Neuroscience 1 - Trans 1 - Nerves and Neurons.pdf

1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO...

1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO A. CLASSIFICATION OF NEURONS CHAPTER OUTLINE Morphologic Class Arrangement of Location  To define the neuron and name its processes Neurites  To learn the varieties of neurons and identify them in the Number, Length, and Mode of Branching of Neurites different parts of the nervous system Unipolar Single neurite Posterior root ganglion  To review the cell biology of a neuron and understand the divides a short function of a nerve cell and its processes distance from the  To review the structure of the plasma membrane as it is cell body related to its physiology Bipolar Single neurite Retina, sensory  To learn the transport of materials from the cell body to emerges from either cochlea, and vestibular the axon terminals end of cell body ganglia  To understand the structure and function of synapses and Multipolar Many dendrites and Fiber tracts of brain neurotransmitters one long axon and spinal cord,  To review the supporting function of the neuroglial cells peripheral nerves, and for nerve cells and the possible role that they play in motor cells of spinal neuronal metabolism, function, and neuronal death cord Size of Neuron The purpose of this chapter is to prepare the student to Golgi Type 1 Single long axon (1 Same with the understand how the basic excitable cell—the neuron— m or more) multipolar neurites communicates with other neurons. It also considers Golgi Type 2 Short axon that with Cerebral and certain injuries to the neuron and the effects of drugs on dendrites cerebellar cortex the mechanism by which neurons communicate with one resembles a star another. I. NEURON  Name given to the nerve cell and all its processes  Composed basically of excitable specialized cells, whose function is to receive sensory stimuli and to transmit them to effector organs, whether muscular or glandular  They are found in the brain and spinal cord and in ganglia.  do not undergo division and replication B. THE MAIN STRUCTURES OF A NERVE CELL BODY, DENDRITES AND AXON STRUCTURE SHAPE APPEARANCE LOCATION FUNCTION Nucleus Large (due to high protein Pale, chromatin widely Centrally placed, displaced Controls cell activity production), Rounded scattered; single to periphery in cell injury Mature neurons no longer prominent nucleulous; duplicate due to lack of Barr body (X compacted centrioles and only chromatins) present in functions in gene female expression Cytoplasmic Organelles: Nissl substance Granules of Broad cisternae; Throughout cytoplasm and Synthesizes protein rough ribosomes are proximal part of dendrites, endoplasmic basophilic absent from axon hillock reticulum and axon, fatigue and injury result in concentration at periphery Golgi Complex Wavy Smooth Close to the nucleus Adds carbohydrate to threads; endoplasmic protein molecule; clusters of reticulum packages products for flattened transport to nerve cisternae terminals; forms cell and small membranes Trans 1 | Snell 1 of 1  1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO vesicles Mitochondria Spherical, Double membrane Scattered Form chemical energy rod shaped with cristae Neurofibrils Linear fibrils (10 nm Run parallel to Run from dendrites Determines the shape diameter) each other; through of the neuron composed cell body to axon bundles of microfilaments, each 10 nm in diameter Microfilaments Linear fibrils Filaments 3–5 nm Form a dense network Role in formation and in diameter beneath the plasma retraction of cell membrane processes and in cell transport Microtubules Linear Run between Run from dendrites Cell transport tubes neurofibrils, 25 through nm in diameter cell body to axon Lysosome Vesicles 8 nm in diameter; Throughout cell Cell scavengers three forms: primary, secondary, and residual bodies Centrioles Paired hollow cylinders Wall made up of Confined to cytoplasm of Take part in cell bundles of microtubules cell body division; maintain microtubules Lipofuscin Granules Yellowish brown Scattered through Metabolic by-product cytoplasm believed to formed from lysosomes Melanin Granules Yellowish brown Substantia nigra of Related to formation of midbrain dopamine Axon hillock Fan shaped Plasma Membrane Not defined; fluid Clear to yellowish brown Within the cell body Site for initiation and conduction of nerve impulse. II. PROCESSES THAT OCCURS IN THE NEURON  Chromatolysis o movement of Nissl substances at the periphery of the cytoplasm giving the impression that the Nissl substance has disappeared due to fatigue or neuronal damage.  Cell transport o involves the movement of membrane organelles, secretory material, synaptic precursor membranes, large dense core vesicles, mitochondria, and smooth endoplasmic reticulum. o can take place in both directions in the cell body and its processes o 2 kinds: o Rapid transport (100 to 400 mm per day) is brought about by two motor proteins associated with the microtubule adenosine triphosphate (ATP)-ase sites; these are kinesin for anterograde (away from the cell) movement and dynein for retrograde movement. The direction and speed of the movement of an organelle can be brought about by the activation S1T1 2 of 2  1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO of one of the motor proteins or of both of the motor proteins simultaneously. o Slow transport (0.1 to 3.0 mm per day) involves the bulk movement of the cytoplasm and includes the movement of mitochondria and other organelles. Slow axonal transport occurs only in the anterograde direction. The molecular motor has not been identified but is probably one of the kinesin family. III.EXCITATION OF THE PLASMA MEMBRANE FOR THE CELL NERVE BODY  Depolarization o Na+ ions diffuse through the plasma membrane into the cell cytoplasm as a result of nerve cell excitation  Action Potential o results from depolarization o very brief, lasts about 5 msec  Once generated, the action potential spreads over the plasma membrane, away from the site of initiation, and is conducted along neuritis as the nerve impulse. It is self-propagated, and its size and frequency do not alter.  