Nervous Tissue PDF

Nervous tissue Dr. M. J. S. Jayarathna, MBBS Lecturer Department of Anatomy Faculty of Medicine University of Peradeniya  Objectives: List components of the nervous tissue Distinguish between neurons and neuroglial cells Describe the general structure of a neuron Ex...

Nervous tissue Dr. M. J. S. Jayarathna, MBBS Lecturer Department of Anatomy Faculty of Medicine University of Peradeniya  Objectives: List components of the nervous tissue Distinguish between neurons and neuroglial cells Describe the general structure of a neuron Explain the functions of the parts of a neuron State the types and functions of neurons Classify neurons based on their structure and function State the types and functions of neuroglia Distinguish between myelinated and non- myelinated nerve fibers Name the types of sensory receptors and state their functions Describe a ganglion Describe the motor end plate Nervous tissue A tissue designed to deliver; Rapid and precise communication, between different parts of the body, by the action of specialized nerve cells called neurons Nervous tissue Nervous system is divided in to two main parts; Central nervous system (CNS) : Brain : Spinal cord Peripheral nervous system (PNS) :Nerves run between the CNS and other tissues :Nerve relay stations (ganglia) Functionally; Somatic nervous system (voluntary function) Autonomic nervous system (involuntary function) Histologically; Neurons Their supporting tissues (neuroglia/ glial cells) Components of the nervous tissue and their differences Neurons Neuroglia/ glial cells Primary type of cell that associates with Supporting role the nervous system Highly specialized Mechanical and metabolic support Interconnected Defense and immunological function Recieve and process information Provide sheath for peripheral nerves Generate appropriate response signals Lines cavities of the brain Excitable cells Participate in forming blood- brain barrier  Synapticend Synaptic endbulb/ bulb/ Terminalboutons/ Terminal boutons/ Axon Hillock Ultrastructure of the neuron Main structures and their functions in a nerve cell body Structure Function Nucleus Contains DNA Control of cell functions/ activity Plasma membrane Responsible for reception and transmission of impulses Selective permeability Cytoplasm Contains organelles and inclusions Nissle granules Protein synthesis Mitochondria Energy production and cellular respiration Golgi apparatus Storage of secretary products for transportation Add carbohydrate to protein molecule Form cell membrane Neurofibrils Stability, support, maintaining shape of the cell body Axon Carry impulses away from the cell body (efferent) Dendrites Carry impulses to the cell body (afferent) Lysosomes Cell scavengers Types of neurons; according to morphology Apolar Unipolar Bipolar Multipolar Golgi Type I Golgi Type II No process Only one Two processes Many processes Single long Short axon process (axon and (one axon, many axon Dendrites dendrite) dendrites) resembles a star 80% 2 poles Many poles Inhibitory in function Not common Grey matter of Posterior root Olfactory nerve Anterior horn Fiber tract of Cerebral and spinal cord ganglion of cells cells, motor cells brain and spinal cerebellar Cerebral cranial nerves Ganglion of 8th of spinal cord cord cortex cortex except 8th cranial nerve Fiber tracts of Peripheral (numerous) Retina brain and spinal nerves cord Motor cells of Peripheral spinal cord nerves According to number, length and mode of branching neurites According to size of neuron Types of neurons according to the number, length and mode of branching of the neurites Classification of neurons; according to structure-function Afferent/ sensory neurons Responsible for bringing sensory information from the outside world To the brain or spinal cord E.g., special senses (vision, hearing, smell, taste), and sense of touch, pain, and temperature Efferent/ motor neurons Nerve fibers responsible for carrying signals from the brain To the peripheral nervous system To initiate an action They carry the impulses to muscles and organs Classification of neurons; according to structure-function Neuroglial or supporting cells Neuroglial cells Ependymal cells Capsular/ satellite cells Schwann cells Other supporting cells Macroglia Microglia Astrocytes Oligodendrocytes Glioblasts Protoplasmic Fibrous Found in grey matter Found in white matter. Thick, symmetrical Thin, irregular. Types of neuroglia Diagrammatic representation of the arrangement of different types of neuroglial cells  Functions of different neuroglial cells Cell type Description Function Ependymal cells Line ventricles, spinal canal Absorptive and secretary function (Ependymocytes, Tanycytes, Cover surfaces of choroid plexus Produce and secrete CSF Choroidal epithelial cells) Circulate/ absorb CSF Capsular / satellite cells Surrounds sensory/ autonomic ganglia Supportive function Schwann cells Found on PNS Forms myelin sheath Astrocytes Star shaped with numerous processes Forms blood-brain barrier Mechanical and metabolic support