Chapter 12 Nervous Tissue

1. Chapter 12Nervous Tissue • Controls and integrates all body activities within limits that maintain life • Three basic functions • sensing changes with sensory receptors • fullness of stomach or sun on your face • interpreting and remembering those changes • reacting to those changes with effectors • muscular contractions • glandular secretions

2. Major Structures of the Nervous System • Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors

3. Organization of the Nervous System • CNS is brain and spinal cord • PNS is everything else

4. Nervous System Divisions • Central nervous system (CNS) - consists of the brain and spinal cord • Peripheral nervous system (PNS) • consists of cranial and spinal nerves that contain both sensory and motor fibers • connects CNS to muscles, glands & all sensory receptors

5. Subdivisions of the PNS • Somatic (voluntary) nervous system (SNS) • sensory neurons from cutaneous, skeletal muscle & tendons, and special sensory receptors to the CNS • motor neurons to skeletal muscle tissue • Autonomic (involuntary) nervous systems (ANS) • sensory neurons from visceral organs to CNS • motor neurons to smooth & cardiacmuscle and glands • sympathetic division (speeds up heart rate) • parasympathetic division (slow down heart rate) • Enteric nervous system (ENS) • involuntary sensory & motor neurons control GI tract • neurons function independently of ANS & CNS

6. Neurons • Functional unit of nervous system • Have capacity to produce action potentials • electrical excitability What other tissue has this capacity? • Cell body • single nucleus with prominent nucleolus • rough ER (Nissl bodies) • free ribosomes for protein synthesis • neurofilaments give cell shape and support • microtubules move material inside cell • lipofuscin pigment clumps (harmless aging) • Cell processes = dendrites & axons

7. Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axons or Dendrites Cell body

8. Dendrites • Conduct impulses towards the cell body • Typically - numerous - short - highly branched - unmyelinated • Surfaces specialized for contact with other neurons (synapses) • Contain neurofibrils

9. Axons • Conduct impulses away from cell body • Single,long, thin cylindrical process of cell, may be myelinated • Arises at axon hillock • Impulses arise from initial segment (trigger zone) • Side branches (collaterals) • End in fine processes called axon terminals • Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Synaptic boutons

10. Axonal Transport • Cell body is location for most protein synthesis • neurotransmitters & repair proteins • Axonal transport system moves substances • slow axonal • movement in one direction only - away from cell body = anterograde • movement at 1-5 mm per day • fast axonal • moves organelles & materials along surface of microtubules • at 200-400 mm per day • transports in either direction (anterograde and retrograde) • for use or for recycling in cell body

11. Axonal Transport & Disease • Fast axonal transport route by which toxins or pathogens reach neuron cell bodies • tetanus (Clostridium tetani bacteria) • disrupts motor neurons causing painful muscle spasms • Bacteria enter the body through a laceration or puncture injury • more serious if wound is in head or neck because of shorter transit time

13. Functional Classification • Sensory (afferent) neurons • transport sensory information from skin, muscles, joints, sense organs & viscera to CNS • Motor (efferent) neurons • send motor nerve impulses to muscles & glands • Interneurons (association) neurons • connect sensory to motor neurons • 90% of neurons in the body (exclusively in CNS)

14. Structural Classification • Based on number of processes found on cell body • multipolar = several dendrites & one axon • most common cell type • bipolar neurons = one main dendrite & one axon • found in retina, inner ear & olfactory • unipolar neurons = one process only (develops from a bipolar) • are always sensory neurons

15. Association or Interneurons • Named for histologist that first described them or for their appearance

16. Neuroglial Cells • Half of the volume of the CNS • Smaller cells than neurons • 50X more numerous • Cells can divide • rapid mitosis in tumor formation (gliomas) • 4 cell types in CNS • astrocytes, oligodendrocytes, microglia & ependymal • 2 cell types in PNS • schwann and satellite cells

17. Astrocytesin CNS • Star-shaped cells • Most common (?) glial cell • Their processes extend into the basement membrane of capillaries and pia mater Functions: • Form blood-brainbarrier by covering blood capillaries • Metabolize neurotransmitters • Regulate K+ balance • Provide nutrients and structural support

18. Oligodendrocytesin CNS • Analogous to Schwann cells of PNS • Each oligodendrocyte wraps around more than one axons in CNS

19. Microgliain CNS • Small cells found near blood vessels • Phagocytic role - clear away dead cells • Derived from cells that also gave rise to macrophages & monocytes

20. Ependymal cellsin CNS • Form epithelial membrane lining cerebral cavities & central canal • Produce CSF (cerebrospinal fluid)

21. Satellite Cellsin PNS • Flat cells surrounding neuronal cell bodies in peripheral ganglia • Support neurons in the PNS ganglia

