Anatomy and Physiology (an integrative approach) ch. 12

Describe the three general functions of the nervous system.

collect information: receptors are specialized nervous system structures that monitor the changes of the internal and external environment called stimuli.

process and evaluate information: After processing sensory input, the brain and spinal cord determine the response (if any is required).

Initiate response to information: the brain and spinal cord initiate a response as motor output via nerves to effectors.

Identify the structural components included in the CNS and those in the PNS.

Central Nervous System (CNS): includes both the brain and spinal cord.

Peripheral Nervous System (PNS): includes nerves (bundles of neuron axons) and ganglia (clusters of neuron cell bodies along nerves).

List the functional organization of the nervous system.

Sensory nervous system

Somatic sensory

visceral sensory

Motor Neurons

somatic motor

autonomic motor

Sensory nervous system

responsible for receiving sensory information from receptors that detect stimuli and transmitting this information to CNS.

somatic sensory

detect stimuli that we can consciously perceive (5 senses)

visceral sensory

detect stimuli that we typically do not consciously perceive (blood vessels and internal organs)

Motor Nervous System

responsible for initiating and transmitting motor output from CNS to effectors.

somatic motor

initiates and transmits motor output from the CNS to skeletal muscle

autonomic motor

innervates and regulates cardiac muscle, smooth muscle, and glands without our conscious control.

Describe five distinguishing features common to all neurons.

excitability: the stimulus causes a local change in the resting membrane potential in the excitable cell.

conductivity: involves an electrical change that is quickly propagated along the plasma membrane as voltage gated channels open sequentially during an action potential.

secretion: neurons release neurotransmitters in response to conductive activity.

extreme longevity: neurons formed as a baby are still functional in the elderly

Amitotic: during fetal development, mitotic activity is lost in most neurons except the olfactory epithelium of the nose and parts of the brain.

Describe the three basic anatomic features common to most neurons.

cell body: enclosed by a plasma membrane and contains cytoplasm surrounding a nucleus

dendrites: relatively short, small, tapering, unmyelinated processes that branch off the cell body.

axon: typically a longer process that stems from the cell body to make contact with other neurons, muscle cells, or gland cells.

Identify the structures unique to neurons.

axon collaterals

telodendria

synaptic knobs

synaptic vesicles

cytoskeleton

neurofilaments

neurofibrils

axon collaterals

side branches of axon

telodendria (axon terminal)

fine terminal extensions of axon, mostly located at distal end.

synaptic knobs

extreme tips of the axon terminals

synaptic vesicles

within the synaptic knobs that contain neurotransmitters

cytoskeleton

composed of microfilaments, intermediate filaments, and microtubules.

neurofilaments

intermediate filaments

neurofibrils

bundles of neurofilaments

Distinguish between fast axonal transport and slow axonal transport, and give examples of the different substances moved by each.

fast axonal transport: approximately 400 mm per day; movement along microtubules.

slow axonal transport: approximately 0.1-3 mm per day; movement resulting from the flow of the axoplasm (axoplasmic flow)

Name and describe the four structural categories of neurons.

multipolar neurons: have many dendrites and a single axon that extends from the cell body.

unipolar neurons: a single short neuron process that emerges from the cell body; start out as bipolar neurons but during development their two processes fuse together to make a single process.

bipolar neurons: have two processes that extend from the cell body; one dendrite and one axon.

anaxonic neurons: only dendrites; no axon.

Identify the three functional categories of neurons and where each is primarily located.

sensory neurons (afferent): neurons of sensory nervous system. responsible for conducting sensory input from both somatic and visceral receptors to the CNS.

motor neurons (efferent): neurons of motor nervous system. conducts motor output from the CNS to both somatic effectors and visceral effectors (all multipolar).

interneurons: entirely within the CNS. receive stimulation from many other neurons and carry out the integrative function of the nervous system. (receive, process, store info, and decide). facilitate communication between sensory and motor neurons.

Describe the structure of a nerve, including the three layers of connective tissue wrappings.

a cable-like bundle of parallel axons that are components of the peripheral nervous system (PNS).

Epineurium: thick layer of dense irregular CT that encloses the entire nerve.

Perineurium: layer of dense irregular CT that wraps fascicles (bundles of axons).

Endoneurium: delicate layer of areolar CT that seperates and electrically insulates each axon.

Explain how nerves are classified structurally and functionally.

