Nervous System: Nervous Tissue PDF

BIO 100 – Chapter 12 Nervous System: Nervous Tissue Nervous System – interprets and controls all sensory input from receptors and motor output to effectors. It is composed of the brain, spinal cord, nerves, and ganglia Nervous Tissue – composed of ex...

BIO 100 – Chapter 12 Nervous System: Nervous Tissue Nervous System – interprets and controls all sensory input from receptors and motor output to effectors. It is composed of the brain, spinal cord, nerves, and ganglia Nervous Tissue – composed of excitable neurons that initiate and transmit graded potentials and action potentials, and glial cells that support and protect them Synapse – functional junction of a neuron with either another neuron or an effector. Either chemical or electrical Glial Cells – distinct cell type of nervous tissue Establishing and Changing a RMP – depends upon various types of pumps and channels within a neuron’s plasma membrane General Functions of the Nervous System: The nervous system collects information through receptors and sensory input, processes and evaluates information, and responds through motor output to effectors (muscles or glands) Organization of the Nervous System: Organized structurally by CNS + PNS Organized functionally by sensory component + motor component Nerves + Ganglia: Nerve – collection of axons in the PNS that are wrapped in connective tissue The entire nerve is enclosed with an epineurium, fascicles of axons are ensheathed with a perineurium, and each axon is wrapped with an endoneurium Ganglion – cluster of neuron cell bodies located along a nerve General Characteristics of a Neuron: Excitability, conductivity, secretion, longevity Typically amitotic Neuron Structure: Cell body Tapering dendrites Long axon Neuron Transport: Neurons transport substances between the cell body and synaptic knobs by fast axonal transport and slow axonal transport Classification of Neurons: Structurally – multipolar, bipolar, unipolar, anaxonic (3) Functional – sensory neurons, motor neurons, interneurons General Characteristics of Glial Cells: Nonexcitable cells that primarily support and protect the neuron Types of Glial Cells: Within the CNS: astrocytes, ependymal cells, microglia, oligodendrocytes Within the PNS: satellite cells + neurolemmocytes Myelination: The process by which part of an axon is wrapped and insulated with myelin Neurolemmocytes myelinate axons in the PNS + oligodendrocytes myelinate axons in the CNS Axon Regeneration: Regeneration of damaged neurons is limited to PNS axons A PNS axon can regrow to re-establish innervation if the cell body in intact and a critical amount of neurilemma remains Types of Pumps and Channels:  Pumps and channels are membrane proteins that facilitate movement of ions across the neuron plasma membrane Distribution of Pumps and Channels: Some membrane transport proteins are located along the entire neuron and some are in specific functional neuron segments Neurons at Rest: RMP is -70mV Closed gated channels Na+, K+ and Cl- concentration gradients along the length of the axon Ca2+ concentration gradient at the synaptic knob Physiologic events that occur in a neuron’s functional segment begin with stimulation of the receptive segment until the release of neurotransmitter from the transmissive segment. Receptive Segment: Includes the dendrites + cell body Involves formation + propagation of graded potentials: both EPSP + IPSP Initial Segment: Summation of EPSP + IPSP that reach the initial segment determines whether the threshold value (-55mV) is reached and an action potential is initiated Conductive Segment: Involved in the propagation of an action potential along an axon, a process that involves depolarization and repolarization Brief period of time that an axon is either incapable of generating an action potential or a greater than normal amount of stimulation is required to generate another action potential is called the refractory period Saltatory conduction occurs if the axon is myelinated Transmissive Segment: Involves exocytosis of neurotransmitter from synaptic vesicles which are located within synaptic knobs Graded Potentials VS Action Potentials: Graded potentials are short-lived electrical signals that occur in the dendrites and cell body due to opening of chemically gated channels, whereas action potentials are self-propagating electrical signals that are initiated in the initial segment, propagated along an axon, and result from the sequential opening of voltage-gated channels. Velocity of Action Potential Propagation: The velocity of action potentials (or nerve signals) is greater in larger and myelinated axons. Nerve fibers, which are axons and their myelin sheath, are classified into three groups based upon the velocity of the nerve signal propagation Frequency of Action Potentials: Frequency of action potentials occurs with increased stimulation of the neuron Classification of Neurotransmitters: Neurotransmitters are conventionally described as molecules synthesized by neurons, which are then stored within vesicles in synaptic knobs and, when released, bind to specific receptors in a target cell to trigger a physiologic response. Major classes of neurotransmitters include acetylcholine, biogenic amines, amino acids, and neuropeptides Features of Neurotransmitters: Acetylcholine (ACh) is discussed in detail because it exhibits all of the classical features of neurotransmitters and is the most understood Neuromodulation: Neuromodulation is the release of chemicals other than neurotransmitters that either increase the responsiveness to a neurotransmitter (facilitation) or decrease the responsiveness to a neurotransmitter (inhibition)