Explaining behavior at the level of the neuron

1. Explaining behavior at the level of the neuron • Your brain and the rest of your nervous system is made up of neurons. • Neurons are brain cells • All neurons are separated from one another, but communicate electrochemically.

2. The neuron • The neuron consists of three parts • The cell body - contains the nucleus and much of the machinery that keeps a neuron alive and working. • The dendrites - widely branching structures that receive transmissions from other neurons. • The axon - a single, long, thin, straight fiber with branches near its tip

4. Myelin sheath • The axon is coated in an insulating substance known as Myelin. • Myelin allows for faster transmission of impulses along an axon. • Myelin has breaks in it known as the Nodes of Ranvier • Once an impulse reaches the end of an axon (the terminal buttons), molecules are released that can either excite or inhibit the receiving cell.

6. Resting Potential • Normally there is an electrical polarization across the membrane of an axon. • This means that there is a negative charge on the inside of the cell and a positive charge on the outside. • At resting potential the inside of the neuron is at -70 millivolts.

7. Four Factors Determine the Ionic Distribution That Underlies the Resting Potential • Differential Permeability of the Membrane • The Sodium/Potassium Pump • Diffusion • Electrostatic Pressure

8. Differential Permeability of the Membrane • Ions pass through membrane at special pores called ion channels • When neurons are at rest, the membrane is: • extremely resistant to the passage of Sodium (Na+) ions • only slightly resistant to the passage of Potassium (K+) ions and Chloride (Cl-) ions

9. The sodium potassium pump • There are little pumps that pump Sodium (NA+) out of the cell, and potassium (K+) in. • The pumps move 3 Sodium molecules out for every 2 potassium molecules they move in • Sodium and potassium both have a +1 charge • more Sodium is being moved out than potassium is being moved in • the build up of Sodium on the outside of the membrane makes it positive and the inside negative.

10. Diffusion also known as -Random Motion • Ions in solution are in random motion • Thus, any time that there is an accumulation of a particular class of ions in one area, • the probability is increased that random motion will move ions out of this area (because there are more ions available to leave) • the probability is decreased that random motion will move more ions into the area (because there are fewer ions available to come in)

11. Electrostatic Pressure • Like charges repel and opposite charges attract • Therefore electrostatic pressure disperses any accumulation of positive or negative charges in an area

13. Generation of Action Potentials • action potentials (APs; neuron firing) are triggered at the axon hillock when a neuron is depolarized to the point that the membrane potential at the axon hillock reaches about -65 mV • this is the threshold of excitation for many neurons • they are all-or-none (they occur full blown or not at all)

14. How does an impulse travel down an axon? • The action potential is an excitation that travels along an axon at a constant strength, no matter how far it must travel. • It is slower then a straight electrical impulse, but has the advantage of maintaining it’s strength no matter how far it must travel.

15. Travel of the action potential • When a neuron fires, certain gates open up that allow Sodium to flow in. • When sodium flows in the electrical charge is neutralized across the membrane. • Then the sodium channels close, and potassium channels open, allowing potassium to leave the cell. • This returns the cell to it’s resting potential (-70 mv).

18. Travel of the action potential. • The axon only has these Sodium gates at breaks in the myelin sheath called the Nodes of Ranvier. • The sodium gates are voltage dependent - that is they open up when the voltage across the membrane drops • Thus, the action potential moves like a wave • jumping from one Node of Ranvier to the next down to the end of the axon. • Saltatory Conduction

20. What happens when an action potential reaches the end of an axon? • The end of an axon has several branching areas called the terminal buttons • Each edge of the terminal button is called the presynaptic membrane. • The presynaptic membrane is separated from the other neuron by what is called the synaptic cleft.

22. The Synapse • The axon that has fired releases a chemical into the synaptic cleft. • This chemical crosses the gap and binds to what is called the postsynaptic membrane. • The chemicals are called neurotransmitters. • They bind at the postsynaptic membrane at what are called receptors.

25. Neurotransmitters • There are many types of neurotransmitters • Acetylcholine • Serotonin • Dopamine

26. Dopamine • Dopamine is one neurotransmitter that has been associated with many neurological disorders • Parkinson’s disease. • Muhammed Ali • Schizophrenia • Sometimes these diseases can be treated by increasing dopamine levels in the brain

27. Behavior and the Nervous System • Psychologists distinguish between the central nervous system and the peripheral nervous system. • Central nervous system consists of the brain and the spinal cord • Peripheral nervous system is composed of bundles of axons between the spinal cord and the rest of the body.

28. Peripheral nervous system • The peripheral system can be further divided • Somatic nervous system = nerves that communicate with the skin and muscles. • Autonomic nervous system = nerves that communicate with the heart, stomach, and other organs.

29. Autonomic nervous system • The autonomic nervous system is a system that we do not have as much control over. • It largely controls things we wouldn’t want to have to think about • breathing • heart rate

30. Divisions of the autonomic nervous system • The sympathetic system - controls fight or flight - increases heart and breathing rate. • The parasympathetic system - decreases heart rate, controls digestion, basically runs the body during normal functioning.

32. Organization and functioning of the brain • The cerebral cortex - the outer surface of the brain. The wrinkled area. • Right and left hemisphere • crosses over - communicates via the corpus callosum

33. The four lobes of the cerebral cortex • Frontal Lobe - thought to be involved in planning and working memory • primary motor cortex • Parietal Lobe - body sensations • primary somatosensory cortex • Occipital lobe - vision • Temporal lobe - hearing - advanced visual processing - emotion

35. What if we cut the corpus callosum? • Travel from the eye to the brain is divided. • Information from the left side of each eye travels to the left hemisphere of the brain • Information from the right side of each eye travels to the right hemisphere of the brain.

36. Hemisphere division continued • The left hemisphere of the brain controls speech for most people • Most people are only able to describe information that reaches the left hemisphere. • However information that reaches the right hemisphere quickly crosses the corpus callosum to the left hemisphere, so that it can be described verbally.

37. What if the corpus callosum is cut • In one experiment researchers showed a woman with a severed corpus callosum a picture of a naked woman to the left field of vision = to the right of her face • When asked what she had seen, she laughed and said a nude picture.

38. Continued. • When the nude picture was shown only in her right field of vision (left of her face), she laughed and turned a little red in the face. • When asked why she was laughing, she said I don’t know, oh that silly machine. • The right side of the brain knew what it saw and caused her to laugh, the left side of the brain heard the laughter, and tried to interpret why it occurred.

39. continued • If she had been allowed to point at several alternatives with her left hand (controlled by the right hemisphere), she would have been able to correctly point at the picture she had seen, even though she would say that she didn’t know what she had seen.

40. Unified consciousness? • We all experience a unified consciousness. • That is, we experience a single self. • The split brain experiments show that that unified consciousness depends on the two hemispheres being able to communicate. • If the corpus callosum is severed then each hemisphere begins to act and experience things independently of the other.