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NERVOUS SYSTEM Maintenance of homeostasis Major regulatory and control center FUNCTIONS Sensing changes (INPUT) Processing information (INTEGRATING) Responding (OUTPUT) SEAT OF ALL MENTAL ACTIVITY • CONSCIOUSNESS • MEMORY • THINKING Divisions of the Nervous System Central Nervous System Peripheral Nervous System Central Nervous System Involves the brain and spinal cord Part of the dorsal body cavity Contains gray and white matter, and has fluid filled spaces Peripheral Nervous System Cranial and Spinal nerves Afferent Nervous System - input Efferent Nervous System output • somatic (voluntary) NS • autonomic (involuntary) NS •sympathetic and parasympathetic divisions Cells of the Nervous System Neuroglial Cells Neurons Neuroglial Cells Found in CNS • Astrocytes • Star-shaped; most abundant; anchor to blood vessels • Microglia – • Protect Neuroglial Cells Found in CNS • Oligodendrocytes • Produce insulating myelin sheath in the CNS • Ependymal • Line cavities • Circulate cerebrospinal fluid Neuroglial Cells Found in PNS • Schwann cell •Form myelin sheath in PNS cells • Satellite cell •Surround cell •Function relatively unknown Neuron Structure Cell body (Soma) Processes • Dendrites • Receiving branches • Axon • Sending branch Classifications of Neurons Structure Function Area served Neurons by structure Pseudounipolar (unipolar) – one process Bipolar – one dendrite & one axon Multipolar • 3 or more processes; most common • 1 axon and 2 or more dendrites Neurons by function Afferent neurons - sensory neurons Efferent neurons - motor or secretory neurons Association neurons - interneurons or intermediate neurons Neurons by the area served Visceral neurons • serving the internal organs • visceral afferent, visceral efferent Somatic neurons • serving the body wall • somatic afferent, somatic efferent Neuron Characteristics Excitable Conductive Transmits information Cannot reproduce Regeneration • cell body intact • presence of neurolemma sheath • regeneration tunnel aligned • very, little scar tissue Nerves Bundles of nerve cell processes in the PNS Link the PNS to the CNS Sensory, motor, or mixed Connective tissue • epineurium, perineurium, endoneurium Tract - bundles of nerve fibers in the CNS White / Gray Matter White matter contains myelinated processes • PNS - myelinated nerves • CNS - myelinated tracts Gray matter- consists of cell bodies and unmyelinated processes • Nuclei- collection of cell bodies (CNS) • Ganglia - cell bodies outside CNS Neurophysiology Basic electrical principles • Voltage – measures potential difference between to charges • Resistance – hindrance to charge flow • Insulators – resist charge flow • Conductors – allow charge flow • Ohm's law: Current = Voltage/Resistance • Ions – charged atoms flow across membranes Ion channels Passive channels – always open Active (gated) channels. • Chemically-gates channels – open when combined with appropriate neurotransmitter • Voltage-gates channels – open in response to changes in membrane potential Establishing the resting membrane potential. The neuron membrane is positively charged on the outside and negatively charged on the inside. Inequality of charged particles occurs only at membrane. The total number of positive and negative ions and molecules inside and outside the cell are equal. Distribution of charged particles Sodium ions (Na+) Potassium ions (K+) Chloride ions (Cl-) Negatively charged proteins (A-) Factors that help establish the RMP. Differences in the resting permeability of the plasma membrane to Na+ and K+ ions – ATPase, Na+ ion/K+ ion pump Impermeability of the plasma membrane to proteins – Membrane potentials that act as signals Communication in neurons and muscle cells involves changing the membrane potential. Factors that change membrane potential. Changing the permeability of the plasma membrane to any ion. Changing the concentration of ions across the plasma membrane. Depolarization - membrane potential decreases Hyperpolarization – membrane potential increases Graded membrane potential. Short-lived depolarization or hyperpolarization of the plasma membrane. Caused by opening of gated ion channels in the plasma membrane. Magnitude of the change in potential is directly related to intensity of the stimulus. Action potential Rapid reversal of membrane potential. Occurs only in neurons and muscle cells. Also called a nerve impulse in a neuron. Only axons can generate an action potential. Steps in generating an action potential. Resting membrane potential. • Voltage-gated Na+ channels closed. • Voltage-gated K+ channels closed. Depolarization. • Opening