A&P Nervous System ch.11

voltage

the measure of potential energy generated by separated electrical charges; measured in either volts or millivolts. Voltage is always measured between two points and is called the potential difference or simply the potential between the points. The greater the difference in charge between two points, the higher the voltage.

Dendrites

a neuron's bushy, short, tapering, diffusely branching extensions that receive messages and conduct impulses toward the cell body; 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; Dendrites convey incoming messages toward the cell body. These electrical signals are usually not action potentials (nerve impulses) but are short-distance signals called graded potentials.

chemical synapse is made up of two parts:

1) A knoblike axon terminal of the presynaptic neuron, which contains many tiny, membrane-bound sacs called synaptic vesicles, each containing thousands of neurotransmitter molecules 2) A neurotransmitter receptor region on the postsynaptic neuron's membrane, usually located on a dendrite or the cell body

2 ways an Action potentials can be propagated (depending on whether myelin is present or absent from the axon):

1) Continuous conduction 2)Saltatory conduction

Repolarization

During which phase of an action potential are voltage-gated K+ channels open, while voltage-gated Na+ channels are closed?

summate

EPSPs and IPSPs can add together to influence the activity of a postsynaptic neuron.

Changes in membrane potential can produce two types of signals:

Graded potentials and action potentials

facilitated

If summation yields only subthreshold depolarization, the neuron is facilitated. It does not fire an action potential, but is more easily excited by successive depolarization events because it is already near threshold.

converging

In a __________ circuit, multiple presynaptic neurons innervate a single postsynaptic neuron.

Hyperpolarized

In a neuron, a membrane potential of -90 mV is considered __________.

Amino acids

It is difficult to prove a neurotransmitter role when the suspect is an amino acid, because amino acids occur in all cells of the body and are important in many biochemical reactions. The amino acids for which a neurotransmitter role is certain include glutamate, aspartate, glycine, and gamma (γ)-aminobutyric acid (GABA), and there may be others.

absolute refractory period

a patch of neuron membrane is generating an AP and its voltage-gated sodium channels are open, the neuron cannot respond to another stimulus, no matter how strong. This period begins with the opening of the Na+ channels and ends when the Na+ channels begin to reset to their original resting state. 1) Ensures that each AP is a separate, all-or-none event 2) Enforces one-way transmission of the AP. Because the area where the AP originated has just generated an AP, the Na+ channels in that area are inactivated and no new AP is generated there. For this reason, the AP propagates away from its point of origin.

the neuron is in the absolute refractory period

Which of the following explains the inability of a neuron to respond to a second stimulus of equal strength to the first stimulus to which it has already responded?

large diameter

Which of the following factors is associated with increased conduction velocity in an axon?

Acetylcholine

Which of the following is NOT a biogenic amine neurotransmitter?

bipolar

Which of the following is a rare type of sensory neuron that is found in special sensory structures such as the retina of the eye?

voltage-gated ion channels

Which of the following is characteristic of an action potential?

leakage channels

Which of the following membrane ion channels in the neuron are always open?

astrocytes

Which of the following neuroglia are most responsible for helping determine capillary permeability in the central nervous tissue?

Endorphins

Which of the following neurotransmitters act(s) as a natural opiate?

All voltage-gated Na+ and K+ channels are closed

Which of the following statements best describes the resting membrane state in the neuron?

The postsynaptic membrane becomes more permeable to potassium and chloride

Which of the following statements concerning inhibitory synapses (IPSPs) is correct?

Acetylcholinesterase

Which of the following substances is NOT a neurotransmitter?

Depolarization

a decrease in membrane potential: The inside of the membrane becomes less negative (moves closer to zero) than the resting potential. For instance, a change in resting potential from −70 mV to −65 mV is a depolarization.

synapse

a junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell—it's where the action is.

Schwann Cells (Neuroglia of PNS)

a.k.a., neurolemmocytes; surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerve fibers; functionally similar to oligodendrocytes. Schwann cells are vital to regeneration of damaged peripheral nerve fibers.

neuron cell body

a.k.a., perikaryon or soma; consists of a spherical nucleus (with a conspicuous nucleolus) surrounded by cytoplasm; the cell body ranges in diameter from 5 to 140 μm; Most neuron cell bodies are located in the CNS, where they are protected by the bones of the skull and vertebral column.

