Peripheral Nerves - Histology
Fundamental regulation of human activity is mediated by the nervous system. This vast network of neurons is functionally divided into central and peripheral divisions. The central nervous system is comprised of the brain and spinal cord, while the peripheral nervous system includes all spinal and cranial nerve fibres providing end organ innervation. Of note, the peripheral nerve is also further subdivided into the somatic and autonomic divisions; the latter of which can either be sympathetic (fight or freight response) or the parasympathetic (rest and relax response).
On gross inspection, nerves can be perceived as solitary, solid, white bands. Histological inspection reveals that each nerve fibre is actually composed of a large number of axons whose cell bodies reside in central or peripheral ganglia, the spinal cord or the brain. The primary purpose of this article is to evaluate the histology of peripheral nervous tissue. Aspects of peripheral nerve embryology and clinically relevant scenarios will also be discussed.
The peripheral nervous system is comprised of visceral, cranial and spinal nerve fibres and their corresponding ganglia (collection of cell bodies). The cluster of ectodermal cells dorsal to the neural tube in the developing embryo gives rise to the neural crest cells. The neural crest cells give rise to both somatic and visceral sensory cells of the peripheral nervous system. These cells undergo significant morphology during development as they gradually change from bipolar neurons to unipolar neurons. The unipolar processes branch into central and peripheral arms; the former extending to the brain or spinal cord, while the later continues toward afferent sites. The vestibulocochlear nerve (CN VIII) is the exception to this rule, as the sensory neurons found in this nerve remains bipolar throughout life.
With the exception of the olfactory (CN I), optic (CN II) and vestibulocochlear (CN VIII) nerves, the neural crest cells leave the brain to form the associated sensory ganglia. The motor cranial nerves – oculomotor (CN III), trochlear (CN IV), abducens (CN VI), spinal accessory (CN XI) and hypoglossal (CN XII) – contain proprioceptive sensory fibers; but are not associated with defined sensory ganglia. CN I and CN II are not true nerves, but rather projections of the brain.
Anatomy of a Nerve Cell
A nerve cell or neuron consists of an irregularly shaped body with projections (that tapers off in rounded bulbs) known as dendrites. All the standard cellular components can be found in a neuron. Additionally, neurons have an elongated projection that extends from the cell body known as an axon.
The length of an axon is variable, and they are often encased in interrupted insulating sheaths known as myelin sheaths (produced by Schwann cells). The points of interruption are called nodes of Ranvier. Together, the nodes of Ranvier and the myelin sheath can alter the rate of conduction along a neuron. If the myelin sheath is thick, and the neuron has numerous nodes of Ranvier, then the rate of conduction will be greater than if the myelin sheath was thin and the neuron had fewer nodes of Ranvier.
The axon terminates in an arborisation of dendrites known as telodendria (s. telodendron). The telodendria come in close contact with (but do not touch) either neighbouring neurons or end organs. These points of interface are known as synaptic clefts. At these junctions, the preceding neuron releases its neurotransmitters into the synaptic cleft. The neurotransmitters will then bind to corresponding receptors on the effector end plate of the adjacent cell.
Neurons of the peripheral nervous system are classified based on their function and structure. Neurons that detect environmental (temperature, touch, proprioception) and visceral (pain) stimuli are known as sensory neurons. The sensory neurons form afferent nerve fibres that take data to the spinal cord and brain to be processed. These neurons have a short stalk projecting from the cell body that shortly thereafter bifurcates; these are pseudo-unipolar neurons. The description given in the previous paragraph represents the multipolar neuron (motor neuron). They deliver motor commands from the brain and spinal cord to the desired end organ; their axons form the efferent fibres.
The Schwann cells are the main supporting cell in the peripheral nervous system. Its role is to enclose each axon into fatty white myelin. In an unmyelinated neuron, the action potential is propagated sequentially along the axon from one sodium channel to the next (immediately adjacent to it). When there are high-resistance, low capacitance, myelin sheaths with only periodic interruptions (nodes of Ranvier) that may be more than 1 mm apart, the electrical impulse will find the path of least resistance to spread to. This form of cellular excitation is known as saltatory conduction; it facilitates more rapid conduction than in unmyelinated cells.
