Physiological Basis of Behaviour Psychology 100 Josée L. Jarry, Ph.D., C.Psych. Department of Psychology University of Toronto May 14, 2003

Nervous system Coordinates and directs the body’s action in the environment Receives information about the environment Organises and integrates information with already stored information Uses integrated information to send messages to the body to respond Provides the basis for conscious experience

Central and Peripheral Nervous Systems Central nervous system Brain and spinal cord Peripheral nervous system Nerves extending from the central nervous system Nerves bundles of axons of many neurons that connect the central nervous system to the body’s muscles, glands, and sensory organs.

The Neuron (1) Carry information rapidly Integrate information from various sources Cell body Contains the cell nucleus and all the basic structures common to all cells. Dendrites Thin extensions branching out from the cell body Increase the surface of the cell to facilitate receipt of signals from other neurons

The Neuron (2) Axon Thin, tube like extension Carries signals from the cell body to other cells Branches into the terminal arborisation Each branch ends with a small swelling called the axon terminal or terminal button Axon terminals release chemical substances onto other cells, either other neurons, muscle cells, or glands

Myelin sheet Many neurons are surrounded by a myelin sheath Casing made of the membranes of fatty cells wrapped tightly around the axon It is designed to insulate the neuron such that its electrical impulses are not lost before reaching the axon terminal Forms a tube made of segments with small gaps called nodes de Ranvier.

Three functional classes of neurons (1) Sensory neurons Carry information from the sensory organs to the central nervous system. 2 or 3 million Motor neurons Carry information from the central nervous system to the body’s muscles and glands

Three functional classes of neurons (2) Interneurons Within the central nervous system Relay information from one set of neurons to the other Organise and integrate information 100 billion to a trillion

Glial cells Are the cushioning of the nervous system Constitute half of the volume of the central nervous system Several functions myelin sheet surround blood vessels in the brain to protect the neurons from certain toxic substances clean up waste products and dead neurons provide nutrients to neurons.

Neural impulse or Action Potential signals or messages sent down the axon electrical bursts that starts at one end of the axon and travel in one direction only toward the axon terminals All or nothing an action potential is always the same strength or magnitude retains its full strength all the way down the axon

Molecular Basis of the Action Potential Intracellular fluid water solution inside of the cell Extracellular fluid water solution that bathes the outside of the cell It is the flow of these fluids through the membrane that produces the action potential.

Chemicals in the Intra and Extra cellular fluid Protein anion(-) large and trapped in the intracellular fluid Potassium (K+) more concentrated in the intracellular fluid than in the extracellular fluid Sodium (Na+) more concentrated in the extracellular fluid Chloride (Cl-)

Resting potential Sodium-potassium pumps constantly pump sodium outside of the neuron, and potassium inside the neuron Sodium channels prevent sodium from entering the membrane Potassium channels prevent potassium from flowing out of the membrane The balance of the ions on both sides of the membrane results in a negative charge inside the cell of approximately -70 millivolts

Diffusion & Electrostatic Pressure Resting potential Na+ is attracted inside the cell by the forces of diffusion and electrostatic pressure K+ is attracted outside the cell by the forces of diffusion but not electrostatic pressure Action Potential Na+ rushes inside the cell by the forces of diffusion and electrostatic pressure K+ rushes outside the cell by the forces of diffusion and electrostatic pressure

Depolarization & Repolarization Depolarization phase Na+ channels open causing sodium to rush inside The electrical charge across the membrane reverses itself The inside of the cell becomes temporarily positive Repolarization phase Na+ channels close, and K+ channels open causing potassium to rush out the sodium-potassium pumps then proceed to move potassium back inside and sodium outside the cell.

Conduction of the Action Potential The action potential triggers the opening of the sodium channels in the area just ahead of it In myelinated axons, the action potential regenerates itself at the nodes de Ranvier Between nodes, travels by passive cable properties Allows faster conduction with less energy expenditure

Synaptic transmission (1) Synaptic cleft Space that separates two neurons Presynaptic membrane The membrane of the neuron from which the signal arrives Postsynaptic membrane The membrane of the neuron that receives the signal

Synaptic transmission (2) Vesicles Tiny globes containing a chemical substance called neurotransmitter Neurotransmitter Released by the presynaptic neuron in the synaptic cleft Transmits signals to the postsynaptic neuron

Excitatory & Inhibitory Synapses Excitatory synapse increases the rate of firing of the postsynaptic neuron transmitter causes the sodium gates to open, allowing sodium to enter the cell Inhibitory synapse decreases the rate of firing of the postsynaptic neuron transmitter causes potassium to leave the cell or Cl- to enter the cell makes the inside even more negative makes the neuron less susceptible of firing.

Integration of Excitatory & Inhibitory Inputs Depolarizations and hyperpolarizations spread passively across the cell body Affect the electrical charge at the junction of the cell body and the axon If reaches critical value, the axon will increase its rate of action potential The greater the depolarization, the greater the number of AP per second The rate of firing reflects the integration of all excitatory and inhibitory input

The Cerebral Cortex: Brain Bark Frontal lobe Motor cortex Parietal lobe Somatosensory cortex Occipital lobe Visual cortex Temporal lobe Auditory cortex Association areas

Topographical Organization The entire body is mapped on the sensory and motor areas of the cortex Adjacent parts of the body are connected to adjacent parts of the cortex Fine sensation and motor control have the largest representation These areas have a certain degree of plasticity

Asymmetry of Higher Functions Sensory and motor areas are quite symmetrical Equally represented in the right and left hemisphere Less symmetry in the association areas Most evident for language and non-verbal, visual-spatial analysis of information