1. The Nervous System Chapter 8 – Overview and Neural Tissue

2. The Nervous system has three major functions :  Sensory – monitors internal & external environment through presence of receptors  Integration – interpretation of sensory information (information processing); complex (higher order) functions  Motor – response to information processed through stimulation of effectors  muscle contraction  glandular secretion

3. Two Anatomical Divisions  Central nervous system (CNS) Brain Spinal cord  Peripheral nervous system (PNS) All the neural tissue outside CNS Afferent division (sensory input) Efferent division (motor output)  Somatic nervous system  Autonomic nervous system General Organization of the nervous system

4. Brain & spinal cord

5. Histology of neural tissue Two types of neural cells in the nervous system:  Neurons - For processing, transfer, and storage of information  Neuroglia – For support, regulation & protection of neurons

6. Neuroglia (glial cells) CNS neuroglia: astrocytes oligodendrocytes microglia ependymal cells PNS neuroglia: Schwann cells (neurolemmocytes) satellite cells

7. Astrocytes create supportive framework for neurons create “blood-brain barrier” monitor & regulate interstitial fluid surrounding neurons secrete chemicals for embryological neuron formation stimulate the formation of scar tissue secondary to CNS injury

8. Oligodendrocytes create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons Microglia “brain macrophages” phagocytize cellular wastes & pathogens

9. Ependymal cells line ventricles of brain & central canal of spinal cord produce, monitor & help circulate CSF (cerebrospinal fluid)

10. Schwann cells surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons creates myelin sheath around most axons of PNS Satellite cells support (structurally & functionally) groups of cell bodies of neurons within ganglia of the PNS

11. Neuron structure

12. Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons) The presence of myelin speeds up the transmission of action potentials along the axon Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “nodes of Ranvier” Regions of the nervous system containing groupings of myelinated axons make up the “white matter” “gray matter” is mainly comprised of groups of neuron cell bodies, dendrites & synapses (connections between neurons) of Ranvier

13. Anatomical organization of neurons Neurons of the nervous system tend to group together into organized bundles The axons of neurons are bundled together to form nerves in the PNS & tracts/pathways in the CNS. Since most axons are myelinated, these regions will look white in appearance (“white matter”) The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS. These parts are not myelinated, therefore will look gray in appearance (“gray matter”)

14. Neural Tissue Organization Figure 8-6

15. Classification of neurons Structural classification based on number of processes coming off of the cell body: Multipolar neuron multiple dendrites & single axon most common type

16. Bipolar neuron two processes coming off cell body – one dendrite & one axon only found in eye, ear & nose Unipolar neuron single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

17. Classification of neurons Functional classification based on type of information & direction of information transmission: Sensory (afferent) neurons – transmit sensory information from receptors of PNS towards the CNS most sensory neurons are unipolar, a few are bipolar Motor (efferent) neurons – transmit motor information from the CNS to effectors (muscles/glands/adipose tissue) in the periphery of the body all are multipolar Association (interneurons) – transmit information between neurons within the CNS; analyze inputs, coordinate outputs are the most common type of neuron (20 billion) are all multipolar

18. Reflex arc (p. 283-286) Reflex – a quick, unconscious response to a stimulus to protect or maintain homeostasis. e.g. stretch reflex, withdrawal reflex Reflex arc – neural pathway involved in the production of a reflex. Structures include: receptor sensory neuron integrating center (brain or spinal cord; may or may not involve association neurons (interneurons)) motor neuron effector

19. Stretch reflex - simplest type of reflex - no association neuron involved Figure 8-29

20. Simplified Withdrawal reflex Figure 8-28

21. Neuron Function Neurons at rest have an unequal distribution of charged ions inside/outside the cell, which are kept separate by the plasma membrane The sum of charges makes the outside of the membrane positive, & the inside of the membrane negative more Na+ ions outside more K+ ions inside large negatively charged proteins & phosphate ions inside

22. Because of the difference of ionic charges inside/outside the cell, the membrane of the resting neuron is “polarized” The difference in charges creates a potential electrical current across the membrane known as the “membrane potential (transmembrane potential)”

