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Ch12
nervous system
91
Anatomy
Undergraduate 3
06/13/2013

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Term
nervous system
Definition
the body's most rapid means of maintaining homeostasis (maintenance of constant internal environment)
Term
Central Nervous System (CNS)
Definition

-brain: 100 billion neurons (each synapse with 1,000-10,000 other neurons)

-spinal cord

Term
peripheral nervous system (PNS)
Definition
communication between CNS and rest of body
Term
components of PNS
Definition

-cranial nerves (12 pairs) emerge from brain

-spinal nerves (31) pairs emerge from spinal cord

-ganglia:masses of neuron cell bodies outside of CNS

-enteric plexuses: network of neurons in GI tract

-sensory receptors: structure that monitors inter/exter environment

Term
divisions of nervous system
Definition

-somatic

-autonomic

Term
somatic nervous system
Definition

-controls skeletal muscle

-voluntary

Term
autonomic nervous system
Definition

-sympathetic division (responds to short term stress)

-parasympathetic division(returns body to normal functions following stress)

-controls smooth muscle, cardiac muscle and glands

-involuntary

Term
enteric nervous system
Definition

-controls smooth muscle and glands of the digestive system

-involuntary

Term
sensory (afferent)
Definition
input: senses changes in external and internal environment and transmits changes via sensory neurons/afferent neurons to CNS
Term
integrative (processing)
Definition
interprets changes (SOLELY IN CNS)
Term
motor (efferent)
Definition
output: responds to changes in form of muscular contraction/gland secretion via motor neurons/efferent neurons
Term
a motor neuron that stimulates a muscle cell to contract belongs to what structural classification of the nervous system?
Definition

A. Efferent

B. Afferent

C. CNS

D. PNS

Term
neurons
Definition
posseses electrical exitability, respond to stimuli, and specialize in conducting action potential (nerve impulse)
Term
cell body
Definition

-clustered into ganglia in PNS

-clustered into nuclei in brain

-clustered into horns in spinal cord

-contains nucleus

-contain nissl bodies-rough er-site of protein synthesis

Term
neruofibrils
Definition

part of cytoskeleton, bundles of intermediate filament that provide shape and support

 

 

Term
microtubules
Definition
assist in moving materials between cell body and axon
Term
dendrites
Definition
extensions that recieve info along with the cell body in motor neurons and interneurons or generate input in sensory neurons (once extension becomes myelinated, then it is called an axon)
Term
axons
Definition

-conduct action potentials toward axon terminal 

-distal end of axons swell into synaptic end bulb that contain neurotransmitters in synaptic vessicles

-bundles of neuron axons in cns:tracts(axons bundled with neuroglia)

-bundles of neuron axons in pns:nerves(axons bundled with endoneurium, perineurium, epineurium)

-frequently myelated in both cns and pns

Term
structural classification:
Definition
classification of neurons according to number of processes from the cell body
Term
unipolar neuron
Definition

one process from cell body

-sensory/afferent in function

-begins as a bipolar neuron in embryo but fuses into single extension

Term
bipolar neuron
Definition

2 extensions from cell body

-1 main dendrite 1 axon

-ex: rods and cones (shape of dendrite) of retina, olfactory nuerons, inner ear neurons

Term
multipolar neuron
Definition

many extensions from cell body

-most of CNS (internueurons) and all motor neurons

Term
functional classification
Definition
classification according to direction which impulses are conducted relative to CNS
Term
sensory (afferent) neuron:
Definition

strictly PNS-transmit impulses toward CNS from receptors

-include both unipolar (mostly) and bipolar neurons

-in unipolar neurons, cell bodies are just outside the spinal cord in a structure called posterior dorsal root ganglia

Term
neuroglia
Definition

support, connect, protect the neurons in both cns and pns

-cancer of NS-(gliomas) involves neuroglia and not neurons bc neuroglia have retained mitotic aility but neround have not retained mitotic ability beyond infancy

-neruoglia outnumber neurons by 5-50x

Term
astrocytes
Definition

star shaped

-twine around neurons to form supporting network

-attach neurons to blood vessels

-create blood-brain barrier

-produce "scar tissue" if there is damage to cns

 

