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Chapter 4

PS268 - Chapter Four

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Wilfrid Laurier University
Bruce Mc Kay

PS268 – Chapter Four – The Nervous System - Psychoactive drugs can influence homeostasis. Homeostasis can be loosely translated as “staying the same” and it describes the fact that many biological factors are maintained at or near certain levels Components of the Nervous System - Nervous system is composed of two types of cells: glial cells (glia) and nerve cells (neurons) - Glia o Glia communicates with neurons and modulate their activity o A function of glia is to create the blood-brain barrier, a barrier between the blood and fluid that surrounds neurons o This semipermeable structure protected the brain from potentially toxic chemicals circulating in the blood o Lipophilic – the extent to which chemicals can be dissolved in oils and fats - Neurons o Primary elements of the nervous system responsible for analyzing and transmitting information o 100 billion neurons in the nervous system o Each neuron have four morphologically defined regions: a cell body, dendrite, axon and presynaptic terminal o The dendrites are treelike features extending from the cell body and contain within their membranes the specialized structures (receptors) that recognize and respond to specific chemicals’ signal. o The long slender axon extends from the cell body and is responsible for conducting the electrical signal to the presynaptic terminals. o The presynaptic terminals are the bulbous structures located at the end of the axon where chemical messengers are stored in small, round packages called vesicles Neurotransmission - Action Potential o This electrical signal initiates a chain of events that allows one neuron to communicate with another through the release of neurotransmitters o The action potential occurs as a result of opening ion channels that allow electrically charged particles (ions) to access to the inside of the cell o This change moves the cell’s membrane away from its resting potential to a more positively charged voltage o Hyperpolarization refers to the state that occurs when the membrane potential of a cell is pushed below the resting potential o The action potential occurs when the neuron’s membrane is depolarized to the threshold of excitation, during the time Na+ channels open, allowing Na+ ions to move across the membrane intracellularly and further depolarize adjacent regions of the neuron o *terms on pg68* The Peripheral Nervous System - Is made up of the nerves outside the brain and spinal cord and consists of the somatic and autonomic nervous system - Somatic Nervous System o The nerves that are on the front lines interacting with the external environment are referred to as the somatic system o The cranial nerves that relate to vision, hearing, taste, smell, chewing and movements are included as spinal nerves carrying information from the skin and joints and controlling movements of the arms and legs o The neurotransmitter at neuromuscular junctions in the somatic system is acetylecholine which acts on receptors that excite the muscle - Autonomic Nervous System o Your body’s internal environment is monitored and controlled by the autonomic nervous system (ANS) which regulates the visceral or involuntary functions of the body such as heart rate and blood pressure o Many psychoactive drugs have simultaneous effects in the brain and on the ANS o ANS is also where chemical neurotransmission was first studied o The ANS is divded into sympathetic and parasympathetic brances  The inhibition of the heart rate by vagus nerve is an example of parasympathetic branch - Central Nervous System o Consists of the brain and spinal cord o These two structures form a central mass of nervous tissue with sensory nerves coming in and motor nerves going out The Brain Major Structures - Some areas of the cortex are known to be involved in processing visual information; other areas are involved in processing auditory or somatosensory information - Relatively smaller cortical areas are involved in the control of muscles (motor cortex) and large areas are referred to as association areas - Underneath the cerebral cortex on each side of the brain and hidden from external view are the basal ganglia comprising of three primary components: the caudate, nucleus, the putamen and the globus pallidus - The basal ganglia are important for maintenance of proper muscle tone - The lowerbrain stem contains a couple of small areas of major importa - Too much output from these structures results in muscular rigidity in the arms, legs and facial muscles. This can occur if parkinson’s disease damages the basal ganglia or as a side effect of some psychoactive drug that act on the basal ganglia - The hypothalamus is a small structure near the base of the brain just above the pituitary gland o Hypothalamus is an important link between the brain and the hormonal output of the pituitary and is thus involved in feeding, drinking, temperature regulation and sexual behaviour - The limbic system consists of a number of connected structures that are involved in emotion, memory for location and level of physical activity o It included the amyglada, nucleus accumbens and hippocampus o The limbic system involves important mechanisms for behavioural control at more primitive level than that of the cerebral cortex - The dopaminergic pathway from the ventral tegmental area to the nucleus accumbens is called the mesolimbic pathway and it plays a key role in reward and reinforcement - The midbrain, pons and medulla are the parts of the brainstem that connect the larger structures of the brain to the spinal cord - One area is the vomiting centre locatedi n the medulla - Often when the brain detects foreign substances in the blood, such as alcohol, this centre is activated and vomiting results. - Chemical Pathways: o Dopamine – we think of many p
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