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What is the function of the nervous system?
- Communicate
- Detects change in internal and external environments, evaluate the information, and initiate an appropriate response.
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What is the nervous system made up of?
The brain, the spinal cord, and the nerves
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Central nervous system (CNS)
- Structural and functional center of entire nervous system
- Consist of brain and spinal cord
- Integrates sensory information, evaluates it, and initiates an outgoing response
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Peripheral nervous system (PNS)
- Nerves that lie in "outer rigions" of nervous system
- Cranial nerves-originate from brain
- Spinal nerves-originate from spinal cord
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Afferent division-consist of all incoming _______ pathways.
Efferent division-consist of all outgoing _______ pathways.
sensory; motor
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Somatic Nervous System (SNS)
Somatic motor division-carries information to somatic _______ (skeletal muscles)
Somatic sensory division-carries feedback information to somatic _______ centers in the CNS
effectors; integration
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Autonomic nervous system (ANS)
Efferent division of ANS-carries information to the autonomic or visceral effectors (smooth and cardiac muscles and glands)
Visceral sensory division-carries feedback information to autonomic integrating centers in the CNS
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Sympathetic divions of ANS
prepares the body to deal with immediate threats to the internal environment; produces "fight-or-flight" response
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Parasympathetic divison of ANS
coordinates the body's normal resting activities; sometimes called the "rest-and-repair" division
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Glia
glial cells support the neurons
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Astrocytes (type of glia)
- star-shaped, largest, and most numorus type of glia
- cell extension connect to both neurons and capillaries
- transfer nutrients from blood and neurons
- form tight sheaths around brain capillaries, which with tight junctions between capillary endothelial cells, constitute the blood-brain barrier (BBB)
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Microglia (type of glia)
- small, usually stationary, cells
- in inflamed brain tissue, they enlarge, move about, and carry on phagocytosis
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Ependymal cell (type of glia)
- resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS
- some produce fluid; others aid in circulation of fluid
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Oligodendrocytes (type of glia)
- smaller than astrocytes with fewer processes
- hold nerve fibers together and produce the myelin sheath
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Schwann cells (type of glia)
- found only in PNS
- suppport nerve fibers and form myelin sheaths
- gaps in the myelin sheath are called nodes of Ranvier
- Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowth
- Satellite cells are Schwann cells that cover and support cell bodies in the PNS
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Neurons
excitable cells that initiate and conduct impulses that make possible all nervous system functions
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Cell body or Perikaryon (components of neurons)
- ribosomes, rough endoplasmic reticulum, Golgi apparatus
- provide protein molecules (meurotransmitters) needed for transmission of nerve signals from one neuron to another
- neurotransmitters are packaged into vessicles
- provide proteins form maintaining and regenerating nerve fibers
- mitochondria provide energy (ATP) for neuron; some are transported to end of axon
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Dendrites (components of neurons)
- each neuron has one or more dendrites, which branch from the cell body
- conduct nerve signals to the cell body of the neuron
- distal ends of dendrites of sensory neurons are receptors
- Dendricts Spine
-small knob-like protrusions on dendrites of some brain neurons; serve as connection points for axons of other neurons
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Axon (components of neurons)
- singel process extending from the axon hillock, sometimes covered by a fatty layer called a myelin sheath.conducts nerve impulses away from the cell body of the neuron
- distal tips of axons are telodendria, each of which terminates in a synaptic knob
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cytoskeleton (components of neurons)
- microtubules and microfilaments, as well as neurofibrils (bundles of neurofilaments)
- allow the rapid transport of small organelles
- vesicles (some containing neurotransmitters), mitochondria
- motor molecules shuttle organelles to and from the far ends of a neuron
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Functional regions of the neuron
- input zone-dendrites and cell body
- summation zone-axon hillock
- conduction zone-axon
- output zone-telodendria and synaptic knobs of axon
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Classification of neurons
Structural classification-classification according to number of processes extending from cell body
- Multipolar-one axon and several dendrites
- Bipolar-only one axon and one dendrite; least numerous kind of neuron
- Unipolar (pseudounipolar)-one process comes off neuron cell body but divides almost immediately into two fibers: central fiber and peripheral fiber
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Reflex arc
A signal conduction route to and from the CNS, with the electrical signal beginning in receptors and ending in effectors
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Three-neuron arc
- most common; consist of afferent neurons, interneurons, and efferent neurons.
- Goes from afferent neurons to CNS to efferent neursons.
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Two-neuron arc
Simplest form; consist of afferent and efferent neurons
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synapse
where nerver signals are transmitted from one neuron to another
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Two type of synapses
- Electrical and chemical
- chemical synapses are typocal in the adult
Chemical synapses are located at the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron.
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Nerves-bundles of peripheral nerve fibers held together by several layers of connective tissue
Tracts-within the CNS, bundles of nerve fibers are called tracts rather than nerves
- Endoneurium-delicate layers of fibrous connective tissue surrounding each nerve fiber
- Perineurium-connective tissue holding together fascicles (bundles of fibers)
- Epineurium-fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve
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What is CNS referred to?
What is PNS referred to?
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Mixed nerves
- contain sensory and motor neurons
- sensory nerves-nerves with predominantly sensory neurons
- motor nerves-nerves with predominantly motor neurons
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Repair of Nerve fibers
- mature neurons are incapable of cell division; therefore, damage to nervous tissue can be permanent
- neurons have limited capacity too repair themselves
- repair if not extensive, the cell body and neurilemma are intact
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Nerve Impulses
- Membrane potentials-all living cells maintain a difference in the concentration of ions across their membranes
- Slight excess of positively charged ions on outside of membrane
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difference in electrical charge is called ________ because it is a type of stored energy
potential
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Polarized membrane-membrande that exhibits a membrane potential
Measured in millivolts (mV).
sign of membrane's boltage indicates the charge on the inside surface of a polarized membrane.
