Introduction to the Nervous System

1. Introduction to the Nervous System to be copied

2. Nervous Tissue & Homeostasis • excitable characteristic of nervous tissue allows for generation of nerve impulses (action potentials) that provide communication & regulation of most body tissue. • together with endocrine system: responsible for maintaining homeostasis

3. ENDOCRINE Differences in Nervous & Endocrine Control of Homeostasis NERVOUS rapid responder action potentials slow, prolonged response releases hormones

4. Structures of the Nervous System • total mass of 2 kg (~3% of total body mass) • Skull • Spinal Cord • Spinal Nerves • Cranial Nerves • Ganglia • Enteric Plexus • Special Senses & other Sensory Receptors

5. Functions of the Nervous System • 3 basic functions: • Sensory • Integrative • Motor

6. Sensory Function • sensory receptors detect internal & external stimuli • sensory (afferent) neurons carry this sensory information to spinal cord & brain thru cranial & spinal nerves

7. Integrative Function • integrate: process • nervous system takes information from sensory neurons & processes that information, analyzes it, stores some of it & makes decisions for appropriate responses • served by interneurons (connect 1 neuron to another neuron • Perception: • conscious awareness of sensory stimuli • occurs in brain

8. Motor Function • served by motor (efferent) neurons • carry info from brain/spinal cord  effectors (muscle or gland) thru cranial or spinal nerves • results in muscles contraction or gland secreting

9. Organization of the Nervous System

10. Histology of the Nerrvous System • 2 cell types • Neurons • Neuroglia

11. Neurons • nerve cells that possess electrical excitability: • ability to respond to a stimulus & convert it into an action potential • stimulus: any change in environment that is strong enough to initiate an action potential

12. Action Potential • electrical signal that propagates along surface of neurolema (membrane) • begins & travels due to movement of ions between interstitial fluid & inside of neuron thru specific ion channels • once begun it travels rapidly @ constant strength

13. Parts of a Neuron

14. Parts of Neuron: Cell Body • contains nucleus, cytoplasm, typical organelles, • + Nissl bodies clusters of RER • make materials for: • growth of neuron • regenerate damaged axons in PNS

15. Nerve Fiber • general term for any neuronal process or extension that emerges from cell body • most neurons have 2: • Dendrites • Axons

16. Dendrites • “little trees” • input portion of neuron • usually, short, tapering, highly branched • their cytoplasm contains Nissl bodies, mitochondria

17. Axon • propagates action potentials  • another neuron • muscle fiber • gland cell

18. Parts of an Axon • joins cell body @ cone-shaped elevation: axon hillock • part of axon closest to hillock = initial segment • jct of axon hillock & initial segment where action potential arises so is called the trigger zone

20. Parts of an Axon • axoplasm: cytoplasm of an axon • axolemma: plasma membrane of axon • axon collaterals: side branches along length of axon (most @ 90°) • axon terminals: axon divides into many fine processes

21. Synapse • site of communication between 2 neurons or between a neuron & effector cell • synaptic end bulbs: tips of some axon terminals swell into bulb-shaped structures • synaptic vesicles: store neurotransmitter • many neurons have >1 neurotransmitter, each with different effects on postsynaptic cell

22. Axonal Transport • 2 types: • for moving materials from cell body  axon terminals • slow • 1-5 mm/d • replenishes new axoplasm to developing or regenerating axons • fast • 200 – 400 mm/d • moves materials to/from cell body • organelles or membranes needed in axon terminal

23. Structural Classification Types of Neurons Functional Classification Sensory Interneurons Motor use # processes extending from cell body Multipolar neurons Bipolar neurons Unipolar neurons

24. Multipolar Neurons • several dendrites with 1 axon • includes most neurons in brain & spinal cord

25. Bipolar Neuron • 1 main dendrite & 1 axon • retina, inner ear, olfactory area of brain

26. Unipolar Neuron • are sensory neurons that begin in embryo as bipolar • during development axon & dendrite fuse then divide into 2 branches (both have characteristic structure & function of an axon) • 1 branch ends with dendrites (out of CNS) • 2nd branch ends in axon terminal (in CNS) • cell bodies of most found in ganglia

27. Unipolar Neuron

28. Pyramidal Cells • in cerebral cortex of brain

29. Neuroglia (Glia) • ~50% vol of CNS • “glue” • do not generate or propagate action potentials • multiply & divide in mature nervous systems • glioma: • brain tumors derived from glial cells • very malignant, grow rapidly

30. Glial Cells of the CNS • ASTROCYTES • OLIGODENDROCYTES • MICROGLIA • EPENDYMAL CELLS

31. Astrocytes • star-shaped • largest & most numerous of glial cells • functions: • physically support neurons • assist in blood-brain-barrier (bbb) • in embryo: regulate growth, migration, &interconnections between neurons • help maintain appropriate chemical environment for propagation of action potentials

32. Oligodendrocytes • “few trees” • smaller & fewer branches than astrocytes • Functions: • form & maintain myelin sheath on axons in CNS • 1 oligo. myelinates many axons

33. Microglia • small cells with slender processes giving off many spine-like projections • function: • phagocytes • remove cellular debris made during normal development • remove microbes & damaged nervous tissue

34. Ependymal Cells • single layer of cuboidal to columnar cells • ciliated & have microvilli • function: • line ventricles of brain & central canal of spinal cord • produce, monitor, & assist in circulation of cerebrospinal fluid (CSF) • form bbb

35. Neuroglial Cells of the PNS • Schwann cells • Satellite cells

36. Schwann Cells • functions: • myelinate axons in PNS • 1 Schwann cell myelinates 1 axon • participate in axon regeneration

37. Satellite Cells • flat cells that surround cell bodies of neurons in PNS ganglia • functions: • structural support • regulate exchange of materials between neuronal cell bodies & interstitial fluid

38. Myelination • myelin sheath: made up of multilayered lipid & protein (plasma membrane) covering • function: • electrically insulates axon • increases speed of nerve impulses

39. Myelinated & Unmyelinated Axons

40. Nodes of Ranvier • gaps in myelin sheath • 1 Schwann cell wraps axon between nodes of Ranvier

41. Myelin • amount increases from birth to maturity • infant‘s responses slower & less coordinated as older child or adult in part because myelination is a work in progress thru infancy

42. Demyelination • loss of myelin sheath • see in disorders: • multiple sclerosis • Tay-Sachs • side effect of radiation therapy & chemotherapy

43. Gray Matter of the Nervous System • contains: • neuronal cell bodies • dendrites • unmyelinated axons • axon terminals • neuroglia

44. White Matter of the Nervous System • composed of: • myelinated axons