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Chapter 3

Chapter 3 Notes

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Psychology 1000

Chapter 3 Notes: Biological Foundations of Behaviour Chapter Outline The Neural Bases of Behaviour Neurons The Electrical Activity of Neurons How Neurons Communicate: Synaptic Transmission Applications Understanding How Drugs Affect Your Brain The Nervous System The Peripheral Nervous System The Central Nervous System The Hierarchical Brain: Structures and Behavioural Functions Research Foundations Wilder Penfield and a Cortical Map Focus on Neuroscience The Neuroscience of the World around You Frontiers Human Aggression, Criminal Behaviour, and the Frontal Cortex The Neural Bases of Behaviour Neurons Neurons: These specialized nerve cells are the basic building blocks of the nervous system that are linked together in circuits. • Supported in their function by glial cells • Generate electricity that creates nerve impulses Glial cells: • Surrounds neurons and hold them in place • Manufacture nutrient chemicals that neurons need • Form the myelin sheath • Absorb toxins and waste materials that might damage neurons • Protect the brain from toxins • Modulating the communication among neurons Blood-brain barrier: Specialized barrier prevents many substances, including a wide range of toxins, from entering the brain. • Walls of blood vessels within the brain contain small gaps • Covered by a specialized type of glial cell Three main parts of a neuron: • Cell body (soma): -Contains the biochemical structures needed to keep the neuron alive -Its nucleus carries the genetic information that determines how the cell develops and functions -Surface of the cell body has receptor areas -Information is combined and processed in cell body • Dendrites: -Branchlike figures emerging from the cell body -Specialized receiving units -Collect messages from neighbouring neurons and send them to the cell body • Axon: -Extending from one side of the cell body -Conducts electrical impulses away from the cell body to other neurons, muscles, or glands. -Branches out at its end to form a number of axon terminals -Connect with dendritic branches from neurons p.70 for diagram The Electrical Activity of Neurons Nerve impulses/actions involves 3 basic steps: 1. At rest, the neuron has an electrical resting potential due to the distribution of positively and negatively charged chemicals (ions) inside and outside the neuron. 2. When stimulated, a flow of ions in and out through the cell membrane reverses the electrical charge of the resting potential, producing an action potential, or nerve impulse. 3. The original distribution of ions is restored, and the neuron is again at rest. Details on the process: • Cell membrane is selective: allowing only certain substances to pass through the ion channels • Ion channel: passageway in the membrane that can open to allow ions to pass through • The chemical environment inside the neuron differs from its external environment • The process whereby a nerve impulse is created involves the exchange of electrically charged atoms (ions). + - • Outside the neuron: + charge Na ions, and – charge Cl ions. + • Inside the Neuron: - charge protein molecules (anions), and +charge K • The ↑ [] of Na outside the cell, and –charge protein ions inside = uneven distribution of + and – ions that makes the interior of the cell negative compared to the outside (The internal difference of 70mV, is called resting potential) • At rest, the neuron is said to be in a state of polarization Action Potential Action potential: a sudden reversal in the neuron’s membrane voltage, during which the membrane voltage momentarily moved from –70mV (inside) to +40mV. Depolarization: shift from negative to positive voltage. Action potential process: a) In a resting state, the neuron’s Na and K channels are closed, and the [] of Na + ions is 10x higher outside the neuro+ than inside it. b) When the neuron is stimulated, Na channels open up. Attracted by the – protein ions inside, Na ions flood into the axon (state of depolarization). c) The interior now becomes + (40mV) in relations to the outside, creating the action + + potential. To restore the resting potential, the cell closes its Na channels, and K flow out through their channels (restoring the negative potential) Supplementary details: + + • Eventually, the excess Na ions flow out of the neuron, and the escaped K ions are recovered. • The action potential flow down the length of the axon to the axon terminals. + • After an impulse passes a point on the axon, K ions flow out of the interior. • During this absolute refractory period the membrane is not excitable and cannot generate another action potential (places a limit on the rate at which nerve impulses can occur) • In humans, the limit is about 300 impulses per second. • “It’s all or nothing”: actions potentials occur at a uniform and maximum intensity, or they do not occur at all. • The negative potential inside the axon has to be changed from –70mV to –50mV (the action potential threshold) by the influx of sodium ions into the axon before the action potential will be triggered. • Gradient potentials: changes in the negative resting potential that do not reach the –50mV action potential threshold. The Myelin Sheath Myelin sheath: fatty, whitish insulation layer derived from glial cells during development covers many axons. Nodes of Ranvier: Interrupts the myelin sheath at regular intervals, where the myelin id either extremely thin or absent. Unmyelinated axons: action potential travels down the axon like a burning fuse. Myelinated axons: electrical conduction can skip from node to node. Multiple sclerosis: this disease occurs when the person’s own immune system attacks the myelin sheath. Damage to the myelin sheath disrupts the delicate timing of nerve impulses. How Neurons Communicate: Synaptic Transmission Synapse: a functional (but not physical) connection between a neuron and its target. Neurotransmission: neurons release chemicals, and it was these chemicals that carried the message from one neuron to the next call in the circuit. Synaptic cleft: tiny gap or space between the axon terminal of one neuron and the dendrite of the next neuron. Neurotransmitters Neurotransmitters: chemical substances that carry messages across the synapse to either excite other neurons or inhibit their fring. The process of chemical communication (5 steps): 1. Synthesis: chemical molecules are formed inside the neuron. The molecules are stored in chambers called synaptic vesicles within the axon terminal. 