Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

1. Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

2. Nervous System • Master controlling and communicating system of the body • Cells communicate by electrical signals and chemical signals • Rapid and specific • Usually cause an immediate response • Neurons = nerve cells

3. Functions • 3 overlapping functions 1. Sensory input – NS uses its millions of sensory receptors to monitor changes • Sensory input 2. Integration – NS process and interprets sensory input and decides what should be done – integration 3. Motor Output – response by activating effector organs – muscles and glands

4. Sensory input Integration Motor output Figure 11.1

5. Divisions • Central NS – brain and spinal cord • Dorsal body cavity • Integrating and command center of NS • Interprets sensory input and dictates motor responses based on reflexes – current and past experience

6. Divisions • Peripheral NS – • Part of NS outside the CNS • Consists of nerves – bundles of axons – extend brain and spinal cord • Spinal nerves – carry impulses to and from the brain • Cranial nerves – impulses to and from the brain

7. Divisions of PNS • 2 Functions – • Sensory, or afferent, division – “carrying towards” • Nerve fibers (axons) and convey impulses to CNS • Somatic afferent fibers – transmit impulses from skeletal muscle • Visceral afferent fibers – transmit impulses from visceral organs

8. Divisions of PNS • Motor, or efferent, division – “carrying away” • Impulses from CNS to effector organs • Activate muscles to contract and glands to secrete • Effect response • Somatic NS – somatic motor nerve fibers conduct impulses from CNS to skeletal muscle • voluntary NS • Automatic NS (ANS) – visceral motor nerve fibers • Regulate activity of smooth muscle, cardiac muscle and glands • “a law unto itself” • Cannot control pumping of heart or food through digestive tract • 2 functional subdivisions – sympathetic and parasympathetic NS – work in opposition to each other

9. Peripheral nervous system (PNS) Central nervous system (CNS) Cranial nerves and spinal nerves Brain and spinal cord Communication lines between the CNS and the rest of the body Integrative and control centers Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Autonomic nervous system (ANS) Somatic nervous system Skin Visceral motor (involuntary) Somatic motor (voluntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Conducts impulses from the CNS to skeletal muscles Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Bladder Parasympathetic motor fiber of ANS Motor (efferent) division of PNS Figure 11.2

11. Histology of Nervous Tissue • Highly cellular • Less than 20 % of CNS extracellular space • Densely packed and tightly intertwined • 2 principal cells • 1. Supporting cells – neuroglia • Small cells that surround and wrap neurons • 2. Neurons – excitable nerve cells that transmit signals

12. Neuroglia • “nerve glue” • Glial cells • 6 types – each own unique function • Supportive scaffold for neurons • Produce chemicals that guide young neuron’s growth • Wrap around and insulate neuronal process to speed up action potential conduction

13. Neuroglia in CNS • Astrocytes • Microglia • Ependymal cells • Oligoderocytes • Most have branching processes (extensions) and a central cell body • Distinguished by smaller size and darker staining nucleus

14. CNS Neuroglia - Astrocytes • “star cells” • Most abundant and most versatile • Radiating processes cling to neurons and synaptic endings • Cover nearby capillaries • Support and branch neurons • Anchor them to nutrient supply • Role in making exchanges – capillaries – neurons • “mopping up” leaked K ions and recapturing released neurotransmitters • Connected by gap junctions • Signal each other with Ca

15. Capillary Neuron Astrocyte (a) Astrocytes are the most abundantCNS neuroglia. Figure 11.3a

16. CNS Neuroglia - Microglia • Small oviod cell with long thorny processes • Processes touch neurons – monitor health • When neurons injuries or in trouble – migrate towards them • Transform into macrophages – phagocytize foreign debris • Protective role

17. Neuron Microglial cell (b) Microglial cells are defensive cells inthe CNS. Figure 11.3b

18. CNS Neuroglia – Epedymal Cells • “wrapping garment” • Shape – squamous columnar • Many ciliated • Line central cavities of brain and spinal cord • Permeable barrier between cerebral spinal fluid and tissue fluid of CNS • Cilia – circulated fluid

19. Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinalfluid-filled cavities. Figure 11.3c

20. CNS Neuroglia – Oligodendrocytes • Fewer processes • Line up along thicker neuron fibers and wrap processes around them • Covering sheaths – myelin sheaths

21. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers. Figure 11.3d

22. Neuroglia in PNS 1. Satellite Cells –surround neuron cell bodies located in PNS • Thought to have same functions as astrocytes 2. Schwann Cells – surrond and form myelin sheaths in PNS • Function similar to oligodendrocytes • Vital to regeneration of damaged nerves

23. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers. Figure 11.3d

24. Neurons • Nerve cell • Structural unit of NS • Highly specialized cells • Conduct messages – nerve impulses • Large complex cells • Cell body and processes • Plasma membrane – electrical signaling • Cell-cell interactions

25. Neurons • Special characteristics – 1. extreme longevity – can function optimally over a lifetime ~100 years 2. amitotic – loose ability to divide • Cannot be replaced if destroyed • Exceptions – olfactory epithelium and hippocampel regions – stem cells • Cannot survive for more than a few minutes without oxygen 3. High metabolic rate – require continuous and abundant oxygen and glucose

26. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Impulse direction Nucleus Node of Ranvier Nissl bodies Axon terminals (secretory region) Axon hillock Schwann cell (one inter- node) Neurilemma (b) Terminal branches Figure 11.4b

