1. CHAPTER 44
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2. The Nervous System
Chapter 44

3. 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

4. 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
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

7. Parasympathetic nervous
CNS
Brain and Spinal Cord
Sensory Pathways
Motor 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)

8. 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

9. Nervous System Organization

10. 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

11. Nervous System Organization

12. 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

13. 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

15. 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

16. 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

17. Nerve Impulse Transmission

18. Nerve Impulse Transmission
Uniqueness of neurons compared with other cells is not the production and maintenance of the resting membrane potential
Rather the sudden temporary disruptions to the resting membrane potential that occur in response to stimuli
2 types of changes
Graded potentials
Action potentials

19. Nerve Impulse Transmission
Graded potentials
Small transient changes in membrane potential due to activation of gated ion channels
Each gated channel is selective
Most are closed in the normal resting cell

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

21. 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

22. Nerve Impulse Transmission

23. 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

24. 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

25. 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

27. 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

29. 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

30. Nerve Impulse Transmission

31. 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

32. Electrical synapses Chemical 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

33. 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

34. Synapses

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

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

37. 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)

39. 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

40. 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

41. 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
Deter the membrane potential from reaching threshold

42. Synaptic Integration

43. 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

44. 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

45. 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

47. Drug Addiction Nicotine
Binds directly to a specific receptor on postsynaptic neurons of the brain
Binds to a receptor for acetylcholine

48. 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
All of the subsequent evolutionary changes in nervous systems can be viewed as a series of elaborations on the characteristics already present in flatworms

50. Vertebrate Brains All vertebrate brains have three basic divisions:
Hindbrain
Midbrain
Forebrain
In fishes,
Hindbrain – largest portion
Midbrain – processes visual information
Forebrain – processes olfactory information

51. Vertebrate Brains

52. Vertebrate Brains
Relative sizes of different brain regions have changed as vertebrates evolved
Forebrain became the dominant feature

53. Vertebrate Brains 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

54. Cerebrum

55. 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

56. 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

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

59. 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

60. 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

61. 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

63. 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

64. Complex Functions of the Brain
Alzheimers 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 ()

65. Spinal Cord
Cable of neurons extending from the brain down through the backbone
Enclosed and protected by the vertebral column and the meninges

66. Spinal Cord 2 zones Inner zone is gray matter
Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia
Outer zone is white matter
Contains cables of sensory axons in the dorsal columns and motor axons in the ventral columns

67. Spinal Cord It serves as the body’s “information highway”
Relays messages between the body and the brain
It also functions in reflexes
The knee-jerk reflex is monosynaptic
However, most reflexes in vertebrates involve a single interneuron

68. Knee-jerk reflex is monosynaptic

69. Most reflexes in vertebrates involve a single interneuron

70. The Peripheral Nervous System
Consists of nerves and ganglia
Nerves are bundles of axons bound by connective tissue
Ganglia are aggregates of neuron cell bodies
Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells

71. The Peripheral Nervous System
Sensory neurons
Axons enter the dorsal surface of the spinal cord and form dorsal root of spinal nerve
Cell bodies are grouped outside the spinal cord in dorsal root ganglia
Motor neurons
Axons leave from the ventral surface and form ventral root of spinal nerve
Cell bodies are located in the spinal cord

72. The Somatic Nervous System
Somatic motor neurons stimulate the skeletal muscles to contract
In response to conscious command or reflex actions
Antagonist of the muscle is inhibited by hyperpolarization (IPSPs) of spinal motor neurons

73. The Autonomic Nervous System
Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata
In both, efferent motor pathway has 2 neurons
Preganglionic neuron – exits the CNS and synapses at an autonomic ganglion
Postganglionic neuron – exits the ganglion and regulates visceral effectors
Smooth or cardiac muscle or glands

75. The Autonomic Nervous System
Sympathetic division
Preganglionic neurons originate in the thoracic and lumbar regions of spinal cord
Most axons synapse in two parallel chains of ganglia right outside the spinal cord
Parasympathetic division
Preganglionic neurons originate in the brain and sacral regions of spinal cord
Axons terminate in ganglia near or even within internal organs