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Resting membrane potential (RMP)
- slight excess of positive ions on a membrane's outer surface is produced by ion transport mechanisms and the membran's permeability characteristics
- the membrane's selective permeability helps maintain slight positive ions outside
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local potentials
slight shift away from the resting membrane in a specific region of the plasma membrane
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excitation
when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move towards zero (depolarization)
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inhibition
when a stimulus triggers the opening of additional K+ channels, incrasing the membrane potential (hyperpolarization)
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Grade potentials
local potentials are called grade potentials because the magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulus
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action potential-the membrane potential of the neuron that is conducting and impulse; also known as a nerve impulse
Mechanisms that produce action potential (all-or-none response)
- stimulus triggers Na+ channel, allowing Na+ inside cell=>depolarization.
- threshold potential is reached, voltage-gated Na+ channels open => more depolarization.
- voltage-gate Na+ stays open for only 1 millisecond before close.
- after action potential peaks, membrane moves to resting when K+ channels open, allowing outward diffusion of K+; process is known as repolarization.
- brief period of hyperpolarization occurs and then the resting membrane potential is restored by the sodium-potassium pumps.
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Absolute Refractory period-brief period (about 1/2 millisecond) where local area of neuron's membrane resists restimulation and will not respond to a stimulus, no matter how strong.
Relative refractory period-membrane is repolarized and restores the resting membrane potential; few seconds after the absolute refractory period; membrane will respond only to a very strong stimulus.
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at the peak of the action potention, the plasma membrane's polarity is now the reverse of the RMP
cycle continues to repeat.
- action never moves backwards, as a consequance of the refractory period.
- in myelinated fibers, action potentials in the membrane only occurs at the nodes of Ranvier; the type of impulse conditions is called saltatory conduction.speed of nerve condition depends on diameter and on the presence or absence of a myeline sheath.
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two types of synapses (junctions):
electrical synapses
chemical synapses
- electrical synapses-cells joined by gap junctions allow an ction potential to simply continue along postsynaptic membrane.
- chemical synapsus-presynaptic cells release chemical transmitters (Neurotransmitters) across a tiny gap to the postsynaptic cell, possibly inducing an action potential there.
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Synaptic knob
tiny bulge at the end of a terminal branch of a presynaptic neuron's axon that contains vesicles housing neurotransmitters.
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Synaptic cleft
space between a synaptic knob and the plasma membrane of a postsynaptic neuron
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arrangements of synapses
- axodendritic=axon signals postsynaptic dendrite; common
- axosomatic=axon signals postsynaptic soma; common
- axoaxonic=axon signals postsynaptic axon; may regulate action potential of postsynaptic axon
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plasma membrand of a postsynaptic neuron
has protein molecules that serve as receptors for the neurotransmitters
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sequence of mechanism of synaptic transmission
- action potential reach synaptic knob, causing calcium ions to diffuse into the knob rapidly
- increase calcium concentration triggers that release of neurotransmitter via exocytosis
- neurotransmitter molecules diffuse across synaptic cleft and bind the receptor molecules, causing ion channels to open
- opening of ion channels produces a postsynaptic potential, either a excitatory postsynaptic potential (EPSP) or an inhabitory postsynaptic potential (IPSP).
- neurotransmitter->terminated by either neurotransmitter molecules being transported back into the synaptic knob (reuptake) and/or metabolized into inactive compounds by enzymes and/or diffused and taken up by nearby glia.
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Spatial summation vs. temporal summation
- spatial summation-adding together the effects of several knobs being activated simultaneously and stimulating different locations on the postsynaptic membrane, producing an action potential
- temporal summation-when synaptic knobs stimulate a postsynaptic neuron in rapid succession, their effects can summate over a brief period of time to produce an action potential.
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Neurotransmitters-neurons communicate with one another
Classification of neurotransmitters
- Function determined by the postsynaptic receptor
- chemical structure-mechanism by which neurotransmitters cause a change
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Acetylcholine (small-molecule neurotransmitters)
- unique chemical structure; acetate (acetyl coenzyme-A) with choline
- deactivated by acetylcholinesterase, with the choline molecules being released and transported back to presynaptic neuron to combine with acetate (to form new acetylcholine)
- present in various locations
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Amines (small-molecule neurotransmitters)
- synthesized from amino acid molecuels
- two catergories: monoamines and catecholamines
- found in various regions of the brain; affecting learning, emotions, motor control, etc.
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Amino acids (small-molecule neurotransmitter)
- believes to be among the most common neurotransmitters of the CNS
- in the PNS, amino acids are stored in synaptic vesicles and used as neurotransmitters
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other small transmitters (small-molecule neurotransmitters)
- Nitric oxide (NO) derived from an amino acid
- NO from a postsynaptic cell signals that presynaptic neuron, providing feedback in a neural pathway.
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Neuropeptides (large-molecule neurotransmitters)
- peptides made up of 2 or more amino acids
- secreated by themselves or with 2nd or 3rd neurotransmitter; in this case it acts as a neuromodulator, a "cotransmitter" that regulates the effects of the neurotransmitter released among with it.Neurotrophins (nerve growth) stimulate neuron development but also can act as neurotransmitters and neuromodulators
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Nerver tissue development begins in actoderm. Occurs most rapidly in womb and in first 2 years.
Nervous cells organize into body network
- Synapse-form and reform until nervous system is intact
- formation of new synapses and strengthening or elimination of old synapses stimulate learning and memory.
- Aging causes degeneration of the nervous system, which may lead to senility
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