Fundamentals of the Nervous System and Nervous Tissue

1. Fundamentals of the Nervous System and Nervous Tissue Chapter 12

2. Introduction • The nervous system is the master controlling and communicating system of the body • It is responsible for all behavior • Along with the endocrine system it is responsible for regulating and maintaining body homeostasis • Cells of the nervous system communicate by means of electrical signals

3. Nervous System Functions • The nervous system has three overlapping functions • Gathering of sensory input • Integration or interpretation of sensory input • Causation of a response or motor output

4. Introduction • Sensory input • The nervous system has millions of sensory receptors to monitor both internal and external change • Integration • It processes and interprets the sensory input and makes decisions about what should be done at each moment • Motor output • Causes a response by activating effector organs (muscles and glands)

5. Organization of the Nervous System

6. Organization • There is only one nervous system; however, for convenience the nervous system is divided into two parts • The central nervous system • Brain and spinal cord • Integrative and control centers • The peripheral nervous system • Spinal and cranial nerves • Communication lines between the CNS and the rest of the body

7. Organization • Basic divisions of the nervous system • Central Nervous Systems • Peripheral Nervous System

8. Organization • The peripheral nervous system has two fundamental subdivisions • Sensory (afferent) division • Somatic and visceral sensory nerve fibers • Consists of nerve fibers carrying impulses to the central nervous system • Motor (efferent) division • Motor nerve fibers • Conducts impulses from the CNS to effectors • (glands and muscles)

9. Organization

10. Organization • The motor division of the peripheral nervous system has two main subdivisions • The somatic nervous system • Voluntary (somatic motor) • Conducts impulses from the CNS to skeletal muscle • Branchial motor • Motor innervation of pharyngeal arch muscles • The autonomic nervous system (ANS) • Involuntary • Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

11. Organization

12. Innervation of Visceral Organs

13. Organization • The autonomic nervous system has two principle subdivisions • Sympathetic division • Mobilizes body systems during emergency situations • Parasympathetic division • Conserves energy • Promotes non-emergency functions • The two subdivisions bring about opposite effects on the same visceral organs • What one subdivision stimulates, the other inhibits

14. Peripheral Nervous System • Visceral organs are served by motor fibers of the autonomic nervous system and by visceral sensory fibers • The somata (limbs and body wall) are served by motor fibers of the somatic nervous system and by sensory somatic sensory fibers • Arrows indicate the direction of impulses

15. Histology of the Nervous Tissue • Nervous tissue is highly cellular • Less that 20% of the CNS is extracellular space • Cells are densely packed and tightly intertwined • Nervous tissue is made up of two cell types • Neurons • Excitable cells that transmit electrical signals • Support cells • Smaller cells that surround and wrap the delicate neurons • These same cells are found within CNS and PNS

16. Supporting Cells • All neurons associate closely with non-nervous support cells of which there are 6 types • Support cells of the CNS • Astrocytes • Microglial • Ependymal • Oligodendrocyte • Support cells of the PNS • Schwann cells • Satellite cells

17. Supporting Cells • While each support cell has a unique specific function, in general these cells provide a supportive scaffolding for neurons • In addition, they all cover nonsynaptic parts of the neurons thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other

18. Clinical Insight • The importance of support cells insulating nerve fibers is illustrated in the disorder call tic douloureux (doo loo-roo) • In this condition the support cells around the sensory nerve fibers of the trigeminal nerve degenerate and are lost • Impulses that carry touch sensations proceed to influence and stimulate the uninsulated pain fibers in the same nerve

19. Supporting Cells in the CNS • The supporting cells of the CNS are collectively called neuroglia or simply, glial cells • Neuroglia usually refer to the CNS but some authors include the PNS

20. Supporting Cells in the CNS • Like neurons, glial cells have branching processes and a central cell body • Neuroglia can be distinguished from neurons by their much smaller size and darker staining nuclei • They outnumber neurons in the CNS by a ratio of 10 to 1 • Make up half of the mass of the brain

21. Astrocytes • Star shaped • Most abundant type of glial cell • Radiating projections cling to neurons and capillaries, bracing the neurons to their blood supply • Astrocytes play a role in exchanges of ions between capillaries and neurons

22. Astrocytes • Astrocytes take up and release ions to control the environment around neurons • Concentrations of ions must be kept within narrow limits for nerve impulses to be generated & conducted • Astrocytes recapture and recycle potassium ions and released neuro- transmitters

23. Astrocytes • Astrocytes contact both the neuron and the capillary in order to sense when the neuron are highly active and releasing large amounts of neurotransmitters (glutamate) • Astrocytes then extract blood sugar from the capillaries they contact to obtain the energy they need to fuel the process of glutamate uptake

24. Microglial • Smallest and least abundant type of neuroglial cell • The ovid cells have relatively long “thorny” processes • Their branches touch nearby neurons to monitor health of the neuron

25. Microglial • These are small ovid cells with relatively long “thorny” processes • Microglial derive from blood cells and migrate to the CNS during embryonic and fetal development

26. Microglial • These cells are phagocytes, the marcophages of the CNS • Microglial move to and then engulf microorganisms and injured or dead neurons

27. Microglial • When invading micro- organisms are present or damaged neurons have died, the micro- glial transforms into a special type of macro- phage that protects the CNS by phagocytizing the microorganisms or neuronal debris • Important because cells of the immune system can enter CNS

28. Ependymal • Range in shape from squamous to columnar and many are cilated • Line the central cavities of the brain and spinal cord • Form a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS • Beating cilia circulates cerebrospinal fluid

29. Oligodendro- cytes • Fewer branches than astrocytes • Cells wrap their cytoplasmic extensions tightly around the thicker neurons in the CNS • Produce insulating coverings called myelin sheaths

30. Supporting Cells of the PNS • There are two supporting cells in the PNS • Satellite cells • Schwann cells • These cells are similar in type and differ mainly in location

31. Satellite Cells • Somewhat flattened satellite cells surround cell bodies within ganglia • Thought to play some role in controlling the chemical environment of neurons with which they are associated, but function is largely unknown

32. Schwann Cells • Surround and form myelin sheaths around the larger nerve fibers in PNS • Similar to the oligodendrocytes of CNS • Schwann cells are vital to peripheral nerve fiber regeneration

33. Neurons • Neurons are the structural units of the nervous system • Neurons are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another

34. Neuron Characteristics • Extreme longevity • Live and function optimally for a lifetime • Amitotic • As neurons assume their role in the nervous system they lose their ability to divide • Neurons cannot be replaced if destroyed • High metabolic rate • Require continuous and abundant supplies of oxygen and glucose • Homeostatic deviations often first appear in nervous tissue which has specific needs

35. Neurons • The plasma membrane of neurons is the site of electrical signaling, and it plays a crucial role in most cell to cell interaction • Most neurons have three functional components in common • A receptive component • A conducting component • A secretion or output component • Each component is associated with a particular region of a neuron’s anatomy

36. Neuron structure • Typically large, complex cells, they all have the following structures • Cell body • Nuclei • Chromatophilic (Nissl) bodies • Neurofibrils • Axon hillock • Cell processes • Dendrites • Axon • Myelin sheath or neurilemma

37. Neuron structure • Cell Body • Nuclei • Chromatophilic (Nissl) bodies • Neurofibrils • Axon hillock • Neuron Processes • Dendrites • Axons • Myelin sheaths • Axon terminals

38. Neuron structure • The cell body consists of a large, spherical nucleus with a prominent nucleolus surrounded by cytoplasm • The cell ranges from 5 to 140m in diameter • The cell body is the biosynthetic center of the neuron

39. Neuron structure • The cell body contains the usual organelles with the exception of centrioles (not needed in amitotic cells) • The rough endoplasmic reticulum or Nissl bodies is the protein and membrane making machinery of the cell • The cell body is the focal point for neuron growth in development

40. Neuron structure • Neurofibrils are bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic bodies • Neurofibrils keep the cell from being pulled apart when it is subjected to tensile stresses

41. Neuron structure • In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons

42. Neuron Cell Bodies • Most neuron cell bodies are located with the CNS where they are protected by the bones of the skull and vertebral column • Clusters of cell bodies in the CNS are called nuclei • The relatively rare collection of cell bodies in the PNS are called ganglia

43. Neuron Processes Motor neuron • Cytoplasmic extension called processes extend from the cell body of all neurons • The CNS contain both neuron cell bodies and their processes • The PNS consists chiefly of processes

45. Neuron Processes Motor neuron • Bundles of neuron processes in the CNS are called tracts • Bundles of neuron processes in the PNS are called nerves

46. Dendrites • Dendrites are short, tapering diffusely branching extensions • Motor neurons have hundreds of dendrites clustering close to the cell body • Dendrites are receptive cites and provide an enormous surface area for the reception of signals • In many areas of the brain the finer dendrites are highly specialized for information collection

47. Dendrites • Dendritic spines represent areas of close contact with other neurons • Dendrites convey information toward the cell body • These electrical signals are not nerve impulses but are short distance signals call graded potentials

48. Axons • Each neuron has a single axon • The axon arises from the cone shaped axon hillock • It narrows to form a slender process that stays uniform in diameter the rest of its length • Length varies; short or absent to 3 feet in length

49. Axons • Each axon is called a nerve fiber • Axons are impulse generators and conductors that transmit nerve impulses away from the cell body

50. Axons • Chromatophilic bodies and the Golgi apparatus are absent from the axon and the axon hillock • The axons also lack ribosomes and all organelles involved in protein synthesis so they must receive their proteins from the cell body