CHAPTER 11

Efferent Division of PNS

*MOTOR* Transmits impulses FROM the CNS to effector organs which are the muscles and glands, activating muscles to contract or glands to secrete. "Carrying away" or "bring about"

Astrocytes

*Most abundant neuroglia in the CNS* Star cells. Supports and braces neurons and anchors them to nutrient supply lines.

Anterograde Movement

*Transport* along the axon. Movement *away* from the cell body.

ACTION POTENTIAL peak membrane potential

+30 to +50 mV

DEPOLARIZATION

A decrease in membrane potential. The inside of the membrane becomes *less negative* (moves closer to zero) than the resting potential. [-70mV to -65 mV is a depolarization] Also includes events that reverse and move above zero to become positive.

Synapses

A junction between a neuron and its target cell (another neuron, muscle, or gland). Signals between neurons and other cells are communicated across synapses.

Action potential

A large transient depolarization event, including polarity reversal, that is conducted along the membrane of a muscle cell or a nerve fiber.

Graded (local) potential

A local change in membrane potential that varies directly with the strength of the stimulus, declines with distance.

GRADED POTENTIAL location of event

cell body and dendrites, typically

Axon Terminals

AKA Terminal Boutons (buttons) are knoblike distal endings of the terminal branches

Astrocytes

CENTRAL NERVOUS SYSTEM NEUROGLIA

Long Axon

Called a nerve fiber.

Cranial Nerves

Carry impulses to and from the brain

Spinal Nerves

Carry impulses to and from the spinal cord

GRADED POTENTIAL stimulus for ion channels

Chemical (neurotransmitter) or sensory stimulus (light, pressure, temperature)

INTO

Chemical and electrical gradients would tend to move Na+ into or out of cell?

Concentration gradient

Chemical force that pushes K+ out of the cell

Unidirectoinal communication

Chemical synapse - chemical event depends on release, difusion, and receptor binding of neurotransmitter molecules -- for comunication between neurons

Nuclei

Clusters of neuron cell bodies found in the CNS

Nonmyelinated fibers

Dendrites are always nonmyelinated Conduct impulses more slowly

Axon hillock

Density of voltage-gated Na+ channels the greatest

Axon

Depends on 1) its cell body to renew the necessary proteins and membrane components and 2) an efficient transport mechanism to distribute them. Quickly decay if cut or damaged.

Axon

Each neuron contains one of these. Arises from a cone-shaped area of the cell body called the hillock (little hill) and narrows to form slender process. Lacks rough endoplasmic reticulum and a Golgi apparatus

Connexons

Electrical synapse contain protein channels called ______

ABSOLUTE REFRACTORY PERIOD

Ensures each action potential is a separate, all-or-none event and enforces one-way transmission of the action potential.

-90 mV

Equilibrium potential for K+

Potassium, sodium

Excitable cells, like neurons, are more permeable to ____ than to ____ at rest.

GRADED POTENTIAL function [postsynaptic (EPSP) potential]

Excitatory --short-distance signaling; depolarization that spreads to axon hillock; moves membrane potential toward threshold for generating an action potential

Neurons

Extreme longevity. *Amitotic* (lose ability to divide). Cannot be replaced if destroyed. Exceptionally high metabolic rate needing abundant glucose and oxygen. Cell body and one or more slender processes.

HYPERPOLARIZATION

Reduces the probability of producing nerve impulses

Excitable

Refers to a cell that responds to stimuli by generating an electrical signal at the cell membrane. Neurons and muscle cells are excitable.

Electrical Currents in the Body

Reflect the flow of ions (not free electrons) across cellular membranes. Slight difference in the numbers of positive and negative ions on the two sides of cellular plasma membranes (a charge separation) so there is a POTENTIAL across those membranes. The membranes provide the resistance to current flow

Gray Matter

Regions of Central Nervous System with mostly nerve cell bodies and unmyelinated axons

White Matter

Regions of the Central Nervous System with dense arrays of myelinated axons

Bidirectional Transport Mechanism

Responsible for axonal transport using ATP-dependent kinesin and dynein, depending on direction of transport.

