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Chapter 7

Chapter 7. Kee

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Michelle French

Chapter Eight: Nerve Cells and Electrical Signaling Overview of the Nervous System: nervous system divided into two main anatomical parts: central nervous system and peripheral nervous system central nervous system + consists of brain and spinal cord + receives and processes information from sensory organs and the viscera to determine the state of external environment (sensory information) and internal environment (visceral information) + integrates information and then send signals to certain organs (ie: muscles and glands) instructing them to perform appropriate tasks + site of learning, memory, emotions, thoughts, language, and other complex functions peripheral nervous system: + consists of neurons that provide communication between the CNS and organs throughout the body + subdivided into two divisions: afferent and efferent + neurons of the afferent division transmit sensory and visceral information from the organs to the CNS - information transmitted to CNS includes somatic senses (associated with skin, muscles, and joints), the special senses (vision, hearing, equilibrium, smell, and taste), and visceral information pertaining to the internal environment (ie: fullness of stomach, blood pressure, blood pH) + neurons of efferent division transmit information from CNS to organs in periphery, effector organs, that perform functions in response to commands from neurons. - usually muscles and glands + a neuron capable of transmitting messages to an effector organ/receiving information from a sensory organ innervate that organ efferent division can be further subdivided into two main branches: somatic and autonomic nervous systems + somatic nervous system: consists of motor neurons which regulate skeletal muscle contractions + autonomic nervous system: consists of neurons that regulate the function of internal organs and other structures (ie: sweat glands and blood vessels) that are not under voluntary control. - autonomic nervous system can be divided into two branches: the parasympathetic & sympathetic nervous systems which tend to have opposite effects on organs body also has an enteric nervous system which consists of an intricate network of neurons in the gastrointestinal tract that can function independently of the rest of the nervous system but communicates with the autonomic nervous system Cells of the Nervous System: nervous system contains two main classes of cells: neurons and glial cells neuron is the functional unit, the smallest unit of a tissue that can carry out the function of that tissue + excitable cells cells capable of producing large, rapid electrical signals called action potentials - communicate by transmitting electrical impulses glial cells: + constitute 90% of the cells in nervous system provide various types of support to the neurons, including 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 and the axon cell body (soma): contains the nucleus and most of the cell's organelles + it carries out most of the functions that other cells perform, such as protein synthesis and cellular metabolism + although mature neuron retain their nuclei, they lose the ability to undergo cell division in most areas of the nervous system, adults have all the neurons they will ever have - however, in few areas of the adult human brain new neurons can develop from undifferentiated cells dendrites branch from cell body and receive input from other neurons at specialized junctions called synapses. + cell bodies themselves can also receive synpatic input + at a synapse, a presynaptic neuron releases a chemical messenger called a neurotransmitter that usually communicates with the dendrite/cell body of a postsynaptic neuron or other cell neurons have another branch that comes off the cell body axon or nerve fiber. +send information + a neuron only has one axon, but axons can branch and thus send signals to more than one destinations + branches of an axon are called collaterals, the extent of branching varies among neurons and is indicative of the amount of communication with other cells. Axon functions in rapid transmission of information over relatively long distances in form of electrical signals action potentials, which are brief, large changes in membrane potential during which the inside of the cell becomes positively charged relative to the outside + beginning and end of an axon are specialized structures called the axon hillock and the axon terminal axon hillock: + site where axon originates from cell body + specialized in most neurons for the initiation of action potentials axon terminal: + specialized to release neurotransmitter on arrival of an action potential + released neurotransmitter molecules carry a signal to a postsynaptic cell, usually to a dendrite or the cell body of another neuron or to the cells of an effector organ + the neuron whose axon terminal is releasing neurotransmitter is the presynaptic cell Localization of Ion Channels in Neurons: because different regions of a neurons generally have specialized functions, each region tends to have specific types of ion channels, most of which are gated + opening/closing of ion channels changes the permeability of the plasma membrane for a specific ion, resulting in a change in the electrical properties of the cell or the release of a neurotransmitter + leak channels non-gated channels, found in plasma membrane through a neuron, are always open and are responsible for the rest membrane potential ligand-gated channels open/close in response to binding of a chemical messenger to a specific receptor in the plasma membrane + in neurons, ligand-gated channels are most densely located in the dendrites and cell body, areas that receive communication from presynaptic neurons in the form of neurotransmitters voltage-gated channels open/close in response to changes in membrane potential + voltage-gated sodium channels and voltage-gated potassium channels are located throughout the neuron, but more densely in the axon and in greatest density in the axon hillock necessary for initiation and propagation of action potentials + voltage-gated calcium channels located most densely in axon terminals open in response to the arrival of an action potential at the axon terminals. + when these channels open, calcium enters the cytosol of the axon terminals and triggers the release of neurotransmitter Structural Classification of Neurons: neurons can be classified structurally according to the number of processes (axons and dendrites) that project from the cell body neurons commonly found in humans include bipolar neurons and multipolar neurons bipolar neurons: generally sensory neurons with two projections, an axon and a dendrite, coming off the cell body + typical bipolar neurons function in senses of olfaction (smell) and vision + most sensory neurons are a subclass of bipolar neurons called pseudo-unipolar neurons because the axon and dendrite projections appear as a single process that extends in two directions from the cell body. + dendrite modified to function much like axon, and is a functional continuation of the axon peripheral axon because it originates in the periphery with sensory receptors and functions as an axon in that it transmits action potentials + axon process is called central axon because it ends in CNS where it forms synapses with other neurons multipolar neurons: most common neurons + have multiple projections from the cell body; one projection is an axon, all the others are dendrites Functional Classification of Neurons: three functional classes of neurons: efferent neurons, afferent neurons, and interneurons efferent neurons: + transmit information from CNS to effector organs + efferent neurons include motor neurons extending to skeletal muscle and neurons of autonomic nervous system + the axon leaves the CNS and becomes part of the peripheral nervous system as it travels to the effector organ it innnervates afferent neurons: + transmit either sensory information from sensory receptors (detect information pertaining to the outside environment) or visceral information from visceral receptors (detect information pertaining to conditions in the interior of the body) to the CNS for further processing + most are pseudo-unipolar neurons, with cell body located outside the CNS in a ganglion (cluster of neural cell bodies located outside the CNS) + endings of peripheral axon located in peripheral organ (sensory organ/visceral organ) where they are either modified into sensory receptors or receive communication from separate sensory receptor cells + central axon terminates in the CNS where it releases neurotransmitter to communicate with other neurons interneurons: + 99% of neurons in the body + located entirely in CNS + perform all functions of CNS, including processing
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