chapter 11
outer collar of perinuclear cystoplasm
(formerly called neurilemma) cytoplasm of Schwann cell that end up a a bulge just external to the myelin sheath
multipolar neurons
(polar = end pole) have 3 or more process--one axon and the rest dendrites. They are the most common neuron type in humans, with more than 99% of neurons in this class. Are the major neuron type in the CNS
ependymal cells CNS
"wrapping garment" -- range in shape from squamous to columnar. , and many are ciliated. Line central cavities of the brain and the spinal cord, where they form a fairly permeable barrier b/w the cerebrospinal fluid that fills those cavities and the tissue fluid bathing the cells of the CNS beating of their cilia helps to circulate the cerebrospinal fluid that cushions the brain and spinal cored
pseudounipolar neurons
(pseudo= false) b/c they originate as bipolar neurons.
postsynaptic potentials excite or inhibit the receiving neuron
1. binding of neurotransmitter at excitatory chemical synapses results in local graded potentials called EPSPs, caused by the opening of the channels that allow simultaneous passage of Na+ and K+ 2. Neurotransmitter binding at inhibitory chemical synapses results in hyper polarization called IPSPs, caused by the opening of K+ or Cl- channels. ISPs drive the membrane potential farther from the threshold 3. EPSPs and ISPSs summate temporally and spatially. The membrane of the axons's initial segment acts as a neuronal integrator 4. Synaptic potentiation which enhances the postsynaptic neuron's response is produced by intense repeated stimulation. Ionic calcium appears to mediate such effects, which may be the basis of learning 5. Presynaptic inhibition is mediated by axoaxonal synapses that reduce the amount of neurotransmitter released by the inhibited neuron
bipolar neurons
2 processes -- an axon and a dendrite--that extend from opposite sides of the cell body. Are rare neurons are found in some of the special sense organs such as in the retina of the eye and in the olfactory mucosa
synaptic delay
414
excitatory postsynaptic potentials
417 -- depolarizing graded potentials
presynaptic inhibition
420-- occurs when the release of excitatory neurotramitter by one neuron is inhibited by the activity of another neuron via an axoaxonal syanpse
summate
420==add together influence such as ESPS and ISPS
axonal transport
B/c axons are often very long, the task of moving molecules along their length right appear difficult. However, through the cooperative efforts of motor proteins and cytoskeletal elements ( mostly microtubules) substances travel continuously along the axon in both directions: Anterograde movement Retrograde movement
continuous conduction
How an action potential can be propagated: in nonmyelinated axons occurs by continuous conduction b/c the voltage-gated channels in the membrane are immediately adjacent to each other. Continuous conduction is relatively slow
gut-brain peptides
a neuropeptides are produced by non neural body tissues and are widespread in the gastrointestinal tract (somatostatin and cholecystokinin)
bidirectional transport
a single bidirectional transport mechanism is responsible for axonal transport. It uses different ATP dependent motor proteins (kinesis or dynein) depending on direction of transport. These proteins propel cellular components along the microtubule like trains along tracks speeds up to 40 cm
Acetylcholine 423
acetylcholinesterase
endocannabinoid 424
act at the same receptors as tetrahydrocannabinol (THC) the active ingredient in marijuana
nitric oxide and carbon monoxide
activate guanylate cyclase, the enzyme that makes the second messenger cyclic GMP, NO, and Co
G-protein coupled receptors 425
activated G proteins typically work by controlling the production of second messengers such as cyclic AMP, Cyclic GMP, diacylglycerol
purines 423
adenosine triphosphate (ATP) Adenosine
chemically gated channels
also called ligand-gated open with appropriate chemical (in this case a neurotransmitter) binds
Neuron
also called nerve cells are the structural units of the nervous system. Neurons have extreme longevity amitotic -as they assume their roles as communicating links of the nervous system, they lose there ability to divide. We pay a high price for this feature b/c neurons cannot be replaced if destroyed. Olfactory epithelium and some hippocampal regions of the brain contain stem cells that can produce new neurons throughout life high metabolic rate--require continuous and abundant supplies of oxygen and glucose. They cannot survive for more than a few minutes w/o oxygen all have a cell body and one or more slender processes
Schwann cells PNS
also called neurolecmmmocytes--surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerve fibers. In this way, they are functionally similar to oligodendrocytes. Schwann cells are vital to regeneration of damaged peripheral nerve fibers
ANS
also called the involuntary nervous system
Leakage (non-gated) channels
alway open
terminal branches
an axon usually branches profusely at its end terminus: 10,000 or more also called terminal arborizations
chromatophillic substance/or Nisst bodies
another name for rough ER--stains darkly with basic dyes
Interneurons
are association neurons. In between motor and sensory neurons in neutral pathways and shuttle signals through CNS pathways where integration occurs. Make up over 99% of the neurons of the body Almost all multipolar, but there is considerable diversity in size and fiber-branching patterns, purkinje and pyramidal cells
neurofibils
are bundles of intermediate filaments (neurofilaments) maintain cell shape and integrity. They form a network throughout the cell body and its processes
chemical synapses
are sites of neurotransmitter release and binding. When the impasse reaches the presynaptic axon terminals, voltage-gated Ca2+ channels open, and Ca2+ enter the cell and mediates neurotransmitter release. Neurotransmitters diffuse across the synaptic cleft and attach to postsynaptic membrane receptors, opening ion channels. After binding, the neurotransmitter are removed from the synapse vy diffusion, enzymatic breakdown, or reuptake into the presynaptic terminal or astrocytes.