Oligodendrocytes Found on CNS Forms myelin sheath Predominant type in white matter Supportive function Glioblasts Found deep in ependyma Proliferate in to mature macroglial cells Microglia Smaller glial cells Defense and immunological function Flat cell body with few short processes Inactive in normal CNS Probably migratory cells Other supporting cells Ensheath the motor and sensory nerve endings Neuron types and layers of the cerebral cortex 5 neuron types 6 layers Neuron types of the cerebral cortex 1. Pyramidal cells 2. Stellate (granule)cells 3. Cells of Martinotti 4. Fusiform cells 5. Horizontal cells of Cajal Neuron types in the cerebral cortex 1. Pyramidal cells Pyramid shaped cell bodies Apex directed towards the cortical surface Axon from the base of the cell pass to underlying white matter and synapse in deep layers of the cortex Thick branching dendrite from apex pass towards surface Short dendrites from the edges of the base ramify laterally. Betz cells, the huge upper motor neurons of the motor cortex are the largest of the pyramidal cells in the cortex 2. Stellate (granule)cells Small Short vertical axon and several short branching dendrites Star shape cell body Subtypes: basket, neurogliaform 3. Cells of Martinotti Small polygonal cells Few short dendrites Axon extends towards the surface and bifurcates to run horizontally, in the most superficial layer 4. Fusiform cells Spindle-shaped Oriented at right angles to the surface of the cerebral cortex Axon arises from the side of the cell body and passes superficially Dendrites from each end of the cell body 5. Horizontal cells of Cajal Small, spindle-shaped Oriented parallel to the surface Least common cell type Only found in the most superficial layer Axons pass laterally to synapse with the dendrites of the pyramidal cells Layers of the cerebral cortex 1. Plexiform (molecular) layer Most superficial Contains dendrites and axons of cortical neurons Horizontal cells of Cajal and neuroglia 2. Outer granular layer Small pyramidal cells, stellate cells Contain various axons and dendritic connections of deep layers 3. Pyramidal cell layer Moderate size pyramidal cells Cells increase in size deeper in the layer Martinotti cells are also present 4. Inner granular layer Densely packed stellate cells 5. Ganglionic layer Large pyramidal cells, small number of stellate cells, cells of Martinotti Has Betz cells of the motor cortex 6. Multiform layer Wide variety of differing morphological forms found in this layer Numerous small pyramidal cells, cells of Martinotti, stellate cells; especially superficially Fusiform cells in the deeper part Neuron types and layers of cerebellum 5 neuron types 3 layers Neuron types of cerebellum 1. Purkinje cells 2. Stellate cells 3. Basket cells 4. Golgi cells 5. Granular cells Neuron types of cerebellum 1. Purkinje cells Very large cell bodies Fine axon extending down through the granular cell layer Extensively branching dendritic system 2. Stellate cells Scattered in the molecular layer 3. Basket cells 4. Golgi cells Scattered in the superficial part of the granular cell layer 5. Granular cells Layers of cerebellum 1. Outer molecular layer Few neurons Large numbers of unmyelinated fibres 2. Middle layer Single layer of huge neurons called Purkinje cells 3. Inner granular cell layer Extremely cellular Nerve fibers Nerve fiber is the name given to an axon (or a dendrite) of a nerve cell Bundles of nerve fibers found in the CNS - Nerve tracts Bundles of nerve fibers found in the PNS - Peripheral nerves Two types : myelinated and nonmyelinated Structural/ histological division of nerve fibers Myelinated nerves Non-myelinated nerves Nerve fibers which are enveloped by myelin sheath No myelin sheath Variable number of concentric layers of supporting cell plasma membrane Large fibers Small fibers Have nodes of Ranvier (constrictions in the myelin No nodes of Ranvier sheath due to interruptions at regular intervals) Formed by Schwann cells in PNS and oligodendrocytes in CNS One Oligodendrocyte may form myelin sheaths for as In PNS, enveloped by the plasma membrane/ many as 60 nerve fibers, only one Schwann cell cytoplasm of Schwann cells (mesaxon) maintains each segment of one nerve fiber In CNS, not associated with oligodendrocytes Structural and metabolic support In areas with synapses/ motor transmission, axon emerges from the mesaxon for a short distance, thus exposing the active region of the axon Act as an insulator Nerve fibers – EM view Peripheral nerve Consists of parallel bundles of nerve fibers, surrounded by connective tissue sheaths Epineurium - Dense connective tissue sheath surrounding the nerve trunk Within the sheath are bundles of nerve fibers Perineurium - Connective tissue sheath surrounding the nerve fiber bundles Endoneurium - Loose, delicate connective tissue between the individual nerve fibers Sensory receptors Nerve endings or specialized cells Convert (transduce) stimuli from the external or internal environments into afferent nerve impulses These impulses pass