Histology of Peripheral Nervous System PDF

HISTOLOGY OF THE PERIPHERAL NERVOUS SYSTEM Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected] ANATOMICAL ORGANIZATION Central Nervous System (CNS) • Brain (cerebrum) • Cerebellum • Spinal cord (Medulla Spinalis) Peripheral nervous system (P...

HISTOLOGY OF THE PERIPHERAL NERVOUS SYSTEM Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected] ANATOMICAL ORGANIZATION Central Nervous System (CNS) • Brain (cerebrum) • Cerebellum • Spinal cord (Medulla Spinalis) Peripheral nervous system (PSS) • Ganglions   • Nerves   • Autonomic (sympathetic and parasympathetic) ganglia Dorsal root (spinal or sensory) ganglia Cranial nerves (those emerge from brain) Spinal nerves (those emerge from spinal cord) Nerve endings NERVOUS SYSTEM • Formed by a network of many billion nerve cells(neurons) and many more supporting cells called glial cells. • Nerve tissue is distributed throughout the body as an integrated communications network. • Cells in both central and peripheral nerve tissue are of two kinds: Neurons Glial cells Features of Nerve Tissue  Nervous tissue contains nerve cells (neurons), neuroglia cells and very few connective tissues.  Nerve cells are separated from connective tissue by a wall called blood brain barrier.  Nerve cells do not exhibit mitosis(non-divided). Neuroglia cells exhibit mitotic activity. NEURON Cell body (Perikaryon, soma): Contains nucleus & cytoplasm Projections a. Dendrites Multiple, receives stimuli b. Axon Single, conducts impulses away  Synaptic boutons are small swellings that are found at the terminal ends of axons. CELL BODY(SOMA) • The cell body (perykarion) is the dilated region of the neuron that contains a large, euchromatic nucleus with a prominent nucleolus and surrounding perinuclear cytoplasm. • The perinuclear cytoplasm contains abundant rough-surfaced endoplasmic reticulum and free ribosomes. On light microscopy, the rough endoplasmic reticulum with rosettes of free ribosomes appears as small bodies, called Nissl bodies DENDRITES • Dendrites, which receive information from other neurons at specialized areas of contact called synapses. • The dendrites are projections that branch many times, forming small, tree-shaped structures protruding from the cell body that provide locations for other neurons to communicate with the cell body. • Information flows through a neuron from the dendrites, across the cell body, and down the axon. • This gives the neuron a polarity—meaning that information flows in this one direction. AXON • The axon arises from the cell body as a single thin process, much longer than the dendrites. • Its thickness is directly related to conduction velocity, which increases with axonal diameter. • Some axons possess collateral branches. • The portion of the axon between the cell body and the beginning of the myelin sheath is the initial segment. • The axonal cytoplasm is called axoplasm. GLIAL CELLS • Support neuronal survival and activities and are 10 times more abundant than neurons in the mammalian brain. • There are six major kinds of glial cells, four in the CNS and two in the PNS. GLIAL CELLS • In the CNS  Astrocytes  Oligodendrocyte  Microglia  Ependymal cells In the PNS Schwann cells Satellite cells SCHWANN CELLS • Envelop nerve fibers in the peripheral nervous system. • Schwann cell produces a myelin sheath. • assist in the regeneration of damaged fibers • Allows for faster action potential propagation along an axon in the PNS SATELLITE CELLS – Cuboidal or squamous glial cells of ganglia arranged around the cell bodies of neurons – Function; metabolic and mechanical support to ganglia cells. THE PERIPHERAL NERVOUS SYSTEM • Main components of the peripheral nervous system (PNS) are the nerves, ganglia and nerve endings. • Peripheral nerves are bundles of nerve fibers (axons) individually surrounded by Schwann cells and connective tissue. Nerve Fibers • A nerve fiber is the basic structural and functional unit of peripheral nerves • As axons course through body tissues, they are associated with Schwann cells. • The axon with its associated Schwann cells forms a nerve fiber. MYELINATED FIBERS • The myelin sheath is a spiral layer of insulation around a nerve fiber. • Myelin is a large lipoprotein complex, its composition is like that of plasma membranes in general. • 20% protein and 80% lipid (phospholipids, glycolipids, and cholesterol) MYELINATION IN PNS • Myelinated axons: Schwann cells wrapping repeatedly around an axon to form the multilayered myelin sheath. • Unmyelinated axons: Several unmyelinated axons wrapping by one cytoplasm of schwann cell. • Although one Schwann cell can myelinate only one axon, several unmyelinated axons can be enveloped by a one Schwann cell. Myelination of PNS axons • Schwann cell engulfs one portion along the length of a large diameter axon. • Schwann cell membrane fuses around the axon and one thin extension of schwann cell elongates greatly and wraps itself repeatedly around the axon to form multiple compacted layers (myelin sheat). • Myelin layers are rich in lipid and provide insulation and facilitate formation of action potential along the axolemma. • One Schwann cell is responsible for the formation of one internodal length of myelin. Schmidt-Lanterman (Myelin cleft) • The axon enveloped by the myelin sheat,which, in addition to membrane, contains some Schwann cell cytoplasm in spaces called Schmidt-Lanterman(myelin cleft) between the major dense lines of membranes. • This cytoplasm moves along the myelin sheat, opening temporary spaces(clefts) that allow renewal of some membrane