22. Schwann Cellin PNS • Cells encircling PNS axons • Each cell produces part of the myelin sheath surrounding an axon (only 1) in the PNS

23. Axon Coverings in PNS • Axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells • Neurolemma is cytoplasm & nucleusof Schwann cell • gaps called nodes of Ranvier • Myelinated fibers appear white • jelly-roll like wrappings made of lipoprotein = myelin • acts as electrical insulator • speeds conduction of nerve impulses • Unmyelinated fibers • slow, small diameter fibers • only surrounded by neurilemma but no myelin sheath wrapping

24. Myelination in PNS • Schwann cells myelinate axons in the PNS during fetal development • Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath • Tube guides growing axons that are repairing themselves

25. Myelination in the CNS • Oligodendrocytes “myelinate” axons in the CNS • Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons • No myelin sheath is formed, no neurolemma • Little regrowth after injury is possible due to the lack of a distinct tube or neurolemma

26. Gray and White Matter • White matter = myelinated processes (white in color) • Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) • In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter • In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS

29. Electrical Signals in Neurons • Neurons are electrically excitable due to the voltage difference across their membrane • Communicate with 2 types of electric signals • action potentials that can travel long distances • graded potentials that are local membrane changes only • In living cells, a flow of ions occurs through ion channels in the cell membrane

30. Two Types of Ion Channels • Leakage (nongated) channels are always open • nerve cells have more K+ than Na+ leakage channels • as a result, membrane permeability to K+ is higher • explains resting membrane potential of -70mV in nerve tissue • Gated channels open and close in response to a stimulus and play role in neuron excitability 1. Voltage-gated open in response to change in voltage 2. Ligand-gated open & close in response to particular chemical stimuli (hormone, neurotransmitter, ion) 3. Mechanically-gated open with mechanical stimulation

31. Gated Ion Channels

32. A, Establishment of a "diffusion" potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside through a membrane that is selectively permeable only to potassium. B, Establishment of a "diffusion potential" when the nerve fiber membrane is permeable only to Na ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions.

33. Resting Membrane Potential • Negative ions along inside of cell membrane & positive ions along outside • potential energy difference at rest is -70 mV • cell is “polarized” • Resting potential exists because • concentration of ions is different inside & outside • extracellular fluid rich in Na+ and Cl- • cytosol full of K+, organic phosphate & amino acids • membrane permeability differs for Na+ and K+ • 50-100 greater permeability for K+ than for Na+ • inward flow of Na+ can’t keep up with outward flow of K+ • Na+/K+ pump removes Na+ as fast as it leaks in

35. Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber; note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The lower panel displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber.

40. Action and Graded PotentialsGraded potentials • Small deviations from resting potential of -70mV • hyperpolarization = membrane has become more negative • depolarization = membrane has become more positive • occur most often in dendrites and cell body of a neuron

41. How do Graded Potentials Arise? • Source of stimuli • mechanical stimulation of membranes with mechanical gated ion channels (pressure) • chemicalstimulation of membranes with ligand gated ion channels (neurotransmitter) • voltage change: graded/postsynaptic/receptor or generator potential • Characteristics of graded potentials: • ions flow through ion channels and change membrane potential locally • amount of change varies with strength of stimuli (not so for AP) • Potential amplitude decreases over distance, dies out (not so for AP)

42. Action Potential • Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state • Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization) • All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second) • Travels (spreads) over surface of cell without dying out

43. Depolarizing Phase of Action Potential • Chemical or mechanical stimuluscaused a graded potential to reachat least (-55mV or threshold) • Voltage-gated Na+ channels open& Na+ rushes into cell • in resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) • when threshold (-55mV) is reached, both are open & Na+ enters • inactivation gate closes in few ten-thousandths of second • only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably (up to +30mV) • Positive (viscous) feedback process

44. Repolarizing Phase of Action Potential overshoot • When threshold potential of-55mV is reached, voltage-gated K+ channels begin to open • K+ channel opening is muchslower than Na+ channel opening • When K+ channels finally open fully, the Na+ channels have already closed (Na+ inflow stops) • K+ outflow returns membrane potential to -70mV (repolarization) • If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase • K+ channels close and the membrane potential returns to the resting potential of -70mV

45. Refractory Period of Action Potential Absolute Relative • Period of time during whichneuron can not generateanother action potential • Absolute refractoryperiod • even very strong stimulus willnot initiate another AP • inactivated Na+ channels must return to the resting state before they can be reopened • large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible • Relative refractoryperiod • a suprathreshold stimulus will be able to start an AP • K+ channels are still open, but Na+ channels have closed Note: Graded Potentials do not have refractory period!

46. The Action Potential: Summarized • Resting membrane potential is -70mV • Depolarization is the change from -70mV to +30 mV • Repolarization is the reversal from +30 mV back to -70 mV)