1. Ch. 37: Neurons & Nervous Systems
Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Ch. 37: Neurons & Nervous Systems

2. Invertebrate Nervous Organization
Biology, 9th ed, Sylvia Mader
Invertebrate Nervous Organization
Chapter 39
Neurons & Nervous Systems
Sponges
Cellular level of organization; can respond to stimuli
Cnidarians
Tissue level of organization; can contract & extend body; move tentacles to capture prey
1. Hydras
Nerve net composed of neurons in contact with one another & with contractile cells in body wall
2. Sea anemones & jellyfishes
Two nerve nets: 1 fast & 1 slow

3. Evolution of the Nervous System
Biology, 9th ed, Sylvia Mader
Evolution of the Nervous System
Chapter 39
Neurons & Nervous Systems

4. Invertebrate Nervous Organization
Biology, 9th ed, Sylvia Mader
Invertebrate Nervous Organization
Chapter 39
Neurons & Nervous Systems
Planarians (flatworms)
Bilateral symmetry (left & right side symmetry)
Ladderlike nervous system: 2 ventral nerve cords with transverse nerves that connect 2 nerve cords
Cephalization - a concentration of ganglia and sensory receptors in the head
Bilateral symmetry & cephalization are two important trends for the evolution of an active life

5. Evolution of the Nervous System
Biology, 9th ed, Sylvia Mader
Evolution of the Nervous System
Chapter 39
Neurons & Nervous Systems

6. Invertebrate Nervous Organization
Biology, 9th ed, Sylvia Mader
Invertebrate Nervous Organization
Chapter 39
Neurons & Nervous Systems
Annelids, Arthropods and Molluscs
Complex animals with true nervous systems
Typical invertebrate nervous system:
-Brain & ventral nerve cord
-Ganglion in each segment of annelids & arthropods
•Brain receives sensory information, controls activity of ganglia and nerves so muscle activity of entire animal is coordinated

7. Evolution of the Nervous System
Biology, 9th ed, Sylvia Mader
Evolution of the Nervous System
Chapter 39
Neurons & Nervous Systems

8. Vertebrate Nervous Organization
Biology, 9th ed, Sylvia Mader
Vertebrate Nervous Organization
Chapter 39
Neurons & Nervous Systems
Central nervous system (CNS)
Develops from an embryonic dorsal tubular nerve cord
Cephalization and bilateral symmetry result in several types of paired sensory receptors that gather information about environment
Vertebrates have thousands or billions more neurons than invertebrates

9. Vertebrate Nervous Organization
Biology, 9th ed, Sylvia Mader
Vertebrate Nervous Organization
Chapter 39
Neurons & Nervous Systems
Vertebrate brain is organized into three areas
Hindbrain
-Regulates organs below level of consciousness (heart & lung functions)
-Also coordinates limb activity, posture & balance
Midbrain
-Optic lobes coordinate responses to visual input
Forebrain
-Receives sensory input from hind & midbrains.
-Cerebrum of mammals associated with higher mental capabilities. Human cerebral cortex is especially large & complex.

10. Biology, 9th ed, Sylvia Mader
Human Nervous System
Chapter 39
Neurons & Nervous Systems
Nervous system has three specific functions:
Receives sensory input
- Receptors respond to external & internal stimuli by generating nerve impulses that travel to CNS
Performs integration
-CNS sums up input it receives from entire body
Generates motor output
-Nerve impulses from CNS go to muscles & glands

11. Organization of the Human Nervous System
Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Organization of the Human Nervous System

12. Biology, 9th ed, Sylvia Mader
Human Nervous System
Chapter 39
Neurons & Nervous Systems
Central nervous system (CNS)
Includes the brain and spinal cord
Lies in the midline of the body
Peripheral nervous system (PNS)
Contains cranial nerves and spinal nerves that
Gather info from sensors and conduct decisions to effectors
Lies outside the CNS
PNS consists of two parts:
-Somatic nervous system
-Autonomic nervous system - divided into
sympathetic & parasympathetic systems

13. Biology, 9th ed, Sylvia Mader
Nervous Tissue
Chapter 39
Neurons & Nervous Systems
Two principal types of cells:
1. Neurons (nerve cells)
Functional units of the nervous system
2. Neuroglia
•Provide support & nourishment to neurons

14. Biology, 9th ed, Sylvia Mader
Nervous Tissue
Chapter 39
Neurons & Nervous Systems
Neurons consist of 3 major parts:
Cell body contains nucleus & organelles
Dendrites are short & highly branched; receive signals from sensory receptors or other neurons; transmit signals to cell body
Axon conducts nerve impulse to other neurons or cells
Axons can be bundled to form nerve fibers
Covered by myelin sheath, white insulating layer

15. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems

16. Biology, 9th ed, Sylvia Mader
Nervous Tissue
Chapter 39
Neurons & Nervous Systems
Neuroglia greatly outnumber neurons in brain
In CNS, there are several different types
Myelin sheath formed from tightly spiraled neuroglia called oligodendrocytes.
In PNS, Schwann cells create myelin sheath
Gaps called nodes of Ranvier, appear between the Schwann cells.

17. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems

18. Biology, 9th ed, Sylvia Mader
Types of Neurons
Chapter 39
Neurons & Nervous Systems
Motor (Efferent) Neurons
Accept nerve impulses from the CNS
Transmit them to muscles or glands. Said to “innervate” these structures.
Sensory Neurons
Accept impulses from sensory receptors
Transmit them to the CNS
Interneurons
Convey nerve impulses between various parts of the CNS

19. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Motor
Interneuron
Sensory

20. Biology, 9th ed, Sylvia Mader
Nerve Impulses
Chapter 39
Neurons & Nervous Systems
Nerve Impulses
These are waves of electrical activity
Involve movement of unequally distributed ions on either side of an axon’s membrane.
Can be measured with tiny electrodes inserted into axons.
A voltage difference can be measured between the inside & outside of the axon.
This is called a membrane potential

21. Nerve Impulses: Resting Potential
Biology, 9th ed, Sylvia Mader
Nerve Impulses: Resting Potential
Chapter 39
Neurons & Nervous Systems
Resting Potential
The membrane potential when the axon is not conducting an impulse
The inside of a neuron is more negative than the outside. Resting potential = -65 mV
Due to following factors:
Higher concentration of Na+ outside the axon
Higher concentration of K+ inside axon
Unequal distribution is partly due to activity of sodium-potassium pump
Also due to large, negatively charged proteins in cytoplasm of axon

22. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Resting Potential of the Axonal Membrane Uneven distribution of Na+ and K+ create membrane potential of -65 mV
More K+
More Na+

23. Biology, 9th ed, Sylvia Mader
Action Potential
Chapter 39
Neurons & Nervous Systems
In order for a nerve impulse to begin an action potential needs to be generated.
An action potential is a rapid change in polarity across a portion of the axon. It will be generated only by a stimulus larger than the threshold value.
•This is the minimum change in polarity across the axon membrane required to generate an action potential.
Stimuli cause gated ion channels to open in the axon membrane. There are two types of gated channels:
•Sodium gates: allow Na+ ions to enter axon
•Potassium gates: allow K+ ions to leave axon

24. Biology, 9th ed, Sylvia Mader
Action Potential
Chapter 39
Neurons & Nervous Systems
Once a stimulus is detected by the axonal membrane, depolarization will begin.
•Sodium ion gates open causing Na+ to flow into axon
•This causes the inside of the neuron to become positively charged (depolarized)
•If threshold depolarization occurs, many more sodium channels will open & the action potential begins.
•The membrane potential rapidly swings up to mV

25. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Depolarization of the Axonal Membrane Na+ ions move inward; membrane potential becomes positive

26. Biology, 9th ed, Sylvia Mader
Action Potential
Chapter 39
Neurons & Nervous Systems
The reversal in polarity causes sodium channels to close & the potassium channels to open.
•K+ ions now move from the inside of the axon to the outside
•As K+ ions leave, the membrane potential changes from +40 mV back to -65 mV.
•Repolarization has occurred.
•The entire action potential takes only two milliseconds

27. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Repolarization of the Axonal Membrane K+ ions rush outside & membrane potential returns to -65 mV

28. Biology, 9th ed, Sylvia Mader
Action Potential
Chapter 39
Neurons & Nervous Systems
As soon as an impulse has occurred at a particular spot on an axon, that region undergoes a short refractory period.
•During this time Na+ gates cannot open yet.
•Thus, that locale is unable to conduct an impulse.
•This ensures that the impulse can only move in one direction.