Term
microglia
Definition

derived from monocytes

-become phagotic and remove injured brain or cord tissue

Term
ependymal cells
Definition

epithelial cells that line ventricles of brain and central canal of cord

-ciliated to assist in circulation of CSF

Term
oligodendrocytes
Definition

similar to astrocytes but have fewer extensions

-produce myelin sheath in CNS

 

CAMEO: Cns, Astrocyte, Microglia, Ependymal, Oligodendrocyte

Term
schwann cells
Definition
support axons and produce myelin sheath in PNS
Term
satellite cells
Definition
support cell bodies in pns
Term
myelin sheath
Definition

produced around an axon by the following two neuroglia cells:

oligodendrocyes and schwann cells

Term
oligodendrocytes
Definition

-cns

-can myelinate up to 15 different neurons (axons)

Term
schwann cells
Definition

-pns

-can have up to 500 schwann cells along the longest neurons (mylenate only one axon)

Term
nodes of ranvier
Definition

gaps in myelinated neuron where myelin is absent.

-produced by both schwann cells as well as oligondendrocytes, so nodes of ranvier are present in both CNS and PNS

Term
white matter of myelin sheath
Definition

cell processes (axons) with myelin

-nerve fiber: general term for any process projecting from the cell body of a neuron 

Term
gray matter in myelin sheath
Definition

parts of neuron, especially cell bodies and dendrites, that lack myelin.

-always located in protected areas of CNS

-ganglia would also be gray bc cell bodies are NOT myelinated.

Term
action potential
Definition

an electric signal that propagates along the membrane of a neuron or mucle fiber and allow for communication over long distances.

 

Term
excitability
Definition

found in action potentials.

-ability to respond to a stimulus and convert it into an action potential

-stimulus: any condition capable of altering the cell's membrane potential enough to cause an AP

-nerve conduction of action potentials involves an electrochemical mechanism

Term
ion channels
Definition

-proteins in the cell membrane

-don't require ATP-movement of ions is by channel-mediated facilitative diffusion

-nongated

-leakage channels: randomly open

--cell membranes of muscle/neurons have more K+ leakage channels than Na+ leakage channels

Term
gated ion channels
Definition

open and close in response to some stimulus (chemical, electrical, mechanical)

-chemical (ligand) gated: open adn close in response to chemical like neurotransmitters, hormones, (dendrites and cell bodies).

Term

Mechanically gated ion channels

 

Definition
open and close in response to mechanical vibration or pressure such as sound waves or pressure of touch/stretch (dendrites of sensory neurons)
Term
voltage gated ion channels
Definition
open and close in response to voltage (axons only)
Term
Na+K+ pump (Na+K+ ATPase)
Definition
movement of 3 Na+ ions out of the cell and 2 K+ ions into the cell by active transport which requires ATP
Term
Resting membrane potential (RMP)
Definition

nonconducting neuron has a rmp of -70mV

-small amounts of K+ move to the outside through leakage (nongated) channels with anions following (cannot diffuse through the membrane and get stuck at the membrane)

-the inside of the cell has a more negative charge thane the outside which is positive; overall the inside of the membrane is -70mV

Term
Reason for resting membrane potential
Definition

-the inside of the membrane has non-diffusible anions (-) phosphate and protein anions

-K- ions are more numerus on the inside than on the outside

-Na+ and Cl- ions are more numerous outside

Term
All or None Principle
Definition

-neuron transmits action potentials according to all or none principle

-if the stimulus is strong enough to generate an action potential, the impulse is conducted down the neuron at a constant and maximum strength for the existing conditions

-stimulus must raise membrane potential to less negative than -55mV (threshold position)

Term
Graded potentials-local potentials
Definition

-affected at site of stimulation and effect decreases with distance

-spreads passively

-the stronger the stimulus, the greater the change in potential and the larger area affected

-the potential change could be positive or negative

Term
Excitatory stimulus
Definition

increases Na+ into cell making membrane hypopolarized

-partially depolarizes and makes membrane less negative

-causes depolarization (but not to -55mV)

-single excitatory stimulus usually does not initiate nerve impulse but membrane is closer to the threshold and more likely to reach threshold with next excitatory stimulus

