Nervous Tissue PDF

The state University of Medicine and Pharmacy Nervous Tissue Department of histology, cytology and embryology Four Basic Types of Tissues in the Body ----------------------------------------------- Epithelium (90% of tumors) Co...

The state University of Medicine and Pharmacy Nervous Tissue Department of histology, cytology and embryology Four Basic Types of Tissues in the Body ----------------------------------------------- Epithelium (90% of tumors) Connective Tissue Muscular Tissue Four Basic Types of Tissues in the Body ----------------------------------------------- Epithelium (90% of tumors) Connective Tissue Muscular Tissue Nervous Tissue  Nerve is like Epithelium Origin of nerve is ectoderm like epidermis (epithelium) of skin The nervous system develops from the outermost of the three early embryonic layers, the ectoderm, beginning in the third week of development. With signals from the underlying axial structure, the notochord, ectoderm on the mid- dorsal side of the embryo thickens to form the epithelial neural plate. The sides of this plate fold upward and grow toward each other medially, and within a few days fuse to form the neural tube. Cells of this tube give rise to the entire CNS, including neurons and most glial cells.  Nervous Tissue Functions: specialized for the transmission, reception, and integration of electrical impulses Distinguishing features: Neurons – very large excitable cells with long processes called axons and dendrites. The axons make contact with other neurons or muscle cells at a specialization called a synapse where the impulses are either electrically or chemically transmitted to other neurons or various target cells (e.g., muscle). Others secrete hormones.  Nervous tissue Distribution: comprise the central nervous system. Individual peripheral nerves are found throughout the body. Individual neurons and clusters of neurons (called ganglia) are found in most organs.  Function of the Nervous System is Communication Dependent upon special signaling properties of neurons Long processes of neurons (e.g., 1 meter motor neuraxon)  Function of the Nervous System is Communication Characteristics of neurons Irritability - protoplasm capable to react to various physical and chemical agents Conductivity - ability to transmit the resulting excitation from one locality to another  Activity of the Nervous System Information Receive: receptors afferent pathway Process: CNS (centralization is paramount) Transmit: efferent pathways effect Voluntary (conscious ) = somatic Involuntary = autonomic Sympathetic - fight or flight Parasympathetic - vegetative  CNS and PNS divisions Central Nervous System (CNS) – Brain and spinal cord Grey matter (neuron’s somas and some processes) White matter (myelinated and unmyelinated axons)  – Myelination is from oligodendrocytes Axon – Glial cells: hillock » Astrocytes » Microglial cells Part of the blood-brain barrier (BBB) Peripheral Nervous System (PNS) – Nerves, sensory ganglia, and autonomic ganglia (or plexus) – Schwann cells: myelin producing glial cells of the PNS  CELLS OF THE NERVOUS SYSTEM: ONLY 2 TYPES!!! ▪ 1. Neurons ▪ 2. Glial Cells (or Supporting Cells) Astrocytes (CNS) Oligodendrocytes (CNS) Ependymal Cells (CNS) Microglial Cells (CNS) Schwann Cells (PNS) Satellite Cells (PNS)  Neuron Structure 1. Cell body = perikaryon = contains nucleus and is the metabolic center of the cell 2. Processes – that extend from the cell body (dendrites and axon) 3. Nerve endings (synapses, special receptors)  Cell body has: Nucleus with large nucleolus Neurofibrils “Nissl bodies” (chromophilic substance)  Neurofibrils are present in the perikaryon, dendrites and axon and are unique to neurons. = “Skeleton” of the neurons  Nissl bodies - large clumps of basophilic material around the nucleus, an aggregation of many parallel cisternae of the rough endoplasmic reticulum with the rosettes of free polisomal ribosomes Function – protein synthesis (neurotransmitters) Neuron processes - Extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body (usually only 1!) All processes end with the nerve endings Slide 8  Axon Slender processes of uniform diameter arising from the axon hillock Axon hillock lacks RER, ribosomes & Nissl substance Nissl substance is also absent in cytoplasm of axon Usually there is only one unbranched axon per neuron Axon terminals (terminal boutons) Axolemma Axoplasm Structural Classification of Neurons - According