Another factor that affects the rate of conduction along nerve fibres is diameter. There is a size based classification system that stratifies nerve fibers based on size and speed. The wider diameter fibres are the type A fibres; they all are myelinated and their conduction velocity ranges from 6 – 120 m/s. Type A fibers are further subclassified such that:
- Aα fibres (12-20 μm; 70-120 m/s) conduct impulses the fastest and serve skeletal muscles.
- Aβ fibres (5-12 μm; 40-70 m/s) have the second fastest conduction rate and are involved in vibration, pressure and tactile sensation.
- Aγ fibres (3-6 μm; 10-50 m/s) are the third fastest conductors of the A fibres and are a part of the proprioceptive pathway.
- Aδ fibres (2-5 μm; 6-30 m/s) are the slowest of the A fibres and are responsible for sharp localized pain detection as well as tactile and thermal sensation.
Type B fibres (<3 μm; 3 - 15 m/s) form the preganglionic autonomic pathway and like the type A fibres, are also myelinated.
Type C fibres are the smallest type of nerve fibres (0.4-1.2 μm; 0.5-2.0 m/s), unmyelinated and they conduct visceral pain as well as thermal impulses.
Thin layers of reticular fibres forming loose vascular connective tissue, known as the endoneurium or the sheath of Henle, surrounds a cluster of unmyelinated axons or a single myelinated axon. A thin layer of flattened connective tissue called the perineurium encircles a cluster of axons known as a nerve fascicle.
A thick outer layer of connective tissue then binds all the fascicles together; this is called the epineurium. The epineurium has some degree of inherent laxity that permits stretching of the fascicles at joints and during general movement. Unlike the neurons which are neuroectodermal in origin, the surrounding connective tissue layers arise from the mesoderm.
Epineurium is formed from areolar connective tissue of mesodermal origin. By convention, the thickness of the epineurium is related to the number of fasciculi it envelops. Fibroblasts, types I and III collagen and some adipocytes provide support to the enveloped nerves. In the perineurium, there are layers of fibroblastic polygonal cells and collagen fibres, that are circumscribed by basal lamina. It is a metabolically active border and as such, the cytoplasm of the constituent cells usually include vesicles and the cytoplasmic membrane demonstrates pinocytotic activity. Endoneurium is rich in type III collagen that is arranged along the long axis of the nerve fibres. This layer is primarily populated by Schwann cells as well as endothelial cells. Occasional mast cells and local macrophages may also be encountered.
The skin is one of the most extensive organs in (rather, on) the human body. Not only does it serve as an impermeable membrane to isolate the external from the internal environment, but it also serves as a stimulus detector. As peripheral nerve fibers enter the subcutaneous connective tissue, they arborize horizontally to form the subcutaneous (deep to the skin in loose connective tissue), dermal (deep part of the dermis with dense collagenous reticular tissue) and papillary (in the dermal papillary layer) plexuses.
There are regional differences in the concentration of nerve endings across the skin. The skin of the palm, soles and aspects of the face contain a larger number of sensory nerve endings compared to the skin of the back. Consequently, stimuli in the areas of the palms and face are more readily detectable in these areas than in the back.
The skin is equipped with non-encapsulated and encapsulated nerve endings that detect thermal, painful and tactile stimuli. The non-encapsulated fibers may either exist freely in the interstitial space or as nerve endings closely applied to adjacent cells; while the encapsulated nerve endings are completely enclosed. Non-encapsulated nerve endings include:
- Peritrichial ending divide in the connective tissue and gains access to the designated hair follicle. A single follicle can be supplied by as many as 20 axons; while each axon supplies several follicles. They transmit light touch via displacement of the hair shafts.
- Merkel’s ending terminate as flattened expansions adjacent to Merkel cells in the stratum basale of the epidermis. These endings respond to touch.
The encapsulated nerve endings include the following:
- Vater-Pacini corpuscles (or simply Pacinian corpuscle) are demyelinated portions of an axon that has become encapsulated by ellipsoid layers of cytoplasm. They are found in glabrous and hairy skin, as well as in the subcutaneous tissue. Pacinian corpuscles detect vibrational stimuli.