Non-gated Channels

Responsible for resting membrane potential (leak channels)

-70 mV

Resting membrane potential (more negative charges on the inside)

Chromatophilic substance

Rough endoplasmic reticulum of the neuron cell body, Nissel bodies, stains dark with basic dyes

GRADED POTENTIAL distance traveled

Short distance, typically within cell body to axon hillock (0.1-1.0 mm) Because current is lost through the "leaky" plasma membrane, voltage declines with distance from the stimulus; the voltage is *decremental.*

Neurotransmitters

Signaling chemicals usually stored in vesicles. Either EXCITE or INHIBIT neurons (or effector cells)

RELATIVE REFRACTORY PERIOD

What is the interval following the absolute refractory period when most sodium channels have returned to their resting state and some potassium channels are still open, but repolarization is occurring.

Postsynaptic neuron

What transmits impulses away from the synapse (info receiver)

Temporal summation

When many nerve impulses arrive in rapid succession at the synapse between a *single* stimulatory presynaptic neuron and a postsynaptic neuron

Spatial summation

When nerve impulses from several presynaptic neurons stimulate a *single* postsynaptic neuron at the same time

POSTSYNAPTIC POTENTIAL

When the stimulus is a neurotransmitter released by another neuron, the graded potential is called this.

Embryonic nervous tissue

Where are electrical synapses most abundant?

Myelin Sheath

Whitish, fatty (protein-lipoid) Segmented; Protects axon; Electrically insulates fibers from one another; Increases the speed of nerve impulse transmission

Ependymal Cells

Wrapping garment. Squamous to columnar. Many are ciliated. Line central cavities of brain/spinal cord. Form fairly permeable barrier between CS fluid and tissue fluid bathing cells of the CNS. Cilia help circulate CS fluid.

Ganglia

clusters of neuron cell bodies that lie along the nerves in the PNS (knot on a string) (swelling)

Dendritic spines

thorny appendages with bulbous or spike ends representing points of close contact (synapses) with other neurons.

Neurofibrils

Bundles of intermediate filaments or neurofilaments maintain cell shape and integrity

Tracts

Bundles of neuron processes in the CNS

Nerves

Bundles of neuron processes in the PNS

Terminal Branches

Terminal arborizations - 10,000 or more branches from the end of a neuron.

Outer Collar of Perinuclear Cytoplasm

The Schwann cell nucleus and most of the cytoplasm that get squeezed out of the cell membrane as it wraps tightly around the axon.

Relative refractory period

The cell can generate another action potential but only if the membrane is more depolarized.

Trigger Zone

In motor neurons, the place where the nerve impulse is generated from: the junction of the axon hillock and axon.

POTASSIUM INTRACELLULAR AND EXTRACELLULAR

140 and 5

SODIUM INTRACELLULAR AND EXTRACELLULAR

15 and 140

REFRACTORY PERIOD

A time during which a neuron cannot respond to another stimulus.

GRADED POTENTIAL positive feed back cycle

Absent

Sodium, potassium, voltage-gated

Action potential changes are a result from a change in membrane permeability first to ______, and then to _________, due to the opening of what type of ion channels? _____________

- 70 30

Action potential changes membrane potential from ____ mV (resting) to _____ mV and back again to the resting membrane potential.

Motor Output

Activation of effector organs - muscles and glands - as the response to the input/stimuli

Leakage or Nongated Channels

Always open

ACTION POTENTIAL amplitude

Always the same size (all-or-none); does not decay with distance. [change in voltage from -70 mV to +30 mV or a total amplitude of about 100 mV]

HYPERPOLARIZATION

An increase in membrane potential - the inside of the membrane becomes *more negative* (moves further from zero) than the resting potential [-70mV to -75mV]

Neuron Processes

Armlike extending from the cell body. CNS contains both neurons and their processes. PNS contains mostly the processes. Tracts/CNS and Nerves/PNS

ACTION POTENTIAL location of event

Axon hillock and axon; also called a *nerve impulse*

Secretory Region

Axon terminals

Myelinated fibers

Axons. ONLY axons have myelination

Myelin Sheath in CNS

Both myelinated and unmyelinated fibers are present; Formed by oligodendroctyes; Nodes are widely spaced NO NEURILEMMA

Axon Collaterals

Branches that extend from the axon at about right angles.

Na+ - K+ Pump

Compensates for the leakage of Na+ and K+ ions

Axon Function

Conducting region of the neuron, generating nerve impulses and transmitting them typically *AWAY* from the cell body along the plasma membrane (axolemma).

Saltatory Conduction

Conduction along a myelinated axon

diameter, myelination

Conduction velocity (rate of impulse propogation) depends on two things: the axon's _____________ and the degree of ________________.

Increased diameter of axon Presence of myelin sheath

Conduction velocity is increased by what two characteristics?