Structural:

Cranial nerves- extend from brain

Spinal nerves- extend from spinal cord

Functional:

sensory nerves- contain sensory neurons that relay information to the CNS

motor nerves- contain motor neurons that relay information from the CNS

mixed nerves- contain both sensory and motor neurons.(most common)

Define a synapse.

the specific location where a neuron is functionally connected either by another neuron or an effector

Describe the essential structural and functional differences between a chemical synapse and an electrical synapse.

chemical synapse: between two neurons is composed of a presynaptic neuron (signal producer) and a postsynaptic neuron (signal receiver) that are seperate by a fluid-filled gap called a synaptic cleft. transmission between the two synapse occurs when a neurotransmitter are released from synaptic vesicles. A synaptic delay occurs between the neurotransmitter release, its diffusion accross cleft, and its bonding to receptors.

electrical synapse: composed of a presynaptic neuron and a postsynaptic neuron physically bound together. Gap junctions are present in the plasma membranes of both neurons and facilitate the flow of ions between the cells. There is no synaptic delay.

List the distinguishing features of glial cells.

found in both the CNS and the PNS.

do not transmit electrical signals

assist neurons with their functions

physically protect, help nourish neurons, as well as provide organized supporting scaffolding for all the nervous tissue.

during fetal development, glial cells provide a guide for neurons to develope

maintains the structure of synapses and modify transmission that occurs there

Far outnumber neurons (about half of the vol of nervous system)

name the four types of glial cells within the CNS, and the two types of glial cells of the PNS.

CNS

Astrocytes

ependymal cells

microglia

oligodendrocytes

PNS

satellite cells

neurolemmocytes

astrocytes (CNS)

exihbit a starlike shape due to projections from their surface; have contact with both capillaries (smallest blood vessels) and neurons.

help form the blood brain barrier (BBB)

reguate interstitial fluid composition

form structural support

assist neuronal development

occupy the space of dying neurons

ependymal cells (CNS)

ciliated simple cuboidal or simple columnar epithelial cells that line the internal cavities of the brain and the central canal of the spinal cord.

ependymal cells and nearby blood cappillaries together form a network called the choroid plexus, which produces cerebrial spinal fluid and helps circulate it.

microglia (CNS)

typically small cells that have slender branches extending from the main portion of the cell.

they are classified as phagocytic cells of the immune system.

protects the CNS against microorganisms and other potentially harmful substances by engulfing infectious agents and removing debris from dead or damaged nervous tissue that results from infections, inflammation, trauma, and a brain tumor.

oligodendrocytes (CNS)

large cells will bulbous body and slender cytoplasmic extensions or processes. creates the myelin sheath for CNS.

satellite cells (PNS)

flattened cells arranged around neuronal cell bodies in a ganglion that physically seperates cell bodies from their surrounding interstitial fluid.

electrically insulates the cell body and regulates continuous exchange of nutrients and waste products between neuron cell bodies and their environment

neurolemmocytes (Schwaan cells) (PNS)

create the myelin sheath of PNS

Define myelination, and describe the composition and function of a myelin sheath.

the process by which part of an axon is wrapped with myelin.

myelin is the insulating covering around the axon that consist of repeating concentric layers of plasma membrane of glial cells.

The main purpose of a myelin sheath is to increase the speed at which impulses propagate along the myelinated fiber.

Distinguish between the myelination process carried out by neurolemmocytes and by oligodendrocytes

neurolemmocytes (Schwaan cells) myelination (PNS): takes many schwaan cells to myelinate the entire axon. There are gaps between the neurolemmocytes called neurofibril nodes.

Oligodendrocytes myelination (CNS): can myeinate multiple axons at one time. Neurofibril nodes are also located between adjacent oligodendrocyte "wraps".

Identify factors that influence regeneration of PNS axons

1. the amount of damage

2. the distance between the site of the damaged axon and the structure it innervates.

explain why axon regeneration in the CNS is limited.

1. oligodendrocytes do not release a nerved growth factor

2. the large number of axons crowded within the CNS tends to complicate regrowth activites

3. Both astrocytes and connective tissue coverings may form some scar tissue that obstructs axon regrowth.

Describe the events of Wallerian degeneration and axon regrowth

wallerian degeneration: the axon severed from the cell body and the myelin sheath surrounding the axon breaks down

axon regrowth: see image

Distinguish between a pump and a channel, and identify the pumps and channels located along the entire neuron plasma membrane

pump: maintains specific concentration gradients by moving substances up a concentration gradient, a process that requires cellular energy.

channel: provide the means for a substance to move down its concentration gradient. (Na+, K+, Cl-)

List and describe the four functional neuron segments, including the distribution of channels and pumps in each.

receptive segment: includes both dendrites and cell body , which are regions of the neuron that recieve stimuli to excite the neuron. contains chemically gated channels. (cation channels, K+ channels, and Cl- channels)

initial segment: composed of axon hillcock. contains both voltage- gated K+ channels and voltage gated Na+ channels.

conductive segment: equivalent to the length of the axon and its branches called telodendria. contains both voltage gated K+ channels and Voltage gated Na+ channels.

transmissive segment: includes the synaptic knobs and contains voltage gated Ca2+ channels and Ca2+ pumps .