- End bulbs are a diverse group of spherical structures with coils of terminal axons in the cellular capsule. They have been identified in the dermis of glabrous skin and in mucous membranes. Several subtypes of end bulbs have been identified, including end bulbs of Krause, genital corpuscles and mucocutaneous endings.
- Meissner’s corpuscles receive innervation from at least three myelinated axons that coalesce in the collagenous capsule. These corpuscles are more abundant at the ridges of the dermal papillary in the fingers. They are most significant in discerning texture with the fingers as they are able to detect mechanical deformities.
- Ruffini ending represent a collection of axonal terminations enclosed by a capsule. These act as stretch receptors.
Another important sensory modality mediated by the peripheral nervous system is proprioception. This refers to a sense of spatial awareness of parts of one’s body. Special sensory endings are located in muscles, joints and tendons that relay this information to the central nervous system. Adequate relay of this information is integral to the efficient coordination and execution of motor function.
Joints are the points of articulation between two bones, with varying ability to articulate with each other. Highly mobile synovial joints (lined with synovial membrane) contain Ruffini-like endings (responds to the initiation of movement) and Pacinian corpuscles (responds to the cessation of movement). Golgi tendon organ-like receptors detect over-distension in the ligaments and consequently take part in inhibiting muscle contraction. The pain perceived during injury or pathological processes within a synovial joint is thought to be mediated by free nerve endings that exist within the connective tissue around the joint.
Tendons are the fibrous extremities of muscle tissue that lack the ability to contract. They serve as anchors that attach muscles to bone. At the musculotendinous junction axons of Aβ fibres lose their myelin sheath as they enter the Golgi tendon organs (neurotendinous spindles) before terminating on the intrafusal tendon fibres. The intrafusal fibres detect increasing tension in the tendon and this is transmitted to the Golgi tendon organs. Subsequently, afferent impulses are relayed to spinal interneurons, which inhibit the alpha motor neurons of the anterior horn of the spinal cord resulting in muscle relaxation.
Like tendons, muscle tissue also contains intrafusal fibres as well as extrafusal fibres. The intrafusal fibres are in contact with neuromuscular spindles; which are organs of proprioception located in skeletal muscle tissue. The neuromuscular spindles are in line with muscular septa (separates muscle fibres). This architectural arrangement results in lengthening of the neuromuscular spindles when the muscle fibres are stretched.
Afferent information from the viscera is mediated primarily by non-encapsulated nerve endings. The exception to this rule is Pacinian corpuscles that exist primarily in the mesenteries. These sensory fibres detect satiety and fullness of the urinary bladder and rectum. Additionally, they also relay pain sensation related to pathological processes of the viscera.
The telodendria form synapses with specialized areas of effector organs (muscle fibres and secretory cells) known as neuroeffectors. Most endocrine organs are stimulated directly by hormones released into the bloodstream from the hypothalamus. Exocrine glands, smooth and cardiac muscles are regulated by the parasympathetic and sympathetic divisions of the peripheral nervous system (autonomic pathway). The effector endings at these synapses are known as varicosities at the distal ends of the unmyelinated axons. There are no definitive specialized areas on the effector cells as seen in skeletal muscle cells. However the varicosities form synapses with these cells and release either adrenergic (sympathetic system) or cholinergic (parasympathetic system) neurotransmitters at the neuroeffector junctions.
Skeletal muscles cells also have modified synaptic interactions with telodendria known as myoneural junctions (motor end plates). A single motor neuron may innervate several to several hundred muscle cells; collectively the unit is referred to as a motor end plate. The telodendria are rich in synaptic vesicles and mitochondria. The synaptic vesicles contain acetylcholine (ACh), which is released from the telodendria under the influence of the action potential. Acetylcholine subsequently binds to acetylcholine receptors (AChR) present at the motor end plate of the muscle cell.
Hematoxylin and eosin (H&E) is the most ubiquitously used stain in histology. However, there are features of nervous tissue that is poorly visualized with H&E, therefore other staining techniques were employed. The myelin sheath is made up of fat, which does not stain with H&E. Instead, osmium tetroxide comes in handy as it stains fat cells black. This also allows for better visualization of the nodes of Ranvier as those areas would remain unstained. Gold chloride is another stain used to highlight neuromuscular junctions, neurofilaments and myelin. This technique depends on the deposition of gold chloride in the muscle tissue.