoglossal nerve (XII): Primarily a motor nerve. This nerve carries impulses to the muscles that move and position the tongue.

Spinal nerves

Thirty one pairs of spinal nerves exist: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

  • Proximal branches: Each spinal nerve branches into a posterior root and an anterior root. The spinal or posterior root ganglion is occupied by cell bodies from afferent neurons. The convergence of posterior and anterior roots forms the spinal nerve. The cauda equina is formed by the roots arising from segments L2 to Co of the spinal cord.
  • Distal branches: After emerging from the vertebral column, the spinal nerve divides into a posterior ramus, an anterior ramus, and a small meningeal branch that leads to the meninges and vertebral column. The posterior ramus innervates the muscles and joints of the spine and the skin of the back. The anterior ramus innervates the anterior and lateral skin and muscles of the trunk, plus gives rise to nerves leading to the limbs (see image below).
  • Nerve plexuses: The anterior rami merge to form nerve plexuses in all areas except the thoracic region (see the image below).
    Unlabelled brachial plexus (for studying). Unlabelled brachial plexus (for studying).
  • Cutaneous innervation and dermatomes: Each spinal nerve except C1 receives sensory input from a specific area of the skin called a dermatome. A dermatome map is a diagram of the cutaneous regions innervated by the branches of each spinal nerve, such a map is an oversimplification, however – each of you is unique as to what you feel.

Autonomic nervous system

The visceral reflexes are mediated by the autonomic nervous system (ANS), which has two divisions (sympathetic and parasympathetic). The target organs of the ANS are glands, cardiac muscle, and smooth muscle: it operates to maintain homeostasis. Control over the ANS is, for the most part, involuntary. The ANS differs structurally from the somatic nervous system in that 2 neurons leading from the ANS to the effector exist, a preganglionic neuron and a postganglionic neuron.

  • Anatomy of the sympathetic division: The sympathetic division is also called the thoracolumbar division because of the spinal nerve it uses. Paravertebral ganglia occur close to the vertebral column. Preganglionic ganglia are short, while postganglionic neurons, traveling to their effector, are long. When 1 preganglionic neuron fires, it can excite multiple postganglionic fibers that lead to different target organs (mass activation). In the thoracolumbar region, each paravertebral ganglion is connected to a spinal nerve by 2 communicating rami, the white communicating ramus and the gray communicating ramus. Nerve fibers leave the paravertebral ganglia by gray rami communicantes and splanchnic nerves.
  • Anatomy of the parasympathetic division: The parasympathetic division is also referred to as the craniosacral division because its fibers travel in some cranial nerves (III, VII, IX, X) and sacral nerves (S2-4). The parasympathetic ganglia (terminal ganglia) lie in or near the target organs. The parasympathetic fibers leave the brainstem by way of the oculomotor, facial, glossopharyngeal, and vagus nerves. The parasympathetic system uses long preganglionic and short postganglionic fibers.
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Microscopic Anatomy

A motor unit consists of an anterior horn cell, its motor axon, the muscle fibers it innervates, and the connection between them (neuromuscular junction). 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 posterior root ganglia.

Nerve fibers outside the spinal cord join to form anterior (ventral) motor roots and posterior (dorsal) sensory root nerve roots. The anterior and posterior roots combine to form a spinal nerve. Thirty of the 31 pairs of spinal nerves have anterior and posterior roots; C1 has no sensory root.

The 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 μm away in peripheral structures. The intercostal nerves are segmental.

The term peripheral nerve refers to the part of a spinal nerve distal to the nerve roots. Peripheral nerves are bundles of nerve fibers. They range in diameter from 0.3-22 μm. Schwann cells form a thin cytoplasmic tube around each fiber and further wrap larger fibers in a multilayered insulating membrane (myelin sheath).

Peripheral nerves have multiple layers of connective tissue surrounding axons, with the endoneurium surrounding individual axons, perineurium binding axons into fascicles, and epineurium binding the fascicles into a nerve. Blood vessels (vasa vasorum) and nerves (nervi nervorum) are also contained within the nerve. Nerve fibers in peripheral nerves are wavy, such that a length of peripheral nerve can be stretched to half again its length before tension is directly transmitted to nerve fibers. Nerve roots have much less connective tissue, and individual nerve fibers within the roots are straight, leading to some vulnerability.

