The Nervous System

1. The Nervous System Chapter 13

2. Neurons • Basic units of communication in nearly all nervous systems • Monitor information in and around the body and issue commands for responsive actions

3. Three Classes of Neurons • Sensory neurons • Interneurons • Motor neurons

4. Neuroglia • Make up more than half the volume of the vertebrate nervous system • A variety of cells that metabolically assist, structurally support, and protect the neurons

5. Structure of a Neuron dendrites INPUT ZONE cell body axon OUPUT ZONE TRIGGER ZONE CONDUCTING ZONE axon endings

6. Resting Potential • Charge difference across the plasma membrane of a neuron • Fluid just outside cell is more negatively charged than fluid inside • Potential is measured in millivolts • Resting potential is usually about -70mv

7. Maintaining Resting Potential Na+ pumped out K+ leaks out Na+ leaks in K+ pumped in K+ and Na+ can’t diffuse across bilayer

8. Ion Concentrations at Resting Potential • Potassium (K+) • Concentration inside the neuron is higher than the concentration outside • Sodium (Na+) • Concentration inside the neuron is lower than the concentration outside

9. Action Potential • A transitory reversal in membrane potential • Voltage change causes voltage-gated channels in the membrane to open • As a result of ion flow through these channels, the inside of neuron briefly becomes more positive than outside

10. Action Potential (1) interstitial fluid cytoplasm Electrical disturbance arrives from input zone

11. Action Potential (2) Na+ Na+ Na+ Opening of sodium gates

12. Action Potential (3) K+ K+ K+ Na+ Na+ Na+ Opening of potassium gates

13. Action Potential (4) K+ K+ K+ K+ Na+ Na+ Na+ Sodium-potassium pump restores gradients

14. Positive Feedback more Na+ ions flow into the neuron more gated channels for Na+ open neuron becomes more positive inside

15. All or Nothing • All action potentials are the same size • If stimulation is below threshold level, no action potential occurs • If it is above threshold level, cell is always depolarized to the same level

16. Repolarization • Once peak depolarization is reached, Na+ gates close and K+ gates open • Movement of K+ out of cell repolarizes the cell • The inside of the cell once again becomes more negative than the outside

17. Recording of Action Potential +20 0 -20 Membrane potential (millivolts) threshold -40 resting membrane potential -70 5 0 2 3 1 4 Time (milliseconds)

18. Propagation of Action Potentials • An action potential in one part of an axon brings a neighboring region to threshold • Action potential occurs in one patch of membrane after another

19. Chemical Synapse • Gap between the terminal ending of an axon and the input zone of another cell plasma membrane of axon ending of presynapic cell synaptic vesicle plasma membrane of postsynapic cell synaptic cleft membrane receptor

20. Synaptic Transmission (1) • Action potential in axon ending of presynaptic cell causes voltage-gated calcium channels to open • Flow of calcium into presynaptic cell causes release of neurotransmitter into synaptic cleft

21. Synaptic Transmission (2) • Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell • Binding of neurotransmitter to receptors opens ion channels in the membrane of postsynaptic cell

22. Synaptic Integration what action potential spiking would look like threshold -65 Membrane potential (milliseconds) EPSP integrated potential resting membrane potential -70 IPSP -75

23. Nerves axon • A nerve is a bundle of axons enclosed within a connective tissue sheath • Permit long-distance communication between the brain or spinal cord and the rest of the body myelin sheath

24. Myelin Sheath • A series of Schwann cells • Sheath blocks ion movements • Action potential must “jump” from node to node

25. Reflexes • Automatic movements made in response to stimuli • In the simplest reflex arcs, sensory neurons synapse directly on motor neurons • Most reflexes involve an interneuron

26. Stretch Reflex Stimulus Biceps stretches. Response Biceps contracts.

27. Central Nervous System Divisions of the Nervous System brain spinal cord sensory nerves axons of motor nerves somatic subdivision (motor functions) autonomic subdivision (visceral functions) parasympathetic sympathetic

28. Central and Peripheral Nervous Systems • Central nervous system (CNS) • Brain • Spinal cord • Peripheral nervous system • Nerves that thread through the body

29. Peripheral Nervous System • Somatic nerves • Motor functions • Carry signals to and from skeletal muscle, tendons, and skin • Autonomic nerves • Visceral functions • Carry signals to and from internal organs and glands

30. Two Types of Autonomic Nerves • Sympathetic nerves • Parasympathetic nerves • Most organs receive input from both • Usually have opposite effects on organ

31. Sympathetic Nerves • Originate in the thoracic and lumbar regions of the spinal cord • Ganglia are near the spinal cord • Promote responses that prepare the body for stress or physical activity (fight-or-flight response)

32. Parasympathetic Nerves • Originate in the brain and the sacral region of the spinal cord • Ganglia are in walls of organs • Promote housekeeping responses, such as digestion

33. Both Systems Are Usually Active • Most organs are continually receiving both sympathetic and parasympathetic stimulation • For example, sympathetic nerves signal heart to speed up, and parasympathetic stimulate it to slow down • Which dominates depends on situation

34. Function of the Spinal Cord • Expressway for signals between brain and peripheral nerves • Sensory and motor neurons make direct reflex connections in the spinal cord • Spinal reflexes do not involve the brain

35. Structure of the Spinal Cord spinal cord ganglion meninges (protective coverings) nerve vertebra

36. Three Brain Divisions • Hindbrain • Midbrain • Forebrain

37. Hindbrain • Medulla oblongata • Cerebellum • Pons

38. Midbrain • Coordinates reflex responses to sight and sounds • Roof of midbrain is the tectum

39. Forebrain • Olfactory lobes • Cerebrum • Thalamus • Hypothalamus and pituitary gland

40. Reticular Formation • Mesh of interneurons extends from top of spinal cord, through brain stem, and into higher integrating centers of cerebral cortex

41. Cerebrospinal Fluid • Surrounds the spinal cord • Fills ventricles within the brain • Blood-brain barrier controls which solutes enter the cerebrospinal fluid

42. Anatomy of the Cerebrum • Largest and most complex part of human brain • Outer layer (cerebral cortex) is highly folded • A longitudinal fissure divides cerebrum into left and right hemispheres

43. Lobes of the Cerebrum Primary somatosensory cortex Primary motor cortex Parietal Frontal Occipital Temporal

44. Limbic System • Controls emotions and has role in memory

45. Memory • Brain’s capacity to store and retrieve information about past sensory input • Stored in stages • Temporary storage in cerebral cortex • Short-term memory • Long-term memory

46. Separate Memory Circuits • Fact memory • Skill memory

47. States of Consciousness • EEGs are electrical recordings of the summed frequency and strength of membrane potentials at the surface of the brain • Part of the reticular formation promotes chemical changes that influence alertness

48. Nervous System Disorders • Meningitis • Encephalitis • Concussion • Epilepsy • Multiple sclerosis • Alzheimer’s disease • Parkinson’s disease

49. Drugs and Addiction • A drug is a substance introduced into the body to provoke a specific physiological response • In addiction, a drug assumes an “essential” biochemical role in the body

50. Stimulants • Increase alertness and body activity, then cause depression • Caffeine • Nicotine - mimics acetylcholine • Cocaine - blocks reabsorption of neurotransmitters • Amphetamines - induces dopamine release