Histology of the Nervous System (King Saud University 2023-24) PDF

HISTOLOGY of the Nervous System Term 3, 2023-24 Anatomy & Physiology of the Nervous System (APNS-211) Basic Science Department COSHP, KSAU-HS, KSA  LEARNING OUTCOMES By the end of this teaching session, each student should be able to: 1....

HISTOLOGY of the Nervous System Term 3, 2023-24 Anatomy & Physiology of the Nervous System (APNS-211) Basic Science Department COSHP, KSAU-HS, KSA  LEARNING OUTCOMES By the end of this teaching session, each student should be able to: 1. Define and classify nervous system 2. Recognize the principle cellular components of the nervous system 3. Identify different stains used to study the nervous tissue 4. Describe histological features of different parts of nervous system 5. Recognize the various names for connective tissue layers in nerves 6. Correlate the histological features with the tissue function  Nervous System Definition: Nervous tissues receive, transmit, and integrate information from outside and inside the body to control the activities of the body The neurons are highly specialized cells interconnected and function together to process information and then generate appropriate response signals LO-1  Divisions of Nervous System Central Nervous System (CNS) Brain & Spinal cord  Protected & enclosed within skull &vertebral canal respectively  Covered by meninges  Cerebro-spinal fluid (CSF) baths surface of brain & spinal cord  Composed of neurons and supporting cells (glial cells ) Peripheral Nervous System (PNS)  Paired spinal & cranial nerves  Ganglia Functionally, the nervous system is divided into the:  Somatic nervous system that supply the body wall and limbs. It is involved with the voluntary functions  Autonomic nervous system that supply the viscera and glands. It exerts control over many involuntary organs LO-1  Cells of the nervous tissue There are two primary cell types Neurons which are excitable cells and transmit electrical signals Glial Cells / Neuroglia are supporting cells. These are:  Astrocytes  Oligodendrocytes  Ependymal cells  Microglia  Schwann cells LO-2  Structure of a Neuron A neuron is an excitatory cell of the nervous system All neurons have the same basic structure and consist of: A large cell body containing the nucleus surrounded by cytoplasm known as the perikaryon, Nissl bodies (H+E), neurofilaments and microtubules It has processes of two types that extend from the cell body, namely a single axon and one or more dendrites LO-2 Dr. Ismail Memon  Neuron processes, Axon Each neuron has a single axon arising from a cone- shaped portion of the cell body called the axon hillock The axon is a cylindrical process up to 1 meter in length terminating on other neurons or effector organs by way of small swellings called terminal boutons It conducts action potentials to influence other neurons or effector organs Axons are commonly referred to as nerve fibers LO-2  Neuron processes Dendrites Dendrites are highly branched, tapering processes attached to the body of the nerve cell These end on specialised sensory receptors (as in primary sensory neurons) or form synapses with neighboring neurons from which they receive stimuli Acts as the major sites of information input into the neuron LO-2  Types of Neurons There are many types neurons. Based on the arrangement of the cell processes the neurons are classified into: Multipolar neurons in which numerous dendrites and a axon project from the cell body e.g. motor neurons, interneurons Bipolar neurons in which single dendrite arises from pole of cell body opposite to the origin of the axon e.g. sensory neurons of special sensory organs; retina of eye, hair cells of ear Pseudo-unipolar neurons in which single axon & dendrite arise from common stem of cell and then separate e.g. primary spinal sensory neuron (dorsal root ganglia neurons) LO-2  Glial cells (Neuroglia) In addition to the neurons the nervous tissue consists of a number of non neuronal support cells, collectively known as neuroglia or neuroglial cells Four principal types of neuroglia cells are recognized in the CNS: – Oligodendrocytes – Astrocytes – Microglia – Ependymal cells LO-2  Glial cells (Neuroglia) Main functions of glial cells are: Supply nutrients & oxygen to neurons Insulate one neuron from another Surround neurons & hold them in place Destroy & remove the debris of dead neurons LO-2  Glial cells Oligodendrocytes These are cells that produce myelin in the CNS, similar to schwann cells of PNS A single oligodendrocyte extends to several axons (up to 50) wrapping