Neurons: The Matter of the Mind

1. Chapter 7 Neurons: The Matter of the Mind Lecture Presentation Betty McGuireCornell University

2. Neurons: The Matter of the Mind • Cells of the nervous system • Structure of neurons • Nerve impulses • Synaptic transmission

3. Cells of the Nervous System • Overview of the nervous system • Function • Integrates and coordinates the body’s activities • Divisions • Central nervous system (CNS) • Brain and spinal cord • Peripheral nervous system (PNS) • All of the nervous tissue outside the brain and spinal cord

4. Cells of the Nervous System • The nervous system is composed of two types of specialized cells • Neurons • Neuroglial cells

5. Cells of the Nervous System • Neurons (nerve cells) • Excitable cells that generate and transmit messages • Neuroglial cells (glial cells) • Outnumber neurons 10 to 1 • Several types, each with a specific job • Provide structural support, growth factors, and insulating sheaths around axons

6. Cells of the Nervous System • Three categories of neurons • Sensory (or afferent) neurons • Motor (or efferent) neurons • Interneurons (or association neurons)

7. Cells of the Nervous System • Sensory neurons • Carry information toward the CNS from sensory receptors • Motor neurons • Carry information away from the CNS to an effector (muscle or gland)

8. Cells of the Nervous System • Interneurons • Located between sensory and motor neurons within the CNS • Found only in the brain and spinal cord • Integrate and interpret sensory signals • Account for more than 99% of the body’s neurons

9. Sensory receptor for pain Muscle (effector) Impulse direction Cell body Motor neuron Sensory neuron Interneuron

10. Structure of Neurons • The shape of a typical neuron is specialized for communicating with other cells • Parts of a neuron • Dendrites = many short, branching projections • Axon = a single long extension • Cell body • Contains nucleus and other organelles • Functions to maintain the neuron

11. Structure of Neurons • Dendrites • Receive signals from other cells • Carry information toward the cell body of a neuron • Axon • Carries information away from the cell body to either another neuron or an effector

12. Dendrites receive information from other neurons or from the environment. The cell body controls the cell‘s metabolic activities. Axon endings release chemicals called neurotransmitters that affect the activity of nearby neurons or an effector (muscle or gland). Nucleus Cell body The cell body integrates input from other neurons. An axon conducts the nerve impulse away from the cell body. Axon endings Receiving portion of neuron Sending portion of neuron

13. Structure of Neurons • Nerves • Consist of parallel axons, dendrites, or both from many neurons • Covered with tough connective tissue • Classified as sensory, motor, or mixed (sensory and motor together) depending on the type of neurons they contain

14. Structure of Neurons • Myelin sheath • Found on most axons outside the CNS and some of those within the CNS • Provides electrical insulation that increases the rate of conduction of a nerve impulse • Composed of the plasma membranes of glial cells

15. Structure of Neurons • Myelin sheath (cont.) • In the PNS, Schwann cells (a type of glial cell) form the myelin sheath • Gaps between adjacent Schwann cells are called nodes of Ranvier • Messages travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction

16. Nucleus Dendrites Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next. Node of Ranvier Schwann cell (a) Myelin sheath (c)Myelin sheath surrounding cut end of axon Schwann cell Axon (b) Nucleus Myelin sheath Nucleus of Schwann cell

17. Nucleus Dendrites Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next. Node of Ranvier Schwann cell (a) Myelin sheath

18. Schwann cell Axon (b) Nucleus Nucleus of Schwann cell Myelin sheath

19. (c) Myelin sheath surrounding cut end of axon

20. Structure of Neurons • Multiple sclerosis (MS) • Results from the destruction of the myelin sheath that surrounds axons in the CNS • The resulting scars (scleroses) interfere with the transmission of nerve impulses • Can result in paralysis and loss of sensation, including loss of vision

21. Structure of Neurons Web Activity: Myelinated Neurons and Saltatory Conduction

22. Nerve Impulses • A nerve impulse, or action potential, is an electrochemical signal involving sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through ion channels • Ions pass through channels without using cellular energy

23. Nerve Impulses • Each ion channel is designed to allow only certain ions to pass through it • Sodium channels permit sodium ions to pass • Potassium channels permit potassium ions to pass • Ion channels may be permanently open or regulated by a “gate,” which is a protein that changes shape and opens or closes a channel

24. Nerve Impulses • Ions also are transported across the membrane by the sodium-potassium pump • Special proteins in the cell membrane that actively transport sodium and potassium ions across the membrane • These pumps use cellular energy to eject sodium ions from within the cell and to bring potassium ions into the cell

25. Nerve Impulses • When a neuron is not conducting a nerve impulse, it is in a resting state • There is a slight difference in charge across the membrane, which is called the resting potential • The inner surface of the membrane is about 70 mV more negative than the outer surface • There are more sodium ions outside the membrane than inside • There are more potassium ions inside the membrane than outside • This state is maintained by the sodium-potassium pump

26. Nerve Impulses • When the neuron is stimulated, there is a sudden reversal of charge across the membrane because the sodium gates open and sodium ions enter the cell • Threshold • Minimum charge that causes the sodium gates to open • Depolarization • Reduction of the charge difference across the membrane

27. Nerve Impulses • Next, the potassium gates open and potassium ions rush out of the cell • This causes the cell to return to its original state (i.e., for the interior of the neuron to become more negative relative to the outside) • Repolarization • Restoration of the charge difference across the membrane

28. Nerve Impulses • Action potential • Sudden reversal of the charge across the membrane followed immediately by its restoration • These changes occur in a wave along the axon

