The Nervous System

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 • 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)

4. Nervous System Organization • Central nervous system (CNS ) • Brain and spinal cord • Peripheral nervous system (PNS) • 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 • Neuroglia • Support neurons both structurally and functionally • 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. Nerve Impulse Transmission • The inside of the cell is more negatively charged than the outside • Sodium–potassium pump • Brings two K+ into cell for every three Na+ it pumps out • Ion leakage channels • Allow more K+ to diffuse out than Na+ to diffuse in

12. Nerve Impulse Transmission • Two major forces act on ions in establishing the resting membrane potential • Electrical potential produced by unequal distribution of charges • Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other

13. Nerve Impulse Transmission • Sodium–potassium pump creates significant concentration gradient • Concentration of K+ is much higher inside the cell • Membrane not permeable to negative ions • Leads to buildup of positive charges outside and negative charges inside cell • Attractive force to bring K+ back inside cell • Equilibrium potential – balance between diffusional force and electrical force

14. Nerve Impulse Transmission

15. Nerve Impulse Transmission • Depolarization makes the membrane potential more positive • Hyperpolarization makes it more negative • These small changes result in graded potentials • Size depends on either the strength of the stimulus or the amount of ligand available to bind with their receptors • Can reinforce or negate each other • Summation is the ability of graded potentials to combine

16. Nerve Impulse Transmission

17. Nerve Impulse Transmission • Action potentials • Result when depolarization reaches the threshold potential (–55 mV) • Depolarizations bring a neuron closer to the threshold • Hyperpolarizations move the neuron further from the threshold • Caused by voltage-gated ion channels • Voltage-gated Na+ channels • Voltage-gated K+ channels

18. Nerve Impulse Transmission • Voltage-gated Na+ channels • Activation gate and inactivation gate • At rest, activation gate closed, inactivation gate open • Transient influx of Na+ causes the membrane to depolarize • Voltage-gated K+ channels • Single activation gate that is closed in the resting state • K+ channel opens slowly • Efflux of K+repolarizes the membrane

19. 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 • Intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

21. Nerve Impulse Transmission • Propagation of action potentials • 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 • Signal does not go back toward cell body

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

25. Nerve Impulse Transmission

26. Synapses • Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells • Presynaptic cell transmits action potential • Postsynaptic cell receives it • Two basic types: electrical and chemical

27. Electrical synapses • Involve direct cytoplasmic connections between the two cells formed by gap junctions • Relatively rare in vertebrates • Chemical synapses • Have a synaptic cleft between the two cells • End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

28. Synapses • Chemical 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 • Produces graded potentials in the postsynaptic membrane • Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

29. Synapses

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

31. Neurotransmitters • Amino acids • Glutamate • 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)

33. 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

34. 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

35. Drug Addiction • Habituation • Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it • Cell decreases the number of receptors because there is an abundance of neurotransmitters • In long-term drug use, means that more of the drug is needed to obtain the same effect

36. 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

38. Drug Addiction • Nicotine • Binds directly to a specific receptor on postsynaptic neurons of the brain • Binds to a receptor for acetylcholine • Brain adjusts to prolonged exposure by “turning down the volume” by • Making fewer receptors to which nicotine binds • Altering the pattern of activation of the nicotine receptors

39. Cerebrum • The increase in brain size in mammals reflects the great enlargement of the cerebrum • Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum • Each hemisphere receives sensory input from the opposite side • Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes

40. Cerebrum

41. Cerebrum • Cerebral cortex • Outer layer of the cerebrum • Contains about 10% of all neurons in brain • Highly convoluted surface • Increases threefold the surface area of the human brain • Divided into three regions, each with a specific function

42. Cerebrum • Cerebral cortex • Primary motor cortex – movement control • Primary somatosensory cortex – sensory control • Association cortex – higher mental functions • Basal ganglia • Aggregates of neuron cell bodies – gray matter • Participate in the control of body movements

44. Each of these regions of the cerebral cortex is associated with a different region of the body

45. Other Brain Structures • Thalamus • Integrates visual, auditory, and somatosensory information • Hypothalamus • Integrates visceral activities • Controls pituitary gland • Limbic system • Hypothalamus, hippocampus, and amygdala • Responsible for emotional responses

46. Complex Functions of the Brain • Sleep and arousal • One section of reticular formation is the reticular-activating system • Controls consciousness and alertness • Brain state can be monitored by means of an electroencephalogram (EEG) • Records electrical activity

47. Complex Functions of the Brain • Language • Left hemisphere is “dominant” hemisphere • Different regions control various language activities • Adept at sequential reasoning • Right hemisphere is adept at spatial reasoning • Primarily involved in musical ability • Nondominant hemisphere is also important for the consolidation of memories of nonverbal experiences

49. Complex Functions of the Brain • Memory • Appears dispersed across the brain • Short-term memory is stored in the form of transient neural excitations • Long-term memory appears to involve structural changes in neural connections • Two parts of the temporal lobes, the hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory

50. Complex Functions of the Brain • Alzheimer disease • Condition where memory and thought become dysfunctional • Two causes have been proposed • Nerve cells are killed from the outside in • External protein: b-amyloid • Nerve cells are killed from the inside out • Internal proteins: tau ()