components as needed for maintanance of the sheat. Nodes of Ranvier • Between adjacent Schwann cells on an axon the myelin sheat shows small nodes of Ranvier(nodal gaps) • specialized regions in the axonal membrane that are not insulated by myelin. • Act as a partial barrier to the movement of materials in and out of the periaxonal space between the axolemma and the Schwann sheat. Unmyelinated axons • Small diameter axons are engulfed by one Schwann cell. • The axons are seperated and each becomes enclosed within its own fold of Schwann cell surface. • NO myelin is formed by wrapping • Small diameter axons utilize action potentials whose formation and maintanence do not depend on the insulation provided by the myelin sheat required by large diameter axons. NERVE ORGANIZATION • Axons and Schwann cells are enclosed within layers of connective tissue – Endoneurium: Each nerve axon, surrounded by the endoneurium. This is a thin, protective layer of connective tissue. – Perineurium: Each nerve fascicle containing more axons, is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. Regulate diffusion into the fascicle make up blood-nerve barrier – Epineurium: The epineurium is the outermost layer of dense connective tissue enclosing the (peripheral) nerve. ENDONEURIUM • • • • Thin loose connective tissue rich in reticular fibers Mast cells, macrophages and a few fibroblasts are present Surrounding individual fibers. In contact with basal lamina of Schwann cell PERINEURIUM • Dense irregular connective tissue • Surrounds a group of fibers(fascicles) • Specialized to contribute to bloodnerve barrier • Two or more cell layers thick EPINEURIUM • Dense irregular connective tissue • Surrounds the entire nerve from outside PERIPHERAL NERVE • Establish communication between centers in the CNS and the sense organs and effectors (muscles, glands…) • They generally contain both afferent(sensory) and efferent(motor) fibers. Nerve Fibers • Afferent fibers carry information from internal body regions and the environment to the CNS. • Efferent fibers carry impulses from the CNS to effector organs • Ovoid structures GANGLIONS • Cluster of cell bodies of neurons located outside the CNS • Surrounded by a connective tissue capsule • Serve as relay stations to transmit nerve impulses, at least one nerve enters and another exits from each ganglion. • Direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion. GANGLIONS 1. Sensory ganglia: i. Dorsal root ganglia (Spinal Ganglia) ii. Ganglia associated with cranial nerves (V, VII, VIII, IX, X) 2. Autonomic ganglia: i. Sympathetic chain ganglia – Paravertebral – Prevertebral ii. Parasympathetic ganglia (=İntramural = terminal gang.) SENSORY GANGLIA • Receive afferent impulses that go to the Central Nervous System. • Neurons of these ganglia are pseudounipolar and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord. • Satellite cells: glial cells of ganglia arranged around the cell bodies of neurons • Little connective tissue containing blood vessels AUTONOMIC GANGLIA • Found within autonomic nerves. • The ganglia contain postganglionic visceral efferent neurons that receive synaptic input from preganglionic visceral efferent neurons. • Autonomic ganglion contains multipolar neuron cell bodies with eccentric nuclei (white arrows); axons (green arrows) arise from each cell body. • Satellite glial cells (red arrows) are sparse and form poorly-defined capsules around individual cell bodies Comparison of the Histology of the Ganglion Autonomic Nervous System • Autonomic nerves effect the activity of smooth muscle,the secretion of some glands,heart rate and many other involuntary activities by which the body maintains constant internal environment. • Autonomic nerves make up the autonomic nervous system • Has two parts;  Sympathetic  Parasympathetic divisions. SYMPATHETIC GANGLIA • Nerve cells of the sympathetic division of the autonomic nervous system are located in chains of ganglia on either side of the vertebral column. • Sympathetic Ganglion deliver information to the body about stress and danger, and are responsible for the fight-or-flight response. PARASYMPATHETIC GANGLIA Ganglia in the sacral region of the spinal cord associated with parasympathetic system • Parasympathetic nerves have their nerve cell bodies in small ganglia located within the organ they innervate. • Parasympathetic ganglia may lack distinct capsules altogether,perikarya and associated satellite cells , forming a loosely organized plexus within the surrounding connective tissue. • Ganglia in the intestines; Meissner’s Plexus (submucosal plexus) Aurbach’s plexus (myenteric plexus) Meissner's (or submucosal) Plexus Provides secretory innervation of goblet cells in the epithelium and motor innervation of a thin layer of smooth muscle. Auerbach's (or myenteric) Plexus - Provides motor innervation to the inner circular and outer longitudinal layers of muscle cells Sensory Nerve Endings • The nervous system has a variety of specialized receptors. • Afferent information from the viscera is mediated by encapsulated and non-encapsulated nerve endings. • A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. • Meissner's and Pacinian corpuscles are two that can be readily seen by H&E. Pacinian corpuscle • Pacinian Corpuscle (or lamellar corpuscles) - nerve endings in skin responsible for sensitivity to vibration and pressure. • 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. Meissner’s corpuscles • Meissner Corpuscle - nerve endings in skin responsible for sensitivity to light touch. • 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.