23. At rest, the transmembrane potential can also be referred to as the “resting membrane potential” (RMP) The RMP of a neuron = -70mV

24. Because Na + & K + can move through leakage channels of nerve cells, the resting membrane potential is maintained by the sodium-potassium exchange pump For ions to cross a cell membrane, they must go through transmembrane channels “leakage channels” – open all the time, allow for diffusion “gated channels” – open & close under specific circumstances (e.g. voltage changes)

25. This change in membrane potential is known as hyperpolarization If a stimulus opens gated K + channels, positive charges leave cell  membrane potential becomes more negative (-70mV  -90mV) When a stimulus is applied to a resting neuron, gated ion channels can open

26. When a stimulus causes Na + gates open, Na + diffuses into the cell This changes the electrical charge inside the cell membrane, bringing it away from its RMP of -70mV toward 0mV This change in membrane potential is known as depolarization

27. If a stimulus only affects Na + gates at a specific site of the axon, the depolarization is small & localized only to that region of the cell. This is known as a graded potential But if the stimulus reaches a certain level (threshold level), voltage controlled Na + gates will begin to open in sequence along the length of the axon. The depolarization will propagate along the entire surface of the cell membrane This propagated change in the membrane potential is known as an action potential (nerve impulse)

28. Action potentials APs involve the movement of Na + ions into the cell (causing depolarization of the membrane), followed immediately by K + ions moving out of the cell through voltage controlled K+ gates (causing repolarization of the membrane), that propagates down the length of the cell APs are due to voltage changes that open & close gated Na + & K + channels within excitable cells Only nerve cells & muscle cells are excitable, i.e. can generate APs. Once an AP begins, it will propagate down the entire cell at a constant & maximum rate. This is known as the “all or none” principle

29. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-8 1 of 5 +30 0 60 _ 70 _ Transmembrane potential (mV) Threshold Resting potential 1 2 3 4 REFRACTORY PERIOD DEPOLARIZATIONREPOLARIZATION Local current Depolarization to threshold Sodium ions Activation of voltage- regulated sodium channels and rapid depolarization Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels The return to normal permeability and resting state Time (msec) 0123 Action Potential Conduction

30. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-8 2 of 5 +30 0 60 _ 70 _ Transmembrane potential (mV) Threshold Resting potential 1 DEPOLARIZATION Local current Depolarization to threshold Sodium ions Time (msec) 0123 Nerve cell at rest (RMP= - 70mV) Stimulus applied to cell Na + gates at axon hillock cause localized depolarization (graded potential) If stimulus is strong enough, flow of Na + ions into cell reach threshold level triggering opening of voltage gated Na + channels & formation of an action potential (nerve impulse)

31. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-8 3 of 5 +30 0 60 _ 70 _ Transmembrane potential (mV) Threshold Resting potential 1 2 DEPOLARIZATION Local current Depolarization to threshold Sodium ions Activation of voltage- regulated sodium channels and rapid depolarization Potassium ions Time (msec) 0123 Once threshold is reached, Na + will quickly diffuse into the cell causing a rapid depolarization of the membrane (- 70 mV  0 mV  +30 mV) this depolarization will spread to adjacent parts of the membrane, activating more voltage controlled Na + gates in succession

32. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-8 4 of 5 +30 0 60 _ 70 _ Transmembrane potential (mV) Threshold Resting potential 1 2 3 DEPOLARIZATIONREPOLARIZATION Local current Depolarization to threshold Sodium ions Activation of voltage- regulated sodium channels and rapid depolarization Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels Time (msec) 0123 When the transmembrane potential reaches +30mV, Na+ gates will close & K + gates will open K + will quickly exit cell resulting in repolarization of membrane & return to resting state

33. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-8 5 of 5 +30 0 60 _ 70 _ Transmembrane potential (mV) Threshold Resting potential 1 2 3 4 REFRACTORY PERIOD DEPOLARIZATIONREPOLARIZATION Local current Depolarization to threshold Sodium ions Activation of voltage- regulated sodium channels and rapid depolarization Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels The return to normal permeability and resting state Time (msec) 0123

