Introduction to the Nervous System and Neurons

1. Chapter 10 The Nervous System

2. Introduction • Types of neural tissue: • 1. Neurons – react to changes around them & send impulses • 2. Neuroglia – support tissue with a variety of functions • Functions of nervous system: • 1. Sensory – use sensory neurons to gather info. inside & outside the body • 2. Motor – use motor neurons to help the body react to stimuli • 3. Integrative – integrate signals from sensory & motor neurons to produce thought, memory, etc.

3. Divisions of the Nervous System • Central Nervous System (CNS) – consists of the brain & spinal cord • Peripheral Nervous System (PNS) – consist of nerves that connect the CNS to other body parts

4. Structure of a Neuron • Dendrites – pick up impulses • Cell body – contains cell parts • Axon – sends impulses • Schwann cells – wrap around the axon • Myelin – lipid covering formed by Schwann cells; speeds rate of impulse • Axon terminals – end of axon

5. Structure of A Neuron • Axon hillock – slight elevation where axon originates • Node of Ranvier – gap in myelin

6. Structure of A Neuron • Neurofibrils – network of fine threads that extend into the axon; for support • Nissl bodies – consist of rough ER Neurilemmal sheath – formed by the cytoplasm & nucleus of the Schwann cell that remain on the outside

7. Nerve anatomy video • https://www.youtube.com/watch?v=DLN1UsvmVvM

8. Direction of Impulse • Impulse always travels from dendrites, through cell body, & down axon • Axon synapses w/next neuron or an effector (muscle or gland)

9. Structural Classification of Neurons • Bipolar – has 2 processes from the cell body, 1 at each end; in sense organs • Unipolar – has 1 process from c.b. that divides into 2; in PNS • Multipolar – have many processes from c.b; in CNS and motor neurons.

10. Functional Classification of Neurons • Sensory (afferent) – unipolar & carry impulses from body parts to brain or s.c. • Interneurons (association neurons) – multipolar & in CNS; form links b/t other neurons • Motor (efferent) – multipolar & carry impulses from brain or s.c. to muscle or gland

11. Types of Neuroglia • Support tissue w/a variety of functions: 1. Astrocytes –star-shaped; found b/t neurons & b.v.; support, transport & communication b/t nerves & b.v. • Transport glucose to Neuron and store glycogen • Separate neurons from each other.

12. Types of Neuroglia 2.Microglia – small w/few processes; found throughout CNS; support & phagocytosis of harmful sub- stances

13. Types of Neuroglia 3. Oligodendrocytes – resemble astrocytes but w/fewer processes; form myelin sheath in CNS

14. Types of Neuroglia 4. Ependyma – columnar & cuboidal shaped cells; form inner lining of brain & s.c.; provide a layer for diffusion to occur

15. Types of Neuroglia

16. Cell suicide • Microglia can destroy cells that are old &/or damaged • A – healthy neuron • B – neuron being destroyed & DNA breaking apart • C – microglia removing debris

17. Resting Potential • A resting neuron is one not sending an impulse & is in resting potential • The cell membrane of this neuron is polarized b/c of an un= distribution of ions on either side • Outside the neuron – greater concentration of Na+ ions • Inside the neuron – greater concentration of K+ ions & negatively charged proteins

18. Resting Potential • K+ leak out of K+ channels at a slow rate leaving behind negatively charged proteins • This makes the charge on the inside of the membrane negative • The voltage meter (next pg.) shows a charge of -70 mv & refers to the charge of a neuron in resting potential

19. Movement of Ions • Ions follow the laws of diffusion (movement from high to low concentrations) when moving thru membranes • Ions enter & leave the membrane thru channels or gates that are specific for that ion

20. Ion Channels • 3 types • Passive- always open • Ligand gated- opened by a chemical compound. (neurotransmitter) • Voltage gated- opened in response to a change in electric potential.

21. Resting Potential The charge outside the cell is positive b/c: 1. the high concentration of Na+ ions 2. the movement of K+ ions to the outside

22. Resting Potential Animation • Resting Potential Animation

23. Sodium Potassium Pump • Membrane protein used for the active transport of Na+ and K+ across membrane. • Requires ATP • Removes 3 Na+ ions and accepts 2 K+ for every ATP molecule used. • Maintains resting potential.