voltage-gated channels
are typically found only on axons. no voltage gated channels means no AP Voltage gated channels open and close in response to changes in the membrane potential. They are initially activated by local currents (graded potentials) that spread toward the axon along the dendritic and cell body membranes In many neurons, the transition from local graded potential to long-distance action potential takes place at the initial segment of the axon. In sensory neurons, the action potential is generated by the peripheral (axonal) process just proximal to the receptor region.
neuron processes
armlike processes extend from the cell body of all neurons.
axon diameter
as a rule, the larger the axon's diameter the faster it conducts impulses. Larger axons conduct more rapidly b/c they offer less resistance to the flow of local currents
conduction velocity
axon diameter degree of myelination
secretory region
axon terminals
myelinated fibers
axons bearing a myelin sheath conduct nerve impulses rapidly
absolute refractory period
begins with the opening of Na+ channels and ends when the Na+ channels begin to reset to their original resting state 410
hyperpolorization
below -70
repolarization
between 0 and +30 MV
neuronal pools 426
billions of neurons in the CNS are organized in this pool. This functional groups of neurons integrate incoming information from receptors or different neuronal polls and then forward the processed information to other destinations
oligodendrocytes CNS
branch, have fewer processes (oligo= few; dander-branch) than astrocytes. line up along the thicker nerve fibers in the CNS and wrap their processes tightly around the fibers, producing and insulating covering called a myelin sheath ***IN THE CNS, IT IS THE OLIGODENDROCYTES THAT FORM THE MYELIN SHEATHS UNLIKE schwann cell, that forms only one segment of a myelin sheath, an oligodendrocytes has multiple flat processes in the PNS, myelin sheath gaps separate adjacent sections of an son's myelin sheath
axon collatrals
branches that extend from the axon at more or less right angles
action potentials are (page 405)
brief, long-distance signals within a neuron is a brief reversal of membrane potential with a total amplitude (change in voltage) of about 100 mV (fro -70 -+30 mV) depolarization is followed by depolarization and often a short period of hyperpolorization. Action potentials do not decay with distance
graded potentials
brief, short-distance signals within a neuron Usually in dendrites of cell bodies Can be depolarization or hyperpolorization changes cause current flows to decrease in magnitude with distance. called graded b/c their magnitude varies directly with stimulus strength. The stronger the stimulus, the more the voltage changes and the father the current flows. Triggered by some change (stimulus) i neuron's environment that opens gated ion channels. Given different names depending on where they occur and the functions they perform: receptor potential or generator potential -- produced when a sensory receptor is excited by its stimulus (light pressure, chemical) postsynaptic potential -- is produced when the stimulus is a neurotransmitter released by another neuron by a fluid-filled gap called a synapse.
