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The Nervous System

45.1 The nervous system consists of neurons and supporting cells.
Neuron Organization
• Sensory neurons carry impulses into the central nervous system (CNS), and motor neurons carry impulses away from the CNS. (p. 940)
• Sensory and motor neurons together form the peripheral nervous system (PNS). (p. 940)
• Neurons are supported by neuroglia. Two of the most important neuroglia in vertebrates are Schwann cells and oligodendrocytes, which produce myelin sheaths. (p. 941)

45.2 Nerve impulses are produced on the axon membrane.
The Resting Membrane Potential
• When a neuron is not being stimulated, it maintains a resting membrane potential. (p. 942)
• The inside of a cell is more negatively charged in relation to the outside of the cell, thus cations outside the cell are attracted to anions inside the cell. (p. 942)
• Due to combined activities of fixed anions, the sodium-potassium pump, and the membrane leak channels, the resting membrane potential for an average neuron remains stable at -70 mV. (p. 942-943)
Graded Potentials Generate Action Potentials
• Graded potentials are caused by the activation of gated ion channels, and the ability of graded potentials to combine is referred to as summation, which results in the inward diffusion of Na+ and the outward diffusion of K+. (p. 944)
• Threshold refers to the level of depolarization required to produce an action potential. (p. 944)
• Action potentials are all-or-none events, and are always separate events. (pp. 944-947)
• Action potentials are propagated as events are reproduced at different points along the membrane. (p. 947)

45.3 Neurons form junctions called synapses with other cells.
Structure of Synapses
• Electrical synapses are characteristic of invertebrate nervous systems, but somewhat rare in vertebrates, while the vast majority of vertebrate synapses are chemical. (p. 948)
• Most synapses have a synaptic cleft separating the two cells. (p. 948)
• Neurotransmitters diffuse rapidly across the cleft and bind to receptor proteins in order to transmit a nerve impulse. (p. 948)
Neurotransmitters and Their Functions
• Acetylcholine binds to receptor proteins in the postsynaptic membrane and causes ion channels to open, producing an excitatory postsynaptic potential (EPSP). (p. 949)
• Glutamine is the major excitatory neurotransmitter in the vertebrate CNS, producing EPSPs, and glycine and GABA are inhibitory neurotransmitters that lead to an inhibitory postsynaptic potential (IPSP). (p. 950)
• Biogenic amines are a class of neurotransmitters that include dopamine, norepinephrine, and serotonin. (p. 950)
• If receptor proteins within synapses are exposed to high levels of neurotransmitter molecules for prolonged periods, the nerve cell often responds by decreasing the number of receptor proteins in the postsynaptic membrane. (pp. 951-953)

45.4 The central nervous system consists of the brain and spinal cord.
The Evolution of the Vertebrate Brain
• Cnidarians exhibit the simplest nervous systems, and in flatworms there is some associative activity. All subsequent evolutionary changes in nervous systems can be viewed as a series of elaborations on characteristics already present in flatworms. (p. 954)
• The vertebrate brain is composed of three major regions: forebrain, midbrain, and hindbrain. (p. 954)
• The hindbrain was the major component in early brains, while starting with amphibian evolution, processing of sensory information is increasingly centered in the forebrain. (pp. 955-956)
The Human Forebrain
• The cerebrum is divided into right and left cerebral hemispheres, which are further divided into frontal, parietal, temporal, and occipital lobes. (p. 957)
• Much of neural activity occurs within the cerebral cortex. The activities of the cerebral cortex fall within the general categories of motor, sensory, and associative. (p. 957)
• The thalamus is a primary site of sensory integration, receiving visual, auditory, and somatosensory information. (p. 958)
• The hypothalamus integrates visceral activities and controls the pituitary gland. (p. 958)
• The left hemisphere is usually the dominant hemisphere for language. (p. 959)
• No one part of the brain appears to control memory. Short-term memory appears to be transient, while long-term memory appears to involve structural changes in neural connections. (p. 960)
The Spinal Cord
• The spinal cord is a cable of neurons extending from the brain down the backbone. It functions in message relaying and reflexes. (pp. 961-962)

45.5 The peripheral nervous system consists of sensory and motor neurons.
Components of the Peripheral Nervous System
• The peripheral nervous system consists of nerves (cablelike collections of axons) and ganglia (aggregations of neuron cell bodies located outside the central nervous system). (p. 963)
• Somatic motor neurons stimulate skeletal muscles to contract, and autonomic motor neurons innervate smooth muscle, cardiac muscle, and glands. (p. 963)
The Autonomic Nervous System
• The autonomic nervous system is composed of the sympathetic and parasympathetic divisions and the medulla oblongata of the hindbrain. (p. 964)
• When the sympathetic division becomes activated, epinephrine and norepinephrine prepare the body for a fight-or-flight response. Parasympathetic nerve actions produced by ACh slow the heart and increase secretions of the digestive organs, thus causing an antagonistic reaction to the fight-or-flight response. (pp. 964-966)

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