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Chapter Nine Nervous System Part 1 Functions of the Nervous System Sensory input- information gathered by sensory receptors about internal and external changes Integration- interpretation of sensory input Motor output- activation of effector (muscles and glands) produces a response Divisions of the Nervous System Central Nervous System (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and from the CNS Peripheral Nervous System Functions Sensory (afferent) division 1 Somatic afferent fibers- convey impulses from skin, skeletal muscles, and joints 2 Visceral afferent fibers- convey impulses from visceral organs Motor (efferent) division Transmits impulses from the CNS to effector organs Somatic (voluntary) nervous system Conscious control of skeletal muscles Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers Regulated smooth muscle, cardiac muscle, and glands Functional divisions Sympathetic good pic Parasympathetic next slide Up next! The neuron! (AKA Nerve Cell) Histology of nervous system Neurons- excitable cells that transmit electrical signals Characteristics Long lived, amitotic, high metabolic rate, electrical signaling and cell-to-cell interactions during development Parts of a neuron Cell body- biosynthetic center of a neuron, network of neurofibrils (neurofilaments) Axon hillock- cone shaped area where the axon arises Clusters of cell bodies in the CNS are called nuclei, and in the PNS are called ganglia Parts of a neuron Processes- bundles are classed tracts (CNS) and nerves (PNS) Dendrites Short, tapering and diffusely branched Receptive (input) region of the neuron Convey electrical signals toward the cell body Parts of a neuron Axons One long axon per cell body Long axons are called nerve fibers Knoblike axon terminals Secretory region of the neuron Release neurotransmitters to excite or inhibit other cells Conducting region of the neuron, generates and transmits nerve impulses away from the cell body Nerve bundles Unmyelinated axons are thin nerve fibers and one Schwann cell can incompletely enclose 15 or more unmyelinated axons End of Day 1 Structural classifications of neurons Multipolar- 1 axon and several dendrites (most abundant) Bipolar- 1 axon and 1 dendrite (rare) Unipolar (pseudounipolar)- single, short process that has two branches Peripheral process- most distal branch Central process- branch entering the CNS Functional classification of neurons Sensory (afferent)- transmit impulses from sensory receptors toward the CNS Motor (efferent)- carries impulses from the CNS to effectors Interneurons (association neurons)- Shuttle signals through CNS pathways Neuroglia (glial cells)- supporting cells Astrocytes (CNS) Most abundant, versatile, and highly branched glial cells Cling to neurons, synaptic endings, and capillaries Support and brace neurons Determine capillary permeability Guide migration of young neurons Control chemical environment Information processing in the brain Neuroglia (glial cells)- supporting cells Neuroglia (glial cells)- supporting cells Microglia (CNS) Migrate toward injured neurons Phagocytize microorganisms and neuronal debris Ependymal cells (CNS) May be ciliated Line the central cavities of the brain and spinal column Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities Neuroglia (glial cells)- supporting cells Oligodendrocytes (CNS) Branched cells Processes wrap CNS nerve fibers, forming insulating myelin sheaths Myelin sheath- concentric layers of Schwann cell membrane Neurilemmal- peripheral bulge of Schwann cell cytoplasm Nodes of Ranvier-myelin sheath gaps between Schwann cells, sites where axon collaterals can emerge CNS Formed by processes of oligodendrocytes, Nodes of Ranvier not present, no neurolimma, thinnest fibers are unmyelinated White matter Dense collections of myelinated fibers Grey matter Mostly neuron cell bodies and unmyelinated fibers Neuroglia (glial cells)- supporting cells Satellite cells (PNS) Surround neuron cell bodies in the PNS Schwann cells (PNS) Surround peripheral nerve fibers and form myelin sheaths Vital to regeneration of damaged peripheral nerve fibers Membrane potentials Role of membrane ion channels Proteins serve as membrane ion channels Two main types of channels Leakage (nongated) channels- always open Membrane potentials Gated channels Three types Chemical gated (ligand-gated) channels- open with binding of a specific neurotransmitter Voltage-gated channels- open and close in response to changes in membrane potentials Mechanically gated channel- open and close in response to physical deformation of receptors When gated channels are open Ions diffuse quickly across the membrane along their electrochemical gradients Chemical gradients go from high to low Electrical gradients go from low to high Ion flow creates an electrical current and voltage changes across the membrane Resting membrane potential Potential difference across the membrane of a resting cell Approximately -70mV in neurons Generated by Differences in ICF (intracellular fluid) and ECF (extracellular fluid) Differential permeability of the plasma membrane Resting membrane potential Differences in ionic makeup ICF has lower concentration of Na+ and Cl- than ECF ICF has higher concentration of K+ and negatively charged proteins (A-) than ECF Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+ Membrane potentials act as signals Changes when concentrations of ions across the membrane change and permeability of membrane to ions changes Signals used to receive, integrate, and send information