nerves
bundles of axons
motor (efferent) neurons
carry impulses away from the CNS to the effector organs (muscles and glands) of the body. ARE MULTIPOLAR--
efferent
carrying away --transmits impulses from the CNS to effect organs, which are the muscles and glands
afferent
carrying toward--consits of nerve fibers (axons) that convey impulses to the central nervous system from sensory receptors located throughout body
gated channels
chemically gated voltage gated mechanically gated
nuclei
clusters of cell bodies in the CNS
ganglia
clusters of cell bodies in the PNS
ganglia
collections of neuron cell bodies
electrochemical gradient 2 componenets
concentration gradient electrical gradient
non-myelinated fibers c
conduct impulses more slowly. DENDRITES ALWAYS NON-MYELINATED When schwann cells surround peripheral nerves-- a single schwann cell can partially enclose 15 or more axons, each of which occupies a separate recess int he schwann cell surface are called nonmyelinated and are typically thin fibers
central nervous system
consists of the brain and spinal cord, which occupy the dorsal body cavity.
Action potentials can be propagated in one of two ways:
continous conduction saltatory conduction
depolorization
decrease in membrane potential: inside of membrane less negative (moves closer to zero) than the resting potential;
2 types of neuron processes
dendrits and axons
Resting membrane potential depends on
difference in ion concentration and permeability
catecholamines
dopamine, norepinephrine and epinephrine
human body is
electrically neutral--it has the same number of positive and negative charges.
Central process
enters the CNS is an axon b/c it conducts impulses away from the cell body 3 facts favor classifying it as an axon 1) it generates and conducts an impulse 2)when large, it is heavily mmyelinated 3)it has a uniform diameter and is indistinguishable microscopically from an axon. **SO IS CENTRAL AND PERIPHERAL PROCESSES AN AXON OR A DENDRITE? We have chosen to emphasize the newer definition of an axon as generating and transmitting an impulse. For unipolar neurons, we will refer to the combined length of the peripheral and central process as an axon. In its place of "dendrites," unipolar neurons have receptive endings (sensory terminals) at the end of the peripheral process
neuropeptides
essentially strings of amino acids include a broad spectrum of molecules with diverse effects.
neural integration 426
first level--neuronal pools and their patterns of communicating with other parts of the nervous system
Acetylcholine
first neurotransmitter identified, best understood b/c it is released at neuromuscular junctions, which are much easier to study than synapses buried in the CNS
synapse
fluid filled gap
relative refractory period
follows the absolute refractory period. Most Na+ channels have returned to their resting state, some K+ channels are still open, and repolzarization is occurring. 410
neuroblasts 428
future neurons
Nodes of Ranvier
gaps in myelin sheath-to prevent adjacent schwann cells from touching one another occur along myelinated axon Axon collaterals can emerge at gaps
ohm's law
give the relationship b/w voltage, current, and resistance current (I) = Voltage (V)/resistance(R) Ohm's law tells 3 things: *Current (I) is directly proportional to voltage: the greater the voltage (potential difference), the greater the current *There is no net current flow b/w points that have the same potential *Current is inversely related to resistance: The greater the resistance, the smaller the current
Amino acids 423
glutamates aspartate glycine gamma aminobutyric acid GABA
lipofuscin
golden-brown pigment sometimes inside neuron cel bodies which is a harmless by-product of lysomal activity, it is sometimes called the "aging pigment" b/c it accumulates its neurons of elderly individuals
growth cone
growing of an axon 428
unipolar neurons
have single short process that emerges from the cell body and divides. T-like into proximal and distal branches. The more distal (peripheral process) is often associated with sensory receptor. The central process enter the CNS. The unipolar neurons are more accurately called pseudo unipolar neurons (pseudo = false) b/c they originate as bipolar neurons. During early embryonic development, the two processes converge and partially fuse to form the short single process that issue from the cell body. Unipolar neurons are found chiefly in ganglia in the PNS, where they function as sensory neurons
inhibitory postsynaptic potenitals
hyper polarizing changes in potential
facilitated 420
if summation yields only sub threshold depolarization the neuron is facilitated. It does not fire an action potential, but is more easily excited by successive depolarization events b/cit is already near threshold
substance P
important mediator of pain signals
threshold
in an action potential when depolarization reaches a critical level (-55 - -50 Mv)
Potassium K+ plays the most important role
in generating the membrane potential
biogenic amines
incllude catecholamines and indoamines
Biogenic amines 423
indolamines
parallel processing
inputs are segregated into many pathway and different parts of the neural circuitry deal simultaneously with the information delivered by each pathway 426
concentration gradient
ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration
electrical gradient
ions move toward an area of opposite electrical charge
resting membrane potential
is approximately -70 Mv (inside negative) in neurons
resistance
is the hindrance to charge flow provided by substances through which the current must pass. Substances with high electrical resistance are insulators, and those with low resistance or conductors
peripheral nervous system
is the part of the nervous system outside the CNS. contains nerves and ganglia
voltmeter
is used to measure the potential difference between two points. When one micro electrode of the voltmeter is inserted into a neuron and the other is in the extracellular fluid, it recored a voltage across the membrane of approximately -70 mV. The minus sign indicates that the cytoplasmic side (inside) of the membrane is negatively changed relative to the outside.
