The Nervous System

1. The Nervous System

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 • Nerve = nerve fibers/CT exits CNS thru foramina of skull and vertebral column 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 • Autonomic – glands, cardiac, smooth muscle • Somatic – skeletal muscle

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

6. Motor (Efferent) Division Autonomic Nervous System • Visceral Motor Division • 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. Neurons • Communication cells of NS • About 1 trillion in your body • Unique cells of the NS • Three physiological functions • Excitability (irritability) • Conductivity • Secretion

9. Neurons – Common Features • Cell Body (cyton, soma, perikaryon) • Metabolic center • Contains nucleolus • Cytoplasm with normal organelles EXCEPT centrioles • Nissl Body (dense collection of ribosomes) and Neurofibrils - Cytoskeleton (intermediate filaments) • Axon Hillox – Functions in summation to be discussed later Figure 7.4

10. 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 • Axolemma - membrane • Axoplasm - cytoplasm • All axons form 100-1000’s axon terminals

11. Neurons – Common Features • Terminal button – axon terminals • Release of synaptic vesicles containing ACh or other neurotransmitters • Communication with other neurons, glands, muscle cell via neurotransmitter

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

13. Functional Classification of Neurons • Grouped based on direction impulse is traveling relative to CNS • Sensory (afferent) – tend to be unipolar • Motor (efferent) - multipolar • Association (Interneuron)-multipolar • Anaxonic –multiple dentrites, no axon. Do not produce action potentials – Why? • Brain and retina (contrast) Figure 7.6

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

15. Motor Neurons (Efferent) • Cell bodies located in CNS • Carry signals from CNS to viscera, muscle, and/or gland • Remember somatic and visceral?

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

17. 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 • Anaxonic – lack axon, no action potential, brain and retina Figure 7.8

18. Axonal Transport • All proteins needed by neuron made in soma – need to be transported throughout neuron. • Movement along axon = Axonal Transport • Microtubles of cytoskeleton – like railroad • Motor? Protein carries organelle or substance on back

19. Axonal Transport • Anterograde Transport – soma down axon • Motor = Kinesin • Fast = 20-400mm/day • Mitochondria, vesicles. Organelles, Ca++ ions • Retrograde Transport – up axon to soma • Motor = Dynein • Fast = 20-400mm/day • Synaptic vesicles, informs soma of conditions, tetanus toxin, herpes, rabies, polio • Slow Axonal Transport – axoplasmic flow • 0.5-10mm/day, always anterograde • Enzymes, cytoskeleton components, renews axoplasmic components, new axoplasm for developing or regenerating neurons • Regeneration governed by slow axonal transport speed

20. 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

21. Structure of Nerve • Endoneurium • Fascicle • Perineurium • Epinerium • Vasa nervorum – BV

22. 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

23. 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

24. Oligodendrocytes - CNS • Wrap extensions around nerve fibers producing fatty myelin sheaths • Presence defines white or grey matter Figure 7.3d

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

26. 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 • Dietary fat important in early development Figure 7.5

27. Regeneration of Nerve Fiber Damage to PNS? Can regenerate if soma and some neurolemma intact Severed end degraded by phagocytes Regeneration tube formed by neurolemma Sprouts from axon until one in regeneration tube, others reabsorbed Directs neuron to original site of communication

28. Unmyelinated Neurons • In PNS, Schwann cells still line neurouns • Axons fit in grooves of Schwann cells • Multiple neurons in single cell • Schwann cell very close = no nodes of Ranvier • Formation of neurolemma but no myelin sheath

29. Nerve Impulses – Action Potentials • Plasma membrane of resting neuron polarized (-70mV) • More negative inside than outside • More K+ inside, more Na+ outside • If inside remains more negative than outside cells remains inactive Figure 7.9

30. Resting Membrane Potential • Determined by 3 factors • Diffusion of ions down [gradient] • Selective permeability of membrane • Electrical attraction of cations and anions • K+ has greatest influence, can pass through membrane via channels • K+ diffuses out of cell leaving anions behind • ICF becomes more negative • Negativity of ICF attracts K+ cations back • Equilibrium between diffusion and electrical attraction • Na+ trickles • Slowly trickles into cell due to electrical attraction • Na+/K+ pump - 3Na+ out for every 2K+ in. Requires ATP

31. Local Potentials vs. Action Potential • Signals are received by dendrites – ligands, light, pressure • Ligand gated Na+ channels open – only 50-75 gates/μm2 Na+ rushes in, reduces negative charge inside = DEPOLARIZATION, • Na+ diffuse to trigger zone – Axon Hillock and Axon, 350-500 gates/μm2 • Differences • Local are graded = more intense open more channels • Decremental = get weaker as they move from site of stimulation • Reversible = Stimulation ceases, movement of K+ returns to resting potential • Excitatory or inhibitory = Neurtransmitter, ACh can depolarize others can hyperpolarize

32. 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 • If local potential reaches trigger zone, opens these gates • Depolarization at site of influx (Threshold -55 mV)

33. 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 • Voltage gated Na+ channels open at -55mV

34. Depolarization • Rush of Na+ drives potential pass 0 mV • Voltage gated channels close • Potential peaks at +35 mV • Must repolarize to conduct another impulse

35. Repolarization • K+ gates are fully open • Repelled by positive inside, flow out of cell • Returns potential to negative • Overshoots by 1-2 mV, hyperpolarization • Na+/K+ pump return K+ and Na+ to original side for next stimulation • Refractory period – only one direction

36. Action Potential vs. Local Potential • All or none • Not graded • Nondecremental • Irreversible

37. Myelinated vs. Unmyelinated • Events just describe for unmyelinated nerves • Voltage gated channels down entire axon • New action potential over and over again until reaches axon terminal • Signal 2m/sec • Myelinated nerves transmit signal much faster, signal jumps from node of Ranvier to node of Ranvier • <25 gates/μm2 internodes, 2,000-12,000 gates/μm2 at nodes of Ranvier • Myelin would block ion movement anyway • Signal is decremental, can’t travel more than 1mm • Yeah there is a node about every 1mm or less • Each node boost signal to original level • SALTATORY CONDUCTION • Signal 120m/sec

38. 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 • Axodendritic, axosomatic, axoaxonic • Nerve Impulse in Electrochemical event • Electrical where? • Chemical where?

39. Structure of Chemical Synapse • Presynaptic neuron synaptic Knob – synaptic vesicles with neurotransmitter ready for release • Postsynaptic neuron – ligand receptors

40. Neurotransmitters • Three categories • Acetycholine – acetic acid and choline • Monoamines – epinephrine, norepinephrine, dopamine, histamine, and serotonin • Catecholamines – first three, will see in endocrine • Neuropeptides – some function as hormones, can be made by digestive tract, can cause cravings for fat or sugar Can have different effects on different cells!!!

41. Excitatory Cholinergic Synapse • Acetylcholine – can be excitatory or inhibitory • Action potential reaches synaptic knob • Opens voltage gated Ca++ channels • Ca++ enters knob, triggers release of synaptic vesicles via exocytosis • Vesicles refilled, reserve pool • ACh diffuses across synaptic cleft, binds to ligand gated channels on postsynaptic neuron • Change in postsynaptic potential

42. Inhibitory GABA-ergic Synapse • Γ-aminobutyric acid GABA • Same process for release of vesicles • GABA binds to ligand channels on postsynaptic cell. • Receptors are chlorine channels • Lots of negative chlorine into cells = hyperpolarization!

43. Excitatory Adrenergic Synapse • Norepinephrine • Act through second messengers like cAMP • NE binds to receptor • Activates G protein • Activates Adenylate Cyclase • Makes cAMP • Can do lots of things including open ligand channels – postsynaptic potential

44. Cessation of Signal • Stop release of neurotransmitters • SNeurotransmitters diffuse from synapse into ECF • Astrocytes degrade • Reuptake by presynaptic cell