Organization of the Nervous System: Neurons, Impulse Transmission, and Function

1. The Nervous System Chapter 44

2. Nervous System Organization All animals must be able to respond to environmental stimuli -Sensory receptors = Detect stimulus -Motor effectors = Respond to it -The nervous system links the two -Consists of neurons and supporting cells

3. Nervous System Organization Vertebrates have three types of neurons -Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) -Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) -Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)

5. Nervous System Organization The CNS consists of the brain and spinal cord The peripheral nervous system (PNS) consists of sensory and motor neurons -Somatic NS stimulates skeletal muscles -Autonomic NS stimulates smooth and cardiac muscles, as well as glands -Sympathetic and parasympathetic NS -Counterbalance each other

6. CNS Brain and Spinal Cord Motor Pathways Sensory Pathways Sensory neurons registering external stimuli Sensory neurons registering external stimuli PNS Somatic nervous system (voluntary) Autonomic nervous system (involuntary) Sympathetic nervous system "fight or flight" Parasympathetic nervous system "rest and repose" central nervous system (CNS) peripheral nervous system (PNS)

7. Nervous System Organization Neurons have the same basic structure -Cell body = Enlarged part containing nucleus -Dendrites = Short, cytoplasmic extensions that receive stimuli -Axon = Single, long extension that conducts impulses away from cell body

8. Nervous System Organization

9. Nervous System Organization Neurons are supported both structurally and functionally by cells called neuroglia -Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons -In the CNS, myelinated axons form white matter -Dendrites/cell bodies form gray matter -In the PNS, myelinated axons are bundled to form nerves

10. Nervous System Organization

11. Nerve Impulse Transmission A potential difference exists across every cell’s plasma membrane -Negative pole = Cytoplasmic side -Positive pole = Extracellular fluid side When a neuron is not being stimulated, it maintains a resting potential -Ranges from -40 to -90 millivolts (mV) -Average about -70 mV

12. Nerve Impulse Transmission The inside of the cell is more negatively charged than the outside because of: 1. Sodium-potassium pump = Brings two K+ into cell for every three Na+ it pumps out 2. Ion leakage channels = Allow more K+ to diffuse out than Na+ to diffuse in

14. Nerve Impulse Transmission There is a buildup of positive charge outside and negative charge inside the membrane -This electrical potential is an attractive force to bring K+ ions back into the cell -Balance between diffusional and electrical forces leads to the equilibrium potential The resting membrane potential can be viewed using a voltmeter and two electrodes

15. Nerve Impulse Transmission

16. Nerve Impulse Transmission There are two types of potentials: -Graded potentials and action potentials Graded potentials are small transient changes in membrane potential due to activation of gated ion channels -Most are closed in the normal resting cell

17. Nerve Impulse Transmission Chemically-gated or ligand-gated channels -Ligands are hormones or neurotransmitters -Induce opening and cause changes in cell membrane permeability

18. Nerve Impulse Transmission Depolarization makes the membrane potential more positive, whereas a hyperpolarization makes it more negative -These small changes result in graded potentials -Can reinforce or negate each other Summation is the ability of graded potentials to combine

19. Nerve Impulse Transmission

20. Nerve Impulse Transmission Action potentials result when depolarization reaches the threshold potential The action potential is caused by voltage-gated ion channels -Two different channels are used: -Voltage-gated Na+ channels -Voltage-gated K+ channels

21. Nerve Impulse Transmission When the threshold voltage is reached, sodium channels open rapidly -Transient influx of Na+ causes the membrane to depolarize In contrast, potassium channel opens slowly -Efflux of K+ repolarizes the membrane

22. Nerve Impulse Transmission The action potential has three phases: -Rising, falling and undershoot Action potentials are always separate, all-or-none events with the same amplitude -Do not add up or interfere with each other The intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

24. Nerve Impulse Transmission Each action potential, in its rising phase, reflects a reversal in membrane polarity -Positive charges due to influx of Na+ can depolarize the adjacent region to threshold -And so the next region produces its own action potential -Meanwhile, the previous region repolarizes back to the resting membrane potential