synaptic vessicle
knoblike axon terminal of the presynaptic neuron which contains many tiny membrane bound sacs contains neurotransmitters
CNS myelin sheaths
lack an outer collar of perinuclear cytoplasm b/c cell extensions do the coiling and the squeezed-out cytoplasm if forced back toward the centrally located nucleus instead of peripherially
Group B fibers
lightly myelinated fibers of intermediate diameter transmit impulses at an average rate of 15 m/s about 30 mi/h
acetylcholinesterase
located in the synaptic cleft,and on postsynaptic membranes. Presynaptic terminals recapture the release choline and reuse it to synthesize more ACH
nervous system
master controlling and communicating system of the body. (electrical and chemical signaling) consists mostly of nervous tissue which is highly cellular. Less than 20% of the CNS is extracellular space, which means that the cells are densely packed and tightly intertwined. made of neuroglia and neurons
autonomic
means a "law unto itself" and b/c we generally cannot control such activities as the pumping of our heart or the movement of food through our digestive tract, the
voltage
measure of potential energy generated by separated electrical charges is measured in either volts (V) or millivolts (1 mV = 0.001 V) Voltage is always measured between two point and is called the potential difference or simple the potential between the points the greater the difference in charge b/w 2 points, the higher the voltage
hydrogen sulfide (H2S)
most recently discovered gasotransmitter that appears to act directly on ion channels and other proteins to alter their function
Group A fibers
mostly somatic sensory and motor fibers serving skin, skeletal muscles and joints. Have largest diameter, thick myelin sheaths, and conduct impulses at speed up to 150 m/s (over 300 miles per hour)
anterograde movements
movement away from the cell body, substances moved in this direction include mitochondria, cytoskeletal elements, membrane components (vesicles) used to renew the axon plasma membrane, and enzymes needed to synthesize certain neurotransmitter. Some neurotransmitter are synthesized int he cell body, packaged into vesicles, and then transported to the axon terminals)
retrograde movements
movement toward the cell body. Substances moved in this direction are mostly organelles returning to the cell body to be degraded or recycled. Retrograde transport is also an important means of intracellular communication. It allows the cell body to be advised or conditions at the axon terminals. It also delivers vesicles of the cell body contains signal molecules (such as nerve growth factor, which activates certain nuclear genes promoting growth)
Structural classification of Neurons
multipolar neurons bipolar neurons unipolar neurons
Axon
neuron never has more than a single axon. arises from a cone-shaped area of the cell body called the axon hillock initial segment of the axon narrows to form a slender process that is uniform in diameter or the rest of its length. may have occasional branches along its length. these branches are called axon collaterals e terminal branches per neuron is not unusual
Peptides 423
neuropeptides substance P endorphins, beta endophin dynorphin enkephalins gut-brain peptides
Gasses and Lipids/ Gasotransmitters
nitric oxide carbon monoxide hydrogen sulfide endocannabinoids
dendrites
of motor neurons short tapering diffusing branching extensions Typically motor neurons have hundreds of twig like dendrites clustering close to the cell body. Virtually all organelles present in the cell body also occur in dendrites Main receptive or input regions, provide an enormous surface area for receiving signals from other neurons In many brain areas, the finer dendtries are highly specialized for collecting information. They bristle with dendritic spines--thorny appendages with bulbous or spiky ends which represent point of close contact (synapse) with other neurons Dendrites convey incoming messages toward the cell body, these electrical signals are usually not action potentials (nerve impulse) but are short-distance signals called graded potentials
peripheral process
often associated with sensory receptor
satellite cells PNS
one of the PNS-- surround neuron cell bodies located in peripheral nervous systems and are thought to have many of the same function in the PNS as astrocytes do in the CNS. name comes from a fancied resemblance to the moons satellites around a planet
voltage-gated
open and close in response to changes in the membrane potential
mechanically gated
open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure)
concentration and electrical gradient often
oppose each other, each trying to drive ions in the opposite direction. Which ever gradient is the strongest wins and drives the net flow of ions in its directions use ohm's law
adenosines
part of ATP acts outside of cells on adenosine receptors. potent inhibit int he brain (caffeine
gated channels
part of the protein forms a molecular "gate" that changes shape to open and close the channel in response to specific signals.
