Nervous System Cells-chapter 12
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What is the function of the nervous system?
- Detects change in internal and external environments, evaluate the information, and initiate an appropriate response.
What is the nervous system made up of?
The brain, the spinal cord, and the nerves
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
Peripheral nervous system (PNS)
- Nerves that lie in "outer rigions" of nervous system
- Cranial nerves-originate from brain
- Spinal nerves-originate from spinal cord
Afferent division-consist of all incoming _______ pathways.
Efferent division-consist of all outgoing _______ pathways.
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
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
Sympathetic divions of ANS
prepares the body to deal with immediate threats to the internal environment; produces "fight-or-flight" response
Parasympathetic divison of ANS
coordinates the body's normal resting activities; sometimes called the "rest-and-repair" division
glial cells support the neurons
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)
Microglia (type of glia)
- small, usually stationary, cells
- in inflamed brain tissue, they enlarge, move about, and carry on phagocytosis
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
Oligodendrocytes (type of glia)
- smaller than astrocytes with fewer processes
- hold nerve fibers together and produce the myelin sheath
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
excitable cells that initiate and conduct impulses that make possible all nervous system functions
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
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
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
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
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
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
A signal conduction route to and from the CNS, with the electrical signal beginning in receptors and ending in effectors
- most common; consist of afferent neurons, interneurons, and efferent neurons.
- Goes from afferent neurons to CNS to efferent neursons.
Simplest form; consist of afferent and efferent neurons
where nerver signals are transmitted from one neuron to another
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.
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
What is CNS referred to?
What is PNS referred to?
- contain sensory and motor neurons
- sensory nerves-nerves with predominantly sensory neurons
- motor nerves-nerves with predominantly motor neurons
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
- 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
difference in electrical charge is called ________ because it is a type of stored energy
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.
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
slight shift away from the resting membrane in a specific region of the plasma membrane
when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move towards zero (depolarization)
when a stimulus triggers the opening of additional K+ channels, incrasing the membrane potential (hyperpolarization)
local potentials are called grade potentials because the magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulus
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.
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.
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.
two types of synapses (junctions):
- 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.
tiny bulge at the end of a terminal branch of a presynaptic neuron's axon that contains vesicles housing neurotransmitters.
space between a synaptic knob and the plasma membrane of a postsynaptic neuron
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
plasma membrand of a postsynaptic neuron
has protein molecules that serve as receptors for the neurotransmitters
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.
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.
Neurotransmitters-neurons communicate with one another
Classification of neurotransmitters
- Function determined by the postsynaptic receptor
- chemical structure-mechanism by which neurotransmitters cause a change
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
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.
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
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.
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
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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