into CNS At CNS, they initiate appropriate voluntary or involuntary responses  Morphological Sensory classification receptors - classification Simple Compound Free, branched or unbranched nerve endings Organization of associated non-neural tissues Rarely visible in LM Compliment the function of Receptors for temperature, the neural receptors cutaneous pain, touch, pressure Eye, Ear, receptors for smell and taste Sensory receptors - Functional classification Exteroceptors Proprioceptors Interoceptors Respond to stimuli from Located within the Respond to stimuli Outside the body. skeletal system. from viscera. Eg: touch Provide conscious and Eg: Chemoreceptors light pressure unconscious information of blood deep pressure about; Vascular/pressure cutaneous pain orientation Baroreceptors temperature tension Distention of hollow smell movement viscera taste skeletal position Visceral pain sight Eg: Vestibula apparatus of Hunger ear hearing Thirst Tendon stretch Malaise receptors Well-being Neuromuscular spindles Specialized sensory receptors 1. Free nerve endings Simplest form Numerous small terminal branches Slow conduction Nerve endings are devoid of myelin Merkel nerve endings/discs ; for light touch sensation 2. Meissner’s corpuscles Small, encapsulated, sensory receptors In the epidermis of the skin Light discriminatory touch 3. Pacinian corpuscles Large, encapsulated Pressure/ coarse touch, vibration, tension In deeper layers of skin, ligaments and joint capsules, mesenteries, serous membranes, erogenous areas, some viscera Have appearance of an onion 4. Ruffini corpuscles Spindle- shaped Found in soles of the foot 5. Neuromuscular spindle Stretch receptor organs within skeletal muscles Regulation of muscle tone vie spinal stretch reflex Encapsulated, lymph filled, fusiform structures Lie parallel to muscle fibers Contains 2 – 10 modified skeletal muscle fibers called intrafusal fibers ( 2 types: nuclear bag fibers and nuclear chain fibers) 6. Krause end bulbs Lining of the oropharynx and conjunctiva of the eyes Ganglion Discreet aggregations of neuron cell bodies located outside the CNS 3 types: 1. Spinal ganglion Lie on the posterior nerve roots of the spinal cord Contain the cell bodies of primary sensory neurons (unipolar) Surrounded by satellite/ capsular cells Encapsulated by condensed connective tissue 2. Sympathetic ganglion Multipolar cells Eccentric nuclei Cytoplasm contains lipofuscin granules Satellite cells are smaller in number and irregularly placed 3. Parasympathetic ganglion Cell bodies of terminal effecter neurons Located near or within the effecter organs May be a well-organized moderate- sized one (otic ganglia) or a few clumped cell bodies forming tiny ganglia Synapses Highly specialized intercellular junctions Allow communication by linking of each nervous pathway For a given synapse, impulse conduction is unidirectional Response may be either excitatory or inhibitory Has pre-synaptic and post-synaptic membranes In-between there is synaptic cleft Synaptic vesicles in the terminal bouton contains neurotransmitters Types: 1. Axodendritic: axon of one neuron synapse with the dendrite of another neuron 2. Axosomatic: axon synapse with the cell body of another cell 3. Axoaxonic: axon synapse with the axon of another neuron Components of a synapse Motor end plate / neuromuscular junction Highly specialized intercellular junction Allow communication between neurons and skeletal muscle fibers Neurotransmitter of somatic neuromuscular junctions – acetylcholine Deactivated by acetyl cholinesterase between successive nerve impulses Special features of motor end plate: One motor neuron may innervate more than one muscle fiber Motor neuron and the muscle fibers which it supplies together constitute a “motor unit” Terminal part of the axon is divided into several branches Each branch terminate as a motor end plate on a different skeletal muscle fiber Axonal branch don’t have myelin sheath Axon divides into terminal boutons Muscle cell surface occupied by the motor end plate is called as “sole plate.” Post synaptic membrane form the secondary synaptic cleft Overlying pre-synaptic membrane is also irregular Blood brain barrier Consists of structures situated at the capillary level Allows a selective entrance of blood contents to the nervous tissue Prevent toxic and harmful substances to reach the brain Components of blood brain barrier: Capillary endothelium without openings Basement membrane of the endothelium Termination of processes of astrocytes covering the capillaries Intercellular spaces containing fluid Processes and cell bodies of neurons Summary: 1. Nervous tissue: introduction 2. Components of the nervous tissue 3. Structure of the neuron 4. Neuron types 5. Cerebral neurons 6. Cerebellar neurons 7. Neuroglial cells 8. Nerve fibers and peripheral nerve 9. Sensory receptors 10. Ganglion 11. Synapse 12. Motor end plate 13. Blood brain barrier Thank you! References: Wheater’s functional histology Snell’s neuroanatomy Snell’s clinical anatomy Last’s anatomy