Sheet: microscopic structures of the ns Done by: Rasha Rakan and Nisreen Obiedat

Transcription

1 Sheet: microscopic structures of the ns Done by: Rasha Rakan and Nisreen Obiedat

2 Lecture Objectives Classify the types of neurons. Describe the structure of the different parts of neurons. Describe the types of glia cells and their functions. Describe the process of myelination of myelinated axons. Describe the structure of peripheral nerves. Describe the structure of ganglia (sensory and autonomic).

3 Nervous Tissue Relatively little intercellular space Cells (neurons or neuroglia) rest on a continuous basal lamina Some cells have microvilli, cilia Strong tendency for cells to be bound to one another Clasping structures between connected cells called synapses

4 Neurons Functional unit of nervous system Have capacity to produce action potentials and transmit it electrical excitability Cell body Cell processes = dendrites (transmit change in voltage towards the cell body)& axons (transmit change in voltage away from the cell body)

5 Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axons or Dendrites Cell body

6 Diversity in Neurons Both structural and functional features are used to classify the various neurons in the body. On the basis of the number of processes extending from the cell body (structure), neurons are classified as multipolar and bipolar Most neurons in the body are interneurons and are often named for the histologist who first described them or for an aspect of their shape or appearance.

7 Structural Classification of Neurons Based on number of processes found on cell body multipolar = several dendrites & one axon most common cell type bipolar neurons = one main dendrite & one axon found in retina, inner ear & olfactory unipolar neurons = one process only(develops from a bipolar) are always sensory neurons

8 Functional Classification of Neurons Peripherally : Sensory (afferent) neurons transport sensory information from skin, muscles, joints, sense organs & viscera to CNS Motor (efferent) neurons send motor nerve impulses to muscles & glands Inside the CNS regardless of their function: Interneurons (association) neurons connect sensory to motor neurons 90% of neurons in the body

9 Association or Interneurons All interneurons and motor neurons are bipolar if it is preganglionic or somatic motor neuron their cell body is inside the CNS and the rest is outside If the motor neuron was postganglionic then all of its parts are outside the CNS Sensory neurons are either bipolar (only special sensors, because the receptors are relatively close to the cell body like retina where the dendrites are in the external layer and the cell body in internal layer) or unipolar ( somatic or visceral sensory neurons, because their cell bodies are in dorsal root ganglia while their receptors are in periphery) The explanation is: that the receptor is far from the cell body, if that space between the cell body and the receptors was dendrites it isn t be able to generate AP, as soon as it reaches the trigger zone it will disappear.

10 Cell bodies of sensory cells in general are outside the CNS with few exceptions that we will mention later Neuronal Structure & Function

11 Cell body Also called perikaryon A trophic center Receives a great number of nerve endings that convey excitatory or inhibitory stimuli Could be small (4-5μm) or large (150 μm) that is visible by naked eye it has nucleus and nucleolus, so it is responsible for all process and nourishment of other parts of the neuron. If we cut the cell body the axon will degenerate because it lost its supply. Cell organelles: Single, large, euchromatic (pale-staining) nucleus with prominent nucleolus Nissl bodies (chromatophilic substance) it appears as dark dots, and it is the reason of grayish colors of neurons (that s why we call it gray matter) rough ER & free ribosomes for protein synthesis it synthesizes neurotransmitters( all neurons are active means the are productive and are abundant with rough ER, Golgi apparatus and mitochondria) neurofilaments (10nm) give cell shape and support. When stained by silver form neurofibrils that can be seen by light microscope intermediate fibers of cytoskeleton, they are well developed, supports the cell body and helps the body with its inside processes microtubules (22nm) move material inside cell very well developed because it is responsible for transporting materials from cell bodies towards periphery lipofuscin granules (harmless aging.)they are residual lysosomal products. Indicates that that the cell is aging cell, brownish in color

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13 Neuronal processes The dendrites are receiving or input portions of a neuron. It can t generate AP but it can transmit change of voltage.the membrane of cell body is able of summation of voltage changes to generate AP. Theaxonconducts nerve impulses from the neuron to the dendrites or cell body of another neuron or to an effector organ of the body (muscle or gland).

14 Dendrites Extend from cell body In proximal portion, continues cytoplasmic character of perikaryon Usually irregular contour and spines (microvilli-like projections) Usually multiple Rarely myelinated, usually unsheathed Ramifies by branching at acute angles If cell body is in CNS then dendrites remain in CNS Conducts impulses towards the cell body

15 Axons Conduct impulses away from cell body Long, thin cylindrical process of cell One per neuron Do not contain ribosomes Dependence on perikaryon If axon is severed, its peripheral parts degenerate and die Arises at axon hillock (its contents are not similar to cell body contents) Impulses arise from initial segment (trigger zone) Branches to form collaterals at obtuse angles they are the base of networking inside CNS. Any info inside a neuron has to be copied to other neurons. It can reach hundreds in number end in fine processes called axon terminals Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Axons are different in sizes and according to that not all of them are mylinated. A and B fibers are large mylinated and thus AP are quickly transmitted. ( 100 time more than small unmylinated fibers like type C.