Graded potentials- incoming short-distance signals Depolarization Reduction in membrane potential Inside of the membrane becomes less negative than the resting potential Increases the probability of producing a nerve impulse Hyperpolarization An increase in membrane potential Inside of the membrane becomes more negative than the resting potential Reduces the probability of producing a nerve impulse Occur when a stimulus causes gated ion channels to open Decrease in magnitude with distance as ions flow and diffuse through leakage channels Action potentials- long-distance signals of axons Brief reversal of membrane potential with an amplitude of ~100mV Occur in muscle cells and axons of neurons Does not decrease in magnitude over distance Nerve fiber classification Group A fibers Large diameter, myelinated somatic sensory and motor fibers Group B fibers Intermediate diameter, lightly myelinated ANS fibers Group C fibers Smallest diameter, unmyelinated ANS fibers The synapse A junction that mediated information transfer form one neuron to another neuron or an effector cell Presynaptic neuron- conducts impulses toward the synapse Postsynaptic neuron- transmits impulses away from the synapse synapses Types of synapses Axodendritic- between the axon of one neuron and the dendrite of another Axosomatic- between the axon of one neuron and the soma of another Less common Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma) Synapses Electrical synapses Less common than chemical synapses Neurons are electrically coupled (joined by gap junctions) Communication is very rapid and may be unidirectional or bidirectional Important in embryonic nervous tissue and some brain regions Synapses Chemical synapses Specialized in the release of neurotransmitters Composed of two parts Axon terminal of the presynaptic neuron Receptor region on the postsynaptic neuron Synaptic cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft Is a chemical event that involves the release, diffusion, and binding of neurotransmitters that ensures unidirectional communication between neurons Neurotransmitters and their receptors Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function Acetylcholine (Ach) Released at neuromuscular junctions and some ANS neurons Synthesized by enzyme choline acetyltransferase Degraded by the enzyme acetylcholinesterase (AChE) Classified by chemical structure and by function Biogenic amines include Catecholamines Dopamine, norepinephrine (NE), and epinephrine Indolamines Serotonin and hisamine Broadly distributed in the brain Play roles in emotional behaviors and the biological clock Classified by chemical structure and by function Amino acids GABA (Gamma-aminobutyric acid) Glycine Aspartate Glutamate Classified by chemical structure and by function Peptides (neuropeptides) Substance P Mediator of pain signals Endorphins Act as natural opiates; reduce pain perception Gut-brain peptides Somatostatin and cholecystokinin Classified by chemical structure and by function Purines such as ATP Act in both the CNS and PNS Provoke pain sensation Gases and lipids Nitric oxide (NO) Involved in learning and memory Carbon monoxide (CO) Regulator of cGMP in the brain Endocannabinoids Lipid soluble and involved in learning and memory Neurotransmitters Direct action Neurotransmitter binds to channel-linked receptor and opens ion channels Promotes rapid responses Ach and amino acids Indirect action Neurotransmitter binds to a G-protein-linked receptor and acts through an intracellular second messenger Promotes long –lasting effects Biogenic amines, neuropeptides, and dissolved gases Neurotransmitters Types of neurotransmitter receptors Channel-linked receptors G protein-linked receptors Basic concepts of neural integration Neuronal pools integrate incoming information and forward the processes information to other destinations Simple neuronal pool Single presynaptic fiber branches and synapses with several neurons in the pool Discharge zone- neurons most closely associated with the incoming fiber Facilitated zone- neurons farther away from incoming fiber Types of circuits in neuronal pools Diverging circuit One incoming fiber stimulated an ever-increasing number of fibers, often amplifying circuits May affect a single pathway or several Common in both sensory and motor systems Types of circuits in neuronal pools Converging circuit Opposite of diverging circuits; strong stimulation or inhibition Also common in sensory and motor systems Types of circuits in neuronal pools Reverberating (oscillating) circuit Chain of neurons containing collateral synapses with previous neurons in the chain Parallel after-discharge circuit Incoming fiber stimulates several neurons in parallel arrays to stimulate a common output cell Neural processing Serial processing Input travels along one pathway to a specific destination Works in all-or-none manner to produce a specific response Ex. reflexes Parallel processing Input travels along several pathways Important in higher-level mental functioning Ex. Smell reminds you of an odor and associated experience Developmental aspects of neurons Originates from the neural tube and neural crest formed from ectoderm Neural tube becomes the CNS Cell death About 2/3 of neurons die before birth Death results in cells that fail to make functional synaptic contacts Many cells also die due to apoptosis (programmed cell death) during development Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs, impulse condition slows and eventually ceases Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence.