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Nerve & Muscle. Divisions of Nervous System

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Nerve & Muscle. Divisions of Nervous System INTRODUCTION OF EXCITATORY TISSUES By Dr. Mudassar Ali Roomi (MBBS, M.Phil.) Assistant Professor Physiology RED CNS BLUE PNS Divisions of nervous system: CNS & PNS Central nervous system (CNS): • Brain (in skull) & Spinal cord (in vertebral canal): Divided into gray matter & white matter. • Formed by neurons & supporting cells (neuroglia) • Gray matter: nerve cell bodies, proximal parts of axons & dendrites. • White matter: nerve fibers / axons. Major parts of Brain: 1. Prosencephalon: Forebrain: (cerebral hemispheres, thalamus, hypothalamus). 2. Mesencephalon: Midbrain 3. Rhombencephalon: Hindbrain: (Pons, cerebellum, medulla oblongata) Brain stem: Midbrain, Pons, Medulla oblongata. Peripheral nervous system (PNS) • Formed by neurons & their processes outside CNS. • Cranial nerves (from brain) • Spinal nerves (from spinal cord) • 2 divisions of PNS: SOMATIC / VOLUNTARY NERVOUS SYSTEM: Nerves supplying skeletal muscles Controls movements of body by acting on skeletal muscles. AUTONOMIC / VISCERAL / VEGETATIVE/ INVOLUNTARY NERVOUS SYSTEM: Function: Controls viscera 2 sub-divisions: SYMPATHETIC PARASYMPATHETIC Cells of nervous tissue TWO TYPES OF CELLS ARE PRESENT IN THE NERVOUS TISSUE: • Nerve cells or Neurons: which are excitable and conduct electrical impulses • Glial (neuroglial) cells or supporting cells: which support, nurture, and protect the neurons. 18 November 2013 6 The Neuron • Structural and functional unit of nervous system. • Features: – Nucleus & cytoplasmic organelles. – No centrosome cannot divide. – Branches Axon / nerve fiber & Dendrites. 18 November 2013 7 Nerve cell body (perikaryon or soma) • It contains nucleus, cytoplasmic organelles, inclusions and cytoskeletal components. • Nucleus: it is large spherical and pale staining (vesicular) and centrally placed in most of the cases. It contains abundant euchromatin (chromatin is finely dispersed) and a large nucleolus (owl-eye nucleus). 18 November 2013 8 Nerve cell body (perikaryon or soma) • (a) Nissl bodies/granules/substance (chromatophilic substance) - basophilic patches, cytoplasmic structures are concentrations of RER. *** • The amount of chromatophilic substance varies according to the type and functional state of the neuron and is particularly abundant in large nerve cells such as motor neurons • (b) Neurofilaments - a variety of intermediate filament. • (c) Surrounding the nucleus are elements of the Golgi apparatus, mitochondria, lysosomes, and microtubules. • (d) Pigment is sometimes present, e.g., melanin in substantia nigra neurons (midbrain), and brown granules of lipofuscin pigment (wear and tear pigment) in old neurons. *** 18 November 2013 9 dendrites (a) Definition: These are the neuronal processes that receive information and transmit it to the cell body. (b) Contain mitochondria, microtubules, and granular ER. (c) Nissl granules are present in the proximal part of the dendrites. (d) Lack Golgi apparatus*** (e) dendrites have spine-like side processes. function: Dendrites integrate the excitatory influences and increase the receptive area of a neuron. 18 November 2013 10 axon • (a) Contains axoplasm flowing centrifugally from the pyramidal shape starting-point of the axon - the axon hillock. *** • initial segment is the site b/w the apex of the axon hillock and the start of the myelin sheath. Action potential is generated here.*** • (b) Has mitochondria, neurofilaments, microtubules, travelling vesicles, and, in some neurons, secretion droplets, in the axoplasm. • (c) Membrane of the axon is called as axolemma, swelling out into a bag at its ends (synaptic boutons) which holds vesicles. • Axonal transport: Kinesin, are responsible for anterograde transport. Dynein allows retrograde transport .