29. Resting Neuron Plasma membrane is charged, with the inside negative relative to the outside. Neuron cytoplasm Cytoplasm Plasma membrane Step 1: The loss of the charge difference across the membrane (depolarization) occurs as sodium ions (Na+) enter the axon. Action Potential The charge difference across the membrane reverses and then is restored. Na+ Cytoplasm Na+ flows inward Step 2: The return of the membrane potential to near its resting value (repolarization) occurs as potassium (K+) ions leave the axon. K+ Na+ Cytoplasm K+ K+ flows outward Restoration of Original Ion Distribution The sodium-potassium pump restores the original distribution of ions. K+ Na+ Cytoplasm K+ Na-K pump restores the original ion distribution

30. Action Potential Restoration of Original Ion Distribution Resting Neuron +30 Step 1: Sodium ions enter neuron. Step 2: Potassium ions leave neuron. 0 Membrane potential Sodium-potassium pump is active. Threshold Resting potential –70 0 1 2 3 4 5 Time (milliseconds)

31. Nerve Impulses • An action potential • Does not diminish, once started • Does not vary in intensity with the strength of the stimulus that triggered it • Is “all-or-nothing”

32. Nerve Impulses • For a very brief period following an action potential, the neuron cannot be stimulated again • This is called the refractory period • It occurs because the sodium channels are closed and cannot be reopened

33. Nerve Impulses Web Activity: Nerve Impulse

34. Synaptic Transmission • Communication between a neuron and an adjacent cell occurs by neurotransmitters • Synapse • Junction between a neuron and another cell • Synaptic cleft • Gap between two cells • Neurotransmitters diffuse across the gap

35. Synaptic Transmission • In the case of two neurons, the presynaptic neuron sends a message to the postsynaptic neuron • Synaptic knob • Swelling at the end of the axon of the presynaptic neuron

36. Plasma membrane of an axon ending of a sending (presynaptic) neuron Synaptic vesicle Synaptic knob Synaptic cleft Plasma membrane of a receiving (postsynaptic) neuron Ion channel Receptor for neurotransmitter (a) Axon of presynaptic neuron Synaptic vesicles (b)

37. Plasma membrane of an axon ending of a sending (presynaptic) neuron Synaptic vesicle Synaptic knob Synaptic cleft Plasma membrane of a receiving (postsynaptic) neuron Receptor for neurotransmitter Ion channel (a)

38. Axon of presynaptic neuron Synaptic vesicles (b)

39. Synaptic Transmission • Specific steps • The nerve impulse reaches the synaptic knob of the presynaptic neuron • The synaptic knob releases neurotransmitter into the synaptic cleft • Prompted by calcium ions moving into the knob • Membranes of synaptic vesicles (packets of neurotransmitter) fuse with plasma membrane at the synaptic knob, spilling contents into the cleft • The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the membrane of the postsynaptic neuron, causing an ion channel to open

40. Synaptic Transmission • At an excitatory synapse, binding of the neurotransmitter to the receptor causes sodium channels to open • Sodium ions enter the postsynaptic cell, increasing the likelihood that an action potential will begin

41. Dendrites Nucleus Axon Impulse Cell body Impulse Step 1:The impulse reaches the axon ending of the presynaptic membrane. Synaptic knob Step 2:Synaptic vesicles release neurotransmitter into the synaptic cleft. Synaptic cleft Synaptic vesicle Membrane of postsynaptic neuron Neurotransmitter Synaptic vesicle Step 3:Neurotransmitter diffuses across synaptic cleft. Receptor (of sodium ion channel) on postsynaptic membrane Step 4: Neurotransmitter molecules bind to receptors on the postsynaptic neuron. Step 5:Sodium ion channels open. Step 6:Sodium ions enter the Postsynaptic neuron, causing Depolarization and possible action potential.

42. Synaptic Transmission • At an inhibitory synapse, binding of the neurotransmitter to the receptor opens different ion channels • The postsynaptic cell’s interior becomes more negatively charged than usual, reducing the likelihood that an action potential will begin

43. Synaptic Transmission • A neuron may have as many as 10,000 synapses with other neurons at the same time • Some synapses have excitatory effects and some have inhibitory effects • Summation • Combined effects of excitatory and inhibitory effects at any given moment • Determines whether an action potential is generated • This level of integration provides fine control over neuronal responses

44. Myelin sheath Receiving cell body Excitatory synapse Inhibitory synapse Axon

45. Synaptic Transmission • Neurotransmitters have temporary effects • Once released into a synapse, neurotransmitters are quickly removed • Some are deactivated by enzymes • The enzyme acetylcholinesterase removes acetylcholine from synapses • Others are pumped back into the synaptic knob of the presynaptic axon

46. Synaptic Transmission Web Activity: The Synapse

47. Synaptic Transmission • There are dozens of neurotransmitters • Some neurotransmitters produce different effects on different types of cells • Example:Acetylcholine • Acts in both the PNS and the CNS • Released at every neuromuscular junction • Myasthenia gravis • Autoimmune disease that attacks the acetylcholine receptors at neuromuscular junctions, resulting in little muscle strength

48. Synaptic Transmission • In the CNS, different neurotransmitters are associated with different behavioral systems • Norepinephrine regulates mood, hunger, thirst, and sex drive • Serotonin promotes a feeling of well-being • Dopamine regulates emotions and complex movements

49. Synaptic Transmission • Changes in the levels of neurotransmitters cause disorders • Alzheimer’s disease • Associated with decreased levels of acetylcholine • Clinical depression • Associated with decreased levels of serotonin, dopamine, and norepinephrine • Parkinson’s disease • Associated with decreased levels of dopamine