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Physiology (114)
PSL201Y1 (46)
Yue Li (10)


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Yue Li

7 NERVE CELLS AND ELECTRICAL SIGNALING Overview of the Nervous System Central Nervous System (CNS): consists of the brain and spinal cord. Receives and processes information from sensory organs and the viscera to determine the state of the external environment (sensory information) and internal environment (visceral information). Peripheral nervous system (PNS): neurons that provide communication between the central nervous system and organs throughout the body. o Afferent: neurons transmit sensory and visceral information from the organs to the CNS. Information transmitted includes somatic senses, special senses, and visceral info. o Efferent: neurons transmit information from the central nervous system to organs in the periphery called effector organs (muscles and glands). o A neuron capable of transmitting messages or receiving information to an effector organ is said to innervate that organ. o Efferent is subdivided into somatic nervous system (motor neurons for skeletal muscle contractions) and autonomic nervous system (regulate the function of internal organs and other structures; have parasympathetic and sympathetic nervous system). Cells of the Nervous System Neurons are the functional unit of the tissue. They are the excitable cells that communicate by transmitting electrical impulses. Excitable cells: cells capable of producing large rapid electrical signals called action potentials. Glial cells constitute 90% of the cells in the nervous system and provide structural and metabolic support. NEURONS Most neurons contain three main components: a cell body and two types of neural processes that extend from the cell body the dendrite(s) and an axon. Cell body (soma): contains the cell nucleus and most of the cells organelles; carries out most of the functions (protein synthesis, cellular metabolism). In most areas of the nervous system, adults have all the neurons they will ever have. In few areas in the brain, new neurons can develop from undifferentiated cells. Dendrites: branch from the soma and receive input from other neurons at specialized junctions called synapses. Axon (nerve fiber): sends information. Neuron usually has one axon, but they can branch and send signals to more than one destination. The branches of an axon are called collaterals. Action potentials: brief, large changes in membrane potential during which the inside of the cell becomes positively charged relative to the outside. Axon hillock: the site where the axon originates from the cell body, is specialized in most neurons for the initiation of action potentials. Axon terminal: specialized to release neurotransmitter on arrival of an action potential. LOCALIZATION OF ION CHANNELS IN NEURONS o Leak channels: found in the plasma membrane throughout a neuron, are always open and are responsible for the resting membrane potential. o Ligand-gated channels: open or close in response to the binding of a chemical messenger (neurotransmitter) to a specific receptor in the plasma membrane; densely located in the dendrites and cell body. o Voltage-gated channels: open or close in response to changes in membrane potential. Have voltage-gated sodium and potassium channels in the axon and axon hillock. Calcium channels are in the axon terminals (triggers the release of neurotransmitter). FUNCTIONCAL CLASSIFICATION OF NEURONS o Efferent neurons transmit information from the CNS to the effector organs. Are located in the CNS, however the axon leaves the CNS and becomes part of the PNS as it travels to the effector organ it innervates.7 NERVE CELLS AND ELECTRICAL SIGNALING o Afferent neurons transmit either sensory info from sensory receptors (from outside environment) or visceral receptors (from inside environment). Most are pseudo- unipolar neurons, with the soma located outside the CNS. The peripheral axon is located in the peripheral organ. The central axon terminates releases neurotransmitters in CNS. o Interneurons account for 99% of all neurons in the body. Located in the CNS. They perform all the sensory information from afferent neurons, sending out commands o effector organs through efferent neurons. STRUCTURAL ORGANIZATION OF NEURONS IN THE NERVOUS SYSTEM o In the CNS, cell bodies of neurons are often grouped into nuclei and the axons travel together in bundles called pathways, tracts, or commissures. o In PNS, cell bodies of neurons are clustered together in ganglia, and the axons travel together in bundles. GLIAL CELLS Their functions include providing structural integrity to the nervous system and chemical and anatomical support that permits neurons to carry out their functions. There are astrocytes, ependymal cells, microglia, oligodendrocytes, and Schwann cells. Only Schewann cells are in the PNS. Oligodendrocytes and Schwann cells form an insulating wrap of myelin around the axons of neurons. Enables neurons to transmit action potentials more efficiently and rapidly. Oligodendrocytes form myelin around axons in the CNS; one oligodendrocyte sends out projections providing the myelin segments for many axons. Schwann cells form myelin around axons in the PNS, but each Schwann cell provides myelin for only one axon. Because the lipid bilayer of a plasma membrane has low permeability to ions, the several layers of membrane of the myelin sheath substantially reduce leakage of ions across the cell me
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