Somatic Nervous System (Motor Division)

a.k.a., voluntary nervous system; composed of somatic motor nerve fibers that conduct impulses from the CNS to skeletal muscles; it allows us to consciously control our skeletal muscles.

all-or-none phenomenon

action potentials either happen completely, or not at all

plasma membrane of the cell body

acts as part of the receptive region that receives information from other neurons.

A change in membrane potential can be produced by...

(1) anything that alters ion concentrations on the two sides of the membrane, or (2) anything that changes membrane permeability to any ion.

rate of impulse propagation depends largely on two factors

1) Axon diameter 2)Degree of myelination

two types of synapses

1) electrical and 2) chemical

Acetylcholine (ACh)

ACh is released by all neurons that stimulate skeletal muscles and by many neurons of the autonomic nervous system. ACh-releasing neurons are also found in the CNS.

Saltatory conduction

AP is generated in a myelinated fiber, the local depolarizing current does not dissipate through the adjacent membrane regions, which are nonexcitable. Instead, the current is maintained and moves rapidly to the next myelin sheath gap, a distance of approximately 1 mm, where it triggers another AP. Consequently, APs are triggered only at the gaps; about 30 times faster than continuous conduction.

myelin sheath gaps (a.k.a., Ranvier)

Adjacent Schwann cells do not touch one another, so there are gaps in the sheath; occur at regular intervals (about 1 mm apart) along a myelinated axon. Axon collaterals can emerge at these gaps.

True

An action potential is a phenomenon that either happens completely or doesn't happen at all.

Anterograde movement (axon)

Anterograde movement is movement away from the cell body. Ex.-mitochondria, cytoskeletal elements, membrane components (vesicles) used to renew the axon plasma membrane, and enzymes needed to synthesize certain neurotransmitters.

differences in the plasma membrane's permeability to those ions

At rest the membrane is impermeable to the large anionic cytoplasmic proteins, very slightly permeable to sodium, approximately 25 times more permeable to potassium than to sodium, and quite permeable to chloride ions. These resting permeabilities reflect the properties of the leakage ion channels in the membrane. 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 only K+ flowed. Therefore, at resting membrane potential, the negative interior of the cell is due to a much greater ability for K+ to diffuse out of the cell than for Na+ to diffuse into the cell. Because some K+ is always leaking out of the cell and some Na+ is always leaking in, you might think that the concentration gradients would eventually "run down," resulting in equal concentrations of Na+ and K+ inside and outside the cell. This does not happen because the ATP-driven sodium-potassium pump first ejects three Na+ from the cell and then transports two K+ back into the cell. The sodium-potassium pump (Na+-K+ ATPase) stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium.

nuclei (CNS)

Clusters of cell bodies in the CNS

ganglia (PNS)

Clusters of cell bodies in the PNS

True

Dendrites and axons are both armlike processes that extend from neuron cell bodies.

the nodes of Ranvier

From where can collaterals emerge on a myelinated nerve?

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.

myelin sheath

Many nerve fibers, particularly those that are long or large in diameter, are covered with a whitish, fatty (protein-lipoid); Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses.

Cytoskeletal elements (in neuron cell body)

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.

6 types of Neuroglia

Neuroglia of CNS • oligodendrocytes • ependymal cells • microglia • astrocytes Neuroglia of PNS • schwann cells • satellite cells

Protein and membrane making machinery (in neuron cell body)

Neuron cell bodies (not axons) have the organelles needed to synthesize proteins—rough endoplasmic reticulum (ER), free ribosomes, and Golgi apparatus.

Multipolar

Neurons can be classified structurally by the number of processes extending from their cell bodies. Which of the following is the most common neuron type in humans?

Gases and lipids

Not so long ago, it would have been scientific suicide to suggest that small, short-lived, toxic gas molecules might be neurotransmitters. Nonetheless, the discovery of these unlikely messengers has opened up a new chapter in the story of neurotransmission. Ex. Nitric oxide, carbon monoxide, and endocannabinoids.

retrograde movement

Organelles slated for degradation or recycling are moved through the axon by ___________.

pigment inclusions (in neuron cell body)

Pigments found inside neuron cell bodies include black melanin, a red iron-containing pigment, and a golden-brown pigment (lipofuscin).

Potassium diffuses out of the cell

Relative to extracellular fluid, the interior of a neuron has a negative charge because __________.

synaptic potentiation

Repeated or continuous use of a synapse (even for short periods) enhances the presynaptic neuron's ability to excite the postsynaptic neuron, producing larger-than-expected EPSPs.

2 subdivisions of the Peripheral Nervous System (PNS)

Sensory (afferent) division & Motor (efferent) division

insulators

Substances with high electrical resistance

conductors

Substances with low electrical resistance

axosomatic synapses

Synapses between axon endings of one neuron and the cell body (soma) of another neuron

axodendritic synapses

Synapses between the axon endings of one neuron and the dendrites of other neurons

increase the frequency of action potentials

The CNS can discriminate between strong and weak stimuli as strong stimuli __________.

Rough Endoplasmic Reticulum

The Nissl bodies in the neuron cell body are otherwise known as which cellular organelle(s)?

differences in the ionic composition of the intracellular and extracellular fluids

The cell cytosol contains 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 the extracellular fluid, the positive charges of Na+ and other cations are balanced chiefly by chloride ions (Cl−). Although there are many other solutes (glucose, urea, and other ions) in both fluids, potassium (K+) plays the most important role in generating the membrane potential.

current

The flow of electrical charge from one point to another; can be used to do work, ex.-power a flashlight. The amount of charge that moves between the two points depends on two factors: voltage and resistance.

Potassium

The movement of which ion out of the cell through leakage channels establishes the negative membrane potential?

Outer collar of perinuclear cytoplasm (formerly called neurilemma)

The portion of the nucleus and most of the cytoplasm of the Schwann cell that ends up as a bulge just external to the myelin sheath.

Chromatophilic substance (Nissl bodies)

The rough ER (in cell body), stains darkly with basic dyes.

skin

The sensory, or afferent, division of the peripheral nervous system transmits information from the __________ to the CNS

pump three sodium ions out of the cell for every two ions of potassium it brings into the cell

The sodium-potassium ion pump will __________.

propagation

The spread of the action potential down an axon, caused by successive changes in electrical charge along the length of the axon's membrane.

Bundles of neurons are called tracts in the CNS and nerves in the PNS

What is the difference between nerves and tracts?

dendrites

Which of the following are the main receptive or input regions found in neurons?

chemical gated (ligand-gated) channel

Which type of ion channel opens when a chemical binds to it?

tract (CNS)

a bundle of axons in the central nervous system

nerve

a bundle of axons in the peripheral nervous system

Neuroglia

a.k.a., glial cells; Supporting cells; small cells that surround and wrap the more delicate neurons; Like neurons, most neuroglia have branching processes (extensions) and a central cell body. They can be distinguished, however, by their much smaller size and their darker-staining nuclei.

Autonomic Nervous System (Motor Division)

a.k.a., involuntary nervous system; consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscle, and glands; "a law unto itself,"; we generally cannot control such activities as the pumping of our heart or the movement of food through our digestive tract; the ANS has two functional subdivisions, the sympathetic division and the parasympathetic division (these divisions work in opposition to each other—whatever one stimulates, the other inhibits.)

terminal branches of axon

a.k.a., terminal arborizations; Branched endings of an axon that transmit messages to other neurons; can have 10,000 or more branches per neuron.

nerve impulse

an AP In a neuron, typically generated only in axons. A neuron generates a nerve impulse only when adequately stimulated. The stimulus changes the permeability of the neuron's membrane by opening specific voltage-gated channels on the axon. These voltage-gated channels are generally found only on axons, where they are critical for AP formation—no voltage-gated channels means no AP.

