Fundamentals of the Nervous System and Nervous Tissue

1. PART 1 Fundamentals of theNervous System andNervous Tissue

2. Nervous System • Master control and communication system • Has three overlapping functions • Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input

3. Nervous System • 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 • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center

5. Basic Divisions of the Nervous System • Peripheral nervous system (PNS) • Outside the CNS • Consists of nerves extending from brain and spinal cord • Cranial nerves • Spinal nerves • Peripheral nerves link all regions of the body to the CNS • Ganglia are clusters of neuronal cell bodies

6. Basic Divisions of the Nervous System Figure 12.2

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

8. 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

9. Types of Sensory and Motor Information Figure 12.3

10. Basic Divisions of the Nervous System • Somatic sensory • General somatic senses – receptors are widely spread • Touch • Pain • Vibration • Pressure • Temperature (receptors discussed in Chapter 14)

11. Basic Divisions of the Nervous System • Somatic sensory (continued) • Proprioceptive senses – detect stretch in tendons and muscle • Body sense – position and movement of body in space • Special somatic senses (Chapter 16) • Hearing • Balance • Vision • Smell

12. Basic Divisions of the Nervous System • Visceral sensory • General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, and reproductive organs • Special visceral senses • Taste

13. Basic Divisions of the Nervous System • Somatic motor • General somatic motor – signals contraction of skeletal muscles • Under our voluntary control • Often called “voluntary nervous system” • Branchial motor • Typical skeletal muscle derived from somitomeres

14. Basic Divisions of the Nervous System • Visceral motor • Regulates the contraction of smooth and cardiac muscle • Makes up autonomic nervous system • Controls function of visceral organs • Often called “involuntary nervous system” • Autonomic nervous system (Chapter 15)

15. Nervous Tissue • Cells are densely packed and intertwined • Two main cell types • Neurons – transmit electrical signals • Support cells (neuroglial cells in CNS) • 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 • Nerve impulse (action potential)

17. The Neuron • Other 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 • Neurons die after 5 minutes without oxygen

18. The Cell Body • Cell body (soma) • Perikaryon – around nucleus • Size of cell body varies from 5–140µm • Contains usual organelles plus other structures • Chromatophilic bodies (Nissl bodies) • Clusters of rough ER and free ribosomes • Stain darkly and renew membranes of the cell

19. The Cell Body • Neurofibrils – bundles of intermediate filaments • Form a network between chromatophilic bodies

20. The Cell Body • Most neuronal cell bodies • Located within the CNS • Protected by bones of the skull and vertebral column • Ganglia – clusters of cell bodies • Lie along nerves in the PNS

21. Structure of a Typical Large Neuron Figure 12.4

22. Neuron Processes • Dendrites • Extensively branching from the cell body • Transmit electrical signals toward the cell body • Chromatophilic bodies – only extend into the basal part of dendrites and to the base of the axon hillock • Function as receptive sites for receiving signals from other neurons

23. Neuron Processes • Axons • Neuron has only one • Impulse generator and conductor • Transmits impulses away from the cell body • Chromatophilic bodies are absent • No protein synthesis in axon

24. Neuron Processes • Axons (continued) • 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

25. 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)

26. Neuron Processes • Nerve impulse • 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

27. 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 PLAY Synapse

28. Two Neurons Communicating at a Synapse Figure 12.6

29. 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 • Uncommon types of synapses

30. Some Important Types of Synapses Figure 12.7

31. Synapses • Elaborate cell junctions • 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

32. Structure of a Synapses Figure 12.8a, b

33. Signals Carried by Neurons • Plasma membranes of neurons conduct electrical signals • Resting neuron – membrane is polarized • Inner, cytoplasmic side is negatively charged • Stimulation of the neuron  depolarization

34. Signals Carried by Neurons Figure 12.9a, b

35. Signals Carried by Neurons • Strong stimulus applied to the axon triggers • Nerve impulse/action potential • Membrane becomes negative externally • Impulse travels the length of the axon • Membrane repolarizes itself

36. Signals Carried by Neurons Figure 12.9c–d

37. 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

38. 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