34. Continuous propagation (continuous conduction) Involves entire membrane surface Proceeds in series of small steps (slower) Occurs in unmyelinated axons (& in muscle cells) Propagation of an Action Potential Figure 8-9(a)

35. Propagation of an Action Potential Saltatory propagation (saltatory conduction)  Involves patches of membrane exposed at nodes of Ranvier  Proceeds in series of large steps (faster)  Occurs in myelinated axons Figure 8-9(b)

36. Action Potential Propagation Neurophysiology: Continuous Saltatory Propagation Neurophysiology: Continuous Saltatory Propagation Neurophysiology: Action Potential PLAY Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

37. “The Big Picture” “Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials. An action potential occurs due to a rapid change in membrane polarity (depolarization followed by repolarization) Depolarization is due to the influx of sodium ions (Na + ); repolarization is due to the efflux of potassium ions (K + )

38. Conduction across synapses Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter In order for neural control to occur, “information” must not only be conducted along nerve cells, but must also be transferred from one nerve cell to another across a synapse

39. The Structure of a Typical Synapse Figure 8-10

40. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-11 1 of 5 An action potential arrives and depolarizes the synaptic knob Action potential ACh is removed by AChE (acetylcholinesterase) Extracellular Ca 2+ enters the synaptic cleft triggering the exocytosis of ACh ACh binds to receptors and depolarizes the postsynaptic membrane EXTRACELLULAR FLUID Synaptic knob PRESYNAPTIC NEURON Synaptic vesicles ER AChE POSTSYNAPTIC NEURON CYTOSOL ACh Ca 2+ Synaptic cleft Chemically regulated sodium channels Initiation of action potential if threshold is reached Receptor Na 2+ Propagation of action potential (if generated) Events at a Typical Synapse

41. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-11 2 of 5 An action potential arrives and depolarizes the synaptic knob Action potential EXTRACELLULAR FLUID Synaptic knob PRESYNAPTIC NEURON Synaptic vesicles ER AChE POSTSYNAPTIC NEURON CYTOSOL An action potential arrives & depolarizes the synaptic knob (end bulb) Before repolarization can occur, Ca +2 gates open & Ca +2 diffuses into end bulb Repolarization occurs

42. Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-11 3 of 5 An action potential arrives and depolarizes the synaptic knob Action potential Extracellular Ca 2+ enters the synaptic cleft triggering the exocytosis of ACh EXTRACELLULAR FLUID Synaptic knob PRESYNAPTIC NEURON Synaptic vesicles ER AChE POSTSYNAPTIC NEURON CYTOSOL ACh Ca 2+ Synaptic cleft Chemically regulated sodium channels Ca +2 causes the synaptic vessicles to fuse with the end bulb membrane causing the exocytosis of the neurotransmitter

43. Figure 8-11 4 of 5 An action potential arrives and depolarizes the synaptic knob Action potential Extracellular Ca 2+ enters the synaptic cleft triggering the exocytosis of ACh ACh binds to receptors and depolarizes the postsynaptic membrane EXTRACELLULAR FLUID Synaptic knob PRESYNAPTIC NEURON Synaptic vesicles ER AChE POSTSYNAPTIC NEURON CYTOSOL ACh Ca 2+ Synaptic cleft Chemically regulated sodium channels Initiation of action potential if threshold is reached Receptor Na 2+ The neurotransmitter diffuses across the synaptic cleft & binds to its receptors on the post synaptic membrane, causing an effect on the post synaptic cell

44. The effect on the post synaptic neuron will depend on whether the neurotransmitter released is  Excitatory (e.g. Ach, norepinephrine (NE))  Inhibitory (e.g. seratonin, GABA) Excitatory neurotransmitters cause Na + gates to open in the post synaptic membrane  depolarization (impulse conduction) Inhibitory neurotransmitters cause K + or Cl - gates to open in the post synaptic cell  hyperpolarization (no impulse conduction)

45.  The effects of neurotransmitters on the post synaptic neurons are usually short lived because most neurotransmitters are rapidly removed from the synaptic cleft by enzymes or reuptake Play - Neurophysiology: Synapse