16 How to differentiate between dendrites and axons 1. Dendrites are multiple, axons are only one for each neuron 2. Dendrites proximal part is conical in shape and the part that connects them to the cell body has the same contents found inside the cell body (like Nissl bodies) this doesn t exist in axons 3. Dendrites start to branch with sharp-angled branches, while axons branches have 90 degree angle. 4. Dendrites are unmylinated and shorter than axons A summry

17 Neuroglial Cells Half of the volume of the CNS Smaller cells than neurons 50X more numerous Cells can divide rapid mitosis in tumor formation (gliomas) 4 cell types in CNS astrocytes, oligodendrocytes, microglia & ependymal 2 cell types in PNS schwann and satellite cells

18 Astrocytes Star-shaped cells Form blood-brain barrier by covering blood capillaries Metabolize neurotransmitters ;forming part of the recycle process Regulate K+ balance Provide structural support Have many types

19 Microglia Small cells found near blood vessels Phagocytic role -- clear away dead cells Derived from cells WBC s that also gave rise to macrophages & monocytes

20 Ependymal cells (CSF) They re low columnar epithelial cells lining the ventricles and they form part of the choroid plexus *choroid plexus: blood capillaries +epyndymal cells.

21 Satellite Cells Flat cells surrounding neuronal cell bodies in peripheral ganglia Support neurons in the PNS ganglia Mainly in the dorsal root ganglia

22 Oligodendrocytes Most common glial cell type Each forms myelin sheath around more than one axons in CNS Analogous to Schwann cells of PNS Huge cells with many branches so myelinate more than one axon

23 Myelination A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons The sheath electrically insulates the axon and increases the speed of nerve impulse conduction Myelination works as a capacitor which means that the action potential travels from node of ranvier to another without dropping below the threshold *cell membrane is the most structure in the cell rich in lipid.

24 Schwann Cell Cells encircling PNS axons Each cell produces part of the myelin sheath surrounding an axon in the PNS Participate in myelination of one axon only unlike oligodendrocytes which participate in myelination of more than one axon

25 Axon Coverings in PNS All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells Neurilemma is cytoplasm & nucleus of Schwann cell gaps called nodes of Ranvier Myelinated fibers appear white jelly-roll like wrappings made of lipoprotein = myelin acts as electrical insulator speeds conduction of nerve impulses Unmyelinated fibers slow, small diameter fibers only surrounded by neurilemma but no myelin sheath wrapping Axolemma: cell membrane of an axon

26 Myelination in Schwann cells myelinate (wrap around) axons in the PNS during fetal development Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath Tube guides growing axons that are repairing themselves : the neurolemma help in preserving the axon pathway so if any axon is damaged the neuroleema help it in regenerating

27 Myelination in the CNS Oligodendrocytes myelinate axons in the CNS Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons No neurilemma is formed so there is Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma

28 Gray and White Matter White matter = myelinated processes (white in color) Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS A nucleus is a mass of nerve cell bodies and dendrites inside the CNS.

29 Spinal Nerves Connective tissue coverings of spinal nerves: Epineurium :around the entire nerve perineurium :around each fascicle Endoneurium : around each axon Fascicles : bundles of axons

30 Spinal Nerves

31 Ganglia Dorsal root (sensory) ganglia Nuclei are large, prominent and central Neurons arranged in rows Two population of neurons based on size Large 100 μm diameter Small μm diameter Surrounded by Satellite (nurse) cells and well developed capsule Fixation causes perikaryon to shrink away from capsule

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33 Ganglia Autonomic ganglia Nuclei are eccentric No neuronal clusters One size neuron-small Poorly defined capsule, fewer satellite cells Lipofscin granules multipolar

34 nucleus Dorsal root ganglia sensory Centric Autonomic ganglia eccentric Neuronal clusters Size of neuron Satellite cells Present : arrows Small (10-15)micro m Large (100)micro m More absent Small size only fewer Capsule(C.T) Type of neurons Lipofuscin granules Well defined Unipolar general sensory neurons Poor defined Multipolar autonomic motor neurons & post ganglionic neurons present

35 Spinal Cord Anterior median fissure Posterior median sulcus Gray and white commissures Central canal Anterior, posterior & lateral gray horns Anterior horns contain motor neurons Posterior horns receive sensory fibers from neurons in the spinal ganglia Anterior, posterior & lateral white columns

36 Internal Anatomy of Spinal Cord

37 Internal Anatomy of Spinal Cord