Perikaryon
peri-around, cary - nucleus)a sperhical nucleus surrounded by a cytoplasm
axolemma
plasma membrane
plasma membranes
provide the resistance to current flow
kinesis or dynein
see bidirectional transport
postsynaptic potential
see graded potential
receptor potential or generator potential
see graded potential
functional classification of neuron
sensory (afferent) neurons motor (efferent) neuron interneuron
synaptic cleft
separates presynaptic from postsynaptic
indolamines
serotonin and histamine
microglial cells CNS
small and opioid with relatively long "thorny" processes. transform into a special type of macrophage that phagocyitizes the microorganisms or neuronal debris.
black melanin
sometimes inside neuron cell bodies include black melanin -- a red, iron-containing pigment
insulators
substances with high electrical resistance
conductors
substances with low resistance
Neuroglia
support and maintain neurons. Once known as the flu or scaffolding that supports the neurons but are now known to have many other impt. unique functions
neuromodulator 424
term used to describe a chemical messenger released by a neuron that does not directly cause EPSP or IPSP but instead effects the strength of synaptic transmission
conducting region
the axon is the conducting region of the neuron generates nerve impulse and transmits them typically away fromothe cell body along the plasma membrane, or axolemma In motor neurons, the nerve impulse is generated at the initial segment of the axon(trigger zone) and conducted along the axon to the axon terminals , which are secretory regions
current
the flow of electrical charge from one point to another and can be used to do work--
1. sensory input 2. integration 3. motor output
the nervous system uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. Gathered info is called sensory input the nervous system processes and interprets sensory input and decides what should be done at each moment -- a process called integration. nervous system activates effector organs--the muscles and glands-- to cause a response, called motor output
degree of myelination
the presences of a myelin sheath dramatically increases the speed of propagation. The conduction velocity increases with the degree of myelination--lightly myelinated fibers conduct more slowly than heavily myelinated fibers.
depolarization
threshold (between -55 - -50 MV)
dendritic spins
throny appendages with bulbous or spiky ends which represent points of close contact (synapses with other neurons
sensory (afferent) neurons
transmit impulses fro sensory receptors in the skin or internal organs TOWARD or into the CNS. Virtually all sensory neurons are unipolar and their cell bodies are located in sensory ganglia outside the CNS
synapses
transmit signals between neurons a functional junction between neurons. information-transmitting neuron is the presynaptic neuron; the information-receiving neuron is the post=synaptic neuron
axon
trigger zone
ATP
universal form of energy (neurotransmitter)
saltatory conduction
when an AP is generated in a myelinated fiber, the local depolarizing current does not dissipate through the adjacent membrane regions, which are non excitable. Instead, the current is maintained and moves rapidly to the next myelin sheath gap, a distance of approximately 1mm, where it triggers another AP (saltare = to leap) b/c the electrical signal appears to jump from gap to gap along the axon. Is 30 x faster than continuous conduction
electrochemical gradient
when gated ion channels diffuse quicklyacross the membrane. The direction the ion moves (into or out of the cell) is determined by the electrochemical gradient
determental
when the current dies out within a few millimeters of its origin
endorphins
which include beta endorphin dynorphin, and enkephalins act as natural opiates, reducing your perception of pain under stressful conditions
myelin sheath
whitish, fatty (protein-lipoid) segment that covers nerve fibers, particularly long or large in diameter. Protect and electrically insulates fibers increase transmission speed of nerve impulses conduct nerve impulses rapdily ASSOCIATED ONLY WITH AXONS
serial processing
whole system works in a predictable all or nothing manner. one neuron stimulates the next, which stimulates the next, and so on eventually causing a specific anticipated response 426
Neurons act together making complex behaviors possible