Describe the distribution of substances between the inside and the outside of a neuron.

inside: greater concentration of K+, Pi, and proteins (more negative)

outside: greater concentration of Na+ and Cl- (more positive)

Integrate the concepts of voltage, current, and resistance with neuron structure and function.

Voltage: the measure of the amount of difference in electrical chanrge between two areas and represents potential energy. the larger the difference in charge, the higher the voltage.

Current: the movement of charged particles across the barrier that separates the charge difference. the greater movement of charged particles, the greater the current.

resistance: the opposition to the movement of charged particles. the larger the resistance, the lower the current.

Define resting membrane potential, and state its typical value for neurons.

resting membrane potential (RMP): when the neuron is at rest (Rmp for neuron is typically -70mV but can range from -40mV and -90mV).

The elecrical charge difference is called a membrane potential.

The value of the voltage difference is typically negative because the voltage of the cytosol at the plasma membrane is relatively negative compared to the voltage measured in the interstitial fluid outside the plasma membrane.

Describe how the resting membrane potential is established and maintained in neurons.

The establishing and maintaining of the RMP is dependent upon the distribution of ions.

A RMP is chiefly a consequence of movement of ions across the plasma membrane through leak channels.

Explain depolarization and hyperpolarization.

depolarization: changes in the membrane potential in the positive direction.

hyperpolarization: the opening of either chemically gated K+ channels to allow K+ to exit the neuron or chemically gated Cl- channels for Cl- to enter the neuron. changes in membrane potential in the negative direction.

graded potentials

occur in the receptive segment of a neuron (dendrites and cell bodies) and are due to the opening of chemically gated channels. The chemically gated channels open temporarily to allow passage of a relatively small amount of a specific type of ion across the plasma membrane. This results in the membrane potential to go through either depolarization or hyperpolarization.

action potentials (depolarization & repolarization)

generated within the initial segment and propagated along the conductive segment of the neuron. An action potential is initiated when voltage-gated channels open in response to a minimum voltage change (threshold value).

Define a postsynaptic potential

graded potentials that occur in postsynaptic neurons.

Compare and contrast the action of excitatory and inhibitory neurotransmitters in developing postsynaptic potentials (graded potentials) in the receptive segment.

EPSP: postsynaptic potentials that result in the neuron becoming more positive.

IPSP: postsynaptic potentials that result in the neuron becoming more negative.

Explain an excitatory postsynaptic potential (EPSP).

1. neurotransmitter binds to receptors that are chemically gated cation (+) channels

2. The amount of neurotransmitter determines how many channels open. More Na+ moves into the neuron, and K+ moves out of the neuron.

3. Consequently, the inside of the neuron becomes slightly more positive due to the gain of positively charged ions. This state of the neuron is called EPSP.

4. The flow of Na+ slows down as it travels down the concentration gradient.

Explain an inhibitory postsynaptic potential (IPSP)

1. neurotransmitter will bind to either a chemically gated K+ channel or a chemically gated Cl- channel dependent upon the neurotransmitter and channels available.

2. If the neurotransmitter binds to a K+ channel then K+ will move out of the neuron causing a loss of positive ions. If the neurotransmitter binds to Cl- channels then Cl- will move into the neuron causing a more negative charge in the neuron. The amount of neurotransmitters depends upon the amount of channels open.

3. consequently, the inside of the neuron becomes slightly more negative due to a loss of positively charged ions and a gain of negatively charged ions. This state of the neuron is IPSP.

4. The current of ions slow down as it travels down the concentration gradient.

Define summation

The changes in the membrane potential associated with these graded postsynaptic potentials are "added" in the initial segment.

describe the two types of summation that can occur in the initial segment.

1. Spatial summation: occurs when multiple presynaptic neurons repeatedly releases neurotransmitter at various locations onto the receptive segment, thus generating EPSPs, IPSPs, or both in the postsynaptic neuron.

2. temporal summation: occurs when a single presynaptic neuron repeatedly releases neurotransmitter to produce multiple EPSPs in the postsynaptic neuron at the same location repeatedly within a very short period of time.

define depolarization and repolarization

depolarization: refers to the gain of positive charge within a neuron that occurs to such an extent to change the plasma membrane potential from negative to positive. This reversal of polarity is due to the opening of voltage-gated Na+ channels and the subsequent movement of Na+ into the cell.

repolarization: the return of polarity from positive back to negative. Repolarization is due to the opening of voltage-gated K+ channels and the subsequent movement of K+ out of the cell.

depolarization and its propagation

1. voltage gated Na+ channels are closed and membrane potential is at -70mV (RMP)

2. Na+ ions flow into the region from adjacent areas causing the Na+ voltage gated channels to open. The positive movement of ions causes the RMP (-70mV) to raise to -55mV (threshold)

3. Na+ channels stay open with rapid flow to allow proper depolarization. The movement of Na+ is very small but just enough to change the RMP.