Peripheral nerves receive collateral arterial branches from adjacent arteries. These arteries that contribute to the vasa nervorum anastomose with arterial branches entering the nerve above and below in order to provide an uninterrupted circulation along the course of the nerve.

Individual nerve fibers vary widely in diameter and may also be myelinated or unmyelinated. Myelin in the peripheral nervous system derives from Schwann cells, and the distance between nodes of Ranvier determines the conduction rate. Because certain conditions preferentially affect myelin, they would be most likely to affect the functions mediated by the largest, fastest, most heavily myelinated axons.

Sensory neurons are somewhat unique, having an axon that extends to the periphery and another axon that extends into the central nervous system via the posterior root. The cell body of this neuron is located in the posterior root ganglion or one of the sensory ganglia of sensory cranial nerves. Both the peripheral and the central axon attach to the neuron at the same point, and these sensory neurons are called "pseudounipolar" neurons.

Before a sensory signal can be relayed to the nervous system, it must be transduced into an electrical signal in a nerve fiber. This involves a process of opening ion channels in the membrane in response to mechanical deformation, temperature or, in the case of nociceptive fibers, signals released from damaged tissue. Many receptors become less sensitive with continued stimuli, and this is termed adaptation. This adaptation may be rapid or slow, with rapidly adapting receptors being specialized for detecting changing signals.

Several structural types of receptors exist in the skin. These fall into the category of encapsulated or nonencapsulated receptors. The nonencapsulated endings include free nerve endings, which are simply the peripheral end of the sensory axon. These mostly respond to noxious (pain) and thermal stimuli. Some specialized free nerve endings around hairs respond to very light touch; also, some free nerve endings contact special skin cells, called Merkel cells.

These Merkel cells (discs) are specialized cells that release transmitter onto peripheral sensory nerve terminals. The encapsulated endings include Meisner corpuscles, Pacinian corpuscles, and Ruffini endings. The capsules that surround encapsulated endings change the response characteristics of the nerves. Most encapsulated receptors are for touch, but the Pacinian corpuscles are very rapidly adapting and, therefore, are specialized to detect vibration. Ultimately, the intensity of the stimulus is encoded by the relative frequency of action potential generation in the sensory axon.

In addition to cutaneous receptors, muscle receptors are involved in detecting muscle stretch (muscle spindle) and muscle tension (Golgi tendon organs). Muscle spindles are located in the muscle bellies and consist of intrafusal muscle fibers that are arranged in parallel with most fibers comprising the muscle (ie, extrafusal fibers). The ends of the intrafusal fibers are contractile and are innervated by gamma motor neurons, while the central portion of the muscle spindle is clear and is wrapped by a sensory nerve ending, the annulospiral ending. This ending is activated by stretch of the muscle spindle or by contraction of the intrafusal fibers (see section V). The Golgi tendon organs are located at the myotendinous junction and consist of nerve fibers intertwined with the collagen fibers at the myotendinous junctions. They are activated by contraction of the muscle (muscle tension).

Both the sympathetic and parasympathetic portions of the autonomic nervous system have a 2-neuron pathway from the central nervous system to the peripheral organ. Therefore, a ganglion is interposed in each of these pathways, with the exception of the sympathetic pathway to the suprarenal (adrenal) medulla. The suprarenal medulla basically functions as a sympathetic ganglion. The 2 nerve fibers in the pathway are termed preganglionic and postganglionic. At the level of the autonomic ganglia, the neurotransmitter is typically acetylcholine. Postganglionic parasympathetic neurons also release acetylcholine, while norepinephrine is the postganglionic transmitter for most sympathetic nerve fibers. The exception is the use of acetylcholine in sympathetic transmission to the sweat glands and erector pili muscles as well as to some blood vessels in muscle.