1. CNS neurons are organized into several types of neuronal pools,, each with distinguishing patterns of synaptic connection called circuits 2. In serial processing, one neuron stimulates the next in sequence, producing specific, predictable response, as in spina reflexed. A reflex is a rapid, involuntary motor response to a stimulus 3. Reflexes are mediated over neural pathways called reflex arcs, the minimum number of elements in a reflex are is five: receptor, sensory neuron, integration center , motor neuron, and effector 4. Parallel processing, which underlies complex mental functions, impulses travel along several pathways to different integration centers 5. The four basic circuit types are diverging, converging, reverberating, and parallel after discharge
Development aspect of neurons
1. Neuron development involves proliferation, migration, and the formation of interconnections. The formation of interconnections involves axons finding their targets and forming synapses, and the synthesis of specific neurotransmitters 2. Axon outgrowth and synapse formation are guided b other neurons, glial cells, and chemicals such as N-CAM and nerve growth factor. Neurons that do not make appropriate synapses dies, and approximately two-thirds of neurons formed in the embryo undergo programmed cell death
Effect of neurotransmitter depends on its receptor
1. The major classes of neurotransmitter based on chemical structure are acetylcholine, biogenic adenines, amnio acids, peptides, purines, dissolved gasses and lipids 2. functionally,, neurotransmitters are classified as 1) inhibitory or excitatory or both 2) direct or indirect action. Direct-acting neurotransmitters bind to and open ion channels. Indirect-acting act through second messengers. Neuromodulators also act indirectly presynaptically or postsynapticaly to change synaptic strength 3. Neurotransmitter receptors are either channel-linked receptors that open ion channels, leading to fast changes in membrane potential, or G protein-coupled receptors that oversee slow synaptic responses mediated by G proteins and intracellular second messengers. Second messengers can act directly on ion channels or active kinases, which in turn active or inactivate other proteins, causing a variety of effects
action potential steps: voltage gated
1. resting state--no ions move through voltage gated channels, both Na+ and K+ closed 2. depolarization is caused by Na+ flowing into the cell (sodium channel opened) 3. Repolarization - is caused by K+ flowing out of the cell--this is when Na+ channels are inactivating and + channels open, allowing K+ to exit 5. Hyperpolarization - caused by K+ continuing to leave the cell. (some K+ channels remain open and Na+ channels are resetting). FROM BOOK SUMMARY: In the AP graph, an AP begins and ends at resting membrane potential Depolarization to approximately +30 mV (inside positive) is caused by Na+ influx. Depolarization ends when Na+ channels inactive. Repolarization and hyper polarization are caused by K+ efflux If the threshold is reached, and AP is generated, If not, depolarization remains local In nerve impulse propagation, each AP provides the depolarizing stimulus for triggering an AP in the next membrane patch. Regions that have just generated APs are refractory ; for this reason, the nerve impulse propagates in one direction only APs are independent of stimulus strength: strong stimuli causes APs to be generated more frequently but not with greater amplitude During the absolute refractory period, a neuron cannot respond to another stimulus b/c it is already generating an AP. During the relative refractory period, the neuron's threshold is elevated b/c depolarization is ongoing In nonmyelinated fibers, APs are produced in a wave all along the axon, that is, but continuous conduction. In myelinated fibers, APs are generated only at myelin sheath gaps and are propagated more rapidly by salutatory conduction
neuron cell body
consists of spherical uncles (within a conspicuous nucleus) surrounded by a cytoplasm. Also called a perikaryon o soma, the cell body ranges in diameter from 5 - 140 um most neurons, the plasma membrane of the cells body acts as part of the receptive region that receives information from other neurons cell body major biosynthetic center and metabolic center of a neuron. In addition to abundant mitochondria, it contains may sturustes you are already familiar with *protein-and membrane-making machinery. Neuron cell bodies (not axons) have the organelles needed to synthesize proteins-rough endoplasmic reticulum (ER), free ribosomes, and golgi apparatus. The rough ER also called the chromatophillic substance or Nisst bodies stains darkly with basic dyes *Cytoskeletal elements-- microtubules and neurofibrils which are bundles of intermediate filaments (neurofilaments) maintain cell shape and integrity They form a network throughout the cell body and its processes *pigment inclusions. Pigments sometimes found inside neuron cell bodies include black melanin, a red iron-containing pigments, and a golden-brown pigment called lipofuscin.