*** 18 November 2013 11 Axon structure 3 LAYERS: (outside inside) • Epineurium: sheath covering whole nerve that contains many fasiculi of nerve fibers. • Perineurium: covers each fasciculus of the nerve • Endoneurium: covers each nerve fiber within the fasiculus. Internal structure of axon: • Axoplasm: (long central core of cytoplasm in the axon) Receives synthesized proteins & neurotransmitters from cell body by axonal flow • Axolemma: (tubular sheath like membrane covering the axoplasm) • Axis cylinder of nerve fiber: (axoplasm + axolemma) CLASSIFICATION OF NEURONS Neurotransmitter Released Cholinergic GABAergic Sympathatic Parasympathatic Adrenergic Noradrenergic Cell Processes Multipolar Bipolar Unipolar Pseudo-unipolar Motor, pyramidal, purkinje Sensory, retina, olfactory mucosa, rods & cones Sensory, dorsal roots & cranial cochlear, vest. ganglia ganglia Cell Size Golgi I Golgi II Motor paramidal cell, Interneuron spinal cord Function 18 November 2013Motor Interneurons Sensory 14 Classification of neurons on the basis of structure • Myelinated covered by myelin sheath • Unmyelinated not covered by myelin sheath Myelinated Fibers • myelin is a whitish lipoprotein complex. Sphingmyelin is the main lipid in it • These are axons of large diameter grow in the PNS 11/18/2013 16 Myelinated Fibers (cont.) • Membranes of Schwann cells have a higher proportion of lipids (80%) than do other cell membranes. • Between adjacent Schwann cells the myelin sheath shows small nodal gaps along the axon, also called nodes of Ranvier. • The length of axon covered by one Schwann cell is called the internodal segment and may be more than 1 millimeter. 11/18/2013 17 Process of myelination (Myelinogenesis) 11/18/2013 18 Process of myelination (cont.) 11/18/2013 19 Myelination of nerve fibers In PNS In CNS 11/18/2013 20 Morphological Classification of neurons- according to the number of cell processes • Unipolar neurons : rare in humans. Found in the mesencephalic nucleus of the 5th cranial nerve • pseudounipolar neurons, which have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS. Examples: found in the dorsal root ganglia of spinal nerves • Bipolar neurons, with one dendrite and one axon. These neurons are found in cochlear and vestibular ganglia, retina and olfactory epithelium. • Multipolar neurons, which have one axon and two or many dendrites. Most human neurons are of this type. Some examples are: pyramidal cells of cerebral cortex, Purkinje cells of cerebellar cortex, and anterior horn cells of the spinal cord. 18 November 2013 21 18 November 2013 22 Classification of Neurons by Functional Role Motor (efferent) neurons • (CNS Periphery) • Motor neurons control effector organs and muscle fibers. Sensory (afferent) neurons • (periphery CNS) • Sensory neurons receive sensory stimuli from the internal or external environment and relay them to the CNS. 18 November 2013 23 Classification of Neurons-according to the length of their axons Golgi type I neurons: possess many dendrites and a very long axon that leaves the grey matter in which its cell body is present. Examples: • pyramidal cells of the cerebral cortex • anterior horn cells of the spinal cord. Golgi type II neurons: possess many dendrites and a relatively short axon that does not leave the part of grey matter in which the cell body of the neuron is present. Example: • Interneurons. 18 November 2013 24 Classification On the basis of distribution: • Somatic supply skeletal muscles • Visceral / Autonomic supply internal organs Classification on the basis of source of origin: • Cranial from the brain • Spinal from the spinal cord COMPOUND ACTION POTENTIAL basis for physiological classification of nerve fibers. • Action potential recording from a nerve trunk. • 3 main peaks: A, B, C. • A peak is further divided into alpha, beta, gamma, delta. • A & B types are myelinated & C type is unmyelinated. November 18, 2013 27 Electrophysiologic classification of nerve fibers November 18, 2013 28 Sensory classification of nerve fibers November 18, 2013 29 Sensory classification Vs electrophysiological classification • Type A (myelinated): A-alpha (Group Ia & Ib), A-beta (Group II), A-gamma (Group II), A-delta (Group III). • Type B (myelinated) • Type C (unmyelinated): Group IV Diameter is directly proportional to conduction velocity. Range of velocity: 120m/sec (in large myelinated fibers) to 0.5 m/sec (in smallest unmyelinated fibers). Sensory classification Vs electrophysiological classification Ia / A Ib /A II /A beta III /A IV / C alpha alpha delta Fibers from Fibers from Fibers from Fibers carrying Unmyelinated annulospiral golgi tendon most discrete temperature, fibers carrying endings of organs. cutaneous tactile crude touch & pain, itch, muscle spindles. receptors & pricking pain. temperature & flower spray crude touch. 17 micron dia endings of muscle spindle. 16 µm in dia. 8 µm in dia. 3 µm in dia. 0.5-2 µm in dia .
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