Hyperpolarization

an increase in membrane potential: The inside of the membrane becomes more negative (moves farther from zero) than the resting potential. For example, a change from −70 mV to −75 mV. Depolarization increases the probability of producing nerve impulses, whereas hyperpolarization reduces this probability.

leakage or nongated channels

are always open.

Myelinated fibers

axons bearing a myelin sheath; conduct nerve impulses rapidly; Note that myelin sheaths are associated only with axons.

neurofibrils

bundles of neurofilaments that provide support for dendrites and axon

motor or efferent neurons

carry impulses away from the CNS to the effector organs (muscles and glands) of the body. Motor neurons are multipolar. Except for some neurons of the autonomic nervous system, their cell bodies are located in the CNS.

Neurotransmitters

chemical messengers, along with electrical signals, are the "language" of the nervous system—the means by which neurons communicate, integrate, and send messages to the rest of the body. Ex. Acetylcholine, biogenic amines, amino acids, peptides, purines, & gases and lipids.

parasympathetic division of ANS

conserves energy; promotes housekeeping functions during rest

Sensory (afferent) division

consists of nerve fibers (axons) that convey impulses to the central nervous system from sensory receptors located throughout the body.

2 types of neuron processes

dendrites and axons; Armlike processes extend from the cell body of all neurons. The brain and spinal cord (CNS) contain both neuron cell bodies and their processes. The PNS consists chiefly of neuron processes (whose cell bodies are in the CNS).

threshold point

depolarization must reach threshold values if an axon is to "fire."

relative refractory period

follows the absolute refractory period; most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarization is occurring. The axon's threshold for AP generation is substantially elevated, so a stimulus that would normally generate an AP is no longer sufficient. An exceptionally strong stimulus can reopen the Na+ channels that have already returned to their resting state and generate another AP.

Oligodendrocytes (Neuroglia of CNS)

form myelin sheaths around CNS nerve fibers; Though they also branch, the oligodendrocytes have fewer processes than astrocytes. Oligodendrocytes line up along the thicker nerve fibers in the CNS and wrap their processes tightly around the fibers, producing an insulating covering called a myelin sheath.

Microglial Cells (Neuroglia of CNS)

glial cells, defensive cells in the CNS; small and ovoid with relatively long "thorny" processes that touch nearby neurons, monitoring their health, and when they sense that certain neurons are injured or in other trouble, the microglial cells migrate toward them. Where invading microorganisms or dead neurons are present, the microglial cells transform into a special type of macrophage that phagocytizes the microorganisms or neuronal debris. (cleans debris) This protective role is important because cells of the immune system have limited access to the CNS.

Lipofuscin

golden-brown pigment in cell body; a harmless by-product of lysosomal activity, is sometimes called the "aging pigment" because it accumulates in neurons of elderly individuals.

excitatory postsynaptic potentials (EPSPs)

graded post-synaptic depolarizations, which increase the likelihood that an action potential will be generated

Multipolar neurons

have three or more processes—one axon and the rest dendrites; They are the most common neuron type in humans, with more than 99% of neurons in this class; the major neuron type in the CNS.

inhibitory postsynaptic potentials (IPSPs)

hyperpolarizations that move the membrane potential farther from threshold

receptor potential or a generator potential (graded potential)

is produced when a sensory receptor is excited by its stimulus (e.g., light, pressure, chemicals).

postsynaptic potential

is produced when the stimulus is a neurotransmitter released by another neuron. Here, the neurotransmitter is released into a fluid-filled gap called a synapse and influences the neuron beyond the synapse.

axon terminals

knoblike distal endings of the terminal branches

2 principle types of nervous cells

less than 20% of the CNS is extracellular space, which means that the cells are densely packed and tightly intertwined made up of 1) Neurons and 2) Neuroglia

nerve fibers

long axons

action potential

long-distance signals of axons that always have the same strength; brief reversal of membrane potential with a total amplitude (change in voltage) of about 100 mV (from −70 mV to +30 mV); Unlike graded potentials, action potentials do not decay with distance

Central Nervous System (CNS)

made up of the brain and the spinal cord; is the integrating and control center of the nervous system. It interprets sensory input and dictates motor output based on reflexes, current conditions, and past experience.

