Neurons.pptx...............................

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1. Nervous System & Neuron kamran

2. The Nervous System General Overview The integration of all three major functions

3. What are the major functional components of the Nervous System? Brain Spinal Cord Afferent Tracts Efferent Tracts Ganglia The Nervous System Functional Components Central Nervous System Peripheral Nervous System

4. The Nervous System Organization of the CNS The Brain Functional organization Neuronal circuits organized into Gray matter & White Matter Gray Matter Consists of Non myelinated axon terminals Dendrites Cell Bodies Functions in signal processing (integration) Neurocrine / neurohormone secretion White Matter Consists of Myelinated axons Functions in Formation of tracts ascending or descending (SC) commissural, association, projection (B)

5. The Nervous System Cells of the Nervous System Cells are grouped into two functional categories Neurons Do all of the major functions on their own, are Afferent Interneurons Efferent Neuroglia Play a supporting role to the neurons Divided into CNS and PNS Neuroglia CNS Astrocytes Oligodendrocytes Microglia Ependymal cells – PNS Neurolemmocytes Satellite cells

6. The human CNS contains about 10 11 (100 billion) neurons A lso contains 2–10 times this number of glial cells About 40 % of the human genes participate, at least to a degree, in formation of CNS The neurons are the basic building blocks of the nervous system Neurons perform the specialized function of integration & transmission of nerve impulse

7. Neurons and glial cells along with brain capillaries form a functional unit that is required for normal brain function , including synaptic activity, ECL fluid homeostasis , energy metabolism, and neural protection Disturbances in the interaction of these elements are the pathophysiological basis for many neurological disorders ( eg , cerebral ischemia, seizures, neurodegenerative diseases, and cerebral edema )

8. GLIAL CELLS Previously, glial cells (or glia ) were viewed as CNS connective/ supporting tissue; also called neuroglia or glia (glia = glue) Today theses cells are recognized for their role in communication within the CNS in partnership with neurons non-excitable and do not transmit nerve impulse (action potential ) Unlike neurons, glial cells continue to undergo cell division in adulthood and their ability to proliferate is particularly noticeable after brain injury ( eg , stroke )

9. Neuroglia Functional Classification

10. Most commonly, neuroglial cells constitute the site of tumors in nervous system Two major types of glial cells in the nervous system ; microglia and macroglia Microglia; smallest neuroglial cells; derived from macrophages outside of the nervous system; often called the macrophages of CNS Scavenger cells that remove debris resulting from injury, infection, and disease ( eg , multiple sclerosis, AIDS-related dementia, Parkinson disease, and Alzheimer disease )

11. Three types of macroglia : oligodendrocytes , Schwann cells, and astrocytes Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively Astrocytes are star-shaped neuroglial cells present in all the parts of the brain; Two types of astrocytes are found in human brain : i . Fibrous astrocytes- occupy mainly the white matter ii. Protoplasmic astrocytes- found in gray matter and have a granular cytoplasm

12. Both send processes to blood vessels of brain, particularly, the capillaries, forming tight junction with capillary membrane; Tight junction in turn forms the blood-brain barrier Also send processes that envelop synapses and the surface of nerve cells Protoplasmic astrocytes have a membrane potential that varies with the external K+ concentration but do not generate propagated potentials Produce substances that are tropic to neurons , and help maintain the appropriate conc. of ions and neurotransmitters by taking up K+ and the neurotransmitters glutamate and γ- aminobutyrate

13. Neurones Neurons occur in a variety of sizes and shapes Most of them contain four parts: (1) a cell body, (2) dendrites, (3) an axon , and (4) axon terminals The cell body (soma) contains the nucleus & ribosome and is the metabolic center of the neuron The dendrites form a series of highly branched outgrowths from the cell body; receive most of the inputs from other neurons The branching dendrites ( some neurons may have as many as 400,000!) increase the cell’s receptive surface area and thereby increase its capacity to receive signals from a myriad of other neurons

14. Particularly in the cerebral and cerebellar cortex, the dendrites have small knobby projections called dendritic spines The axon, sometimes also called a nerve fiber, is a single long process that extends from the cell body to its target cells (from μ m to m in length) The part of the cell body where the axon is joined is known as the initial segment, or axon hillock The initial segment is the “trigger zone” where, in most neurons, the electric signals are generated & are then propagated away from the cell body along the axon or, sometimes, back along the dendrites