to amount of processes 1. Unipolar neurons – are found during early embryogenesis. They have one axon Structural Classification of Neurons 2. Bipolar neurons – one axon and one dendrite Structural Classification of Neurons 3. Pseudounipolar neurons – have a short single process leaving the cell body Structural Classification of Neurons 4. Multipolar neurons – many extensions from the cell body  Functional Classification of Neurons 1. Sensory (afferent) neurons Carry impulses from the sensory receptors to the cell body 2. Motor (efferent) neurons Carry impulses from cell body which lie in the central nervous system to effector cells 3. Interneurons (=association neurons) - 99,9% in the central nervous system Connect sensory and motor neurons Neurons form reflex arc  Supporting Cells (Neuroglia or Glia) = Macroglia + Microglia  Macroglia in the CNS 1. Ependymal cells Line cavities of the brain and spinal cord Synthesize cerebrospinal fluid  2. Astrocytes Star-shaped cells Support neurons Form barrier between capillaries and neurons (BBB) Control the chemical environment of the brain (CNS) 2 types: Protoplasmic and Fibrous  BBB Open Sesame For all its complexity, traffic between the bloodstream and brain is governed by an exceedingly simple set of rules. To cross, a compound must either be smaller than 500 kilodaltons (like most antidepressants, antipsychotics and sleep aids), able to use one of the natural gateways embedded in the barrier itself (like the Parkinson's drug l-dopa), or be lipophilic, meaning it has an affinity for lipids and can thus bind to and slip across the lipid cell membrane (like alcohol, cocaine and heroin). By most estimates, 98 percent of all medications fail to meet any of these criteria, which means they cross the barrier in quantities so minuscule as to be medically useless—or they do not cross at all.  3. Oligodendrocytes Produce myelin sheath around nerve fibers in the central nervous system Nourish neurons  Microglia - arise from monoblast of the blood Spider-like phagocytes Dispose of debris  Supporting Cells of the PNS Schwann cells - form myelin sheath in the peripheral nervous system  Supporting Cells of the PNS Satellite cells – surround cell bodies of neurons in sensory ganglia  Nerve fibers 1. Unmyelinated 2. Myelinated  Unmyelinated nerve fiber: Axones and dendrites are invaginated in Schwann cell cytoplasm Myelinated nerve fibers Myelinated nerve fibers Myelinated nerve fibers Myelinated nerve fibers Schwann cells produce myelin sheath  Myelinated nerve fiber structure Nodes of Ranvier – spaces between 2 Schwann cells – free from myelin Nodes of Ranvier provide saltatory conduction of nerve impulse  Synapse The specialized region of contact between 2 neurons  Classification of synapses: by nature: chemical synapse electrical synapse by localisation axodendritic synapse axosomatic synapse axoaxonic synapse By action: excitatory synapse inhibitory synapse  Sensory Nerve endings (afferent neurons receptors) Classifications: By location: 1. Exteroceptors, 2. Interoceptors, 3. Proprioceptors By type of stimuli: 1. Chemoreceptors, 2. Mechanoreceptors, 3. Photoreceptors, 4. Thermoreceptors  Sensory nerve endings (afferent neuron receptors) Classification: By type of the structure: 1. A. Free nerve endings B. Hair follicle nerve ending C. Merkel nerve endings (Merkel’s disk) 2. Encapsulated: Tactile corpuscle of Meissner Corpuscle of Pacini Ruffini endings 3. Muscle spindle 1. A. Free nerve endings – pain, thermal receptors 1. B. Hair follicle nerve endings – respond to very light touch 1. C. Merkel nerve endings – light touch receptors 1. C. Merkel nerve endings – light touch receptors 2. Encapsulated = Tactile corpuscle of Meissner 2. Encapsulated = Tactile corpuscle of Meissner 2. Encapsulated = Tactile corpuscle of Meissner  2. Encapsulated. Corpuscle of Pacini (lamellar body) is specialized to detect gross pressure changes and vibration 2. Encapsulated. Corpuscle of Pacini lamellar body are specialize to detect vibration  2. Encapsulated. Ruffini ending Dense branches of nerve-endings encapsulated in connective tissue. Is sensitive to skin stretch  2. Encapsulated. Ruffini ending Dense branches of nerve-endings encapsulated in connective tissue. Is sensitive to skin stretch 3. Muscle spindle (detects muscle stretch) 3. Muscle spindle (detects muscle stretch) 3. Muscle spindle (detects muscle stretch) 3. Golgi tendon organ (detects muscle tension) Motor end plate