Term
inhibitory stimulus
Definition

increases K+ outward or increases Cl- inward

-makes membrane more negative

-makes the membrane hyperpolarized (as low as -90mV)

-generation of action potential is now more difficult

Term
action potential (ap)
Definition

rapid change in membrane potential (polarity) that can spread down the length of the axon

-the membrane will depolarize and then repolarize

-only muscle and neurons can produce AP

-in neurons, an AP lasts about 1ms or less

-propagation of APs down axons=nerve impulses

Term

First step in generating an action potential

 

Definition

-depolarization: when a stimulus is applied, if the sum of stimuli is excitatory(mechanical gated of chemical gated ion channels open and cause a net flow of Na- into the cell) and depolarization occurs to threshold potential (threshold=-55mV)

-at -55mV, voltage gated Na+ channels open and Na+ rushes in (na+inflow) making the inside of the cell positive

Term
2nd step of action potential
Definition

repolarization: membrane potential returns to a negative value

-it is due to K+ ions flowing outward through voltage gated K+ channels

-channels open in response to positive membrane and remain open until membrane potential returns to a negative value

-ion distribution is reverse of that at resting

-inside= (-)

Term
3rd step of action potential
Definition

refractory period: period of time during which an excitable cell cannot generate another action potential

-because of ion distribution has not returned to resting, sufficient potential has not built up on either side of the membrane to generate a new action potential

-refractory period begins at depolarization and continues until the resting membrane ion distribution is restored.

Term
Leakage channels
Definition

allow ions to flow into and out of the cell

-Na+K+ pump also operates in restoring the resting ion distribution by pumping Na+ out of the cell and K+ into the cell

 

Term
propagation of action potentials
Definition

-each action potential acts as a stimulus for development of another action potential in an adjacent segment of membrane

-Na+ inflow during the depolarization phase of an action potential diffuses to an adjacent membrane segment

-increase in Na+ concentration raises the membrane potential of that membrane segment to the threshold potential, generating a new action potential

Term
AP Facts
Definition

-action potentials do not travel but are regenerated

in sequence along an axon like tipping dominoes

-refractory period prevents action potential from going backwards

-action potentials continue to be regenerated in sequence until the potential reaches the end of an axon

Term
saltatory conduction
Definition

action potentials are only generated at the nodes of ranvier

-ap will skip from node to node 

-ionic movement is inhibited beneath myelin sheath

-conserves energy because Na-K pump is not needed as extensively because only nodes of ranvier are depolarized and repolarized

-conducts up to 50x faster than unmyelinated neuron

Term
speed of impulse conduction (propagation) is determined by:
Definition

-presence of myelin sheath: the further the nodes are apart, the faster the transition

-diameter of fiber: the greater the diameter the greater density of voltage gated Na+ channels. the greater the diameter, the faster the transmission

temperature: the greater the temperature the faster the transmission

-localized cooling can block impulse conduction; therefore pain can be reduced by application of ice

Term
types of nerve fibers based on transmission speed
Definition

-A fibers: myelinated and large diameter; fastest conduction; in areas where split second responses can mean survival, speeds up to 280mph

-B fibers: myelinated and smaller diamter; speeds up to 32mph

-C fibers: unmyelinated and smaller diameter; speeds up to 4mph

---note: B and C fibers are going to and from viscera

Term
Synapse
Definition
connection between axon terminal and another neuron, muscle (neuromuscular junction), or gland (neuroglandular junction)
Term
electrical synapse
Definition
ionic current spreads directly from one cell to another through gap junctions (found in cardiac and smooth muscle)
Term
chemical synapse
Definition

neurotransmitter is secreted from one cell and a second cell responds to it

-flow of information is one direction

--Structure of chemical synapse

-synaptic end bubld of first neuron (presynaptic neuron)=presynaptic membrane

-presynaptic electrical signal is converted to chemical signal

-presynaptic neuron releases neurotransmitter

Term
synaptic cleft
Definition

20-50 nm gap between neuron and next structure

-impulse cannot jump cleft, therefore, will need chemical transmission in form of neurotransmitter

 