Sympathetic preganglionic neurons are located between T1 and L2 in the lateral horn of the spinal cord. Therefore, sympathetics have been termed the "thoracolumbar outflow." These preganglionic visceral motor fibers leave the cord in the anterior nerve root and then connect to the sympathetic chain through the white rami communicans. This chain of connected ganglia follows the sides of the vertebrae all the way from the head to the coccyx. These axons may synapse with postganglionic neurons in these paravertebral ganglia. Alternatively, preganglionic fibers can pass directly through the sympathetic chain to reach prevertebral ganglia along the aorta (via splanchnic nerves).

Additionally, these preganglionics can pass superiorly or inferiorly through the interganglionic rami in the sympathetic chain to reach the head or the lower lumbosacral regions. Sympathetic fibers can go to viscera by 1 of 2 pathways. Some postganglionic can leave the sympathetic chain and follow blood vessels to the organs. Alternatively, preganglionic fibers may pass directly through the sympathetic chain to enter the abdomen as splanchnic nerves. These synapse in ganglia located along the aorta (the celiac, aorticorenal, superior, or inferior mesenteric ganglia) with postganglionic. Again, postganglionics follow the blood vessels.

Sympathetic postganglionics from the sympathetic chain can go back to the spinal nerves (via gray rami communicans) to be distributed to somatic tissues of the limbs and body walls. For example, the somatic response to sympathetic activation will result in sweating, constriction of blood vessels in the skin, dilation of vessels in muscle and in piloerection. Damage to sympathetic nerves to the head results in slight constriction of the pupil, slight ptosis, and loss of sweating on that side of the head (called Horner syndrome). This can happen anywhere along the course of the nerve pathway including the upper thoracic spine and nerve roots, the apex of the lung, the neck or the carotid plexus of postganglionics.

Parasympathetic nerves arise with cranial nerves III, VII, IX, and X, as well as from the sacral segments S2-4. Therefore, they have been termed the "craniosacral outflow." Parasympathetics in cranial nerve III synapse in the ciliary ganglion and are involved in pupillary constriction and accommodation for near vision. Parasympathetics in cranial nerve VII synapse in the pterygopalatine ganglion (lacrimation) or the submandibular ganglion (salivation), while those in cranial nerve IX synapse in the otic ganglion (salivation from parotid gland).

The vagus nerve follows a long course to supply the thoracic and abdominal organs up to the level of the distal transverse colon, synapsing in ganglia within the organ walls. The pelvic parasympathetics, which appear as the pelvic splanchnic nerves, activate bladder contraction and also supply lower abdominal and pelvic organs.

Physiology

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.

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Pathophysiological Variants

The sensory and motor cell bodies are in different locations, and therefore, a nerve cell body disorder typically affects either the sensory or motor component but rarely both.

Damage to the myelin sheath (demyelination) slows nerve conduction. Demyelination affects mainly heavily myelinated fibers, causing large-fiber sensory dysfunction (buzzing and tingling sensations), motor weakness, and diminished reflexes. The hallmark of acquired demyelinating polyneuropathy is severe motor weakness with minimal atrophy.

Because the vasa nervorum do not reach the center of a nerve, centrally located fascicles are most vulnerable to vascular disorders (eg, vasculitis, ischemia). These disorders result in small-fiber sensory dysfunction (sharp pain and burning sensations), motor weakness proportional to atrophy, and less severe reflex abnormalities than in other nerve disorders. The distal two-thirds of a limb is affected most. Initially, deficits tend to be asymmetric because the vasculitic or ischemic process is random. However, multiple infarcts may later coalesce, causing symmetric deficits (multiple mononeuropathy).

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, weakness).

Damage to the myelin sheath (eg, by injury or Guillain-Barré syndrome) can often be repaired by surviving Schwann cells in about 6-12 wk.

After axonal damage, the fiber regrows within the Schwann cell tube at about 1 mm per 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 virtually impossible when the cell body dies and is unlikely when the axon is completely lost.

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Other Considerations

Somatic reflexes

Reflexes are quick, involuntary, stereotyped reactions of peripheral effectors to stimulation. A spinal reflex is made up of a reflex arc, including somatic receptors, afferent nerve fibers, interneurons, efferent nerve fibers and skeletal muscles.

