Chapter 2: Neurophysiology

Type A fibers

- are the largest fibers present in mammalian nerves -comprises the most rapidly conducting axons -divided into four groups, a through d.

Nerve cells (neurons) carry out three tasks:

1. receiving information 2. Processing information 3. Sending information

the outside of the cell contains excess Na+ ions at a ratio of ____

14:1

Nonmyelinated fibers are free ending nerves; the smallest nerve fibers, which are also known as ____

C fibers.

___ is responsible for receiving sensory information, processing the information, and initiating an outgoing response

CNS

Nerves are found only in the

PNS

This process increases the conduction velocity of action potentials without needing to increase the diameter of an axon.

Saltatory conduction

Unmyelinated fibers and myelinated axons of the CNS do not ___

regenerate

____ consists of all incoming information traveling along sensory or afferent pathways

Afferent division

Both A and C fibers are found abundantly in the oral cavity, with ___ fibers in the greater distribution.

C

___ are long cylinder fibers with high-electrical resistance cell membranes that have no myelin sheath. They are surrounded by low-resistance extracellular fluid.

Nonmyelinated nerve fibers

The outer layer of cytoplasm is called

neurilemma

is an excitable cell that is the basic functional unit of the nervous system, specialized in sending impulses and making all nervous system functions possible

neuron

what is the main difference between motor (efferent) neurons and sensory (afferent) neurons?

the location of the cell body along the axon

In myelinated nerves, the local anesthetic blocks only at

the nodes of Ranvier 6

each fascicle is wrapped in a layer of connective tissue called

the perineurium

the electrical potential of nerve axoplasm in the resting state is approximately ____ mV with a range of ___ to ___ mV, and it will remain this way until a stimulus comes along.

-70 mV; -40 to -95 mV

Myelin is formed by either one of two types of glial cells:

-Schwann cells ensheathing peripheral neurons -Oligodendrocytes insulating those of the CNS

Type B fibers

-are myelinated and moderate in size and are almost indistinguishable from Ad fibers but are not responsible for dental pain

Type C fibers

-are unmyelinated and the most numerous fibers in the PNS. -responsible for carrying the sensations of dull or burning dental pain

What factors influence the ability of the local anesthetic to block the nerve conduction?

-axonal conduction velocity -frequency of firing -degree of myelination -fiber diameter

functional regions of a sensory neuron

-dendritic (input) zone -cell body (soma) -axon hillock (summation zone) - axon -output zone (synaptic knobs)

Conduction Blocking by Anesthetics

-nerve fibers smaller in diameter tend to be more sensitive than larger fibers -larger nerve fibers require greater amount of anesthetic to prevent depolarization -Inferior alveolar (IA) and posterior superior alveolar (PSA) blocks require more anesthetic than infiltration

steps to the mechanism that produces an action potential

1. the membrane is in its resting state and polarized. Na+ in the extracell. fluid. K+ in the intracell fluid. The electrical potential of the nerve axoplasm is approx. -70 mV. 2. A stimulus triggers Na+ channels in axonal membrane to open and allows Na+ diffusion. Slow depolarization begins. 3. As threshold potential is reached, it triggers action potential, causing more voltage-gated Na+ channels to open. 4.As more Na+ enters the cell , the membrane rapidly depolarizes even further 5. The magnitude of the action potential peaks (at +30 mV) when voltage-gated Na+ channels close 6. Repolarization begins when K+ channels open, allowing outward diffusion of K+ 7. After the absolute and relative refractory periods that occur repolarization, the resting potential is restored by the Na+/K+ pump.

___ the largest nerve fibers and be either motor or sensory. This fiber may require a stronger minimal stimulus compared with smaller C fibers. Are responsible for sharp pain.

A fibers

____ is the interval during which a second action potential absolutely cannot be initiated , no matter how large a stimulus is applied

Absolute refractory period

an ____, also known as nerve impulse, is a spike of positive and negative ionic discharge that travels along the membrane of a cell.

Action potential

-if a neuron responds at all, then it must respond completely and will be no longer dependent on the stimulus to continue. Greater intensity of stimulation does not produce a stronger signal but can produce more impulses per second

All or none principle

____ pathways carry information to the autonomic or visceral effectors that control involuntary smooth muscle, cardiac muscle, glandular tissue, and other involuntary tissue.

Autonomic nervous system

-the part of the axon where it emerges from the soma -decides whether or not to send the impulse further down the axon -if the total strength of the signal exceeds the threshold potential, the structure will fire a signal -Most easily excitable part of the neuron and the spike initiation zone for the axon.

Axon Hillock (Summation Zone)

___ these fibers have medium diameters, and they are lightly myelinated motor fibers

B fibers

___ These fibers are the most numerous and the smallest, they are usually unmyelinated, and they are primarily responsible for dull, aching pain.

C fibers

Unipolar sensory neuron (CNS):

Cell body does not participate in nerve conduction and is therefore located off the axon.

____ ions assist in maintaining the membrane potential and are concentrated in the extracellular fluid with the Na+ ions

Chloride (Cl-)

___ consists of all outgoing information along motor or efferent pathways.

Efferent division

___ is composed of about 75% lipid, about 20% and about 5% carbohydrates.

Myelin

___ is a many layered coating, largely composed of a fatty substance called myelin that wraps around the axon of a neuron and very efficiently insulates it.

