1. The Biological Bases of Behavior: The Neuron What is the nervous system?

2. Nervous system  Is a complex communication network in which signals are constantly being transmitted, received and integrated.  It handles information to and from the different parts of your body.  The part of the nervous system located in the skull is referred to as the brain; that found in the spine is called the spinal cord. The brain and the spinal cord are continuous through an opening in the base of the skull; both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system.

4. The Neuron  Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them. Glial cellsGlial cells

5. Nervous Tissue  Glia cells are the glue that provide structural support, nourishment and insulation for neurons.  Neurons are individual cells in the nervous system that receive, integrate and transmit information.

6. Structure of the Neuron  The cell body or soma is the part that contains the cell nucleus and much of the chemical machinery common to most cells.  Dendrites are the parts of a neuron that look like trees and are specialized to receive information. Most receive information from MANY cells, sometimes thousands so they have extensive dentritic systems.

8. The structure: continued  The axon is a long thin fiber that transmits signals away from the soma to other neurons or to muscles or glands. They can be several feet long and can branch off to communicate with a number of other cells.  They are wrapped in a myelin sheath, or a fatty white substance called myelin. It is an insulating material, derived from glia cells that encases the axons.  It speeds up the transmission of signals that move along the axon. Without the sheath, signals are not sent effectively and there is a loss of muscle control (Multiple Sclerosis).

9. And more still…. The terminal buttons are small knobs located at the end of the neuron and are responsible for sending the signal on to other neurons through the secretion of chemicals called neurotransmitters. At the end of the terminal button is a gap known as a synapse. This is a junction where information is transferred from one cell to another. Neurotransmitters are used to carry the signal across the synapse to other neurons.

10. The Neural Impulse  Hodgkin and Huxley (1952) learned that neural impulses are complex electrochemical reactions.  Inside and outside of the neuron are electrically charged atoms and molecules called ions.  While the flow of positive and negative molecules goes through the membrane, they do not move at the same rate, leaving a negative charge in the cell interior. At rest, the cell is like a battery, a source of potential energy  The resting potential of a neuron is its stable, negative charge when the cell is inactive

11. The Action Potential As long as the voltage remains the came the cell is quiet with no messages being sent. Once the neuron is stimulated, channels in the cell membrane open allowing positively charged ions to rush in. For an instant, the neuron is less negative, or even positive, creating an action potential. An action potential is a very brief shift in a neuron’s electrical charge that travels along an axon.

12. Absolute Refractory Period During an action potential, a second stimulus will not produce a second action potential (no matter how strong that stimulus is) During an action potential, a second stimulus will not produce a second action potential (no matter how strong that stimulus is) The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin. The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin. The relative refractory period is the time after the absolute period that the neuron can fire but its threshold for firing is elevated, requiring a more intense stimulation to initiate an action potential. The relative refractory period is the time after the absolute period that the neuron can fire but its threshold for firing is elevated, requiring a more intense stimulation to initiate an action potential.

13. All-or-None Law  Even though action potential are an all or nothing event, neurons can convey information about the strength of a stimulus.  They do so by varying the rate at which they fire action potentials.

14. Sending Signals  Two neurons do not actually touch. They are separated by the synaptic cleft, a microscopic gap between the terminal button of one neuron and the cell membrane of another neuron.  The arrival of an action potential at an axon’s terminal buttons triggers the release of neurotransmitters- chemicals that transmit information from one neuron to another.  The chemicals are stored in small sacs called synaptic vesicles.  The chemicals are released when a vesicle fuses with the membrane of the presynaptic cell and its contents spill into the synaptic cleft.  Once released, the neurotransmitters diffuse across the synaptic cleft to the membrane of the receiving cell.

15. Receiving Signals  When the neurotransmitter and a receptor molecule combine, the cell reaction is call a postsynaptic potential (PSP), a voltage change at a receptor site on a postsynaptic cell membrane.  These do not follow an all-or-none law as action potentials do. Instead they are graded, or vary in size and they increase or decrease the probability of a neural impulse in the receiving cell in proportion to the amount of voltage change.  Two types of messages can be sent from cell to cell: excitatory and inhibitory

16. Postsynaptic Potentials  An excitatory PSP is a positive voltage shift that increase the likelihood that the postsynaptic neuron will fire action potentials.  An inhibitory PSP is a negative voltage shift that decreases the likelihood that the postsynaptic neuron will fire action potentials.

17.  The direction, excitatory or inhibitory, depends on which receptor sites are activated in the postsynaptic neuron. The effects last only a fraction of a second, then the neurotransmitters drift away from the receptor sites or are deactivated by enzymes that metabolize (convert) them into inactive forms.  Most are reabsorbed into the presynaptic neuron through reuptake, a process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane, a type of recycling of the bodies building blocks.

18. Neurotransmitters  Acetylcholine: (Ach) Acetylcholine is particularly important in the stimulation of muscle tissue. Contributes the regulation of attention, arousal and memory. The poison curare blocks transmission of acetylcholine. Some nerve gases inhibit the breakdown of acetylcholine, producing a continuous stimulation of the receptor cells, and spasms of muscles such as the heart.  Norepinephrine: (NE) This compound is secreted principally from the adrenal gland. Contributes to the modulation of mood and arousal. Cocaine and amphetamines elevate activity at the NE synapses.  Dopamine: (DA) Dopamine facilitates critical brain functions and voluntary movement, pleasurable emotions and, when unusual quantities are present, abnormal dopamine neurotransmission may play a role in Parkinson's disease (decreased levels), certain addictions, and schizophrenia (over activity). Cocaine and amphetamines elevate activity at the DA synapses. Parkinson's disease Parkinson's disease

19.  Serotonin: Involved in regulation of sleep and wakefulness, eating aggression. Serotonin is assumed to play a biochemical role in mood and mood disorders, including anxiety, depression, and bipolar disorder. Prozac and similar antidepressant drugs affect serotonin circuits. depression  GABA: Serves as widely distributed inhibitory transmitter. Valium and similar anti-anxiety drugs work at GABA sites.  Endorphins: Resemble opiate drugs in structure and effects. Contribute tot pain relief and perhaps to some pleasurable emotions. Neurotransmitters Continued

20. Agonists and Antagonists  An agonist is a chemical that mimics the action of a neurotransmitter  i,.e.: Nicotine is an agonist for Ach and binds to the receptor site, fooling the body that Ach has been secreted and bound.  An antagonist is a chemical that opposes the action a a neurotransmitter.  For example: The drug curare is an antagonist for Ach and binds to the receptor site, blocking the action so the person cannot move.