4. The voltage gated Na+ channels only stay open for a short amount of time and close changing the current activation state to inactivation state. This temporarily prevents their reopening.

repolarization and its propagation

5. Once threshold is reached (-55mV), it triggers K+ channel to open. They dont fully open until depolarization has ended. Rapid K+ exit from the axon causes repolarization. Enough K+ leaves the axon to change the RMP from +30mV to a negative RMP of -70mV. This triggers Na+ channels to change to its resting state. Now they are available to send a new nerve signal.

6. K+ channels typically stay open longer until RMP returns to -70mV. For a brief time the RMP is negative.

7. Voltage gated K+ channels are closed and RMP of -70mV is restablished with leak channels and pumps.

Graph and explain the electrical changes that occur in an axon.

Define refractory period, and explain the difference between the absolute refractory period and relative refractory period associated with transmitting an action potential.

The brief time period after an action potential has been initiated during which an axon is either incapable of generating another action potential or a greater than normal amount of stimulation is required to generate another action potential.

absolute: the time after an action potential onset when no amount of stimulus, no matter how strong, can initiate a second action potential.

relative: occurs immediately after the absolute refractory period, during the hyperpolarization phase of the action potential. Another action potential can now be initiated in an axon only if the stimulation of the plasma membrane is greater than the stimulus normally needed to generate an action potential.

Describe events that occur when an action potential reaches the transmissive segment.

transmissive segment is synaptic knob.

The main activity that occurs at the transmissve segment is the release of neurotransmitter from synaptic vesicles.

Explain the role of Ca2+ in neurotransmitter release.

the voltage change associated with the depolarization triggers the opening of voltage-gated Ca2+ channels. Calcium ions move into the synaptic kniob and they eventually bind to proteins associated with with synaptic vesicles. These synaptic vesicles use exocytosis to relaese neurotransmitters.

Compare and contrast continuous conduction and saltatory conduction in the mechanism and velocity of transmission of an action potential.

continuous conduction: propagation of action potential along unmyelinated nerve fibers. (slower conduction velocity of action potential)

saltatory conduction: propagation of action potential along myelinated nerve fibers. (increasing conduction velocity of action potential)

Identify the critera used to distinguish the groups of nerve fibers.

Group A: has a conduction velocity that may be as fast as 150 meters per second; these fibers have both a large diameter and are myelinated. (all somatic motor neurons that extend from the CNS to skeletal muscles are included in this group)

Group B: conducts at approximately 15 meters per second ( generally small in diameter, unmyelinated, or both)

Group C: conducts at 1 meter per second ( generally small in diameter, unmyelinated, or both)

Identify the four classes of neurotransmitters

Acetylcholine (ACh)

Biogenic amines

Amino acids.

Neuropeptides (or peptides)

Explain the two ways in which neurotransmitters are removed from the synaptic cleft.

To remove the neurotransmitter it can either be destroyed by enzymes (acetylcholinesterase) in the synaptic cleft or it can be taken back into the presynaptic axon terminal (at left) by a process called reuptake.

Define neuromodulation, including faciliation and inhibition

is the release of chemicals from cells that locally regulate or alter the response of neurons to neurotransmitters.

facilitation: occurs when there is greater response from a postsynaptic neuron because of the release of neuromodulators.

(increased # of neurotransmitters or increased # of receptors on postsynaptic)

inhibition: occurs when there is less response from a postsynaptic neuron because of the release of neuromodulators. (decreased #of neurotransmitters or decreased # of receptors on postsynaptic)

Identify the four different types of neuronal pools

1. converging

2.diverging

3. reverberating

4. parallel-after-discharge

converging circuits

involves inputs that come together (converge) at a single postsynaptic neuron. This neuron receives input from several presynaptic neurons. (stimulus is senses- smelling food, seeing food)

diverging circuits

spreads information from one presynaptic neuron to several postsynaptic neurons, or from one pool to multiple pools. (maintains posture)

reverberating circuits

utilize feedback to produce a repeated, cyclical stimulation of the circuit: This is termed reverberation. Once activated, a reverberating circuit may continue to function until the cycle is broken by either inhibitory stimuli or synaptic fatigue. (continue breathing while sleeping)

parallel-after-discharge circuits

input is transmitted simultaneously along several neuron pathways to a common postsynaptic cell. Note that neuron pathways in a parallel-after-discharge circuit vary in the number of neurons within the pathway and thus the number of synapses within the pathway. (used for performing precise mathematical calculations)