Histology of Nervous system

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 foreman. 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 Riviera 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 superiority 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  with post ganglionic. Again, post ganglionics follow the blood vessels.

Sympathetic post ganglionics from the sympathetic chain can go back to the spinal nerves (via gray rami communicants) 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. 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).