Nervous Tissue

1. Nervous Tissue

2. Nervous Tissue • Consists of two main cell types: - neurons - sensory control, and regulation - neuroglia - support, protection, and homeostasis

3. Parts of a Neuron • cell body • dendrites • axon

4. Cell Body • contains nucleus, surrounded by granular cytoplasm • contains organelles (i.e., lysosomes, mitochondria, Golgi complex, etc.) • Nissl bodies (chromatophilic substance) - arrangement of rough endoplasmic reticulum (site of protein synthesis) • neurofibril - provides support and shape for cell

5. Two Kinds of Nerve Fibers • dendrites - neurons usually contain many • axons - neurons contain only one

6. Dendrites dendro = tree • highly branched processes • not myelinated • extend out from cell body • receive stimuli • carry a nerve impulse toward the cell body

7. Axon • long, thin, cylindrical projection • may or may not be myelinated • contains mitochondria and neurofibrils but no rough ER; no protein synthesis • cytoplasm - axoplasm • surrounded by axolemma (lemma = sheath or husk) • carries a nerve impulse away to other cells i.e., (nerves, muscles, glands)

8. Parts of an Axon • long nerve cell fibers are covered with myelin - fatty substance - protects and insulates fibers - increases transmission rate of - nerve impulses - forms myelin sheath

9. Parts of an Axon (cont.) • neurolemma sheath surrounds myelin • Schwann cells are surrounded by neurolemma - Schwann cellsarealso called neurolemmocytes • narrow gaps in myelin sheath between adjacent Schwann cells are called nodes of Ranvier

10. Parts of an Axon (cont.) • axon hillock - cone-shaped elevation where axon joins cell body • initial segment - first portion of axon • trigger zone - junction of the hillock and initial segment where nerve impulses arise

11. Parts of an Axon (cont.) • axon collateral - side branch along length of axon • axon terminals - fine branched end portions of an axon and its collaterals - when impulse reaches axon terminal, neurotransmitter is released

12. Parts of an Axon (cont.) • neurons are close to each other but separated by synaptic gap - this junction is called a synapse

13. Parts of an Axon (cont.) • synaptic end-bulbs - bulb-shaped structures - tips of axons • synaptic vesicles - membrane-enclosed sacs within synaptic end-bulbs - store neurotransmitters (different neurons have different neurotransmitters)

14. Neuroglia

15. Neuroglia • constitute one-half of volume of CNS • outnumber neurons 5-50 times • can multiply and divide, unlike neurons • Functions: - nerve glue - supports - insulates - protects

16. Types of Neuroglial Cells • astrocytes • oliogodendrocytes • microglia • ependymal cells • Schwann cells • satellite cells

17. Astrocytes (astro = star) • star-shaped • many processes • participate in metabolism of neurotransmitters • maintain potassium (K+) balance for generation of nerve impulses • participate in brain development • aid in formation of blood-brain barrier • link between neurons and blood vessels

18. Oliogodendrocytes(oligo = few, dendro = tree) • few processes • smaller than astrocytes • form supporting network by twining around neurons • produce a lipid and protein wrapping called a myelin sheath

19. Microglia(micro = small, glia = glue) • small • protect CNS from disease • phagocytic - engulf microbes and clear away debris

20. Ependymal Cells(ependyma = upper garment) • shape ranges from squamous to columnar • may be ciliated • line brain ventricles and central canal of spinal cord ventricles are spaces that produce and circulate cerebrospinal fluid

21. Schwann Cells (Neurolemmocytes) • found in peripheral nervous system • form myelin sheath • support, protect, and nourish neurons • plasma membrane of neurolemmocytes forms myelin sheath around axon in PNS • myelin sheath insulates speed of impulse transmission

22. Satellite Cells • found in peripheral nervous system • support neurons in ganglia (clusters of neuron cell bodies) of PNS

23. Neuron Classification • structural classification - based on number of processes extending from cell body • functional classification - based on direction in which a neuron transmits a nerve impulse

24. Structural Classification • unipolar (pseudounipolar) - have a single process extending from cell body - always are sensory neurons - originate in embryo as bipolar - during development, axon and dendrite fuse together into a single process - single process divides into two branches a short distance from cell body

25. Structural Classification • bipolar - have one dendrite and one axon - found in retina, internal ear, and olfactory area of brain • multipolar - several dendrites and one axon - most neurons in brain and spinal cord are of this type

26. Functional Classification • based on the direction in which a neuron transmits a nerve impulse • Two types of neurons in PNS: - afferent (toward CNS) - efferent (away from CNS)

27. Afferent • carry impulses (sensory information) toward CNS • also known as sensory neurons Efferent • carry impulses away from CNS • affect activity of muscles or glands • also known as motor neurons

28. Sensory = Afferent Motor = Efferent

29. Afferent or Sensory Neurons • Four categories: - general somatic - special somatic - general visceral - special visceral

30. Afferent or Sensory Neurons (cont.) • general somatic - carry impulses from skin, skeletal muscles, and joints (pain, temperature, touch, vibration, and pressure)

31. Afferent or Sensory Neurons (cont.) • special somatic - carry or relay impulses from retina and internal ear (vision, hearing, and balance)

32. Afferent or Sensory Neurons (cont.) • general visceral - carry impulses from internal organs and receptors of body fluids (distension of internal organs and chemical conditions in body)

33. Afferent or Sensory Neurons (cont.) • special visceral - carry impulses from tongue and olfactory mucosa (taste and smell)

34. Efferent or Motor Neurons • general somatic - carry impulses to most skeletal muscles • general visceral - carry impulses to smooth muscle, cardiac muscle, and glands • special visceral - carry impulses to skeletal muscles that control facial expression

35. Associated Neurons • located in CNS • carry impulses from one neuron to another inside brain and spinal cord • establish interrelationship with other neurons forming neuronal circuits