26. Nerve Impulse Transmission Two ways to increase velocity of conduction: 1. Axon has a large diameter -Less resistance to current flow -Found primarily in invertebrates 2. Axon is myelinated -Action potential is only produced at the nodes of Ranvier -Impulse jumps from node to node -Saltatory conduction

27. Nerve Impulse Transmission

28. Synapses Synapses are intercellular junctions -Presynaptic cell transmits action potential -Postsynaptic cell receives it Two basic types: electrical and chemical Electrical synapses involve direct cytoplasmic connections between the two cells formed by gap junctions -Relatively rare in vertebrates

29. Synapses Chemical synapses have a synaptic cleft between the two cells -End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

30. Synapses Action potential triggers influx of Ca2+ -Synaptic vesicles fuse with cell membrane -Neurotransmitter is released by exocytosis -Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins -Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

31. Synapses

32. Neurotransmitters Acetylcholine (ACh) -Crosses the synapse between a motor neuron and a muscle fiber -Neuromuscular junction

33. Neurotransmitters Acetylcholine (ACh) -Binds to ligand-gated receptor in the postsynaptic membrane -Produces a depolarization called an excitatory postsynaptic potential (EPSP) -Stimulates muscle contraction -Acetylcholinesterase (AChE) degrades ACh -Causes muscle relaxation

34. Neurotransmitters Amino acids -Glutamate is the major excitatory neurotransmitter in the vertebrate CNS -Glycine and GABA (g-aminobutyric acid) are inhibitory neurotransmitters -Open ligand-gated channels for Cl– -Produce a hyperpolarization called an inhibitory postsynaptic potential (IPSP)

35. Neurotransmitters

36. Neurotransmitters (Cont.)

37. Neurotransmitters Biogenic amines -Epinephrine (adrenaline)and norepinephrine are responsible for the “fight or flight” response -Dopamine is used in some areas of the brain that control body movements -Serotonin is involved in the regulation of sleep

38. Neurotransmitters Neuropeptides -Substance P is released from sensory neurons activated by painful stimuli -Intensity of pain perception depends on enkephalins and endorphins Nitric oxide (NO) -A gas ; produced as needed from arginine -Causes smooth muscle relaxation

39. Synaptic Integration Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body -Small EPSPs add together to bring the membrane potential closer to the threshold -IPSPs subtract from the depolarizing effect of EPSPs -And will therefore deter the membrane potential from reaching threshold

40. Synaptic Integration

41. Synaptic Integration There are two ways that the membrane can reach the threshold voltage -Spatial summation -Many different dendrites produce EPSPs -Temporal summation -One dendrite produces repeated EPSPs

42. Drug Addiction Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it -This process is called habituation -The cell decreases the number of receptors because there is an abundance of neurotransmitters

43. Drug Addiction Cocaine affects neurons in the brain’s “pleasure pathways” (limbic system) -Binds dopamine transporters and prevents the reuptake of dopamine -Dopamine survives longer in the synapse and fires pleasure pathways more and more -Prolonged exposure triggers the limbic system neurons to reduce receptor numbers -The cocaine user is now addicted

45. Drug Addiction Nicotine binds directly to a specific receptor on postsynaptic neurons of the brain -Brain adjusts to prolonged exposure by “turning down the volume” in two ways: 1. Making fewer nicotine receptors 2. Altering the pattern of activation of the nicotine receptors

46. The Central Nervous System Sponges are only major phylum without nerves Cnidarians have the simplest nervous system -Neurons linked to each other in a nerve net -No associative activity Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity -Two nerve cords run down the body -Permit complex muscle control

48. Vertebrate Brains All vertebrate brains have three basic divisions: -Hindbrain or rhombencephalon -Midbrain or mesencephalon -Forebrain or prosencephalon In fishes, -Hindbrain = Largest portion -Midbrain = Processes visual information -Forebrain = Processes olfactory information

49. Vertebrate Brains

50. Vertebrate Brains The relative sizes of different brain regions have changed as vertebrates evolved -Forebrain became the dominant feature