2. Storage: The molecules are stored in chambers called synaptic vesicles within the axon terminal. 3. Release: When an action potential comes down the axon, these vesicles move to the surface of the axon terminal and the molecules are released into the fluid-filled space between the axon of the sending neuron and the membrane of the receiving neuron. 4. Binding: The molecules cross the synaptic space and bind to receptors sites-large protein molecules embedded in the receiving neuron’s cell membrane. Binding produces a chemical reaction that can have 2 effects on a postsynaptic neuron: • Depolarize (excite) by stimulating the inflow of Na or other +charged ions. These neurotransmitters are called excitatory transmitters. This stimulation may exceed the action potential threshold and cause the postsynaptic neuron to fire an action potential. • The chemical reaction created by the docking of a neurotransmitter at its receptor sit will hyperpolarize the postsynaptic membrane by stimulating ion channels that allow positively charged K ions to flow out of the neuron or –charged ions flow into the neuron. This makes membrane potential even more –. Hyperpolarization makes it more difficult for excitatory transmitters at other receptors sites to depolarize the neuron to its action potential neurons. Transmitters that create hyperpolarization are thus inhibitory in their function. 5. Deactivation: Once a neurotransmitter molecules bonds to its receptor, it continues to activate ir inhibit the neuron until it deactivates. Deactivation occurs in 2 ways: • Deactivated by other chemicals located in the synaptic space that break them down into their chemical components. • Reuptake: the transmitter molecules are reabsorbed into the presynaptic axon terminal. When the receptor molecule is vacant, the postsynaptic neuron returns to its former resting state. Specialized Transmitter Systems Acetylcholine (ACh): Neurotransmitter, which is involved in memory and muscle activity. • Reduction in ACh weakens or deactivates neural circuitry that stores memories. • Excitatory transmitter at the synapse where neurons activate muscle cells. Dopamine: Neurotransmitter, that mediates a wide range of functions, including motivation, reward, and feelings of pleasure; voluntary motor control; and control of thought processes. Serotonin: Neurotransmitter that influences mood, eating, sleep, and sexual behaviour. Endorphins: Neurotransmitter that reduces pain and increases feelings of well-being. Neuromodulators: These substances circulate through the brain and either increase or decrease the sensitivity of thousands of neurons to their specific transmitters. Applications Understanding How Drugs Affect Your Brain The Nervous System Three types of neurons: 1. Sensory neurons: carry input messages from the sense organs to the spinal cord and brain. 2. Motor neurons: transmit output impulses from the brain and spinal cord to the body’s muscles and organs. 3. Interneurons: link the input and output functions. They perform connective associative functions within the nervous system. Two major subsystems of the nervous system: 1. Central nervous system: consisting of all the neurons in the brain and spinal cord. 2. Peripheral nervous system: composed of all the neurons that connect the central nervous system with the muscles, glands, and sensory receptors. The Peripheral Nervous System The peripheral nervous system has two major divisions 1. Somatic nervous system: consists of the sensory neurons that are specialized to transmit messages from the eyes, ears, and other sensory receptors, and the motor neurons that send messages from the brain and spinal cord to muscles that control our voluntary movements. *Allows you to sense and respond to your environment. 2. Autonomic Nervous System: regulates the body’s internal environment, which controls the glands and the smooth (involuntary) muscles that form the heart, the blood vessels, and the lining of the stomach and intestines. Concerned with involuntary functions (i.e. respiration, circulation) Two subdivisions of the autonomic nervous system (typically, these two divisions affect the same organ or gland in opposing ways: 1. Sympathetic nervous system: has an activation or arousal function, and it tends to act as a total unit. 2. Parasympathetic system: is more specific in its opposing actions. It slows down body processes and maintains or returns to the state of rest. *By working together to maintain equilibrium in our internal organs, the two divisions can maintain homeostasis (delicately balanced or constant internal state). The Central Nervous System The Spinal Cord • Most nerves enter and leave the central nervous system by way of the spinal cord • 40-45 cm long in adults • 2.5cm in diameter • Vertebrae: bones of the spine (protects the neurons in the spinal cord) • Sensory nerves enter the back side of the spinal cord along its lengths • Motor nerves exit the spinal cord’s front side • Spinal cord reflex system: spinal reflexes can be triggered at the level of the spinal cord without any involvement of the brain to reduce reaction time. The Brain • 1.4 kilograms of protein, fat, and fluid inside your skull • Most active energy consumer of all your body organs • 2% of your total body weight • Consumes 20% of total oxygen in a resting state • Never rests: its rate of energy metabolism is relatively constant day and night Unlocking the Secrets of the Brain The methods: Neuropsychological tests: measure verbal and non-verbal behaviours that are known to be affected by particular types of brain damage. Destruction and stimulation techniques: produce brain damage (lesions) under controlled conditions, in which specific nerve tissue is destroyed, or surgically remove a portion of the brain to study their consequences. Electrical recording: Neuron’s electrical activity can be measured by inserting small electrodes into particular areas of the br
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