Peripheral Nervous System (PNS)

mostly consists of pairs of cranial nerves, spinal nerves, and associated ganglia; nerves (bundles of axons) that extend from the brain and spinal cord, and ganglia (collections of neuron cell bodies). Spinal nerves carry impulses to and from the spinal cord, and cranial nerves carry impulses to and from the brain.

Dynein

motor protein in retrograde transport; carries cargo like organelles and vesicles from the periphery to the center of the cell; proteins propel cellular components along the microtubules

Kinesin

motor proteins in anterograde transport; uses the energy of ATP hydrolysis to move toward the plus end of a microtubule; proteins propel cellular components along the microtubules

Electrical synapses

much less common than chemical synapses; consist of gap junctions; Their channel proteins (connexons) connect the cytoplasm of adjacent neurons and allow ions and small molecules to flow directly from one neuron to the next. These neurons are electrically coupled, and transmission across these synapses is very rapid; unidirectional or bidirectional; Electrical synapses between neurons provide a simple means of synchronizing the activity of all interconnected neurons; Electrical synapses are far more abundant in embryonic nervous tissue; As the nervous system develops, chemical synapses replace some electrical synapses and become the vast majority of all synapses.

structural classification of neurons

multipolar, bipolar, unipolar; grouped structurally according to the number of processes extending from their cell body

Neurons

nerve cells that are excitable (respond to stimuli by changing their membrane potential) and transmit electrical signals; Although neurons vary in structure, they all have a cell body and one or more slender processes.

presynaptic neuron

neuron conducting impulses toward synapse (sends information)

Presynaptic inhibition

occurs when the release of excitatory neurotransmitter by one neuron is inhibited by the activity of another neuron via an axoaxonal synapse; the end result is that less neurotransmitter is released and bound, forming smaller EPSPs.

Voltage-gated channels

open and close in response to changes in the membrane potential

Mechanically gated channels

open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure).

Chemically gated channels (a.k.a., ligand-gated channels)

open when the appropriate chemical/ neurotransmitter binds

axolemma

plasma membrane of axon

Integration

process in which the nervous system processes and interprets sensory input and decides what should be done at each moment

threshold stimuli

produce depolarizing currents that push the membrane potential toward and beyond the threshold voltage.

main receptive (or input) regions of Dendrites

provide an enormous surface area for receiving signals from other neurons.

Continuous conduction

relatively slow Action potential propagation in nonmyelinated axons; the voltage-gated channels in the membrane are immediately adjacent to each other.

A single basic bidirectional transport mechanism

responsible for axonal transport. It uses different ATP-dependent "motor" proteins (kinesin or dynein), depending on the direction of transport. These proteins propel cellular components along the microtubules like trains along tracks at speeds up to 40 cm (15 inches) per day.

synaptic vesicles

saclike structures found inside the synaptic knob containing chemicals

functional classification of neurons

sensory (afferent), motor (efferent), interneurons; neuron type according to direction of impulse conduction

receptive endings

sensory terminal of unipolar neurons at end of peripheral process

nerve growth factor (NFG)

signal molecule that activates certain nuclear genes promoting growth.

subthreshold stimuli

stimuli that produce no observable contractions

Satellite Cells (Neuroglia of PNS)

surround neuron cell bodies located in the peripheral nervous system; are thought to have many of the same functions in the PNS as astrocytes do in the CNS. Their name comes from a resemblance to the moons around a planet.

Motor (efferent) division

the PNS transmits impulses from the CNS to effector organs, which are the muscles and glands. These impulses activate muscles to contract and glands to secrete. In other words, they effect (bring about) a motor response. The motor division has two main parts 1) somatic nervous system & 2) autonomic nervous system

Axon Hillock

the cone-shaped area on the cell body from which the axon originates

resting membrane potential

the electrical charge of a neuron when it is not active; This potential difference in a resting neuron (Vr) is said to be polarized. The value of the resting membrane potential varies (from −40 mV to −90 mV) in different types of neurons.

