NERVOUS TISSUE Part 1 PDF

HNU Faculty of Medicine Histology & Cell Biology Department Year 1 Principle of Human Body Structure HBS-101 NERVOUS TISSUE (PART 1) Objective 1. Recognize the different classifications of neurons. 2. Describe the histologi...

HNU Faculty of Medicine Histology & Cell Biology Department Year 1 Principle of Human Body Structure HBS-101 NERVOUS TISSUE (PART 1) Objective 1. Recognize the different classifications of neurons. 2. Describe the histological structure of the neuron. 3. Compare between the dendrites & the axon at the structural & functional levels. 4. Differentiate between spinal & sympathetic ganglia. 5. Discuss histological structure of synapses 6. Describe histological structure & functions of neuroglial cells.  Cell body Neurons (Perikaryon) Dendrites(multiple processes) Axon (single process.)  Structure: Cell body (Perikaryon) It is part of neuron containing nucleus & surrounding cytoplasm. Size: Varies from 4 um as (granular cells in cerebellar cortex) to 100um as in motor neurons. Shape: Pseudo-unipolar neuron: the cell body has rounded shape. Bipolar: the cell body is fusiform shape. Multipolar: the cell body is stellate, pyramidal or pyriform. Nucleus: large spherical, euchromatic with a prominent nucleolus (characters of active cell). The cytoplasm: 1. It contains highly developed rER& numerous polyribosomes appears under L.M as basophilic granular areas called Nissl bodies. varies according to neuronal function. 2. Golgi complex: around the nucleus. 3.Mitochondria: scattered throughout the cytoplasm. 4. Neurofilaments: (intermediate filaments) are abundant in Perikaryon & processes. visible by L.M (stained brown by Ag).They provide structural support. 5. Microtubules: - Arranged in parallel bundles in Perikaryon & processes. - Involved in axonal transport of neurotransmitter substances& enzymes 6. Centrioles: absent, as neurons cannot divide. 7. Inclusions in form of : ►Lipofuscin pigment is golden brown. It is a residue of undigested material by lysosomes. increases with aging. ►Melanin: Dark brown or black pigment) - found in neurons of substantia nigra of midbrain. ►Lipid droplets: in cytoplasm represent energy reserve or products of abnormal metabolism.  Processes Both dendrites & axon have mitochondria, neurofibrils & microtubules Dendrites Axon 1. Usually numerous 1. Single 2. Short. 2. Long 3. Thick. 3. Thin. 4. Branching like a tree. Branches arise at acute angle 4. Not branching except at the end. arise at right angle 5. Become thinner as they subdivide into branches. 5. has a constant diameter. 6. contain Nissl granules. 6. Does not Contain Nissl bodies. 7.Covered by spine like processes called dendritic spines 7.No spines that for synaptic contacts f r 8. Carries nerve impulses to the cell body. 8.Carry nerve impulses away the cell o m body.  Classification of neurons: A-They are classified according to number of processes into: 1.Pseudo-unipolar: nerve body (perikaryon) is rounded, has a single process & just after leaving the perikaryon divided into two branches which form a T-shape, one proceed as a peripheral branch (dendrite) & the other proceed centrally & act as (axon). Site: Spinal ganglia & mesencephalic nucleus of trigeminal nerve. 2. Bipolar: Have one dendrite & one axon Site : - In cochlear & vestibular G in ear. - Retina in eye & olfactory in nose. 3. Multipolar: have one axon & many dendrites. They take different forms: ► Stellate → Anterior horn cells in spinal cord. ► Pyramidal → Pyramidal cells in cerebral cortex. ► Pyriform: as Purkinje cells in cerebellar cortex  Classification of neurons: B- According to function: 1. Sensory: (Afferent) neurons receive sensory stimuli as cells of dorsal root ganglion. 2. Motor: (Efferent) neurons control effector organs such as Muscle& glands as anterior horn in spinal cord. 3. Interneurons connect neurons as in retina & spinal cord.  Classification of neurons: C- According to length of axon: 1.Golgi type I: neurons have long axon that leaves grey matter & enters white matter as Motor neurons in spinal cord. Pyramidal cells in cerebral cortex. Purkinje cells in cerebellar cortex. 2.Golgi type II: neurons have short axon that doesn't leave grey matter as interneuron in cerebral & cerebellar.  Length of Axon Golgi Type I (pyramidal cells of cerebral cortex) Golgi Type II (granular cells of cerebellum) sensory neuron receptor Interneuron motor neuron effector  Nerve fiber ► A nerve fiber consists of an axon covered by 2 sheathes (myelin & Schwann’s cells). ► The axon is covered by axolemma & contains cytoplasm called axoplasm. ►It arises from a conical extension of cell body called axon hillock.  Types of nerve fibers 1. Unmyelinated nerve fibres: Have no myelin sheath. It is subdivided into: A- Without sheath of Schwann cells (neurolemma) as in gray matter (Naked). B- With sheath of Schwann cells as in sympathetic post ganglionic fibers. 2. Myelinated nerve fibres: Have myelin sheath. It is subdivided into: A- Without sheath of Schwann cells as in white matter. B- With sheath of Schwann cells as in peripheral nerve fibers. The sheath of Schwann It consists of flattened cells with flattened nuclei that form thin chain around myelin of nerve fiber. Functions: 1. Formation of myelin sheath in the peripheral nerves. 2. Electric insulation. 3. Regeneration where axon grows from the proximal stump along the path formed by Schwann cells.  Myelin Sheath Structure: Formed of many layers of modified cell membranes with higher lipids than cell membrane. L.M →Lipoprotein dissolved after routine fixation & can be stained black by osmic acid. E/M→ appears as fused spiral laminae of plasmalemma. It shows gaps called nodes of Ranvier that represent the spaces between adjacent sheath cells. Sheath of myelin is thus divided into segments by the nodes which are called inter-nodal segments. Each Schwann cell wraps around one segment of a single axon while each oligodendroglia cell warps around one segment of many axons (10-60). Functions: Speed up the speed of nerve impulse  Peripheral Nervous System It consists of Nerves, ganglia and nerve endings.  Peripheral Nerve (Nerve trunk Epineurium→ The whole nerve is covered by dense C.T Perineurium→ The nerve fibers are arranged in the form of bundles. Endoneurium→ Inside the bundle the nerve fibers are connected by (sheath of Henle). It consists of reticular fibers formed by Schwann cells. It is formed of flattened epithelium-like cells joined by tight junctions. This forms a barrier to protect the nerve fibers.  Test yourself 1.What is meant by nerve fiber? a) It means cell body of neuron. b) It means the axon covered by two sheathes. c) It means myelin sheath only. d) It means gaps between myelin segments. 2. What is the type of nerve cell with rounded cell body & single divided process? a) Unipolar. b) Pseudo-unipolar. c) Bipolar. d) Multipolar Reference 26