Myelin sheath

___ are the gaps formed between the myelin sheaths generated by different cells.

Nodes of Ranvier

C fibers are the most numerous fiber type in the ___

PNS

____ includes nerve tissues that lie in the periphery, or "outer regions," of the nervous system, consisting of 31 pairs of spinal nerves arising from the spinal cord and 12 pairs of cranial nerves arising from the brain

Peripheral nervous system (PNS)

____ means that the electrical charge on the outside of the membrane is positive while the electrical charge on the inside of the membrane is negative

Polarization

___ ions are predominantly in the intracellular fluid

Potassium (K+)

Beta (b)

Proprioception (awareness of position/ equilibrium)

___ is the interval immediately after the absolute refractory period, during which initiation of a second action potential is inhibited but not impossible if a larger stimulus is applied

Relative refractory period

____ occurs once the peak of the action potential is reached and the membrane potential begins to move back toward the resting potential (-70 mV).

Repolarization

___ is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node opening the stimulud gated channels and allowing Na+ ions to rush into the cell, causing depolarization.

Saltatory conduction

____ ions are predominantly in the extracellular fluid

Sodium (Na+)

___ is a subdivision of the efferent division of the PNS and controls the bodys voluntary and reflex activities through somatic sensory and somatic motor components.

Somatic nervous system

although sensory and motor nerves are slightly different in structure, their main components of the neuron are the same:

The cell body with a nucleus , axon, dendritic zone with free nerve endings, and terminal arborization.

___ is the minimum magnitude a voltage fluctuation in the summation or conduction zone must have to initiate an action potential that will trigger the opening of a voltage-gated ion channel to cause depolarization

The threshold potential

Change in the voltage across the nerve membrane can alter the RMP and the function of the voltage-dependent ion channels. If the voltage changes by a large enough amount, an electrochemical pulse called an ____ is generated.

action potential

___ is the membrane potential of an active nerve conducting an impulse

action potential

____ signal jumps along the axon, from node to node, rather than propagating smoothly, as it does in axons that lack a myelin sheath.

action potential

Each neuron has a threshold level, or the point at which there is no holding back called the

all-or-none principle

Visceral sensory pathways conduct information toward

autonomic effectors

the sympathetic and parasympathetic pathways conduct information toward

autonomic effectors

-located at a distance from the axon in a sensory neuron and is not involved in impulse transmission -responsible for protein synthesis and provides metabolic support for the neuron

cell body (soma)

Multipolar motor neuron (CNS):

cell body participates in impulse conduction and is therefore located at the terminal arborization

Nerve fibers that extend from the cell body toward the CNS are termed

central fibers

nerve fibers that extend from the cell body toward the CNS are called

central fibers

the relationship between the relative amounts of ions inside and outside the nerve membrane is known as the

concentration gradient

Fasciculi located in core region are called

core bundles

-the most distal section of the neuron and is arborization of nerve endings. -respond to stimulation -cellular extensions referred to as dendritic tree. -majority of input to the neuron occurs

dendritic (input) zone

Within a nerve, each axon is surrounded by a layer of connective tissue called the

endoneurium

Nodes of Ranvier (Schwann cells) are present at

even intervals along myelinated nerve fibers

the nerve fibers are bundled together into groups called __.

fascicles

the sympathetic division prepares the body to deal with an emergency situation and is involved with the _____ response

fight or flight

Myelin is made up primarily of a glycolipid called ____ and completely insulates the axon from the outside

galactocerebroside

Myelin is formed by

glial cells

Somatic sensory pathways conduct information toward

integrators in the CNS

How fast does a nerve fiber conduct an impulse?

it depends on the presence or absence of a myelin sheath and the diameter of the nerve fiber.

the ___ the diameter of the axon, the ___ the speed of the impulse will travel across the nerve fiber

larger; faster

Alpha (a)

largest, fastest; responsible for muscle movement and light touch

Fasciculi located in mantle regions are called

mantle bundles

neurons possess protein "tunnels" known as ____ through which specific ions such as Na+ and K+ can pass.

membrane channels

the difference in the electrical charge on the outside of the cell versus the inside of the cell is called the

membrane potential

____ contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same bundle

mixed nerves

Larger-diameter A fibers require ____ anesthetic volume than smaller C nerve fibers to provide complete nerve blockade

more

In verebrates, the axons of many neurons are sheathed in ____, which increases the speed of impulse conduction

myelin

contains many cable-like bundles of peripheral axons, which are encapsulated together.

nerve

Each nerve is a cordlike structure containing many axons. These axons are often referred to as

nerve fibers

each nerve is a cordlike structure containing many axons. These axons are often referred to as

nerve fibers

Chemical synapses use chemical transmitters called

neurotransmitters

the cell bodies of sensory neurons do not participate in nerve conduction and, therefore, are located

off the axon

Delta

pain, temperature

Nerve fibers that extend from the cell body away from the CNS are termed

peripheral fibers

nerve fibers that extend from the cell body away from the CNS are called

peripheral fibers

the parasympathetic division coordinates the bodys normal resting activities and is known as the _____ response.

rest or digest

When a neuron is not stimulated, it is sitting with no impulse to carry or transmit, and its membrane is considered polarized and in its resting state, also called the

resting membrane potential

Anesthetics block conduction in the following order:

small myelinated axons, nonmyelinated axons, large myelinated axons

Somatic motor pathways conduct information toward

somatic effectors

the autonomic nervous system is further divided into the ____ and the ____ divisions

sympathetic; parasympathetic

the cell body of the motor neuron participates in impulse conduction and is therefore located at the

terminal arborization

___ is the structural and functional center of the entire nervous system that includes the brain, contained within the skull, and the spinal cord, contained within the vertebral canal.

the central nervous system (CNS)

within each nerve, each axon is surrounded by a layer of connective tissue called

the endoneurium

the entire nerve is wrapped in a layer of connective tissue called

the epineurium

Gamma

touch, pressure

In the CNS, the analogous structures known as

tracts