Chemical synapses

the most common type of synapse. They are specialized to allow the release and reception of chemical messengers known as neurotransmitters.

postsynaptic neuron

the neuron transmitting the electrical signal away from the synapse (receives information)

axon collaterals

the occasional branches along its length that axons may have; They extend from the axon at more or less right angles.

dendritic spines

thorny appendages with bulbous or spiky ends—which represent points of close contact (synapses) with other neurons

sensory or afferent neurons

transmit impulses from sensory receptors in the skin or internal organs toward or into the central nervous system. Except for certain neurons found in some special sense organs, virtually all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS. Only the most distal parts of these unipolar neurons act as the receptive region, and the peripheral processes are often very long. For example, fibers carrying sensory impulses from the skin of your big toe travel for more than a meter before they reach their cell bodies in a ganglion close to the spinal cord. In some sensory neurons, the receptive endings function directly as sensory receptors. In other sensory neurons, receptive endings are associated with larger sensory receptors that include other cell types.

Graded potentials

usually incoming signals operating over short distances that have variable (graded) strength; short-lived, localized changes in membrane potential, usually in dendrites or the cell body. They can be either depolarizations or hyperpolarizations. These changes cause current flows that decrease in magnitude with distance.

The Axon: Functional Characteristics

-Conducting region of neuron -Generates nerve impulses -Transmits them along axolemma (neuron cell membrane) to axon terminal, usually away from the cell body -Secretory region -Neurotransmitters released into extracellular space -Either excite or inhibit neurons with which axons in close contact -Carries on many conversations with different neurons at same time -Lacks rough ER and Golgi apparatus -Relies on cell body to renew proteins and membranes -Efficient transport mechanisms -Quickly decay if cut or damaged

axon structure

- Type of neuron process - Each neuron as a SINGLE axon - The axon arises from a cone-shaped area of the cell body called the AXON HILLOCK - May be short or as long as 3-4ft - Long axons are called NERVE FIBERS - Each neuron has one axon but axons may have occasional branches along their length called AXON COLLATERALS - Can have 10,000 or more branches called TERMINAL BRANCHES per neuron - The end of each terminal branch is called AXON TERMINALS (or TERMINAL BOUTONS)

Myelination in the CNS

-contains both myelinated and nonmyelinated axons. -oligodendrocytes that form myelin sheaths has multiple flat processes that can coil around as many as 60 axons at the same time. -myelin sheath gaps separate adjacent sections of an axon's myelin sheath. -CNS myelin sheaths lack an outer collar of perinuclear cytoplasm because cell extensions do the coiling and the squeezed-out cytoplasm is forced back toward the centrally located nucleus instead of peripherally. -the smallest-diameter axons are nonmyelinated, covered by the long extensions of adjacent glial cells.

Information Transfer across Chemical Synapses

1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3 Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 5 Binding of neurotransmitter opens ion channels, creating graded potentials. 6 Neurotransmitter effects are terminated. The effects of neurotransmitters generally last a few milliseconds before being terminated in one of three ways, depending on the particular neurotransmitter: -Reuptake: by astrocytes or the presynaptic terminal, where the neurotransmitter is stored or destroyed by enzymes (as with the neurotransmitter norepinephrine) -Degradation: by enzymes associated with the postsynaptic membrane or present in the synaptic cleft (as with acetylcholine) -Diffusion: away from the synapse

Nerve fibers may be classified according to diameter, degree of myelination, and conduction speed.

1) Group A fibers - are mostly somatic sensory and motor fibers serving the skin, skeletal muscles, and joints. They have the largest diameter & thick myelin sheaths. 2) Group B fibers - are lightly myelinated fibers of intermediate diameter. 3) Group C fibers - have the smallest diameter. They are nonmyelinated, so they are incapable of saltatory conduction and conduct impulses at a leisurely pace. The B and C fiber groups include autonomic nervous system motor fibers serving the visceral organs; visceral sensory fibers; and the smaller somatic sensory fibers that transmit sensory impulses from the skin (such as pain and small touch fibers).