voltage-gated Na+ channel in the axon. • Influx of Na+ ions results in depolarization of the axonal membrane. • Closing of Na+ channels stops the influx of Na+ ions. Repolarization. • Open voltage-gated K+ channel in the axon. • Efflux of K+ ions results in repolarization of the axonal membrane. • Membrane potential moves back to RMP. • Na+/K+ ATP pump helps re-establish the Na+ and K+ ion concentrations inside and outside the neuron. Propagation of the action potential. An action potential travels away from its point of origin. An action potential is selfpropagating. (domino effect) • Changes in membrane potential in one section stimulates depolarization in next section of the membrane Threshold and the all-ornone phenomenon All action potentials are the same regardless of the strength of stimulus. Strong stimuli lead to more action potentials during a time frame. Weak stimuli lead to fewer action potentials during a time frame. Refractory period. Absolute refractory period • Time period in which no impulse can be generated Relative refractory period – • Time period in which only a strong stimulus will generate an impulse Conduction velocities of axons Speeds up to 100 m/s or more Influenced by • Axon diameter: larger = faster • Myelinated versus unmyelinated axons • Saltatory Conduction = impulse leaps from one node of Ranvier to the next Synapse = junctions between neurons Axodendritic – axon to dendrite Axosomatic - axon to cell body Axoaxonic - axon to axon Dendrodendritic – dendrite to dendrite Presynaptic neurons – conducts to the synapse Postsynaptic neuron – conducts away from the synapse Synapses Electrical synapse direct connections allow current to flow from one cell to the next Chemical synapse – use chemical neurotransmitters to conduct impulses across the synapse Parts of a chemical synapse Presynaptic neuron's axon terminal contains synaptic vesicles containing a neurotransmitter. Synaptic clefts - Fluid-filled space between the pre- and postsynaptic neurons. Postsynaptic neuron has receptors for neurotransmitters released from the synaptic vesicle. Information flow across a chemical synapse. Action potential opens calcium channels in the presynaptic membrane. Synaptic vesicles fuse with the membrane of the axon terminal and neurotransmitter is released into the synaptic cleft. Neurotransmitter binds with receptors on the postsynaptic membrane. Ions channels open leading to the depolarization or hyperpolarization of the postsynaptic membrane. Termination of the neurotransmitter effect. Neurotransmitter degraded by an enzyme. Neurotransmitter taken up by the presynaptic terminal. Diffusion of the neurotransmitter from the synaptic cleft. Postsynaptic potentials. Excitatory postsynaptic potential (EPSP). • Causes depolarization of postsynaptic membrane. • Opens channels allowing Na+ and K+ ions to cross the membrane. Inhibitory postsynaptic potential (IPSP). • Causes hyperpolarization of postsynaptic membrane. • Opens K+ or chloride channels allowing one of both of these ions to cross the membrane. Modification of synaptic events. Summation = effects add up • Temporal summation – rapid stimulation • Spatial summation – stimulation from mutiple presynaptic terminals Synaptic potentials – continued use increases ability to excite the postsynaptic membrane Neurotransmitters Acetylcholine Biogenic amines. • Catecholamines. • Dopamine • Norepinephrine • Epinephrine • Indolamines. • Serotonin • Histamine - Neurotransmitters Amino acids. • Gamma amino butyric acid • Glutamate • Glycine • Aspartate Peptides. • Substance P • Endorphins • Enkephalins - Novel messengers • ATP • NO • CO Neurotransmitters by function. Excitatory - cause depolarization Inhibitory – cause hyperpolarization SENSORY SYSTEMS Levels of Sensation Sensation - is the arrival of a sensory impulse to the brain Perception - is the interpretation of the sensation Sensory Receptors Simple receptors - General senses Complex receptors - Special senses Selectivity Types by location • Exteroceptors - outside • Interoceptors – visceral (more general) • Proprioceptors – musculoskeletal (more specific Receptors classified by stimulus Mechanoreceptor – touch, pressure, vibration, etc. Photoreceptor - light Thermoceptor - temperature Chemoreceptor - chemicals Nociceptor – damage / pain Cutaneous Sensation Tactile sensations • • • • • touch pressure vibration cold, heat pain Crude / Discriminative Touch Types of Tactile Receptors Meissner’s corpuscles – light touch Hair root plexuses – light touch Merkel discs – light touch Pacinian corpuscles – deep pressure Itch/tickle Thermoreceptors - heat Nociceptors - pain