Fundamentals of the Nervous System and Nervous Tissue

1. Fundamentals of theNervous System andNervous Tissue PART 1

2. Nervous System • Master control and communication system

3. Nervous System: Functions • Three overlapping functions • Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input • CNS Processes and interprets sensory input • Makes decisions – integration • Dictates a response by activating effector organs • Response – motor output

4. Basic Divisions of the Nervous System: CNS • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center

5. Basic Divisions of the Nervous System: PNS • Peripheral nervous system (PNS) • Outside the CNS • Nerves extending from brain and spinal cord • Cranial nerves • Spinal nerves • Link all regions of the body to the CNS

6. Sensory Input and Motor Output • Sensory signals picked up by sensory receptors • Carried by afferent nerve fibers of PNS to the CNS • Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors • Innervate muscles and glands

7. Sensory Input and Motor Output • Divided according to region they serve • Somatic body region • Visceral body region • Results in four main subdivisions • Somatic sensory • Visceral sensory • Somatic motor • Visceral motor

8. Somatic Sensory • Somatic sensory • General somatic senses – receptors are widely spread • Touch, pain, vibration, pressure, and temperature • Proprioceptive senses – detect stretch in tendons and muscle • Body sense – position and movement of body in space • Special somatic senses • Hearing, balance, vision, and smell

9. Visceral Sensory • Visceral sensory • General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, reproductive organs • Special visceral senses – taste

10. Somatic Motor • Somatic motor • General somatic motor – signals contraction of skeletal muscles • Under voluntary control • Often called “voluntary nervous system”

11. Visceral Motor • Visceral motor • Regulates the contraction of smooth and cardiac muscle and gland secretion • Makes up autonomic nervous system • Controls function of visceral organs • Often called “involuntary nervous system”

12. Peripheral Nervous System Summary Figure 12.3

13. Types of Sensory and Motor Information Figure 12.3

14. Types of Sensory and Motor Information Figure 12.3

15. Nervous Tissue • Cells are densely packed and intertwined • Two main cell types • Neurons – transmit electrical signals • Support cells (neuroglial cells) – nonexcitable • Surround and wrap neurons

16. The Neuron • The human body contains billions of neurons • Basic structural unit of the nervous system • Specialized cells conduct electrical impulses along the plasma membrane • Graded potentials • Action potentials

17. The Neuron: Special Characteristics • Longevity – can live and function for a lifetime • Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception • High metabolic rate – require abundant oxygen and glucose

18. Neuron Structure

19. The Cell Body or Soma (also called Perikaryon) • Size varies from 5–140µm • Contains nucleus, organelles plus other structures • Chromatophilic bodies (Nissl bodies) • Clusters of rough ER and free ribosomes • Stain darkly and renew membranes of the cell • Neurofibrils – bundles of intermediate filaments • Form a network between chromatophilic bodies

20. Nissl Body Staining

21. The Cell Body • Most neuronal cell bodies • Located within the CNS (clustered in nuclei) • Protected by bones of the skull and vertebral column • Ganglia – clusters of cell bodies in PNS

22. Cell Body Structure Figure 12.4

23. Neuron Processes: Dendrites • Dendrites • Extensively branching from the cell body • Transmit electrical signals (graded potentials) toward the cell body • Chromatophilic bodies – only extend into the basal part of dendrites • Function as receptive sites

24. Dendritic Spines

25. Dendritic Spines

26. Neuron Processes: Axons • Axons (nerve fibers) • Neuron has only one, but it can branch • Impulse generator and conductor • Transmits action potentials away from the cell body • Chromatophilic bodies absent • No protein synthesis in axon

27. Neuron Processes: Axons • Axons • Neurofilaments, actin microfilaments, and microtubules • Provide strength along length of axon • Aid in the transport of substances to and from the cell body • Axonal transport

28. Neuron Processes • Axons • Branches along length are infrequent • Axon collaterals • Multiple branches at end of axon • Terminal branches (telodendria) • End in knobs called axon terminals (also called end bulbs or boutons) Neuron Structure

29. Neuron Processes: Action Potentials • Nerve impulse (action potential) • Generated at the initial segment of the axon • Conducted along the axon • Releases neurotransmitters at axon terminals • Neurotransmitters – excite or inhibit neurons • Neuron receives and sends signals

30. Synapses • Site at which neurons communicate • Signals pass across synapse in one direction • Presynaptic neuron • Conducts signal toward a synapse • Postsynaptic neuron • Transmits electrical activity away from a synapse

31. Two Neurons Communicating at a Synapse Figure 12.6

32. Types of Synapses • Axodendritic • Between axon terminals of one neuron and dendrites of another • Most common type of synapse • Axosomatic • Between axons and neuronal cell bodies • Axoaxonic, dendrodendritic, and dendrosomatic • Less common types of synapses • Function not as well understood

33. Types of Synapses Figure 12.7

34. Synapses • Axodendritic synapses – representative type • Synaptic vesicles on presynaptic side • Membrane-bound sacs containing neurotransmitters • Mitochondria abundant in axon terminals • Synaptic cleft separates the plasma membrane of the two neurons

35. Structure of a Synapses PLAY Synapse Figure 12.8a, b

36. Synapse

37. Signals Carried by Neurons: Resting Membrane Potential • Plasma membranes of neurons conduct electrical signals • Resting neuron – membrane is polarized • Inner, cytoplasmic side is negatively charged

38. Changes in Membrane Potential • Signals occur as changes in membrane potential

39. Directional Signals • Stimulation of the neuron  depolarization • Inhibition of the neuron  hyperpolarization

40. Action Potentials Figure 12.9a, b

41. Action Potentials on Axons • Strong depolarizing stimulus applied to the axon hillock triggers • Action potential • Membrane becomes positive internally • Action potential travels the length of the axon • Membrane repolarizes itself

42. Action Potentials on Axons Figure 12.9c–e

43. Graded Potentials on Dendrites and the Cell Body • Natural stimuli applied to dendrites and the cell body • Receptive zone of the neuron • Membrane stimulation causes local depolarization • A graded potential – inner surface becomes less negative • Depolarization spreads from receptive zone to the axon hillock • Acts as the trigger that initiates an action potential in the axon

44. Synaptic Potentials • Excitatory synapses • Neurotransmitters alter the permeability of the postsynaptic membrane • Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane • Drives the postsynaptic neuron toward impulse generation

45. Synaptic Potentials • Inhibitory synapses • The external surface of the postsynaptic membrane becomes more positive • Reduces the ability of the postsynaptic neuron to generate an action potential

46. Classification of Neurons • Structural Classification • Functional Classification

47. Structural Classification of Neurons Classification based on number of processes • Multipolar • Bipolar • Unipolar (pseudounipolar)

48. Multipolar Neurons • Possess more than two processes • Numerous dendrites and one axon Figure 12.10a–c

49. Bipolar Neurons • Possess two processes • Rare neurons – found in some special sensory organs Figure 12.10a–c

50. Unipolar (Pseudounipolar) Neurons • Possess one single process • Start as bipolar neurons during development Figure 12.10a–c