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NERVE TISSUE Neuron – Nerve Cell

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NERVE TISSUE Neuron – Nerve Cell Department of Histology and Embryology, P. J. Šafárik University, Medical Faculty, Košice NERVE TISSUE: Sylabus for foreign students Author: doc. MVDr. Iveta Domoráková, PhD. Revised by: prof. MUDr. Eva Mechírová, CSc. NERVE TISSUE FUNCTION: Reception, transmission, processing of nerve stimuli. Coordination of all functional activities in the body: - motor function (body movement) - sensory (rapid response to external stimuli) - visceral, endocrine and exocrine glands - mental functions, memory, emotion A) Anatomically nervous system consists of: 1. CNS (central nervous system) – brain, spinal cord 2. PNS (peripheral nervous system) – peripheral nerves and ganglia B) Functionally nervous system is divided into the: 1. Somatic nervous system (sensory and motor innervation) 2. Autonomic nervous system (involuntary innervation of smooth muscles, glands) C) Microscopic structure of the nerve tissue - two types of cells: 1. Nerve cells – neurons 2. Glial cells (supporting, electrical insulation, metabolic function) Neuron – nerve cell - is the structural and functional unit of the nerve tissue - receives stimuli from other cells - conducts electrical impulses to another cells by their processes - chainlike communication - ten bilion of neurons in humans A. Neurons according the shape: Pyramidal (E) star-shaped (D) pear-shaped (G) oval (B) B. Types of neurons according number of the processes 1. multipolar (D,E,G)) 2. bipolar (A) 3. pseudounipolar (B) 4. unipolar C. Neurons - according the function Motor (efferent) neurons – convey impulses from CNS to effector cells Sensory (afferent) neurons - convey impulses from receptors into CNS Interneurons – integrated network between motor and sensory neurons Neurosecretory neurons – synthesize and release hormones (e.g. oxytocin – hypothalamus) PERIKARYON (SOMA), cell body trophic centre (troph-; nutrition) nucleus oval, euchromatic, prominent nucleolus perinuclear cytoplasm contains: Nissl substance – basophilic (rER); protein-producing cell (membrane, neurotransmitters, enzymes) Golgi apparatus mitochondria lysosomes transport vesicles lipofuscin (pigment of age), melanin (pigments) cytoskeleton – keep the shape of cell; transport LM: neurofibrils (argyrophilic, brown or black after silver impregnation) EM: neurofilaments (intermediate filaments) neurotubules (microtubules) Part of the perikaryon from which axon extends is called axon hillock. It is an area free of rER and GA. DENDRITES Are receptor processes that receive stimuli from other neurons or from the external environment. Dendrites give up from perikaryon, they branch (arborisation) and become thinner. Their cytoplasm is basophilic because of presence of Nissl bodies in their thick part. Cytoplasm of dendrites contains: rER (Nissl substance) mitochondria neurofilament and neurotubule no GA !! AXONS Are effector processes that transmit stimuli away from the cell body to another neurons or effector cells. Golgi type I neurons – have long axon (e.g. motor neurons in the spinal cord – 100cm) Golgi type II neurons – have short axon Cytoplasm (axoplasm) of axons contains: neurotubules, neurofilaments (transport of vesicles, metabolites) mitochondria smooth endoplasmic reticulum (synthesis, transport) vesicles Description of axon: o axolemma o axoplasm o axon hillock o initial segment - bare area of axon, not coverd by myeline sheath; Function: propagation of nerve impulses = action potential o myeline sheath (myelinated axons) o node of Ranvier o internodal segment o collaterals o terminal arborization o terminal buttons (endings); synaptic vesicles o synapses Substances required in the axon and dendrites are synthesized in the cell body and need transport to their destination. AXONAL TRANSPORT – with the help of microtubules, mitochondria (energy) direction: • anterograde – from perikaryon to the axon terminal • retrograde – carries material from axon to the perikaryon Neurons communicate with other neurons and effector cells by SYNAPSES Synapses are specialized junctions between neurons that facilitate transmission of impulses from one (presynaptic) neuron to another (postsynaptic) neuron. A. Synapse between neuron and effector organ: 1. skeletal muscle fiber = myoneuronal junction (motor end-plate) 2. gland cells B. Synapses between neurons TYPES OF SYNAPSES: 1. Axo-dendritic synapse 2. Axo-somatic synapse 3. Axo-axonal synapse According the mechanism of conduction of nerve impulse: Chemical synapse – synaptic vesicles; neurotransmitters Electrical synapse – gap junctions – ion transport – direct spread of electrical current (smooth muscles, cardiac muscles cells) Synaps is composed of 3 parts: 1. presynaptic membrane of synaptic boutton (axoplasm contains synaptic vesicles, mitochondria) 2. synaptic cleft 3. postsynaptic membrane (contains receptors for attachment of neurotransmitters) Synaptic transmission Process of myelination: A. myelinated nerve fiber in peripheral nerve a) axon has myeline sheath (concentric layers of myeline) b) on the surface is Schwann sheath (rest of the Schwann cell cytoplasm with nucleus) B. non-myelinated nerve fibers in the peripheral nerve - several axons (5-10) are invaginated to the superficial part of Schwann cell cytoplasm. C. Myelinated axons in the central nervous system (CNS) – myelin sheath is formed by processes of oligodendrocytes. One inetrnodal segment is formed by one process of oligodendrocyte. One oligodendrocyte can form more internodal segments by its processes. D. Non-myelinated axons in the CNS – axons are surrounded by neuropil (processes of other neurons and glial cells). Neuroglial cells in CNS Neuroglial cells in PNS LM: neuron surrounded by satellite cells EM: myelinated axon .
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