The Nervous System

1. The Nervous System Chapter 7

2. What is the nervous system? • Master controller and communicator • Electrical impulses – rapid, specific, immediate • 3 Functions • Sensory input • Integration • Motor output Figure 7.1

3. Organization • Structural vs. Functional Classification • Central – integrating and command center • Peripheral – communication lines, carry signals from sensory receptors to CNS and from CNS to motor receptors Figure 7.2

4. Functional Organization • Only the PNS – 2 divisions • 1) Sensory (afferent) – nerves convey signals TO CNS • Somatic fibers – sensory fibers from skin, skeletal muscles, and joints • Visceral sensory fibers – visceral organs • 2) Motor (efferent) – nerves carry impulses from CNS to effector organs, muscles, glands • 2 divisions – somatic and autonomic

5. Somatic Nervous System • Conscious control of skeletal system – voluntary • Some reflexes are not voluntarily controlled – EXAMPLES?

6. Autonomic Nervous System • Regulates involuntary activity – cardiac and smooth muscle • 2 divisions • Sympathetic – stimulate • Parasympathetic - inhibits

7. Structure and Function of Nervous Tissue • Nervous tissue = neurons and supporting cells • All supporting cells called neuroglia • All support, insulate, and protect but have individual roles • Called glia or glial cells • DO NOT TRANSMIT IMPULSES • Never loose ability to divide unlike neurons – tumors?

8. Astrocytes - CNS • Supporting glial cell • Star shaped • ~ ½ nervous tissue • Swollen ends • Anchor and brace • Barrier between capillary and neuron – protection • Control chemical environment of brain – mop up ions and neurotransmitters Figure 7.3a

9. Microglia and Ependymal cells - CNS • Microglia - Spider like phagocytes • Ependymal – line central cavities of brain and spinal cord, beating cilia circulates cerebrospinal fluid cushions CNS Figure 7.3b Figure 7.3c

10. Oligodendrocytes - CNS • Wrap extensions around nerve fibers producing fatty myelin sheaths Figure 7.3d

11. Schwann and Satellite Cells - PNS • Schwann cells – produce myelin sheaths around nerve cells of PNS • Satellite Cells – protective, cushioning cells Figure 7.3e

12. Neurons – Common Features • Cell Body • Metabolic center • Contains nucleolus • Cytoplasm with normal organelles EXCEPT centrioles • RER (Nissl substance) and Neurofibrils (intermediate filaments) Figure 7.4

13. Neurons – Common Features • Processes • Vary in length – microscopic to multiple feet • Dendrites – Convey incoming signals toward cell body, may have 100’s • Axons – Convey signal away from cell body, has only 1 • Some may have collateral branch • All axons form 100-1000’s axon terminals • Neurotransmitters • Synaptic Cleft

14. Neurons – Common Features • Myelin Sheath – covers most neurons • White, fatty material • Protects, insulates, increases transmission rate • Which cells in the PNS? • Gaps between cells = nodes of Ranvier

15. Differences in Myelin Sheath CNS and PNS • PNS – Schwann Cells, wrap around like jelly-roll • Initially wrapped loose, but gets tighter, all cytoplasm on outside, called neurilemma • Neurilemma remains when nerve damaged, plays role in nerve repair • CNS – oligodendrocytes, lacks neurilemma, can wrap around 80+ neurons, nerve regeneration lacking in CNS Figure 7.5

16. Terminology • Nuclei – cluster of cell bodies in CNS, remember metabolic center, little or no division after birth, if cell body damaged, nerve dies, protected by skull and spine • Ganglia – cluster of cell bodies in PNS • Tracts – bundles of nerve fibers running through CNS • Nerves – bundle of nerve fibers running though PNS • White Matter – dense collection of myelinated fibers • Gray Matter – mostly unmyelinated fibers and cell bodies

17. Functional Classification of Neurons • Grouped based on direction impulse is traveling relative to CNS • Sensory (afferent) • Motor (efferent) • Association (Interneuron) Figure 7.6

18. Sensory Neurons (Afferent) • Cell bodies always found in ganglion outside CNS • Associated with special receptors – special senses and more simple receptors • Cutaneous sense organs • Meissner’s corpuscle, Pacinian • Proprioceptors • Pain receptors

19. Motor Neurons (Efferent) • Cell bodies located in CNS • Carry signals from CNS to viscera, muscle, and/or gland

20. Inerneurons • Connect motor and sensory neurons • Cell body always in CNS • LOOK at where dendrites and axons are for all nerves!

21. Structural Classification of Neurons • Based on number of processes • Multipolar – several, all motor and association neurons, most common • Bipolar – one axon, one dendrite, rare in adults, some special senses eye, nose • Unipolar – single process from cell body, divides into central and peripheral process. Unique only dendrites on peripheral process, axon conducts signal both toward and away from cell body Figure 7.8

22. Nerve Impulses – Action Potentials • Plasma membrane of resting neuron polarized • More negative inside than outside • More K+ inside, more Na+ outside • If inside remains more negative than outside cells remains inactive Figure 7.9

23. Action Potential Initiation and Generation • Many different types of stimuli light, pressure, sound but most stimulated by neurotransmitters of other cells • Stimulus changes permeability of cell • Stimulation opens Na+ channels, diffusion from out to in • Depolarization at site of influx

24. Stimulation Strong Enough Action Potential Started • If stimulus enough and Na+ enough, the local depolarization activates neuron to send long distance signal = action potential or nerve impulse • All or none

25. Repolarization • Rush of Na+ in changes membrane permeability again, Na+ no longer allowed in, K+ can flow out • Restores original gradient • Must repolarize to conduct another impulse • Na+/K+ pump restores concentrations of ions, uses ATP

26. Myelinated vs. Unmyelinated • Events just describe for unmyelinated nerves • Myelinated nerves transmit signal much faster, signal jumps from node of Ranvier to node of Ranvier

27. We have been discussing irritability, what about conductivity? • How does one cell “talk” to another? • Neurotransmitters • Action potential reaches axon terminals of one cell, triggers release of neurotransmitters into synaptic cleft • Bind receptors on next cell and off we go • Nerve Impulse in Electrochemical event • Electrical where? • Chemical where?