Muscle spindle

The muscle spindle is a stretch receptor located in muscle. It is a cigar-shaped organ containing 3-12 modified muscle fibers wrapped in a fibrous capsule. Muscle spindles have 3 types of nerve fibers: Primary afferent, secondary afferent, and gamma motor neurons.

The stretch reflex

When a muscle is stretched, it contracts to maintain tone. This is the stretch (myotatic) reflex. Stretch reflexes involve specific muscles and sometimes feed back to a set of synergists and antagonists. These reflexes are important in coordinating vigorous and precise movements. The tendon reflex (knee jerk) is an example of a monosynaptic reflex arc. For reflexes like the knee jerk to work, reciprocal inhibition of antagonistic muscles must occur simultaneously.

The flexor (withdrawal) reflex

Flexor reflexes are important when a limb must be pulled away from harm. These types of reflexes involve a polysynaptic reflex arc, a pathway in which signals travel over many synapses on their way back to the muscle.

The Golgi tendon reflex

Golgi tendon organs are proprioceptors located at the junction of a muscle and its tendon. Golgi tendon organs produce an inhibitory response called the Golgi tendon reflex when muscle contracts too tightly. This prevents damage to the tendon.

Development of the nervous system

Before the formation of the nervous system in the embryo, 3e main cell layers become differentiated. The innermost layer, the endoderm, gives rise to the gastrointestinal tract, the lungs, and the liver. The mesoderm gives rise to the muscle, connective tissues, and the vascular system. The third and outer most layer, the ectoderm, formed of columnar epithelium, gives rise to the entire nervous system and skin.

During the third week of development, the ectoderm on the dorsal surface of the embryo between the primitive knot and the buccopharyngeal membrane becomes thickened to form the neural plate.

The plate, which is pear shaped and wider cranially, develops a longitudinal neural groove. The groove now deepens so that it is bounded on either side by neural folds. With further development, the neural folds fuse, converting the neural groove into a neural tube. Fusion starts at about the midpoint along the groove and extends cranially and caudally so that in the earliest stage, the cavity of the tube remains in communication with the amniotic cavity through the anterior and posterior neuropores.

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Etiology

Disorders can result from damage to or dysfunction of the cell body, myelin sheath, axons, muscle or neuromuscular junction. Disorders can be genetic or acquired (due to toxic, metabolic, traumatic, infectious, or inflammatory conditions). Peripheral neuropathies may affect one nerve (mononeuropathy), several discrete nerves (multiple mononeuropathy, or mononeuritis multiplex), or multiple nerves diffusely (polyneuropathy). Some conditions involve a plexus (plexopathy) or nerve root (radiculopathy). More than one site can be affected; eg, in the most common variant of Guillain-Barré syndrome, multiple segments of cranial nerves, usually the facial nerves, may be affected.

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Evaluation

Clinical evaluation typically starts with history, and the focus should remain on type of symptom, onset, progression, and location, as well as information about potential causes (eg, family history, toxic exposures, past medical disorders). Physical and neurologic examination should further define the type of deficit (eg, motor deficit, type of sensory deficit, combination). Sensation (using pinprick and light touch for small fibers and vibration for large fibers), proprioception, motor strength, and deep tendon reflexes are evaluated. Cranial nerve and autonomic function are also evaluated. Whether motor weakness is proportional to the degree of atrophy is noted, as are type and distribution of reflex abnormalities.

Physicians should suspect a peripheral nervous system disorder based on the pattern and type of neurologic deficits, especially if deficits are in the territories of 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:

  • Patterns of generalized weakness that suggest a specific cause (eg, predominant ptosis and diplopia, which suggest early myasthenia gravis)
  • Symptoms and signs other than weakness that suggest a specific disorder or group of disorders (eg, cholinergic effects, which suggest organophosphate poisoning)
  • Deficits in a stocking-glove distribution, which suggest diffuse axonal disorders or polyneuropathy
  • Fasciculations
  • Hypotonia
  • Muscle wasting without hyperreflexia
  • Weakness that is progressive, chronic, and unexplained

Clues that the cause may not be a peripheral nervous system disorder include upper motor neuron signs including hyperreflexia and hypertonia. Hyporeflexia is consistent with peripheral nervous system deficits but is nonspecific.