Term

cell membrane of second neuron (postsynaptic neuron)=postsynaptic membrane

-postsynaptic neuron has receptors for neurotransmitter

-postsynaptic neuron receives chemical signal (neurotransmitter) and in turn may generate an electrical signal (action potential)

Definition
Term
exocytosis of neurotransmitter
Definition

-when nerve impulse (action potential) arrives at synaptic end bulb, the depolarization phase opens voltage cated Ca2+ channels

-extracellular Ca2+ flows inward

-Increase in Ca2+ inside the neuron, triggers exocytosis of synaptic vessicles

-neurotransmitter enters synaptic cleft

Term
neurotransmitters
Definition

interact with receptor sites of chemically gated ion channels on the postsynaptic membrane to produce EPSP: excitatory postynaptic potential: a type of graded potential

--typically results from the opening of chemically gated Na+ channels

Term
IPSP
Definition

inhibitory postysnaptic potential-a type of graded potential

--typically results from the opening of chemically gated K+ channels of Cl- channels

Term
Summation of EPSP and IPSP
Definition

inhibition or excitation

-spatial (multiple synapse stimulation)

Term
whether a neurotransmitte is excitatory or ibhibitory is determined by the
Definition
postsynaptic membrane receptor
Term
ACh
Definition

main neurotransmitter of PNS (not common in CNS)

-excitatory for skeletal muscle

-inhibitory for cardiac muscle

-important in hippocampus for memory consolidation (destroyed in Alzheimers)

Term
adenosine
Definition

excitatory in pns and cns

-caffeine accts as a competitive inhibitor at adenosine receptors in the brain

Term
dopamine (da)
Definition

-secreted in the limbic system, cerebral cortex, basal ganglia, hypothalamus (brain)

-excitatory for emotional response but inhibitory in basal ganglia

-responsible for pleasurable expiriences

-low levels are associated with parkinsons

-excess is associated with schitzophrenia

-seems to be the neurotransmitter involves in addiction to heroin, meth, cocaine, marijuana, aocohol, nicotine, and caffeins.

-cocaine prevents DA reuptake

Term
norepinephrine (ne)
Definition

-found in the brain and secreted by sympathetic nervous system

-affects mood

-low levels are associated with depression

-methamphetamines (speed)-prevents NE reuptake

 

Term

epinephrine=adrenaline

 

Definition

-secreted by the adrenal gland

-enhances sympathetic nervous system response

Term
serotonin
Definition

-produced from amino acid, tryptophan

-high amounts in milk and turkey

-secreted in limbic system, hypothalumus, cerebellum, spinal cord.

--affects mood, sleep, memory

Term
glycine
Definition

-common inhibitory neurotransmitter in spinal cord (1/2) of inhibitory synapses in cord use glycine)

-tetanus toxin inhibits clycine, causing "lockjaw"

-strychnine blocks glycine receptors, causing the disphragm to continously contract which leads to suffocation

Term
GABA (Gamma aminobutyric acid)
Definition

-1/2 of inhibitory synapses in spinal cord use gaba

-common inhibitory neurotransmitter in brain (as many as 1/3 of brain isynapses use gaba)

-prevents chaos in nervous system

-gaba reduces anxiety

-valium, xanax, alcohol and barbituates enhance the action of gaba

-treatment for epilepsy is a drug that increases gaba

Term
glutamate
Definition

-common in excitatory in cns

-increase in glutamate after stroke may lead to death of neurons bc of oxygen deprivation to glutamate transporters that wrk by active transport (requires oxygen for atop synthesis)

 

Term

aspartate (aspartic acid)

 

Definition
-common excitatory in cns
Term
peptides
Definition
series of covalently linked amino acids
Term
substance P
Definition
neurotransmitter in pain pathways (mediates our perception of pain)
Term
enkephalins and endorphins
Definition

endogenous morphine like substances

-both are structurally similar to morphine and bind to morphine receptors

Term
circuits
Definition

-typical neuron recieves input from 1000 to 10000 synapses

-each presyn. neuron may branch and synapse with up to 25,000 or more different postsynaptic neurons

-convergence-single postsynaptic neuron controlled by converging signals coming from 2 of more presynaptic neurons

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