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