29. Complete Action Potential Graph
Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Complete Action Potential Graph
Refractory period

30. Propagation of Action Potentials
Biology, 9th ed, Sylvia Mader
Propagation of Action Potentials
Chapter 39
Neurons & Nervous Systems
In unmyelinated fibers, an action potential at one locale stimulates adjacent ions channels to open & continue the action potential along the axon.
In myelinated fibers, an action potential at one node causes an action potential at the next node. This is called Saltatory Conduction.
It is much faster than in unmyelinated fibers:
-100 m/sec versus 1.0 m/sec
The strength of the nerve impulse does NOT decrease with distance traveled along an axon

31. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Unmyelinated neurons

32. Biology, 9th ed, Sylvia Mader
All-or-None Response
Chapter 39
Neurons & Nervous Systems
Conduction of a nerve impulse is an all-or-nothing event
Either a fiber conducts a nerve impulse or it does not.
-If the threshold value is met an action potential will be generated.
-If the threshold value is not met no action potential will occur
•Increased intensity of stimulus above threshold does NOT alter the intensity or speed of impulse produced.
-Intensity of message is determined by how many nerve impulses are generated per second.

33. Biology, 9th ed, Sylvia Mader
A Synapse
Chapter 39
Neurons & Nervous Systems
A synapse is a region where axon terminals of one neuron nearly touch the dendrites or cell bodies of another neuron.
•At a synapse, the membrane of the first neuron is called the presynaptic membrane.
•The membrane of the second neuron is called the postsynaptic membrane.
The small gap between neurons is the synaptic cleft
Electrical nerve impulses cannot cross a synaptic cleft. How do impulses move between neurons?

34. Transmission Across a Synapse
Biology, 9th ed, Sylvia Mader
Transmission Across a Synapse
Chapter 39
Neurons & Nervous Systems
Transmission across a synapse is carried out by molecules called neurotransmitters
•These are stored in synaptic vesicles inside of axon terminal.
When a nerve impulse reaches an axon terminal:
• Gated channels for calcium ions, in the axon terminal membrane, suddenly open
-Ca2+ enters the terminal
Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane
Neurotransmitter molecules are released into the synaptic cleft
-They diffuse across cleft to postsynaptic membrane & bind with specific receptor proteins.

35. Synapse Structure and Function
Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Synapse Structure and Function

36. Synapse Structure and Function
Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Synapse Structure and Function

37. Biology, 9th ed, Sylvia Mader
Chapter 39
Neurons & Nervous Systems
Synapse Function & Games
Click on link to get to a good internet activity

38. Biology, 9th ed, Sylvia Mader
Synaptic Integration
Chapter 39
Neurons & Nervous Systems
A single neuron is on the receiving end of
Many excitatory signals - those that drive neurons closer to an action potential. Have a depolarizing effect on neuron.
Many inhibitory signals - those that drive a neuron further from an action potential. Have a hyperpolarizing effect on neuron.
Neurons integrate these incoming signals •Integration is the summing up of excitatory & inhibitory signals from different synapses or from a rapid firing rate from one synapse.

39. Biology, 9th ed, Sylvia Mader
Synapse Integration
Chapter 39
Neurons & Nervous Systems

40. CNS: Brain and Spinal Cord
Biology, 9th ed, Sylvia Mader
CNS: Brain and Spinal Cord
Chapter 39
Neurons & Nervous Systems
Spinal cord and brain are wrapped in three protective membranes, meninges
Spaces between meninges are filled with cerebrospinal fluid which cushions & protects the CNS
Fluid is continuous with that of central canal of spinal cord and the ventricles of the brain
Meningitis is a bacterial or viral inflammation of the meninges which can be life-threatening.

41. Biology, 9th ed, Sylvia Mader
Spinal Cord
Chapter 39
Neurons & Nervous Systems
The spinal cord has two main functions:
Center for many reflex actions
Means of communication between the brain and spinal nerves
The spinal cord is composed of central gray matter and peripheral white matter
Cell bodies and short unmyelinated fibers give the gray matter its color
Myelinated long fibers of interneurons running in tracts give white matter its color

42. Biology, 9th ed, Sylvia Mader
Chapter 39
The Spinal Cord
Neurons & Nervous Systems

43. Biology, 9th ed, Sylvia Mader
The Brain
Chapter 39
Neurons & Nervous Systems
Cerebrum is the largest portion of the brain in humans
Communicates with, and coordinates the activities of, the other parts of the brain
Carries out higher thought processes required for learning, memory, language & speech
Longitudinal fissure divides into left and right cerebral hemispheres
-Right = artistic & musical ability, emotions, spatial relationships
-Left = mathematics, language, analytical reasoning

44. Biology, 9th ed, Sylvia Mader
The Human Brain
Chapter 39
Neurons & Nervous Systems

45. Biology, 9th ed, Sylvia Mader
Cerebral Cortex
Chapter 39
Neurons & Nervous Systems
A thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres
It contains:
•motor areas - organizes commands to muscles
•sensory areas - where sensory information is processed
•association areas - where integration occurs

