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)