Refractory Period o The duration of nonexcitable state where another action Ionic and electrical changes that occur in a neuron when it is potential cannot be elicited immediately stimulated o Controls the maximum frequency that the action potentials can be conducted along the plasma membrane. The greater the strength of the initial stimulus, the larger the IV. SODIUM AND POTASSIUM CHANNELS initial depolarization and the greater will be the spread into the surrounding areas of the plasma membrane.  Summated  Sodium and potassium channels are formed of the protein o Effect of multiple excitatory stimuli molecules that extend through the full thickness of the plasma o Example – sub threshold stimuli may pass over the surface membrane. of the cell body and be summated at the origin of the axon  Gating and so initiate an action potential o involves the twisting and distortion of the channel, thus  Hyperpolarization creating a wider or narrower lumen o Reduces the excitatory state of the cell o occurs in response to such stimuli as voltage change, the o Influx of Cl- ions through the plasma membrane into the presence of a ligand, or stretch or pressure. neuron by inhibitory stimuli  Non stimulated state o K+ channel gate is wider than Na++ o K+ diffuse out of cytoplasm more readily  Stimulated o Sodium channel is first wide open, o then the gates of the potassium channels are opened, and o the gates of the sodium channels are nearly closed again  Absolute refractory period o occurs at the onset of the action potential when a second stimulus is unable to produce a further electrical change o due to the inability to get the sodium channels open Ionic and electrical changes that occur in a neuron  Absolute refractory period during hyperpolarization o When a very strong stimulus can produce an action potential, presumably the sodium channels are opened S1T1 3 of 5  1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO V. THE NERVE CELL PROCESSES Neurites: the processes of nerve cell Division: 1. Dendrites - Short processes of the cell body - Dendritic spines – small projections of dendrites - Extensions of the cell body to increase the surface area for the reception of axons from the neurons - Conduct nerve impulse toward the cell body A. AXON TRANSPORT: brought about by microtubules assisted by the microfilaments 1. Fast anterograde transport - 100-400 mm/day - transport of proteins and transmitter substances or their precursors 2. Slow anterograde transport - 0.1-3.0 mm/day - Transport of axoplasm and includes the microfilaments and microtubules 3. Retrogade transport - Explains how the cell bodies of nerve cells respond to changes in the distal end of the axons - Ex: activated growth factor receptors, pinocytotic vesicles, worn-out organelles 2. Axon - Longest process of the cell body B. SYNAPSES - Axon hillock – small conical elevation on the cell body - Synapse: the site where two neurons come into close - Terminals – terminal distal ends of the terminal proximity and functional interneuronal branches of the axon communication occurs - Varicosities – swellings of axons (especially - Takes place at a one direction only autonomic nerves) near their termination; appears - Depend on the site of the synapse: axodendritic, like a string of beads axosomatic, or axoaxonic - Large diameter: conduct impulse rapidly; smaller - 2 types: chemical and electrical diameter: conduct impulse slowly - Axolemma – plasma membrane bounding the axon - Axoplasm - cytoplasm of the axon; has no Nissl granules or golgi complex - Initial segment – most excitable part of the axon; site which action potential originates S1T1 4 of 5  1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO 1. Chemical Synapses - A neurotransmitter passes across the narrow space between the cells and becomes attached to a protein molecule in the postsynaptic membrane (receptor) - Ex. of neurotransmitters: acetylcholine (Ach), norepinephrine, epinephrine, dopamine, glycine, serotonin, GABA, enkephalins, substance P, and glutamic acid - All skeletal neuromuscular junctions – use only acetylcholine *Action of Neurotransmitters (Steps): 1. Neurotransmitters are released from the nerve ending by the arrival of the nerve impulse (action potential) 2. Influx of calcium ions – synaptic vesicles fuse with presynaptic membrane 3. Neurotransmitters are ejected in the ECF in the synaptic cleft – diffuse across the gap to the postsynaptic membrane  The excitatory and the inhibitory effects on the postsynaptic membrane of the neuron will depend on the summation of the postsynaptic responses at the different synapses. o Ex: Depolarization: action potential will be initiated, nerve impulse will travel along the axon S1T1 5 of 5  1A NEUROANATOMY NEUROBIOLOGY OF NEURONS DR. ALLAN D. VIADO *Distribution & Fate of Neurotransmitters Acetycholine - Location: neuromuscular junction, autonomic ganglia, & parasympathetic nerve endings - Effect: limited by the destruction of the transmitter in the synaptic cleft by the enzyme acetylcholinesterase (AChE) Norepinephrine - Location: sympathetic nerve endings, hypothalamus (CNS) Dopamine - Location: basal nuclei (ganglia) *Neuromodulator at Chemical Synapses Neuromodulator: the substances capable of modulating and modifying the activity of the postsynaptic neuron - Release into the cleft have no direct effect on the postsynaptic membrane - Enhance, prolong, inhibit, or limit the effect of the principal neurotransmitter on the postsynaptic membrane - Act through a second messenger system (e.g. G- protein) 2. Electrical Synapses - Gap junctions containing channels that extend from the cytoplasm of the presynaptic neuron to that of the postsynaptic neuron - Rare - Neurons communicate electrically Advantage: bidirectional REFERENCES 1. Snell R.S Clinical Neuroanatomy, 7th Edition 2010. 2. Patestas M., Gartner L. A Textbook of Neuroanatomy. 2006 S1T1 6 of 5