15. The main axon may have branches, called collaterals , along its course Near the ends both the main axon and its collaterals undergo further branching; The greater the degree of branching of the axon and axon collaterals, the greater the cell’s sphere of influence Each branch ends in an axon presynaptic terminal ; composed of a number of synaptic knobs which are also called terminal buttons or boutons They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored

16. The axons of many neurons are myelinated , that is, they acquire a sheath of myelin , a protein–lipid complex that is wrapped around the axon In the PNS, myelin forms when a Schwann cell wraps its membrane around an axon up to 100 times The myelin is compacted when the extracellular portions of a membrane protein called protein zero ( P ) lock to the extracellular portions of P in the apposing membrane Various mutations in the gene for P cause peripheral neuropathies; 29 different mutations have been described that cause symptoms ranging from mild to severe

17. Myelin sheath is not a continuous sheath; absent at regular intervals ( 1-μm constrictions that are about 1 mm apart); the area where myelin sheath is absent is called node of Ranvier Segment of the nerve fiber between two nodes is called internode Myelin sheath is responsible for white color of nerve fibers Unmyelinated neurones are simply surrounded by Schwann cells without the wrapping of the Schwann cell membrane that produces myelin around the axon In the CNS, most neurons are myelinated ; the myelin-forming cells are the oligodendroglia

18. Here oligodendrocytes emit multiple processes that form myelin on many neighboring axons In multiple sclerosis, a crippling autoimmune disease, patchy destruction of myelin occurs in the CNS The loss of myelin is associated with delayed or blocked conduction in the demyelinated axons Neurons are classified by three different methods. A. Depending upon the number of poles B. Depending upon the function C. Depending upon the length of axon

19. Depending Upon The Number Of Poles: Based on the number of poles from which the nerve fibers arise, neurons are divided into three types : 1. Unipolar Neurons: Neurons that have only one pole ; from a single pole, both axon and dendrite arise; this type of nerve cells is present only in embryonic stage in human beings 2. Bipolar Neurons: Neurons with two poles are known as bipolar neurons; Axon arises from one pole and dendrites arise from the other pole 3. Multipolar Neurons: Neurons which have many poles; one of the poles gives rise to axon and all other poles give rise to dendrites

20. Depending Upon The Function On the basis of function, nerve cells are classified into two types: 1. Motor or efferent neurons 2. Sensory or afferent neurons 1. Motor or Efferent Neurons: Neurons which carry the motor impulses from CNS to peripheral effector organs like muscles, glands, blood vessels , etc. Generally, each motor neuron has a long axon and short dendrites 2. Sensory or Afferent Neurons: Neurons which carry the sensory impulses from periphery to CNS; g enerally , each sensory neuron has a short axon and long dendrites

21. Depending Upon The Length Of Axon Depending upon the length of axon, neurons are divided into two types: 1. Golgi type I neurons 2. Golgi type II neurons 1. Golgi Type I Neurons: They have long axons; cell body of these neurons is in different parts of CNS and their axons reach the remote peripheral organs 2. Golgi Type II Neurons: Such n eurons have short axons; present in cerebral cortex and spinal cord

22. AXONAL TRANSPORT Neurons-secretory cell; differ from other secretory cells in that the secretory zone is generally at the end of the axon , far away from the cell body Protein synthesis occurs in cell body & transported to the axonal ending by axoplasmic flow The functional and anatomic integrity of the axon is very important; if the axon is cut, the part distal to the cut degenerates ( wallerian degeneration ) Orthograde transport occurs along microtubules that run along the length of the axon and requires two molecular motors , dynein and kinesin

23. Orthograde transport moves from the cell body toward the axon terminals Has both fast and slow components ; fast axonal transport occurs at about 400 mm/day, and slow axonal transport occurs at 0.5 to 10 mm/day Retrograde transport, which is in the opposite direction (from the nerve ending to the cell body), occurs along microtubules at about 200 mm/day Synaptic vesicles recycle in the membrane, but some used vesicles are carried back to the cell body and deposited in lysosomes Some materials taken up at the ending by endocytosis, including nerve growth factor (NGF) and some viruses, are also transported back to the cell body