Nerve Fiber Mylenation in PNS

1) Schwann cell envelops an axon 2) The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers 3) The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

3 special characteristics of neurons

1. Extreme longevity (can function optimally for a lifetime) 2. Amitotic (After maturation they do not divide anymore) 3. High metabolic rate (require continuous and abundant supplies of oxygen -> cannot survive for more than a few minutes without oxygen)

Generating an action potential steps

1. resting state: all gated Na+ and K+ channels are closed 2. Depolarization: Na+ channels open 3. Repolarization: Na+ channels are inactivating, and K+ channels open 4. Hyperpolarization: some K+ channels remain open, and Na+ channels reset

Purines

Purines are one of the classes of nitrogen-containing bases that make up DNA and RNA. Two purines, ATP and adenosine, also have well-established roles as chemical messengers: 1) Adenosine triphosphate (ATP), the cell's universal form of energy, is now recognized as a major neurotransmitter (perhaps the most primitive one) in both the CNS and PNS. Like the receptors for glutamate and acetylcholine, certain receptors produce fast excitatory responses when ATP binds, while other ATP receptors trigger slow, second-messenger responses. Upon binding to receptors on astrocytes, ATP mediates Ca2+ influx. 2) Adenosine, a part of ATP, also acts outside of cells on aden-osine receptors. Adenosine is a potent inhibitor in the brain. Caffeine's well-known stimulatory effects result from blocking these adenosine receptors.

3 overlapping functions of the nervous system

Sensory input, Integration, and Motor Output

biogenic amines

Simple hormones; water soluble; derived from amino acids; e.g, epinephrine, norepinephrine, dopamine, serotonin, and histamine

multiple local potentials occur at different places on the same cell at the same time

Spatial summation occurs when __________.

frequency of action potentials

Strong stimuli generate nerve impulses more often in a given time interval than do weak stimuli. Stimulus intensity is coded for by the number of impulses per second—rather than by increases in the strength (amplitude) of the individual APs.

synaptic cleft

The narrow gap that separates the presynaptic neuron from the postsynaptic cell.

Motor Output

The nervous system activates effector organs (the muscles and glands) to cause a response

Peptides

The neuropeptides, essentially strings of amino acids, include a broad spectrum of molecules with diverse effects. For example, a neuropeptide called substance P is an important mediator of pain signals. By contrast, endorphins, which include beta endorphin, dynorphin, and enkephalins, act as natural opiates, reducing our perception of pain under stressful conditions. Enkephalin activity increases dramatically in pregnant women in labor. Endorphin release is at least partially responsible for the "runner's high." Ex. endorphins, tachykinins, somatostatin, and cholecystokinin.

Degree of myelination

The presence 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.

Ohm's law

V=IR; Voltage (V) = current (I)×resistance (R) 1) Current is directly proportional to voltage: The greater the voltage (potential difference), the greater the current. 2) There is no net current flow between points that have the same potential. 3) Current is inversely related to resistance: The greater the resistance, the smaller the current.

Nuclei exist in the CNS, ganglia in the PNS.

What is the difference between the clusters of cell bodies called nuclei and those known as ganglia?

electrochemical gradient

When gated ion channels open, ions diffuse quickly across the membrane; determines the direction an ion moves (into or out of the cell); Two components: The concentration gradient & the electrical gradient.

ependymal cells

Which cells line the central cavities of the brain and spinal cord and provide a fairly permeable barrier between the CSF and nervous tissue?

the number of processes extending from their cell body

Which criterion is used to structurally classify neurons?

The nervous system processes and interprets sensory input and decides what should be done at each moment

Which of the following best describes the nervous system function of integration?

slow conduction of nerve impulses

Which of the following characteristics is NOT associated with a myelinated nerve fiber?

diverging circuit

Which of the following circuit types is exemplified by impulses that travel from a single neuron of the brain, activate one hundred or more motor neurons in the spinal cord, and excite thousands of skeletal muscle fibers?

influx of Ca++

Which of the following conditions would cause synaptic potentiation?

processing and interpreting sensory input and determining what should be done at each moment in the body.

Which of the following defines integration?

multiple sclerosis

Which of the following diseases is directly related to demyelination?

the autonomic nervous system

Which of the following divisions of the nervous system is also known as the involuntary nervous system?

action potential

Which of the following electrical events occurs when a certain threshold is reached?