Dendrites

Convey incoming messages *TOWARD* the cell body. Usually not action potentials (nerve impulses) but short-distance signals called graded potentials.

SALTATORY CONDUCTION

Current passes through a myelinated axon only at the nodes of Ranvier. Voltage-gated sodium chanels concentrated at those nodes. Action potentials triggered only at the nodes and jump from one node to the next. Much faster than unmyelinated axon conduction.

GRADED POTENTIAL peak membrane potential EPSP

Depolarizes; moves toward 0 mV

ACTION POTENTIAL summation

Does not occur; all-or-none phenomenon

Na+

During depolarization phase, voltage-gated ____ channels open

Oligodendrocytes

Fewer processes than astrocytes. Line up along thicker nerve fibers in the CNS and wrap processes tightly around fibers producing insulating covering called MYELIN SHEATH.

Electrical gradient

Force tends to pull K+ back into the cell

Myelin Sheath in PNS

Formed by Schwann cells Schwann cell envelopes an axon in a trough Encloses the axon with its plasma membrane Has concentric layers of membrane that make up the myelin sheath

Axoaxonal synapse

Found in the hippocampus, a brain region associated with emotions and memory

Interneurons (association neurons)

Functional Classification Located BETWEEN motor and sensory neurons Most common Primary neuron in the CNS

Efferent (Motor) Neuron

Functional Classification Transmit impulses *FROM CNS TO EFFECTORS* (muscles and glands)

Afferent (Sensory) Neuron

Functional Classification Transmit impulses from effectors (skin, muscles, joints, and visceral organs) *TOWARDS THE CNS*

Axon

Generates an action potential, an outgoing signal also called a nerve impulse, and conducts it to the next cell. *TRANSMITTING OR CONDUCTIVE REGION OF THE NEURON*

RECEPTOR POTENTIAL

Generator potential -- when the receptor of a sensory neuron is excited by some form of energy (heat, light, etc) the graded potential is called this.

Absolute refractory period

Neuron cannot generate another action potential because Sodium channels are inactive

Gated Channels

Proteins form a molecular gate that changes shape to open and close the channel in response to specific signals. When they open and ions cross the membrane, electrical currents and voltage changes occur, according to the rearranged Ohm's Law: (V) = (I) x (R)

Astrocytes

Guide migration of young neurons. Control chemical environment around neurons. Mop up leaked potassium ions and recapture/recycle released neurotransmitters.

Lipofuscin

Harmless by-product of lysosomal activity is sometimes called the aging pigment (golden brown) because it accumulates in neurons of the elderly

Nervous Tissue

Highly cellular, less than 20% cellular space; cells are densely packed and tightly intertwined.

Increase

How would an increase in the number of passive K+ channels affect membrane permeability?

Decrease

How would the closing of voltage-gated K+ channels affect membrane permeability?

Increase

How would the opening of voltage-gated K+ channels affect membrane permeability?

GRADED POTENTIAL peak membrane potential IPSP

Hyperpolarizes; moves toward -90 mV

Inhibitory Postsynaptic Potentials (IPSPs)

Hyperpolarizing changes in potential (caused by binding of neurotransmitters at inhibitory synapses) reduce a postsynaptic neurons' ability to generate an action potential.

DEPOLARIZATION

Increases the probability of producing nerve impulses

GRADED POTENTIAL function [postsynaptic (IPSP) potential]

Inhibitory -- short-distance signaling; hyperpolarization that spreads to axon hillock; moves membrane potential away from threshold of generating an AP

Electrical gradients

Ions move toward an area of opposite electrical charge.

GRADED POTENTIAL initial effect of stimulus EPSP

Opens chemically gated channels that allow simultaneous Sodium and Potassium fluxes

ACTION POTENTIAL initial effect of stimulus

Opens voltage-gated channels; first opens Sodium channels, then Potassium channels

Retrograde Movement

Organells returning to the cell body to be recycled or degraded

Excitatory Postsynaptic Potentials (EPSPs)

Local graded depolarization events that occur at excitatory postsynaptic membranes

ACTION POTENTIAL distance traveled

Long distance - from trigger zone at axon hillock through entire length of axon (a few mm to over a meter). APs do not decay with distance.

ACTION POTENTIAL function

Long-distance signaling; constitutes the nerve impulse

GRADED POTENTIAL

Magnitude varies with strength of stimulus. Stronger stimulus=more voltage changes=further current flow

Hyperpolarization

Membrane becomes more negative than -70 mV. Reduces probability of producing nerve impulses

Threshold

Membrane trigger point, at which point an action potential will be generated

ACTION POTENTIAL positive feed back cycle

Present

Integration

Processing and interpretation of input

Sensory Input

Monitoring and gathering of stimuli that occurs both inside and outside the body.

Oligodendroctyes

Mutliple flat processes that can coil around as many as 60 axons at the same time.

Neurons

Nerve cells that are responsive to stimuli (excitable) and transmit electrical signals. *Structural unit of the nervous system.*

Afferent Division of PNS

Nerve fibers (axons) that convey impulses to the CNS from sensory receptors located through the body (skin, skeletal muscles, joints, and visceral organs) "Carrying toward" *SENSORY*

Schwann Cells

Neuroglia of the PNS. Neurolemmocytes surround all nerve fibers in the *PERIPHERAL NERVOUS SYSTEM* and form myelin sheaths around thicker nerve fibers. Similar in function to oligodendrocytes. Vital to regeneration of damaged peripheral nerve fibers.

Satellite Cells

Neuroglia of the PNS. Surround neuron cell bodies. Function like astrocytes of the CNS. Fancied resemblance to moons around a planet.

Dendrites

Neuron Process. Short, tapering, diffusely branching. Motor neurons have hundreds of twiglike processes clustered close to cell body. Main receptive or input regions. *ALWAYS UNMYELINATED*

Central Nervous System (CNS) Brain & Spinal Cord

Occupies dorsal cavity and is integrating and control center. Interprets sensory input and dictates motor output based on reflexes, current conditions, past experiences.

CONTINUOUS CONDUCTION

Occurs on unmyelinated axons only. Action potentials are generated at sites immediately adjacent to each other. Conduction is relatively slow.

Voltage-gated channels

Open and close in response to changes in the membrane potential. Responsible for generation and propagation of the action potential, the outgoing signal from the neuron.

Mechanically gated channels

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

Chemically gated or ligand-gated channels

Open when the appropriate chemical (neurotransmitter) binds. Responsible for synaptic potentials, the incoming signals to the neuron.

Repolarize

Opening of voltage-gated K+ channels cause membrane to do what? (Page 401) and K+ moves OUT of the cell

Depolarize

Opening of voltage-gated channels causes the membrane to ____________ (voltage change).

GRADED POTENTIAL initial effect of stimulus IPSP

Opens chemically gated Potassium or Chlorine channels

Peripheral Nervous System (PNS)

Outside the central nervous system; consists of bundles of paired axons extending from brain and spinal cord.

Autonomic Nervous System (ANS)

Part of the *Motor or Efferent Division of the PNS* Visceral motor nerve fibers regulating the activity of smooth muscles, cardiac muscles, and glands. "A law unto itself". Involuntary Nervous System.

Somatic Nervous System

Part of the PNS *Motor or Efferent Division* Conducts impulses from the CNS to skeletal muscles. *Voluntary* Nervous System. Conscious control of skeletal muscles. Motor nerve fibers

Somatic Sensory Fibers

Part of the PNS *Sensory or Afferent Division* Convey impulses from the skin, skeletal muscles, and joints -- *soma=body*

Visceral Sensory Fibers

Part of the PNS *Sensory or Afferent Division* Transmits impulses from the visceral organs (organs within the ventral body cavity)

Neuron Cell Body

Perikaryon or *soma*. Major biosynthetic center of a neuron. Clustered free ribosomes and rough ER. Most active and best developed in the body. *RECEIVES SIGNALS FROM OTHER CELLS AND SENDS THEM TOWARDS THE AXON*

Axolemma

Plasma membrane of axon

Potassium

Plays the most important role in generating membrane potential

Neurilemma

Portion of the Schwann cell, next to the exposed part of its plasma membrane, also called the *outer collar of perinuclear cytoplasm*

Microglial Cells

Small and ovoid with long thorny processes that touch nearby neurons. Monitor neuronal health and migrate toward injured/troubled neurons. *Can transform into macrophage* phagocytizing microorganisms and neuronal debris. Immune cells have limited access to CNS so these play important protective role.

Neuroglia

Smalls cells that surround and wrap the more delicate neurons. GLIAL. Glue or scaffolding that supports neurons

Frequency of action potentals

Stimulus intensity is measured by number of impulses per second, or _____________, rather than by increases in the strength (amplitude) of the individual action potentials.