27. Neuron – Cell Body • Spherical nucleus surrounded by cytoplasm • Perikaryon or soma – cell body • Ranges in diameter from 5 to 140 um • Major biosynthetic center of the neuron • Usual organelles • Clustered and free ribosomes and rough ER – most active and developed in the body • Rough ER – Nissl bodies – chromatophilic substance • Golgi – well developed and form arci or complete circle around nucleus • Mitochondria - scattered

28. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Impulse direction Nucleus Node of Ranvier Nissl bodies Axon terminals (secretory region) Axon hillock Schwann cell (one inter- node) Neurilemma (b) Terminal branches Figure 11.4b

29. Neuron – Cell Body • Neurofibrils – bundles of intermediate filaments • Maintain cell shape and integrity • Pigment inclusions – black melanin, red iron containing pigment, gold brown pigment • Lipofuscin – ageing pigment – accumulates in neurons of elderly • Most cell bodies in CNS are protected by bones of skull and vertebral column • Clusters of cell bodies in CNS – nuclei • Clusters of cell bodies in PNS - ganglia

30. Neurons - Processes • Arm like • Extend from cell bodies • Bundles of processes – • Tracts in CNS • Nerves in PNS

31. Neurons - Processes • 2 types – 1. dendrites - short , tapering diffusely branching extensions • Main receptive or input regions • SA for receiving signals • Convey incoming messages toward cell body • Usually not AP but short distance signals called graded potentials

32. Neurons - Processes 2. Axon – single • Arises from axon hillock then narrows to form slender processes • Some short or absent • others – long – up to 3 to 4 ft • Long axon = nerve fiber • Axon branches – axon collaterals • Branches profusely at its end • 10000 or more terminal branches – telodendria • Knob like distal ends – axon terminals, synaptic knobs, boutons

33. Neurons - Processes • Axon Cont • Axon – conducting region • Generates nerve impulses and transmits them away from cell body along plasma membrane – axolemma • Nerve impulses from axon hillock to axon to axon terminal – secretory region • Depends on • 1. cell body to renew necessary proteins and membrane components • 2. efficient transport mechanisms to distribute

34. Neurons - Processes • Axon cont • Anterograde movement – movement toward an axon terminal • Retrograde movement – movement in the opposite direction • Viruses and bacteria toxins – damage neural tissues – use retrograde axonal transport to reach cell body – polio, rabies, herpes, tetanus

35. Neurons – Myelin Sheath and Neurilemma • Myelin Sheath – whitish, fatty (protein-lipid) • Protects and insulates fibers • Increases the speed of transmission of nerve impulses • Myelintaed fibers – conduct fast • Unmyelinated fibers – slower • Dendrites always unmyelinated • Formed by Schwann cells – indent to receive an axon, then wrap around them

36. Schwann cell plasma membrane A Schwann cell envelopes an axon. 1 Schwann cell cytoplasm Axon Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Neurilemma 3 Myelin sheath (a) Myelination of a nervefiber (axon) Figure 11.5a

37. Neurons – Myelin Sheath and Neurilemma • Neurilemma – exposed part of plasma membrane • Gaps in sheaths – nodes of Ranvier – myelin sheath gaps occur at regular intervals • ~1 mm apart along axon • Regions of brain and spinal cord – • White matter – dense collections of myelinated fibers • Gray matter – nerve cell bodies and unmyelinated fibers

38. Classification of Neurons • Structural – grouped according to number of processes extending from cell body • 3 major groups – multipolar, bipolar, and unipolar

39. Structural Classification 1. Multipolar– 3 or more processes • 1 axon and the rest dendrites • Most common • 99 % of neurons • Major type in CNS

40. Structural Classification 2. Bipolar Neurons – 2 processes • Axon and dendrite • Extend from opposite sides of the cell • Rare – found in special sense organs • Neurons in retina of eye – olfactory mucosa

41. Structural Classification 3. Unipolar Neurons – single short process • Emerges and divides – T-like proximal and distal • Distal – peripheral process • Proximal – central process • Pseudounipolar neurons – originate as bipolar – fuse during development, chiefly in ganglia of PNS

42. Table 11.1 (1 of 3)

43. Table 11.1 (2 of 3)

44. Functional Classification • Groups neurons according to direction in which nerve impulse travels relative to CNS 1. Sensory, or afferent, neurons – transmit impulses from sensory receptors in skin or internal organs toward the CNS • Almost always unipolar • Cell bodies – sensory ganglia outside CNS • Distal parts - receptor sites • Peripheral process – very long • Big tow – 1 meter till spinal cord • Receptive endings are naked

45. Functional Classification 2. Motor, or efferent, neurons – carry impulses away from CNS to effector organs (muscles/glands) • Multipolar • Cell bodies located in CNS, except for some in ANS

46. Functional Classification 3. Interneurons, association neurons – • In between • In neural pathways • Shuttle signals through CNS where integration occurs • Most confined in CNS • 99 % of neurons in body • Multipolar • Diversity in size and fiber branching patterns

47. Table 11.1 (3 of 3)

48. Membrane Potentials • Neurons – highly irritable or excitable response to stimuli • Stimulation  impulse generated and conducted along length of axon • Action Potential – nerve impulse always the same regardless of source or type of stimulus

49. Membrane Potential – Basic Principals • Human body – electrically neutral same number of + and – • Areas where 1 type of charge predominates – regions that are + or – • Opposite charges attract – energy must be used to separate them • Coming together – liberates energy

50. Membrane Potential – Basic Principals • Voltage – measure of potential energy – volts or mV • Measured between 2 points • Called potential difference or simply potential between 2 points • Greater difference in charge – higher voltage