Chapter 10

1. Human Anatomy & Physiology I Overview of the Nervous System Chapter 10

2. Organization of the Nervous System • Central nervous system - CNS • Brain and Spinal Cord (in dorsal body cavity) • Integration and command center – interprets sensory input and responds to input • Peripheral nervous system - PNS • Paired Spinal and Cranial nerves • Carries messages to and from the spinal cord and brain – links parts of the body to the CNS

3. Divisions of the Nervous System • Central Nervous System • brain • spinal cord • Peripheral Nervous System • peripheral nerves • cranial nerves • spinal nerves

4. Nervous System Functions: Sensory Input – monitoring stimuli occurring inside and outside the body Integration – interpretation of sensory input Motor Output – response to stimuli by activating effector organs

5. Divisions Nervous System

6. Levels of Organization in the Nervous System

7. Divisions of Peripheral Nervous System • Sensory Division • picks up sensory information and delivers it to the CNS • Motor Division • carries information to muscles and glands • Divisions of the Motor Division • Somatic – carries information to skeletal muscle • Autonomic – carries information to smooth muscle, cardiac muscle, and glands

8. Functions of Nervous System • Sensory Function • sensory receptors gather information • information is carried to the CNS • Motor Function • decisions are acted upon • impulses are carried to effectors • Integrative Function • sensory information used to create • sensations • memory • thoughts • decisions

9. PNS - Two Functional Divisions • Sensory (afferent) Division • Somatic afferent nerves – carry impulses from skin, skeletal muscles, and joints to the CNS • Visceral afferent nerves – transmit impulses from visceral organs to the CNS • Motor (efferent) Division • Transmits impulses from the CNS to effector organs, muscles and glands, to effect (bring about) a motor response

10. Classification of Neurons • Sensory Neurons • afferent • carry impulse to CNS • most are unipolar • some are bipolar • Interneurons • link neurons • multipolar • in CNS • Motor Neurons • multipolar • carry impulses away from CNS • carry impulses to effectors

11. Motor Division: two subdivisions • Somatic Nervous System (voluntary) • Somatic motor nerve fibers (axons) that conduct impulses from CNS to Skeletal muscles – allows conscious control of skeletal muscles • Autonomic Nervous System (ANS) (involuntary) • Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands • Two functional divisions – sympathetic and parasympathetic

12. Levels of Organization in the Nervous System

13. Histology of Nerve Tissue Two principal cell types in the nervous system: Neurons – excitable nerve cells that transmit electrical signals Supporting cells – cells adjacent to neurons or cells that surround and wrap around neurons • Cell Types of Neural Tissue • neurons • neuroglial cells

14. Neurons (Nerve Cells) Highly specialized, structural units of the nervous system – conduct messages (nerve impulses) from one part of the body to another • Long life, mostly amitotic, with a high metabolic rate (cannot survive more than a few minutes without O2) Structure is variable, but all have a neuron cell body and one or more cell projections called processes.

15. Generalized Neuron

16. Neuron Structure

17. Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus The major biosynthetic center Has no centrioles Has well-developed Nissl bodies (rough ER) Axon hillock – cone-shaped area where axons arise Clusters of cell bodies are called Nuclei in the CNS and Ganglia in the PNS

18. Processes Extensions from the nerve cell body. The CNS contains both neuron cell bodies and their processes. The PNS consists mainly of neuron processes. Two types: Axons and Dendrites Bundles of neuron processes are called Tracts in the CNS and Nerves in the PNS

19. Dendrites Short, tapering, diffusely branched processes The main receptive, or input regions of the neuron (provide a large surface area for receiving signals from other neurons) Dendrites convey incoming messages toward the cell body These electrical signals are not nerve impulses (not action potentials), but are short distance signals called graded potentials

20. Axons Slender processes with a uniform diameter arising from the axon hillock, only one axon per neuron A long axon is called a nerve fiber, any branches are called axon collaterals Terminal branches – distal ends are called the axon terminus (also synaptic knob or bouton)

21. Axons: Function Generate and transmit action potentials (nerve impulses), typically away from the cell body As impulse reaches the axon terminals, it causes neurotransmitters to be released from the axon terminals Movement of substances along axons: Anterograde - toward axonal terminal (mitochondria, cytoskeletal, or membrane components) Retrograde - away from axonal terminal (organelles for recycling) Anterograde → ←Retrograde

22. Myelin Sheath • Whitish, fatty (protein-lipoid), segmented sheath around most long axons – dendrites are unmyelinated • Protects the axon • Electrically insulates fibers from one another • Increases the speed of nerve impulse transmission

23. Myelin Sheath Formed by Schwann cells in the PNS • A Schwann cell envelopes and encloses the axon with its plasma membrane. • The concentric layers of membrane wrapped around the axon are the myelin sheath • Neurilemma – cytoplasm and exposed membrane of a Schwann cell

24. Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge

25. Myelination of Axons • White Matter • contains myelinated axons • Gray Matter • contains unmyelinated structures • cell bodies, dendrites

26. Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are more widely spaced There is no neurilemma (cell extensions are coiled around axons) White matter – dense collections of myelinated fibers Gray matter – mostly soma and unmyelinated fibers

27. Classification of Neurons • Bipolar • two processes • eyes, ears, nose • Unipolar • one process • ganglia • Multipolar • many processes • most neurons of CNS

28. Classification of Neurons Structural Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process

29. Neuron Classification Functional Sensory (afferent) – transmit impulses toward the CNS Motor (efferent) – carry impulses away from the CNS Interneurons (association neurons) – lie between sensory and motor pathways and shuttle signals through CNS pathways

33. Supporting Cells: Neuroglia • Six types of Supporting Cells - neuroglia or glial cells – 4 in CNS and 2 in the PNS • Each has a specific function, but generally they: • Provide a supportive scaffold for neurons • Segregate and insulate neurons • Produce chemicals that guide young neurons to the proper connections • Promote health and growth

34. Types of Neuroglial Cells • Schwann Cells • peripheral nervous system • myelinating cell • Astrocytes • CNS • scar tissue • mop up excess ions, etc • induce synapse formation • connect neurons to blood vessels • Oligodendrocytes • CNS • myelinating cell • Ependyma • CNS • ciliated • line central canal of spinal cord • line ventricles of brain • Microglia • CNS • phagocytic cell

35. Supporting Cells: Neuroglia Neuroglia in the CNS Astrocytes Microglia Ependymal Cells Oligodendrocytes Neuroglia in the PNS Satellite Cells Schwann Cells Outnumber neurons in the CNS by 10 to 1, about ½ the brain’s mass.

36. Types of Neuroglial Cells

37. Astrocytes Most abundant, versatile, highly branched glial cells Cling to neurons, synaptic endings, and cover nearby capillaries Support and brace neurons Anchor neurons to nutrient supplies Guide migration of young neurons Aid in synapse formation Control the chemical environment (recapture K+ ions and neurotransmitters)

38. Microglia Microglia – small, ovoid cells with long spiny processes that contact nearby neurons When microorganisms or dead neurons are present, they can transform into phagocytic cells

39. Ependymal Cells Ependymal cells – range in shape from squamous to columnar, many are ciliated Line the central cavities of the brain and spinal column

40. Oligodendrocytes Oligodendrocytes – branched cells that line the thicker CNS nerve fibers and wrap around them, producing an insulating covering – the Myelin sheath

41. Schwann Cells and Satellite Cells Schwann cells - surround fibers of the PNS and form insulating myelin sheaths Satellite cells - surround neuron cell bodies within ganglia

42. Regeneration of A Nerve Axon

43. Neurophysiology • Neurons are highly irritable (responsive to stimuli) • Action potentials, or nerve impulses, are: • Electrical impulses conducted along the length of axons • Always the same regardless of stimulus • The underlying functional feature of the nervous system

44. Definitions Voltage (V) – measure of potential energy between two points generated by a charge separation (Voltage = Potential Difference = Potential) Current (I) – the flow of electrical charge Resistance (R) – tendency to oppose the current Units: V (volt), I (ampere), R (ohm) Insulator – substance with high electrical resistance Conductor – substance with low electrical resistance

45. Voltage (V) Current (I) = Resistance (R) Ohm’s Law The relationship between voltage, current, and resistance is defined by Ohm’s Law In the body, electrical current is the flow of ions (rather than free electrons) across membranes A Potential Difference exists when there is a difference in the numbers of + and – ions on either side of the membrane

46. Membrane Ion Channels Types of plasma membrane ion channels • Passive, or leakage, channels – always open • Chemically (or ligand)-gated channels – open with binding of a specific neurotransmitter (the ligand) • Voltage-gated channels – open and close in response to changes in the membrane potential • Mechanically-gated channels – open and close in response to physical deformation of receptors

47. Ligand-Gated Channel Example: Na+-K+ gated channel Closed when a neurotransmitter is not bound to the extracellular receptor Open when a neurotransmitter is attached to the receptor - Na+ enters the cell and K+ exits the cell

48. Voltage-Gated Channel • Example: Na+ channel • Closed when the intracellular environment is negative • Open when the intracellular environment is positive - Na+ can enter the cell

49. Electrochemical Gradient Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge Together, the electrical and chemical gradients constitute the ELECTROCHEMICAL GRADIENT

50. Voltage (V) Current (I) x Resistance (R) = Ion Channels When gated ion channels open, ions diffuse across the membrane following their electrochemical gradients. This movement of charge is an electrical current and can create voltage change across the membrane. Ion movement (flow) along electrochemical gradients underlies all the electrical phenomena in neurons.