Although many exceptions are possible, certain clinical clues may also suggest possible causes of peripheral nervous system deficits

Neurological History and examination can narrow the diagnostic possibilities and further guide with testing. Usually, nerve conduction studies are done to help identify the level of involvement at the nerve, plexus, root, muscle or neuromuscular junction. In addition, it can occasionally help distinguishing demyelinating from axonal lesions.

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Nerve Roots Supply Dermatomes

With few exceptions, complete overlap exists between adjacent dermatomes. This means that the loss of a single nerve root rarely produces significant loss of skin sensitivity. The exception to this rule is found in small patches in the distal extremities, which have been termed "autonomous zones." In these regions, single nerve roots supply distinct and nonoverlapping areas of skin. By their nature the "autonomous zones" represent only a small portion of any dermatome and only a few nerve roots have such autonomous zones.

For example, the C5 nerve root may be the sole supply to an area of the lateral arm and proximal part of the lateral forearm. The C6 nerve root may distinctly supply some skin of the thumb and index finger. Injuries to the C7 nerve root may decrease sensation over the middle and sometimes the index finger along with a restricted area on the dorsum of the hand. C8 nerve root lesions can produce similar symptoms over the small digit, occasionally extending in to the hypothenar area of the hand. In the lower limb, L4 nerve root damage may decrease sensation over the medial part of the leg, while L5 lesions affect sensation over part of the dorsum of the foot and great toe. S1 nerve root lesions typically decrease sensation on the lateral side of the foot.

Damage to peripheral nerves often produces a very recognizable pattern of severe weakness and (with time) atrophy. Damage to single nerve roots usually does not produce complete weakness of muscles since no muscles are supplied by a single nerve root. Nonetheless, weakness is often detectable.

Examples in the upper extremity include weakness of shoulder abductors and external rotators with C5 nerve root lesions, weakness of elbow flexors with C6 nerve root lesions, possible weakness of wrist and finger extension with C7 nerve root lesions, and some weakness of intrinsic hand muscles with C8 and T1 lesions. In the lower extremity, some weakness of knee extension with L3 or L4 lesions may occur, some difficulty with great toe (and, to a lesser extent, ankle) extension with L5 lesions, and weakness of great toe plantar flexion may occur with S1 nerve root damage (see image below).

Dermatome (cutaneous sensory nerve root) distribut Dermatome (cutaneous sensory nerve root) distribution in the upper and lower extremity.

Motor nerve fibers end in myoneural junctions. These consist of a single motor axon terminal on a skeletal muscle fiber. The myoneural junction includes a complex infolding of the muscle membrane, the ridges of which contain nicotinic acetylcholine receptors. A matrix in the synaptic cleft contains acetylcholinesterase, involved in termination of action of the neurotransmitter.

One motor neuron has connections with many muscle fibers through collateral branches of the axon. This is called the "motor unit" and can vary from a handful of muscle fibers per motor neuron in muscles of very fine control (such as eye muscles) up to several thousands (as in the gluteal muscles).

The autonomic nervous system consists of 2 main divisions, the sympathetic and the parasympathetic nervous systems. The sympathetics are primarily involved in responses that would be associated with fighting or fleeing, such as increasing heart rate and blood pressure as well as constricting blood vessels in the skin and dilating them in muscles. The parasympathetic nervous system is involved in energy conservation functions and increases gastrointestinal motility and secretion. It also increases bladder contractility.

Some areas exist in which blood vessels are under competing sympathetic and parasympathetic control, such as in the nose or erectile tissues. Some areas exist where a competitive balance between sympathetics and parasympathetics exists, such as the effects on heart rate or the pupil. For some functions, sympathetics and parasympathetics cooperate; an example being parasympathetic nerves, which are necessary for erection and sympathetics for ejaculation.

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