46. The Lobes of a Cerebral Hemisphere
Biology, 9th ed, Sylvia Mader
The Lobes of a Cerebral Hemisphere
Chapter 39
Neurons & Nervous Systems

47. Biology, 9th ed, Sylvia Mader
Diencephalon
Chapter 39
Neurons & Nervous Systems
Consists of hypothalamus and thalamus
Hypothalamus is an integration center
- helps maintain homeostasis by regulating:
hunger, sleep, thirst, body temp, water balance
- controls the pituitary gland & is thus a link between nervous & endocrine systems
Thalamus receives all sensory input except smell
- integrates sensory information & sends to cerebrum

48. Biology, 9th ed, Sylvia Mader
Cerebellum
Chapter 39
Neurons & Nervous Systems
Largest part of hindbrain.
Receives sensory input from the eyes, ears, joints, and muscles about position of body parts
Sends motor impulses out the brain stem to the skeletal muscles
Thus, it helps to maintain posture & balance
Ensures smooth, coordinated muscle movements

49. Biology, 9th ed, Sylvia Mader
Brain Stem
Chapter 39
Neurons & Nervous Systems
Contains the midbrain, the pons, and the medulla oblangata
Midbrain
Acts as a relay station for tracts passing between
The cerebrum, and the spinal cord or cerebellum
Pons
Helps regulate breathing and head movements
Medulla oblongata
Contains reflex centers for heartbeat, breathing, & blood pressure, vomiting, coughing, sneezing, hiccuping, and swallowing

50. Biology, 9th ed, Sylvia Mader
Limbic System
Chapter 39
Neurons & Nervous Systems
Complex network of tracts and “nuclei”
Integrates higher mental functions and primitive emotions into a united whole
Accounts for why certain activities are deemed as pleasurable: eating, sex, etc.

51. Biology, 9th ed, Sylvia Mader
The Limbic System
Chapter 39
Neurons & Nervous Systems

52. Peripheral Nervous System
Biology, 9th ed, Sylvia Mader
Peripheral Nervous System
Chapter 39
Neurons & Nervous Systems
The peripheral nervous system lies outside the CNS and contains nerves, bundles of axons.
Two types of nerves:
•Cranial nerves
-12 pairs attached to brain.
-Concerned with head, neck & facial region
•Spinal nerves
-31 pairs; one from each vertebrae
-Dorsal root contains axons of sensory neurons
-Ventral root contains axons of motor neurons

53. Cranial and Spinal Nerves
Biology, 9th ed, Sylvia Mader
Cranial and Spinal Nerves
Chapter 39
Neurons & Nervous Systems

54. Biology, 9th ed, Sylvia Mader
The Reflex Arc
Chapter 39
Neurons & Nervous Systems
Spinal Reflex
Involves only the spinal cord
Sensory receptors activated (touch a sharp pin)
Nerve impulses move along sensory axons toward spinal cord
•Sensory neurons enter cord dorsally & pass signals to interneurons
•Interneurons synapse with motor neurons
•Motor neuron axons leave cord ventrally & carry impulse to an effector
-Effectors bring about a response (muscle contracts)

55. A Reflex Arc Showing the Path of a Spinal Reflex
Biology, 9th ed, Sylvia Mader
A Reflex Arc Showing the Path of a Spinal Reflex
Chapter 39
Neurons & Nervous Systems

56. Peripheral Nervous System
Biology, 9th ed, Sylvia Mader
Peripheral Nervous System
Chapter 39
Neurons & Nervous Systems
Autonomic system
Controls the smooth muscles, cardiac muscles, and glands
Usually unaware of its actions
Divided into two divisions
Sympathetic division
Parasympathetic division

57. Sympathetic and Parasympathetic Divisions
Biology, 9th ed, Sylvia Mader
Sympathetic and Parasympathetic Divisions
Chapter 39
Neurons & Nervous Systems
Sympathetic division
Especially important during fight or flight responses during emergencies
Accelerates heartbeat and dilates bronchi
Inhibits digestive tract
Neurotransmitter is norepinephrine (NE)
Parasympathetic division
Promotes all internal responses associated with a relaxed state
Promotes digestion and retards heartbeat
Neurotransmitter is acetylcholine (ACh)

58. Autonomic System Structure and Function
Biology, 9th ed, Sylvia Mader
Autonomic System Structure and Function
Chapter 39
Neurons & Nervous Systems