GRADED POTENTIAL summation

Stimulus responses can summate to increae amplitude of graded potential. Temporal (increased frequency of stimuli) Spatial (stimuli from multiple sources)

More often

Strong stimuli generate nerve impulses *more often or less often* in a given time interval than do weak stimuli?

Unipolar Neuron

Structural Classification 1 process Found primarily in the Peripheral Nervous System

Bipolar Neuron

Structural Classification 2 processes Rare, found in some special sense organs (retina)

Multipolar Neuron

Structural Classification 3 or more processes Most common Primary neuron in CNS

Insulators

Substances with high electrical resistance

Conductors

Substances with low resistance

Selective permeability

The characteristic of a cell membrane that permits some particles to cross it but prevents other particles from crossing.

Electrochemical gradient

The combined difference in concentration and charge; influences the distribution and direction of diffusion of ions.

Concentration gradients

The difference in the concentration of a particular substance between two different areas. Ions move down the concentration gradient from the area where their concentration is high to the area where their concentration is low.

Current

The flow of electrical charge from one point to another Amount of charge that moves between the two points depends on two things: voltage and resistance.

Equilibrium potential for potassium

The membrane potential at which the electrical and chemical forces that drive the ions across the cell membrane are equal and opposite. (-90 mv)

Resistance

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

Neuron Function

The ion flow along an electrochemical gradient is the basis of neuron function.

Synapse

The junction that mediates information from one neuron to another neuron or an effector cell -- can be electrical or chemical!

Voltage

The measure of potential energy generated by separated charge. Measured in either volts (V) or millivolts (1mV = 0.001 V). Always measured between two points and is considered the *potential* between the points. The greater the difference in charge, the higher the voltage.

Propogation of a nerve impulse

This means that the action potential is regenerated anew at each membrane patch, and every subsequent action potentail is identical to the one that was initially generated.

The neuron cannot respond to another stimulus no matter how strong it is.

What happens when a patch o neuron membrane is generating an AP and its voltage-gated sodium channels are open?

Axodendritic

What is a synapse called that occurs between the axon of one neuron and the dendrite of another

Axosomatic

What is a synapse called that occurs between the axon of one neuron and the soma (cell body) of another

THEY ARE CELLS WITH EXCITABLE MEMBRANES

What is special about neurons and muscle cells that they can generate action potentials?

OHM'S LAW Current (I) = voltage(V) / resistance (R)

The relationship between voltage, current, and resistance. Current (I) is directly proportional to voltage. The greater the voltage (potential difference), the greater the current. There is no net current flow between points that have the same potential. Current is inversely related to resistance. The greater the resistance, the smaller the current.

Resting membrane potential

The voltage that exists across the plasma membrane during the resting state of an excitable cell. [from 290 to 220 millivolts] Sodium and Potassium determine resting membrane potential; Na-K pump maintains resting membrane potential along with non-gated channels [potassium leak channels]

Nodes of Ranvier

Tiny areas of bare axon between neighboring segments of myelin sheath. IN a myelinated axon, charge flows across the membrane only at the nodes of Ranvier so the action potential appears to jump along the axon.

Function of an EPSP

To help trigger an action potential distally at the axon hillock of the postsynaptic neuron

Electrical, chemical

Transmission of nerve impulses along an axon and across electrical synapses is a purely ___________ event, but _____________ synapses convert the electrial signals to neurotransmitters (chemical signals). The chemical signals travel across the synapse and are converted back to electrical signals.

PROPAGATING

Transmitting the action potentials

Ion Channels

Watery pores contained within integral proteins, large protein molecules embedded within cell membranes of neurons.

Dendrites and cell body

What areas of neuron generate signals that open voltage-gated channels?

Inactivation gates of the Na+ channels begin to close Voltage-gated K+ channels open

Two processes that stop the potential from rising above +30 mV

GRADED POTENTIAL amplitude

Various sizes (graded); decays with distance

Resting neurons

Very permeable to K+ and only slightly permeable to Na+. Also permeable to Cl- but it contributes little to the resting membrane potential

ACTION POTENTIAL stimulus for ion channels

Voltage (depolarization, triggered by graded potential reaching threshold)

GRADED POTENTIAL repolarization

Voltage independent; occurs when stimulus is no longer present

ACTION POTENTIAL repolarization

Voltage regulated; occurs when Sodium channels inactivate and Potassium channels open

Presynaptic neuron

What conducts impulses toward the synapse (the sender)