24. Action Potential • An abrupt change in the electrical potential across the cell membrane that occurs after a stimulus (a.k.a. nerve impulse): 1. Resting neuron stimulated (remember – a resting neuron is polarized) 2. Na+ channels open & Na+ move into membrane; charge inside cell becomes + (+30mv) & neuron is depolarized 3. Na+ channels close & K+ channels open; K+ move out & charge reverts back to negative (-70mv); cell is repolarized

25. Resting Potential → Action Potential A)Resting potential (polarized) B)Action potential A.P. in the 1st region stimulates adjacent region (de- polarized) C)1st region repolarized

26. Nerve Impulse Cartoon • Impulse Animation

27. Action Potential Animation • Action Potential Animation

28. Graphing Action Potential After repolarization a brief period of delay occurs when Na+ gates cannot temporarily open; called refractory period

29. Graphing Action Potential Hyperpolarization when the cell becomes more negative than -70mv; depends on which ions are allowed to enter the cell, + or – ions (i.e. Cl- ions) Threshold – the minimum amt. of stimulus required to cause an action potential

30. Impulse Conduction • Saltatory conduction – impulse jumps from 1 node of Ranvier to another; why? • Myelin covering – mostly lipids which prevent flow of ions • channels - are located at nodes of Ranvier for ions to diffuse in & out • Myelinated axons (white matter) - conduct impulses faster than unmyelinated axons (gray matter)

31. Saltatory Conduction Animation • Animation • Slide 13

32. The Synapse • Junction b/t 2 neurons • Presynaptic neuron – occurs before the syapse • Postsynaptic neuron – occurs after the synapse • Synaptic knob – enlargement of axon terminal • Synaptic vesicles – store ntm • Synaptic cleft – space b/t neurons

33. Actual Synapse

34. Events at the Synapse • Action potential travels down presynaptic neuron & arrives at synapse • Synaptic knob becomes more permeable to Ca+ & they diffuse inward • This causes vesicles to release ntm • Ntm causes A.P. to enter postsynaptic neuron • A.P. continues to travel down postsynaptic neuron

35. The Synapse

36. Types of Neurotransmitters • The nervous system produces approx. 30 different types of ntm • Some open ion channels, others close them • Monoamines: Neuropeptides: - epinephrine - endorphins - norepinephrine - enkephalins - dopamine - substance P - serotonin Acetylcholine (ACh)

37. Effects of Ntms • Epinephrine & norepinephrine – hormones when released in blood, but ntm in the n.s.; stimulate autonomic n.s.; incr. HR, resp. rate, etc.; “fight-or-flight” response • Dopamine – excitatory or inhibitory; create a sense of well-being; insufficient levels associated with Parkinson’s disease • Serotonin – inhibitory; insufficient levels associated with insomnia • Endorphins & enkephalins – generally inhibitory & influence mood; released under stress to reduce pain (blocks substance P) • Substance P – excitatory; helps in perception of pain • ACh – stimulates muscles to contract

38. Synaptic Potentials • Ion channels that respond to ntm are called chemically gated channels (as opposed to those that are voltage-gated & are involved in sending A.P.) • Changes in chem. gated channels create local changes called synaptic potentials (a small, temporary change in the potential charge of a neuron) • They allow one neuron to influence another

39. The Synapse

40. Synaptic Potentials • 2 types: 1. Excitatory postsynaptic potential(EPSP) – occurs when the neuron is depolarized (or becomes less negative), but the charge is subthreshold (<+30mv). A true A.P. won’t occur, but will be morelikely to occur if the neuron receives more subthreshold stimuli

41. Synaptic Potentials 2. Inhibitory postsynaptic potential (IPSP) occurs when the neuron is hyperpolarized (or becomes more negative than -70mv). An A.P. will be less likely to occur. • The type of ntm secreted will decide the effect that occurs.

42. Effects of Ntm on Synaptic Potentials • If a ntm opens Na+ channels & Na+ diffuse in, the membrane is depolarized (EPSP) • If a ntm opens K+ channels & K+ diffuse out, the membrane is hyperpolarized (IPSP) • A neuron can receive EPSP’s & IPSP’s simultaneously; the neuron responds to the algebraic sum of the + and - charges

43. Synaptic Potential vs. Action Potential • 2 differences: 1. P.S.P. are graded (depends on amt. of ntm) & their effect adds up (called summation) whereas A.P. are all-or-none 2. P.S.P. decr. in intensity w/incr. distance from synapse • Facilitation – when a neuron receives subthreshold stimuli & gets closer to sending an A.P.

44. Convergence vs. Divergence • Convergence – impulses from 2 or more fibers converge on a single neuron (summation will occur) • Divergence – when outgoing impulses are divided onto several branches of an axon

45. Convergence vs. Divergence

46. Importance of Ions • Ca+ are needed for the release of ntm • Ca+ are also needed to close Na+ channels • Insufficient Ca+ levels result in channels remaining open & impulses repeatedly transmitted; results in tetany • May occur in pregnancy (as fetus uses maternal Ca+), when diet lacks Ca+ or Vit D during dehydration

47. Importance of Ions • An incr. in extracellular K+ causes neuron to be less negative; threshold is reached sooner & neurons are very excitable; may result in convulsions • A decr. in extracellular K+ causes neuron to be more neg.; does not allow an A.P. to occur & muscles may become paralyzed