Ions flow directly from one neuron to the next.

Which of the following events occurs at an electrical synapse?

Two factors generate the resting membrane potential:

differences in the ionic composition of the intracellular and extracellular fluids, and differences in the plasma membrane's permeability to those ions.

nonmyelinated fibers

do not contain sheaths; conduct impulses more slowly; Dendrites are always nonmyelinated.

Sensory Input

gathered information; The nervous system uses its millions of sensory receptors to monitor changes occurring both inside and outside the body.

Unipolar neurons (pseudounipolar neurons)

have a 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 a sensory receptor. The central process enters the CNS; Unipolar neurons are more accurately called pseudounipolar neurons because they originate as bipolar neurons. During early embryonic development, the two processes converge and partially fuse to form the short single process that issues from the cell body; found chiefly in ganglia in the PNS, where they function as sensory neurons; Three facts favor classifying it as an axon: (1) It generates and conducts an impulse (functional definition of axon); (2) when large, it is heavily myelinated; and (3) it has a uniform diameter and is indistinguishable microscopically from an axon.

Bipolar neurons

have two processes—an axon and a dendrite—that extend from opposite sides of the cell body. These rare neurons are found in some of the special sense organs such as in the retina of the eye and in the olfactory mucosa.

trigger zone of neuron

initiates action potentials

Interneurons or association neurons

lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs. Most interneurons are confined within the CNS. They make up over 99% of the neurons of the body, including most of those in the CNS. Almost all interneurons are multipolar, but there is considerable diversity in size and fiber-branching patterns. Ex.-Purkinje and pyramidal cells

Ependymal Cell (Neuroglia of CNS)

lines cerebrospinal fluid-filled CNS cavities; range in shape from squamous to columnar, and many are ciliated; They line the central cavities of the brain and the spinal cord, where they form a fairly permeable barrier between the cerebrospinal fluid that fills those cavities and the tissue fluid bathing the cells of the CNS. The beating of their cilia helps to circulate the cerebrospinal fluid that cushions the brain and spinal cord.

sympathetic division of ANS

mobilizes body systems during activity (fight or flight)

Retrograde movement (axon)

movement toward the cell body. Ex.- organelles returning to the cell body to be degraded or recycled; Important means of intracellular communication. It allows the cell body to be advised of conditions at the axon terminals; delivers vesicles to the cell body containing signal molecules (such as nerve growth factor)

secretory region of neuron

nerve impulse is conducted along the axon to the axon terminals

decremental

the flow of charge decreases as the distance from the site of origin of the graded potential increases

Resistance

the hindrance to charge flow, provided by substances through which the current must pass.

axon diameter

the larger the axon's diameter, the faster it conducts impulses. Larger axons conduct more rapidly because they offer less resistance to the flow of local currents, bringing adjacent areas of the membrane to threshold more quickly

structures of a cell body

the major biosynthetic center and metabolic center of a neuron; abundant in mitochondria; Protein- and membrane-making machinery; Cytoskeletal elements; Pigment inclusions.

Nervous System

the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its cells communicate by electrical and chemical signals, which are rapid and specific, and usually cause almost immediate responses.

Astrocytes (Neuroglia of CNS)

the most abundant and versatile glial cells; cling to neurons and their synaptic endings, and cover nearby capillaries; support and brace the neurons and anchor them to their nutrient supply lines; making exchanges between capillaries and neurons, helping determine capillary permeability. They guide the migration of young neurons and formation of synapses (junctions) between neurons; control the chemical environment around neurons, where their most important job is "mopping up" leaked potassium ions and recapturing and recycling released neurotransmitters; respond to nearby nerve impulses and released neurotransmitters; Connect by gap junctions, signal each other with slow-paced intracellular calcium pulses (calcium waves), and by releasing extracellular chemical messengers. They also influence neuronal functioning and therefore participate in information processing in the brain.

axonal transport

through the cooperative efforts of motor proteins and cytoskeletal elements (mostly microtubules), transports materials from the neuronal cell body to distant regions in the dendrites and axons, and from the axon terminals back to the cell body.

synaptic delay

time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell