Chapter 3 - Peripheral nervous system
The peripheral nervous system is divided into somatic and autonomic components.
Somatic nervous system.
The somatic nervous system includes the sensory and motor nerves that innervate the limbs and body wall. Sensory nerve fibers in the peripheral nerves are the peripheral axonal process of neurons in the dorsal root ganglion. The motor axons are the processes of anterior horn cells of the spinal cord.
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. There are also blood vessels (vasa vasorum) and nerves (nervi nervorum) 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 anastamose with arterial branches entering the nerve above and below in order to provide an uninterrupted circulation along the course of the nerve. There is usually sufficient collateral circulation to survive damage to one of the feeding arteries. However, this is unpredictable.
Individual nerve fibers vary widely in diameter and also may 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. Table 1 shows the functional categories of nerve fibers and the relative speed of conduction. On this table, please note that the function of an axon can be deduced from its diameter and from conduction velocity. Because certain conditions preferentially affect myelin, they would be most likely to affect the functions mediated by the largest, fastest, most heavily myelinated axons (see table 1).
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 dorsal root (figure 3). The cell body of this neuron is located in the dorsal 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.
There are several structural types of receptors in the skin. These fall into the category of encapsulated or non-encapsulated receptors. The non-encapsulated endings include free nerve endings, which are simply the peripheral end of the sensory axon. These mostly respond to noxious (pain) and thermal stimuli. There are some specialized free nerve endings around hairs that respond to very light touch and also free nerve endings that contact special skin cells, called Merkle's cells. These Merckle's cells (discs) are specialized cells that release transmitter onto peripheral sensory nerve terminals. The encapsulated endings include Meisner's 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 specialize 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, there are muscle receptors that 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 the majority of fibers comprising the muscle (i.e., 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).
The cutaneous distribution of sensory nerves is shown in figure 4. There is a small area of overlap between sensory distributions of peripheral nerves. It is important to note that there is significant variability in the precise borders of the peripheral distribution of nerves although the general pattern is quite consistent. Nerve roots supply dermatomes (figure 5). With few exceptions, there is complete overlap 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 non-overlapping 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 into 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.
In addition to sensory problems, peripheral nerve injury can affect strength. The principal innervation for the most important muscles is depicted in table 2. 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 there are no muscles supplied by a single nerve root. Nonetheless, there is often detectible weakness. 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, there may be some weakness of knee extension with L3 or L4 lesions, some difficulty with great toe (and, to a lesser extent, ankle) extension with L5 lesions and weakness of great toe plantar flexion with S1 nerve root damage.
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. There is also a matrix in the synaptic cleft containing 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).
Autonomic nervous system
The autonomic nervous system consists of two 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. There are some areas in which blood vessels are under competing sympathetic and parasympathetic control, such as in the nose or erectile tissues. There some areas where there is a competitive balance between sympathetics and parasympathetics, 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.
Both the sympathetic and parasympathetic portions of the autonomic nervous system have a two neuron pathway from the central nervous system to the peripheral organ. Therefore, there is a ganglion interposed in each of these pathways, with the exception of the sympathetic pathway to the adrenal medulla. The adrenal medulla basically functions as a sympathetic ganglion. The two 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, sympathatics have been termed the "thoracolumbar outflow." These preganglionic visceral motor fibers leave the cord in the ventral nerve root and then connect to the sympathetic gangliated chain through the white rami communicans (figure 3). 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 gangliated chain to reach prevertebral ganglia along the aorta (via splanchnic nerves). Additionally, these preganglionics can pass rostrally or caudally through the gangliated chain to reach the head or the lower lumbosacral regions. The sympathetic pathway to the head is shown in figure 6. Sympathetic fibers can go to viscera by one of two pathways. Some postganglionic can leave the gangliated chain and follow blood vessels to the organs. Alternatively, preganglionic fibers may pass directly through the gangliated chain to enter the abdomen as splanchnic nerves. These synapse in ganglia located along the aorta (the celiac, renal, superior or inferior mesenteric ganglia) with postganglionic. Again, postganglionics follow the blood vessels.
Sympathetic postganglioncs from the gangliated 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 and loss of sweating on that side of the head (called Horner's syndrome). This can happen anywhere along the course of the nerve patheway 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 cilliary 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 very close to (or within) the organ walls. The pelvic parasympathatics, which appear as "pelvic splanchnic nerves" activate bladder contraction and also supply lower abdominal and pelvic organs.