group c fibers
have smallest diameter are non-myelinated. so they are incapable of saltatory conduction and conduct impulses at a leisurely pace 1 m/s or 2 mi/h or less page 412
Myelin sheaths are formed
in the PNS by Schwann Cells, which indent to receive an axon and then wrap themselves around it in a jelly roll fashion. Initially the wrapping is loose, but the Schwan cell cytoplasm is gradually squeezed from between the membrane layers When wrapping process is complete, many concentric layers of schwann cells plasma membrane enclose the axon, much like gauze wrapped around an injured finger. This tight coil of wrapped membranes is the myelin sheath, and its thickness depends on the number of spirals. nucleus and most of the cytoplasm of the schwann cell end up as a bulge just external to the myelin sheath--called the outer collar of perinuclear cytoplasm (formerly known as the neurilemma) plasma membrane of myelinating cells contain much less protein that those of most body cells channel and carrier proteins noticeably absent myelin sheath exceptionally good insulators presence of particular protein molecules that interlock to form a sort of molecular velcro b/w adjacent myelin membrans
hyperpolorization
increase in membrane potential. Inside of membrane become more negative (moves farther from zero) than the resting potential
resting membrane potential
potential difference in a resting neuron (V1) and is said to be polarized. The value of the resting membrane potential varies (from -40 - -90 mV) in different types of neurons Resting potential exists only across the membrane; the solution inside and outside the cell are electrically neutral. 2 factors generate the rating membrane potential: *differences in the ionic composition of the intracellular and extracellular fluids *difference in the plasma membranes permeability to those ions DIFFERENCES IN IONIC COMPOSITION: Cell cytosol has a lower concentration of Na+ and a higher concentration of K+ than the extracellular fluid negatively charged (anionic) proteins help to balance the positive charges of intracellular cations (primarily K+) In extracellular fluid, the positive charge of Na+ and other cations are balanced chiefly by chloride ions (Cl-) *potassium plays the most important role in generating the membrane potential DIFFERENCES IN PLASMA MEMBRANE PERMEABILITY: At rest, the membrane is impermeable to large anionic cytoplasmic proteins, very slightly permeable to sodium, approximately 25Xmor permeable to potassium than to sodium , and quite permeable to chloride ions. Potassium ions diffuse out of the cell along their concentration gradient much more easily than sodium ions can enter the cell along theirs, K+ flowing out of the cell causes the cell to become more negative inside, Na+ trickling into the cell makes the cell just slightly more positive than it would be if on K+ flowed. Therefore, at resting membrane potential, the negative interior of the cell is due to much greater ability for K+ to diffuse out of the cell than for Na+ to diffuse into the cell. B/C K+ is ALWAYS leaking out of the cell and some Na+ is always leaking in, you might think that the concentration gradient would eventually run down, resulting in equal concentrations of Na+ and K+ inside and outside the cell. This does not happen b/c the ATP-driven sodium-potassium pump first eject 3 Na+ from the cell and then transports 2 K+ back into the cell. In other words, the sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium CHANGING THE RESTING MEMBRANE POTENTIAL: Neurons use changes in their membrane potential as signals to receive, integrate, and send information. A change in membrane potential can be produced by 1) anything that alters ion concentrations on the two sides of themembranes 2) anything that changes membrane permeability to any ion CHANGES IN MEMBRANE POTENTIALS PRODUCE 2 TYPES OF SIGNALS: Graded potentials-- short distances, variable graded strength action potentials-long distance signals that always have the same strength
Astrocytes CNS
shaped like delicate branching sea anemones, astrocytes "star cells" are the most abundant and versatile glial cells. numerous radiating process cling to neuron and their synaptic endings, and cover nearby capillaries. Support and brace neurons and anchor them to their nutrient supply lines play a role in making exchanges b/w capillaries and neurons helping determine capillary permeability. guide migration of young neurons and formation of synapses b/w neurons. control chemical environment around neurons, where their most import. job is "mopping up" leaked potassium ions and recapturing recycling released neurotransmitter. connected by gap junctions, astrocytes signal each other with slow-paced intracellular calcium pulses (calcium waves) and by releasing extracellular chemical messengers