44.1 What Cells Are Unique to the Nervous Systems?

Chapter 44 – Neurons and Nervous Systems 44.1 What cells are unique to the nervous systems? Nervous systems include neurons and glial cells. Neurons are organized in circuits with sensory inputs, integration, and outputs to effectors. Glial cells serve support functions. Review Figures 44.1 and 44.3 In vertebrates, the brain and spinal cord form the central nervous system, which communicates with the rest of the body via the peripheral nervous system. The CNS increases in complexity from invertebrates to vertebrates and from fish to mammals. Review Figures 44.1 and 44.2 Neurons generally receive information via their dendrites, of which there can be many, and transmit information via their single axons, which end in axon terminals. Review Figure 44.3 Where neurons and their target cells meet, information is transmitted across specialized junctions called synapses. Schwann cells and oligodendrocytes produce myelin, which insulates neurons. Astrocytes create the blood–brain barrier. Review Figure 44.4 44.2 How do neurons generate and conduct signals? See Web/CD Tutorial 44.1 Neurons have an electric charge difference across their plasma membranes, the membrane potential. The membrane potential is created by ion pumps and ion channels. When a neuron is not active, its membrane potential is a resting potential. Review Figures 44.5 and 44.6 The sodium-potassium pump concentrates K+ on the inside of a neuron and Na+ on the outside. Potassium channels allow K+ to diffuse out of the neuron, leaving behind unbalanced negative charges. Review Figures 44.6 and 44.7 Patch clamping allows the study of single ion channels. Review Figure 44.8 The resting potential is perturbed when ion channels open or close, changing the permeability of the plasma membrane to charged ions. Through this mechanism, the plasma membrane can become depolarized or hyperpolarized. Review Figure 44.9 An action potential is a rapid reversal in charge across a portion of the plasma membrane resulting from the sequential opening and closing of voltage-gated Na+ and K+ channels. These changes in voltage-gated channels occur when the plasma membrane depolarizes to a threshold level. Review Figure 44.10, Web/CD Tutorial 44.2 Action potentials are all-or-none, self-regenerating events. They are conducted down axons because local current flow depolarizes adjacent regions of membrane and brings them to threshold. Review Figure 44.11 In myelinated axons, action potentials appear to jump between nodes of Ranvier, patches of axonal plasma membrane that are not covered by myelin. Review Figure 44.12 44.3 How do neurons communicate with other cells? Neurons communicate with each other and with other cells by transmitting information over chemical synapses (with neurotransmitters) or electrical synapses. The neuromuscular junction is a well-studied chemical synapse between a motor neuron and a skeletal muscle cell. Its neurotransmitter is ACh, which causes a depolarization of the postsynaptic membrane when it binds to its receptor. Review Figure 44.13, Web/CD Tutorial 44.3 When an action potential reaches an axon terminal, it causes the release of neurotransmitters, which diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. Review Figures 44.13 and 44.14 Synapses between neurons can be either excitatory or inhibitory. A postsynaptic neuron integrates information by summing excitatory and inhibitory postsynaptic potentials in both space and time. Review Figure 44.15 Ionotropic receptors are ion channels or directly influence ion channels. Metabotropic receptors are G protein-linked receptors that influence the postsynaptic cell through various signal transduction pathways and result in the opening or closing of ion channels. The actions of ionotropic synapses are generally faster than those of metabotropic synapses. Review Figure 44.16 There are many different neurotransmitters and even more types of receptors. The action of a neurotransmitter depends on the receptor to which it binds. Review Table 44.1 (Part 1), 44.1 (Part 2) Web/CD Activity 44.1 With repeated stimulation, a neuron can become more sensitive to its inputs through LTP. The properties of the NMDA glutamate receptor appear to explain LTP. Review Figures 44.17 and 44.18