around about 1 mm of each & forming the myelin sheath Tumor of oligodendrocyte called oligodendroglioma The Schwann cells myelinate the nerve fibers in PNS. A Schwann cell myelinate only a segment of one nerve called the internode LO-2  Glial cells Astrocytes Astrocytes are highly branched cells that pack the interstices between the neurons, their processes and the oligodendrocytes They provide mechanical support and mediate the exchange of metabolites between neurons and the vascular system They also form part of the blood-brain barrier Astrocytes also play an important role in repair of CNS tissue LO-2  Glial cells Microglia The microglia cells have elongated nuclei with little cytoplasm and fine highly branched processes Represent monocyte & macrophage system of CNS Resting microglia cells migrate to CNS during embryological development Can be derived from circulating monocytes which migrate into CNS later in life After brain injury they become active, phagocytic & develop into macrophages LO-2  Glial cells Ependymal Cell Typically cuboidal-columnar cells, often with cilia Line the ventricles of the brain & spinal canal and play role in the production of the cerebrospinal fluid (CSF) Tumor of ependymal cell are called ependymoma LO-2  Different Stains Used To Study The Nervous Tissue 1. Hematoxylin and Eosin (H+E): 2. Heavy Metal Impregnation Technique: Gold and silver are used to study neuron morphology. LO-3  Different Stains Used To Study The Nervous Tissue 3. Immunohistochemistry: Used to study neuron specific proteins e.g; neurofilament protein. 4. Nissl Method: A Stains RNA, identifying the rough endoplasmic reticulum (Nissl Substance). Note the axon (A) which lacks Nissl Substance LO-3  Organization of the neurons in the CNS Cerebrum Layers of neurons are arranged close to the surface of the brain to form the gray matter or cerebral cortex Neurons grouped together deep in the brain form nuclei (thalamus, basal ganglia) Inner white matter conveys axons of the neurons ( bundles of nerve fibers) There are six layers of cerebral cortex: Molecular (plexiform) layer. External granular layer. External pyramidal layer. Internal granular layer. Internal pyramidal layer. Multiform (fusiform) layer. LO-4  Cerebellum The cerebellum is composed of a surface layer, grey matter, the cortex that is arranged forming folds called the folia and central core of nerve fibers, the white matter There are three layers of cerebellar cortex: Outer molecular layer ML: It consists of un- myelinated nerve fibers Middle Purkinje cell layer PL: It consists of neurons (Purkinje cells) with large cell bodies arrange in single layer Inner granular cell layer GL: It consists of small neurons LO-4  Spinal cord Neurons form central core (grey matter) of the spinal cord with the fibers outside (white matter) In cross section the spinal cord consists of: Gray matter, butterfly shaped central mass of neurons that extends to form ventral, dorsal & lateral horns White matter: nerve fibers forming ascending & descending tracts surrounding the gray matter LO-4  Peripheral Nervous System The peripheral nervous system PNS consists of: 31 pairs of the spinal nerves 12 pars of the cranial nerves Ganglia – primary sensory spinal ganglia, ganglia associated with cranial nerves, and autonomic ganglia LO-4  Connective tissue layers of Nerve fibers Each nerve is composed of nerve fibers which are arranged in the nerve fascicles Each nerve fiber with investing schwann cell is surrounded by delicate layer of connective tissue the endoneurium Each nerve fascicle in a nerve is wrapped by a connective tissue covering the perineurium Each nerve is externally covered by connective tissue covering the epineurium Perineurium LO-5  Peripheral nerve- Coverings a- Epineurium LO-5 b- Perineurium  Myelinated & non-myelinated nerve fibers Non-myelinated nerve fibers: Small axons (ANS, Pain fibers) are only enveloped by cytoplasm of schwann cells Myelin sheath: Large axons are wrapped by concentric layers of schwann cell plasma membrane called myelin sheath It functions to: – protect the axons – electrically insulate fibers from one another – increase the speed of nerve LO-6 impulse transmission  Myelin Sheath It’s a segmented sheath around most long neuronal axons formed by the schwann cells in PNS and by the oligodendrocytes in the CNS More than one schwann cells wrap end to end to myelinate a nerve fiber A segment of nerve fiber covered by a single schwann cell is called internode A segment of nerve fiber between two schwann cell is called node of Ranvier LO-6